JOURNAL OF APPLIED PHYSICS 116, 054908 (2014)
Observed damage during Argon gas cluster depth profiles of compound
semiconductors
Anders J. Barlow,a) Jose F. Portoles, and Peter J. Cumpson
National EPSRC XPS Users’ Service (NEXUS), School of Mechanical and Systems Engineering,
Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom
(Received 9 May 2014; accepted 23 July 2014; published online 6 August 2014)
Argon Gas Cluster Ion Beam (GCIB) sources have become very popular in XPS and SIMS in
recent years, due to the minimal chemical damage they introduce in the depth-profiling of polymer
and other organic materials. These GCIB sources are therefore particularly useful for depthprofiling polymer and organic materials, but also (though more slowly) the surfaces of inorganic
materials such as semiconductors, due to the lower roughness expected in cluster ion sputtering
compared to that introduced by monatomic ions. We have examined experimentally a set of five
compound semiconductors, cadmium telluride (CdTe), gallium arsenide (GaAs), gallium phosphide
(GaP), indium arsenide (InAs), and zinc selenide (ZnSe) and a high-j dielectric material, hafnium
oxide (HfO), in their response to argon cluster profiling. An experimentally determined HfO etch
rate of 0.025 nm/min (3.95 10 2 amu/atom in ion) for 6 keV Ar gas clusters is used in the depth
scale conversion for the profiles of the semiconductor materials. The assumption has been that,
since the damage introduced into polymer materials is low, even though sputter yields are high,
then there is little likelihood of damaging inorganic materials at all with cluster ions. This seems
true in most cases; however, in this work, we report for the first time that this damage can in fact be
very significant in the case of InAs, causing the formation of metallic indium that is readily visible
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4892097]
even to the naked eye. V
I. INTRODUCTION
Argon Gas Cluster Ion Beam (GCIB) sources were originally developed for wafer smoothing during semiconductor
processing1,2 and advanced coatings,3 and subsequently the
use of these sources for analytical applications was pioneered in SIMS.4,5 They have proved to be very useful in
XPS.6 These GCIB sources produce ionised argon clusters
having a size distribution ranging from around 500 to 5000
atoms, which can be narrowed by mass filtering provided by
the manufacturer of the source. The average kinetic energy
per atom is much lower than for conventional monatomic
ion sources, leading to greatly reduced damage in sputter
depth-profiling of polymers. The very low damage that these
cluster ions introduce has been shown5 to allow practical
depth-profiling of polymer and other organic samples,7
largely preserving sample chemistry. These GCIB sources
are therefore particularly useful for depth-profiling polymer
and organic materials. The assumption has been that, since
the damage introduced into polymer materials is low, even
though sputter yields are high, then there is little likelihood
of damaging inorganic materials at all with cluster ions.
In this work, we report for the first time that this damage
can be very significant in the case of some compound
semiconductors.
Excellent progress has been made over the last five years
in understanding cluster-surface interactions in an analytical
context.8–10 In our laboratory, we have performed depth
a)
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-8979/2014/116(5)/054908/5/$30.00
profiles of many different organic layers and have characterised their behaviour during GCIB etching. Recently, we have
published measurements of total sputter yields from a large
number of organic and inorganic materials exposed to cluster
ions11,12 indicating that the total sputter yield from inorganics is significantly less than that for organics over the
range of cluster energies of practical significance in XPS and
SIMS. This is of course beneficial when depth-profiling a
polymer, for example, where it is desirable to have a high
sputter yield, with a damage depth much less than the information depth of the analytical technique.
With such low sputter yields, it is unlikely that a GCIB
would be used for depth profiling through thick (i.e., more
than a few tens of nanometres) inorganic layers, due to the
prohibitively long times that would be required. However,
these sources were originally developed for semiconductor
processing by reducing surface roughness, so may be
expected to be useful in depth-profiling the first 10 nm or so
of semiconductor materials at extremely good depth resolution. It is therefore beneficial to understand how inorganic,
and specifically semiconductor, surfaces respond to GCIB
exposure, especially from the point of view of the XPS analyst, a technique where surface damage can be readily
detected. This may also be of importance when attempting
removal of organic contamination from a surface13 or when
depth-profiling through an organic layer deposited on top of
an inorganic substrate14 (for example, an etch-resist residue
on a semiconductor surface). Prior to our experimental work,
we expected the damage to inorganic materials to be unspectactular and perhaps undetectable. We have been surprised to
find that in a few cases, this damage can be very significant,
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C 2014 AIP Publishing LLC
V
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J. Appl. Phys. 116, 054908 (2014)
changing not just the surface chemistry of an inorganic semiconductor but even causing gross surface discoloration to the
naked eye, as shown in Figure 1.
