Characterization of SiGe Bulk Compositional Standards
with Electron Probe Microanalysis
R. B. Marinenko*, J. T. Armstrong*, S. Turner*, E. B. Steel*, and F. A. Stevie**
^National Institute of Standards and Technology (NIST), MS 8371, Gaithersburg, MD 20899
**Analytical Instrumentation Facility, Room 318, EGRC Box 7531,1010 Main Campus Drive, North Carolina
State University, Raleigh, NC27695-7531
Abstract. Bulk SiGe wafers cut from single-crystal boules were evaluated with the electron probe microanalyzer (EPMA)
for micro- and macroheterogeneity for use as primary standards for future characterization of SiGe thin films on Si that are
needed by the microelectronics industry as reference standards. Specimens with nominal compositions of 14 at. %, 6.5 at. %,
and 3.5 at. % Ge were rigorously tested with wavelength dispersive spectrometers (WDS) using multiple point, multiple
sample, and duplicate data acquisitions. The SiGe 14 is a good bulk reference material for evaluation of SiGe thin films.
INTRODUCTION
Silicon germanium technology is having a profound
impact on the wireless and computer industries due to
the development of electronic components that are
smaller, faster, less noisy and require less power than
conventional silicon technology [1]. This trend is expected to continue since the fabrication cost of silicon
germanium devices is nearly as inexpensive as silicon
devices, while little retooling is required. Characterization of the SiGe thin films used in the new devices has
therefore become necessary. NIST is working with the
semiconductor industry to use electron probe microanalysis (EPMA), Auger analysis, analytical electron microscopy (AEM), and secondary ion mass spectrometry
(SIMS) to determine the composition, extent of heterogeneity, and thickness of industry-produced SiGe thin
films on Si. These films will become available to laboratories for characterization as part of a NIST "Interactive Materials" reference program designed to share materials and analytical results among microanalysis laboratories.
From commercial sources we obtained several Si
wafers coated with SiGe thin films of different thicknesses. An abundance of samples could be cut from any
one wafer for distribution to the industrial community
for use as reference standards after characterization of
each wafer for Si and Ge compositions and evaluation of
the extent of compositional and thickness heterogeneity.
This paper discusses the first step in the development of
this project - the evaluation of SiGe bulk specimens for
use as primary standards for the microanalysis of these
SiGe thin films on Si.
EXPERIMENTAL
SiGe specimens cut from single-crystal boules
normal to the growth axis were obtained from Virginia
Semiconductor. The nominal compositions were 3.5 at.
% Ge in Si, 6.5 at. % Ge in Si, and 14 at. % Ge in Si. The
specimens were mounted on brass disks with conductive
carbon tape without carbon coating for testing with the
electron probe microanalyzer (EPMA) using five wavelength dispersive spectrometers (WDS). EPMA uses a
high energy, focused electron beam (up to 40 keV) to
excite characteristic x rays in a specimen. The sample
excitation volume is a sphere with a diameter of approximately 1 |im. It is therefore ideal for determining the
extent of heterogeneity on the micro scale (point-to-point
variability) and on the macro scale (sample-to-sample
variability). For the certification of NIST Standard Reference Materials (SRMs) for microanalysis, this laboratory has developed analytical and statistical procedures
for the testing the micro- and macroheterogeneity of flat,
polished materials [2].
RESULTS AND DISCUSSION
Initial Heterogeneity Testing
The purpose of the initial heterogeneity testing
of these specimens was to determine if any specimens
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
2003 American Institute of Physics 0-7354-0152-7/03/$20.00
238
showed appreciable microheterogeneity. Line traverses
and random sampling procedures were used. On each
specimen two 40 point traverses normal to one another
were run at two randomly selected locations with steps
of 2 (im or 5 Jim between points. Such traverses are routinely run during heterogeneity testing to determine if distinct phases across short distances (5 jim to 100 (im)
within specimens are present. Table 1 lists analysis parameters, average counts/20 s observed per traverse, and
the la uncertainty observed for each sample for the
traverse with the largest uncertainty. These data suggest
that the SiGel4 specimen is the least heterogeneous while
the SiGe3.5 is the most heterogeneous on the |im scale.
