ASTM F1393-92(1997)

Standard Test Method for Determining Net Carrier Density in Silicon Wafers by Miller Feedback Profiler Measurements With a Mercury Probe

SCOPE
1.1 This test method  covers the measurement of net carrier density and net carrier density profiles in epitaxial and polished bulk silicon wafers in the range from about 4 X 10 13  to about 8 X 10 16  carriers/cm (resistivity range from about 0.1 to about 100 [omega]-cm in n-type wafers and from about 0.24 to about 330 [omega]-cm in p-type wafers).
1.2 This test method requires the formation of a Schottky barrier diode with a mercury probe contact to an epitaxial or polished wafer surface. Chemical treatment of the silicon surface may be required to produce a reliable Schottky barrier diode. (1)  The surface treatment chemistries are different for n- and p-type wafers. This test method is sometimes considered destructive due to the possibility of contamination from the Schottky contact formed on the wafer surface; however, repetitive measurements may be made on the same test specimen.
1.3 This test method may be applied to epitaxial layers on the same or opposite conductivity type substrate. This test method includes descriptions of fixtures for measuring substrates with or without an insulating backseal layer.
1.4 The depth of the region that can be profiled depends on the doping level in the test specimen. Based on data reported by Severin (1) and Grove (2), Fig. 1 shows the relationship between depletion depth, dopant density, and applied voltage together with the breakdown voltage of a mercury silicon contact. The test specimen can be profiled from approximately the depletion depth corresponding to an applied voltage of 1 V to the depletion depth corresponding to the maximum applied voltage (200 V or about 80% of the breakdown voltage, whichever is lower). To be measured by this test method, a layer must be thicker than the depletion depth corresponding to an applied voltage of 2 V.
1.5 This test method is intended for rapid carrier density determination when extended sample preparation time or high temperature processing of the wafer is not practical.  Note 1-Test Method F419 is an alternative method for determining net carrier density profiles in silicon wafers from capacitance-voltage measurements. This test method requires the use of one of the following structures: (1) a gated or ungated p-n junction diode fabricated using either planar or mesa technology or (2) an evaporated metal Schottky diode. Although this test method was written prior to consideration of the Miller Feedback Method, the Miller Feedback Method has been satisfactorily used in measuring the round robin samples.
1.6 This test method provides for determining the effective area of the mercury probe contact using polished bulk reference wafers that have been measured for resistivity at 23°C in accordance with Test Method F84 or Test Method F673. This test method also includes procedures for calibration of the apparatus.  Note 2-An alternative method of determining the effective area of the mercury probe contact that involves the use of reference wafers whose net carrier density has been measured using fabricated mesa or planar p-n junction diodes or evaporated Schottky diodes is not included Note-The light dashed line represents the applied reverse bias voltage at which breakdown occurs in a mercury silicon contact; the heavy dashed line represents 80% of this voltage, it is recommended that the applied reverse bias voltage not exceed this value. The light chain-dot line represents the maximum reverse bias voltage specified in this test method. FIG. 1% Relationships between Depletion Depth, Applied Reverse Bias Voltage, and Dopant Density in this test method but may be used if agreed upon by the parties to the test.
1.7 This standard does not purport to address all of the safety problems, 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 stateme...

General Information

• Status: Historical
• Publication Date: 31-Dec-1991
• ICS: 29.045 - Semiconducting materials
• Technical Committee: F01 - Electronics

Relations

• Replaced By: ASTM F1393-02 - Standard Test Method for Determining Net Carrier Density in Silicon Wafers by Miller Feedback Profiler Measurements With a Mercury Probe (Withdrawn 2003)
• Effective Date: 10-Dec-2002

Standards Content (Sample)

