ASTM F1451-92(1999)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn. Contact
ASTM International (www.astm.org) for the latest information.
Designation: F 1451 – 92 (Reapproved 1999)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Measuring Sori on Silicon Wafers by Automated Noncontact
Scanning
This standard is issued under the fixed designation F 1451; 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.
1. Scope 3. Terminology
1.1 This test method covers a noncontacting, nondestructive 3.1 Definitions:
procedure to determine the sori of clean, dry semiconductor 3.1.1 mechanical signature, of an instrument—that compo-
wafers. nent of a measurement that is introduced by the instrument and
1.2 The test method is applicable to wafers 50 mm or larger which is systematic, repeatable and quantifiable.
in diameter, and 100 μm (0.004 in.) approximately and larger in 3.1.2 median surface, of a semiconductor wafer—the locus
thickness, independent of thickness variation and surface of points equidistant from the front and back surfaces.
finish, and of gravitationally-induced wafer distortion. 3.1.3 quality area—that portion of the wafer within which
1.3 This test method employs a two-probe system that the specified parameter is determined.
examines both external surfaces of the wafer simultaneously. 3.1.4 reference plane, of a semiconductor wafer—a plane
1.4 This test method measures sori of a wafer corrected for from which deviations are measured.
all mechanical forces applied during the test. Therefore, the 3.1.5 reference plane deviation (RPD)—the distance from a
procedure described gives the unconstrained value of sori. This point on a reference plane to the corresponding point on a
test method includes a means of canceling gravity-induced wafer surface. A dome-shaped wafer is considered to have
deflection which could otherwise alter the shape of the wafer. positive RPD at its center; a bowl-shaped wafer is considered
The resulting parameter is described by Sori in SEMI Speci- to have negative RPD at its center.
fication M1, Appendix Shape Decision Tree (see Annex A1). 3.1.6 sori, of a semiconductor wafer—the difference be-
1.5 This standard does not purport to address all of the tween the maximum and minimum distances of the front
safety concerns, if any, associated with its use. It is the surface of a free, unclamped wafer from a front-surface least
responsibility of the user of this standard to establish appro- squares reference plane.
priate safety and health practices and determine the applica- 3.1.6.1 Discussion— The front surface may contain regions
bility of regulatory limitations prior to use. with upward or downward curvature or both; under some
1.6 The values stated in SI units are to be regarded as the conditions the front surface may be flat (see figures in
standard. The values given in parentheses are for information Appendix X1).
only. 3.1.7 thickness, of a semiconductor wafer—the distance
through the wafer between corresponding points on the front
2. Referenced Documents
and back surfaces.
2.1 ASTM Standards: 3.1.8 Other definitions relative to silicon material technol-
D 4356 Practice for Establishing Consistent Test Method
ogy can be found in Terminology F 1241.
Tolerances
4 4. Summary of Test Method
F 1241 Terminology of Silicon Technology
2.2 SEMI Standard: 4.1 A calibration procedure is performed. This sets the
M1 Specifications for Polished Monocrystalline Silicon Wa- instrument’s scale factor and other constants. In the methods
fers using representative wafer inversion this procedure also deter-
mines the mechanical signature of the instrument and the effect
of gravity on the wafer (Z − median surface; Z − front
mg fg
This test method is under the jurisdiction of ASTM Committee F-1 on
surface).
Electronics and is the direct responsibility of Subcommittee F01.06 on Silicon
4.2 The wafer is supported by a small-area chuck and is
Materials and Process Control.
scanned along a prescribed pattern by both members of an
Current edition approved Nov. 15, 1992. Published January 1993.
Poduje, N., “Eliminating Gravitational Effect in Wafer Shape Measurements,”
opposed pair of probes.
NIST/ASTM/SEMI/SEMATECH Technology Conference, Dallas, TX Technology for
4.3 One of two measurement methods is used. These are
Advanced Materials/Process Characterization, February 1, 1990.
3 distinguished by their use of either a representative wafer or the
Annual Book of ASTM Standards, Vol 14.02.
sample wafer to calculate gravity effects, and in the application
Annual Book of ASTM Standards, Vol 10.05.
Available from SEMI, 805 East Middlefield Road, Mt. View, CA 94043.
