Download Application of ion scattering techniques to characterize polymer

Materials Science and Engineering R 38 (2002) 107±180
Application of ion scattering techniques to characterize
polymer surfaces and interfaces
Russell J. Compostoa,*, Russel M. Waltersb, Jan Genzerc
a
Laboratory for Research on the Structure of Matter, Department of Materials Science and Engineering,
University of Pennsylvania, Philadelphia, PA 19104-6272, USA
b
Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272, USA
c
Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA
Abstract
Ion beam analysis techniques, particularly elastic recoil detection (ERD) also known as forward recoil
spectrometry (Frcs) has proven to be a value tool to investigate polymer surfaces and interfaces. A review of ERD,
related techniques and their impact on the field of polymer science is presented. The physics of the technique is
described as well as the underlying principles of the interaction of ions with matter. Methods for optimization of
ERD for polymer systems are also introduced, specifically techniques to improve the depth resolution and
sensitivity. Details of the experimental setup and requirements are also laid out. After a discussion of ERD,
strategies for the subsequent data analysis are described. The review ends with the breakthroughs in polymer science
that ERD enabled in polymer diffusion, surfaces, interfaces, critical phenomena, and polymer modification.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ion scattering; Polymer surface; Polymer interface; Elastic recoil detection; Forward recoil spectrometry;
Depth profiling
1. Introduction
Breakthroughs in polymer science typically correlate with the discovery and application of new
experimental tools. One of the first examples was the prediction by P.J. Flory that the single chain
conformation in a dense system (i.e. a polymer melt) is ideal and follows (nearly) Gaussian statistics
[1]. In the Flory model, chains interpenetrate and have a radius of gyration that varies as a(N/6)1/2
where N and a are the segment number and size, respectively. Although polymer scientists now take
this fundamental law for granted, over 20 years passed before this model was proven. By blending a
dilute concentration of deuterated molecules with identical chains (natural abundance of hydrogen),
small-angle neutron scattering (SANS) experiments were able to directly determine chain
conformation [2±4]. SANS is now a standard technique in the polymer scientist's toolbox for
studying the bulk thermodynamic properties of polymer mixtures and solutions. Today new
experimental techniques continue to push the frontiers as demonstrated by the recent imaging of
individual molecules using the scanning force microscope [5].
Relevant to this review, our understanding of polymer surfaces and interface problems of
fundamental and technological importance has been greatly advanced by ion beam analysis IBA,
techniques. When light ions at MeV energies are incident on a target, some ions transfer energy to
lighter target nuclei in an elastic collision such that the target nucleus recoils and ejects from the
*
Corresponding author. Tel.: ‡1-215-985-1386; fax: ‡1-215-573-2128.
E-mail address: [email protected] (R.J. Composto).
0927-796X/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 7 - 7 9 6 X ( 0 2 ) 0 0 0 0 9 - 8
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
target. By detecting the energy of the recoiling nuclei the depth-profile perpendicular to the surface
of the target is measured. In ``cleaning up the tower of babel of acronyms (CUTBA) in IBA'', the
consensus of the ion beam community was to call this technique elastic recoil detection (ERD)
analysis [6]. Although forward recoil spectrometry (FRES) is the acronym most used by polymer
scientists, it is time for the polymer community to adopt the ERD convention to avoid further
confusion.
For several reasons ERD is a natural technique to study polymer systems. Whereas polymers are
predominantly comprised of carbon and hydrogen, many polymers are available in their deuterated
analogs. For example, the same polybutadiene can be deuterated or hydrogenated to produce isotopic
Ê , respectively, the
blends having identical values of N. Because N and a are typically 1000 and 5 A
Ê
natural length scale for many surface and interface phenomena is 100 A, comparable to the depth
resolution of ERD. Since no one technique can provide the necessary depth resolution, lateral
resolution, sensitivity and quantification, physical scientist are increasingly using a combination of
complementary depth profiling techniques to better understand surface and interfacial issues. For
example, consider the enrichment of one component at the surface of a two component blend. ERD,
a direct profiling technique, can be used to determine the surface excess independent of any models,
whereas neutron reflectivity, a model dependent technique provides details about the shape of the
profile. Because patterning is an area of increasing interest, techniques with good depth and lateral
resolution will be needed in the future. Ideally, such techniques will allow scientists to address
interface issues regardless of geometry (e.g. fibers).
The most comprehensive guide to ERD is ``FRES'' by Tirira et al. [7]. This text includes a
detailed analysis of ion interactions in solids, provides a review of cross-sections important in recoil
analysis, and describes variations of ERD including conventional, time-of-flight (TOF) and
coincidence techniques. A general background for ERD is provided in [8,9]. Several brief review
articles covering ion beam analysis of polymers are also available [10±13].
This review is intended to educate polymer scientists about ion beam techniques, particularly
ERD, and make ion beam users aware of breakthroughs in polymer science brought about by ion
beam analysis. Thus, Sections 2 and 3 are dedicated to reviewing the fundamental interaction
between ions and solids, and the basic principles of ion beam techniques with an emphasis on ERD.
To help polymer scientist optimize ERD experiments, Section 4 describes depth resolution,
sensitivity and beam damage. In Section 5, strategies for analyzing and simulating data are
presented. Section 6 reviews related IBA techniques as well as complementary ones. To demonstrate
the impact of ion scattering on polymer science, Section 7 presents selected case studies in diffusion,
surfaces, interfaces, critical phenomena and polymer modification. Although these studies mainly
focus on ERD, the utilization of other ion beam techniques such as nuclear reaction analysis (NRA)
are also included.
2. Overview of ion beam analysis techniques
2.1. Introduction
When a beam of high-energy, MeV, incident atoms strikes a surface, three main interactions are
possible. An incident atom could strike a target atom on the surface of greater atomic number, say an
incident 4 He striking a target 12 C. In this case, the incident atom undergoes an elastic collision and
the 4 He will be repelled back toward the source of the beam, or backscattered. This interaction is the
basis of Rutherford backscattering spectrometry (RBS). In RBS, the energy of the backscattered
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
incident ions are detected and the elemental composition of the sample can be determined. Another
possible outcome occurs when the incident atom posses greater mass than the target atom, 4 He
striking 1 H. In this case, after the collision event due to the physics of an elastic collision, the
incident 4 He atom cannot be backscattered, but will continue in the direction forward and the 1 H will
be recoiled and expelled from the target sample. This interaction is the basis of ERD. The energy of
the expelled recoiled 1 H atom is detected in the forward direction to determine the depth-profile of
1
H.
The third possibility is that the incident atoms penetrate the surface and simply lose energy via
low impact collisions with electrons. Eventually, at some depth below the surface, the incident ion
undergoes an elastic collision and then travels back out of the sample again losing some energy. This
well-defined loss of energy as the incident atom travels through the sample provides the unique
depth profiling capability of RBS.
2.2. Rutherford backscattering spectrometry (RBS)
The basic principles of ion beam analysis of materials were discovered more than 90 years ago
by Rutherford when he bombarded solid targets with alpha particles [14]. His experiments provided
the foundation for RBS. A detailed description of RBS can be found in the literature [11,15±17].
Here, we restrict ourselves to a brief description. RBS can be used for elemental determination and
to probe the depth-profile of heavier elements. In order for backscattering to occur, the target
elements must posses a larger atomic mass than the incident ion. For polymer investigations, the
usual incident ion is 4 He, so any element larger than helium can be identified, although it does
become increasingly more difficult to distinguish elements with very large atomic masses. Most
elements of interest in polymer studies 12 C, 14 N, 16 O, 19 F, 31 P, 32 S and 35 Cl, can be resolved using
RBS or some variation of RBS such as glancing angle RBS.
A schematic of a typical set up is shown in Fig. 1. The RBS experiment consists of a
monoenergetic beam of ions typically produced by a tandem accelerator (cf. Fig. 17), accelerated to
an energy, Ein,0, of a few MeV that are then focused on a sample. After colliding with a heavier atom
in the sample, the projectile is backscattered with an energy Eout,0 and travels outward to a solid state
Fig. 1. Experimental configuration of RBS. An accelerator provides high-energy light ions that strike a planar surface and
are backscattered to a detector. The detector registers the energy of each backscattered ion. The entire beam line and
sample chamber is under vacuum.
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Fig. 2. Schematic representation of an elastic collision between a projectile of mass Mp and velocity vp, and energy, Ein and
a target mass, Mt, which is initially at rest. After the collision, the projectile and the target mass have velocities v0p , and v0t ,
respectively. The angle F is positive as shown. All quantities refer to a laboratory frame of reference.
detector that is placed at angle y with respect to the incident beam. Since the interaction between the
projectiles and the target atoms is elastic, the simple rules of conservation of energy and momentum
transfer can be applied to derive the relations between the scattering geometry and the energies of the
incoming and scattered projectiles. Fig. 2 shows pictorially the motion of the atoms before and after
a scattering event. At the sample surface, the energy of a projectile with mass Mp that is
backscattered from a target atom with mass Mt is related to the energy of the incoming ion via
Eq. (1):
Eout;0 ˆ KEin;0
where K is the kinematic scattering factor defined by
q32
2
Mp cos y ‡ Mt2 Mp2 sin2 y
5
Kˆ4
Mp ‡ Mt
(1)
(2)
Eq. (2) demonstrates that each element at the sample surface can produce a backscattered ion with a
characteristic value of energy, Eout,0. In addition, projectiles scattering from heavier atoms in the
target lose a smaller portion of their energy because the kinematic factor is larger. So larger target
atoms produce a backscattered ion with higher energy, and consequently an elemental signature that
appears at higher energy.
However, a majority of the incident 4 He atoms do not strike a surface atom, but instead
penetrate into the sample before scattering. As the 4 He ions traverse the sample, they collide with
electrons and loss a well-defined amount of energy that is proportional to the distance of the path
traveled. Although this energy loss occurs by discrete interactions, statistically the overall process
can be considered continuous. At some depth, x, in the sample the ion can undergo a scattering event
and the backscattered ion loses more energy as it travels back out of the sample again proportional to
the distance traveled through the sample (cf. Fig. 1). This is the origin of the depth profiling
capability of RBS. Finally, the backscattered ion travels to the detector where the ion is converted to
a count with a specific energy. Fig. 1 steps through the sources of energy loss in an RBS experiment.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
111
For a beam directed normal to the sample surface, the detected energy of an ion backscattered from
depth t is
energy detected ˆ K…incident energy energy lost on inward path†
or
"
ED …x† ˆ K Ein;0
dE
dx
#
He;in
t
dE
dx
t
He;out cos y
energy lost on outward path
(3)
RBS is useful in analyzing polymers such as ionomers that contain heavy elements. For an
incident 2.8 MeV 4 He2‡ beam, Fig. 3 shows an RBS spectra from a poly(styrene-co-Zn sulfonate)
containing 5 mol% sulfonic acid groups. For each acid group there are 0.5 65 Zn counter ions. The
Ê thick and on a silicon wafer substrate. So the elements in the film that will
ionomer film is 5000 A
12
16
backscatter are C, O, 32 S and 65 Zn having kinematic factors of 0.25, 0.36, 0.61 and 0.78,
respectively. Therefore, the front edges energies are 0, 7, 1.0, 1.7, and 2.2 MeV, respectively.
Because the film is on an infinitely thick 28 Si wafer, a silicon shelf appears in the spectrum. The front
edge is suppressed by 0.35 MeV due to energy loss in the film. At lower energies are the 16 O and 12 C
yields, which superimpose on the silicon signal. Because the yields are proportional to elemental
concentration, the polymer stoichiometry can be determined.
2.3. Elastic recoil detection (ERD)
Whereas it is a useful technique for determining the concentrations of heavy elements in the
target, RBS is not suitable for studying the concentration distributions of lighter elements. This
Ê film of poly(styrene-co-Zn sulfonate) with 5 mol% sulfonic acid groups and
Fig. 3. Typical RBS spectrum, from a 5000 A
50% neutralized with 65 Zn. Due to the kinematic factor the heavy elements appear at higher energies. Also the silicon
signal from the substrate is suppressed because it is buried below the film.
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drawback limits the application of RBS analysis of polymers to limited cases. In some experiments,
labeling the polymers with a heavy element marker or even mixing with non-polymeric materials
that bear heavier elements can overcome this obstacle. However, one has to be aware that this
approach could strongly affect the thermodynamic behavior of the system. In spite of this, with just a
small adjustment of the scattering geometry, one can use the same ion beam to study the
concentration profiles of light target elements, i.e. hydrogen. The necessary contrast is achieved by
labeling one of the components with deuterium. In most cases, the deuterium labeling does not
produce drastic changes to the thermodynamic behavior of the analyzed polymers. A technique used
to depth-profile 1 H and/or 2 D in materials is called ERD. Introduced by L'Ecuyer and co-workers in
1976 [18] and later optimized by Doyle and Peercy in 1979 [19], ERD became a valuable tool for
studying the distribution of 1 H and 2 D in many materials including polymers [20].
Traditional ERD provides 1 H and 2 D concentration profiles with a surface resolution of ca.
Ê and can probe ca. 7000 A
Ê below the surface. By decreasing the beam energy and optimizing
800 A
Ê have been measured with a probe depth of 700 A
Ê.
the geometry, surface resolutions as good as 80 A
ERD is also sensitive enough to measure less than a monolayer of hydrogen or deuterium, if a large
dose is used. These attributes make ERD an attractive technique for studying diffusion, surface/
interface segregation, polymer adsorption and phase separation.
To perform ERD, the only change in the RBS experimental apparatus is a detector in the forward
direction of the beam and a goniometer to rotate the sample to glancing angles. A diagram of a sample
chamber for doing RBS and ERD is shown in Fig. 4. The ERD and RBS geometries are depicted with
dotted and solid lines, respectively. Typically ERD uses 4 He incident particles to determine the
concentration profile of both 1 H and 2 D in a polymer sample. Since 4 He has an atomic mass of 4 amu
the only species that it can forward scatter are 1 H and 2 D; hence, the necessity of the detector in the
forward position. Also since the lighter atom is recoiled in the forward direction the sample must be
positioned so that a forward recoil particle can escape the sample and travel to the detector. This is
achieved by tilting the sample holder so that the incident beam impinges on the sample at a low angle.
Fig. 5 shows the path of the incident and exiting beam in a typical ERD set-up. Unlike RBS,
after the recoil event the ion of interest is now the lighter 1 H or 2 D. A beam of 4 He‡ or 4 He2‡ is
accelerated to energies of 1±4 MeV. The beam is focused and then collimated so that when it strikes
the sample surface the shape of the beam is well defined and of a limited width, typically 1 mm. The
4
He beam intersects the plane of the sample with at an angle a1. The 4 He atoms that impinge on the
surface cause some of the 1 H and 2 D atoms to be recoiled and expelled from the sample, but again
just as in RBS most of the 4 He atoms enter the sample and lose energy along the path. On the inward
path, the 4 He ions lose an amount of energy that will depend on the path-length via the angle a1. 1 H
and 2 D atoms are recoiled from the sample at all angles, but only those atoms that are expelled at an
angle a2 reach the detector. On the outward path the 1 H or 2 D ion also loses some amount of energy.
Fig. 4. Design of a sample chamber for both RBS (solid lines) and ERD (doted lines) experiments.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
113
Fig. 5. Experimental configuration of ERD. The high energy 4 He ions recoil 1 H and 2 D out of the sample. A detector then
registers the energy of the recoiled 1 H and 2 D.
Thus, in an ERD experiment, recoiled 1 H and 2 D leave the sample surface with energies
characteristic of their depths and kinematic factors. In addition, a high flux of 4 He ions are forward
scattered by heavy atoms in the substrate (i.e. silicon) with an energy that again depends on the depth
of the scattering event. This high flux of 4 He would overwhelm the relatively low flux of 1 H and 2 D.
In order to reduce the number and energy of the scattered 4 He particles that reach the detector, a thin
filter of either aluminum foil or MylarTM, between 2 and 10 mm, is placed in front of the ERD
detector. The filter stops the low-energy 4 He and slows down the high-energy 4 He such that their
energy does not interfere with the 1 H or 2 D signal. However, the filter also slightly slows down both
the 1 H and 2 D recoiled particles and introduces signal broadening due to straggling. Thus, the
detector ``sees'' both 1 H and 2 D at energies related to the depth from which they where recoiled in
the sample as well as the glancing incident 4 He ions at even lower energies. The final energy of the
recoiled 1 H or 2 D atom that emerges from a sample of thickness t is described by
energy detected ˆ K…incident energy energy lost on inward path†
energy lost on outward path energy lost passing through filter
or
"
ED ˆ K Ein;0
#
dE
t
dx He;in sin a1
dE
t
dx H=D;out sin a2
DEfilter
(4)
where K is now the kinematic factor for forward scattering (cf. Eq. (10)). The energies of these
events are shown pictorially in Fig. 6.
Fig. 6. Schematic representation of a recoil event. A high energy 4 He ion recoiling a 1 H ion out of the sample, as in an
ERD experiment.
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 7. ERD spectra of a thin (20 nm) dPS tracer film on a thick (500 nm) PS matrix. The dPS tracer film has diffused into
the matrix after annealing at 145 8C for 1 h.
Ê , deuterated polystyrene (dPS) film on a thick
Fig. 7 shows the ERD spectrum from a thin, 200 A
Ê PS matrix film that was annealed for 1 h at 145 8C. The high and low-energy peaks
5000 A
correspond to the 1 H and 2 D originating from the dPS and PS, respectively. Because it is the heavier
atom, 2 D is detected at higher energies than the hydrogen. The energy at the front edge corresponds
to the sample surface, whereas the lower energies correspond to 2 D (1 H) below the surface. The dPS
tracer film has diffused into the PS matrix as demonstrated by the diffuse 2 D signal. This profile can
be fitted to Fick's second law to determine the dPS tracer diffusion coefficient.
3. Interaction of ions with matter
Rutherford backscattering and related ion analysis techniques are used to identify species type
and concentration, and determine the depth distribution of a given species. For most ion beam
techniques, light MeV incident probes undergo a single-collision with a positively charged nucleus
located within the target material. The kinematics of this elastic collision can be treated by applying
the principles of conservation of energy and momentum. Thus, by measuring the energy of a
backscattered or forward scattered particle involved in an elastic collision, the mass of a target atom
can be identified. As discussed in Section 3.1, mass analysis is the ability to distinguish between
atomic masses of elements present in a target. Because interactions are based on classical scattering
in a central-force field, the scattering cross-section or collision probability can be derived using
simple Coulomb scattering and related to fundamental parameters such as the energy of the incident
particle. Knowledge of the scattering cross-section underlies converting the measured number of
scattering events from a given target with the actual elemental concentration in the target (Section
3.2). One of the beautiful aspects of MeV ion scattering is that most of us are already familiar with
the key concepts underlying mass sensitivity and elemental concentration. The third unique feature
of ion analysis techniques involves the energy loss of MeV light ions traveling through solids. This is
the second mechanism of energy loss in an ion/solid interaction. The energy loss of incident or
exiting particles occurs in a regular manner through the energy transfer from a heavy incident
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
115
particle to a bound or free electron in the target. Although understanding the energy loss mechanism
is still a topic of current research, knowledge of the energy loss in a solid allows one to convert the
measured energy of backscattering from target atoms deep in the sample to a depth-profile (Section
3.3). In summary, ion scattering is a unique quantitative analysis tool because it is based on relatively
well-understood fundamental interactions of ions with matter.
3.1. Mass analysis
The kinematic factor provides ion scattering with mass sensitivity. The atomic level interaction
that gives rise to the kinematic factor is depicted in Fig. 2, where a projectile ion, 4 He, of mass Mp
and velocity, vp, strikes a stationary target ion, 12 C, of mass Mt and zero velocity at a scattering angle
y. Because this is an elastic collision between two particles (i.e. a billiard ball collision), the incident
energy after scattering can be found by applying conservation of energy and momentum parallel and
perpendicular to the direction of incidence, namely,
energy :
2
1
2 Mp v
ˆ 12 Mp …v0p †2 ‡ 12 Mt …v0t †2
(5)
X-axis : Mp vp ˆ Mp v0p cos y ‡ Mt v0t cos F
(6)
Y-axis : 0 ˆ Mp v0p sin y
(7)
Mt v0t sin F
Using Eqs. (5)±(7), the ratio of particle velocities can be found. Using the non-relativistic energy
E ˆ 0:5 Mv2 and taking M p < M t as in RBS:
"
#2
…Mt2 Mp2 sin2 y†1=2 ‡ Mp cos y
Eout;0
ˆ
ˆK
(8)
Mt ‡ Mp
Ein;0
or
Eout;0 ˆ KEin;0
where K is the kinematic factor for elastic scattering. Physically, K determines the amount of energy
transferred to the target atom. The key observation is that the energy after scattering depends only on
the masses of the incident and target atom and the scattering angle. Eq. (8) reduces to
K ˆ ‰…M t M p †=…M t ‡ M p †Š2 at 1808. Note that the kinematic energy loss is greatest for a direct
backscattering event. Therefore, the optimum mass resolution is achieved when the detector is
placed as close as possible to the backscattered direction. Sometimes annular detectors are used in
order to achieve this condition.
