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Surface and Coatings Technology 142᎐144 Ž2001. 943᎐949
Effects of plasma non-homogeneity on the physical
properties of sputtered thin films
V. Rigato a,U , G. Maggioni a , A. Patelli b, V. Antoni c , G. Serianni c , M. Spolaorec ,
L. Tramontin c , L. Depero d, E. Bontempi d
b
a
I.N.F.N. Laboratori Nazionali di Legnaro, Via Romea 4, I-35020 Legnaro (Pd), Italy
Dipartimento di Elettronica e Informatica, Uni¨ ersita
` di Pado¨ a, INFM, Unita’ di Pado¨ a, Via Gradenigo 6 r A, I-35131 Pado¨ a, Italy
c
Consorzio RFX, Corso Stati Uniti 4, I-35127 Pado¨ a, Italy
d
INFM Unita’ di Brescia and Laboratorio di Strutturistica Chimica, Uni¨ ersita
` di Brescia, ¨ ia Branze 38, 25123, Brescia, Italy
Abstract
The plasma generated in a two-target closed field unbalanced magnetron sputtering system for thin film deposition is
characterized by means of Hall probes and cylindrical Langmuir probes as a function of the position inside the vacuum system.
The plasma potential, electron density and temperature profiles in different locations are measured by two diagnostic systems
equipped with cylindrical Langmuir probes. The plasma non-homogeneity due to the presence of magnetic field gradients is
evaluated. In order to test the effects of measured plasma non-homogeneity on the physical properties of sputter-deposited
coatings, several substrates are put inside the chamber in regions characterized by different plasma density and plasma potential.
The composition, microstructure and morphology of TiN x films grown onto these substrates are then studied by means of nuclear
techniques ŽRBS, n-RBS, NRA, ERDA., X-ray diffraction ŽXRD. and secondary electron microscopy ŽSEM.. The mechanical
properties are determined by micro-scratch test and nanoindentation and correlated to the local plasma parameters. 䊚 2001
Elsevier Science B.V. All rights reserved.
Keywords: Plasma diagnostics; Unbalanced magnetron; Titanium nitride
1. Introduction
Balanced and unbalanced magnetron sputtering deposition techniques are widely applied both in industrial processes and in advanced materials development
and treatment. The ion density of the glow discharge is
often enhanced by means of permanent magnets in
order to produce a region over the target where the
electron loss rate is reduced and the ionization
probability enhanced due to the magnetic confinement
of electrons. The knowledge of the ion flux and energy
U
Corresponding author. Tel.: q39-049-8068-476; fax: q39-049641925.
E-mail address: [email protected] ŽV. Rigato..
of ions bombarding the growing film is of fundamental
importance in ion plating processes mostly for ceramic
coating production where the final coating properties
like micro-structure, stoichiometry, hardness, wear resistance and adhesion are strongly dependent on plasma
potential, electron temperature and density. Therefore
it is of primary importance to study the plasma homogeneity and its effects on the final coating properties.
The measurement of magnetic field and plasma
parameters can be performed by using Hall and Langmuir probes w1x. Rohde et al. w2x studied a dual cathode,
reactive sputtering system by means of a flat-discshaped Langmuir probe in order to better model the
conditions for an actual substrate. However, this kind
of probe is not the most suitable one for measuring the
plasma potential and carrier densities as also recog-
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 5 8 - 0
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V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949
nized by the authors. The present work aims at the
characterization of a DC closed field unbalanced magnetron sputtering device by means of Hall probes and
small cylindrical Langmuir probes best suited for precise plasma characterization.
In the present work TiN x coatings were grown by
reactive sputtering in ArrN2 plasma on static substrates located in regions characterized by quite different plasma density and potential due to the different
magnetic fields. The composition and density of TiN x
films have been studied by means of Rutherford and
non-Rutherford backscattering analysis ŽRBS, non-RBS
and NRA. and elastic recoil detection analysis ŽERDA..
The coating microstructure has been investigated by
X-ray diffraction ŽXRD. and grazing incidence XRD
ŽGIXRD.. The mechanical properties were determined
by micro-scratch test and nanoindentation.
The physical properties of these coatings have been
related to the local plasma parameters.
2. Experimental
2.1. Coating deposition setup
The coating deposition setup consists of a vacuum
chamber Žlength 1194 mm, height 755 mm, depth 734
mm. equipped with two 250 = 140 mm2 rectangular
unbalanced magnetron sources located in opposite
positions. The distance between the cathode surfaces is
460 mm Žsee Fig. 1.. Two DC current-controlled power
supplies ŽAdvanced Energy MDX. are used to negatively bias the cathodes. The target material is titanium.