In this work, GCIB sputter depth profiles were performed on a variety of compound semiconductors, CdTe,
ZnSe, GaP, GaAs, and InAs, and a high-j dielectric material,
HfO. For most of these materials, no detectable damage
(taken as the reduction or preferential sputtering of species
within the material) was observed. For a small subset, however, damage became evident in the XPS spectra after long
etch times and eventually was visible to the naked eye on the
surface of the semiconductor. All XPS analyses were performed in our Theta Probe XPS instrument (Thermo Scientific,
East Grinstead, UK), utilising a monochromatic Al Ka X-ray
source (1486.6 eV) operated at 100 W. While the Theta Probe
is capable of operating in Parallel Angle-Resolved XPS
(ARXPS) mode, the work discussed herein was collected in
the “standard” lens mode, whereby all electrons were collected
from 20 to 80 to the surface normal. Spectra were collected
with pass energy of 40 eV and were the average of 10 sweeps.
Charge neutralisation with a combination of low-energy electrons and argon ions was used throughout all experiments, and
checks made that this low energy monatomic ion exposure
alone did not lead to damage.
The GCIB source is a Thermo Scientific MAGCIS gun,
which throughout this work was operated at 6000 eV in
“small” cluster mode, meaning a broad, semi-log distribution
of Argon cluster size centred around 1000 atoms per cluster
at an input pressure of 4 bars. On the low mass side, this distribution has a half-width of 200 atoms/cluster, and on the
high mass side can extend to several thousand atoms per
cluster. It is expected that the high mass clusters will have
significantly less impact on the etch rate relative to the
desired 1000 atom clusters. Neutral atoms are not expected
to escape from the MAGCIS due to a 2 bend in the optics,
and monoatomic ions are well below the cut-off of the
MAGCIS mass filter. Further to this, multiply charged Ar
clusters are not expected to be stable enough to emerge from
the source.15 An ion beam raster of 1 mm 2 mm was used,
with an X-ray spot size of 200 lm 400 lm, i.e., well within
the area of the sputter crater. Throughout all experiments,
the beam current was stable around 20 6 2 nA (as measured
at the sample plate within the instrument). An etch cycle of
300 s was used for all depth profiles.
Sputter time was converted to depth from experimentally
determined etch rates on the high-j dielectric material, HfO,
using a method detailed previously.12 During sputtering, a
contact shadow mask aligned with the ion beam is used to pattern fine step-edges throughout the area of the sputter crater.
The total etch depth and thus etch rate are then straightforward to measure using white-light interferometry (Newview
5000, Zygo, Middle-field, CT, USA). An example linescan
across one step-edge is provided in Figure 2. The total crater
depth is then taken as the step height in the profile, and multiple measurements were made across step-edges throughout
the etch crater. From this analysis, an average etch rate of
0.025 nm/min was then determined for HfO, using a 6 keV Ar
GCIB of 1000 atom clusters. Upon conversion, this relates to
an etch rate of 6.77 10 24 cm3/ion, 6.77 10 27 cm3/atom
in ion, and 3.95 10 2 amu/atom in ion.
FIG. 1. Photograph of InAs surface after 6 keV and 2 keV GCIB depth profiles. The resultant damage after the 6 keV GCIB exposure is clearly visible.
FIG. 2. Linescan across a step-edge from a white light interferometry topography map representative of the crater depth on HfO after etching.
II. RESULTS
Spectra acquired at two cumulative etch times during the
depth profiles of compound semiconductors are given in
Figure 3. Carbonaceous contamination for all samples was
completely removed after a single etch cycle of 300 s at
6 keV (small clusters), and native oxides were typically
removed after 3 to 6 etch cycles. For ease of comparison, the
spectra from 2100 and 6000 s are shown here for all semiconductors studied. From the HfO experiments, this relates to an
approximate depth of 0.9 nm and 2.5 nm, respectively. For
each compound, a narrow-scan spectrum of most analytical
interest for each element in the semiconductor was collected.
Once the native oxides were removed through etching, the
shape and position of the peaks observed in the spectra for
each of the compounds remained the same throughout all 6000
s of exposure to the 6 keV small cluster beam. Peak positions
and widths were measured for each of the spectra, taken from
the lowest binding energy component of the doublets analysed
(for example, the 5/2 component of a 3d doublet). The minimal
change in either peak position or peak width (measured by
FWHM) suggests that the compound semiconductors CdTe,
GaAs, GaP, and ZnSe and the high-j dielectric HfO are etched
by the 6 keV small cluster beam without damage.