Mass resolution is a practical question particularly when backscattering from polymers. Since
1
H is usually lighter than the incident particle (i.e. 4 He), backscattering from 1 H is not allowed
according to the principles of conservation of energy and momentum. RBS may have trouble
resolving neighboring elements on the periodic table using standard beam energies and angles. We
know this by determining the smallest difference in mass DMt that can be resolved. For elements a
and b, the corresponding energy difference is given by
DEout;0 ˆ …Ka
Kb †Ein;0
(9)
There are several strategies for maximizing DEout,0. First, one could maximize K a K b by using
y ˆ 1808. In practice, the detector has a finite size (diameter) and is usually located as physically
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
close to 1808 as possible. As shown in Fig. 4, detectors are usually placed near 1708 either collinear
with the incident beam or below. Furthermore, the incident beam energy can be increased to maximize
Eout,0.
The mass resolution for RBS is mainly limited by the energy resolution of the detector.
Semiconductor detectors are typically used to measure the energy Eout,0 of backscattered particles.
The physics of charged-particle detectors will be reviewed in Section 4.2. Typical energy resolutions,
DED, range from 10 to 20 keV.
Example 1 (Mass resolution in RBS). Consider backscattering 2.0 MeV 4 He‡ ions from a monolayer (10 nm) of poly(tetrafluoroethylene), C2F4 at 180 8C.
Determine whether the carbon and fluorine signals are resolved in a backscattering spectrum?
Element
Mass
Kinematic factor
C
F
12
19
0.25
0.42
So, DEout;0 ˆ 340 keV. Thus, the maxima of the Gaussian shaped yields would be separated by
340 keV which is @DED. In practice, such measurements are difficult because the C and F peaks are
relatively small and usually superimposed on a large background signal from the substrate (e.g. silicon).
3.1.1. Elastic recoil detection
RBS is most effective for studying a film of heavy elements on a substrate of light elements.
Although some polymer interface problems involve heavy elements (i.e. diffusion of halogenated
solvent), most problems involve systems where two or more polymers are chemically similar (e.g.
mainly hydrogen and carbon). However, in many cases it is relatively straightforward to prepare or
purchase a deuterated version of one component. Under these conditions, mass sensitivity involves
differentiating the hydrogen and deuterium signals. Fig. 5 shows the ERD geometry for a
monoenergetic beam of 4 He ions of energy Ein,0 impinging on a sample at an angle a1 with respect to
the sample surface. The lighter target element recoils after undergoing an elastic collision with the
heavy incident particle and gets sent in the forward direction at an angle f with respect to the
incident beam. As in backscattering, the energy Eout,0 imparted to the target element is determined
by conservation of energy and momentum where
Mp Mt
Eout
ˆKˆ4
cos2 F
2
Ein
…Mp ‡ Mt †
(10)
Note that the maximum energy is transferred to the recoiling target particle at F. Traditionally, the
standard ERD geometry utilizes the same path-length into and out of the sample as shown in Fig. 5,
namely, a1 ˆ a2 ˆ 158 and F ˆ 308. Using 3.0 MeV 4 He and a 10.6 mm thick MylarTM stopper
filter, the accessible depth (about 800 nm) and depth resolution (about 80 nm) is suitable for a wide
variety of polymer surface and interface problems. As shown in Section 4.2, the depth resolution can
be greatly enhanced by varying the incident and/or exiting angles.
Example 2 (Mass resolution in ERD). For 3.0 MeV 4 He incident on monolayer mixture of
deuterated and natural abundance PS, determine the energy difference between forward scattered
1
H and 2 D at F ˆ 308.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
117
Fig. 8. The forward recoil spectra of 1 H and 2 H (deuterium) of a thin dPS:PS mixture using 3.0 MeV 4 He ions. The
detector is at F ˆ 308 and a 10 mm thick MylarTM filter is used (taken from [21]).
From Eq. (10), the kinematic factors are K…1 H† …1=2† and K…2 D† …2=3†. Thus, the 2 D
nuclei recoiling from the surface receive a higher fraction of the incident energy than the 1 H nuclei.
So, Eout …D† Eout …H† ˆ …1=6† 3:0 MeV ˆ 0:5 MeV. Therefore, the 2 D peak is well separated
from 1 H compared to the detector resolution.
Fig. 8 shows a spectrum for 1 H and 2 D recoils from a thin dPS:hPS mixture in a typical ERD
experiment [21]. Rather than being at 2.0 and 1.5 MeV, the peak positions of the 2 D and 1 H signals
are located at much lower energies, 1.59 and 1.15 MeV, respectively. The energies of the detected
recoils are shifted to lower values because of energy loss in the stopper filter, which is placed in front
of the detector. When an ion enters this detector a current pulse whose height is proportional to the
ion energy is produced. Thus, the detector does not distinguish between the forward recoiled 1 H or
2
D and the forward scattered 4 He which glance off the heavier nuclei in the film or substrate. The
purpose of the stopper filter is to slow down the heavier 4 He ions and prevent them from
overwhelming the 1 H and 2 D recoil signals. The recoils themselves get slowed down by the stopper
filter but still reach the detector with sufficient energy. Straggling of the recoiling nuclei through the
filter is a major limitation to the energy and depth resolution of conventional ERD.
3.2. Elemental sensitivity
Scattering of the incident ion from a target atom is a rare event. A vast majority of incident ions
penetrate many microns into the sample or substrate before stopping. The concentration, or more
accurately the number Ns of target atoms (molecules) per unit area, is directly proportional to the
yield, Y, or number of detected particles QD:
Q
Y ˆ QD ˆ s…y†O
Ns
qe
(11)
where Q is the total integrated charge of incident ions on the sample, qe the charge per incident ion,
so Q/qe then the total number of incident ions and O is the solid angle of the detector. The
fundamental parameter which determines the probability of scattering is the average differential
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scattering cross section, defined here as s. Physically, s represents the ``area'' presented to the
incident particle by a given target atom. Typically, the solid angle is kept small, on the order of a
millisteradian, so that the cross section is taken as a constant value. If the solid angle is large, the
integral scattering cross-section must be used to account for the range of the scattering angle, y. A
major difference between MeV ion scattering and most other characterization techniques is that ion
scattering is an absolute technique requiring no calibration. This unique advantage results from the
ion±ion cross-section, which is well defined, readily available and accurately known.
The cross section is based on a Coulombic force between a positively charged incident ion and
target nuclei. Because of this interaction, the incident nuclei deviates from its original path and the
target nuclei will recoil. The two-body scattering cross section is
"
#
2
Zp Zt e2
f‰1 ……Mp =Mt †sin y†2 Š1=2 ‡ cos yg2
4
(12)
s…y† ˆ
4Ein
sin4 y
‰1 ……Mp =Mt †sin y†2 Š1=2
were Zp is the atomic number of the incident projectile ions of mass Mp, Zt the atomic number of
target ions of mass Mt, and e is the electron charge. The cross-section is defined in CGS units (cm2)
Ê . To demonstrate the interplay between
so that Eq. (12) can be solved simply by using e2 ˆ 14:4 eV A
atomic mass, atomic number and their respective physical parameters, namely, kinematic factor and
cross-section, respectively, Fig. 9 shows a backscattering simulation for 2.0 MeV 4 He‡ incident on a
monolayer of 12 C, 24 26 Mg and 79;81 Br (i.e. C0.33Mg0.33Br0.33) at y ˆ 1708. Each signal is well
separated due to the widely different masses. Furthermore, the heavy elements clearly scatter more
strongly than the larger ones due the contrast in Z. The isotopes of Br and Mg demonstrate how the
mass sensitivity decreases as the atomic number increases. Based on Example 1, the reader should be
able to determine why the yields from the Mg isotopes (79% of 24 Mg, 10% of 25 Mg and 11% of
26
Mg) can be resolved and conversely why the isotopes of Br (50.5% of 79 Br and 49% of 81 Br) are
indistinguishable. In Eq. (12), the bracketed mass dependent term accounts for the recoiling of the
Fig. 9. RUMP simulation of a stochiometric 12 C, 24;25;26 Mg and 79;81 Br monolayer. Heavier elements have larger cross
sections and consequently higher yields. Note that isotopes of Mg are distinguished whereas those from Br are not.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 10. The carbon cross-section at y ˆ 170:58 for 4 He energies between 1.6 and 5.0 MeV, normalized by the Rutherford
cross-section, scR . Note that the cross-section is enhanced by a factor of 6 at energies just below 3.583 MeV (taken from
[22]).
target atom and acts to decrease the probability for scattering. For heavy targets like silicon the
correction is small, 4% at 1808, even though the energy transferred to silicon is about 50%.
However, for light elements like those found in polymers, the correction can be significant. For
example, the cross-section correction for 4 He incident on 12 C is 22%.
Because the cross-section scales as Zt2 backscattering is most sensitive to heavy target elements.
In fact, backscattering using light incident ions like 4 He has not been used frequently to study
polymer interfaces because the low Zt and small Mt produces a low yield and small Eout,0,
respectively. Consequently, the backscattering peaks from these light elements in the polymer are
usually small and overlap with much stronger signals coming from either the substrate or heavier
target elements. However, for selected problems, RBS is a powerful technique for characterizing thin
polymer films. In this case, accurate cross-sections are critical for determining the composition of
polymeric films. As a rule of thumb, for incident 4 He ions at energies of 2.0 MeV, the Rutherford
cross-section is usually valid. Fig. 10 shows that the carbon cross section at a y of 170.58 is
Rutherford up to about 2.2 MeV [22]. Note that the cross section is reduced below the Rutherford
value between 2.5 and 3.0 MeV. Although this behavior decreases elemental sensitivity, as long as
one uses the proper cross-sections RBS can still be used. On the contrary, utilizing the enhanced
cross section at, for example, an incident energy of 3.4 MeV can enhance carbon sensitivity. This
method was used to determine the carbon-on-glass concentration in spin polymer films exposed to
thermal and ultra violet-irradiation [23,24].
The profiling of light elements such as 1 H and 2 D in polymers involves knowing the scattering
cross-section for the recoiled elements. Also, the resonant and non-resonant nuclear reaction
techniques require an accurate measurement of the cross-section. Unlike Rutherford scattering, no
simple analytical form of the cross-section is available to describe the probability of reaction
between an incident particle like 4 He and a light target nucleus. A rigorous study of the absolute
cross section for hydrogen forward scattering was carried out by Baglin et al. [25] and compared
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 11. Hydrogen forward recoil cross-sections, sH, plotted as a function of 4 He ion energy for F ˆ 308. The solid lines
through the data points are arbitrary polynomial fits to the data points. For comparison, the cross-section derived from the
Rutherford formula is shown (taken from [25]).
with literature values. The hydrogen cross-section for 1.0±2.9 MeV 4 He at f of 20, 25, 30 and 358
were measured using a thin PS film. PS was chosen because the monomer unit (C8H8) contains
50 at.% hydrogen. Moreover, because it mainly cross-links, PS is relatively stable during exposure to
MeV 4 He radiation. Fig. 11 shows the 1 H cross-sections in mb/sr at 308 along with the calculated
Rutherford values [25]. Although scattering is approximately Rutherford at energies near 1.0 MeV,
the hydrogen cross-section decreases much more weakly with increasing energy compared with the
Rutherford 1/E2 behavior. For example, at 2.9 MeV the cross-section is enhanced by a factor of 2.5
over the Rutherford value. Conventional and TOF±ERD are performed using 3.0 MeV 4 He ions
whereas low-energy ERD (LE-ERD) requires knowledge of the cross-section at energies ranging
from 1 to 2.5 MeV.
The deuterium cross section for 1.0±2.9 MeV 4 He was also measured by Kellock and Baglin
[28] and compared with literature values. Once again polymeric films, in this case dPS (C8D8), were
used as standards. The deuterium cross section was found to have a resonance at 2.135 MeV with a
60 keV full-width half maximum (FWHM). The resonance was strongest at 208 and systematically
decreased as the angle was increased to 408. For simplicity, ERD experiments on deuterated samples
should be carried out either far above or below the resonance range, i.e. 2.0 MeV < Ein < 2:3 MeV.
This is one reason why Genzer et al. developed LE-ERD using 1.3 MeV 4 He‡ [26]. To increase
accessible depth, Genzer et al. increased the energy to 2.0 MeV 4 He‡ which is still below the
resonance range [27]. Fig. 12 shows the deuterium cross section at 308, the conventional scattering
angle in ERD [28]. Note the resonance near 2.1 MeV. As in the hydrogen case, the Rutherford crosssection falls below the experimentally measured one. In contrast to hydrogen, the deuterium crosssection does not agree with the Rutherford prediction near 1.0 MeV. As mentioned by Kellock and
Baglin, Rutherford scattering for 4 He on 2 D is not expected for 4 He energies greater than 0.3 MeV.
Unlike the hydrogen case, the literature values for the 2 D cross-section seem to agree with each
other.
Ê ) film containing a mixture of PS and
As an example, we consider the yield from a thin (200 A
1
2
dPS. Fig. 8 shows the resulting yields of H and D using an incident beam energy, recoil angle, and
integrated charge of 3.0 MeV 4 He, 308, and 20 mC, respectively. From Figs. 11 and 12, the 2 D to 1 H
cross section ratio is 1.90. Correspondingly, the ratio of Ns,D to Ns,H is 0.53 and, thus the film
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 12. Deuterium cross-section as a function of energy plotted at F ˆ 308. The uncertainty of the cross-section scale is
estimated to be 5%. The polynomial fit is continuous, with the data points in the resonance region omitted, as explained
in the text. The dotted line is the cross-section given by the Rutherford formula (taken from [28]).
contains approximately 33 at.% 2 D and 66 at.% 1 H (i.e. PS0.66dPS0.33). As mentioned in Section 3.1,
the 2 D nuclei receive a higher fraction of the incident energy than the 1 H nuclei and, therefore, the
2
D peak is shifted to higher energies. Although the kinematic factors are about two-thirds and onehalf for 2 D and 1 H, respectively, the energies of the detected recoils are shifted to lower values than
the calculated ones. This shift results from energy loss in a stopper filter placed in front of the
detector in ERD experiments. The energy loss of 2 D and 1 H in the stopper filter involves the same
mechanism that imparts ion scattering with depth profiling capabilities.
3.3. Depth sensitivity and energy loss
Ions traversing through a solid lose energy by screened Coulomb collisions with target nuclei
and interactions with bound and free target electrons [29]. For MeV scattering the nuclear energy
loss is approximately three orders of magnitude smaller than the electronic energy loss. Nuclei
energy loss is mainly of interest in ion implantation and sputter±etching techniques like secondary
ion mass spectrometry SIMS. Electronic interactions are the dominant mechanism for energy loss in
the ion scattering analysis techniques of interest to this review and will be the focus of this section.
The Stopping and range of ions in matter (SRIM1) is a flexible software program for calculating the
interaction of ions with solids [30]. SRIM1 calculates the distribution of ions in complex materials
with up to eight layers. One can select the incident beam species and energy in addition to a material
composition. This latter feature is particularly useful for polymer scientists because the stopping
power has been directly measured in only a limited number of polymers. However, before using such
a program, one must understand how energetic ions interact with matter.
As an energetic ion moves through matter, the ion loses energy by exciting or ejecting electrons
from the target atom. This interaction is through a Coulomb force acting on the electron due to the
passing of a charged-particle. Because of the light electron mass, the path of the incident ion does not
change significantly. Moreover, because there are numerous electrons in the target, the incident ion
slows down in a nearly continuous fashion. One can readily calculate the energy loss per unit pathlength, (dE/dx), due to the momentum transfer between incident particle and target electrons. A
second contribution due to incident particles outside the electron orbit undergoing small momentum
transfers resulting from electron excitation is more difficult to derive. At high incident energies, the
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
electronic stopping power is given by
Mp
dE 2pZ 2 e4
2me v2
ln
NZt
ˆ
dx
Ein;0
me
I
(13)
where N is the atomic density, me the electron mass, v the ion velocity, and I the excitation energy of
an electron. Not only does (dE/dx) decrease with increasing incident energy but dE/dx varies
systematically as 1/E. Because of this systematic variation, the detected energy range can be readily
converted into a depth-profile.
The energy loss has been measured for elemental targets and some compounds [30]. For new
compounds and polymers, Bragg's rule can be used to calculate the stopping power from the
individual components. Alternatively, the stopping power can be determined experimentally by
measuring the energy loss, dE, in a film of known thickness, dx [31,32]. To account for the atomic
density, the stopping cross-section is frequently used:
e …eV cm2 † ˆ
1 dE
N dx
(14)
where N is the atomic density. Fig. 13 shows the energy loss, dE/dx, for 1 H ions in PS. The maximum
energy loss is near 0.11 MeV and corresponds, approximately, to the conditions when the incident
ion velocity is equal to the Bohr velocity of an electron, v0 ˆ 2:188 108 cm/s. For ion velocities
much greater than v0 , the fast collision regime described by Eq. (13) is valid. Fig. 13 shows that in
this regime the stopping power displays decreases monotonically for energies greater than 0.6 MeV.
Ê , or about
As seen from the plot, 2.0 MeV 1 H ions will lose 18 eV while passing through each A
1800 eV while traversing a film whose thickness is comparable to the size of a typical polymer
Ê . Note that this energy loss (1.8 keV) is much less than the detector resolution.
chain, Rg100 A
As mentioned earlier, for compounds, molecules, and mixtures, the stopping cross section can
be calculated using Bragg's rule, which assumes that the target atoms contribute independently to the
total energy loss regardless of bonding. For example, in an A/B mixture the stopping power of the
Fig. 13. Stopping power for 1 H ions traversing a PS matrix over a wide range of energies. Note that the stopping power
decreases monotonically >0.6 MeV.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
123
compound is determined from the weighted average of the stopping powers of the two pure
elemental targets:
e ˆ xeA ‡ yeB
(15)
Tabulated values of elemental stopping powers are widely available [33]. Although one can use
Eq. (15), the stopping powers of polymers should be directly measured when possible because
elemental densities used in Eq. (15) will not necessarily match the known polymer density. For
example, Wallace et al. measured the stopping powers of 1±3 MeV hydrogen and helium in both
PS [31] and polyimide [32] using a thin-film backscattering method by sandwiching a polymer
layer between very thin gold layers. This method is particularly sensitive to the energy loss in the
film. In polyimide, the measured 1 H and 4 He stopping powers were found to be in excellent
agreement with both Bragg's rule (Eq. (15)) and the cores-and-bonds model, which accounts for
chemical bonding in compounds. Similar results were found for the 1 H stopping power in PS.
Fig. 14 shows that the 4 He stopping power varies from 27.4 to 14.1 eV/(1015 atoms/cm2) as Ein
increases from 1 to 3 MeV [31]. The measured stopping power agrees well with Bragg's rule at
energies above 1.5 MeV and with the CAB model below 1.5 MeV. Further studies are needed to
clearly differentiate between these models. Leblanc et al. [34] showed that corrections to the
stopping power due to chemical bonding are generally small except near the maximum stopping
power region. Calculations of light ion stopping powers near the maximum have been worked out
by Sigmund [35]. Including bonding effects could be particularly important in the case of LEERD where one uses low-energy 1 MeV 4 He ions. This observation reinforces the need to
directly measure the stopping powers of polymers whenever possible rather than rely solely on
software calculations based on Eq. (15).
To determine a depth-profile from ion analysis one relies on knowledge of the energy loss of the
incident ion along the inward path and the exiting ion along the outward path. The objective is to
relate the energy of the detected particle to the depth at which the incident ion collides with the target
Ê.
atom. As a rule of thumb, the energy loss of MeV 4 He ions in most hard materials is 30 to 60 eV/A
4
For polymers, this value is usually smaller. For example, for 1.5 MeV He ions in PS the energy loss
Ê . The thin-film analysis method is one approach for approximating the energy loss.
is about 20 eV/A
For example, for an incoming ion incident perpendicular to the sample surface, the total energy loss
Fig. 14. Stopping power (filled circles) for helium in polystyrene. The stopping powers predicted by Bragg's rule (dashed
line) and the cores-and-bonds model (solid line) are also shown (taken from [31]).
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
is proportional to the path-length traversed:
dE
DEin;1 ˆ
x
dx in
(16)
Because the film is thin, the energy loss is evaluated using an average value between the initial
energy Ein,0 and the energy before impact. The outward energy loss of 4 He in RBS and light ions in
ERD can be determined in a similar manner. For RBS, the energy width of a backscattering signal is
dE
1 dE
ˆ t‰SŠ
(17)
DE ˆ t K ‡
dx in jcos yj dx out
where t, y and [S] are the thickness, scattering angle and backscattering energy loss factor,
respectively. The key result here is that the energy width is directly proportional to the film
thickness. Thus, by assuming a constant energy loss, Eq. (17) demonstrates that the energy axis is
linearly related to the depth scale. Eq. (17) can also be presented as
t
(18)
ein ˆ t‰eŠ
DE ˆ t Kein ‡
jcos yj
It is important to point out that ions lose energy continuously (cf. Figs. 13 and 14) and, therefore,
using one value to represent energy lose is only valid for very thin-films.
Fig. 15 shows the RBS spectrum from a poly(xylenyl either) (PXE) thin-film (about 1 mm)
which has been stained with bromine [36]. With increasing energy, the carbon and oxygen signals
from the polymer film, and the silicon substrate signals are given. The sharp step in yield at
1.76 MeV is due to 4 He ions scattered from the bromine at the surface. From the bromine energy
width, the thickness of the film was found to be about 1 mm. To improve accuracy, RUMP [37] was
used to simulate the spectrum using a film thickness of 915 nm. Note that the thickness calculation is
based on knowing the composition, which in turn is used to determine the stopping power from
Bragg's rule.
