Download (1–1.5 kV) nitrogen-ion bombardment on sharply pointed tips

Effects of low-energy (1–1.5 kV) nitrogen-ion bombardment on sharply
pointed tips: Sputtering, implantation, and metal-nitride formation
Y.-C. Kima)
Department of Materials Science and Engineering and Materials Research Center, R. R. McCormick School
of Engineering and Applied Science, Northwestern University, 2225 N. Campus Drive, Evanston,
Illinois 60208-3108
C.-J. Yu
Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208
D. N. Seidmanb)
Department of Materials Science and Engineering and Materials Research Center, R. R. McCormick School
of Engineering and Applied Science, Northwestern University, 2225 N. Campus Drive, Evanston,
Illinois 60208-3108
~Received 4 September 1996; accepted for publication 16 October 1996!
Low-energy ~1–1.5 kV! nitrogen-ion bombardment of sharply pointed Fe-3 at. % Mo tips is
performed, at room temperature and 473 K, by applying a negative voltage to the tip in the presence
of nitrogen gas ~1023 –1025 Torr! in an ultrahigh vacuum atom-probe field-ion microscope
~APFIM!. Tip sharpening, as a consequence of sputtering, is observed directly in situ via APFIM.
This sharpening phenomenon can be explained by the inertia effect on the trajectories of the
field-emitted electrons and nitrogen ions that results in sputtering of the shank of the tip. We also
observe that the implanted low-energy ~1–1.5 kV! nitrogen ions react with the molybdenum atoms
in solid solution in this Fe~Mo! alloy to produce immobile molecular diameter nitrides at room
temperature and 473 K. The measured range profile of the implanted 1.5 kV nitrogen species is
compared with results calculated using the TRIM.SP Monte Carlo code. This comparison implies
that the ionized nitrogen molecules ~N11
2 !, generated by the field-emitted electrons, obtain the full
potential energy of the tip before bombarding the tip’s surface and then dissociating into nitrogen
ions ~N11! at or near the surface. The experimental nitrogen range profile is quantitatively fitted to
the Edgeworth expansion of the normal distribution function. © 1997 American Institute of
Physics. @S0021-8979~97!06702-9#
I. INTRODUCTION
The geometry of a sharply pointed tip affects the performance of scientific apparatus and technological equipment
that utilize sharply pointed conducting tips: for example,
scanning tunneling microscopy ~STM!, atom-probe field-ion
microscopy ~APFIM!, and field emitter displays ~FEDs!.
STM ideally requires that the apex of a tip consists of a
single atom, through which the tunneling currents pass, to
obtain the optimum spatial resolution at an atomic scale.
Typically electrochemically sharpened tips are employed in
STM to scan electrically conducting surfaces after some ex
situ or in situ cleaning procedure.1 Alternatively, Fink2 employed field-ion microscopy ~FIM! to obtain a three-atom
‘‘platform’’ on the $111%-plane of a tungsten tip by the
physical process of field evaporation. A single-atom tip was
then obtained by vapor deposition of a tungsten atom on top
of the three-atom platform, via controlled physical evaporation from a tungsten metal source. In addition, Binh heated a
tip in the presence of an applied electric field to produce an
atomically sharp tip.3 The latter is obtained when the effect
of the applied electric field overcomes the effect of temperature, which causes blunting via surface diffusion; a heated tip
wants, of course, to blunt to minimize its free energy.4 Typia!
LG Semiconductors, Cheongju, South Korea.
Electronic mail: [email protected]
b!
944
J. Appl. Phys. 81 (2), 15 January 1997
cal values for temperature and applied dc voltage are 1500–
2000 K and 2–3 kV. However, since surface diffusion of
metal atoms can be significant even at room temperature,5 a
simple resharpening procedure is critical to reactivate
blunted STM tips. Recently, Salemink et al. used a procedure they call ‘‘self-sputtering’’ to resharpen tips; this approach involves applying a negative dc voltage to a tungsten
tip in a partial pressure of neon gas.6 They reported a voltage
drop during the self-sputtering process7 as evidence of tip
resharpening.
