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SEMI MF1528-1104 © SEMI 2004 2 5 Terminology 5.1 Definitions 5.1.1 ion mass spectrometry — th e separation and counting of ions by their ma ss-to-charge ratio. 5.1.2 primary ions — ions c reated and focused by an ion gun…
SEMI MF1528-1104 © SEMI 2004 1
SEMI MF1528-1104
TEST METHOD FOR MEASURING BORON CONTAMINATION IN
HEAVILY DOPED n-TYPE SILICON SUBSTRATES BY SECONDARY
ION MASS SPECTROMETRY
This guide 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 August 16, 2004. Initially available at www.semi.org
September 2004; to be published November 2004. Original edition published by ASTM International as
ASTM F 1528-94. Last previous edition SEMI MF1528-94 (Reapproved 1999).
1 Purpose
1.1 Frequently it is essential to control the boron level
in heavily-doped n-type substrates used to make
epitaxial wafers because the boron contamination can
result in autodoping at the epitaxial silicon-substrate
interface.
1.2 SIMS can measure the boron contamination in
heavily-doped n-type substrates.
1.3 This test method can be used for process control,
research and development, and materials acceptance
purposes.
2 Scope
2.1 This test method covers the determination of total
trace boron contamination in the bulk of single crystal,
heavily doped n-type silicon substrates using secondary
ion mass spectrometry (SIMS).
2.2 This test method can be used for silicon in which
the dopant concentrations are less than 0.2% (1 10
20
atoms/cm
3
) for antimony, arsenic or phosphorus
doping. This test method is especially applicable for
silicon where boron is an unintentional p-type
contaminant at trace levels (<5 10
14
atoms/cm
3
).
2.3 This test method can be used for silicon in which
the boron contamination is greater than two times the
SIMS detection limits that is approximately between 5
10
12
atoms/cm
3
and 5 10
13
atoms/cm
3
depending
upon the instrumentation type.
2.4 In principle, different sample surfaces can be used,
but the precision estimate was taken from data on
polished etched surfaces.
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 Boron adsorbed on the surface can interfere with
the boron measurement.
3.2 Boron adsorbed from the SIMS instrument
chamber to the surface can interfere with the boron
measurement.
3.3 The specimen surface must be flat in the specimen
holder windows so that the inclination of the specimen
surface with respect to the ion collection optics is
constant from specimen-to-specimen. Otherwise, the
accuracy and precision can be degraded.
3.4 The accuracy and precision of the measurement
degrade significantly as the roughness of the specimen
surface increases. This degradation can be avoided by
using polished etched surfaces.
3.5 Variability of boron in the calibration standards can
limit the measurement precision.
3.6 Bias in the assigned boron concentration of the
calibration standard can introduce bias into the SIMS
measured boron.
4 Referenced Standards
4.1 SEMI Standards
SEMI M1 — Specifications for Polished
Monocrystalline Silicon Wafers
SEMI MF1241 — Terminology of Silicon Technology
4.2 ASTM Standard
E 122 — Practice for Choice of Sample Size to
Estimate a Measure of Quality of a Lot or Process
1
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
1 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
SEMI MF1528-1104 © SEMI 2004 2
5 Terminology
5.1 Definitions
5.1.1 ion mass spectrometry — the separation and
counting of ions by their mass-to-charge ratio.
5.1.2 primary ions — ions created and focused by an
ion gun onto the specimen surface to sputter ionize
surface atoms.
5.1.3 secondary ion mass spectrometry — mass
spectrometry performed upon secondary ions from the
specimen surface.
5.1.4 secondary ions — ions that leave the specimen
surface as a result of the primary ion beam sputter
ionizing the specimen surface atoms.
5.2 For definitions of other terms used in silicon wafer
technology refer to SEMI M1 and SEMI MF1241.
6 Summary of Test Method
6.1 Secondary ion mass spectrometry (SIMS) is
utilized to determine the bulk contamination of boron in
single crystal, heavily doped n-type silicon substrates.
6.1.1 Specimens of single crystal silicon (one silicon
specimen with very low boron concentration, such as a
high resistivity n-type float-zone silicon specimen, to be
used as an instrumental BLANK; one calibration
specimen made of a bulk-doped boron silicon wafer;
and the test specimens) are loaded into a sample holder.
6.1.2 An oxygen primary ion beam is used to bombard
each specimen.
6.1.3 The positive secondary ions are mass analyzed.
6.1.4 The BLANK silicon sample is sputtered to
determine instrumental background.
6.1.5 The samples are then analyzed for boron and
silicon in a sequential manner throughout the holder.
6.1.6 The ratio of the measured boron and silicon
secondary ion intensities (B
+
/S
+
) is calculated for each
specimen.
6.1.7 The (B
+
/Si
+
) ratios of the test specimens are then
converted to boron concentrations by linear scaling
from the (B
+
/Si
+
) ratio of the calibration specimen that
has a known boron concentration.
6.1.8 No crater measurement is required.
7 Apparatus
7.1 Magnetic Sector SIMS Instrument, equipped with
an oxygen primary ion source, electron multiplier
detector, and Faraday cup detector or daley detector
capable of measuring positive secondary ions. The
SIMS instrument should be adequately prepared (that
is, baked) so as to provide the lowest possible
instrumental background.
