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SEMI MF1617-0304 © SEMI 2003, 2004 2 from plastic cassette containers. The relative importance of the organic i nterference is dependent upon the sur face organics of the test wafer. 3.1.2 Another significant i n terfer …

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SEMI MF1617-0304 © SEMI 2003, 2004 1
SEMI MF1617-0304
TEST METHOD FOR MEASURING SURFACE SODIUM, ALUMINUM,
POTASSIUM, AND IRON ON SILICON AND EPI SUBSTRATES BY
SECONDARY ION MASS SPECTROMETRY
This test method was technically approved by the Global Silicon Wafer Committee and is the direct
responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by
the North American Regional Standards Committee on December 4, 2003. Initially available at
www.semi.org February 2004; to be published March 2004. Originally published by ASTM International as
ASTM F 1617-95. Last previous edition SEMI MF1617-98 (Reapproved 2002).
1 Purpose
1.1 Secondary ion mass spectrometry (SIMS) can
measure on polished silicon wafer product the
following:
(1) the sodium and potassium areal densities that
can affect voltage flatband shifts in integrated circuits,
and,
(2) the aluminum areal density that can affect the
thermal oxide growth rate.
(3) the iron areal density that can affect gate oxide
integrity, minority carrier lifetime, and current leakage.
1.2 The SIMS measurement facilitates the production
of silicon wafers with upper control limits on sodium,
potassium, aluminum, and iron areal densities.
1.3 This test method can be used for monitoring a
mirror-polished wafer cleaning process, for research
and development, and for materials acceptance
purposes.
1.4 This test method can provide spatial information
for these metal contaminants, including near-edge
substrate contamination levels.
1.5 This test method is especially useful for
determining the surface metal areal densities in the
native oxide or chemically grown oxide of polished
silicon substrates after cleaning.
2 Scope
2.1 This test method covers the determination of total
sodium, aluminum, potassium, and iron on the surface
of mirror-polished single crystal silicon and silicon epi
substrates using SIMS. This test method measures the
total amount of each metal, because this test method is
independent of the metal' s chemistry or electrical
activity.
2.2 This test method can be used for silicon with all
dopant species and dopant concentrations.
2.3 This test method is especially designed to be used
for surface metal contamination that is located within
approximately 5 nm of the surface of the wafer.
2.4 This test method is useful for sodium, aluminum,
potassium, and iron areal densities between 10
9
and
10
14
atoms/cm
2
. The limit of detection is determined by
either the BLANK value or by count rate limitations,
and may vary with instrumentation.
2.5 This test method is complementary to:
2.5.1 Total reflection X-ray fluorescence (TXRF), that
can detect higher atomic number Z, surface metals such
as iron, but does not have useful (<10
11
atoms/cm
2
)
detection limits for sodium, potassium, and aluminum
on silicon.
2.5.2 Vapor phase decomposition (VPD) of surface
metals followed by atomic absorption spectroscopy
(AAS) or inductively coupled plasma mass
spectrometry (ICP-MS) of the VPD residue, where the
metal detection limits are 10
8
to 10
10
atoms/cm
2
. There
is no spatial information available and the VPD
preconcentration of metals is dependent upon the
chemistry of each metal.
NOTICE: This standard does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the user of this standard to establish
appropriate safety and health practices and determine
the applicability of regulatory or other limitations prior
to use.
3 Limitations
3.1 Surface metal contamination of sodium, aluminum,
potassium or iron introduced during handling of the test
specimen or during the measurement itself introduces a
bias to the measurement. (Particulate room
contamination containing these metals is easily
distinguished from metal contamination by the shape of
the SIMS profile that should have the log of the signal
drop linearly with time. A particulate contamination
does not follow this shape under SIMS profile.)
3.1.1 The discrimination of elemental ions from
molecular ions is particularly important for the
27
Al
+
signal that has a significant interference below about
10
11
to 10
12
atoms/cm
2
from ubiquitous C
2
H
3
+
molecular ions that may arise from clean room air or
SEMI MF1617-0304 © SEMI 2003, 2004 2
from plastic cassette containers. The relative
importance of the organic interference is dependent
upon the surface organics of the test wafer.
3.1.2 Another significant interference occurs from
ubiquitous BO
+
when the aluminum is in the range of
10
9
to 10
10
atoms/cm
2
, since surface boron at the 10
12
atoms/cm
2
range is common for all wafers, both n-type
as well as p-type.
1
3.1.3 If the surface contains high levels of sodium,
there may be a NaO
+
molecular interference for
39
K
+
.