Fig. 15. RBS spectra (*) of 2.20 MeV 4 He ions backscattered from pure PXE (F ˆ 0:0). The sample was stained in a
bromine and methanol solution for 24 h. Simulated spectrum (Ð) where the thickness and mer unit of PXE are 915 nm and
C8H7.7OBr0.14. The energies at which the 4 He ions would be backscattered by 12 C, 16 O, 28 Si and 80 Br nuclei at the surface
are marked (taken from [36]).
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
125
Depth profiling in ERD is similar to RBS except that the outgoing energy loss results from a
light ion, either 1 H or 2 D, exiting the target. The energy of the outgoing particle is given by
Eout;0 ˆ KEin;1
‰SŠt
(19)
where
‰SŠ ˆ
KSin
Sout
‡
cos…90 a1 † cos…90 a2 †
(20)
For ERD and LE-ERD, the energy loss of the recoil in the stopper filter, dEs, must be included so
that ED ˆ Eout;0 dEs . Therefore, the 1 H or 2 D energy width is given by
DE ˆ ‰SŠx ‡ ‰dEs;xˆ0
dEs;x Š
(21)
where the second term accounts for energy broadening in the stopper filter. At high energies, where S
decreases as 1/E, the second term is negative and, therefore, the detected energy width is
compressed. For energies near the stopping power maximum of MylarTM the stopping power will
change in a non-linear fashion. Under these conditions, the second term is positive and the energy
width is expanded. As a detailed example of applying Eq. (21), Barbour and Doyle [8] demonstrate
the slab analysis technique to determine the energy of a 2.397 MeV 1 H nucleus as it traverses 12 mm
of MylarTM.
Ê thick. The
Fig. 16 shows a conventional ERD spectrum from a PS:dPS film which is 4500 A
beam parameters are a1 ˆ 158, a1 ‡ a2 ˆ 308 and 3.0 MeV. The integrated charge and MylarTM
thickness are 20 mC and 12 mm, respectively. As always, because of its larger kinematic factor, the
deuterium signal is shifted to higher energies than hydrogen. Because of the energy loss in the
stopper filter, dEs, the 2 D and 1 H front edges are decreased with respect to KD Ein;0 ˆ 2:0 MeV and
KH Ein;0 ˆ 1:5 MeV, respectively. Note that 2 D has a larger energy width than 1 H mainly because of
Fig. 16. ERD spectrum of a homogeneous blend of hydrogenated and deuterated polystyrene. Two distinct peaks,
corresponding to hydrogen (1 H) and deuterium (2 D) are obtained. The solid line is a simulation of the experimental data
(taken from [12]).
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
its larger stopping power in MylarTM. For example, the electronic stopping power can be calculated
using SRIM1. For the same conditions, the back 2 D edge will overlap with the front 1 H edge for a
Ê , which defines the accessible depth. The depth resolution is about
PS:dPS thickness near 7000 A
Ê for this conventional version of ERD. For routine analysis, a 3.0 MeV 4 He ion energy and a
700 A
symmetric beam path geometry provides a good trade-off between large accessible depth and decent
depth resolution. As will be discussed in Section 4.2, depth resolution can be greatly improved by
varying the incident energy and ERD geometry. With some minor drawbacks this technique provides
the best accessible depth because only the individual 1 H and 4 D signals are extracted from the
detector. Furthermore, since no stopper filter is needed, the depth resolution is mainly due to the
detector, geometry and straggling in the sample.
4. Experimental considerations
4.1. Instrumentation
Accelerators originally developed for fundamental nuclear physics studies are readily available
and capable of producing MeV light ions for materials analysis. With only a slight modification,
these accelerators are easily reconfigured for material analysis. Another route is to purchase a
commercial accelerator designed for materials analysis techniques. Fig. 17 shows a typical facility
including the injector source, accelerator tank, magnets, target chamber, and accelerator controls.
Although a detailed description of each component is beyond the scope of this review, a polymer or
materials scientist with little understanding of basic instrumentation will not be able to take
advantage of the versatility of ion scattering nor optimize its sensitivity and depth resolution for a
particular interface problem. Let us point out a simple example. Since 1984, polymer scientists
followed the ``recipe'' established in the seminal introduction of ERD to the polymer community by
Mills et al. [20]. However, with the recent introduction of LE-ERD, polymer scientists have
optimized beam energy and scattering geometry to decrease depth resolution by a factor of 7,
opening up a new range of problems accessible by ERD.
In this section, we briefly review the accelerator components including ion beam production,
acceleration, and selection. Most ERD users are particularly interested in understanding the
scattering chamber, which contains the sample manipulator, beam current measurement system, and
detector system. The processing of the detector signal includes the preamplifier, main amplifier and
multichannel analyzer. Although this aspect of data processing will not be covered, the user must be
Fig. 17. Schematic layout of the ion scattering facility at the University of Pennsylvania.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
aware of certain pitfalls outlined in [17], such as dead time and pulse pileup, which lower the
accuracy of ion beam analysis. For the interested reader, the major components of ion beam analysis
have been previously reviewed in textbooks [7,17,38]. Furthermore, brochures and catalogues
provided by acceleration, detector, and electronics manufactures provide a wealth of theoretical and
practical information.
4.1.1. Accelerator
Ion source: For materials analysis applications, the purpose of the source is to create a high flux
of ions and inject them at low-energy into the accelerator [38]. To achieve nanoampere current over a
cm2 target area requires an ion source that produces tens of microamperes. A radio frequency ion
source, based on electrical discharge in a gas, is ideal for producing predominantly positive ions of
1
H, 4 He, 14 N and 16 O containing a cocktail of atomic and molecular species and charge states.
Duoplasmatron and sputter sources are also used in high-energy applications. Because most
accelerators have positively charged terminals, positive ions must be converted to negative species,
usually by passing through an alkali metal vapor (e.g. rubidium or lithium). The negative ion beam
current at this stage is ca. nanoampere. An extractor and double gap lens assembly draw the negative
ions from this mixture by acceleration to about 25 keV. The positive ions are repelled. A velocity
selector then steers the desired negative ion (4 He ) into the accelerator.
The main purpose of an accelerator is to produce a stable high voltage. Van de Graaff and
Tandetron accelerators are the two main types used in materials analysis. The Van de Graaff was
invented at the behest of Rutherford who needed a high flux of alpha particles to ``accelerate'' his
studies of the nucleus [39]. Whereas the Van de Graaff produces charge via a moving belt, the
Tandetron is a solid state system with no moving parts. Pelletrons provide an inexpensive alternative,
generating a high voltage by means of a metal pellet charge transfer system (i.e. alternating
insulating and metal pellets). The popular tandem accelerators use an external source to create
negative ions which are accelerated from ground to the terminal, stripped of their negative charge
and accelerated through a second tube to ground to give an energy of (n ‡ 1)V eV, where V is the
terminal voltage and n the positive charge state after stripping.
After exiting the accelerator, the beam contains a range of species, charge states, and energies.
The desired ions must then be extracted and focused on target. Typically, the ion beam passes
through a magnetic field set-up by an analyzing-switching magnet whose field is set to allow only
the desired species to pass. For example, one can select either 4 He‡ or 4 He2‡, a selection usually
based on the highest beam current. Beam shaping and steering is further carried out using a series of
magnetic or electrostatic quadupole or einzel lenses, electrostatic steering or scanning in the x- and
y-directions. Apertures or x, y slits placed immediately after the switcher and immediately before the
target chamber are used to help define the size of the beam. Beam profile monitors in front of the
slits provide a remote way to view beam shape and intensity in the x±y-directions. The typical beam
size for a well-tuned system ranges from 0.1 to 3 mm in diameter. For spot sizes less than 0.1 mm, a
microprobe technique requiring special ion optics and source is needed. Such microprobe techniques
have not been applied to polymers because the high current density results in tremendous beam
damage.
4.1.2. End station
For an analysis chamber dedicated to ERD, the system must provide beam current integration, a
detector, and sample manipulator as shown in Fig. 4. The beam current is usually measured by
determining the electrical current, which flows between the target and ground. Accurate
measurement of dose, Q, is arguably the greatest limitation for absolute determination of
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
concentration. Although most polymer depth-profiles rely on standards, the interaction of ions with a
sample should be understood. When MeV ions impinge on a sample, electrons are emitted with
energies ranging from a few eV to over 1 keV [17]. Because an electron leaving the sample is
indistinguishable from a positive ion hitting the target, electrons must either be prevented from
leaving the sample or counted. In the former case, the target is biased at a few kV to prevent low-E
electrons from escaping. For polymers, the production of inner-shell vacancies leading to photon
emission is a key problem. These photons (i.e. UV and visible) leave the transparent polymer sample,
interact with chamber walls, and create a flux of secondary electrons. In fact, the light emitted by
polymers such as PS can be easily seen by eye and facilitates locating the beam footprint. For MeV
4
He‡ , Venkatesan [17] reports that the secondary electron current can be comparable to the ion beam
current. An electrostatically shielded Faraday cup surrounding the sample provides a reliable method
to detect secondary electron emission. Beam current integration can also be measured upstream
using a ``transmission'' design. Here, a rotating wire intercepts the beam several times per second to
provide a relative measure of beam current.
In ERD, the sample is tilted so that the incident beam makes a glancing angle, typically 158
with the plane of the surface. A semiconductor detector is placed in the forward direction to capture
the recoiled 1 H and 2 D nuclei. Silicon surface barrier (SB) and passivated implanted planar silicon
(PIPS) detectors are commonly used because of their reliability, efficiency and energy resolution.
Typical resolutions range from 15 to 20 keV for most ERD applications. The principle of operation
of silicon detectors has been described [17,40]. Briefly, incoming ions traverse a metal window
or dead layer before entering the active detector region. Energy losses in this dead layer and that
due to nuclear collisions are taken into account when calibrating the detector system. Upon
colliding with electrons (i.e. electronic energy loss), charge carriers are produced. For silicon the
formation energy of one electron±hole pair is 3.67 eV. Charge carriers are swept from the depletion
zone, which must be thick enough to stop the highest energy particle. The resultant charge pulse of
a few nanoseconds duration is integrated in a charge-sensitive preamplifier to produce a voltage
pulse for further processing. A good description of semiconductor detectors can be found in
manufacturer catalogues (EG&G Ortec, 100 Midland Road, Oak Ridge, TN 37831; Canberra
Industries Inc., Meriden, CT 06450). An excellent introduction to the processing of detector signals
has been published [17].
An aperture immediately in front of the detector is particularly important in the forward
scattering geometry to reduce kinematic broadening, which in turn limits depth resolution, and
prevent ions from reaching the outside edge of the detector where resolution is poor. For a
rectangular slit, Doyle and Peercy [19] have reported on the optimum conditions needed to minimize
the depth resolution resulting from kinematic broadening. This broadening is mainly due to the finite
size of the detector aperture. Since the kinematic factor varies as cos2 f, a spread in f results in a
range of recoil energies, KEin,0, associated with surface atoms. Recently, calculations by Brice and
Doyle showed that curved slits, having the same area as rectangular ones, improved the energy
resolution by 50% [41].
An absorber or stopper filter is also placed in front of the detector. This filter prevents the large
flux of elastically scattered 4 He from overwhelming the detector. MylarTM (polyethylene
terephthlate) and aluminum are common filter materials because they are available over a wide
range of uniform thickness. Choosing a filter thickness depends on the range of the 4 He in the filter
and the range of the recoiled nuclei in the filter. For example, 2.5 MeV 4 He ions scattered from a
high mass nucleus will be completely stopped by a 10.6 mm filter whereas recoiling 1 H will only lose
about 400 keV. This difference is mainly due to the much lower stopping power for 1 H in MylarTM.
Unfortunately, the absorber filter can spread the 1 H energy by ca. 50 keV because of straggling in the
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
thick filter. Filter straggling is a major component of the overall depth resolution. Thus, the stopper
filter should be as thin as possible to maximize resolution. The program SRIM1 provides a
convenient method to calculate the filter thickness needed to stop a given incident ion or the
straggling of 1 H or 2 D through such a filter.
As discussed in Section 4.2.3, LE-ERD is based on decreasing the incident beam energy in
order to minimize filter thickness. For example, using a 2.85 MeV He2‡ ion in conjunction with
Ê at the surface. However, a combination
8.0 mm of MylarTM results in a FWHM resolution of 600 A
TM
2‡
4
Ê . It is important to
of 1.00 MeV He with a 3 micron Mylar filter provides a resolution of 170 A
Ê , respectively. Presently, the best
note that the accessible depth decreases from 6000 to 1000 A
Ê with an accessible depth of 300 A
Ê [42].
surface resolution achieved with LE-ERD is 80 A
This section demonstrates that the optimum characterization of polymer surfaces and interfaces
will only be achieved if polymer scientists understand the rudiments of accelerator instrumentation.
Furthermore, an understanding of how detector systems work can open up new areas within the
polymer field that have yet to be explored. LE-ERD is an example of such a breakthrough.
4.1.3. Radiation safety notes
Because polymer scientists are not usually trained in nuclear physics a brief discussion of
radiation safety is appropriate [17,38,43]. Although extremely safe under most conditions,
commercial accelerators for materials analysis can produce unwanted radiation, X-rays, g-rays,
and neutrons. The main problem is usually Bremsstrahlung radiation. Electrons are produced by the
interaction of the ion beam with background gas and the beam line, thus the need for good vacuum
and collimation, respectively. These electrons are accelerated through the high voltage region back
towards the ion source resulting in Bremsstrahlung radiation. Users also need to anticipate nuclear
reactions between the incident projectile and target nucleus. Fortunately, for a 4 He or 1 H beam
energy below 3 MeV, nuclear reactions with the target are not serious problems unless the target
contains tritium, lithium or beryllium [38]. Tesmer and Nastasi have tabulated the Coulomb barriers
for the production of neutrons and g. Because this barrier scales with the incident and target particle
atomic numbers, nuclear reactions are mainly important for protons on light element targets.
However, some low-energy nuclear reactions exist and can be quite useful. For example, Kerle et al.
have recently used the 2 H(3 He, 1 H)4 He nuclear reaction to achieve a surface resolution of 6 nm
FWHM [44].
4.2. Depth resolution
As with other experimental techniques, a thorough understanding of the parameters that affect
an ERD experiment is required to optimize the resolution and sensitivity for a particular surface or
interface problem. In ERD, the key parameters are beam energy, stopper filter thickness, scattering
angle, F, and the total charge. The effect of these parameters on depth resolution and sensitivity will
be discussed in Sections 4.2.2 and 4.2.3. Methods for optimization will be discussed as well as how
optimization of one factor, such as resolution, often comes at the expense of another factor. For
instance, the optimization of the depth resolution using LE-ERD leads to a decrease in the depth
below the surface that can be probed.
The major disadvantage of conventional ERD is its relatively poor surface depth resolution
Ê using a 10.5 mm Al filter and a beam energy of 3.0 MeV. This results mainly from
which is ca. 800 A
the energy loss due to straggling inside the thick stopper filter, either MylarTM or Al. There are three
relatively simple ways in which the depth resolution of ERD can be improved while still maintaining
its simplicity: (i) lowering E0, which allows the use of a thinner MylarTM filter and also increases the
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Fig. 18. The volume fraction profile (dotted line) for surface segregation in a polymer blend having surface and bulk
concentrations of 0.8 and 0.1, and a surface excess of 6 nm. The solid lines are obtained by convoluting the dotted line with
Ê , (b) 300 A
Ê , (c) 140 A
Ê , and (d) 50 A
Ê.
a Gaussian instrumental resolution function having FWHM (a) 800 A
stopping power of the sample, (ii) varying F, the scattering angle, and (iii) using heavier projectiles.
The first two effects constitute the basis of LE-ERD.
Direct depth profiling techniques, such as the ion beam techniques discussed in this review,
measure the polymer concentration in direct space perpendicular to the sample surface. Because of
the statistical nature of the measurement each technique has a limited depth resolution. As a result,
the concentration profiles that emerge from the measurements are smeared by the instrumental
resolution of the technique. This effect can be simulated by convoluting the ``real'' profile with a
Gaussian resolution function having a certain characteristic ``instrumental'' broadening characterized usually by the FWHM of the function. In a case of ``ideal'' resolution (FWHM ! 0), the
measured profile would be indistinguishable from the ``real'' one. We illustrate the effect of a finite
instrumental resolution on a simple example that involves surface segregation of a polymer from a
polymer blend. Such a situation is shown in Fig. 18. The ``real'' volume fraction of the segregated
species (dotted line) varies smoothly from its bulk value of 0.1 to 0.8 at the surface. The solid lines in
Fig. 18 denote the volume fraction profiles convoluted with Gaussian functions having resolutions
Ê , (b) 300 A
Ê , (c) 140 A
Ê , and (d) 50 A
Ê . These values are the depth resolutions at the
of: (a) 800 A
sample surface corresponding to the different techniques mentioned in this review including (a)
ERD, (b) non-resonant 2 H(3 He, 1 H)4 He NRA in the Payne et al. [45] geometry, (c) LE-ERD or nonresonant 2 H(3 He, 1 H)4 He NRA in the Chaturvedi et al. [46] geometry, and (d) resonant 1 H(15 N, 4 He
g)12 C NRA. Fig. 18 clearly demonstrates that the techniques with the best depth resolution more
closely resemble the ``real'' experimental situation. Depth resolution is, thus, a key factor to consider
when choosing a technique for interfacial studies.
4.2.1. Contributions to ERD depth resolution
Several factors influence the depth resolution of ERD. Through careful design of the
experimental set-up, many of these factors can be mitigated to achieve the best depth resolution,
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
131
which is defined as
dx ˆ
DEtot
…dE=dx†eff
(22)
where DEtot is the total energy resolution and (dE/dx)eff is the ``effective'' stopping power of ions
penetrating the sample. This expression shows that the best depth resolution is achieved when the
energy resolution is a minimum and the effective stopping power a maximum.
To evaluate Eq. (22), the contributions to DEtot must be identified and determined. These are
1.
2.
3.
4.
5.
DED, detector energy resolution [38,39];
DEM, energy broadening due to multiple scattering;
DES, energy straggling in the sample;
DEF, energy straggling in the stopper filter;
DEG, geometrical broadening due to beam divergence and finite acceptance angle of detector.
Assuming they are uncorrelated and can be characterized by a Gaussian distribution, these
factors add in quadrature:
2
2
ˆ DED2 ‡ DEM
‡ DES2 ‡ DEF2 ‡ DEG2
DEtot
(23)
In a real experiment, only the energy resolution of the detector, DED, is constant (although it can
decay with time).1 Factors 2±4 can be calculated for a particular experimental set-up and sample
[26,47±51]. A brief description of each contribution to the total energy resolution is given below. A
comprehensive discussion can be found in references [17,38,48±51].
Straggling, DES, is a result of the statistical nature of the interactions between the incident
particles and the sample. Moreover, as the incident ion beam traverses the sample, the diameter of
the beam broadens because the ions make multiple small-angle collisions with the nuclei in the
sample. The lateral and angular spread, including path-length fluctuations, of the incoming and
outgoing particles have been estimated from theories of multiple scattering [51,52]. Because of the
statistical nature of these interactions, the energy loss by the ions has a Gaussian distribution with a
width corresponding to the energy straggling. The energy spread due to straggling can be
approximated from the Bohr theory [53], which predicts that straggling is independent of energy and
increases as the square root of the film thickness (i.e. either sample or filter).
As discussed elsewhere [26,48,49], multiple scattering, DEM, is particularly important for
glancing angle geometry and recoil collisions below the surface. Energy straggling is similar to
multiple scattering in that an energetic particle loses energy via many individual encounters with
electrons in the sample and stopper filter.
The error associated with geometric broadening, DEG, occurs because the ion beam has finite
width and the detector has finite area. These two contributions produce two sources of error that
can be evaluated following the procedure of Turos and Mayer [48]. The first contribution comes
from the strong dependence of the kinematic factor K on F as can be seen in Eq. (10). Due to
the finite width of the beam and detector, different spots on the sample will have different values
of F, and thus, different values of K, as depicted schematically in Fig. 19. The second mechanism
reflects a distribution of path-lengths as well as distribution of energy losses in the material. For
a well collimated incident beam, only geometric broadening due to the beam width and finite
detector acceptance angle are important. As shown by Turos and Mayer [48], kinematical spread and
1
Depending on the manufacturer, the energy resolution of the detector usually ranges from 10 to 20 keV.
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 19. Geometric broadening, DEG, is due to the variation in the recoil angle, F, caused by the finite width of the beam
and the spread due to the solid angle of the detector.
path-length differences compensate each other near a1 ˆ 12:58 and, therefore, geometric broadening
is minimized near this angle (cf. Fig. 19). However, at lower values of a1, this broadening makes a
significant contribution to the overall resolution. A detailed discussion of geometric broadening is
given in [41,48] and [54]. For most samples, the effect of lateral inhomogeneities, such as surface
roughness, on DEtot can be ignored. However, in cases where a glancing scattering geometry is
applied, the effect of surface roughness on depth resolution may become important. Geometric
broadening is reduced by having a well collimated, narrow beam incident on the sample.
