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SEMI MF1617-0304 © SEMI 2003, 2004 5 contaminants from the free s urface or native oxide layer on the specimen should ordinarily reach a m aximum within the first 1 nm of the depth profile, a nd decrease monotonical ly t…

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SEMI MF1617-0304 © SEMI 2003, 2004 4
Absorption Spectroscopy (VPD/AAS) or Vapor Phase
Decomposition/Inductively-Coupled-Plasma Mass
Spectrometry (VPD/ICP-MS) and shown by qualitative
SIMS to be spatially uniform, since spatial non-
uniformity can introduce variability or bias to the SIMS
quantitative measurement. With a spin coat
contaminated reference sample, it may be possible to
avoid the need for the crater depth measurement and its
bias/variability. For this reference sample, the areal
density D to be used in Equation 1 (see Section 12.1) is
the elemental areal density determined by the
VPD/AAs or VPD/ICP-MS, corrected for isotopic
abundance since the SIMS measurement of SI
i
is
isotopic.
10.1.1.2 Silicon Wafers — Dipped in an intentionally
contaminated SC-1 (NH
4
OH:H
2
O
2
:H
2
O) bath where
the calibration of the metal deposition is by VPD/AAS
or VPD/ICP-MS and shown by qualitative SIMS, or by
TXRF for iron, to be spatially uniform, since spatial
non-uniformity can introduce variability or bias to the
SIMS quantitative measurement. With an SC-1 dipped
reference sample, it may be possible to avoid the need
for the crater depth measurement and its
bias/variability. For this reference sample the areal
density D to be used in Equation 1 (see Section 12.1) is
the elemental areal density determined by the
VPD/AAS or VPD/ICP-MS, corrected for isotopic
abundance since the SIMS measurement of Si
i
is
isotopic.
10.1.1.3 Ion Implants — Such as
23
Na,
27
Al,
39
K, and
56
Fe, (or
54
Fe) implanted into silicon and shown by
qualitative SIMS, or by TXRF for iron to be spatially
uniform, since spatial non-uniformity can introduce
variability or bias to the SIMS quantitative
measurement. It is necessary to demonstrate that RSFs
obtained from these reference materials and the
instrumentation and operation used for the
measurement are consistent with RSFs from spin coated
reference samples.
5
For this reference sample, the areal
density D to be used in Equation 1 (see Section 12.1) is
the implanted isotopic dose.
11 Procedure
11.1 Specimen Loading and SIMS Instrument Set-up
11.1.1 Cleave or dice individual specimens (unknowns,
reference, BLANK) to fit within the sample holder.
The reference sample must contain
23
Na,
27
Al,
39
K, and
56
Fe (or
54
Fe) or there must be multiple reference
samples, each with one or more of these elements. Do
this preparation in such a way as to minimize
contamination of the specimens with surface metals.
11.1.2 Load the specimen(s) into the SIMS sample
holder.
11.1.3 Transfer the loaded holder into the SIMS
instrument sample chamber.
11.1.4 Turn on the instrument in accordance with the
manufacturer' s instructions.
11.1.5 Set the appropriate analytical conditions
including the method for molecular ion mass
interference discrimination.
11.1.5.1 Select a primary ion current, primary beam
raster size, and secondary spectrometer transmission,
which will result in an appropriate sputter rate (less
than 0.015 nm/s).
11.1.5.2 Select the secondary spectrometer conditions
so that the maximum secondary ion-count rates result in
dead time losses below 10%.
6
11.1.6 Confirm that analytical conditions are
appropriate to the measurement goals by measurement
of known reference standard(s) and BLANK sample.
11.1.6.1 Confirm that the analytical sputter rate is such
that each element monitored in a single profile is
sampled at a frequency greater than or equal to once
every 0.2 nm of sputtered depth.