In this article we report on the effects of nitrogen-ion
bombardment on tip geometry, and the formation of molecular diameter nitrides caused by an internal nitridation reaction between implanted 1–1.5 kV nitrogen ions and molybdenum atoms in an Fe-3 at. % Mo tip. The principle
experimental tool employed is APFIM and all experiments
are performed in situ. First, a negative voltage is applied to a
tip in the presence of nitrogen gas. The tip emits electrons
via field emission and the field-emitted electrons in turn ionize the nitrogen molecules. The positively charged nitrogen
molecules are then, of course, attracted to the negatively
charged tip. This results in both sputtering of the tip and
implantation of nitrogen ions. Next APFIM is employed to
measure the tip radius and the range profile of the implanted
nitrogen atoms. APFIM results yield direct evidence for tip
resharpening during ion bombardment as a result of sputter-
0021-8979/97/81(2)/944/7/$10.00
© 1997 American Institute of Physics
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experimental procedures employed. First, an Fe-3 at. % Mo
ingot is produced in an arc melter in a high purity argon gas
atmosphere, and the ingot is then drawn into 0.15 m diameter
wire. Next a 1-cm-long piece of wire is electrochemically
polished in two steps: ~a! beaker polishing, and ~b! loop polishing @Fig. 1~a!#.9 Beaker polishing utilizes a chemical solution of 20% perchloric acid in acetic acid at 18 Vdc and is
employed to form a taper at the end of a wire. The tapered
end is then positioned in a 5-mm-diam stainless steel loop
employing 2% perchloric acid in 2-butoxyethanol as the
electrolyte in the form of a meniscus at 9 Vdc. This procedure is monitored with an optical microscope at a magnification of 200x.
B. Ion bombardment, sputtering, ion implantation,
and atom-probe analysis
FIG. 1. Schematic diagram of experimental procedures employed.
ing of the shank as well as the range profile of the implanted
nitrogen. Thus tips that are blunted as a result of the fieldevaporation process can be resharpened employing this
simple low-energy irradiation treatment. This is a useful
technique for the in situ sharpening or resharpening of tips.
Furthermore, APFIM results demonstrate that '50% of the
implanted nitrogen ions react with the Mo atoms that are in
solid solution. And we demonstrate, via APFIM, that molybdenum nitride clusters with a diameter on the molecular scale
are produced as a result of the implantation process. The
small diameter of the molecular clusters is a result of the fact
that they have a negligible diffusivity at room temperature
and, therefore, they cannot agglomerate into larger clusters
or precipitates via random-walk diffusion processes. Also,
the diameter of the molybdenum nitride clusters did not increase for an implantation at 473 K, indicating they are also
immobile at this temperature. Thus we have developed a
controlled and quantitative methodology for internal nitridation, employing 1–1.5 kV nitrogen ions, of an Fe-based alloy
that can produce a region that is mechanically hard on the
scale of <100 Å. This is quite different from the classical
approach of ion-nitriding steel in a glow discharge in a nitrogen gas mixture.8
II. EXPERIMENTS
A. Specimen preparation
Figure 1 is a schematic diagram illustrating the principle
J. Appl. Phys., Vol. 81, No. 2, 15 January 1997
A specimen with a sharply pointed tip is then inserted
into the main chamber of our APFIM10 through an ultrahigh
vacuum ~UHV! loadlock, which is baked at 150 °C prior to
the insertion of the specimen into the main chamber; this
procedure is employed to maintain UHV conditions. Typically the ambient pressure in the main chamber is ,2310210
Torr. Then the tip is cooled to 60 K, which is the temperature
of analysis ~T a !, and a positive voltage is applied to it to
obtain an FIM image at a partial pressure of 531026 Torr
Ne. Initially a tip is rough on an atomic scale, so the process
of pulsed field evaporation is utilized to form an atomically
smooth surface that produces an FIM image with excellent
atomic resolution. A stable neon FIM image is observed of
the Fe-3 at. % Mo alloy at 60 K; the calculated ionization
field of neon ~'3.5 V Å21! is close to the calculated evaporation for pure iron ~'3.6 V Å21!.