The measurements were repeated on InAs, and the
results are presented in Figure 4. The native oxide on the
InAs sample was rapidly removed by the 6 keV small cluster
beam, with minimal oxide left after the first 300 s etch cycle.
At this point, the In3d and As3d spectra were measured and
observed to be as expected for InAs, with the In3d5/2 peak
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Barlow, Portoles, and Cumpson
J. Appl. Phys. 116, 054908 (2014)
FIG. 3. Analytical peaks of interest for
two total etch times, 2100 s and 6000 s,
during the depth profiles of (a) CdTe,
(b) GaAs, (c) GaP, (d) ZnSe, and (e)
HfO. Peak widths for each of the spectra are given in the inset tables and
were taken from the lowest binding
energy components of the spectra.
position measured as 444.3 eV in good agreement with previous work.16,17 After a total etch time of 2100 s, however,
broadening of the In3d peaks was observed, and reflected in
the measured FWHM of the 3d5/2 component. After the full
6000 s depth profile, a noticeable “shoulder” was evident on
the low binding energy side of the In3d5/2 peak. Minimal
change was observed in the As3d peak, however.
Analysis of the In4d doublet also showed a similar low
binding energy shoulder after the full depth profile as shown
in Figure 4(b). In both instances, the extra feature matches
in binding energy what would be expected for In metal, suggesting that the cluster ion beam is reducing the In within
the InAs to its metallic state. The In3d5/2 peak was fitted
with two synthetic components, one at 444.3 eV for InAs
and the other at 443.8 eV for In metal.16,17 Plotted in Figure
4(c) is the change in ratio of these features as the depth profile progressed. Note that we saw no evidence of X-rays
accelerating the damage or sputter yield in any of these inorganic samples, whereas we have previously seen that X-ray
enhancement of cluster ion sputter yields in certain
polymers.18,19
To reduce ion-induced damage, the energy-per-atom
can be reduced. For a 6 keV small cluster beam, we expect
a distribution of energies around 6 eV per atom in the
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J. Appl. Phys. 116, 054908 (2014)
FIG. 4. Spectra from the depth profile of InAs. The In3d and As3d spectra are given in (a) for three steps during the 6 keV small cluster depth profile. A feature
becomes evident in the In3d spectra resulting from a reduction of In to the metallic state. This change in peak shape is also evident in (b) for the In4d spectra.
(c) Peak fitting the In3d5/2 component with two synthetic peaks for InAs and In metal shows how this damage develops with etch time and depth. Reducing the
ion energy to 2 keV in (d) allows the depth profile to be performed with no damage detected in the spectra.
cluster (1000 atoms per cluster), which can be reduced to
2 eV per atom by reducing the ion energy to 2 keV. Figure
4(d) shows the In3d spectra for 3 stages during a depth profile of InAs using a 2 keV small Ar GCIB. Previously, damage was evident after 2100 s. With the reduced energy, even
after 10 000 s of ion etching, no damage was evident in the
In3d spectra. This is reflected by a visual inspection of the
InAs surface after etching as was introduced in Figure 1.
The 6 keV sputter locations are clearly visible as regions of
damage where the InAs has been reduced to form metallic
In at the surface. In the region of the 2 keV depth profile,
however, there is no visible damage at the surface. Of
course, one must remark that at reduced ion energy, it
should be expected that the sputter yield will also decrease,
thus leading to longer sputter times being required for a
depth profile. However, the strength of a GCIB on inorganic
surfaces lies in its ability to remove surface contamination
and oxides with minimal and shallow damage to the surface
of interest. The 2 keV small cluster beam was able to
remove the surface oxide after 2000 s of etching.
Continuing exposure to this beam up to 10 000 s saw no
adverse impact to the chemistry at the surface, suggesting a
useful margin in the removal of contamination or oxide.
The higher energy 6 keV beam removed the oxide in 300 s;
however, the peak shape showed signs of broadening immediately beyond this time, suggesting that the reduction of
the InAs was occurring after a relatively brief exposure,
leaving less margin for choice of sputtering time sufficient
to remove the contamination and oxide.
It might have been expected that exposure of an inorganic material to a GCIB would not result in damage at the
surface which would in turn influence an analytical result,
especially given the large difference in sputter yields
between inorganics and organics for these ion beams.
