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SEMI MF1188 © SEMI 2003, 2005 2 Despite the fact t hat IOC-88 has been adopte d by all of the ma jor standards devel oping organizati ons active in the sem iconductor field, many ol der calibration fa ctors that have bee…
SEMI MF1188-0305 © SEMI 2003, 2005 1
SEMI MF1188-0305
TEST METHOD FOR INTERSTITIAL OXYGEN CONTENT OF SILICON
BY INFRARED ABSORPTION WITH SHORT BASELINE
These test methods were technically approved by the Global Silicon Wafer Committee and are the direct
responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by
the North American Regional Standards Committee December 10, 2004 Initially available at
www.semi.org
January 2005; to be published March 2005. Original edition published by ASTM International as ASTM F
1188-88. Last previous edition SEMI MF1188-02.
INTRODUCTION
Prior to 2002, ASTM Test Method F 1188 used a long baseline for the 1107 cm
1
oxygen
absorption peak. This long baseline was drawn between the average transmittances in the regions
from 900 to 1000 cm
1
and from 1200 to 1300 cm
1
. The same baseline was used in ASTM Test
Method F 121, the predecessor to Test Method F 1188, and also in the analysis of infrared data
from the Grand Round Robin (GRR) experiment.
1
The GRR experiment resulted in a calibration
factor between the infrared absorption coefficient at 1107 cm
1
and the absolute interstitial oxygen
content known as “IOC-88.” This calibration factor is utilized in the current editions of all known
standardized test methods for oxygen content of silicon, including DIN 50438, Part 1; and JEITA
EM-3504; as well as the current Chinese National Standard (GB).
The long baseline method is being retained in the form published in the 1993a edition of this test
method in Appendix 1. However, the long baseline is subject to uncertainties due to perturbations
in the IR absorption at the end-point regions arising from effects other than absorption by
interstitial oxygen. Use of a shorter baseline, drawn between 1040 and 1160 cm
1
, which is less
affected in this manner,
2
results in improved precision of the method. Use of the shorter baseline
is introduced into this test method to replace the long baseline method. Although the use of this
baseline results in a somewhat smaller net absorption coefficient for the same oxygen content, this
change is not very large, and can be neglected in most cases. Optionally, to completely eliminate
the effect of the baseline changes, the measurement equipment may be calibrated with suitable
certified reference materials (CRMs) or reference materials derived from CRMs.
NIST SRM
3
2551 oxygen-in-silicon reference material,
4,5
which is the principal CRM used for
calibrating commercial infrared spectrophometers for oxygen measurements, was certified using a
subset of the GRR specimens as the absolute references. Both the GRR specimens and the SRM
specimens were measured using a short baseline similar to that now incorporated in this test
method. This resulted in (1) reduced measurement uncertainty for the SRM specimens, and (2) a
strong metrological foundation provided by the GRR specimens. Consequently, the IOC-88
results are available to the users of the SRMs without requiring a change in the calibration factor
even though the short-baseline method is used.
1 Baghdadi, A., Bullis, W. M., Coarkin, M. C., Li Yue-zhen, Scace, R. I., Series, R. W., Stallhofer, P., and Watanabe, M., “Interlaboratory
Determination of the Calibration Factor for the Measurement of the Interstitial Oxygen Content of Silicon by Infrared Absorption,” J.
Electrochem. Soc. 136, 2015–2034 (1989). Baghdadi, A., Scace, R. I., and Walters, E. J., “Semiconductor Measurement Technology: Database
for and Statistical Analysis of the Interlaboratory Determination of the Calibration Factor for the Measurement of the Interstitial Oxygen Content
of Silicon by Infrared Absorption,” NIST Special Publication 400-82, July 1989.
2 Series, R. W., “Determination of Oxygen and Carbon in Silicon,” RSRE Memorandum No. 3479 (Royal Signals & Radar Establishment,
Malvern, Worcs, UK, 1982).
3 SRM is a registered trademark of the National Institute of Standards and Technology.
4 Available as a set of four 25 mm square, 2 mm thick, silicon specimens, mirror polished on both sides. One of the specimens is a high
resistivity float zoned specimen with negligible oxygen content. The other three specimens were cut from n-type crystals grown by a modified
Czochralski process. Their room temperature resistivity is > 3 ·cm, and they have nominal oxygen content of 17, 23, and 26 parts per million
atomic (IOC-88). The specimens are mounted on aluminum disks for ease of use in production spectrophotometers.
5 Details of the certification of this SRM are given in Rennex, B. G., “Standard Reference Materials: Certification of a Standard Reference
Material for the Determination of Interstitial Oxygen Concentration in Semiconductor Silicon by Infrared Spectrophotometry,” NIST Spec. Publ.
260-121 (Aug. 1994).
SEMI MF1188 © SEMI 2003, 2005 2
Despite the fact that IOC-88 has been adopted by all of the major standards developing
organizations active in the semiconductor field, many older calibration factors that have been used
in earlier standards issued by ASTM and other standards development organizations are still in use
in the industry today. Tables showing the relations between the IOC-88 interstitial oxygen
calibration factor and other standardized calibration factors that have been used in the silicon
semiconductor industry can be found in SEMI M44. It should be emphasized that these factors are
at times referred to in the literature by common names and at other times by the designation of the
standard where they were used. Furthermore, in the jargon of the industry, interstitial oxygen
content is frequently described as being determined in accordance with a particular standardized
method whereas, in actual fact, only the calibration factor is taken from the standard while the
measurement itself is made by whatever method is employed within the instrumentation used.
1 Purpose
1.1 The presence of oxygen can be beneficial to certain manufacturing operations by preventing the formation of
process-induced defects. Oxygen is introduced into silicon wafers during the crystal growing process. Hence, it is
important to control the oxygen content of silicon crystals.