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 1393 – 92 (Reapproved 1997)
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
Determining Net Carrier Density in Silicon Wafers by Miller
Feedback Profiler Measurements With a Mercury Probe
This standard is issued under the fixed designation F 1393; 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
1.1 This test method covers the measurement of net carrier
density and net carrier density profiles in epitaxial and polished
bulk silicon wafers in the range from about 4 3 10 to about
8 3 10 carriers/cm (resistivity range from about 0.1 to about
100 V-cm in n-type wafers and from about 0.24 to about 330
V-cm in p-type wafers).
1.2 This test method requires the formation of a Schottky
barrier diode with a mercury probe contact to an epitaxial or
polished wafer surface. Chemical treatment of the silicon
surface may be required to produce a reliable Schottky barrier
diode. (1) The surface treatment chemistries are different for
n- and p-type wafers. This test method is sometimes considered
destructive due to the possibility of contamination from the
Schottky contact formed on the wafer surface; however,
repetitive measurements may be made on the same test
specimen.
1.3 This test method may be applied to epitaxial layers on
the same or opposite conductivity type substrate. This test
method includes descriptions of fixtures for measuring sub-
strates with or without an insulating backseal layer.
1.4 The depth of the region that can be profiled depends on
the doping level in the test specimen. Based on data reported
by Severin (1) and Grove (2), Fig. 1 shows the relationship
between depletion depth, dopant density, and applied voltage
together with the breakdown voltage of a mercury silicon
contact. The test specimen can be profiled from approximately
NOTE 1—The light dashed line represents the applied reverse bias
the depletion depth corresponding to an applied voltage of 1 V
voltage at which breakdown occurs in a mercury silicon contact; the heavy
to the depletion depth corresponding to the maximum applied
dashed line represents 80 % of this voltage, it is recommended that the
voltage (200 V or about 80 % of the breakdown voltage,
applied reverse bias voltage not exceed this value. The light chain-dot line
whichever is lower). To be measured by this test method, a
represents the maximum reverse bias voltage specified in this test method.
layer must be thicker than the depletion depth corresponding to FIG. 1 Relationships between Depletion Depth, Applied
Reverse Bias Voltage, and Dopant Density
an applied voltage of 2 V.
1.5 This test method is intended for rapid carrier density
determination when extended sample preparation time or high
temperature processing of the wafer is not practical.
This test method is under the jurisdiction of ASTM Committee F-1 on
NOTE 1—Test Method F 419 is an alternative method for determining
Electronics and its the direct responsibility of Subcommittee F01.06 on Silicon
net carrier density profiles in silicon wafers from capacitance-voltage
Materials and Process Control.
measurements. This test method requires the use of one of the following
Current edition approved May 15, 1992. Published July 1992.
structures: (1) a gated or ungated p-n junction diode fabricated using either
DIN 50439, Determination of the Dopant Concentration Profile of a Single
Crystal Semiconductor Material by Means of the Capacitance-Voltage Method and planar or mesa technology or (2) an evaporated metal Schottky diode.
Mercury Contact, is the responsibility of DIN Committee NMP 221, with which
Although this test method was written prior to consideration of the Miller
Committee F-1 maintains close liaison. DIN 50439 is available from Beuth Verlag
Feedback Method, the Miller Feedback Method has been satisfactorily
GmbH, Burggrafenstrasse 4-10, D-1000, Berlin 30, Germany.
used in measuring the round robin samples.
The boldface numbers in parenthesis refer to the list of references at the end of
this test method. 1.6 This test method provides for determining the effective
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 1393
area of the mercury probe contact using polished bulk refer- contact does not result in excessive series resistance as
ence wafers that have been measured for resistivity at 23°C in determined in 11.4 (see also 6.2).
accordance with Test Method F 84 or Test Method F 673. This 3.2.3.1 Discussion—A low-resistance contact may gener-
test method also includes procedures for calibration of the ally be achieved by using a metal semiconductor contact with
apparatus. an area much larger than that of the mercury probe contact.
3.2.4 mercury probe contact—a Schottky barrier diode
NOTE 2—An alternative method of determining the effective area of the
formed by bringing a column of mercury into contact with an
mercury probe contact that involves the use of reference wafers whose net
appropriately prepared polished or epitaxial silicon surface.
carrier density has been measured using fabricated mesa or planar p-n
junction diodes or evaporated Schottky diodes is not included in this test
4. Summary of Test Method
method but may be used if agreed upon by the parties to the test.
4.1 A calibration procedure using polished bulk wafers of
1.7 This standard does not purport to address all of the