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 1451
of gravity correction to the median or top surface. analysis and correction algorithms. Internal system monitoring
4.3.1 Method 1—Median Surface, Representative Wafer may also be used to correct nonrepetitive and repetitive system
(MR): mechanical translations. Failure to provide such corrections
4.3.1.1 The sample wafer is scanned in the normal orienta- may cause errors.
tion (front surface up) and data are taken. 6.2 If a measured wafer differs substantially in diameter,
4.3.1.2 The paired displacement values are used to construct thickness, fiducials or crystal orientation from that used for the
a median surface. gravitational compensation procedure, the results may be
4.3.1.3 The median surface is mathematically corrected for incorrect. Estimates of the errors for differences in diameter
gravitational effects and for mechanical signature of the instru- and thickness are shown in Appendix X2. If the crystal
ment, by subtracting Z , which is obtained from a represen- orientation of the sample to be measured differs from the
mg
tative wafer during calibration. crystal orientation of the gravity-compensation wafer, then the
4.3.1.4 One half the thickness at each point is added to the measured sori value may differ from the actual sori value by up
corrected median surface to construct the corrected front to 15 %. Error tables for fiducial variation have not been
surface. generated.
4.3.2 Method 2—Median Surface, Sample Wafer (MS): 6.3 Different methods for implementing gravitational com-
4.3.2.1 The sample wafer is scanned in the normal orienta- pensation may give different results. Varying levels of com-
tion (front surface up) and data are taken. pleteness of implementing a method may also give different
4.3.2.2 The wafer is scanned in the inverted orientation results.
(front-side down) and data are taken (see Note 3 in 11.2.1).
NOTE 1—The recommended method for gravitational compensation is
4.3.2.3 The two measurements at each point are subtracted
Representative Wafer Inversion, since it allows the use of a single wafer
and divided by two. This constructs a gravity-corrected median
to establish the compensation that is subsequently applied to sample
surface.
wafers. The sample wafer inversion method requires that every wafer be
measured twice, once in a normal and once in an inverted position, which
4.3.2.4 One half the thickness at each point is added to the
increases measurement time and subjects the sample to additional han-
corrected median surface to construct the corrected front
dling. Theoretical modeling requires only a single measurement per
surface.
sample, however it does not address machine signature issues, nor is a
4.4 A least-squares reference plane is constructed from the
rigorous theory presently known to exist.
corrected front surface.
6.4 Mechanical variations in wafer holding devices between
4.5 The reference plane deviation (RPD) is calculated at
systems may introduce measurement differences (see 7.1.1).
each measured point.
6.5 Most equipment systems capable of this measurement
4.6 Sori is reported as the algebraic difference between the
have a definite range of wafer thickness combined with sori
most positive RPD and the most negative RPD.
(dynamic range) that can be accommodated without readjust-
5. Significance and Use ment. If the sample moves outside this dynamic range during
either calibration or measurement, results may be in error. An
5.1 Sori can significantly affect the yield of semiconductor
overrange signal can be used to alert the operator and mea-
device processing.
surement data examiners to this event.
5.2 Knowledge of this characteristic can help the producer
6.6 The quantity of data points and their spacing may affect
and consumer determine if the dimensional characteristics of a
the measurement results (see 7.1.2).
specimen wafer satisfy given geometrical requirements.
5.3 Changes in wafer sori during processing can adversely
7. Apparatus
affect subsequent handling and processing steps. These
7.1 Sori-Measuring Equipment, consisting of wafer holding
changes can also provide an important process monitoring
device, multiple-axis transport mechanism, probe assembly
function.
with indicator, and system controller/computer, including data
5.4 This test method is suitable for measuring the sori of
processor and suitable software. The system must be equipped
wafers used in semiconductor device processing in the as-
with an overrange signal. Instrument data reporting resolution
sliced, lapped, etched, polished, epitaxial or other layer condi-
shall be 100 nm or smaller.
tion and for monitoring thermal and mechanical effects on the
7.1.1 Wafer-Holding Device, for example a chuck whose
sori of wafers during device processing.
face is perpendicular to the measurement axis, and on which
5.5 Until the results of a planned interlaboratory evaluation
the wafer is placed for the measurement scan. The diameter of
of this test method are established, use of this test method for
the wafer holding device shall be 22 mm (0.9 in.) diameter, 33
commercial transactions is not recommended unless the parties
mm (1.3 in.) diameter, or other value as agreed upon between
to the test establish the degree of correlation that can be
using parties.
obtained.