Energy straggling in the stopper filter, DEF, is the largest error in a conventional ERD
experiment. The straggling process in the filter is the same as the straggling in the sample. While this
filter is needed to reduce the energy of the scattered 4 He ion, it also decreases the energy of the
recoiled 1 H and 2 D ions while increasing the energy spread. Several different experimental designs
have focused on reducing or eliminating this error.
4.2.2. Optimizing depth resolution in ERD
The depth resolution can be improved by increasing (dE/dx)eff as shown in Eq. (22). Fig. 20
shows pathways for the incoming ion and the outgoing recoils in ERD. The energy losses of the 4 He
projectiles and the recoiled 1 H on their inward and outward paths, respectively are denoted as (dE/
dx)in,He and (dE/dx)out,H. The scattering geometry is defined via F and a1. Because the incident beam
enters the end station at a fixed geometry, and the detector position is typically fixed, the scattering
angle, F, is fixed (e.g. typically 308).2 However, the angle between the incident beam and sample, a1,
can be varied by rotating the sample. As shown below, optimizing a1 can greatly improve depth
resolution. The effective energy loss is then obtained from Eq. (22), where …dE=dx†eff ˆ ‰SŠ is
comprised of …dE=dx†in;He ˆ Sin and …dE=dx†out;H ˆ Sout for the inward and outward paths.
There are three ways to maximize (dE/dx)eff. First, because (dE/dx)He is about an order of
magnitude greater than (dE/dx)H, the overall (dE/dx)eff can be greatly increased by increasing the
path-length of the incident 4 He ions. This is accomplished by decreasing a1 which changes the pathlength, t/sin a1, as seen in Fig. 21. On the other hand, increasing the path-lengths of the ions inside
the sample may produce a substantial increase in the multiple scattering, which deteriorates the
depth resolution. Thus, in order to improve the depth resolution by tilting the sample with respect to
the incoming ion beam, a compromise between these opposite tendencies has to be found. The
2
In [48], Turos and Mayer showed that the optimum angle for the analysis of the forward recoiled particles is at
F 308.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 20. Schematic of the energy loss in an ERD experiment as the incident 4 He ion traverses the sample, collides with a
1
H atom and finally the 1 H(D) ion leaves the sample. The parameters are defined in the text.
second way to maximize (dE/dx)eff, is by decreasing Ein,0, which causes both (dE/dx)in,He and (dE/
dx)out,H to increase [55]. However, the trade-off here is that at lower beam energies there is a
decrease in the depth that is accessible. Finally, increasing the atomic number of the projectile will
also increase (dE/dx)eff. This modification increases the kinematic scattering factor K (cf. Eq. (8))
and also leads to higher (dE/dx)in,He and (dE/dx)out,H. However, heavy projectiles may lead to
undesirable modification of the sample as discussed in Section 7.4.
The depth resolution can also be improved by reducing the magnitude of DEtot by focusing on
the largest contribution to DEtot, which is the straggling in the filter, DEF. To reduce DEF, Genzer
et al. [26] decreased the beam energy, Ein,0, from 3 to 1.3 MeV which consequently allowed for a
reduction in the stopper filter thickness from 11 to 3 mm. As a result, the filter straggling decreased
by a factor of 1.5. A lower beam energy also increases (dE/dx)eff, by a factor of 1.6, which
further enhances the resolution. By using a lower beam energy and thinner filter, the depth resolution
improves by a factor of 4 in LE-ERD.
Fig. 22 shows the total and individual energy resolutions calculated for 2 D in PS using 1.3 MeV
4
He (LE-ERD) at F ˆ 308, and a1 ˆ 208 with a MylarTM filter thickness of 3.0 mm [26]. By
increasing the thickness of the stopper filter to ca. 11 mm (ERD) the contribution due to DEF
increases substantially (from 13 to 22 keV). The energy resolution is 20 keV at the surface and
Fig. 21. This schematic shows how to increase (dE/dx)eff by decreasing a1 which consequently increases the path-length of
the incoming 4 He ions. Although the path-length out decreases, (dE/dx)eff increases overall because (dE/dx) of 4 He is four
times greater than that of 1 H at the same velocity (i.e. dE/dx is proportional to Z12 at the same velocity).
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 22. Theoretical energy resolution of 2 D in deuterated polystyrene as a function of depth in the sample. The
experimental parameters are E0 ˆ 1:3 MeV, a 3.0 mm thick MylarTM stopper filter, F ˆ 30, and a1 ˆ 208. The calculated
curves correspond to the detector resolution (thin solid line), energy straggling in the stopper filter (thick dashed line),
energy straggling in the sample (thin dotted line), geometrical broadening (dash-dotted line), multiple scattering (dash-dotdotted line), and total energy resolution (thick solid line) (taken from [26]).
Ê . Examination of the individual contributions shows
slowly increases to 32 keV at a depth of 1000 A
Ê
that near the surface (<500 A), the total energy resolution is dominated by straggling in the stopper
Ê ), multiple
filter, the detector resolution, and geometry. However, beneath the surface (>400 A
scattering dominates and drives the total energy resolution to larger values. The increase in multiple
scattering is even stronger at glancing angles because of the increase in the total path-length in the
sample.
4.2.3. Improved depth resolution with LE-ERD
Ê
Using four alternating layers of normal PS and its deuterated analogue (dPS), each 300 A
thick, Genzer et al. [26] demonstrated that lowering Ein,0 and consequently using a thinner MylarTM
filter produced radical improvement in depth resolution. Fig. 23a shows the ERD spectrum of a Si/
PS/dPS/PS/dPS sample analyzed with a 3.0 MeV 4 He‡ beam and a 10.35 mm thick Al filter at
a1 ˆ 158 and F ˆ 308. Because of the straggling in the stopper filter, the deuterium yields from the
two dPS layers overlap, suggesting that the near-surface spatial resolution is greater than the
thickness of the PS spacer. Likewise, the two PS layers cannot be resolved. The fit to the
Ê , respectively [26].
experimental data reveals that the depth resolution for 2 D and 1 H is 750 and 780 A
TM
stopper filter thickness to
By decreasing the incident beam energy to 1.3 MeV and the Mylar
4.5 mM, the depth resolution improves dramatically, as demonstrated by the two distinguishable
deuterium maxima shown in Fig. 23b. Qualitatively, this observation indicates that the 2 D resolution
Ê ) of the PS spacer. The depth resolutions of 2 H and 1 H are
is comparable to the thickness (250 A
Ê
250 and 300 A, respectively, which is a three-fold improvement relative to conventional ERD.
The depth resolution can be further improved by varying a1, the angle between the incoming
4
He beam and the sample, to achieve glancing incident and/or exit geometries. From the scattering
geometry, the range for a1 is 0 > a1 > 30 . In a typical set up F 308, and thus, a1 can range from
>08 to <308. In the standard ERD geometry (a1 ˆ 158) the path-lengths of the incoming and
outgoing ions are the same. However, in the glancing exit geometry (a1 > 158) the path-length of
the outgoing particle is longer than the incident particle. Correspondingly, at glancing incident
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 23. The ERD and LE-ERD spectra of 1 H and 2 D for 4 He‡ ions incident at a1 ˆ 158 and F ˆ 308, respectively. The
incident beam energy is (a) E0 ˆ 3:0 MeV and stopper filter is 10.35 mm thick aluminum filter and (b) E0 ˆ 1:3 MeV and
stopper filter is a 4.5 mm thick MylarTM filter. The solid line represents fit to experimental data (taken from [26]).
angles (a1 < 158), the incoming 4 He particle travels a longer path in the sample than the outgoing
particles.
Fig. 24 shows LE-ERD spectra from the sample above as a function of increasing incident
angle, a1 ˆ 10, 7.5 and 58. Here, a 4.5 mm thick MylarTM filter is used with a 1.3 MeV energy beam.
Using Fig. 23b as a guide, the depth resolution has further improved, as demonstrated by the two
distinguishable 2 D (and 1 H) maxima in Fig. 24. Upon decreasing a1 from 0 to 58 the depth resolution
Ê , respectively.
of 2 D and 1 H at the surface improves from 200 and 215 to 145 and 155 A
Similar improvements in the depth resolution can be achieved using the glancing exit geometry,
which has the advantage of a smaller incident beam footprint on the sample [26]. One drawback of
Ê for conventional ERD to
LE-ERD is that the probing depth in the sample decreases from 7000 A
Ê
800 A. Thus, LE-ERD is best suited to study surface or near-surface phenomena. However, as
discussed in Section 7.3, LE-ERD combined with sputtering can be used to probe interfaces buried
much deeper than the nominal probing depth of LE-ERD. In order to maximize depth resolution, the
lowest beam energy and thinnest MylarTM filters should be chosen to meet the probing depth
requirements.
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 24. LE-ERD spectra for 1.3 MeV 4 He‡ ions incident on Si/PS/dPS/PS/dPS sample at a1 ˆ 10 (open circles), 7.58
(crosses), and 58 (closed triangles). A 4.5 mm thick MylarTM stopper filter is used (taken from [26]).
4.3. Elemental sensitivity
After depth resolution, elemental sensitivity is the next critical parameter. In an ERD
experiment, sensitivity is defined as the minimum amount of hydrogen or deuterium that can be
detected. In theory, the detection limit of ERD is infinitesimal if the beam current is small, and the
operator's time is unlimited. That is to say, as long as any 2 D exists in the sample, an incident 4 He
particle will eventually recoil it from the sample into the detector. However, in practice, analysis time
per sample is an issue as well as the collection of a statistically significant number of counts. For
practical purposes, two main factors limit the sensitivity (1) the 1 H or 2 D signal must be resolved
from the background signal, and (2) the time to obtain an acceptable number of counts must be
reasonable.
The absolute sensitivity (AS) defines the minimum of 1 H or 2 D counts to achieve a certain
precision. Usually expressed in the number of 1 H or 2 D per cm2, AS can be evaluated from
AS ˆ
100% 2 sin a1
p
Os…Q=qe †
(24)
where p is the percent precision and is determined by the total detected yield:
p
Y
100%
pˆ
100% ˆ p
Y
Y
(25)
Eqs. (24) and (25) show that in addition to the required precision the absolute sensitivity strongly
depends on the detector solid angle, O, the scattering cross-section, s, and the total number of
incident particles, Q/qe. In principle, the AS can be improved by moving the detector closer to the
sample to increase O. However, the energy spread of the recoils limits how much one can increase O
before the depth resolution begins to increase. Two practical methods to improve AS involve
increasing the incident beam flux or decreasing the incident beam energy to increase s. Other routes
include increasing Q (and Y) by increasing beam current or exposure time. In this case, the amount of
current that can be tolerated by a sample should be carefully considered.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
137
The background signal in an ERD spectrum is mainly due to pulse pile-up, which occurs at high
beam current. When two ions strike the detector at the same time, the detector is unable to
distinguish between particles and registers one count at the combined energy of the two ions. Pulse
pile-up creates a background signal that can mask a 1 H or 2 D signal, particularly at low
concentrations. Pulse pile-up can be reduced by using a thicker filter that prevents the large flux of
forward scattered 4 He ions from reaching the detector. The great majority of particles that are
recorded by the detector are forward scattered 4 He ions. For example, a 2.8 MeV 4 He beam incident
Ê PS film on silicon will result in 40 forward scattered 4 He ions for every recoiled 1 H
on a 4000 A
that reaches the detector covered by a 8 mm MylarTM filter. Because 2 D ions recoil at higher energies
than 1 H ions, the 2 D signal has a lower background and is, therefore, easier to resolve. Moreover, the
2
D resonance near 2.15 MeV (cf. Fig. 12) can be utilized to dramatically increase the sensitivity to
2
D. The AS can be understood by calculating the number of counts from a 10 nm molecular layer of
PS. The atomic density of 1 H in PS is ‰HŠPS ˆ …1:05 g=cm3 8 atoms=monomer 6:023 1023
atoms=mole†=…104 g=mole†, or 4:86 1023 atoms/cm3. For a 3.0 MeV 4 He ion incident at 308, the
1
H yield is about 760 counts per 10 mC of integrated charge.
4.4. Sample modification
During ion beam analysis, the 1 H and/or 2 D that recoil from the sample are removed from the
polymer. This modification during the analysis may complicate the data analysis. An estimate of the
number of atoms that are elastically recoiled from the sample during an ERD experiment can be
obtained from a simplified version of Eq. (26):
Y sO
Q
t
N
qe sin a1
(26)
In Eq. (26), N is the bulk density of the sample and t is the sample thickness. The geometry is shown
schematically in Fig. 25. As an example, the total number of 1 H nuclei recoiled from the volume
intersected by an incident beam having a 0.02 cm2 cross-section is calculated. For Q ˆ 10 mC, the
total number of recoiled 1 H in the forward direction is about 2:5 106 . This value is quite small
Fig. 25. Schematic illustrating the volume of the sample irradiated by the incoming beam.
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
compared to the total number of 1 H atoms in this volume, which is 3:7 1015 atoms. Thus, the
elastic collisions that produce the ERD signal do not significantly change the 1 H and/or 2 D
concentration in the polymer.
The structure of polymers can be changed by the large amount of energy deposited in the
sample through inelastic energy loss processes. Ion interactions with the polymer can lead to chain
scission, cross-linking, and molecular emission. The extent to which these effects occur depends on
the primary ion beam type, energy, flux, etc. and the properties of the polymer (structure, molecular
weight, etc.). Some general rules for predicting sample modification have been established ([56] and
the references therein). For example, ion beam bombardment of poly(methyl methacrylate) causes
chain scission because the quaternary carbon atoms (bonded to four other carbon atoms) tend to
degrade the main chain. As a consequence small molecule fragments can volatilize. In contrast,
because PS mainly cross-links upon irradiation the material loss is small (<1±3%) and usually
uniform throughout the sample. As a general rule, polymers with stabilizing phenyl rings tend to be
stable under the ion beam.
Although irradiation can be used to improve bulk and surface properties (cf. Section 7.4),
polymer modification by the incident beam may also complicate depth profiling analysis. To acquire
accurate spectra, sample modification during irradiation should be reduced as much as possible. In
some cases, cooling the sample with liquid nitrogen helps to decrease the concentration of volatile
species emitted from the sample. Scanning the ion beam over the sample also decreases polymer
degradation by spreading the damage over a larger area. In addition, sample degradation can be
reduced by using a lower dose and higher beam energy; in both cases, however, the signal will be
lower and the statistics poorer.
4.5. Sample requirements and preparation
The sample requirements are rather modest. The sample must be stable under ``modest''
vacuum, typically 10 6 Torr in the sample chamber. The vacuum is required because the 4 He‡ ions
have a small mean free path through a gas at atmospheric pressure. In special cases, an
environmental chamber can be used to expose samples to a vapor (e.g. water) at pressures of up to
10 3 Torr. A second requirement is that the sample must allow the charge from the incident ion beam
to reach the sample holder, which is connected to the current integration system. Typically samples
are mounted with conducting tape to the holder. If the sample is a thick insulator, charging can be
avoided by coating with a metal, rubbing with an eraser or providing a conducting pathway from the
surface to the holder. A third requirement is sample geometry, which is mainly limited by the sample
holder size, typically 5 cm2. The cross-section of the incident beam, 1±2 mm2, sets the minimum
sample size. Samples can be up to 3 cm thick.
For ERD analysis, polymer films are usually mounted on flat, solid substrates. Because of their
low price and availability, silicon substrates are commonly used. Polymer films are typically
deposited on the substrate by spin-coating from a polymer solution. This technique, originally
developed by the semiconductor industry for depositing photoresist films, produces smooth, laterally
Ê ). Because of the grazing incident and exit
uniform films having small variations in thickness (<15 A
geometry, sample roughness will tend to spread out the back edge of a signal and/or degrade the
depth resolution. Film thickness and quality depends on the (i) substrate properties such as surface
energy and roughness, (ii) polymer, (iii) molecular weight, (iv) solvent, (v) solution concentration,
and (vi) spinning speed and time, etc. The relationship between these parameters and the sample
thickness can be established empirically [57]. The thickness of films prepared by spin-coating can
range from nanometers [58,59] to a micron. For very thick samples, the thickness becomes less
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
uniform. For thicknesses up to several micrometers, films can be prepared by pulling a substrate at a
constant rate from a concentrated polymer solution or by using a doctor blade to spread the solution.
To prepare a bilayer (or a multilayer), two possible strategies can be utilized. In both cases, the
bottom layer is prepared using the procedure described earlier. If the polymer to be placed on the top
dissolves in a solvent that does not dissolve the bottom layer, the top layer can be prepared by spincoating a solution directly on the bottom layer. Otherwise, the top layer is spin-coated on a
microscope glass slide, scored around the edges, floated on deionized water, and picked up with the
bottom layer/silicon sample. If the top polymer does not float off glass, one can use a different
supporting media, such as single crystal NaCl or mica.
5. Data analysis
5.1. Data conversion
The aim of many ERD experiments is to obtain a depth-profile of a light element in a sample. In
Section 3, the origin of the depth and concentration resolutions in an ERD experiment was
Fig. 26. (a) ERD spectrum of a thin film of PS on a dPS matrix after annealing for 240 s at 171 8C; (b) volume fraction of
PS derived from the ERD spectrum in (a). The solid line is a theoretical fit to the data using a D of 5:3 10 13 cm2/s
(taken from [111]).
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R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
examined. However, as was previously mentioned, the data output from the ERD detector is simply a
column of numbers that corresponds to the quantity of counts that were recorded in each channel.
Fig. 26a shows an ERD plot of the normalized yield at each channel number from a thin dPS layer,
Ê on top of a thick PS. Normalized yield is the raw counts divided by the beam dose, detector
200 A
solid angle, and the channel energy width. This raw data in counts per channel must be converted
into a concentration versus depth-profile as shown in Fig. 26b. Note how the concentration versus
depth plot is the mirror image of the raw data; this is because moving to lower energy, to the left, is
equivalent to moving deeper into the sample, to the right. Another feature is the apparent observation
of dPS at negative depths. This anomaly is a direct result of surface broadening due to the finite
system resolution of 80 nm.
This section will outline how to perform this conversion from counts versus channel to
concentration versus depth. The first step is to convert from channel number to the energy of the
detected ions. Each channel number corresponds to a specific energy range, and a calibration is used
to convert from channel number to the energy of the detected particle. Using known parameters such
as the stopping powers of the projectile and the recoiled ion and the kinematic factor, the energy can
be converted to depth in the sample. Likewise the number of counts registered in each channel and
can be converted to the concentration of the species at that depth.
The resultant ERD spectrum is governed by a set of parameters defining the scattering
geometry, incident beam characteristics and the parameters of the target and the stopper filter; the
following is a complete catalog of the information necessary to conduct the energy to depth and the
counts to concentration conversion. The scattering geometry can be completely defined by the angle
between the incident beam and the sample, a1, and the recoil angle between the incident beam and
detector, F. For convenience in determining the path-length of the ions in the sample, the outgoing
beam angle a2, is also introduced (a2 is simply the difference between a1 and F). This geometry is
described in Fig. 20. The relevant parameters of the incident projectile are its mass, Mp, initial
energy, Ein,0, and charge, Zp. The mass, Mt, and scattering cross-section, s(E, F), of the recoiled
target are known as well as the stopping powers of the incident particles, Sin, and the recoiled
particles, Sout in both the samples and the stopper filter.
The stopping powers are a direct measure of the energy loss rate of the projectiles and the recoils as
they penetrate through the sample. Generally, Sin, and Sout are not constant, but are strong functions of
the energies of the incoming or outgoing ion. The stopping power of many materials has been tabulated
[60] or they can be determined from SRIM1, the stopping and range of ions in matter, a program
developed by Ziegler et al. [30]. This program provides a plot of particle energy versus stopping power,
(dE/dx). From this data, a polynomial equation can be fited to the relevant energy region of the dE/dx
curve, as shown in Fig. 27 for both 1 H and 4 He. As for the stopper filter, the type of material and its
thickness determines the amount of energy lost by the recoiled ions as they pass through the filter.
The stopping power of the stopper filter can be determined theoretically or by SRIM1. Because
the stopper filter is usually several microns thick, the precision in determining the energy loss of the
recoils is limited by the uncertainty in filter thickness. A direct experimental determination of the
stopping power is the best. For a known beam energy incident on a sample with a surface target
atom, say 1 H, the stopping power of the filter can be determined by measuring the recoiled particle
energy after passing through the filter. Using the incident beam energy and kinematic factor as in
Eq. (10), the energy of the recoiled 1 H immediately after leaving the sample surface can be
determined. The energy of same recoiled 1 H after it leaves the filter corresponds to the front edge of
the 1 H spectrum. The energy lost due to the stopper filter is then simply the difference:
Efilter ˆ KEin;0
ED
(27)
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 27. Stopping power for 1 H and 4 He in PS over a wide range of 4 He energies. For the energy to depth conversion, a
polynomial has been fit over the energy region of interest.
By repeating this experiment with various incident beam energies, a plot of recoiled particle energy
versus stopping power can be constructed, and fited with a polynomial equation to describe how the
stopping power in the filter varies with the energy of the recoiled particles. The results of such
experiments are shown in Fig. 28 for both 1 H and 2 D.