To analyze a specimen chemically on an atomic scale
our time-of-flight atom probe is employed. First, the Ne imaging gas is pumped out to reestablish UHV conditions in
the main chamber. Then the specimen is literally dissected
one atom at a time employing the pulsed field-evaporation
technique and the mass-to-charge state ratio ~m/n! of each
field-evaporated ion is measured via time-of-flight mass
spectrometry @Fig. 1~c!#. The quantity V DC is the applied dc
voltage and V pulse is the pulse voltage employed to dissect
the specimen on an atomic scale, so that the total voltage
applied during pulse field evaporation is the sum
V DC1V pulse . And the ratio V pulse/V DC is the pulse fraction, f ,
which is employed during pulsed field evaporation.
To induce sputtering of a tip a negative potential, 1.5
kV, is applied to a specimen in the presence of nitrogen gas
~partial pressure of 131025 Torr! at room temperature for 10
min @Fig. 1~b!#. The field-emitted electrons ionize the nitrogen gas molecules, resulting in positively charged molecules
that are accelerated toward the tip and sputter it, thereby
creating a sharper tip; a certain fraction of the nitrogen molecules is also implanted in the tip. These procedures are repeated to verify the reproducibility of the sharpening or resharpening process through sputtering. Some of the
experimental conditions employed are listed in Table I.
Kim, Yu, and Seidman
945
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FIG. 2. Tip radius variation during atom-probe data collection and ion bombardment procedures. The tip radius ~r t 5V/ k E e ! increases during the atom-probe
data collection procedure and decreases with ion bombardment as indicted by the vertical arrows. V is the total applied voltage, k a constant ~'5!, and E e the
calculated evaporation electric field of iron ~'3.6 V Å21!.
III. RESULTS
A. Molecular nitrogen-ion irradiation, tip sharpening,
and detection of molybdenum nitride molecules
Figure 2 displays the variation in the calculated tip radius versus the cumulative number of field-evaporation
events collected during an extended APFIM run on one
specimen. The vertical arrows in this figure indicate when
the tip was irradiated with ionized nitrogen molecules ~N11
2 !.
Note that the tip radius increases during the atom-probe data
collection procedure and then decreases as a result of the
irradiation. Experimentally a sharper tip manifests itself in a
decrease in the applied dc voltage required to sustain additional field evaporation. The tip radius is calculated using the
well-known relation between the total applied voltage and tip
radius ~r t !, E e 5~V DC1V pulse!/k r t , where E e is the evaporation field, and k is a constant ~'5! that takes into account the
nonspherical shape of a tip;11 the calculated evaporation field
for Fe is '3.6 V Å21, based on the so-called ionic model for
field evaporation which is essentially a 0 K model.12
Figure 3 shows schematically the trajectories of both
field-emitted electrons and molecular nitrogen ions, based on
approximate calculations of the trajectories of field-emitted
electrons and gas ions.13 The trajectories of the field-emitted
electrons deviate from the electric field lines associated with
a sharply pointed tip due to the so-called inertia effect. The
same inertia effect applies to the trajectories of the N11
2 ions.
ions
impinge
on
surface
area
that is
Therefore, some N11
2
outside the region of field-emitting electrons and sputter it—
that is, the shank of the tip—and this is the suggested physical mechanism of tip sharpening or resharpening.
Figure 4 exhibits an APFIM mass spectrum of the cu-
TABLE I. Ion bombardment conditions for an Fe-3 at. % Mo tip.
Tip temperature Tip voltage Nitrogen partial Irradiation
~K!
~Vdc!
pressure ~Torr! time ~min.!
First run
Second run
Third run
946
298
298
473
1500
1500
1000
1025
1023
1025
J. Appl. Phys., Vol. 81, No. 2, 15 January 1997
10
10
60
FIG. 3. Schematic diagram of the trajectories of field-emitted electrons that
deviate from the electric field lines associated with the sharply pointed tip
due to the inertia effect. The same inertia effect applies to the trajectories of
1
the N1
2 ions created by the field-emitted electrons. Therefore, some N2 ions
bombard regions outside of the field-emitting tip surface, that is, in the
region of the shank, resulting in tip sharpening or resharpening.