In principle,
11
B
12
C
+
and
11
B
28
Si
+
can be a molecular
interferences for
23
Na
+
and
39
K respectively.
3.1.4 The
54
Fe
+ 1
signal can have interferences from
27
Al
2
+ 1
or
54
Cr
+ 1
.
3.1.5 Discrimination of molecular ion interferences can
be achieved using magnetic mass spectrometers
operated under high mass resolution or in some cases
using quadrupole mass spectrometers via energy
filters.
2
3.2 Bias in the relative sensitivity factors (RSFs)
derived from reference materials can introduce bias into
the SIMS measured areal densities.
3.3 Mass interferences can introduce bias if the
instrument mass resolution, or subsequent detection
scheme, is not sufficient to exclude the interference.
3.4 The SIMS sodium, aluminum, potassium, and iron
instrumental backgrounds can limit the detection of low
levels of surface metals.
3.5 The accuracy and precision of the measurement can
be significantly degraded by analysis of specimens
whose surfaces are not all at the same inclination with
respect to the ion collection optics of the SIMS
instrument. The specimen holder must be constructed
and maintained such that after specimen(s) are loaded
into the holder, the inclination of the surface of each
specimen is constant from specimen to specimen.
3.6 The accuracy and precision of the measurement
significantly degrade as the roughness of the specimen
surface increases. This degradation can be avoided by
using chem-mechanical polished surfaces.
3.7 If an oxygen leak is not used in conjunction with
the measurement, there may be a bias due to the effect
1 Mollenkopf, H., “Chemicals and Cleanroom Filtered Air Effects on
Boron Contamination and Its Near Surface Detection in Silicon
Wafers,” Extended Abstracts, Vol 93-2, Abstract No. 170, The
Electrochemical Society, Pennington, NJ, 1993, pp. 273–274.
2 Frost, M. R., “On the Use of Quadrupole SIMS for the
Measurement of Surface Metallic Contamination,” Contamination
Control and Defect Reduction in Semiconductor Manufacturing III,
ECS Proceedings Volume 94-9, edited by D. N. Schmidt (The
Electrochemical Society, Pennington, NJ, 1994) pp. 339–350.
of different chemical native-oxide thicknesses upon ion
yields. This effect has not been studied.
4 Referenced Standards
4.1 ASTM Standards
E 122 — Practice for Choice of Sample Size to
Estimate a Measure of Quality for a Lot or Process
3
E 673 — Terminology Relating to Surface Analysis
4
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 All terms in this test method are in conformance
with those given in ASTM Terminology E 673.
6 Summary of Test Method
6.1 Specimens of mirror-polished single crystal silicon
are loaded into a sample holder. The holder is
transferred into the analysis chamber of the SIMS
instrument.
6.2 A primary ion beam, typically O
2
+
, is used to
bombard each specimen with a sputter rate less than
0.015 nm/s (0.9 nm/min).
6.3 The area of analysis may be different for different
instruments and may range from 100 µm × 100 µm to 1
mm × 1 mm.
6.4 Depending upon instrumentation, a molecular
oxygen jet or leak may be focused on the analysis area.
6.5 The positive secondary ions
23
Na,
27
Al,
39
K, and
54
Fe are mass analyzed by a mass spectrometer, and
detected by an electron multiplier (EM) or equivalent
high-sensitivity ion detector as a function of time until
the signals reach background levels or 1% of the initial
signal rates of each element. The instrumentation must
be able to discriminate the elemental ion signals from
molecular interferences.
6.6 A BLANK silicon sample is used to evaluate
whether the lower limit of detection arises from
molecular ion interferences, elemental instrumental
backgrounds, or count rate limitations. The matrix
positive secondary ion count rate for silicon (
28
Si,
29
Si,
or
30
Si) is measured by a faraday cup (FC) or
appropriate detector during, or at the end of, the profile.
If multiple detectors are used during the test, the
relative sensitivities of the detectors are determined by
measuring standard ion signals (either the same positive
3 Annual Book of ASTM Standards, Vol 14.02, ASTM International,
100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone:
610-832-9500, Fax: 610-832-9555, Website:
www.astm.org
4 Annual Book of ASTM Standards, Vol 03.06.
SEMI MF1617-0304 © SEMI 2003, 2004 3
secondary ion count rate or ion count rates of known
relative intensity such as natural
28
Si/
30
Si) on each
detector.