The ultimate pressure of the vacuum chamber was
- 2 = 10y4 Pa. The discharge gas was high purity
argon at the working pressure of 0.1 Pa. High purity
nitrogen was used as reactive gas. For the TiN x coatings deposition the current on each magnetron source,
Imagn , was set at 3.5 A corresponding to a total discharge current Itot s 7 A Žtotal power Pmagn s 3.7 kW..
A closed-loop optical emission monitor ŽOEM. was
Fig. 1. Experimental setup used for the deposition of TiN x coatings.
The picture shows the Langmuir probersample-holder system Žtop
view.. Dimensions in mm.
used to control the nitrogen gas flow to the deposition
chamber. An optical fiber transmits the light emitted by
the plasma from a region just in front of one Ti
cathode to a CCD spectrometer. The intensity of a
spectral line depends on the number of excited species
per unit volume, so the control of the line intensity
allows control of the deposited film stoichiometry in
reactive sputtering by selecting the desired concentration of metal atoms in the plasma. This technique also
allows us to record the entire visible spectrum emitted
by the plasma and to choose the more suitable metal
line to control the process. For the TiN x coatings
deposition the 500-nm Ti line has been chosen and the
OEM setting Ži.e. the ratio between the intensity of this
line after and before nitrogen introduction into the
chamber. was 65%.
TiN x coatings were deposited on silicon substrates
Žfor analytical purposes. and on M42 tool steel Žfor
mechanical characterization .. The substrates were put
in four different Ž x, y, z . locations Žsee Fig. 1.: A s s
Ž0,220,0.; Bs s Žy50,220,0.; Cs s Žy100,220,0. and Ds
s Žy150,220,0..
Before the TiN x coating deposition a 100᎐200-nmthick Ti underlayer was deposited in pure argon plasma.
Both Ti and TiN x films were grown by DC biasing the
substrate at Vs s y40 V.
2.2. Magnetic field measurements
The magnetic field characterization was performed
using Hall-effect sensors suitable for static field measurements. They were mounted on a mechanical supporting structure that permitted us to record simultaneously all three components. Due to the position of
the permanent magnets of the two opposing magnetron
sources the magnetic field configuration is characterized by symmetries with respect to the Ž x,y ., Ž y,z . and
Ž x,z . planes Žsee Fig. 1., hence the magnetic field
mapping was limited to the following two regions: 0 F x
F 80 mm and 20 F y F 230 mm; 20 F y F 230 mm and
0 F z F 90 mm.
2.3. Langmuir probes measurements
Two Langmuir probe systems were used: the first
one consisted of three tungsten tips having length of 3
mm and diameter of 0.2 mm. The distance between the
tips was 8 mm. Each electrode emerged from a stainless steel cylindrical pipe Ž150 mm length, 4 mm diameter. electrically isolated from it. A 25-mm-diameter
stainless steel cylinder supported all three pipes and
was mounted on the shaft of a manipulator allowing
translation and rotation of the whole system. This first
Langmuir probe system was used only in the pure
argon plasma.
The second system, used during TiN deposition, con-
V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949
sisted of four probes. Each probe was located close to
each sample in order to measure the plasma parameters during the process. The probes position Žwith reference to Fig. 1. were: A p s Ž0,240,0.; Bp s
Ž y 50,240,0 . ; C p s Ž y 100,240,0 . and D p s
Žy150,240,0.. The length and diameter of the tungsten
wire was 4 mm and 0.2 mm, respectively. A 350-mmhigh, 6-mm-diameter stainless steel vertical tube supported each probe in each sample holder.
The analysis of Langmuir probe data has been performed according to the method described by Serianni
et al. w3x based on the following hypotheses: Ža. the ion
current portion of the Langmuir probe I᎐V characteristic curve depends on the applied voltage according to
the Orbital Motion Limited theory and for voltages low
enough with respect to the floating potential a linear
trend was assumed; Žb. the electron current can be
affected by the presence of two Maxwellian electron
populations in the plasma; and Žc. the magnetic field
due to magnetron sources reduces the portion of I᎐V
Langmuir curve useful to evaluate the plasma parameters.
2.4. Coating characterization
Compositional analysis has been carried out by ion
beam analysis using the 7-MeV CN and the HVEC
2.5-MeV Van de Graaff accelerators at the Laboratori
Nazionali di Legnaro of INFN.