Kim, Yu, and Seidman
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FIG. 4. An APFIM mass spectrum of the cumulative number of events vs
the mass-to-charge state ratio ~m/n! for an Fe-3 at. % Mo alloy irradiated in
situ with nitrogen ions created in the field-emission ~FE! mode. Nitrogen
evaporates in the singly charged state, Fe in the doubly charged state, Mo in
both the doubly and triply charged states, and MoN in both the doubly
charged and triply charged states. The hydrogen signal is from the ambient
pressure in the APFIM.
FIG. 5. Cumulative numbers of N11, MoN21, and MoN31 species after the
second ion bombardment procedure in the form of an integral profile. The
formation of MoN21 ions occurs in this Fe~Mo! alloy about 20–40 Å away
from the tip surface, where the formation of MoN31 ions stops. This effect
can be understood by considering the electric-field variation at the tip apex
during data collection. The analysis temperature ~T a ! was 60 K and the
pulse fraction, f , was 0.15.
mulative number of events versus the mass-to-charge state
ratio ~m/n! for an Fe-3 at. % Mo alloy which was irradiated
in situ with N11
2 . Note the following different charge states
of the field-evaporated species: nitrogen in the singly
charged state ~N11! as an ion and not a molecular ion; Fe in
the doubly charged state ~Fe21!; Mo in both the doubly and
triply charged states ~Mo21 and Mo31!; and molybdenum
nitride in both the doubly and triply charged states ~MoN21
and MoN31!. The hydrogen signal is from the ambient pressure in the APFIM. It is important to note that the implanted
nitrogen atoms are collected as three different species ~N11,
MoN21, and MoN31! during the atom-probe analyses that
follow each irradiation. Figure 5 is an integral profile exhibiting the cumulative numbers of these three species as a function of depth ~Å! from the irradiated surface, after the
completion of the second irradiation, 1.5 kV, procedure
~Table I!. Note that the signal from the MoN21 species begins to appear at a depth of 20–40 Å. Also at this depth the
MoN31 signal commences to level off. This effect can be
understood by considering the variation of the electric field
at the apex of a tip during the atom-probe data collection. As
data collection progresses the image magnification decreases
due to the increase of the tip radius; this occurs because an
FIM is a point projection microscope and the magnification
is given by the ratio of the tip-to-screen distance ~a constant!
to r t . Therefore, the projected area of the probe hole,
through which the field-evaporated ions pass on their way to
a channel electron multiplier array detector, on the surface of
a tip increases with increasing r t and this should, at first
blush, result in a higher collection efficiency. Experimentally, however, the computer algorithm that controls the
APFIM requires a constant data collection rate and therefore
the voltage on the tip is automatically increased. And this
concomitantly results in a higher electric field during the
initial data collection period. Furthermore, it is known that
postionization of field-evaporated ions may occur in high
electric fields yielding ions with higher charge states.14
Therefore, MoN31 is collected initially when the tip is sharp
and concomitantly E is large. However, when the tip blunts
the electric field decreases and MoN21 is favored.
The MoNn1 molecules are detected as individual events,
and they are both preceded and followed by Fe21 events. The
detection of individual MoNn1 molecules indicates that the
bonding between Mo and N is strong enough to withstand
the evaporation field required to dissect atomically this Fe-3
at. % Mo alloy, even though the evaporation field is high
enough to overcome the Fe-Fe and Fe-N ‘‘bonds.’’ The relative weakness of the Fe-N bonds compared to Mo-N bonds
can be understood by considering the Gibbs free energy of
formation of nitrides. That is, the Gibbs free energy of formation of molybdenum nitride is greater than that of iron
nitride.15 The manner in which the MoNn1 molecules are
detected strongly suggests that their state in the specimen is
also in the form of a molecule, and that clustering of MoN
into large clusters or small precipitates does not occur in this
alloy due to the small diffusivity of N and MoN clusters at
room temperature. Typically, 50% of the Mo in solid solution is internally nitrided to form MoN. Thus, this implantation technique gives rise to a very high number density of
MoN molecules in the apex region of a tip on a scale of less
than 100 Å.