However, it is now apparent that some materials may suffer
from damage due to these beams within a realistic experimental time frame. Thus, the proper use of GCIB sources
becomes a balance between minimising damage to a surface
of interest, while also minimising the length of time needed
for a piece of analysis. Elsewhere, we have derived a
“figure of merit” for removal of organic overlayers on inorganic substrates,15 indicating that organic overlayers are
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Barlow, Portoles, and Cumpson
preferentially removed from the surfaces of inorganic substrates particularly for cluster beams with energy below
around 4 eV/atom. This was obtained purely by consideration of the relative sputter rates of typical organic and inorganic materials. Our present work suggests that this low
energy-per-atom, or lower, may be even more desirable for
cases, like InAs, where the underlying inorganic material
may be chemically modified by clusters of higher energyper-atom.
III. CONCLUSIONS
Five compound semiconductors and a high-j dielectric material were depth profiled using an Ar GCIB.
CdTe, GaAs, GaP, ZnSe, and HfO all responded well
to a 6 keV, 1000 atom Ar cluster beam with no appreciable damage observed over a 6000 s depth profile.
Carbonaceous contamination was removed during the first
300 s etching step, and oxides were typically removed after 3 to 6 etch cycles. Damage was, however, observed in
InAs, and XPS indicated this to be a reduction of indium
to its metallic state. This was observed to begin occurring
almost immediately after the first 300 s etch cycle, though
became most prominent after 2100 s of exposure. After
reducing the energy-per-atom to 2 eV, the depth profile
could be performed for a cumulative etch time of 10 000 s
without any appreciable damage.
ACKNOWLEDGMENTS
X-ray photoelectron spectra were obtained at the
National EPSRC XPS Users’ Service (NEXUS) at
Newcastle University, an EPSRC Mid-Range Facility. The
J. Appl. Phys. 116, 054908 (2014)
authors would like to thank Mr. Mike Foster for technical
assistance and Dr. Naoko Sano for useful discussions.
1
I. Yamada, J. Matsuo, N. Toyoda, and A. Kirkpatrick, Mater. Sci. Eng., R
34, 231–295 (2001).
L. P. Allen, T. G. Tetreault, C. Santeufemio, X. Li, W. D. Goodhue, D.
Bliss, M. Tabat, K. S. Jones, G. Dallas, and D. Bakken, J. Electron. Mater.
32, 842–848 (2003).
3
A. J. Perry, S. J. Bull, A. Dommann, D. Rafaja, B. P. Wood, and M.
Michler, Surf. Coat. Technol. 133–134, 253–258 (2000).
4
N. Winograd, Z. Postawa, J. Cheng, C. Szakai, J. Kozole, and B. J.
Garrison, Appl. Surf. Sci. 252, 6836–6843 (2006).
5
C. M. Mahoney, Mass Spectrom. Rev. 29, 247–293 (2010).
6
T. Miyayama, N. Sanada, S. R. Bryan, J. S. Hammond, and M. Suzuki,
Surf. Interface Anal. 42, 1453–1457 (2010).
7
N. Winograd, Surf. Interface Anal. 45, 3–8 (2013).
8
A. Delcorte, B. J. Garrison, and K. Hamraoui, Anal. Chem. 81, 6676–6686
(2009).
9
C. Anders, H. M. Urbassek, and R. E. Johnson, Phys. Rev. B 70, 155404
(2004).
10
T. Seki, T. Murase, and J. Matsuo, Nucl. Instrum. Methods Phys. Res.,
Sect. B 242, 179–181 (2006).
11
P. J. Cumpson, J. F. Portoles, N. Sano, and A. J. Barlow, J. Appl. Phys.
114, 124313 (2013).
12
P. J. Cumpson, J. F. Portoles, and N. Sano, J. Vac. Sci. Technol., A 31,
020605 (2013).
13
P. J. Cumpson and A. J. Barlow, “Optimum conditions for cleaning surfaces by argon gas cluster ions prior to XPS analysis,” Surf. Interface Anal.
(unpublished).
14
P. J. Cumpson, J. F. Portoles, N. Sano, A. J. Barlow, and M. Birch, Surf.
Interface Anal. 45, 1859–1868 (2013).
15
Discussion with Thermo-Scientific, East Grinstead, UK.
16
M. Procop, J. Electron Spectrosc. Relat. Phenom. 59, R1–R10 (1992).
17
G. Hollinger, R. Skheyta-Kabbani, and M. Gendry, Phys. Rev. B 49,
11159–11167 (1994).
18
P. J. Cumpson, J. F. Portoles, N. Sano, and A. J. Barlow, J. Vac. Sci.
Technol., B 31, 021208 (2013).
19
P. J. Cumpson, J. F. Portoles, and N. Sano, Surf. Interface Anal. 45,
601–604 (2013).
2
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