Contaminants like carbon and oxygen have been
studied by non-Rutherford BS, using a 3.0-MeV and
2.0-MeV proton beam and by Žd,p. nuclear reaction
analysis. The hydrogen content analysis has been performed by ERDA with a 2.2-MeV ␣-particle beam. The
titanium depth concentration profiles have been determined using a 5.85-MeV ␣-particle beam. Coating
thickness has been measured by cross-section SEM.
The grazing incidence X-ray diffraction spectra and
the ␪᎐2␪ patterns have been collected by a Bruker ‘D8
Advance’ diffractometer equipped with a Gobel
¨ mirror.
The angular accuracy was 0.001⬚ and the angular resolution was better than 0.01⬚. The Cu K ␣ line of a
conventional X-ray source Ž40 kV, 40 mA. was used.
The XRD experimental data were fitted using the
Topas P program w4x. The lattice parameters were
calculated by the Unit Cell program w5x.
The mechanical properties have been characterized
by a CSEM Nano Hardness Tester ŽNHT. and Micro
Scratch Tester ŽMST.. The nano-indentations have been
performed with a maximum load of 10 mN using a
Berkovitch diamond tip. The maximum depth of the
indentation was approximately 140 nm, less than 1r10
of the total thickness of the TiN x films. The scratch
tests have been performed using a 200-␮m radius
Rockwell C diamond tip with a progressive load from 0
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Fig. 2. Magnetic field map on the plane z s 0.
to 30 N. The scratch tracks have been observed by an
optical microscope and by SEM.
3. Results and discussion
3.1. Magnetic field measurements
The results of the magnetic measurements obtained
in the horizontal plane Ž z s 0. are presented in Fig. 2;
the figure is split into two parts where different scales
are used: the magnetic field ranged from approximately
2 to 20 mTorr in a closed field arrangement.
3.2. Langmuir probe measurements
3.2.1. Pure argon plasma
The effect of the probe assembly insertion on the
magnetron voltage was negligible and comparable to
the bias variations usually observed during target
lifetime. A larger perturbation effect was observed in
the plasma measurements at x ) 0 ŽFig. 3.: in this case,
the whole probe system was lying between the cathodes.
In Fig. 3 the profiles of electron temperature and
density and plasma potential are shown for y s 230 mm
and z s 0 mm at 800 W. Two electron populations with
different temperature and density Ž‘hot’ and ‘cold’ electrons. have been found, according to Serianni et al. w6x.
At the center Ž xs 0 mm. the temperatures and densities of cold and hot electrons tend to converge. The
probe I᎐V characteristics show a gradual change
between an outer region where a double slope in the
logarithmic plot of electron current is clearly detected
and an inner region where the double slope is less
marked due to higher values of temperature of cold
electrons and density of hot electrons. The hot electron
density increases, more rapidly than an exponential
curve, by approximately two orders of magnitude going
from xs y300 mm to x s 0 mm; the cold electron
density increases almost exponentially by approxi-
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V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949
As already mentioned the region where x) 0 mm is
affected by the perturbation of the plasma due to the
probe insertion so that a non-symmetric profile of
temperature, density and potential around xs 0 mm
are obtained. The density of both cold and hot electrons is reduced in the perturbed portion of the plasma
as can be seen from Fig. 3.
Fig. 3. Electron temperature Ža., density Žb. and plasma potential Žc.
profiles in pure Ar plasma: open and solid circles refer to hot and
cold electrons, respectively Ž y s 230, z s 0; Pma gn s 800 W; Imagn s 1
A; Ar; p s 0.1 Pa..
mately an order of magnitude in the same direction.
The density ratio of cold and hot electrons ranges from
; 300 to ; 10 going towards the center Ž xs 0 mm.
and the temperature ratio Žhot over cold electron temperature. changes from ; 10 to ; 2.
The floating potential decreases monotonically towards the magnetic axis from y5 to y17 V. The
plasma potential, resulting from the fitting procedure,
displays a maximum at xs 0 mm; an electric field of
approximately 100 Vrm can be estimated between
᎐100 and 0 mm, yielding, with B ( 4 mTorr, an E= B
drift velocity of ; 25 kmrs directed along the z axis. It
is worth noting that a plot of Vp y Vf vs. the weighed
temperature ŽTw s Tc q Ž n h rn c . ⭈ T h . is well fitted by a
straight line with a slope of 5.2᎐5.6 VreV. This value is
in very good agreement with that computed by assuming comparable electron and Ar ion collecting areas in the framework of a single Maxwellian theory;
this last value has been used in the linearization of the
Orbital Motion Limited ŽOML. theory w6x.
