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SEMI MF657-0705 © SEMI 2003, 2005 1 SEMI MF657-0705 TEST METHOD FOR MEASURING WARP AND TOTAL THICKNESS VARIATION ON SILICON WAFERS BY NONCONTACT SCANNING This test method was technically approved by the global Silicon Wa…
SEMI MF525-0705 © SEMI 2003, 2005 17
RELATED INFORMATION 3
SOURCES OF SYSTEMATIC ERROR
NOTICE: This related information is not an official part of SEMI MF525. It was derived from information
developed during the original preparation of the standard in ASTM Committee F-1 on Electronics in 1977. This
related information was approved for publication by full letter ballot procedures.
R3-1 In addition to random errors, there are a number of sources of systematic error which can be identified but
which cannot be estimated here; their estimation must be done by the individual laboratory.
R3-2 Calibration Specimen Nonuniformity — The four-point probe method for measuring the resistivity of the
calibration specimens responds to the average resistivity of a specimen over an area which is several times the total
spacing of the four-point probe. Within this area there may be significant variation of resistivity. Spreading
resistance measurements respond to the local resistivity of the calibration specimens. Calibration specimens shall
therefore have uniform resistivity such that the resistivity assigned to the specimen by use of the four-point probe
method satisfactorily represents the resistivity value at the location where spreading resistance calibration
measurements will be taken; otherwise systematic errors are incurred in calibration.
R3-3 Choosing a Model for the Calibration Relation — The empirical relation between spreading resistance and
resistivity values of the calibration specimens are commonly approximated by a number of different relations:
single-piece log-log least-squares fit, piecewise log-log fit, and polynomial fit. The best form or model to fit the
calibration data has not been established. Any of the chosen models may have significant high-side or low-side
systematic errors at various resistivity values compared to the unknown “true” relation.
R3-4 Loss of Control of the Spreading-Resistance Probes — Wear, contamination, or other degradation of the
probes may cause sudden shifts in measurement response at some or all resistivity values. Such shifts may not be
accompanied by recognizable loss of measurement precision and merely add an additional systematic error between
calibration and test specimen measurement values.
R3-5 Loss of Control of Specimen Preparation Process — Contamination of specimen-polishing materials or post-
polishing chemicals as well as excess polishing-induced damage or unrecognized differences in technique such as
applied pressure, specimen area, or post-polishing storage environment may cause undetected systematic errors in
test or calibration specimen values.
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notice.
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respecting the validity of any patent rights or copyrights asserted in connection with any items mentioned in this
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SEMI MF657-0705 © SEMI 2003, 2005 1
SEMI MF657-0705
TEST METHOD FOR MEASURING WARP AND TOTAL THICKNESS
VARIATION ON SILICON WAFERS BY NONCONTACT SCANNING
This test method was technically approved by the global Silicon Wafer Committee. This edition was
approved for publication by the global Audits and Reviews Subcommittee on April 6, 2005. It was available
at www.semi.org in June 2005 and on CD-ROM in July 2005. Original edition published by ASTM
International as ASTM F 657-80. Last previous edition SEMI MF657-92 (Reapproved 1999).
1 Purpose
1.1 Warp and thickness variation of silicon wafers can significantly affect the yield of semiconductor device
processing.
1.2 Knowledge of these characteristics can help the supplier and customer determine if the dimensional
characteristics of a particular wafer satisfy given geometrical requirements.
1.3 Changes in wafer warp during processing can adversely affect subsequent handling and processing steps
1.4 This test method is suitable for measuring the warp and TTV of silicon wafers used in semiconductor device
processing in the as-sliced, lapped, or polished condition and for monitoring thermal and mechanical effects on the
warp of silicon wafers during device processing.
2 Scope
2.1 This test method covers a noncontacting, nondestructive procedure to determine the warp and total thickness
variation (TTV) of clean, dry silicon wafers in a free (unclamped) condition. The procedure uses a three-point back
surface reference plane for determining warp.