Plots of x-ray counts vs. point sequence also confirmed
that these three specimens exhibit different levels of heterogeneity.
Random sampling tests on each specimen were
run to evaluate the overall specimen heterogeneity. Each
specimen was divided into rectangular sectors, or samples,
excluding the extreme specimen edges. Four or five randomly selected points were sampled within each sector
and x-ray counts were acquired three times on the same
point for an acquisition time of 20 s or 40 s. Data from
the five fixed spectrometers were acquired simultaneously
and corrections were made for current drift (usually less
than 1% relative). Data were processed with a nested
design analysis of variance procedure used previously at
NIST [2,3,4]. From the repeated readings taken from the
randomly selected points within each sample (or sector),
and from the known mass fraction of each element as
well as the background counts on each spectrometer, variance components were calculated for between points
(a 2), for between samples (a 2), and for the experimental uncertainty (a 2). The overall variance (o 2) in
Ew
W
mass fraction due to the heterogeneity contribution was
calculated for each group of analyzed samples or sectors
as,
= U
(T 2 + Urr 2 +
Sw
Pw
(1)
the sum of the between specimens, between points, and
experimental variances. Table 2 lists the expanded uncertainties (3a, where a = V aw2 ) in % mass fraction determined from data for each specimen - they are 1 % to 2 %
relative for Si and Ge in SiGel4 and for Si in SiGe6.5, but
for Ge in the SiGe6.5 they are 2 % to 3.5 % relative. For
the SiGe3.5 these uncertainties are 4.5 % relative for Si
and as much as 15 % relative for Ge. The consistently
higher uncertainty for Ge is due to the much lower counting statistics observed for Ge compared to Si, due in part
to the lower concentration of this element in the specimens and the lower spectrometer efficiency for Ge.
SiGe6.5 was is two large pieces, designated "top"
and "bottom." Data taken from the top portion are shown
in Fig 1 where the total x-ray counts acquired in 20 s are
plotted vs. point number. Data from the three repeated readings taken from four randomly selected locations in each
of the four sectors, totalled 48 points. The error bars are
the expanded uncertainties (±3a) determined from Poisson counting statistics, where a is equal to the square root
of the average number of counts/20 s. This specimen definitely shows heterogeneities between sectors as did the
statistics in Table 2. A similar plot (not shown here) of
data taken from the SiGel4 sample showed considerably
less heterogeneity, while for SiGe3.5 there is considerably greater heterogeneity, as expected from Table 2. For
this reason, the SiGe3.5 was excluded from subsequent
heterogeneity tests. Since this was an initial test, the extent of heterogeneity of SiGe6.5 was not sufficient at this
point to exclude it from further testing.
Having five spectrometers with only two elements to analyze allows the acquisition of duplicate data
on more than one spectrometer and/or the acquisition of
data from two different x-ray lines of the same element as
can be seen in Table 2 and Fig. 1. This type of information helps to sort out instrumental problems such as spurious noise signals that occasionally occur over the duration of an experiment from true compositional changes in
the specimen.
Throughout these analyses we used plots of x-
Table 1. Uncertainties (la) for 40-Step Traverses
Specimen Steps
Element/Line/Crystal/Spectrometer
(urn)
SiKocTAPl GeKccLiF2 GeKccLiF3 SiKoPET4
SiGel4
2
Cts/20s
o(rel %)
1862735
0.25%
2784659
0.51%
5
SiGe3.5
Cts/20s
3169570
a(rel %)
1.39%
40 pts/traverse, 20 kV, 80 nA, 20 s counting times
SiGe6.5
5
Cts/20s
o(rel %)
GeLocTAPS
123457
134272
201235
444393
0.38%
62545
0.60%
43240
4.19%
0.40%
68594
0.67%
47606
4.09%
0.30%
109363
0.47%
74876
4.34%
0.31%
635956
0.40%
710021
1.09%
239
Final Heterogeneity Testing
ray counts for a given element vs. acquisition sequence
or plots of Si x-ray counts vs. Ge x-ray counts to visualize the consistency of the data, but only a few are presented here. At the voltage and current used in these initial tests, the count rate for SiKa on TAP1 was too high,
swamping the counter with too many photons. Occasional spurious signals were also observed that could not
be attributed to concentration changes. For subsequent
tests the voltage and current were lowered to 15 kV and
60 nA, respectively, in hopes of reducing erratic signals.