3.2.2. Argonr N2 plasma
A set of local plasma parameter measurements has
been performed during the growth of TiN x films using
a reactive plasma. In this case the second diagnostic
system with four Langmuir probes, described above,
has been used. The reactive gas flow rate was set by the
optical emission gas feeding device to a value such that
the optical emission signal was 65% of the original
value in pure argon plasma. This value has been chosen
in order to carry out the deposition in the metallic
regime just before the transition to the poisoned point
w7x. The measured plasma parameters did not show
appreciable differences when compared with those obtained in a pure Ar plasma in the same experimental
conditions.
The results obtained during TiN x deposition for the
hot and cold electron temperatures and the respective
densities are shown in Fig. 4. The electron densities
increase towards the center of the discharge Ž xs 0
mm., with more than an exponential increase for the
hot electron density. The ratios n crn h and TcrT h range
from 500 to 10 and from 20 to 3, starting from xs y150
mm towards xs 0 mm. We can then estimate a local
ion current density js , impinging on each substrate
varying from 0.5 mArcm2 at xs y150 mm to 3.5
mArcm2 at x s 0 mm. During the reactive deposition,
we obtain plasma potential values in the range between
q3 and q6 V moving from xs y150 mm to xs 0
mm. The energy of ions impinging on the growing films,
as calculated by plasma potential and substrate bias, is
in the range 40᎐50 eV.
3.3. Coating properties
Cross-section SEM images Žnot reported in this paper. show that the TiN x coatings deposited in the
different positions are characterized by a void-free,
dense structure. Fig. 5 shows the titanium deposition
rate in the argon plasma as a function of x position
Ž y s 220 mm; z s 0 mm. as measured by RBS analysis.
The Ti deposition rate in ArrN2 plasma in the four
sample positions is also reported. The Ti deposition
profile deviates from the expected Ti sputtering profile
typical of magnetron sources: in the 100-mm-wide central region the deposition rate clearly decreases; the
deposition profile shows a maximum value at approximately xs y75 mm. At xs y100 mm the deposition
V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949
Fig. 4. Temperature and density profiles measured along the x-axis
for hot electrons Žopen circles. and cold electron Žsolid circles.
during TiN x deposition Ž Pmagn s 3.7 kW; Imagn s 3.5 A; ArrN2 ;
pAr s 0.1 Pa..
rate starts to decrease as expected. The experimental
findings indicate that the Ti re-sputtering in the central
region is quite pronounced as deduced from the current density data measured by the Langmuir probes: as
a matter of fact the current density at xs 0 mm is
approximately seven times that at xs y150 mm. A
similar trend is observed for Ti deposition rate in
ArrN2 plasma. From the measured deposition rate
947
and current density the ion-to-Ti-atom ratio at the
substrate, JirJ Ti , has been evaluated. These data are
reported in Fig. 6a: as it can be seen the JirJ Ti values
range from approximately 2 Ž xs y150 mm. to 10
Ž xs 0 mm..
The stoichiometry of TiN x coatings deposited in the
four x positions is characterized by a value NrTis
0.95" 0.05 for all the samples. The contaminant concentration resulted below 1 at.%. From the thickness
data and the composition analysis the coatings density
has been determined. As is shown in Fig. 6b the density
decreases from 5.5 grcm3 to 4.3 grcm3 in passing from
xs y150 to 0 mm.
The color of the coatings was bright gold.
From the ␪᎐2␪ X-ray scans a trend of the average
crystallite size has been determined as a function of the
position using the Scherrer formula: the crystallite size
is higher at xs 0 mm Ž91 nm. and decreases to 18 nm
at xs 100 mm Žsee Fig. 6a..
From the grazing angle X-ray spectra collected at
␪ s 0.2⬚, 0.5⬚, 1.0⬚ and 2.0⬚ Žnot shown in this paper. it
was deduced that at increasing grazing angles the TiN
reflections shift down to lower 2␪ values, indicating an
increase of the lattice parameter a of this phase at the
substraterfilm interface. The trend is observed in all
the samples but it is much more pronounced in the
samples deposited at x s y100 mm and xs y150
mm. At a fixed grazing angle, the lattice parameter
increases as a function of x position: at ␪ s 2.0⬚ it
˚ at x s 0 mm to as 4.275 A
˚
increases from as 4.246 A
at xs y100 mm ŽFig. 6c..
