ASTM F374-00a

Standard Test Method for Sheet Resistance of Silicon Epitaxial, Diffused, Polysilicon, and Ion-implanted Layers Using an In-Line Four-Point Probe with the Single-Configuration Procedure

SCOPE
1.1 This test method covers the direct measurement of the average sheet resistance of thin layers of silicon with diameters greater than 15.9 mm (0.625 in.) which are formed by epitaxy, diffusion, or implantation onto or below the surface of a circular silicon wafer having the opposite conductivity type from the thin layer to be measured or by the deposition of polysilicon over an insulating layer. Measurements are made at the center of the wafer using a single-configuration of the four-probe, that is, with the current being passed through the outer pins and the resulting potential difference being measured with the inner pins.
1.2 This test method is known to be applicable on films having thickness at least 0.2 µm. It can be used to measure sheet resistance in the range 10 to 5000 [omega], inclusive.
1.2.1 The principle of the test method can be extended to cover lower or higher values of sheet resistance; however, the precision of the method has not been evaluated for sheet resistance ranges other than those given in 1.2.
Note 1--The minimum value of the diameter is related to tolerances on the accuracy of the measurement through the geometric correction factor. The minimum layer thickness is related to danger of penetration of the probe tips through the layer during measurement.
1.3 Procedures for preparing the specimen, for measuring its size, and for determining the temperature of the specimen during the measurement are also given. Abbreviated tables of correction factors appropriate to circular geometry are included with the method so that appropriate calculations can be made conveniently.
Note 2--The principles of this test method are also applicable to other semiconductor materials, but neither the appropriate conditions nor the expected precision have been determined. Other geometries can also be measured, but only comparative measurements using similar geometrical conditions should be used unless proper geometrical correction factors are known.
Note 3--Some relaxations of test conditions are mentioned in order to assist in applying the principles of the method to nonreferee applications, for which a complete nonreferee method has not yet been developed. The relaxed test conditions given are consensus conditions only and their effect on measurement precision and accuracy has not been explored.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.5 This standard does not purport to address all of the safety concerns, 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 limitations prior to use.Specific hazard statements are given in Section 9.

General Information

• Status: Historical
• Publication Date: 09-Dec-2000
• ICS: 29.045 - Semiconducting materials
• Technical Committee: F01 - Electronics

Relations

• Replaced By: ASTM F374-02 - Standard Test Method for Sheet Resistance of Silicon Epitaxial, Diffused, Polysilicon, and Ion-implanted Layers Using an In-Line Four-Point Probe with the Single-Configuration Procedure (Withdrawn 2003)
• Effective Date: 10-Dec-2002

Standards Content (Sample)

