ASTM F1391-93(2000)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: F 1391 – 93 (Reapproved 2000)
Standard Test Method for
Substitutional Atomic Carbon Content of Silicon by Infrared
Absorption
This standard is issued under the fixed designation F 1391; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This test method is a replacement for Test Method F 123. This current version uses a new coefficient
for the conversion between the absorption-coefficient, carbon peak-height, and the substitutional
carbon content. Also, the curve fitting is done in the carbon-only absorbance spectrum, and it
incorporates an improved method of determining the baseline.
−1
1. Scope from 2000 to 500 cm (5 to 20 μm).
2 1.6 This test method provides procedure and calculation
1.1 This referee test method covers the determination of
sections for the cases where thickness values of test and
substitutional carbon concentration in single crystal silicon.
reference specimens are both closely matched and not closely
Because carbon may also reside in interstitial lattice positions,
matched.
when in concentrations near the solid solubility limit, the
1.7 This standard does not purport to address all of the
results of this test method may not be a measure of the total
safety concerns, if any, associated with its use. It is the
carbon concentration.
responsibility of the user of this standard to establish appro-
1.2 The useful range of carbon concentration measurable by
priate safety and health practices and determine the applica-
this test method is from the maximum amount of substitutional
bility of regulatory limitations prior to use.
carbon soluble in silicon down to about 0.1 parts per million
15 −3
atomic (ppma), that is, 5 3 10 cm for measurements at
2. Referenced Documents
room temperature, and down to about 0.01 ppma, that is,
15 −3
2.1 ASTM Standards:
0.5 3 10 cm at cryogenic temperatures (below 80 K).
E 131 Terminology Relating to Molecular Spectroscopy
1.3 This test method utilizes the relationship between car-
F 1241 Terminology of Silicon Technology
bon concentration and the absorption coefficient of the infrared
absorption band associated with substitutional carbon in sili-
3. Terminology
con. At room temperatures (about 300 K), the absorption band
−1 3.1 Definitions:
peak is at 605 cm or 16.53 μm. At cryogenic temperatures
−1 3.1.1 General definitions for terms related to infrared ab-
(below 80 K), the absorption band peak is at 607.5 cm or
sorption spectroscopy are found in Terminology E 131.
16.46 μm.
3.1.2 Definitions for terms related to silicon materials tech-
1.4 This test method is applicable to slices of silicon with
nology are found in Terminology F 1241.
resistivity higher than 3 V-cm for p-type and higher than 1
3.2 Definitions of Terms Specific to This Standard:
V-cm for n-type. Slices can be any crystallographic orientation
3.2.1 background spectrum,, n—in FT-IR instruments, the
and should be polished on both surfaces.
single-beam spectrum obtained without a specimen in the
1.5 This test method is intended to be used with infrared
infrared light path which is usually obtained with only nitro-
spectrophotometers that are equipped to operate in the region
gen, dry air, or a vacuum in the beam.
3.2.2 baseline,, n—a straight line interpolation between
This test method is under the jurisdiction of ASTM Committee F-1 on
points on either side of the carbon peak of the absorbance
Electronics and is the direct responsibility of Subcommittee F01.06 on Silicon
Materials and Process Control.
spectrum, drawn to represent the spectrum that would have
Current edition approved Dec. 15, 1993. Published February 1994. Originally
been obtained in the absence of the impurity (see Fig. 1).
published as F 1391 – 92. Last previous edition F 1391 – 92.
3.2.3 baseline absorbance,, n—the value of the baseline at
DIN 50438/2 is also a method for measuring the substitutional carbon content
the wavenumber corresponding to the carbon peak which is
of silicon. It differs in some aspects, including different conversion coefficients,
from this proposed test method (see “Related Materials Section,” Vol 10.05). It is the
used for evaluating the absorbance peak height.
responsibility of DIN Committee NMP 221, with which Committee F-1 maintains
close technical liaison. DIN 50438/2, Testing of Inorganic Semiconductor Materials:
Determination of the Impurity Content in Silicon by Means of Infrared Absorption;
Carbon, is available from Beuth Verlag GmbH, Burggrafenstrasse 4-10, D-1000 Annual Book of ASTM Standards, Vol 03.06.
Berlin 30, Federal Republic of Germany. Annual Book of ASTM Standards, Vol 10.05.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
F 1391
cryogenic temperatures, the test slices’ thicknesses can range
from 2.0 to 4.0 mm.