These findings show that the coatings are characterized by a compressive stress which is less pronounced
for the samples deposited at x s 0 mm.
Fig. 5. Titanium deposition rate in Ar plasma ŽI. and in reactive ArrN2 plasma ŽB. Ž Pmagn s 3.7 kW; Imagn s 3.5 A; pAr s 0.1 Pa. as measured
by Rutherford Backscattering.
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V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949
Fig. 6. Ža. JirJ Ti ion-to-Ti-atom ratio incident at the substrate and
average crystallite size of deposited TiN x coatings; Žb. deposition rate
Ž␮mrh. and density of TiN x coatings; Žc. nanohardness and lattice
parameter Žat ␪ s 2.0⬚. of TiN x coatings as a function of x position.
Also the texture of the coatings showed a trend with
the position. As shown in Fig. 7, which reports the
␪᎐2␪ X-ray scans for the different samples, the films
obtained at xs 0 mm Ž JirJ Ti ; 10. are characterized
by a very strong Ž200. preferred orientation. On the
contrary the samples obtained under a less intense ion
bombardment Ž xs y100 mm, x s y150 mm. are
characterized by a Ž111. preferred orientation. Thus it
can be concluded that the films obtained at xs 0 mm
Ž JirJ Ti ; 10. are characterized by a quite different microstructure than those obtained at higher distances
from the center, due to the effect of the more intense
ion bombardment as already reported in Sundgren w8x.
This finding is also confirmed by the nano-indentation and microscratch tests performed on the same
samples. The Vickers hardness of the coatings, ranging
from 2000 to 2500 kgfrmm2 , is reported in Fig. 6c. It
clearly increases with the lattice parameter and is a
minimum for the samples deposited at x s 0 mm. This
behavior can be related to the increase of film density
accompanied by the increase of the compressive stress
of the films.
Microscratch measurements on samples deposited at
xs 0 mm performed with 200-␮m radius tip, show
lateral spallation failures, while on the bottom of the
track the TiN x film is still present. It has been verified
by energy dispersive X-ray analysis that the TiN x layer
detaches from the Ti underlayer and not from the M42
substrate indicating that the TirTiNx interface is characterized by a weaker adhesion, most probably due to
the different growth mechanism of the Ž200. preferentially oriented TiN x layer onto a crystallized Žapprox.
100-nm-thick. Ti underlayer Žalso subject to intense ion
plating during deposition.. The sample deposited at
xs y50 mm did not show any failure up to 30 N, while
conformal cracking occurred in the samples deposited
at x s y100 mm and xs y150 mm. at 11 and 20 N,
respectively. This failure mode can be related to the
higher internal stress of these samples.
Fig. 7. ␪᎐2␪ X-ray scans of TiN x coatings grown in different positions, from x s 0 mm Žbottom. to xs y150 mm Žtop..
V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949
4. Conclusions
A two-cathode DC unbalanced magnetron sputtering
device has been characterized by means of Hall and
Langmuir probes. Langmuir probe measurements show
the existence of two main plasma regions characterized
by different electron density, temperature and plasma
potential related to the magnetic field. In the high
density region the plasma density is approximately 10 17
ionsrm3 in the reported experimental conditions. Outside this high density region a decrease of plasma
density, temperature and plasma potential are measured. Different ion numbers JirJ Ti have been calculated as a function of the position in the case of
sputtering of Ti and TiN x films. The JirJ Ti values
ranged from 2 to 10 by going towards the high density
region.
The physical properties of the TiN x films statically
deposited in regions characterized by different values
of JirJ Ti are quite different. The films obtained with
most intense ion plating in the central high density
region show a lower density, higher average crystallite
size, Ž200. preferred orientation, lower hardness and a
less stressed micro-structure.
The coatings deposited in the region characterized
by JirJ Ti of approximately 2 showed a preferred Ž111.
orientation and a much smaller average crystallite size.
As a result they were harder and denser. The samples
deposited in the region characterized by JirJ Ti of approximately 7 show the best adhesion properties. The
sample composition is independent on the position and
is characterized by a NrTi value of 0.95" 0.05.
These findings obtained during a static deposition of
949
TiN x with a dual cathode closed field system indicate
that the final properties of coatings grown on this
particular system are strongly dependent on position.
This effect can be averaged out if samples are moved
inside the chamber provided that motion parameters
are properly chosen.
The present study should be extended to systems
with four and more opposed polarity unbalanced magnetrons which should be characterized by a wider region of homogeneous confined plasma.
Acknowledgements
The authors wish to thank V. Cervaro for the technical support.
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