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 374 – 00a
Standard Test Method for
Sheet Resistance of Silicon Epitaxial, Diffused, Polysilicon,
and Ion-implanted Layers Using an In-Line Four-Point Probe
with the Single-Configuration Procedure
This standard is issued under the fixed designation F 374; 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.
for which a complete nonreferee method has not yet been developed. The
1. Scope
relaxed test conditions given are consensus conditions only and their effect
1.1 This test method covers the direct measurement of the
on measurement precision and accuracy has not been explored.
average sheet resistance of thin layers of silicon with diameters
1.4 The values stated in SI units are to be regarded as the
greater than 15.9 mm (0.625 in.) which are formed by epitaxy,
standard. The values given in parentheses are for information
diffusion, or implantation onto or below the surface of a
only.
circular silicon wafer having the opposite conductivity type
1.5 This standard does not purport to address all of the
from the thin layer to be measured or by the deposition of
safety concerns, if any, associated with its use. It is the
polysilicon over an insulating layer. Measurements are made at
responsibility of the user of this standard to establish appro-
the center of the wafer using a single-configuration of the
priate safety and health practices and determine the applica-
four-probe, that is, with the current being passed through the
bility of regulatory limitations prior to use. Specific hazard
outer pins and the resulting potential difference being measured
statements are given in Section 9.
with the inner pins.
1.2 This test method is known to be applicable on films
2. Referenced Documents
having thickness at least 0.2 μm. It can be used to measure
2.1 ASTM Standards:
sheet resistance in the range 10 to 5000 V, inclusive.
D 5127 Guide for Ultra Pure Water Used in the Elecrronics
1.2.1 The principle of the test method can be extended to
and Semiconductor Industry
cover lower or higher values of sheet resistance; however, the
E 1 Specification for ASTM Thermometers
precision of the method has not been evaluated for sheet
F 42 Test Method for Conductivity Type of Extrinsic Semi-
resistance ranges other than those given in 1.2.
conducting Materials
NOTE 1—The minimum value of the diameter is related to tolerances on
F 1529 Test Method for Sheet Resistance Uniformity
the accuracy of the measurement through the geometric correction factor.
Evaluation by In-Line Four-Point Probe with the Duel-
The minimum layer thickness is related to danger of penetration of the
Configuration Procedure
probe tips through the layer during measurement.
2.2 SEMI Standard:
1.3 Procedures for preparing the specimen, for measuring 5
C 3.15 Specifications for Nitrogen Gas
its size, and for determining the temperature of the specimen
C 28 Specification for Hydrofluoric Acid
during the measurement are also given. Abbreviated tables of 5
C 31 Specification for Methanol
correction factors appropriate to circular geometry are included
with the method so that appropriate calculations can be made 3. Terminology
conveniently.
3.1 Definitions:
3.1.1 four-point probe—an electrical probe arrangement for
NOTE 2—The principles of this test method are also applicable to other
determining the resistivity of a material in which separate pairs
semiconductor materials, but neither the appropriate conditions nor the
expected precision have been determined. Other geometries can also be
of contacts are used (1) for passing current through the
measured, but only comparative measurements using similar geometrical
specimen and (2) measuring the potential drop caused by the
conditions should be used unless proper geometrical correction factors are
current.
known.
3.1.1.1 Discussion—It may consist of a unitized probe head
NOTE 3—Some relaxations of test conditions are mentioned in order to
holding all four probes or it may have each of the four
assist in applying the principles of the method to nonreferee applications,
individual probes attached to a separate cantilevered arm.
This test method is under the jurisdiction of ASTM Committee F01 on Annual Book of ASTM Standards, Vol 11.01.
Electronics and is the direct responsibility of Subcommittee F01.06 on Silicon Annual Book of ASTM Standards, Vol 14.03.
Materials and Process Control. Annual Book of ASTM Standards, Vol 10.05.
Current edition approved Dec. 10, 2000. Published February 2001. Originally Available from the Semiconductor Equipment and Materials Institute, 625 Ellis
published as F 374 – 74 T. Last previous edition F 374 – 00. St., Suite 212, Mountain View, CA 94043.
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 374
3.1.2 probe head, of a four-point probe—the mounting that 6. Interferences
(1) fixes the position of the four-point probe in a specific
6.1 Photoconductive and photovoltaic effects can seriously
pattern such as an in-line (collinear) or square array and (2)
influence the observed resistivity, particularly with nearly
contains the pin bearings and springs or other means for
intrinsic material. Therefore, all determinations should be
applying a load to the probe pins.
made in a dark chamber unless experience shows that the
3.1.3 probe pin, of a four-point—one of the four needles
material is insensitive to ambient illumination.
supporting the probe tips; mounting in a bearing contained in
6.2 Spurious currents can be introduced in the testing circuit
the probe head and loaded by a spring or dead weight.
when the equipment is located near high-frequency generators.
3.1.4 probe tip, of a four-point probe—the part of the pin
If equipment is located near such sources, adequate shielding
that contacts the wafer.
must be provided.
3.1.5 probe tip spacing, of a four-point probe—the distance
6.3 Minority carrier injection during the measurement can
between adjacent probe tips.
occur due to the electric field in the specimen. With material
3.1.6 sheet resistance, R [V or V per square]—of a
s
possessing long lifetime of the minority carriers and high
semiconductor or thin metal film, the ratio of the potential
resistivity, such injection can result in a lowering of the
gradient (electric field) parallel with the current to the product
resistivity for a distance of several centimeters from the point
of the current density and thickness.
of injection. Carrier injection can be detected by repeating the
3.1.6.1 Discussion——The sheet resistance is formally
measurements at lower current. In the absence of injection, no
equal to the bulk resistivity divided by the thickness of the
increase in resistivity should be observed at the lower current.
material, taken in the limit as the thickness approaches zero.
The current level recommended (Table 1) should reduce the
probability of difficulty from this source to a minimum, but in
4. Summary of Method
cases of doubt the measurements of 12.4 through 12.8 should
4.1 A in-line four-point probe is used to determine the
be repeated at a lower current. If the proper current is being