2.2 The test method is applicable to circular silicon wafers from 50 mm (or 2.0 in.) to 200 mm in diameter, and
100 m (or 0.004 in. approximately) and larger in thickness, independent of thickness variation and surface finish.
The test method is applicable to wafers of semiconductors other than silicon with these same physical
characteristics.
2.3 This test method is not intended to measure surface flatness; warp, which is not to be confused with flatness, is
a bulk property of the wafer. Warp may be caused by unequal stresses on the two exposed surfaces of the wafer. It
cannot be determined from measurements on a single exposed surface. The median surface may contain regions
with upward or downward curvature or both; under some conditions the median surface may be flat.
2.4 This test method measures warp and TTV of a wafer with no mechanical force except gravity applied during the
test. Therefore, the procedure described gives the unconstrained value of warp or TTV. Gravity-induced deflection
alters the shape of the wafer and is included in the measurement.
2.5 For application to wafers of diameter 3 in. or smaller, the values stated in inch-pound units are to be regarded as
the standard whether or not they appear in parentheses; the values stated in acceptable metric units are for
information only. For application to wafers of diameter larger than 3 in., the values stated in acceptable metric units
are to be regarded as the standard; the values stated in inch-pound units are for information only.
NOTICE: This standard does not purport to address safety issues, if any, associated with its use. It is the
responsibility of the users 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 In this test method, both TTV and warp are determined using a specified partial scan pattern; thus, the entire
surface is not sampled and use of another scan pattern may not yield the same result.
3.2 Most equipment systems capable of this measurement have a definite range of wafer thickness combined with
warp which can be accommodated without readjustment. Any values observed while in an over-range condition are
invalid.
SEMI MF657-0705 © SEMI 2003, 2005 2
3.3 This test method does not completely separate thickness variation from warp. In some cases, the median
surface may be flat but still show a non-zero value for warp.
3.4 Running probes off the test specimen during the scan sequence gives false readings.
3.5 Any change in the reference plane during scanning produces error in the indicated measurement equal to the
axial vector value of the deviation at the probe axes at the points of largest and smallest differences. If such changes
occur, there is the possibility that an incorrect location may be identified as an extremum.
3.6 Non-parallelism of the reference plane to the granite base surface produces an error in the indicated
measurement proportional to the non-parallelism.
3.7 Foreign particles (dirt) between the measuring ring and surface plate introduce error.
3.8 Vibration of the test specimen relative to the probe-measuring axis introduces error.
4 Referenced Standards and Documents
4.1 SEMI Standards
SEMI M1 — Specifications for Polished Monocrystalline Silicon Wafers
SEMI M59 — Terminology for Silicon Technology
4.2 ANSI Standard
ANSI/ASME B46.1 — Surface Texture (Surface Roughness, Waviness, and Lay)
1
4.3 Federal Standard
GGG-P 463 C Surface Plate, Granite
2
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 Definitions
5.1.1 median surface (of a semiconductor wafer) — the locus of points in the wafer equidistant from the front and
back surfaces.
5.1.2 Other terms relating to silicon technology are defined in SEMI M59.
6 Summary of Test Method
6.1 The wafer is supported by three hemispherical points on a reference ring, and both surfaces are simultaneously
scanned along a prescribed pattern by both members of an opposed pair of probes.
6.2 The displacements (distances) between each probe and the nearest surface of the wafer are determined (in pairs)
at intervals along the scan pattern.
6.3 Half the difference between the largest and smallest of the differences of the paired displacements is taken as a
measure of the warp.
6.4 The difference between the largest and smallest of the sums of the paired displacements is taken as a measure of
the total thickness variation.
7 Apparatus
7.1 Warp Measuring Equipment — Consisting of movable reference ring, fixed probe assembly with indicator,
guide, and surface plate as follows:
1 American National Standards Institute, New York Office: 25 West 43rd Street, New York, NY 10036, USA. Telephone: 212.642.4900, Fax:
212.398.0023, Website:
www.ansi.org.
2 Standardization Documents Order Desk, Bldg. 4 Section D, 700 Robbins Ave., Philadelphia, PA 19111-5094.