4.2 A reference slice of known low carbon content is
prepared in the same manner.
4.3 After verifying that the instrument is suitably set up,
transmittance spectra of the sample and reference are obtained
−1
over the range from 700 to 500 cm (14.3 to 20.0 μm) on a
double-beam dispersive or single-beam Fourier Transform
Infrared (FT-IR) spectrophotometer in accordance with manu-
facturer’s instructions. Absorbance spectra are derived from
these transmittance spectra and a carbon-only spectrum is
obtained as the difference between the two absorbance spectra.
4.4 A baseline is drawn between the regions on both sides of
the carbon peak on this difference absorbance spectrum, and
absorbance values of both the peak and baseline are recorded.
The absorbance peak height is the difference between these two
values. This peak height, corrected for sample thickness, is
multiplied by a constant to calculate the substitutional carbon
concentration. Two constants are used, one for measurements
at room temperature (300 K), and one for measurements at
cryogenic temperatures (below 80 K).
FIG. 1 Carbon-Only Absorbance Spectrum
5. Significance and Use
5.1 Carbon may have an important role in defect formation
processes. Some laboratories have attributed carbon as being
3.2.4 Fourier transform infrared (FT-IR) spectrometer,
involved in the formation of swirl. Carbon has also been shown
n—type of infrared spectrometer in which the data are obtained
to serve as a nucleation center for the precipitation of oxygen.
as an interferogram.
5.2 Although electrically inactive, substitutional carbon
3.2.4.1 Discussion—An interferogram is a record of the
causes stress that can be observed by X-ray topography.
modulated component of the interference signal measured by
5.3 Direct effects on the reverse bias characteristics of
the detector as a function of retardation in the interferometer.
power devices and annealing problems in neutron transmuta-
This interferogram is then subjected to a Fourier transforma-
tion doped silicon have been associated with carbon.
tion to obtain an amplitude-wavenumber (or wavelength)
5.4 This test method has applicability in production control,
spectrum. FT-IR instruments are always used in conjunction
materials research, quality assurance, and materials accep-
with a computer to control the interferometer, collect and
tance.
manipulate the data, and for spectral output.
3.2.5 FWHM—acronym for full width at half maximum, the
6. Interferences
width of the absorbance band at half its magnitude as measured
from the baseline. 6.1 Stray light that reaches the detector tends to reduce the
3.2.6 reference spectrum,, n—the spectrum of the reference
calculated absorbance value and thereby reduces the reported
specimen. carbon concentration.
3.2.6.1 Discussion—For true double-beam instruments, it is
6.2 The carbon level of the reference slice should be less
15 3
obtained directly with the reference specimen in the sample
than 2 3 10 atoms/cm (0.04 ppma) to minimize the com-
beam and the reference (second) beam empty. For FT-IR and
parative error at room temperature. The detection limit at
other single-beam instruments, it is the result of ratioing the
cryogenic temperatures (below 80 K) is about 0.01 ppma.
single-beam spectrum of the reference specimen to the back-
Obtaining reference samples much below 0.01 ppma in carbon
ground spectrum obtained in 3.2.1.
content may prove to be difficult. Therefore the measurement
3.2.7 sample spectrum,, n—the spectrum obtained when the
of very low carbon content silicon near the 0.01 ppma
sample specimen is placed in the infrared beam.
detection limit is necessarily a comparative measurement only.
3.2.7.1 Discussion—For true double-beam instruments, it is
6.3 Spectrophotometer technique is critical to a successful
obtained directly with the sample in the sample beam and the
carbon determination. The manufacturer’s instrument instruc-
reference (second) beam empty. For FT-IR and other single-
tion manuals should be studied to familiarize the operator with
beam instruments, it is the result of ratioing the single-beam
the proper use of the spectrometer. Since the transmittance at
spectrum of the sample to the background spectrum obtained in
the carbon peak can be very low, while the transmittance at the
3.2.1.
baseline regions is about 40 %, extremely good photometric
linearity is critical. Wavenumber precision is also critical
4. Summary of Test Method
because the carbon peak lies on the shoulder of a very intense
lattice absorption band.