11.1.6.2 If an oxygen jet is used, confirm that 0-leak
pressure is suitable by measuring a matrix ion depth
profile and demonstrating that matrix secondary ion
yield is constant (within 20%) during the analysis of the
first 10 nm of a specimen. Perform this confirmation
measurement on a typical specimen at the same sputter
rate to be used for the measurement of the surface metal
impurities. If the ion yield shows significant variation,
increase the 0-leak pressure by a factor of 2× and
reconfirm stability.
11.1.6.3 Depending upon the instrumentation used and
if needed, measure the detector efficiency ratio(s) for
any detectors used during the measurement (for
example, the electron multiplier and the faraday cup
detectors). Make this measurement by intercomparing
standard secondary ion signals of the appropriate
intensity (to minimize dead-time losses). The
secondary ion count rates used here may be different
for this measurement than for the analysis, and may be
generated under different sputter rate conditions than
the analysis.
11.2 Analysis of Specimen
11.2.1 Center the primary ion beam, initialize the
SIMS instrument control software, and begin a SIMS
profile. For sodium, aluminum, potassium, and iron,
the secondary ion signals generated by sputtering these
6 Liu, Y. A., and Fleming, R. H., “Reduced Electron Multiplier Dead
Time in Ion Counting Mass Spectrometry,” Rev. Sci. Instr, 64, 1661
(1993).
SEMI MF1617-0304 © SEMI 2003, 2004 5
contaminants from the free surface or native oxide layer
on the specimen should ordinarily reach a maximum
within the first 1 nm of the depth profile, and decrease
monotonically thereafter. If this general profile shape
is not obtained, repeat the measurement on a fresh
surface of the specimen.
11.2.2 When the impurity signal has either: (1)
decreased to at least <1% of the maximum, (2) or
reached a constant background count rate, then measure
and record the silicon matrix ion count rate on the
appropriate detector.
11.2.3 Measure the sputter rate under the analysis
conditions. This can be done by measuring the SIMS
crater depth with a stylus profilometer or equivalent
instrument if available, and combining the depth
measurement with the recorded time elapsed during the
sputter profile. If the analytical craters are too shallow
to be measured by available instrumentation, the sputter
rate must be monitored during a measurement series by
repeated analysis of a previously characterized standard
sample.
11.2.4 Measure the BLANK under the same analytical
conditions as the unknown specimens.
12 Calculations
12.1 The RSF for an element in a matrix can be
determined experimentally from the SIMS profile of a
reference material containing a known areal density of
the impurity of interest as follows:
()
NISId
tDNI
bi
m
−
=RSF (1)
where:
D = the areal density of the impurity in
atoms/cm
2
,
N = the number of data cycles in the profile,
I
m
= the matrix isotope secondary ion intensity,
(counts/s),
d = the sputtered depth, cm,
SI
i
= the sum of the impurity isotope secondary
ion-counts over the depth of the profile,
I
b
= a constant background intensity of the
impurity isotope, and
t = the analysis time for the species of interest,
s/cycle.
12.2 The calibration of concentration in a SIMS profile
is calculated as follows:
=
EM
FC
I
I
C
m
i
i
RSF (2)
where:
C
i
= the impurity atomic concentration,
atoms/cm
3
,
I
i
= the isotope secondary in intensity, counts/s,
I
m
= the matrix isotope secondary ion-intensity,
counts/s,
RSF = relative sensitivity factor, atoms/cm
3
, and
FC
EM
= the ratio of matrix intensities on the faraday
cup (FC) to the electron multiplier (EM),
when two detectors are used.
12.3 The conversion of data cycles or time into depth is
completed by measuring the crater depth and the total
time (or data cycles) of the sputtering that formed the
crater. The depth scale is assumed to be linear with
time or data cycles.
12.4 Calculate the RSFs and impurity concentration
versus depth from Equations 1 and 2.
12.5 Calculate the surface areal density by integrating
the concentration versus depth profile after any
subtraction of a constant background count rate that
may be appropriate.