The SiGel4 and SiGe6.5 wafers were cleaved
into smaller specimens, approximately 0.5 cm x 0.5 cm
each. Five specimens from each were tested to determine if they were sufficiently similar for use as reference
standards. Three were intended for the bulk quantitative
analysis with ICP-OES (Inductively Coupled PlasmaOprtical Emission Spectroscopy) while two would be retained as primary reference standards.
Table2. Extent of Specimen Heterogeneity (Initial Test)
Dimension # Sectors,
Expanded Uncertainty (3 a ) due to
# Points,
Specimen Heterogeneity, % mass fraction
Specimen Sector Size # Repeats SiK
SiK
GeK
GeK
GeL
(mm)
Time/pt
TAP1
LiF2 LiF3 TAP4 PETS
SiGel4
0.21
0.16
0.39
0.33
20 x 11.4
12,4,3
0.31
5x3.8
20s
SiGe6.5
0.84
15x8.3
1.28
0.75
4,4,3
0.51
1.11
Bottom
7.5x4.2
40s
SiGe6.5
17.2 x 10.2
0.63
0.67
0.58
4,4,3
0.38
0.97
Top
7.9x5.1
20s
SiGe3.5
35.4 dia
2.95
3.20
4,5,3
1.13
3.05
0.80
20s
SKTAP1
.
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8
W
$$::;,.::•;.•:•;
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X
4S
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Figure L Plots of x-ray counts
as acquired
and in sequence with the electron
micropiobe with five fixed WD spectrometers. The.data were
from three repeated
from each
Dffour randomly selected points in four different sectors of the top half of specimen SiOe6,5,
240
The Si and Ge data were acquired separately
at different currents to get better counting statistics and
spectrometer efficiency. The SiKa line was analyzed
using both the TAP 1 and TAP5 crystals with the current
lowered to 10 nA at 15 kV to avoid the high count rates
that flooded the TAP1 counter in the initial heterogeneity test discussed above. Ge was tested in a second run
using the TAP1 and TAPS crystals for the GeLa line,
and the LiF2 and LiF3 crystals for the GeKoc line. A15
keV excitation potential and current of 60 nA was used
for the Ge test. The instrument was attended constantly
during the acquisition to be sure that the focus was maintained between specimens.
Plots of all data points taken in sequence from
the five specimens of each of the SiGel4 and SiGe6.5
samples and for the five repeated 7-point analyses of
the Si and Ge wafers are shown in Fig. 2 (counts vs.
point no). Corrections were made to the counts data for
current changes during the experiment. The bars correspond to the ± 3 a uncertainty based on the average number of counts from Poisson counting statistics. The
GeKoc plots are not included in Fig. 2 since they duplicate the GeLa plots at lower count rates. As expected
from previous observations, the data from the SiGel4
specimens show consistency among points and among
specimens and is similar to, though not quite as consistent as, the data taken from the Si and Ge wafers. The
SiGe6.5 data shows less consistency among points and
among specimens with points falling outside of the ±
3a uncertainty bars.
Table 3 lists the expanded uncertainties, 3a,
for the set of data taken at each of the SiKa and GeLa
lines from each of the Si and Ge wafers, and the SiGel4
and SiGe6.5 samples. The value of a was calculated
from the contributions to the heterogeneity cited in equation (1) above. These data confirm that the SiGel4 is a
good material for use as a bulk primary reference standard. The heterogeneity of this specimen is only about
0.5 % mass fraction more than that observed for the pure
Si and Ge wafers. The SiGe6.5 is obviously more heterogeneous, particularly for Ge where the expanded heterogeneity contribution is almost 5 % mass fraction.
Data for the wafers do not include an uncertainty for
SiKaTAPl because the between specimen variance was
less than the between point variance, resulting in a negative number when the latter was subtracted from the
former, terminating the calculations. Also, the value
for GeLa on TAP1 is larger than for GeLa on TAP5
because a single outlier reading from one data point was
not excluded from the calculations.