J. Appl. Phys., Vol. 81, No. 2, 15 January 1997
B. Nitrogen range profiles
Figure 6~a! is an integral profile, along a ^110&-type direction, exhibiting the cumulative number of nitrogen atoms
versus depth in angstroms ~Å! from the surface of a tip after
the second irradiation procedure. ~A negative 1.5 kV potential is applied to the tip at room temperature for 10 min in a
partial pressure of 131023 Torr N2—see Fig. 2 and Table I.!
Kim, Yu, and Seidman
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of the implanted species, the steady-state concentrations of
Frenkel pairs produced, and the diffusivities of all the atomic
species involved; that is, it depends on the microscopic details of a many kinetic processes.17 The measured diffusivity
of nitrogen in Fe at room temperature predicts a negligibly
small root-mean-squared diffusion distance of this gas.18 Figure 6~c! exhibits nitrogen range profiles calculated employing the TRIM.SP Monte Carlo code.19 This code was run for
four different cases: nitrogen atoms and nitrogen molecules
at two different energies ~150 and 1500 eV, respectively!.
The calculated nitrogen range profile of N11 at 1500 eV
matches the experimental results displayed in Fig. 6~b!.
However, the nitrogen ions generated by electron bombard20
Therefore,
ment are mainly composed of N11
2 molecules.
the TRIM.SP Monte Carlo results, in combination with the
experimental data, indicate that the molecular nitrogen ions
11
~N11
ions at or near the surface of
2 ! dissociate into two N
the tip and the nitrogen atoms then lose their energy via a
series of many-body collisions with the Fe and Mo lattice
atoms. In Ref. 13 the author concluded that the average energy of the nitrogen ions that bombard an emitting surface is
10% of the tip potential. However, neither the simulations
for 150 eV N11 nor 150 eV N11
2 agree with our experimental
results @Fig. 6~b!#, indicating that this conclusion is incorrect.
To estimate the kinetic energy distribution of electrons
and nitrogen ions we model the tip and counter electrode by
two concentric spheres; this is a reasonable model since the
radius of the outer sphere is much greater than that of the
inner sphere. The kinetic energy of an electron, E K (r), for
this geometry is given by21
E K ~ r ! 5eV
FIG. 6. Nitrogen range profile in an Fe~Mo! tip: ~a! The cumulative number
of nitrogen events obtained by atom-probe microscopy in the form of an
integral profile. ~b! The range profile of nitrogen events obtained by atomprobe microscopy. It was obtained after the second ion bombardment procedure. ~c! Calculated range profiles obtained employing the TRIM.SP
Monte Carlo code. The code is run for four different cases; nitrogen atoms
and nitrogen molecules with two different energies ~150 and 1500 eV!.
In determining this integral profile all the detected nitrogen
species are counted; that is, those from the N11, MoN21, and
MoN31 peaks in the mass spectrum. The number of atoms
collected per $110%-type plane is typically 16 and this number in combination with the known interplanar distance between $110%-type planes ~d $110%52.02 Å! yields the depth
scale. Figure 6~b! displays a nitrogen range profile based on
the data in Fig. 6~a!. The total range of nitrogen atoms is less
than 120 Å. The average nitrogen concentration, '5 at. % N,
in this distance is obtained from the ratio of the total number
of nitrogen atoms to the total number of collected atoms,
49/913. This value is, of course, significantly higher than the
equilibrium nitrogen solid solubility in iron at room temperature ~,0.001 at. % N!.16 This is not surprising as ion implantation is a nonequilibrium process, where the final concentration is determined by the fluence employed, the diffusivity
948
J. Appl. Phys., Vol. 81, No. 2, 15 January 1997
S D
r 0r t 1 1
2
r 0 2r t r t r
for r t <r<r o ,
~1!