13 Report
13.1 Report the following information:
13.1.1 The instrument used, the operator, and the date
of the measurements,
13.1.2 Identification of test, BLANK, and reference
specimens,
13.1.3 Calibration procedure used,
13.1.4 Method used to discriminate molecular ion
interferences,
13.1.5 Surface areal density of the impurity, and
13.1.6 Surface areal density of the impurity for the
BLANK specimen.
14 Precision and Bias
7
14.1 Precision — Tables 1, 2, and 3 summarize the
statistics from a spoke-wheel round robin including 4
test samples and 10 laboratories. Each laboratory was
provided with a spin coated reference standard of
surface sodium, aluminum, and potassium. The 95%
repeatability, r, and reproducibility, R, limits are
7 Support data are available from SEMI Headquarters. Request
SEMI MF1617 round robin report (formerly ASTM RR: F01-1010).
SEMI MF1617-0304 © SEMI 2003, 2004 6
calculated as 2.8 times their respective standard
deviations (s
r
, S
R
).
14.1.1 SIMS measurements were made on samples
taken from an iron contaminated silicon wafer to
estimate the precision of the surface iron measurement.
Two SIMS instruments were used: a CAMECA IMS 3f
and a CAMECA IMS 4f. All measurements used a 3-
keV oxygen ion beam with an oxygen flood.
Measurements were taken over a one-year period. For
the CAMECA IMS 3f, 23 measurements were taken
with an average iron reading of 1.51 × 10
11
atoms/cm
2
and a 1 standard deviation of 0.148 × 10
11
atoms/cm
2
.
For the CAMECA IMS 4f, 20 measurements were
taken with an average iron reading of 1.71 × 10
11
atoms/cm
2
and 1 standard deviation of 0.188 × 10
11
atoms/cm
2
.
14.2 Bias — Bias cannot be estimated because there
are no accepted absolute standards. However, for
comparison only, the VPD/AAS results for the sodium,
aluminum, and potassium test samples are included in
Tables 1–3. A correlation between SIMS and TXF
quantitive results for iron is shown.
8
15 Keywords
15.1 aluminum; iron; potassium; silicon; SIMS;
sodium; surface contamination
Table 1 Summary Statistics for Sodium (Units of
10
10
Atoms/cm
2
)
Sample VPD/AAS X-Bar s
r
S
R
r R
A 10–15 10.68 2.240 2.791 6.273 7.816
B 29–32 33.64 3.339 4.931 9.348 13.81
C 115–121 112.1 16.19 19.79 45.32 55.41
E 0.6–4 0.665 0.359 0.861 1.005 2.410
Table 2 Summary Statistics for Aluminum (Units of
10
10
Atoms/cm
2
)
Sample VPD/AAS X-Bar s
r
S
R
r R
A 3 3.504 0.6865 1.107 1.922 3.100
B 7–8 9.425 0.7684 2.239 2.152 6.269
C 22–25 28.80 2.298 6.107 6.434 17.10
E not detected 0.795 0.390 0.760 1.093 2.128
8 Smith, S. P., and Metz, J., “Understanding the Correlation of
Surface SIMS and TXRF Measurements of Surface Metal
Contamination on Silicon Wafers,” Science and Technology of
Semiconductor Surface Preparation, edited by G. S. Higashi, M.
Hirose, S. Raghavan, and S. Verhaverbeke, Material Research
Society Symposium Proceedings, Vol 477 (Materials Research
Society, Pittsburgh, PA, l997) pp. 305–310.
Table 3 Summary Statistics for Potassium (Units of
10
10
Atoms/cm
2
)
Sample VPD/AAS X-Bar s
r
S
R
r R
A 7–8 7.829 1.642 2.761 4.596 7.731
B 22–23 24.11 3.607 5.943 10.10 16.64
C 92 82.37 12.77 19.63 35.74 54.97
E 0.1–2 0.407 0.162 0.344 0.454 0.965
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