specimen sheet resistance. A direct current is passed through
used, doubling or halving its magnitude should cause a total
the specimen between the outer probe pins, and the resulting
change in observed resistance which is less than 0.5 %.
potential difference is measured between the inner probe pins.
6.4 Semiconductors have a significant temperature coeffi-
The sheet resistance is calculated from the ratio of the
cient of resistivity. Consequently, the current used should be
measured voltage to current values using correction factors
small to avoid resistive heating. The current level recom-
appropriate to the geometry.
mended (Table 1) should reduce the chances of this difficulty.
4.2 The spacing between the probe tips is determined from
If resistive heating is suspected, it can be detected by a change
measurements of indentations made by the probe tips in a
in readings as a function of time starting immediately after the
polished silicon surface. This test is also used to determine the
current is applied. If such a change is observed, repeat the
condition of the probe tips.
measurements of 12.4 through 12.8 at a lower current.
4.3 The accuracy of the electrical measuring equipment is
6.5 Vibration of the probe head may cause variations in
tested by means of an analog circuit containing a known
contact resistance, which is often manifested in unstable
resistance together with other resistors that simulate the resis-
readings. If difficulty is encountered, the apparatus should be
tance at the contacts between the probe tips and the semicon-
shock mounted.
ductor surface.
6.6 Penetration of either current or voltage probe tip through
the layer to be measured to the substrate can result in erroneous
5. Significance and Use
readings. This can usually be checked by mounting the
5.1 The sheet resistance of silicon epitaxial, diffused, and
specimen in direct contact with a metallic support grounded to
implanted layers is an important materials acceptance and
the current supply and looking for a reduction in measured
process control parameter. The sheet resistance measurement
specimen voltage in at least one polarity. If this condition
may be used by itself or may be combined with a value of layer
obtains, examine the probe tips microscopically for sharp
thickness, obtained separately, to obtain an estimate of the
asperities and remove these by polishing, or reduce probe
resistivity of an epitaxial layer or of the surface concentration
force, or obtain probe pins with blunter tips.
of dopant for diffused layers.
6.7 The accuracy with which the separation of the probe tips
5.2 This test method is suitable for use in materials accep-
is measured affects the accuracy of the calculated sheet
tance, manufacturing control, research, and development.
NOTE 4—An alternate method, Test Method F 1529, will generally
TABLE 1 Current Values Required for Measurements of Sheet
provide superior measurement precision that may be very important for
Resistance
spatial uniformity mapping requirements. That test method will also avoid
A
Sheet Resistance, V I
the need to apply a lateral geometry correction to the measurements.
However, that test method will generally require the use of a fully
2.0–25 10 mA
20–250 1 mA
automated four-probe measurement system.
200–2500 100 μA
2000–25 000 10 μA
Smits, F. M., “Measurement of Sheet Resistivities with the Four-Point Probe,” A
The proper value of current depends on layer thickness and probe spacing in
Bell System Technical Journal, BSTJA, Vol. 37, 1948, p. 711; Swartzendruber, L. J.,
addition to layer sheet resistance. The current used shall be stable to within 0.05 %
“Correction Factor Tables for Four-Point Probe Resistivity Measurements on Thin,
during the time of measurement and shall be selected to give a measured
Circular Semiconductor Samples, NBS Technical Note 199, NBTNA, April 15,
specimen voltage between 5 and 20 mV, inclusive. The overlap in ranges in the
1964. table is intentional since the table illustrates starting points for current selection.
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 374
resistance. The relative accuracy of probe tip spacing measure- shall be in the range from 0.30 to 0.80 N (31 to 81 gf),
ment decreases as the nominal value of the probe tip spacing inclusive, when the four-point probe is against the specimen in
decreases. For referee measurement purposes, use of a four- measurement position. For hemispherical-tipped probe pins
point probe with 1.59 mm (0.0625 in.) nominal spacing is with tip radius less than 100 μm, the force on each probe tip
required. Four-point probes having other nominal probe tip shall be 0.30 6 0.03 N (31 6 3 gf), inclusive, when the
spacings are suitable for nonreferee measurements. four-point probe is against the specimen in measurement
6.8 The accuracy of the final calculated value of sheet position.
resistance is degraded if the four-point probe is not placed at
NOTE 5—The combination of probe tip radius and probe pin load,
the center of the specimen during measurement (see 12.4). For
which is chosen, affects not only the immunity from probe tip penetration
referee measurements, the center of the tip array probe shall
of very thin layers but also the electrical quality of contact and hence the
not be more than 1.0 mm from the center of the specimen as
noise and accuracy of measurement. The presence of higher resistivity
values at the top surface of the silicon layer to be measured may require
measured along a nonflatted diameter.
an increase in the force of probe pin or use of sharper probe tips. An
6.9 The sheet resistance value calculated from the measure-
example of this situation is a buried peak boron implant.
ments may be in error if the thin film intended for the front
7.3.3 Insulation—The electrical isolation between a probe
surface is also formed on the rear surface of the wafer, and if
pin (with its associated spring and external lead) and any other
the wafer edges provide a conducting path between the
probe pin or probe head part shall be at least 10 V.
front-surface and rear-surface films. The effect of complete
7.3.4 Probe Alignment and Separation—The four-point
coverage of the wafer front surface, edge, and rear surface by
probe tips shall be in an equally spaced linear array. The
a thin conducting film is to make the appropriate value of the
separations between adjacent probe tips shall have a nominal
correction factor F equal to the limiting value of 4.532,
value of 1.59 mm (0.0625 in.). (Other nominal probe spacings
regardless of wafer diameter or probe spacing. It is generally
such as 1.0 and 0.6 mm (0.040 and 0.025 in.) are suitable for
difficult or impossible to test for the conductivity type of the
nonreferee measurements.) The spacing between probe pins
wafer edges. However, if a conductivity-type test of the rear
shall be determined in accordance with the procedure in 11.1 in
surface of the wafer shows this surface to be of the same
order to establish the suitability of the probe head as defined in
conductivity type as the f
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