Quantitative EPMA
K-value data (ratio of x-ray counts from unknown to that of a standard) were also taken for Si and
Ge from each of the five SiGel4 and SiGe6.5 specimens using the Si and Ge wafers as reference standards.
The same randomly selected seven or eight points used
to collect the x-ray counts data above from the SiGe
specimens were used for the k-value data. The Si and
Ge wafers were the reference standards for the determination of the k-ratios. Plots of the K-value data showed
the same trends within and between specimens observed
for the counts data in Fig. 2. These analyses provided
numerous datapoints for quantification since a k-value
is the first estimate of composition in the EPMA quantification process.
Special efforts are needed to accurately determine the composition of Si-Ge alloys by quantitative
electron microprobe analysis. The same experimental
data, when processed through the various electron microprobe correction procedures currently in use by different laboratories can yield concentrations that vary by
over 20 % relative. There is considerable uncertainty in
the mass absorption coefficients for this system, particularly for SiKa x-rays by Ge. Using a multiple-keV correction evaluation procedure developed by our laboratory [5], we determined the best combination of mass
absorption coefficients and correction procedures that
correctly predicted the change in measured k-values with
electron beam accelerating potential for the various SiGe alloys we analyzed. Using this correction, we determined that our best estimate of the composition of the
nominal SiGe6.5 alloy is 6.7 at. %±0.1 at. % Ge and the
composition of the nominal SiGe 14 alloy is 14.4 at. %
+0.1 at.% Ge. These results will be augmented with
bulk composition quantition using ICP-OES.
CONCLUSION
The SiGe 14 is represents a good microanalysis reference standard due to the low heterogeneity of
this material, with the maximum expanded uncertainty
due to heterogeneity no greater than 1.5% relative mass
fraction for Ge and considerably less for Si. The SiGe6.5
is not as good as the SiGe 14 material with an expanded
uncertainty due to Ge heterogeneity of about 5 % relative mass fraction. The SiGe 14 material will be used to
set up a set of secondary standards of SiGe films on Si
that will be useful for industry and become part of our
Interactive Materials program.
241
REFERENCES
[1] Subbanna, S., Myerson, B., Barnum, J.C., OConnell, T., and
St. Onge.,S., Future Fab Internatl, Issue 11 (June 29, 2001).
[2] Marinenko, R. and Leigh, S., "Heterogeneity Evaluation of
Research Materials for Microanalysis Standards Certification," Micros. MicroanaL, Cambridge Press, New York (in review).
[3] ISO Guide 35, Certification of reference material — General
and statistical principles,2nd ed., Internal. Org. for Standardization, Switzerland (1989).
[4] Neter, J., Kutner, M.H., Nachtsheim, C. J., and Wasserman,
W., Applied Linear Statistical Models, 4th ed., Irwin/ McGrawHill, Chicago (1996) pp. 1121-47.
[5] Armstrong, J. T., Marinenko, R. B., and Davis, J. M., "A
Simple Method for Determining Optimum Corrections for
High-Accuracy EPMA in Difficult Chemical Systems" in Micros. Microanal. ed. by Voelkl, E., et. al., Cambridge Press, 8,
Suppl. 2, New York (2002,) pp. 438-9.
Table 3. Uncertainty due to Specimen Heterogeneity
Expanded Uncertainty (3 o) in % Mass Fraction
Sample
SiGel4
SiGe6.5
Si&Ge
Wafers
SiKcc TAP1
1.22
1.75
n/a
SiKcc TAPS
1.29
1.55
0.94
GeLa TAP1
1.38
4.67
1.36
GeLa TAP5
1.55
4.64
0.60
Figure 2. Plots of x-ray count data simultaneously taken with the electron microprobe with two different TAP crystals, spectrometers
1 and 5, are compared for specimens SiGel4 (top), SiGe6.5 (middle), and the Si and Ge wafers (bottom). For the SiGe specimens 7
or 8 points were each read twice from five different samples. For the Si and Ge wafers, 7 points were each read twice at the beginning,
in the middle, and at the end of the data acquisition time. All data are plotted in acquisition sequence. Vertical ticks at bottom of plots
denote sample changes for the SiGe specimens, but for the Si and Ge wafers, they designate time breaks between repeated data
acquisitions on the same 7 data points.
242