1.2 Measurement of the intensity of the 1107 cm
1
oxygen-in-silicon band with an infrared spectrophotometer
enables the determination of the value of the absorption coefficient and, hence, by the use of a calibration factor or
linear regression curve, the content of interstitial oxygen.
1.3 This test method can be used as a referee method for determining the interstitial oxygen content of silicon slices.
Knowledge of the interstitial oxygen content of silicon wafers is necessary for materials acceptance and control of
fabrication processes, as well as for research and development.
2 Scope
2.1 This test method covers the determination of the interstitial oxygen content of single crystal silicon by
measurement of an infrared absorption band at room temperature. This test method requires the use of an oxygen-
free reference specimen. It is recommended that a reference material set, such as NIST SRM
3
2551,
4
another
certified reference material set for oxygen content of silicon,
6
or reference materials traceable to the CRMs, be used
to calibrate the spectrophotometer in order to reduce bias.
2.2 This test method requires the use of a computerized spectrophotometer, preferably a Fourier Transform Infrared
(FT-IR) spectrophotometer. This method is incorporated into many modern FT-IR instruments.
2.3 The useful range of oxygen concentration measurable by this test method is from 1 × 10
16
atoms/cm
3
to the
maximum amount of interstitial oxygen soluble in silicon.
2.4 If the spectrophotometer is calibrated using 2 mm thick double-side polished CRMs, this test method is suitable
for use only with 2 mm thick, double-side polished test specimens. It can be extended to the measurement of test
specimens polished on one or both sides with thickness in the range 0.4 mm to 4 mm with the use of working
reference materials traceable to the double-side polished CRMs.
2.5 The oxygen concentration obtained using this test method assumes a linear relationship between the interstitial
oxygen concentration and the absorption coefficient of the 1107 cm
1
band associated with interstitial oxygen in
silicon.
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 guides and determine the
applicability of regulatory or other limitations prior to use.
6 In addition to NIST SRM 2551 (Office of Standard Reference Materials, National Institute of Standards and Technology, 100 Bureau Drive,
Gaithersburg, MD 20899-2322), certified reference materials for measurements of the interstitial oxygen content of silicon can be obtained from
JEITA (see footnote 12) and from BCR (CRM 368 and CRM 369, from the Community Bureau of Reference, Commission of the European
Communities, rue de la Loi 200, B-1040 Brussels, Belgium).
SEMI MF1188-0305 © SEMI 2003, 2005 3
3 Limitations
3.1 The oxygen absorption band overlaps a silicon lattice band. The oxygen-free reference specimen must be
matched within ±0.5% to the thickness of the test specimen in order to properly remove the effects of the silicon
lattice absorption.
3.2 Since both the oxygen band and the lattice band can change with the specimen temperature, the temperature
inside the spectrophotometer sample compartment must be maintained at 27 ± 5°C during the measurement.
3.3 Significant free carrier absorption occurs in n-type silicon with resistivity below 1 ·cm, and in p-type silicon
with resistivity below 3.0 ·cm. For test specimens below these resistivities, the reference crystal must be matched
in resistivity as well as in thickness. The resistivity match must be sufficiently close so that the transmittance of the
test specimen relative to the reference specimen at 1600 cm
1
must be 100 ± 5 %.
3.4 The free carrier absorption in n-type crystals with resistivities less than 0.1 ·cm, or in p-type crystals with
resistivities less than 0.5 ·cm reduces the available energy below that required for the satisfactory operation of
most spectrophotometers.
3.5 The full width at half maximum (FWHM) of the oxygen-in-silicon band at 300 K is 32 cm
1
. Calculations
made from spectral data having a FWHM greater than this value may be in error.
4 Referenced Standards
4.1 SEMI Standards
SEMI C29 — Specifications and Guideline for 4.9% Hydrofluoric Acid 10:1 v/v
SEMI M44 — Guide to Conversion Factors for Interstitial Oxygen in Silicon
4.2 ASTM Standards
E 1 — Specification for ASTM Thermometers
7
E 131 — Terminology Relating to Molecular Spectroscopy
8
E 932 — Practice for Describing and Measuring Performance of Dispersive Infrared Spectrophotometers
8
F 121— Test Method for Intersititial Atomic Oxygen Content of Silicon by Infrared Absorption
9
4.3 JEITA (formerly JEIDA) Standard
EM-3504 (61) — Standard Test Method for Interstitial Atomic Oxygen Content of Silicon by Infrared Absorption
10
4.4 DIN Standard
50438/1 — Determination of Impurity Content in Silicon by Infrared Absorption: Oxygen
11
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 For definitions of terms relating to absorption spectroscopy, refer to ASTM Terminology E 131.
5.2 Definitions
7 Annual Book of ASTM Standards, Vol 14.03, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone: 610-
832-9500, Fax: 610-832-9555, Website:
www.astm.org
8 Annual Book of ASTM Standards, Vol 03.06.
9 Withdrawn in 1988; last available edition in the 1987 edition of Annual Book of ASTM Standards, Vol 10.05.
10 Japan Electronics and Information Technology Industries Association, 3
rd
floor, Mitsui Sumitomo Kaijo Bldg. Annex, 11, Kanda-Surugadai
3-chome, Chiyoda-ku, Tokyo 101-0062, Japan, Website:
www.jeita.or.jp
.
11 Deutches Institut für Normung e.V., standards are available in both English and German editions from Beuth Verlag GmbH,
Burggrafenstrasse 6, 10787 Berlin, Germany, Telephone: 49.30.2601-0, Fax: 49.30.2601.1263, Website:
www.din.de
.