6.7 The net integrated
23
Na,
27
Al,
39
K, and
54
Fe signals
are converted to quantitative areal densities using the
detector efficiency ratios (if multiple detectors are used)
and RSFs measured from reference materials.
34
7 Apparatus
7.1 SIMS Instrument — Equipped with a primary ion
beam, preferably O
2
+
, a mass spectrometer with some
method of discriminating molecular ion interferences
from elemental ions of interest, an electron multiplier
detector, or faraday cup detector, or similar detector
system capable of measuring secondary ion count rates,
or any combination thereof, and the ability to sputter
the surface at less than 0.015 nm/s. An molecular
oxygen jet may be used to stabilize the surface ion
yield; the local oxygen gas flux to the specimen surface
must be stable enough that the secondary ion yield does
not vary during the analysis. The stability of the
oxygen jet effect can be checked by monitoring a
silicon matrix signal during a profile; and if the matrix
signal is not monitored during a profile, the vacuum
chamber pressure can be monitored for fluctuations in
the oxygen gas pressure. The SIMS instrument should
be adequately prepared and maintained so as to provide
the lowest possible instrumental backgrounds.
7.2 Test Specimen Holder — Reserved for the SIMS
measurement. In some instruments the holder can
support multiple 5 by 5-mm samples that are held face
down against metal (tantalum) windows. In other
instruments the holder can support one or more 15 by
15-mm samples by spring clips on the edge of the
specimen or silver paste on the sample back surface.
Some instruments can hold full silicon wafers. In all
cases, the sample holder should be adequately prepared
and maintained so as to provide the lowest possible
contribution to instrumental backgrounds.
7.3 Stylus Profilometer — Or equivalent device (for
example, atomic force microscope) to measure SIMS
crater depths. This is required to calibrate depth scales
for concentration profiles of calibration standard
samples. For routine depth scale calibration of test
specimens, the instrument must be capable of crater
depth measurements of 10 nm with an accuracy and
precision of 10%. In the absence of such an instrument,
a secondary depth calibration standard (such as a silicon
wafer with a known impurity depth profile) must be
prepared and can be used to measure the SIMS sputter
rate during the measurement.
NOTE 1: The accurate measurement of sputter crater depths
on the order of and below 10 nm is still an area of research,
because the sputtered silicon is very reactive to oxygen in the
air when removed from the SIMS chamber for crater depth
measurement. For example, sputter craters expected to be
only 1 nm in depth can actually be thicker than the original
native oxide due to this rapid oxide growth on sputtered
silicon, that is, one observes a bump on the surface where the
erosion was, rather than a crater. If a reference sample is
similar to the unknown in type, that is, intentional surface
contamination rather than shallow ion implant, it may be
possible to avoid the need for a direct shallow crater depth
measurement on each sample.
8 Sampling
8.1 Since this procedure is destructive in nature, a
sampling procedure must be used to evaluate the
characteristics of a group of silicon wafers. No general
sampling procedure is included as part of this test
method, because the most suitable sampling plan will
vary considerably depending upon individual
conditions. For referee purposes, a sampling plan shall
be agreed upon before conducting the test. See ASTM
Practice E 122 for suggested choices of sampling plans.
9 Specimen Requirements
9.1 Sample specimen surfaces must be flat and smooth
(see Sections 3.5 and 3.6).
9.2 Sample specimens must be cleaved or diced to fit
within the sample specimen holder. Specimens need
not be all the same size, but typical specimen sizes are
approximately square with one side of dimension 5 to
15 mm.
9.3 Prepare a BLANK specimen with a level of surface
metal below that expected to be detected by this test
method, and which has surface organics of a level and
type expected for the unknown specimens.
NOTE 2: If measurements are to be made on specimens
taken soon after a cleaning process, the organics on the
surface from clean room air and cassette carriers are very low
and negligible. However, if the specimens are stored for one
day or more, the level of organics may be significant for a
molecular ion interference in the measurement
5
; this
interference must be removed by the detection scheme.
10 Calibration
10.1 The calibration is by RSFs which are determined
empirically from reference materials.
10.1.1 Reference materials may be either:
10.1.1.1 Spin Coat Contaminated Samples —
Calibrated by Vapor Phase Decomposition/Atomic
5 Muller, A. J., Sinclair, J. D., Psota-Kelty, L. A., and Krauttler, H.
W., “Measurement of Airbourne Concentrations and Surface Arrival
Rates of Organic Contaminants in Clean Rooms,” 1993 Proceedings,
Vol 1 (Institute of Environmental Sciences, Mount Prospect, IL,
1993) pp. 373–379.