7.1.2 Multiple-Axis Transport Mechanism, which provides a
means for moving the wafer-holding device, or the probe
6. Interferences
assembly, perpendicularly to the measurement axis in a con-
6.1 Any relative motion along the probe measuring axis
trolled fashion in several directions. This motion must permit
between the probes and the wafer holding device during
scanning will produce error in the measurement data. Vibration
The Representative Wafer method of gravitational correction is covered by a
of the test specimen relative to the probe-measuring axis will
patent held by ADE Corporation, 77 Rowe Street, Newton, MA 02166. Alternate
introduce error. Such errors are minimized by system signature methods are described in 12.3.2.
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 1451
Z 5 the distance between the wafer upper surface and the
u
point halfway between the upper and lower probes.
8. Materials
8.1 Set-up Masters, suitable to accomplish calibration and
standardization as recommended by the equipment manufac-
turer.
8.2 Reference Wafer, with sori value # 20 μm and with a
data set that is used to determine the level of agreement
between the system under test and the data set (see Annex A1).
8.3 Representative Wafer—if using the Representative Wa-
fer Inversion Method, a wafer identical in nominal diameter,
nominal thickness, nominal fiducials, composition and crystal-
FIG. 1 Schematic View of Wafer, Probes, and Fixture
line orientation to those being measured is required for the
calibration procedure. Its sori need not be known.
data gathering over a prescribed scan pattern covering the
9. Suitability of Test Equipment
entire quality area. Maximum data point spacing to be used
9.1 The suitability of the test equipment shall be determined
shall be 4 mm, or other value as agreed upon between using
with the use of a reference wafer and its associated data set in
parties.
accordance with the procedures of Annex A1, or by perfor-
7.1.3 Probe Assembly with Paired Non-contacting
mance of a statistically-based instrument repeatability study to
Displacement-Sensing Probes, Probe Supports, and Indicator
ascertain whether the equipment is operating within the manu-
Unit. The probes shall be capable of independent measurement
facturer’s stated specification for repeatability.
of the distance between the probed site on each surface of the
NOTE 2—Subcommittee F1.95 is currently developing an instrument
sample wafer and the motion plane. The probes shall be
repeatability study format.
mounted above and below the wafer in a manner so that the
probed site on one surface of the wafer is opposite the probed
9.2 Determination of degree of suitability is currently under
site on the other. The common axis of these probes is the investigation.
measurement axis (see Fig. 1). The probe separation D shall be
10. Sampling
kept constant during calibration and measurement. Displace-
10.1 This test method is nondestructive and may be used on
ment resolution shall be 0.1 μm or better. The probe sensor size
either 100 % of the wafers in a lot or on a sampling basis.
shall be 4 by 4 mm, or other value to be agreed upon between
10.1.1 If samples are to be taken, procedures for selecting
using parties. Systems employing either representative wafer
the sample from each lot of wafers to be tested shall be agreed
inversion or sample wafer inversion methods for gravity
upon between the parties to the test, as shall the definition of
compensation must provide precise positioning in both mea-
what constitutes a lot.
surement orientations so that measurements are taken at
identical locations for each orientation of the sample.
11. Calibration and Standardization
7.1.3.1 The following equations are derived from Fig. 1.
11.1 Calibrate in accordance with the manufacturer’s in-
D 5 a 1 t 1 b (1)
structions.
D t 11.2 When using the representative wafer inversion method
Z 52 1 b 1 (2)
m
2 2
(Method 1), perform the following procedures to determine
Z , the representative effect of gravity and machine effects on
D t mg
Z 5 2 a 2 (3)
m
2 2 the representative wafer.
11.2.1 Method 1—MR (Median Surface, Representative
b 2 a
Z 5 (4)
Wafer)—Scan the representative wafer in the normal orienta-
m
tion (front-side is upper surface) and
...