5.1.1. Channel to energy
To convert from channel number to energy requires a calibration plot of particle energy versus
recorded channel number. To reduce unknowns, the calibration is conducted without a stopper filter
in front of the ERD detector. The optimum calibration sample has at least one heavy element that is
Fig. 28. Experimental determination of the stopping power for both 1 H and 2 D though 7.5 mm of MylarTM. The integrated
energy loss through MylarTM is measured from a standard (e.g. hPS:dPS blend) over numerous beam energies.
141
142
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 29. Experimental calibration of a detector. The plot was constructed using five 4 He beam energies over the region of
interest. The offset and channel width (slope) are 96.95 keV and 5.29 keV per channel, respectively.
known to be at the surface. The energy of a 4 He forward scattered from the surface is the product of
the kinematic factor and beam energy. By a one-to-one correspondence, the energy, KE0, for that
particular channel is known. Repeating this procedure for a range of incident beam energies yields a
linear relationship, E ˆ a…channel no:† ‡ b where a is the energy width of each channel, usually in
keV per channel and b is the multichannel analyzer offset. Fig. 29 shows a typical calibration curve.
Calibration samples with two or more surface atoms are particularly useful because each experiment
yields two or more data points for conversion. The calibration energy range should be similar to the
experimental range to ensure accuracy.
5.2. Direct conversion of experimental data
5.2.1. Energy to depth conversion
The procedure that relates the detected recoil energy to the depth t of the scattering event inside
the sample is straightforward for very thin targets. Because the stopping powers are relatively
constant in thin-films, the conversion simply follows the recipe outlined in Section 3.3. However, for
thick targets the energy losses of the incoming 4 He and exiting 1 H or 2 D are significant and,
therefore, the energy dependence of the stopping power in the sample must be included by using the
``thick target approximation'' [11], which is outlined in the following sections.
In the thick target approximation, the sample is divided into N layers, each having thickness
Dx.3 The total sample thickness is N Dx. For sufficiently small Dx, the stopping power inside each
layer can be considered constant. The energy to depth conversion is performed via a sequential
calculation of the energy losses of the incident ion and recoils for each layer. Fig. 30 shows a
schematic representation of the thick target approximation for an incident 4 He ion, with initial
energy Ein,0, recoiling a 2 D or 1 H nucleus at depth t. The energy of the incident ion entering layer j is
Ein,j 1. The energy of this ion in layer j can be derived from the recurrent relation:
Ein;j ˆ Ein;j
3
1
Dx
Sin;j 1 Ein;j
sin a1
1
(28)
To simplify the analysis, we have chosen the thicknesses of every sublayer to be the same. However, in general, each
sublayer can have a different thickness.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
143
Fig. 30. Schematic of the thick target approximation for an ERD experiment. To account for the dependence of the
stopping power on the ion energy, the `thick' sample is divided into N slabs of thickness Dx. If the slab thickness is thin
enough, the stopping power in each slab is taken as a constant.
where Sin,j 1 is evaluated at Ein,j 1. The values of Sin and Sout are derived from the stopping powers
of the constituent elements by applying Bragg's rule as outlined in Section 3.3. At each layer, Ein and
Eout are related as
Ein;j ˆ KEout;j
(29)
where K is defined in Eq. (10). Similar to the incident ion procedure, the energy of a recoiled ion
leaving layer j is Eout,j 1, and the energy of a recoiled ion passing through layer j 1 can be
evaluated from the recurrent relation
Eout;j
1
ˆ Eout;j
Dx
Sout;j Eout;j
sin a2
(30)
where Sout,j is evaluated at Eout,j. As shown in Fig. 30, the energy of the recoils, ED is given by
ED ˆ Eout;0
Efilter …Eout;0 †
(31)
where Efilter is the total energy loss of the recoils in the stopper filter which can be calculated using a
similar procedure or determined experimentally as discussed earlier. An example of the energy to
depth conversion is given in Table 1 for a 2.8 MeV 4 He projectile into dPS with a layer thickness of
Ê , with angles of a1 ˆ a2 ˆ 158, and a stopper filter of 8 mm MylarTM. Note that the stopping
150 A
Ê of the target dPS. Even though the layers are
power changes by 4% over the thickness 3000 A
relatively thick, the stopping power between any two adjacent layers is nearly constant, affirming the
assumption of our thick target approximation.
5.2.2. Counts to concentration
The yield (counts) of the recoiled ions can be converted to the concentration of atoms within a
given layer. The concentration in layer j, Nj, is related to the yield, Yj, as
Yj ˆ
…Q=qe †Nj s…Ein;j 1 ; f†OdEdet
cos F…dE=dx†eff;j
(32)
In Eq. (32), Q/qe is the total number of incident ions, s(Ein,j 1, F) the scattering cross-section
evaluated at Ein,j 1 and F, O the detector solid angle, dEdet is the energy width of a channel in the
144
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Table 1
Ê ) and an 8 mm
Conversion of energy to depth for in an ERD experiment using 2.8 MeV 4 He incident on PS (3000 A
TM
Mylar filter
Layer
Ê)
Depth (A
Ein;x
(4 He, MeV)
…Dx†Sin;x
(4 He, MeV)
K Ein;x
(2 D, MeV)
…Dx†Sout;x
Eout,0
Efilter
(2 D, MeV) (2 D, MeV) (2 D, MeV)
ED
(2 D, MeV)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
0
150
300
450
600
750
900
1050
1200
1350
1500
1650
1800
1950
2100
2250
2400
2550
2700
2850
3000
2.80
2.79
2.78
2.77
2.77
2.76
2.75
2.74
2.73
2.72
2.71
2.70
2.69
2.69
2.68
2.67
2.66
2.65
2.64
2.63
2.62
0
0.0087
0.0087
0.0087
0.0087
0.0087
0.0088
0.0088
0.0088
0.0088
0.0088
0.0088
0.0089
0.0089
0.0089
0.0089
0.0089
0.0090
0.0090
0.0090
0.0090
1.87
1.86
1.86
1.85
1.84
1.84
1.83
1.83
1.82
1.81
1.81
1.80
1.80
1.79
1.78
1.78
1.77
1.77
1.76
1.75
1.75
0
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0018
0.0019
0.0019
0.0019
0.0019
0.0019
1.56
1.55
1.54
1.53
1.52
1.52
1.51
1.50
1.49
1.48
1.47
1.46
1.46
1.45
1.44
1.43
1.42
1.41
1.40
1.40
1.39
1.87
1.86
1.85
1.84
1.84
1.83
1.82
1.81
1.81
1.80
1.79
1.78
1.77
1.77
1.76
1.75
1.74
1.74
1.73
1.72
1.71
0.31
0.31
0.31
0.31
0.31
0.31
0.31
0.31
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.33
0.33
multichannel analyzer, and (dE/dx)eff,j is the effective stopping power of the recoiled nuclei
[11,19,54] originating from the jth layer:
dE
dx
j Y
Sr;i 1 Sr;det
ˆ
‰SŠj
Sr;i
Sr;0
eff;j
iˆ1
(33)
The first term on the right-hand side of Eq. (33) is the ratio of the stopping powers at both interfaces
of each sublayer, the second term represents the ratio of the stopping powers of the recoils after
passing through and before reaching the stopper filter, respectively. The third term in Eq. (33) is the
stopping power of the recoils in layer j defined using the stopping power of the projectile Sp,j and the
recoil Sr,j:
‰SŠj ˆ
kSp;j
Sr;j
‡
cos y cos…f y†
(34)
Eqs. (28)±(34) provide the framework for determining the concentration of light elements. For
samples of a known composition, a simulated ERD spectrum can be calculated using the
previous approach. This spectrum can be convoluted with the resolution function, a Gaussian, to
account for instrumental resolution and then compared directly with the experimental spectrum.
If this comparison is unsatisfactory, a new composition is used to simulate another ERD
spectrum for comparison with the experimental one. This procedure is repeated until satisfactory
agreement is found as discussed later. The analysis of NRA data is in principle similar to that of
ERD [11,61].
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
5.3. Scaling approach to convert experimental data
The scaling approach is a fast and straightforward method for analyzing the experimental ERD
spectrum. In this section, a brief description of the scaling method is given. A more comprehensive
version can be found elsewhere [17,19]. One advantage of this approach, relative to computer
simulation, is that the absolute values of s and dE/dx are no longer required. The key parameters that
must be accurately known are the functional dependencies of s and (dE/dx)eff on E0. The scaling
approach is most useful when the signals originating from 1 H and 2 D do not overlap.
Using the thick target approximation, s and (dE/dx)eff are calculated for a series of layers and
tabulated as a function of the recoil energies. The previous method for converting the recoil yield into
concentration is then followed. Each data point in the recoil spectrum is divided by s(Ein,j)/s(Ein,0)
and multiplied by (dE/dx)eff,j/(dE/dx)eff,0, where s(Ein,0) and (dE/dx)eff,0 are the scattering crosssection and effective stopping power evaluated at the sample surface, respectively, and s(Ein,j) and
(dE/dx)eff,k are their corresponding quantities in layer j. Note that the energy of the recoil originating
from layer j corresponds to the energy of the data point to be converted. In this way, the recoil yield
has been normalized to account for the changing values of s and (dE/dx)eff in the sample. The
absolute values of the concentration are obtained by multiplying a correction factor, which is
determined by either conservation of material or a priori knowledge of the concentration. In some
cases, the placement of a calibration layer on the sample of interest proves useful. For example, a thinfilm with a known concentration, i.e. dPS, is deposited over part of the sample before ERD analysis.
After accounting for s and (dE/dx)eff, the spectrum is normalized using the internal standard.
In some limited cases, the recoiled yield can be directly converted into a concentration profile
very easily. First, an ERD spectrum is taken from the sample to be analyzed (sample A). Then, using
the same experimental conditions (incident beam energy and type of the projectiles, beam flux,
scattering geometry, etc.), an additional ERD spectrum is recorded from a ``normalization'' sample
(sample N), usually a thick polymer film. The concentration profile is then obtained by dividing the
spectrum from sample A by the one from N. Note that the polymer used for normalization must have
a similar stopping power behavior as sample A. After the yield to concentration conversion, the
energy scale is converted into the depth scale using the energy-depth dependence determined using
the thick target approximation.
5.4. Computer simulation
Two methods can be used to analyze experimental ERD spectra. The first one numerically
deconvolves the experimental spectrum as outlined earlier. Although the most direct method, this
approach is rather cumbersome and usually complex. A second way of interpreting spectra is based
on computer simulations. In this approach, a composition profile is first estimated and its simulated
spectrum compared with the experimental one [15,17]. An interactive process then ensues until there
is satisfactory agreement between the simulated and experimental spectra.
In 1994, Vizkelethy [62] catalogued many of the computer simulation programs used for ion
beam analysis. However, many of these programs, mostly DOS based, are dated, unavailable or no
longer used. Table 2 provides a selective list of the current computer programs used for RBS and/or
ERD including RUMP, GISA, SIMNRA and IBA [37,63±66]. These programs may include fitting
routines and non-Rutherford cross-sections including resonance cross-sections. As discussed in
Section 3, the energy spread of the projectiles and recoils in the target, and the recoils in the stopper
filter, DEtot, is included in the experimental spectrum but not the spectrum calculated using the
approach in Section 5.1. An exact calculation of the total energy spread can be made [26]. An
145
146
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Table 2
Selected list of several readily available simulation programs for RBS and ERD analysis
Program name
Operating system (computer)
Technique
Reference
RUMP
SIMNRA
GISA
IBA
ANSIC
Win 95/98/NT
DOS
Win 95/OS-2
RBS,
RBS,
RBS,
RBS,
[37,63]
[65]
[64]
[66]
ERD
ERD
ERD
ERD
alternative approach is to convolute the ``ideal'' spectrum with an energy distribution function that
includes the particular contributions to DEtot. In summary, the accuracy of computer simulations
depends on how well the following parameters are known: (i) scattering cross-sections, (ii) stopping
powers of the incident particles and recoils in the target, and (iii) stopping powers of the recoils in
the stopper filter.
6. Ion beam and complementary techniques
Conventional ERD has been modified to improve upon the nominal depth resolution or
sensitivity. For example, emission angle ERD (EA-ERD) benefits from a superb depth resolution
[67], coincidence detection ERD (CD-ERD) has a superior sensitivity (below ppm) [68,69] or
electromagnetic filter detection ERD (E and B ERD) which benefits from a depth resolution of ca.
Ê and sensitivity better than 0.2 at.% [70,71]. The improvements offered by these techniques
100 A
have not been widely applied to polymer systems. In Section 6.1, ion beam techniques are presented
including TOF±ERD, resonant and non-resonant NRA, and heavy ion ERD (HI-ERD) (Table 3). In
Section 6.2, a brief review of techniques for characterizing polymer surfaces (Table 4) and polymer
interfaces (Table 5) is presented.
6.1. Related ion beam techniques
6.1.1. Time-of-flight±ERD (TOF±ERD)
The principles of TOF±ERD are identical to ERD. The main difference lies in the detection
system. As discussed previously, the main factor limiting the depth resolution of ERD is straggling in
Table 3
Summary of key features and specifications for RBS, ERD and NRA depth profiling techniques
Technique
Incident
beam
Detected
particles
RBS
ERD
LE-ERD
TOF±ERD
NRAb
NRAc
NRAd
4
4
4
1
a
He
He
4
He
4
He
3
He
3
He
15
N
Contrast
due to
He
Heavy atoms
1
H, 2 H
H, 2 H
1
1
H, 2 H
H, 2 H
1
2
4
1
H, H, He H, 2 H
4
2
He
H
1
2
H
H
1
g-Rays
H
Information
content
Depth
Probing
Profiling
Ê ) depth (mm) sensitivity
resolution (A
(1020 cm 3)
Marker depth profile
1
H, 2 H depth profiles
1
H, 2 H depth profiles
1
H, 2 H depth profiles
2
H depth profile
2
H depth profile
1
H depth profile
80±300a
800
80±400a
250
140
300
50
1
0.7
0.1
1
1
8
3
Depends on (i) beam type, (ii) incident beam energy, (iii) sample type, and (iv) scattering geometry.
Based on the 2 H(3 He, 4 He)1 H non-resonant nuclear reactionÐfollowing Chaturvedi et al. [46].
c
Based on the 2 H(3 He, 4 He)1 H non-resonant nuclear reactionÐfollowing Payne et al. [45].
d
Based on the 1 H(15 N, 4 He g) resonant nuclear reaction.
b
0.01±1a
1
<1
1
>1
>1
1
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
147
Table 4
Polymer surface analysis techniques and their characteristics
Technique
Acronym
Chemical
sensitivity
Lateral
resolution
Information
type
Contact angle
Optical microscopy
Scanning near-field
optical microscopy
Scanning tunneling
microscopy
Scanning electron
microscopy
Transmission electron
microscopy
Atomic force microscopy
CA
OM
SNOM
±
±
±
mm
10 mm
Ê
1000 A
Surface hydrophobicity
Surface image
Surface topography
STM
Possible
Ê
1 A
SEM
Ê
Quantitative 50 A
TEM
±
Ê
30 A
Molecular imaging,
surface topography
Surface topography,
surface image
Two-dimensional profile
AFM
Possible
Ê
5 A
SSIMS
Elemental
1 mm
Molecular imaging,
surface topography
Surface composition
XR
XPS
(ESCA)
IR-ATR
±
Chemical
±
10 mm
Surface roughness
Surface composition
Static secondary ion
mass spectrometry
X-ray reflectivity
X-ray photoelectron
spectroscopy
Infra-red attenuated
total reflection
High-resolution electron
energy loss spectroscopy
Auger electron spectroscopy
HREELS
AES
SemiSeveral mm Surface vibrational
quantitative
spectrum
±
1 mm
Surface vibrational
spectrum
Elemental
1 mm
Surface composition,
composition topography
Comment
Reference
[98,99]
[104]
[105]
a,b
[106]
a,c
[102,108]
a,c,d
[108]
[109]
a,c
[100±102]
a
[84,110]
[100]
a,c
[98,99]
a,c
[103]
a,c,e
[97]
a
Sample damage likely.
Requires conducting substrate.
c
Requires vacuum.
d
Requires special sample preparation (microtoming, staining, etc.).
e
Depth profile obtained in combination with sputtering.
b
the stopper filter. Whereas straggling is greatly reduced by using low beam energies and thinner
stopper filters, the probing depth is greatly reduced by this method, called LE-ERD. Rather than
using a stopper filter, TOF±ERD separates the 1 H and 2 D recoils from the forward scattered 4 He by
using an electronic discriminating system.
Introduced by both the Groleau and Thomas groups [72,73], TOF±ERD was first applied to
polymers by Sokolov et al. to study the surface structure of polymer blends [74]. The geometry and
detector system for TOF±ERD are illustrated Fig. 31. The forward scattered 4 He and recoiled 1 H and
2
D pass through a thin carbon filter placed between the sample and detector. As the ions penetrate
the carbon filter, secondary electrons are emitted from the filter. To minimize straggling, the carbon
Ê ). The secondary electrons are then amplified and generate a start
filter is very thin (500±1000 A
signal for an electronic clock. A corresponding stop signal is then generated when an ion arrives at
the detector. Because the distance between the detector and carbon filter, Dx, is known (usually 15±
90 cm) the TOF of the particle, tF, is also known. The TOF of each particle is a function of its mass
with heavier particles having longer flight times than lighter ones. In this way, the 4 He projectiles
and 1 H and 2 D recoils can be discriminated without using a stopper filter. For ions recoiling from the
sample surface, the TOF can be estimated from
r
Mt
(35)
t ˆ Dx
2KE0
148
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Table 5
Polymer interface analysis techniques and their characteristicsa
Technique
Acronym Contrast
Depth
resolution
Information
Comment Reference
Pendant drop technique
Transmission electron
microscopy
Atomic force microscopy
Dynamic secondary
ion mass spectrometry
X-ray reflectivity
PDT
TEM
±
Staining
±
Ê
10 A
Interfacial tension
Cross-sectional view
b,c,d
[85,88]
[108]
AFM
DSIMS
Friction/modulus
Elemental
Ê
5 A
Ê
90±130 A
Cross-sectional view
Elemental depth profile
b,c,e
[109]
[86]
XR
Elemental
(heavy atoms)
Elemental
(1 H, 2 H)
Refractive index
Ê
5 A
Marker depth profile
Ê
10 A
1
Chemical
Ê
10 A
H and 2 H
depth profiles
Film thickness,
refractive index profile
Elemental depth profile
Elemental
(1 H, 2 H)
Elemental
(1 H, 2 H)
10 mm
Elemental depth profile
Ê
10 A
Elemental depth profile
Neutron reflectivity
NR
Ellipsometry
ELLI
X-ray photoelectron
spectroscopy
Infrared densitometry
XPS
(ESCA)
IR-D
Small angle neutron
scattering
SANS
Ê
10 A
b
[84,110]
[84]
[89]
b,c,e
[100]
[90]
f
[92]
a
See Table 3 for ion beam techniques.
Possible beam damage to specimen.
c
Requires vacuum.
d
Requires special sample preparation (microtoming, staining, etc.).
e
Relative concentration.
f
Provides optical transform of profile.
b
where Mt is the ion mass and K and E0 are the kinematic factor and the incident energy, respectively.
Because 1 H and 2 D have slightly different masses and, thus, different kinematic factors, they also can
be discriminated by TOF±ERD. The results are recorded as a three-dimensional spectrum, where the
recoiled yield is a function of both the detected energy and the TOF.
The main advantage of TOF±ERD over the conventional ERD is improved depth resolution.
Ê , similar to RBS. In principle, the depth
The reported surface depth resolution for PS is 250±300 A
resolution of TOF±ERD is determined by the resolution of the SB detector. TOF±ERD also has
the ability to distinguish 1 H from 2 D and to depth-profile these elements to several micrometers
Fig. 31. TOF±ERD geometry showing incident ion, sample, and detection system.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
below the surface. As with conventional ERD or LE-ERD, depth resolution deteriorates with probing
depth. In spite of these advantages, TOF±ERD is not as widely used as ERD for several reasons.
First, extremely sensitive and rather complicated electronics is required to produce and amplify the
start and stop signals. Any small changes in the stability of the electronics will degrade the quality of
the detected signal. Moreover, the carbon foil is extremely sensitive to sudden changes in the
pressure and must be kept in a very dry environment. Also, the sensitivity of TOF±ERD for detecting
1
H and 2 D (40%) is worse than that of ERD because the efficiency in generating the secondary
electrons decreases with decreasing atomic number [74]. Several methods to improve efficiency have
been proposed. For example, Gujrathi and Bultena [75] found that covering the carbon foil with a
thin layer of MgO improved the detection efficiency of detecting 1 H to 90%.
6.1.2. Heavy ion ERD
The depth resolution of conventional ERD can be improved by using heavier projectiles. While
this method has been used extensively to study the hydrogen distribution in solid materials [76], HIERD has not been extensively applied to polymers because of concern for beam damage. In the first
application, Green and Doyle [77] used HI-ERD to study diffusion in polymer blends. Using a 28 Si
Ê and the probing depth 9200 A
Ê . These
beam the depth resolution was found to be 300 A
improvements over ERD were, however, nullified by heavy radiation damage to the polymer which
was 30 times greater than using a 4 He projectile. Radiation damage in polymers and routes to reduce
beam damage are discussed in Section 4.4. Using 2.4 MeV 12 C incident ions, Geoghegan and Abel
[78] applied ERD to study surface segregation from a PS polymer network with a depth resolution of
Ê at the sample surface. In addition, the depth resolution was found to remain relatively constant
80 A
Ê beneath the sample surface. In order to mitigate
up to the maximum accessible depth of 1000 A
beam damage to the sample the dose on any one spot of the sample was limited. Based on knowledge
Ê 3. The
of the damage caused by a 12 C beam [79], the dose on each spot was kept below 0.1 eV A
sample was translated across the beam and the yield from each spot was summed to obtain sufficient
statistics. While HI-ERD has been shown to work on PS systems [78,80], in comparison with other
polymers, PS is particularly stable to beam damage. Other polymers would require an even lower
dose per sample spot possibly making HI-ERD impracticable.