where e is the electron charge, V is the total tip voltage
~V DC1V pulse!, r 0 is the counter electrode radius, and r t is the
tip radius. When the tip radius is 500 Å and the counter
electrode radius is 5 cm, the kinetic energies of the emitted
electrons from the tip rise to 99% of the maximum possible
energy at a distance of 5 mm from the center of the tip, which
is one-ten thousandth of the counter-electrode radius. The
kinetic energy of a nitrogen ion which is ionized at r and
accelerated to the tip is given by Eq. ~1! with a negative sign,
since it is derived by spatially integrating from r to r t when
the velocity, v , changes from 0 to v . Therefore, the fraction
of nitrogen ions that have a kinetic energy that is less than
99% of the maximum possible kinetic energy is negligible.
The energy distribution of the nitrogen ions produced by this
ionization process is similar to that of a conventional ion
generator. The molecular nitrogen ions generated by this ionization process move to the tip apex radially, while ions produced by a conventional generator move along lines that are
normal to the surface. Since the depth significantly affected
is small ~,100 Å! compared to the tip radius ~500 Å!, the
effect of this radial movement can be ignored. Therefore, the
use of TRIM.SP Monte Carlo results are valid in estimating
the nitrogen range profile obtained by this implantation procedure.
The Edgeworth expansion of the normal distribution
function is chosen to fit the experimental nitrogen range proKim, Yu, and Seidman
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TABLE II. Parameters of the Edgeworth, normal, and experimental distributions for 1500 eV N11 implanted in an Fe~Mo! alloy.
Mean ^x& ~Å!
Standard deviation ^Dx& ~Å!
Skewness ^Dx 3&/^Dx 2&3/2
Kurtosis ^Dx 4&/^Dx 2&2
a
Edgeworth
Normal
Experimentala
41
29
0.62
2.92
41
29
0
3.0
41
29
0.72
2.62
The experimental values were calculated employing a bin size of 2 Å.
mental and Edgeworth range profiles are in reasonably good
agreement, while the normal ~or Gaussian! distribution function is not a good representation of the experimental data.
FIG. 7. Normalized range profile of 1500 eV N11 ions implanted in an
Fe~Mo! single-phase solid-solution alloy specimen at room temperature. The
ordinate is normalized with respect to the number of N atoms at the experimental value of the mode. Both the normal and Edgeworth distribution
functions are superimposed on the experimental range profile—see Table II
and Sec. III B. Note that the normal distribution function is a poor representation of the experimental data.
file @Fig. 6~b!#. The Edgeworth expansion has been successfully used to describe both range22,23 and deposited energy24
profiles. The early terms of the Edgeworth expansion distribution function, f ~j!, of the standardized variable
j5~r t 2^x&2x!/^Dx 2&1/2 are
S D
FS D S D
f ~ j !5f~ j !2
1
1
A2 p
f ~ j !~ j !5
G
g 21 ~ 6 !
g2 ~4!
f ~ j !1
f ~ j ! 1... ,
24
72
where25
f~ j !5
g1 ~3!
f ~j!
6
F G
exp 2
j2
,
2
dj
f~ j !,
dj j
j51,2,3,... ,
~2!
~3!
~4!
g 1 5 ^ Dx 3 & / ^ Dx 2 & 3/2,
~5!
g 2 5 ~ ^ Dx & / ^ Dx & ! 23,
~6!
^ Dx j & 5 ^ x2 ^ x & ! j & ,
~7!
4
2 2
and
j51,2,3... .
The quantity ^x& is the average penetration, ^Dx& the standard
deviation ~or the straggling!, ^Dx j & the jth central moment of
the distribution, and the tip radius is assumed to be 500 Å.
Note that f~j! is the normal distribution, g1 the skewness,
and g213 the kurtosis of a non-normal distribution.
Both the normal and Edgeworth distribution functions
are superimposed on the experimental range profile in Fig. 7.