4.1 At room temperature, test slices are prepared that are
polished on both sides to a nominal thickness of 2.0 mm. At 6.4 The FWHM of the carbon absorption band at room
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
F 1391
−1
temperature must be less than 6 cm for acceptable measure- 7.5 Calcium Fluoride Crystal (CaF ), cut to nominal
ments. At cryogenic temperatures (below 80 K), it must be less thickness of 5 mm.
−1
than 3 cm for acceptable measurements. Excessive width
8. Sampling
may be due to improper thickness matching, to stress, or to the
use of a low resolution setting of the instrument. In dispersive
8.1 Unless otherwise specified, a silicon slice used for the
instruments, excessive widths can also result from incorrect carbon test is to be measured at the nominal slice center.
instrument balance setting or too fast a scan speed. In Fourier
9. Test and Reference Specimens
transform instruments, excessive widths can result from the use
of a source-defining aperture that is too wide.
9.1 A single crystal slice of about 2-mm thickness must be
6.5 Specimens that do not exceed the instrument beam size
used at room temperature, and a slice of 2.4 to 3.5-mm
will cause error. Use of apertures at the sample, or preferably
thickness is preferable at cryogenic temperatures.
beam condensers to reduce the beam size at the sample, can
9.2 Both the test and reference specimens must be carefully
correct this problem.
shaped to the following criteria:
6.6 The main two-phonon lattice band of silicon, at about
9.2.1 Thickness variation over the measurement area of
−1
610 cm (16 μm), is very intense; the absorption coefficient for
0.005 mm or less.
−1
this band is about 9 cm at room temperature, and about 5
9.2.2 Identical surface preparation, and
−1 5
cm at 78 K. This broad band peaks close to the wavelength
9.2.3 When the measurement is made with double beam
of the carbon-in-silicon band and so presents a problem in
simple dispersive infrared spectrophotometers, the final thick-
measuring the intensity of the carbon band.
ness of test and reference slices should be equal to within 6
6.7 Reference and test slices must be as close as practically
0.01 mm (see 10.4).
possible to the same temperature to avoid the effects of
9.2.4 When the measurement is made with computer as-
temperature on the intensity of the lattice band.
sisted double-beam dispersive or single-beam FT-IR spectro-
6.8 The minimum detection level of this test method is
photometers, the final thickness of test and reference slices
limited by the signal-to-noise ratio of the spectrum. Thus
should be equal to within 6 0.5 mm (see 10.5).
attaining the highest possible sensitivity by this test method
9.2.5 Surface area large enough such that with respect to the
requires long measurement times and stable spectrophotom-
holders no incident radiation can bypass either the test or
eters.
reference specimen.
6.9 Free carrier absorption in silicon specimens with resis-
9.3 The reference specimen must be selected from float
tivities less than 3 V·cm for p-type, or 1 V-cm for n-type,
zone silicon with a minimal substitutional carbon concentration
reduces the available energy below the level required for
and a carrier concentration such that there is no measurable
satisfactory operation of most spectrophotometers.
free carrier absorption of infrared radiation in the range from
−1
6.10 For samples at cryogenic temperatures (below 80 K),
500 to 2000 cm .
plane parallel, polished surfaces may cause interference fringes
NOTE 1—A satisfactory method of selecting usable reference specimens
on the spectrum. Increasing the sample thickness or inducing a
is to prepare polished slices of equal thickness from many different
wedge (non-flatness) in the sample will reduce these interfer-
low-carbon silicon crystals produced by the float zone method, and then
ence fringes.
compare them to each other in an infrared radiation (IR) spectrophotom-
eter. The selection must be carried out at cryogenic temperatures for
7. Apparatus
cryogenic measurements. The specimen(s) showing the highest relative
−1 −1
transmittance at 605 cm at room temperature, or at 607.5 cm at
7.1 Infrared Spectrophotometer, either a dispersive (com-
cryogenic temperatures (below 80 K), can be used as reference speci-
puterized or non-computerized) or a Fourier transform (FT-IR)
men(s).
spectrophotometer may be used. The resolution of the spectro-
−1
9.4 Both test and reference specimens must have resistivi-
photometer must be at least 2 cm at room temperature, or 1
−1
ties higher than 3 V·cm for p-type silicon and 1 V-cm for
cm at cryogenic temperatures, over the range from 500 to
−1
n-type silicon.
700 cm for either dispersive or Fourier transform infrared
spectrophotometers. The total operating range of the spectro-
10. Procedure
−1
photometer sh
...