7.2 Liquid Nitrogen or Liquid Helium Cooled
Cryopanel, to surround the test specimen holder in the
analysis chamber if the analysis chamber vacuum is
10
8
torr or higher. For instruments with vacuum less
than 10
8
torr the cooling is not required.
7.3 Test Specimen Holder, to keep the specimen(s)
analysis surface planar and perpendicular to the several
kV extraction field, depending on the instrument type.
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.
8.1.1 No general sampling procedure is included as
part of this test method, because the most suitable
sampling plan varies considerably depending upon
individual conditions. See ASTM Practice E 122 for
suggested choices of sampling plans.
8.1.2 For referee purposes, a sampling plan shall be
agreed upon between the parties to the test before
conducting the test.
9 Specimen Requirements
9.1 Sample specimens must be flat and smooth on the
side used for analysis. The surface shall have a polish
etch surface or better (that is, chem-mechanically
polished).
9.2 Sample specimens must be cleaved or diced to fit
within the sample specimen holder.
9.3 A bulk boron-doped silicon calibration specimen is
required.
9.4 A BLANK silicon specimen with boron
concentration below 5 10
12
atoms/cm
3
must be
present.
10 Calibration
10.1 The calibration standard must be present with
bulk boron concentration between 1 and 10 10
14
atoms/cm
3
as determined by some other measurement
that is agreed upon by the parties.
10.2 Each calibration standard specimen must be the
same size and have the same polished surface as the test
specimen.
10.3 Each BLANK specimen must be the same size
and have the same polished surface as the test
specimen.
SEMI MF1528-1104 © SEMI 2004 3
11 Procedure
11.1 Specimen Loading
11.1.1 Load the specimens into the SIMS sample
holder checking to see that the specimens are flat
against the backs of the windows and cover the
windows as much as is possible. A specimen load
includes: one BLANK silicon specimen, one calibration
specimen, and the test specimens.
11.2 Instrument Tuning
11.2.1 Turn on the instrument in accordance with the
manufacturer's instructions.
11.2.2 Fill the liquid nitrogen or helium cold trap if
cooling equipment is used as stated in Section 7.2.
11.2.3 Analytical Conditions
11.2.3.1 Use a focused oxygen primary-ion current and
adjust the contrast diaphragm and field aperture to
maximize the
30
Si
+
-ion count rate. The rate must be
greater than 1 10
8
counts/s as measured on the
Faraday cup or daley detector, and with no raster.
11.2.3.2 Use a first raster test condition of several
hundred micrometers by several hundred micrometers,
depending upon the beam radius, (typical condition is
250-m by 250-m raster) to remove surface boron in
the native oxide. For the actual measurement, use a
second raster test condition reduced several times from
the first raster condition (typical second raster condition
is 50-m by 50-m). Use integration times of 1 s.
11.3 Analysis of Specimen
11.3.1 Position the specimen holder such that the
sputtered crater in the specimen forms near the center
of the window.
11.3.2 Center the primary ion beam and begin a SIMS
profile.
11.3.2.1 Sputter the sample using the first raster
condition for 50 to 100 cycles of the magnet until the
boron intensity is stable to remove any residual surface
contamination found typically in the native oxide of all
wafers.
11.3.2.2 Decrease the rastered area to the second raster
condition and continue sputtering until the boron signal
is stable.
11.3.3 At the end of the profile, measure and record the
11
B
+
count rate on the electron multiplier detector and
the
30
Si
+
matrix intensity on the Faraday cup detector or
daley detector, averaged over the last 15 cycles.
11.3.4 Repeat the above steps for all the specimens in
the holder until all the specimens have been sputtered.
11.3.5 Calculate the ratio (
11
B
+
/Si
+
) of boron count rate
to silicon intensity using the recorded secondary ion
intensities at the end of each profile. Record each
calculated ratio as S
u
.
11.3.6 If the measured ratio (
11
B
+
/Si
+
) for the BLANK
specimen exceeds 20 to 50% of the ratio (
11
B
+
/Si
+
) of
the other specimens, abort the analysis and find the
cause of the high instrumental background.
11.3.7 For all specimens including the BLANK,
calibration, and test specimens, record the specimen
identification, and the (
11
B
+
/Si
+
) ratios, in a table.
12 Calculations
12.1 Relative Sensitivity Factor (RSF) Calibration
12.1.1 Calculate the relative sensitivity factor RSF as
follows:
)/SiB(
[B]
RSF
11
(1)
where:
[B] = assigned boron concentration in the calibration
standard,
11
B
+
= the
11
B ion count on the electron multiplier, and
Si
+
= ion intensity on the Faraday cup, daley detecto
r
or other detector capable of measuring positive
secondary ion currents above the rate of 10
8
counts/s.
12.1.2 Convert the SIMS ion count (
11
B
+
/Si
+
) ratios,
(S
u
), for each test specimen to boron concentration [B]
u
by multiplying the S
u
by the relative sensitivity factor
derived from the calibration standard:
RSFS[B]
uu
(2)
13 Report
13.1 Report the following information:
13.1.1 The instrument used, the operator, the date of
the measurements, and the boron relative sensitivity
factor,
13.1.2 Identification of test and standard specimens
and the location of the test specimen with respect to the
crystal, and
13.1.3 The boron concentration values for the test
specimens and the BLANK silicon specimen.
14 Precision and Bias
14.1 Precision — The precision was estimated for this
measurement using over 900 samples, ten analysts, and