~The Edgeworth distribution function was fitted to the experimental composite range profile employing a commercially available software program.26! Table II lists the four
parameters required to fit the Edgeworth distribution function to the experimental range profile. Note that the experiJ. Appl. Phys., Vol. 81, No. 2, 15 January 1997
IV. SUMMARY
Low-energy ~1.0–1.5 kV! nitrogen-ion bombardment of
a sharply pointed Fe-3 at. % Mo tip is performed by applying
a negative voltage to the tip in the presence of nitrogen gas
~1023 –1025 Torr!. The field-emitted electrons ionize the nitrogen gas molecules to produce N11
2 molecules, which are
attracted to the negatively biased tip. Direct evidence of tip
sharpening and resharpening, as a consequence of sputtering,
is observed during the N11
bombardment via atom-probe
2
field-ion microscopy. The physical origin of the sharpening
process is the inertia effect on the trajectories of the fieldemitted electrons and nitrogen ions, which results in irradiation of the shank of the specimen as well as the sharply
pointed apex of the tip. We also directly observe that the
implanted nitrogen ions react with the Mo in this singlephase Fe~Mo! alloy to produce immobile molecular diameter
nitrides at room temperature and 473 K; physically this corresponds to internal nitridation of Mo on an atomic scale.
The charge state of the collected MoN species changes from
31 to 21 during the atom-probe data collection. This effect
can be understood by considering the variation of the electric
field at the tip apex during the data collection procedure. The
measured range profile of the implanted nitrogen species is
compared with the results obtained employing the TRIM.SP
Monte Carlo code. This comparison clearly demonstrates
that the ionized nitrogen molecules ~N11
2 !, generated by
field-emitted electrons from a tip, obtain their full potential
energy before bombarding the tip’s surface and dissociating
into nitrogen ions ~N11! at or near the surface. The experimental nitrogen range profile is well described by the Edgeworth expansion of the normal distribution function. We
have thus also developed an accurate and quantitative methodology for internal nitridation of Fe~Mo! alloys on a scale
of ,100 Å, and this is quite different from the classical
approach of ion nitriding by exposing steel to a glow discharge in a gas phase mixture. This technique for locally
hardening Fe~Mo! alloy tips on a 100 Å scale, by producing
a high number density of molecular diameter molybdenum
nitrides, is relevant to the problem of micromachining metal
surfaces.27
ACKNOWLEDGMENTS
This work was supported by the MRL program of the
National Science Foundation, at the Materials Research CenKim, Yu, and Seidman
949
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ter of Northwestern University, under Award No. DMR9120521. The authors thank Dr. S. S. Brenner for generously
providing the first Fe-3 at. % Mo wires we used, and D. A.
Shashkov for valuable discussions and his aid in operating
and maintaining the atom probe. We thank Dr. W. Eckstein
for kindly supplying us with his TRIM.SP Monte Carlo code
and Dr. D. Udler for helping us to run it.
C. J. Chen, Introduction to Scanning Tunneling Microscopy ~Oxford University Press, New York, 1993!, Chap. 13.
2
H. W. Fink, IBM J. Res. Dev. 30, 460 ~1986!.
3
V. T. Binh, J. Microsc. 152, 355 ~1988!; Surf. Sci. 202, L539 ~1988!.
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G. L. Kellogg, Surf. Sci. Rep. 21, 1 ~1994!.
6
O. Albrektsen, H. W. M. Salemink, K. A. Morch, and A. R. Tholen, J.
Vac. Sci. Technol. B 12, 3187 ~1994!.
7
We prefer not to use the term self-sputtering as the ions doing the sputtering originate in the gas phase and not the element from which the tip is
fabricated.
8
B. Berghaus and H. Bueck, US Patent No. 2946708 ~26 July 1960!; A.
Brokman and F. R. Tuler, J. Appl. Phys. 52, 469 ~1981!.
9
B. W. Krakauer and D. N. Seidman, Rev. Sci. Instrum. 63, 4071 ~1992!;
B. W. Krakauer, J. G. Hu, S.-M. Kuo, R. L. Mallick, A. Seki, D. N.
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10
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1
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25
The experimental value of ^x& is measured from the surface of the sphere
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origin at the center of the sphere. Note that when x50 the value of f ~j! is
small.
26
KaleidaGraph 3.0 by Abelbeck Software.
27
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13
14
Kim, Yu, and Seidman
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