6.1.3. Non-resonant nuclear reaction analysis
In contrast to ERD, which is based on a billiard ball-like collision, the incident ion in a NRA
experiment penetrates into the target nucleus to excite a nuclear cascade. Figure 32 shows the most
common nuclear reaction for detecting 2 D. When an incident 3 He particle having energy E0 hits a 2 D
Fig. 32. Non-resonant nuclear reaction between incident 3 He particle and target 2 D nuclei. This reaction, denoted as
2
H(3 He, 1 H)4 He, is exothermic (Q ˆ 18:352 MeV).
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Fig. 33. Non-resonant nuclear reaction geometry showing incident ion, sample, and detection system consisting of a
magnet for deflection and solid state detector.
atom in the target material, a nuclear reaction occurs to produce 4 He and 1 H with energies E1 and E2,
respectively. This reaction, denoted as 2 H(3 He, 1 H)4 He, is quite exothermic (Q ˆ 18:352 MeV).
Therefore, the outgoing 4 He and 1 H ions have energies that are much higher than E0. The scattering
cross-section for this nuclear reaction, s, is much smaller than that for elastic scattering of 4 He from
2
D (i.e. ERD). Therefore, the incident beam energy is chosen to produce the maximum s (i.e.
E0 ˆ 600 keV) [81]. In a typical set up with E0 ˆ 700 keV the outgoing 4 He and 1 H have energies of
E1 ˆ 5:84 MeV and E2 ˆ 13:24 MeV. Depending on whether 4 He or 1 H are detected, two variations
of NRA have been developed.
Klein and co-workers [46] at the Weizmann Institute perform depth profiling by detecting 4 He.
As shown in Fig. 33, the outgoing particles travel towards the detector placed at F ˆ 308. Before
reaching the detector the particles are separated by a magnetic field so that only the 4 He particles
reach the detector. Both the neutral 3 He particles, which are not deflected, and the 1 H ions, which are
strongly deflected, are stopped by slits placed in front of the detector as shown in Fig. 33. The main
purpose of the magnetic filter is to prevent the electronic detection system from being saturated by
the signal from the elastically scattered 3 He.4 The depth distribution of 2 D is obtained from the
energy distribution of the outgoing 4 He particles. Similar to ERD, the nuclear reactions originating at
larger depths beneath the sample surface correspond to 4 He with lower energies. In NRA, the depth
resolution is improved because no stopper foil is needed and the energy loss in the sample is larger
Ê.
[61]. The depth resolution at the surface of PS was found to be 140 A
In the NRA technique developed by Payne et al. [45], the 1 H products of the nuclear reaction are
detected by a detector positioned at F ˆ 208. Similar to the Weizmann group approach, a depth-profile
is determined by monitoring the energy of 1 H emitted from the sample. In contrast to the previous
approach, the nuclear reaction originating deeper in the sample produces 1 H with higher energy. As
before, no stopper filter is required. Because the number of elastically scattered 3 He is negligible,
magnetic separation of the particles is no longer necessary. The depth resolution can be further
improved by increasing the path-length of the incoming 3 He. For E0 ˆ 700 keV, the depth resolution at
Ê as the angle between the sample normal and the incident
the surface improves from 1000 to 300 A
beam increases from 0 to 758. While the depth resolution is worse than in the Weizmann group
approach, the set up adopted by Payne et al. benefits from much larger probing depths (up to 8 mm)
[45]. Moreover, the depth resolution in the latter set up deteriorates more slowly with increasing depth.
6.1.4. Resonant nuclear reaction analysis
Whereas non-resonant NRA is useful for profiling 2 D, resonant NRA provides a direct method
for profiling 1 H. In contrast non-resonant nuclear reactions, where the cross-sections for the reaction
4
The number of the elastically scattered 3 He particles is much larger than that of 4 He produced by the nuclear reaction.
By ``filtering-off'' the signal arising from 3 He one can, thus, significantly decrease the ``dead time'' of the detector.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
vary smoothly with the energy of the projectile, the resonant nuclear reactions take place only within
a narrow energy window. One of the most useful reactions is 1 H(15 N, 4 He g)12 C, which has a strong
resonance at an incident energy of 6.40 MeV [82]. This reaction produces 4.44 MeV g-rays, which
are counted by a g-ray detector. The number of g-rays is proportional to the 1 H concentration at the
depth of the reaction. To depth-profile 1 H, the energy of the incident 15 N projectiles is systematically
increased from 6.40 MeV to higher energies. Measuring the 1 H distribution in thin polymer films,
Ê at the sample surface. This value is very
Endisch et al. [83] measured a depth resolution of 50 A
close to the best resolution obtained with the most sensitive depth profiling techniques such as
neutron or X-ray scattering. Unfortunately, NRA based on the 1 H(15 N, 4 He g)12 C reactions suffers
from several drawbacks that limit its ability to study polymers. First, the sample must be very large
(several cm2) because of the large incident beam cross-section (40 mm2). Second, the sample must
be placed inside a bore-hole of a bismuth germanate g-ray detector to increase the reaction yield by
increasing the solid angle to almost p [83]. Third, the nuclear reaction produces an undesirable
background of g-rays that must be distinguished from the g-ray signal. Although no substantial
damage was reported [84], radiation damage to polymers in general is likely due to the g-rays as well
as the heavy projectiles. In spite of these drawbacks, resonant NRA can be an extremely useful
technique to study phenomena on the size scale of a polymer coil.
6.2. Complementary techniques
Because of the great interest in polymer surface and interface problems, polymer scientists
have had a renewed interest in advancing existing techniques as well as developing new methods
with improved depth resolution and sensitivity. The ion beam techniques described in this review
are listed in Table 3. One approach for selecting a technique is to identify whether the problem of
interest is located at the surface or interface. Tables 4 and 5 list the experimental techniques
frequently used in surface and interfacial polymer science. For surface studies (Table 4), chemical
information, lateral resolution, and information type are important parameters. For interface
studies (Table 5), contrast, depth resolution, and information type are important [85±94]. A
detailed discussion of each technique is found in the reference list (last column). Technique
theory, specifications and applications to polymer science have been previously reviewed [84,93±
97].
Some polymer surface characteristics of interest include surface topography, surface chemical
composition, molecular orientation, and molecular imaging. Traditional surface science techniques
include contact angle measurements [98,99], X-ray photoelectron spectroscopy [100], static
secondary ion mass spectrometry [100±102] infrared attenuated total reflection, and high-resolution
electron energy loss spectroscopy [103]. Microscopy (cf. Table 4) also provides a versatile array of
tools for studying surface properties [102,104±109]. These techniques usually provide information
about the sample surface and the near-surface region, a few nanometers below the surface. The
Ê to several millimeters.
lateral resolution of the surface sensitive techniques ranges from several A
Two or more techniques with complementary strengths and weaknesses provide the best method for
a complete surface characterization.
Depth-profiling techniques provide a measure of the concentration profile perpendicular to the
sample surface. These techniques can be subdivided into two main groups depending on whether the
concentration profiles are obtained in reciprocal or real space. In the first category are scattering
techniques based on light (ellipsometry, modified optical schlieren technique (MOST)), X-rays (Xray reflectivity) [84,110] or neutrons (neutron reflectivity, small-angle neutron diffraction) as the
source of incoming radiation (particles). On the other hand, real space depth-profiling techniques
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mainly use beams of accelerated ions to probe the composition. These techniques include dynamic
secondary ion mass spectrometry and the ion beam techniques in this review.
Sensitivity and depth resolution are key parameters for choosing a depth-profiling technique.
Ê ), elemental
While the depth resolution of scattering techniques is usually excellent (several A
sensitivity is highest for abrupt changes in the concentration. In addition, the depth-profile is
obtained from an optical transform of scattering intensity. Only in special cases can this information
be directly converted into a real space profile. In practice, the data are analyzed by model fitting,
which can lead to some uncertainty in interpreting experimental results if more than one model
concentration profile is suitable. In most cases, independent information about the concentration in
the sample is required using real space depth-profiling techniques. In spite of their poorer depth
resolution, real space techniques based on the interaction of ions with the sample provide useful
information about polymers near surfaces and interfaces. Because their depth resolution can be as
small as a single (large) polymer molecule, ion beam techniques can be used as standalone
experimental tools. In addition to their excellent sensitivity (usually <1 at.%), real space depthprofiling techniques are not limited to sharp concentration gradients.
Since the mid-1980s ion beam techniques have provided polymer scientists with a powerful tool
for depth-profiling polymers in thin-films. In Section 7, of this review, selected case studies are
presented to demonstrate the range of polymer surface and interface problems addressed by ERD.
7. Polymer surface and interface case studies
Since it was first applied in the mid-1980s, ERD has become a standard technique in the
characterization toolbox of polymer scientists. In this section, selected case studies are presented to
provide the reader with insight about the range of surface and interface problems that ERD has
already addressed either by itself or in conjunction with complimentary techniques. In particular,
tracer and mutual diffusion of macromolecules as well as small molecule diffusion are presented in
Section 7.1. Section 7.2 discusses polymers at surfaces, which involves understanding the
enrichment of one polymer in a multi-component system at the polymer/air, polymer/polymer, and
polymer/solid interfaces. In Section 7.3, polymer/polymer interfaces problems are addressed
including the application of ERD to determine phase diagrams, understand phase separation, and
investigate wetting behavior. Finally, in Section 7.4, polymer modification by ion beam irradiation is
presented.
7.1. Polymer diffusion
Polymer dynamics was one of the first significant problems to be addressed by ERD. Using
ERD, Green and co-workers [20,111] firmly established the range of tracer and matrix molecular
weights over which the reptation mechanism [112] dominates diffusion in polystyrene (PS).
Although first proposed in 1971, the first definitive studies of reptation were performed many years
later using an innovative version of infrared spectroscopy [113,114]. In a sense, these infrared
spectroscopy studies nucleated the application of a wide range of new techniques to study polymer
diffusion, including fluorescence recovery after pattern photobleaching, forced Rayleigh scattering
(FRS), neutron and X-ray small-angle neutron scattering, nuclear magnet resonance, RBS, NRA and
ERD [11,115]. Due to their outstanding spatial resolution, secondary ion mass spectrometry and
neutron reflectivity have been the most recent additions to the experimentalists toolbox of surface
and interface techniques.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
7.1.1. Self and tracer diffusion
In this section, we focus on how ERD is used to determine tracer and self-diffusion coefficients.
Rather than provide a comprehensive review of polymer diffusion we refer the interested reader to a
review article [116]. We start by discussing the ground-breaking experiments by Green et al.
[20,111] which set the stage for a decade of diffusion, surface, and interface studies. A thin
polystyrene (PS) layer 15 nm thick was deposited on a thicker (650 nm) layer of deuterated PS
(dPS). It is important to point out that a true tracer diffusion measurement requires a low volume
fraction (i.e. trace amount) of the diffusing species in the matrix, and therefore, the top film should
be made as thin as possible.
The volume fraction versus depth profile of polystyrene chains of molecular weight 1:1 105 is
shown in Fig. 26b [111]. The characteristic diffusion distance, x ˆ …Dt†0:5 , of 300 nm is optimum for
these experimental conditions because it is much greater than the depth resolution of 80 nm, but
much less than the matrix film thickness. Furthermore, the measured value of the maximum volume
fraction f is only 0.03 suggesting a trace amount of PS in the dPS matrix. The true maximum is
greater than this because the profile in Fig. 26b is not deconvolved with the instrumental resolution,
about 80 nm FWHM. This instrumental broadening is also responsible for the apparent observation
of PS at negative depths. Whereas Fig. 26b utilizes a thin PS tracer film, a tracer film of dPS has the
advantage of an unlimited PS matrix thickness, cost reduction, and accuracy.
Mills et al. [20] used ERD to depth profile dPS in a thick PS matrix. Using a thin dPS film on a
thick PS matrix allows one to make the matrix film many microns thick. Here, the accessible depth is
now limited by overlap between the 2 D yield from beneath the surface and the 1 H yield from the
surface layer. The ERD spectrum before and after annealing is shown in Fig. 34 [20]. For 3 MeV
4
He2‡ , F ˆ 308, and a 10.6 micrometer MylarTM filter, the accessible depth was about 800 nm. To
determine the diffusion coefficient, D, the dPS volume fraction profile is fited with the solution to
Fig. 34. ERD spectra from a bilayer film consisting of 12 nm of dPS (M w ˆ 225,000) on PS (M w ˆ 2 107 ) before
diffusion (triangles) and after diffusion for 3600 s at 170 8C (crosses) (taken from [20]).
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Fig. 35. The tracer diffusion coefficient, D, of polystyrene of molecular weight M diffusing in polystyrene at 171 8C is
plotted as a function of M. D measured by ERD is in excellent agreement with other ion beam techniques (RBS and NRA)
and FRS (taken from [11]).
the thin film diffusion equation
"
#
h x
h‡x
f…x† ˆ 0:5 erf
‡ erf
…4D t†0:5
…4D t†0:5
(36)
where h is the film thickness and t the annealing time. Fig. 35 shows the tracer diffusion coefficient
of PS at 171 8C measured by ERD and other techniques [11]. For the diffusion of high molecular
weight polymers (M @ M e ) into a high molecular weight matrix, D was observed to vary as M 2, in
agreement with the reptation prediction. These ERD studies supported earlier diffusion studies using
FRS [117].
This brings up the question of technique choice. For example, relative to say FRS, the ERD
equipment is more complex and costly. However, ERD has a higher spatial resolution and provides a
direct spatial profile. To choose a technique to study diffusion, the slow diffusion rate of polymers is
a limiting factor. For entangled polymers, D typically ranges from 10 11 to 10 15 cm2/s at
temperatures ca. 70 8C above the glass transition. Thus, in 1 h, the diffusion distance ranges from
Ê , respectively and therefore, techniques that provide a high spatial resolution, 100 A
Ê,
2 mm to 200 A
are most attractive. Contrast between the diffusing species and medium is also an important
consideration. Some techniques such as FRS require fluorescent labels on the diffusant. ERD simply
requires that one of the polymers is deuterated, usually the diffusant. Although many deuterated
polymers and monomers are available, isotopic contrast can be a deterrent when studying unusual
systems. Another consideration is the sensitivity of polymers to radiation. Polymers exposed to light
ions undergo cross-linking, chain scission or a combination of both. This topic is discussed in
Section 7.4.
Following the entangled PS diffusion studies, ERD was soon applied to study tracer and selfdiffusion in other systems, and as a function of diffusing species architecture and matrix species
molecular weight and architecture. In particular, ERD was used to clarify whether short matrix
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 36. ERD spectra converted to volume fraction versus depth profiles corresponding to (a) a tracer dPS film and (b) a
tracer dPXE film diffused into a PS0.55:PXE0.45 matrix for 1:56 105 s at 206 8C. The solid lines are theoretical fits using
(a) DdPS ˆ 1:2 10 14 and (b) DdPXE ˆ 2:2 10 15 cm2/s for dPS and dPXE, respectively (taken from [119]).
chains provide ``moving'' constraints for the diffusant, and increase diffusivity [118]. ERD also
played an important role in the study of star, cyclic and block copolymer diffusion [116].
Tracer diffusion in chemically dissimilar, yet miscible, polymer pairs was first studied by ERD
[119,120]. The tracer diffusion of dPS and deuterated poly(xylenyl ether) (dPXE) into PS:PXE
blends was measured. Fig. 36 shows the depth profiles, converted from ERD spectra, for tracer films
of (a) dPS and (b) dPXE diffused into a PS:PXE matrix [119]. Note the similarity with Fig. 34 for the
dPS/PS isotopic blend. The main difference between these two studies is that the conversion from
Fig. 36a and b utilizes the stopping powers of 4 He, 1 H, and 2 D in the PS:PXE blend. Because of their
different composition and structure, the stopping power difference between polymers (such as PS
and PS:PXE blends) can be significant. For example, the stopping powers for 2 MeV 4 He in
polyimide and PS are 25 eV/(1015 atoms/cm2) and 18 eV/(1015 atoms/cm2), respectively [31,32].
Thus, each matrix species (composition) requires its own set of stopping powers for conversion. If
the tracer component is very dilute after annealing, the stopping power (and therefore, depth scale) in
the matrix is not significantly changed by the diffusing species. As in the dPS/PS case, the tracer
diffusion coefficients of dPS and dPXE, respectively in PS:PXE matrices scale as M2 , in agreement
with reptation [120].
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ERD has also been used to study tracer diffusion of dPS in PS:poly(vinyl methyl ether) blends
[121], and both dPS and deuterated tetramethylbisphenol-A polycarbonate (dTMPC) in PS:TMPC
blends [122]. In both studies, the diffusion coefficient scaled as M 2. The tracer diffusion coefficient
of dPMMA in PMMA: poly(styrene-ran-acrylonitrile) blends was found to vary as M 2.2, in fair
agreement with reptation [123]. Note that in two of the above studies, the analysis of reptation in
polymers blends was limited by the availability of both components in deuterated form.
7.1.2. Mutual diffusion
The first quantitative studies of mutual diffusion between chemically dissimilar polymers were
carried out by Jones et al. [124] using microprobe analysis and Composto et al. [125] using ERD. In
the latter study, the interdiffusion coefficient was determined from the polymer volume fraction
profile between two blends of dPS:PXE having slightly different dPS composition, fdPS. Fig. 37
shows the volume fraction of dPS across the interface of a diffusion couple before and after
annealing for 1800 s at 206 8C [125]. The initial fdPS of the bottom film is 0.60 while that of the top
film is 0.51. In Fig. 37a, the interfacial broadening between the two films reflects the instrumental
resolution of about 80 nm. The solid line corresponds to a step function convoluted with the
instrumental resolution function, a Gaussian with a FWHM of 80 nm. After diffusion, the interface
broadens as shown in Fig. 37b. Because the glass transition of PS and PXE are quite different, 100
versus 206 8C, respectively, the D between pure couples is a strong function of concentration.
However, by using a composition difference between blends of 10%, a single mutual diffusion
Fig. 37. Volume fraction of deuterated polystyrene in a dPS:PXE thin-film diffusion couple: (a) as deposited, and (b) after
diffusion for 1800 s at 206 8C. The bulk volume fractions of the top and bottom films differ by ca. 10% to minimize the
composition dependence of the mutual diffusion coefficient (taken from [125]).
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
157
coefficient D should control the diffusion across the interface and, hence, we expect a concentration
profile given by
f…z† ˆ 0:5…f2
f1 † erf
h
x
w
‡ erf
h
x
w
‡ f1
(37)
where h is the top film thickness and w ˆ 2(Dt)0.5. The solid line in Fig. 37b represents the best fit of
this equation to the data using D ˆ 1:1 10 13 cm2/s. By varying the PS degree of polymerization,
NPS, D was found to scale as NPS1 [126]. Since PS is the faster-moving species in these blends this
variation of D is strong evidence for the fast theory of mutual diffusion. The mutual diffusion
coefficient is enhanced, relative to the tracer diffusion coefficient, by the negative w, which drives
intermixing between the attractive PS and PXE, segments. Accelerated diffusion was also observed
in the poly(vinyl chloride)/polycaprolactone system studied by Jones et al. [124].
In a related study, Green and Doyle measured the mutual diffusion coefficient of dPS/PS, an
isotopic alloy having a small positive w [127]. By using high molecular weight polymers, the
unfavorable segmental interaction parameter was found to inhibit diffusion. Using NRA, subsequent
studies by Losch et al. [128] were carried out below the critical temperature, TC, but in the one-phase
region using NRA. Bilayers having 10% difference in composition were annealed at T < T C .
Critical slowing down was observed as the average blend composition approached the coexisting
composition.
7.1.3. Small molecule diffusion through polymers
Because of their low vapor pressure, small molecule diffusion studies in a vacuum environment
pose a greater challenge than polymer diffusion studies. A majority of the small molecule studies
have focused on using RBS to depth profile solvents in glassy polymers [129±132]. In such cases, the
solvents contained an internal label (e.g. iodine or chlorine) for analysis. It is instructive to recall the
sample preparation route because the same route can be used to prepare samples for ERD. Polymer
coatings were exposed to vapor in a temperature bath containing solvent. After the diffusion time,
samples were quickly removed, quenched in liquid nitrogen to ``freeze in'' the small molecule
profile, and transferred to a pre-cooled sample holder in a glove bag backfilled with nitrogen. The
sample holder is then quickly attached to a sample manipulator which itself is cooled, usually by
liquid nitrogen.
ERD has been infrequently applied to study small molecule diffusion in part because volatility
will cause profile redistribution unless special sample handling is carried out as described.
Furthermore, small molecule diffusion through glassy polymers is much faster than polymer
diffusion in a melt. For example, the diffusion coefficient of water in polyimide is 5:6 10 9 cm2/s
[133]. Next, we review one innovative adaptation of ERD to study small molecule adsorption in
polymers.
Polyimide (PI) is a commonly used in microelectronics packaging applications. However, PI
films immersed in water debond from the silicon substrate after a few days because of water
adsorption. Wallace and co-workers [25,134] carried out in situ studies of deuterated water
absorption by PI during 1.3 MeV 4 He‡ irradiation. When the pressure in the target chamber was
raised to 1:3 10 2 Pa by the addition of D2O vapor through a leak valve, deuterium was readily
incorporated into the PI. Fig. 38 shows two 1.3 MeV ERD spectra taken after modifying 60 nm PI
films with 5:25 1014 and 3:15 1015 ions/cm2 [134]. The peaks centered at channels 375 and 250
correspond to deuterium and hydrogen, respectively. This study found that water adsorption could be
greatly enhanced >3 wt.% by simultaneously exposing PI to water vapor and ion beam irradiation.
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Fig. 38. ERD spectra showing the deuterium oxide uptake in polyimide at doses of 0:525 1015 ions/cm2 (filled circles)
and 3:15 1015 ions/cm2 (open triangles) (taken from [134]).
Furthermore, within the limits of the depth resolution (25 nm), no excess of D2O at the substrate
could be observed. Certain ERD techniques with better depth resolution should be able to resolve
whether water preferentially segregates to oxide surfaces or is uniformly distributed within the film.
External ion beam analysis in which samples are analyzed under atmospheric conditions offer one
route to increase the applicability of ERD [135]. In this method, the ion beam exits the target
chamber, crosses a short air gap, before encountering a film covering a water reservoir.
7.2. Polymers at surfaces
7.2.1. Surface segregation
In liquid and solid mixtures, the surface composition usually differs from the bulk because the
lower surface energy component tends to bloom to the surface [136]. In 1989, Jones et al. [137]
made the first observation of surface enrichment in polymer blends using an ion scattering technique,
ERD. The surface of a dPS:PS mixture was enriched with dPS as shown in Fig. 39. The dPS surface
excess, z, the hatched area in Fig. 39b, increased strongly as the dPS bulk concentration increased,
which is in general agreement with mean field theory. It was surprising that the small difference
between PS and dPS (i.e. the dPS surface energy is only 0.078 dyn/cm less than PS) can produce a
Ê . This study was the first of many on the dPS:PS system. Later studies
large surface excess, 100 A
invoked higher resolution techniques, including TOF±ERD, to determine the detailed depth
distribution [74,138±140]. The effect of molecular weight on the surface enrichment was also
studied [141,142]. Surface segregation studies were also performed on isotopic blends of deuterated
poly(ethylene propylene) and poly(ethylene propylene) [143].
In addition to isotopic blends, surface segregation in statistical copolymers has received
attention. For example Steiner and co-workers [144] used nuclear reaction analysis (NRA) to
measure z of poly(ethylene-ran-ethylethylene) E-EEx copolymers, where x is the fraction of EE
groups. The component with the higher ethyl branching fraction x was found to enrich the surface.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 39. Volume fraction profile of dPS at the surface of a dPS:PS blend (a) before and (b) after annealing at 184 8C for 5
Ê denoted by the
days. The solid lines are simulations assuming (a) a uniform 0.15 blend and (b) a surface excess of 110 A
cross-hatched area (taken from [137]).
This enthalpically driven segregation was justified by the lower surface energy of the more branched
chains.
Surface enrichment in chemically dissimilar blends has also received much attention. Bruder
and Brenn [145] and Gluckenbiehl et al. [146] studied mixtures of dPS and poly(styrene-co-4bromostyrene), PBrxS, where x is the mole fraction of bromostyrene, and observed surface
enrichment of dPS, the lower surface energy component. Using both neutron reflectivity and LEERD, Genzer et al. [27] measured both the surface concentration and z in dPS:PBrxS blends and
were able to perform a comprehensive test of existing models. As shown in Fig. 40, the dPS surface
excess measured by NR (solid circles) was in excellent agreement with that determined by LE-ERD
(open circles). In this study, the self consistent mean field (SCF) model is in better agreement with
experimental results than the Schmidt and Binder (SB) model. These measurements of z also
provide strong evidence for the scattering length density profile extracted from NR, which relies on a
trial and error fitting procedure. A main advantage of LE-ERD is that it can accurately measure z
Ê)
independent of any model. As a compliment to ERD, NR provides excellent depth resolution (5 A
and is quite sensitive to small variations of steep concentration gradients.
In the same spirit, NR, DSIMS, TOF±SIMS and LE-ERD were used to investigate surface
segregation in binary mixtures of poly(styrene-co-acrylonitrile)'s, SANx and dSAN. The SAN with
the lower AN content segregates to the surface. In one study [147], the dSANx profile was found to
deviate from the predicted exponential profile shape. At high bulk dSANx volume fractions (e.g.
0.6), the experimental profile displays a flattening relative to SCMF model predictions as shown in
Fig. 41.
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Fig. 40. Surface excess of dPS as a function of dPS volume fraction. The solid and open circles are from NR and LE-ERD,
respectively. The dotted, dashed and dash-dotted lines correspond to the Schmidt±Binder prediction, the SCF model with
short range interactions, and SCF with long range interactions, respectively (taken from [27]).
7.2.2. Segregation of polymers to the polymer/polymer interface
In this section, we examine polymer adsorption to the interface between two homopolymers,
which are incompatible with each other. The driving force for segregation is the decrease in
interfacial tension that also produces a small dispersion size and a broader interface. Buried
interfaces are notoriously difficult to characterize because many surface analysis techniques, such
Ê ). Ion scattering, in particularly
as AES and XPS, only probe the very near surface region (<100 A
ERD, allows direct profiling of interfaces even if they are ``buried'' 0.5 mm below the sample
surface.
The first quantitative study of diblock copolymer segregation to interfaces between immiscible
homopolymers was carried out with ERD. Shull and co-workers [148] measured the interfacial
Fig. 41. Volume fraction profile of dSAN23 in SAN27 obtained from NR (solid line), calculated using the monodisperse
SCF (dashed line) and polydisperse SCF (dotted line) models (taken from [147]).
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 42. Volume fraction profile of dPS-b-PVP copolymer at the PS/PVP immiscible interface after 8 h at 178 8C. The
Ê (taken from [148]).
copolymer concentration in the top film is 2.1% and interface copolymer excess is 100 A
excess of a diblock copolymer of deuterated polystyrene and poly(2-vinylpyridine) (dPS-b-PVP) at
interfaces between PS and PVP homopolymers. Fig. 42 shows the copolymer distribution after 8 h at
178 8C [148]. From this profile, the equilibrium volume fraction of copolymer in the PS phase and
Ê , respectively. Upon increasing the equilibrium
the interface copolymer excess are 2.1% and 100 A
Fig. 43. Volume fraction profile of dPS-b-PVP copolymer at the PS/PVP immiscible interface after 8 h at 178 8C at a high
Ê . A large surface excess is also evident
bulk copolymer concentration, 5.8%. The interface excess has increased to 155 A
(taken from [148]).
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volume fraction to 5.8% a new feature is observed, namely a surface excess of copolymer as shown
in Fig. 43 [148]. The authors attribute this surface excess to the formation of micelles, which
segregate to the surface. We conclude by noting that the a priori identification of this surface activity
by NR would be extremely difficult in this case.
Green and Russell [149] used ERD to study the segregation of symmetric diblock copolymers at
the interface between immiscible polystyrene and poly(methylmethacrylate) layers. ERD and SIMS
were used to characterize dPS-b-PVP adsorption at the interface between PS and a random
copolymer of PS and poly(4-hydroxy styrene) P(S-ran-PPHS) [150]. Above a critical copolymer
concentration, the copolymer interfacial excess increases dramatically but without any corresponding surface excess of dPS-b-PVP. Although SIMS is used to locate the dPS and PVP blocks, depth
profiling techniques that laterally average over millimeters or centimeters are incapable of
identifying whether the dPS-PVP copolymer chains are free or assemble into micelles or
microemulsion morphologies. As materials with nanostructures becoming increasingly important,
the combination of depth profiling techniques, like ERD, with laterally profiling techniques, like
AFM, will become more widely used.
The interfacial properties of an immiscible polymer blend A/C can be controlled by adding a
third component B, which is miscible with C but not with A. In 1992, Helfand [151] predicted that
B will enrich the interface if A/B interactions are favored over A/C ones (i.e. wAB < wAC ). In 1995,
Faldi et al. [152] produced the first experimental evidence for interfacial segregation in A/BC
systems. The system included polystyrene (PS or A) and two miscible random copolymers of
poly(styrene-co-4 bromostyrene) (dPBrxS or B, and PBry S or C) having x ˆ 0:09 and y ˆ 0:13
mole fraction of bromostyrene, respectively. Fig. 44 shows the depth profile of B before (open
circles) and after annealing at 170 8C for 41 days (closed circles). The interfacial excess of B,
Ê , is represented by the shaded region. The solubility of B in the A layer contributes to
about 120 A
broadening of the interfacial profile. Fig. 45 shows the experimental profile and the model profile
before (dotted line) and after (solid line) including the system resolution. The solid line
underestimates the measured broadening because it does not include the solubility of B in the A
layer. The profile was corrected for instrumental resolution by convolving the model profile with
Ê from the sample
Gaussian functions having a FWHM that increased linearly from 400 to 650 A
Fig. 44. LE-ERD volume fraction profile of dPBr0.09S (component B) at the A/BC interface as cast (open circles) and after
41 days at 170 8C (closed circles). The B interfacial excess is given by the shaded area (taken from [152]).
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 45. Volume fraction profile of dPBr0.09S (B) determined by LE-ERD (closed circles). The SCF profiles with (solid
line) and without (dotted line) instrumental resolution are also shown (taken from [152]).
Ê , respectively. These FWHM values were determined from standards
surface to a depth of 2000 A
and are specific to the ion beam conditions, namely, 2.0 MeV 4 He at 158 glancing incident and exit
angles using a 7.5 mm MylarTM filter. In complimentary studies, the interfacial width was
measured by NR [153]. In a subsequent study, ERD was used to study the effect of molecular
weight on the interfacial excess of B [154].
7.2.3. Polymer adsorption to the polymer/substrate interface
Polymer adsorption from a melt plays an important role in many materials technologies such as
coatings, adhesives, and microelectronics. One strategy to improve adhesion is to add a polymeric
adhesion agent, i.e. a molecule that migrates to the interface, anchors to the substrate, and entangles
with matrix polymers. The 1990s have witnessed numerous studies aimed at providing a
fundamental understanding of the key factors that influence polymer adsorption from the melt. A
recent review has been published by Composto and Oslanec [155]. This review includes
contributions made by ERD to study the adsorption of end-functionalized homopolymers [156]
and block copolymer adsorption [157±161].
7.3. Polymer/polymer interfaces and critical phenomena
Because of their technological and fundamental importance, multi-component, multi-phase
polymer blend thin films have been the subject of many theoretical and experimental studies both in
the past and in recent years. In this section, we will review experiments in which ion beam
techniques were used to probe the behavior of A/B polymer blends. The first class of studies includes
the investigation of the interfacial region between the A-rich and B-rich phases. In the second part,
we will discuss how the presence of additional interfaces (polymer/surface and polymer/substrate)
influences the overall behavior of systems undergoing phase separation. Focusing mainly on
experiments using NRA, a recent review [162] examines phase coexistence and segregation in thin
polymer films.
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7.3.1. Microscopic phenomena at coexistence
7.3.1.1. Interfacial width. The interfacial width between two coexisting phases in phase separated
polymer blends provides direct information about the thermodynamics of mixing. The polymer
volume fraction profiles vary smoothly across the interfacial region separating the two coexisting
phases. For a blend with an upper critical solution temperature in the infinite molecular weight limit,
Helfand and Tagami [163,164] showed that the width of the interface, w, scales as:
a
w p ;
wAB
(38)
where a is the polymer segment size and wAB the Flory±Huggins interaction parameter. This
approach assumes T ! T C , the critical temperature. Close to TC, Binder [165] found that w scales as:
a
w p
…wAB =wC † 1
(39)
where wC is the value of wAB at the critical point. Eq. (39) predicts that far from TC where wAB is the
high (0.001±0.1), w is on the order on nanometers. On the other hand, Eq. (39) demonstrates that
upon approaching T C …wAB ! wC †, w increases rapidly.
Because of their large size, polymers are model systems for studying coexistence phenomena.
Polymer behavior is also simplified because the entropy of mixing in polymer mixtures is strongly
reduced by a factor of 1/N, where N is degree of polymerization, as compared to small molecule
mixtures. As a result, the thermodynamic behavior of the polymer blends is typically dominated by
the repulsive enthalpic interactions between segments. An additional benefit of the large N is the
slowing down of polymer motion, which in turn allows for detailed dynamic studies [166]. The large
size of polymer coils is also an advantage because spatial dimensions that scale with the size of the
coil (e.g. w) are greatly magnified, particularly near TC. Because of their depth resolution, ion beam
techniques are well suited for studying the interfacial properties of immiscible interfaces.
Rafailovich et al. [167] provided the first study of interface formation in partially immiscible
polymer blends using RBS. This study also reported the first experimental measurements of the
critical slowing down of the mutual diffusion as a function of temperature. Their results indicated
that the interfacial width grew more slowly than the expected Fickian prediction w t1/2.
The equilibrium and kinetic properties were later extensively studied at the Weizmann Institute
and Freiburg. Using NRA, Steiner and co-workers [168,169] found that w increased according to
Eq. (39) for T near TC. Steiner and co-workers [170,171] also followed the kinetics of formation of
the interfacial region between two partially immiscible polymers and found that w increased as
w ta , where a ranges from 0.27 to 0.38 in agreement with previous experiments. Bruder and coworkers [172±174] used conventional ERD to measure w between dPS and poly(styrene-co-4bromostyrene) (PBrxS) where x was the mole fraction of bromostyrene. For x ˆ 0:062 and 0.119
segment/segment interactions were ca. 1- and 1.5-orders of magnitude more repulsive than those in
the isotopic dPS/PS system. Nevertheless, the interfacial width behavior was in good qualitative
agreement with previous experiments [168±171].
7.3.1.2. Phase diagram. Bruder and co-workers [172±174] were the first to show that interdiffusion
experiments can be used to determine the coexisting compositions of partially immiscible polymers.
Using NRA, Budkowski et al. [175] extended these studies to map out the temperature-composition
phase diagram for high-molecular weight dPS/PS mixtures. Fig. 46 shows the dPS volume fraction in
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 46. Volume fraction profile of dPS at the dPS/PS interface determined using 2 H(3 He, 4 He)1 H NRA. The various
symbols indicate the volume fraction of dPS in as-cast sample (diamonds) and samples annealed at 170 8C for 6 days (open
circles) and 29.7 days (closed circles). The arrows in the figure indicate the coexisting compositions of dPS in the PS-rich
(f1) and dPS-rich (f2) phases (taken from [175]).
the dPS rich phase (0.8) and dPS poor phase (0.2). In subsequent studies, Budkowski et al. [176]
investigated the effects of film confinement on the interfacial properties of dPS/PS system. They found
that for film thickness near w the composition profiles of the coexisting layers were modified. This
method of determining phase coexistence between two immiscible polymers was later extended to
other systems, such as polyolefin [144,177] and random copolymer [178].
As discussed in Section 4, the depth resolution of ion beam techniques deteriorates with probing
depth. Therefore, in most cases, the depth resolution at a buried interface is insufficient to measure w
between highly immiscible polymers (i.e. narrow interfaces). A simple sample modification step
prior to ERD analysis significantly improves the depth resolution below the surface. Genzer and
Composto [179] determined the interfacial width between dPS and poly(1,4-butadiene) (PB) using
LE-ERD. Prior to depth profiling, the sample was ``thinned'' by partially sputtering the top dPS layer
Ê of dPS was left over the buried interface.
using a low-energy Ar beam. In this way, only 200 A
Ê , in a good
Using LE-ERD and a grazing exit angle geometry the interfacial width was 60 35 A
agreement with the predicted value. This study shows that ion beam techniques, combined with
sample modification, can be used to measure features on the order of a polymer coil.
7.3.2. Macroscopic phenomena at coexistence
Binary polymer mixtures phase separate upon quenching into the unstable region of the phase
diagram. In the bulk, the concentration fluctuations that govern the phase separation process are
random. However, in thin polymer films the presence of interfaces, such as the polymer/surface or
the polymer/substrate, imposes directionality on the compositional waves in the blend. In particular,
close to the surface (and substrate), the resultant phases are oriented parallel to the surface. Thus,
phase demixing is governed by the interplay between phase separation and wetting. While the first
phenomenon is governed by the polymer/polymer interactions, the latter is controlled by the
interactions between the polymer and the interface. Because of their low interfacial energy, polymer
blends are model systems for studying wetting phenomena. Moreover, because the resultant
morphology has a strong preference to form a layered structure parallel to the surface (and
substrate), ion beam depth profiling techniques are ideal tools for studying wetting.
The surface induced ordering in phase separated polymer blends has been a subject of both
theoretical [180,181] and experimental [182] interest. In this section, we will briefly outline the
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Fig. 47. Volume fraction vs. depth profiles of dPEP in a 50/50 mixture after 172,800 s at 35 8C. The oscillatory profile is
representative of SDSD (taken from [183]).
experiments carried out with ion beam techniques. Interested readers should consult a review by
Krausch [182].
7.3.2.1. Surface directed spinodal decomposition. Jones et al. [183] reported the first experimental
observation of ``surface directed spinodal decomposition'' (SDSD). The system was a mixture of
poly(ethylene propylene) (PEP) and its deuterated analogue (dPEP). Critical mixtures of PEP/dPEP5
were quenched into the two-phase region of the phase diagram and conventional ERD was used to
monitor the volume fraction profiles of both polymers. The resultant spectra revealed the oscillatory
composition profile shown in Fig. 47. Namely, the dPEP-rich phase present at the surface was followed
by a PEP-rich phase, which in turn was followed by another dPEP-rich phase. The authors attributed
SDSD to the preferential surface adsorption of dPEP, the lower surface energy species. Limited depth
resolution of conventional ERD did not allow for a detailed study of the growth rate.
Krausch et al. [184] preformed the first rigorous kinetic study of spinodal decomposition at the
polymer mixture/air interface. Using TOF±ERD, the researchers studied the growth of the wetting
layer formed at the surface of critical mixtures of dPEP/PEP. Their results unambiguously showed
that the thickness of the surface wetting layer grew as t1/3 and the composition profiles exhibited
universal scaling behavior in the near surface region. Fig. 48 shows the normalized dPEP volume
fraction profiles as a function of depth divided by the wetting layer thickness. The dPEP volume
fraction profiles at various times fall onto a single master curve. In subsequent studies, Krausch et al.
used NRA to investigate the interplay between surface fields and domain ordering during SDSD
[185] and TOF±ERD to study off-critical compositions [186]. Using NRA to investigate SDSD in
critical mixtures of dPS and poly(a-methyl styrene) (PaMS), Geoghegan et al. [187] reported the
formation of a four-layer structure dPS-rich/PaMS-rich/dPS-rich/PaMS-rich. Geoghegan reported
that the surface dPS-rich layer grew as t0.14 (or logarithmically), in contrast to prior studies. This
group [188] also investigated the kinetics of SDSD in off-critical mixtures using NRA.
Thin film confinement can perturb the thermodynamic and kinetic behavior of SDSD. Krausch
et al. [189] carried out experiments on dPEP/PEP films with a range of thickness. Using NRA,
Fig. 49 displays the dPEP volume fraction profiles for (a) thick and (b) thin samples after 5.5 h in the
two-phase region. For a thickness much greater than the spinodal wavelength, the SDSD wave
gradually decays into the bulk. Upon decreasing the thickness, the spinodal waves originating from
both interfaces appear as shown in Fig. 49b. The profile was described by the superposition of two
5
By critical mixtures, we understand systems in which their composition coincides with the composition of the critical
point.
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 48. Deviation of the dPEP volume fraction from its average value vs. depth normalized by the wetting layer thickness
l (t) at 296 K for annealing times of: 308 min (solid circles), 594 min (stars), 1168 min (triangles), 1898 min (open squares)
and 2752 min (open circles) (taken from [184]).
damped cosine waves originating from both interfaces. The calculated profiles (cf. solid lines) were
corrected for the instrumental resolution by convoluting with a Gaussian function. In addition, these
results were in excellent agreement with cell-dynamical simulations [180,181,189] that provided
complementary information by ``visualizing'' the in-plane morphology.
Jandt et al. [190] extended the phase separation studies to thin dPEP/PEP films with a sample
thickness that is smaller than the wavelength of the spinodal wave. TOF±ERD and AFM were
simultaneously used to probe the polymer volume fraction profiles and lateral morphology in the
Ê thick was initially
samples, respectively. Their results showed that the surface of films 2000 A
smooth but after a certain annealing time developed a regular roughness pattern because of phase
separation. TOF±ERD measurements revealed that the time at which the new surface morphology
appeared corresponded to the transition from four to two layers. At much longer times the two-layer
films became smooth again.
Using LE-ERD and AFM, Wang and Composto [191,192] identified three distinct regimes of
evolution for deuterated poly(methyl methacryate) dPMMA and poly(styrene-ran-acrylonitrile) SAN
blends undergoing simultaneous phase separation and wetting. Fig. 50 shows the dPMMA volume
Fig. 49. Volume fraction profile of dPEP in a 50/50 mixture determined by NRA at a film thickness d of: (a) d > 1000 nm,
and (b) d ˆ 282 nm. The solid lines represent a model profile convoluted with a Gaussian of increasing FWHM. The
thickness in (b) is denoted by a vertical line (taken from [189]).
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Fig. 50. ERD volume fraction profiles of dPMMA in a 495 nm dPMMA:SAN film having an initial dPMMA volume
fraction of 0.47 after (a) no anneal, (b) 2 h, (c) 8 h, (d) 48 h, (e) 72 h, and (f) 136 h (taken from [191]).
fraction profiles during the early (a±b), intermediate (c±d) and late (e±f) stages of evolution. The
trilayer structure formed during the early stage eventually decays into a pseudo-uniform profile and
then finally a thick wetting layer appears. By selectively etching the dPMMA, AFM was used to
show that the constant composition during the intermediate stage actually corresponds to an average
profile from a two-phase mixture (i.e. dPMMA-rich columns in a SAN-rich matrix). This example
shows the ability of ERD to provide useful depth profiling information even in phase separated
systems.
Whereas previous SDSD studies focused on UCST systems, Kim et al. [193] observed
quantitatively similar behavior in mixtures of dPS and tetramethylbisphenol-A polycarbonate, a
LCST system. Samples of various thicknesses were depth-profiled using both conventional ERD and
TOF±ERD. The polymer volume fraction profiles were similar to those observed in dPEP/PEP
mixtures. These results, thus, demonstrated that SDSD is a general phenomenon regardless of system
thermodynamics.
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7.3.2.2. Wetting and interface induced self-assembly. A binary polymer mixture in contact with the
surface exhibits a preferential attraction of the lower surface energy component at the surface. For
mixtures in the one-phase region of the phase diagram this phenomenon is called surface segregation
(enrichment). In a two-phase mixture, the phase with a higher content of the surface preferred
component enriches the surface of the system forming either a microscopically thin or
macroscopically thick layer. The former situation is the so-called partial wetting cases whereas
the latter describes the complete wetting state. Upon changing external parameters of the system (e.g.
temperature, chemical potential, surface/polymer interactions), a transition from partial to complete
wetting (or vice versa) can occur. While the wetting transitions have been investigated quite
extensively in small molecular systems [194], studies using polymer systems have been limited.
Steiner et al. [195] reported the first experimental study of wetting from binary polymer blends.
Using two random copolymers of polyethylene (PE) and poly(ethyl ethylene) (PEE), bilayer samples
were prepared where the lower surface energy component (deuterated) was deposited on the
substrate and covered with a film of the high surface energy component. NRA was used to
characterize the thickness of the wetting layer growth as shown in Fig. 51. Note the growth of the
surface layer as well as the constant concentration in the depletion zone. In contrast to SDSD studies,
the thickness of the surface layer increased logarithmically with time. Using NRA and optical
microscopy, Steiner et al. [196] later demonstrated that the copolymer layers eventually invert if the
original bilayer is not too thick. This group [197] also investigated the formation of the wetting layer
from dPS/PS mixtures and observed the growth of a dPS-rich wetting layer. Geoghegan and Krausch
have prepared the most recent review about wetting at polymer surfaces and interfaces [198].
As mentioned previously, the wetting behavior of binary polymer mixtures depends on several
parameters, including temperature and polymer/substrate interactions. Although temperature is the
easiest parameter to vary, the polymer/substrate interactions can be tuned by a judicious choice of the
substrate onto which the polymers are cast. Bruder and Brenn [199] published the first experimental
report of polymer/substrate interactions on wetting from phase-separated mixtures. TOF±ERD and
optical microscopy were simultaneously used to determine the polymer volume fraction profiles and
morphology of critical mixtures of dPS and PBr0.119S spin-coated on two different substrates, silicon
wafers with either its native oxide (SiOx) or a chromium (Cr) layer. In both cases, the dPS-rich phase
was found to preferentially enrich the surface. In the case of SiOx both the dPS-rich and PBr0.119Srich phases were found to be in contact with the substrate. Although TOF±FRES only detects the
average composition of a multiphase film, AFM was able to provide the necessary lateral
information about the phase structure. On the other hand, Cr substrates were covered entirely by the
PBr0.119S-rich phase and the system formed a bilayer structure. The kinetics of phase-separation
from dPS/PBr0.097S mixtures on SiOx surfaces was later studied by TOF±ERD and scanning nearfield optical microscopy or SNOM [200].
By systematically varying the polymer/substrate interaction, both the wetting and ``drying''6 of
the substrate by one of the phases can be explored. Genzer and co-workers [201,202] used ERD to
monitor the polymer volume fraction profiles in critical mixtures of dPEP and PEP spin-coated onto
Si wafers covered with Au onto which a self-assembled monolayer (SAM) mixture of
HS(CH2)17CH3 and HS(CH2)15COOH was adsorbed. By varying the mole fraction of the COOH
terminated component in the SAM layer from 0 to 1, the substrate surface energy was varied from
20 to 81 mJ/m2, respectively. Whereas the surface was always enriched by dPEP-rich phase, the
substrate could be wet by the dPEP-rich phase or the PEP as x is varied from 0 to 1, resulting in the
6
While wetting describes a substrate covered by a macroscopically thick layer of A-rich phase, ``drying'' denotes an Arich phase excluded from the substrate, which is wet by the B-rich phase.
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Fig. 51. Composition-depth profile of dPE0.12-co-PEE0.88 copolymer in the bilayer of dPE0.12-co-PEE0.88/PE0.22-coPEE0.78 after annealing 110 8C for (A) 0, (B) 30 min, (C) 8.0 h, and (D) 3 days. Depths of 0 and 650 nm correspond to the
air and silicon surfaces, respectively. The surface wetting layer grows at the expense of material adjacent to the silicon
(taken from [195]).
formation of three or two layer structures, respectively. The precursor to this transition was also
investigated with ERD [203].
Controlling the wetting properties of polymer mixtures by adjusting the polymer/substrate
interactions is a novel route to tailor film morphology. For example, one can extend studies of phase
separation and wetting on laterally homogeneous substrates to substrates that are chemically
patterned. Krausch et al. [204] investigated spinodal decomposition in polymer mixtures on a
patterned substrate. The system was a mixture of dPS and PBr0.5S spun-cast on substrates consisting
of stripes of Cr deposited onto Si substrate that was (i) covered with native oxide, and (ii) hydrogen
terminated. TOF±ERD demonstrated that the dPS-rich phase was always present at the surface. In
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
addition, TOF±ERD spectra showed that both the dPS-rich and PBr0.5S-rich phases had the same
affinity towards SiOx and Cr substrates, but only the PBr0.5S-rich phase wetted the hydrogenterminated substrate. Optical microscopy revealed that the strong differences in the polymer
behavior at the substrates led to lateral ordering in the phase separated mixtures. In the case of Cr
strips on the SiOx substrate, a typical in-plane isotropic pattern of spinodal decomposition was
observed. In contrast, the substrate consisting of Cr on a hydrogen terminated substrate drove the
phase separation to occur in stripe-like domain structures. This study nicely demonstrated that
complete information about the system behavior can be achieved by simultaneously applying a highresolution ion beam depth profiling technique, such as TOF±ERD, and a technique that provides
information about the sample morphology, such as optical microscopy.
Finally, we conclude this section by briefly discussing the self-assembly in thin films of diblock
copolymers. Due to different interactions of the two blocks with the surface and substrate, thin
copolymer films exhibit ordering that propagates from both the surface and substrate into the bulk.
Depending of the thickness of the sample and the amplitude of the oscillations, the two waves may
be either damped or interfere. Russell and co-workers carried out a series of experiments
investigating the self-assembly in thin block copolymer films. These and other studies were
summarized in a review by Krausch [182]. Because of their small spatial features, block copolymer
ordering requires techniques with the best possible depth resolution. Because of their excellent depth
Ê ) NR and XR are suitable tools for investigating the self-assembly in block
resolution (10 A
copolymers. Recently, Stamm et al. [205] showed that resonant NRA based on the 1 H(15 N ag)12 C
nuclear reaction can be used to depth-profile block copolymer films. As discussed in Section 6.1, this
Ê ) and, in contrast to NR or XR,
type of NRA benefits from excellent depth resolution (50 A
provides a direct volume fraction profile. The system investigated was a diblock copolymer of
poly(d8-styrene-b-4-methylstyrene) (dPS-b-PMS). Because the styrene block was labeled with
deuterium the signal coming from PMS was detected with the NRA. In thick films, the PMS block is
always present at the surface and alternating lamellae of dPS and PMS decay slowly into the bulk
[205]. In a subsequent study, Giebler and co-workers [206] extended the study to very thin dPS-bPMS films and found interesting ordering phenomena when the film thickness becomes smaller than
the periodicity of the lamellae.
7.4. Ion beam modification of polymer films
The interaction of ions with polymers underlies applications ranging from semiconductor
fabrication to space technology where polymer foils protect spacecraft components from solar wind.
Whereas polymer properties are usually dictated by their molecular characteristics such as chain
stiffness and length, ion irradiation provides an alternative route to modify their optical, electrical,
and mechanical properties [207±209]. Calcagno et al. [208] have studied the effect of ion irradiation
on the microstructure of polystyrene and several other polymers. At low fluence (1014 ions/cm2),
300 keV protons mainly produce inter-chain crosslinks in PS, suggesting that transport [210] and
mechanical properties are modified. After implanting ions into polymers, Lee et al. [207] observed
substantial improvements in the hardness and wear resistance demonstrating that the interaction of
ions with polymers can be beneficial.
An incident ion beam traversing a polymer undergoes two mechanisms of energy loss. In
nuclear stopping, the ion will colloid with a target atom through a screened Coulomb collision.
Nuclear stopping is dominant at low (keV) energies and, therefore, only of minor importance in
materials characterization of the outer micron using MeV light ions. The electronic stopping power
is determined by the incident ion interacting with the electrons in the polymer. At high energy
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transfer, electrons can be stripped from the molecule resulting in a free radical. Ion beam
modification then leads to oxidation in the case of simple hydrocarbons (e.g. polyvinylchloride),
chain scission (e.g. PMMA, PTFE), and cross-linking (e.g. PS, polyimide). Typically, chain scission
and cross-linking both occur to some degree in polymers although one mechanism typically
dominates.
Irradiation produces two readily visible changes in polymers. Typically, the surface of
hydrophobic polymers becomes hydrophilic. This can be easily demonstrated by gently blowing
moisture across the sample and observing the wetting of droplets in the modified region. This
modified region is usually easily identified because most polymers turn brown or black after
irradiation. The mechanism responsible for this coloration was studied by optical adsorption at
different depths from the surface [211]. The formation of graphite was proposed as a possible reason
for the darkening. Similarly, MeV 4 He ions were found to induce carbonization in a partially cured
phenolic resin [209].
As previously mentioned, ion beams can actually improve polymer properties. As one example
4
He and 16 O incident particles were used to modify the transport and mechanical properties of PS
coatings [212]. Over a 6 mm 6 mm area, PS thin films (1.4 mm) were irradiated by either 400 keV
16 ‡
O or 400 keV 4 He‡ ions at a beam current of 2 or 8 nA, respectively. The fluence was varied from
1:7 1013 to 34:0 1013 ions/cm2. Using SRIM [30], the stopping powers for 4 He and 16 O in PS
were found to be 27 and 589 keV/mm, respectively. Although the 4 He stopping power was mainly
electronic, the nuclear contribution to the 16 O stopping power was significant, about 5%. The depth
range of the 4 He and 16 O ions in PS was 2.18 and 0.904 mm, respectively.
After modification, a thin dPS film (23 nm) was deposited onto the PS matrix. The diffusion
couples were annealed in a vacuum oven at 170 8C and analyzed by ERD. Fig. 52 shows LE-ERD
spectra for dPS/PS diffusion couples annealed for 7800 s [212]. The 1 H yield in the 16 O modified
(34:0 1013 ions/cm2) PS is much lower than in the standard PS. Moreover, for the oxygen modified
matrix the 2 D yield has a Gaussian shape with a 30 keV FWHM (i.e. instrumental resolution)
suggesting that an insignificant amount of dPS has diffused into the modified PS. Fig. 52 also shows
that dPS diffuses readily into the unmodified PS matrix. Using Fick's second law [213], a dPS
diffusion coefficient of 2:6 10 15 cm2/s was determined.
After modification and before placing the dPS on top, the PS surface topography was
characterized using AFM. Post-indentation surface scans were taken to determine if morphological
Fig. 52. LE-ERD spectra of dPS/PS diffusion couples with unmodified PS (solid line) and 16 O modified PS (broken line).
The 16 O fluence into the modified PS is 3:4 1014 ions/cm2. Diffusion into the modified PS is arrested (taken from [212]).
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Fig. 53. Atomic force microscopy image of unmodified PS after a force displacement indentation. The PS mound was
created upon withdrawing the AFM tip from the film (taken from [214]).
changes occurred during the force displacement measurements. Indentation of the unmodified film
produced a PS mound having a height of 316 nm and a half-height diameter of 1 mm (Fig. 53) [214].
These mounds are produced at room temperature, far below the glass transition temperature. Postindentation surface scans of 16 O modified PS showed no evidence of tip indentation suggesting that
the modified PS is hardened either by crosslinking or the formation of a carbon rich material.
Lee and co-workers studied the depth dependent hardness of PS after irradiation with 2 MeV
4
He ions [215] and found that the hardness variation followed the stopping power for ionization.
Namely, hardness increased as a function of depth and displayed a maximum at 6.5 mm below the
surface. Crosslinking due to ionization was used to justify this result.
8. Future trends
In 1976, the first elastic recoil experiment was performed using a heavy, high energy incident
ion, namely, 35.0 MeV Cl, to depth profile lithium in multilayer targets [18]. In 1979, Doyle and
Peercy demonstrated that 2.4 MeV 4 He‡ ions incident at a1 of 158 was an excellent choice for depth
profiling hydrogen in silicon nitride layers [19]. It was this version of ERD that was first applied to
polymer surfaces and interfaces in 1984 by Mills et al. [20]. As reviewed in Section 7, since this
time, ERD has played a central role in the study of polymer and small molecule diffusion, surface
segregation, interfacial segregation, surface directed spinodal decomposition, wetting and phase
separation in thin films.
Future developments in ERD applied to polymer surfaces and interfaces will be driven by the
desire for better depth resolution, simultaneous depth resolution and accessible depth, and most
importantly quantitative depth profiling with excellent lateral resolution. To improve depth
resolution, the challenge has been to selectively detect the light recoiling target nuclei from the
intense forward scattered probe particles without loss of resolution. Conventional ERD uses a filter
foil to exclude these forward scattered particles. However, energy straggling in the foil greatly limits
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Fig. 54. TOF±ERD spectrum of Si/SiO2/Si multilayer with nitrogen segregation to the SiO2 interfaces (taken from [217]).
the depth resolution. Low-energy ERD uses a much thinner stopper foil to optimize depth resolution.
Wang [42] has achieved a surface depth resolution of 8 nm FWHM using a 500 keV 4 He and a
1.45 mm PS filter foil. Whereas LE-ERD provides outstanding surface resolution, this technique is
not appropriate to probe profiles deep below the surface (i.e. 0.5 mm). Good depth resolution and
excellent accessible depth can be achieved by replacing the filter foil with other means of particle
discrimination.
Although it has great potential, TOF±ERD has received relatively limited attention by the
polymer community [74]. Relative to ERD and LE-ERD, the TOF±ERD technique is more costly
(up to US$ 20k) and complex to interpret. For example, in the TOF technique each recoiled species
has a different probability of detection whereas conventional silicon detectors are nearly 100%
efficient for each species. TOF±ERD has been reported to have a depth resolution of 30 nm at the
polymer surface [216]. One exciting variation of TOF±ERD, which has not been applied to
polymers, is called mass and energy dispersive recoil spectrometry [217]. Using a high energy, HI
beam, Whitlow [217] analyzed a Si/SiO2/Si multilayer which had nitrogen segregation at both
SiO2 interfaces. Fig. 54 shows a spectrum in which the 14 N, 16 O and 28 Si yields are clearly
separated according to mass and energy (i.e. depth). Thus, this technique shows excellent promise
for polymer interface studies. However, because the present version relies on heavy incident ions
(e.g. Br), polymer scientists will need to modify experimental conditions to minimize sample
damage.
Green and Doyle used silicon ERD to measure the 1 H and 2 D profiles in polymers [218].
Unfortunately, the incident silicon in these studies resulted in extensive damage and H(D) loss.
Geoghegan and Abel [78] demonstrated carbon ERD results in minimum damage by accumulating
spectra at a low dose over fresh regions of the sample. A comprehensive analysis of HI-ERD has
been previously published [8].
R.J. Composto et al. / Materials Science and Engineering R 38 (2002) 107±180
Many polymer surface and interface problems now require both spatial and lateral resolution.
Microbeam ERD analysis has been performed by Tirira et al. using 3.0 MeV 4 He and a transmission
geometry in which the recoiled nuclei pass through the back of the sample [219]. In this study, the
hydrogen concentration in a film was mapped with a lateral resolution of 10 mm and a depth
resolution of 30±40 nm. Another outstanding feature was the tremendous accessible depth of 6 mm.
In the past, polymer scientists have stayed away from microbeams because of the obvious damage
due to squeezing a nanoampere beam into an area of 100 mm2. However, the same argument was also
made when conventional ERD was first applied to polymers.
This section ends with a brief discussion of ion beams for materials modification. As mentioned
earlier, the interaction of ions with matter leads to both desired (Section 7.4) and undesired (Section
4.4) changes in the properties of thin polymer films. In the future, new efforts should be made to take
advantage of the controlled manor of energy loss of ions in matter. In particular, as opposed to
gamma and electron beams, ion beams can be tuned via energy and incident mass to modify only the
very near surface region while leaving the underlying film unmodified. Although some empirical
studies have been made, designed barrier films with outstanding wear, hardness and corrosion
resistance should evolve. Another possible area for ion beams to contribute is in the area of
electroactive polymers for thin film transistors. Here, ion beams should play a role in the
characterization of metal/polymer, polymer/dielectric; and polymer/substrate interfaces, and
possibly the doping of plastic transistors. In general, ion beam analysis of polymers for information
and communication technologies should increase in the next few years.
9. Conclusion
In this review, we have presented the fundamental parameters underlying ERD, namely the
kinematic factor, cross-section, and stopping-power. The basic principles of ERD and related ion
beam techniques are also reviewed. In choosing an experimental technique for a particular problem,
depth resolution and the means of contrast are of great importance. A brief discussion of
instrumentation is given along with a review of data analysis strategies. Since no single technique is
suitable for every surface and interface problem, a discussion of complimentary depth profiling
techniques is provided as well as their strengths and weaknesses. The aforementioned topics are
aimed to bring a polymer scientists up to speed in the area of ion beam analysis, particularly ERD.
The aim of the case study section is to educate both the polymer and ion beam communities
about the contributions made by ERD to address outstanding problems in the areas of polymer
diffusion, polymer surfaces and interfaces, and polymer modification. Because an exhaustive review
is impossible, only selected studies involving ERD are included. NRA has also made important
contributions and apologies are made to proponents of this technique. Looking towards the future
demands addressing new problems in polymer surfaces and interfaces. In the future, depth profiling
over a small lateral area will be of increasing importance for example to study electroactive polymer
devices, arrays on a chip, etc. Because of radiation damage, new developments in microbeam
analysis are needed before this technique can be applied to polymers with the same regularity as hard
materials. Improvements in depth resolution are also always in demand. It is not clear whether HIERD will make significant inroads because of sample damage. However, other ERD techniques,
such as those based on coincidence methods and E and B filters, have been making important
contributions to characterizing hard materials, but have yet been applied to address polymer surface
and interface problems. These techniques are readily available to the polymer community and should
be evaluated in the near future.
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Acknowledgements
We dedicate this review to the early pioneers who introduced ERD to the polymer community,
namely, J.W. Mayer, C.J. Palmstrùm, P.J. Mills, P.F. Green, and E.J. Kramer. In particular, we
acknowledge the leadership of E.J. Kramer who nucleated the use of ERD among the polymer
community and encouraged future innovations including low-energy ERD. There are many people
who provided sage advice or innovative ideas to the ion scattering facility at the University of
Pennsylvania. J.B. Rothman provided invaluable suggestions that lead to the development of lowenergy ERD and other unique capabilities that we now take for granted. Many others provided their
expertise including D. Jacobson, J.E.E. Baglin, A. Faldi, W.E. Wallace, R. Oslanec, H. Wang, M.
Geoghegan and D. Yates. The NSF MRSEC program (DMR00-79909) at the University of
Pennsylvania has provided longstanding support for the facility under the former and current
directors, E.W. Plummer and M.L. Klein. Russell J. Composto acknowledges financial support for
ion beam experiments from NSF DMR-9974366 and ACS-PRF-34081. Russel M. Walters
acknowledges support from the NSF MRSEC and Aston Fellowship Program. Jan Genzer is a
Camille Dreyfus Teacher-Scholar.
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