known carrier density is used to determine the mercury probe
safety concerns, if any, associated with its use. It is the
contact area.
responsibility of the user of this standard to establish appro-
4.2 The test specimen is placed on the mercury probe
priate safety and health practices and determine the applica-
fixture. A column of mercury is brought into contact with an
bility of regulatory limitations prior to use. Specific hazard
epitaxial or polished wafer surface by a pressure differential
statements are given in Note 4 in 7.2, 7.3, and 8.2.
between the mercury and ambient to form a Schottky barrier
2. Referenced Documents
diode (mercury probe contact).
2.1 ASTM Standards: 4.3 A low-resistance return contact is also made to either the
D 1193 Specifications for Reagent Water front or back surface of the wafer. This contact may be either
F 26 Test Methods for Determining the Orientation of a a metal plate or a second mercury silicon contact with an area
Semiconductive Single Crystal much larger (32 times or larger) than the mercury probe
F 42 Test Methods for Conductivity Type of Extrinsic contact.
Semiconducting Materials
4.4 The quality of the Schottky barrier diode formed is
F 81 Test Method for Measuring Radial Resistivity Varia- determined by viewing the “delta X wave shape” on an
tion on Silicon Slices
oscilloscope and verifying that it is a good square wave per
F 84 Test Method for Measuring Resistivity of Silicon manufacturer’s operating instruction. It can also be evaluated
Wafers with an In-Line Four-Point Probe by measuring its series resistance and its reverse current
F 419 Test Method for Determining Carrier Density in characteristics.
Silicon Epitaxial Layers by Capacitance Voltage of Mea- 4.5 A current is driven through the diode by a radio
surements on Fabricated Junction or Schottky Diodes frequency (RF) generator. The current is compared to a
F 673 Test Method for Measuring Resistivity of Semicon- reference current (magnitude of which is set by the dielectric
ductor Slices or Sheet Resistance of Semiconductor Films constant and area controls) at the error summation point at the
with a Noncontact Eddy-Current Gage input of an amplifier in a servo-controlled feedback loop that
F 723 Practice for Conversion Between Resistivity and (a) keeps the RF current amplitude constant and (b) generates
Dopant Density for Boron-Doped and Phosphorus-Doped an output d-c signal, X, that is proportional to the depletion
Silicon depth. The reverse bias (V) on the diode is step-modulated at a
F 1241 Terminology of Silicon Technology low frequency and at an amplitude proportional to signal X,
2.2 SEMI Standard: keeping dV/dX, the change in electric field, constant. The
SEMI C1 Specifications for Reagents amplitude of the resulting modulation of the X signal (dX)is
therefore proportional to the net carrier density. A d-c signal,
3. Terminology
1/N, (net carrier density) proportional to dX is generated. The
3.1 Definitions:
signal is used for read out information.
3.1.1 For definitions of terms used in silicon wafer technol-
4.6 The net carrier density as a function of depth is
ogy refer to Terminology F 1241.
determined by the profiler circuitry and computer data acqui-
3.2 Definitions of Terms Specific to This Standard:
sition hardware and software.
3.2.1 breakdown voltage—the reverse bias voltage at which
NOTE 3—The net carrier density values obtained by this test method are
the mercury probe contact exhibits a leakage current density of
frequently converted to resistivity, which is generally a more familiar
3 mA/cm .
parameter in the industry. If this is done, the conversion should be made
3.2.2 compensation capacitance, C —the sum of the
in accordance with Practice F 723, using the tabular or computational
comp
stray capacitance of the measurement system and the periph- methods given in paragraph 7.2 of this practice (conversion from dopant
density to resistivity) in order to eliminate the self-consistency errors in
eral capacitance of the mercury probe contact (see 10.3).
the equations given in Practice F 723. The choice of conversion direction
3.2.3 low-resistance contact—an electrically and mechani-
is based on the fact that the net carrier density of the reference wafer used
cally stable contact (3) in which the resistance across the
for determination of the area of the mercury probe contact (see 8.4 and
10.2) is traceable to National Institute of Standards and Technology using
4 the methods of paragraph 7.2 of Practice F 723 so that the more laborious
Annual Book of ASTM Standards, Vol 11.01.
iterative procedure is applied to the less frequently measured reference
Annual Book of ASTM Standards, Vol 10.05.
wafers and the direct conversion procedure is applied to material being
Available from Semiconductor Equipment and Materials International, 805
East Middlefield Road, Mountain View, CA 94043. evaluated by this test method. Note that in applying this conversion
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 1393
procedure in either direction it is assumed that the net carrier density is
7.2.1 Back-Side-Return-Contact Fixture, for use in measur-
equal to the dopant density.
ing polished wafers or epitaxial layers deposited on substrates
of the same conductivity type, a probe fixture that holds the
5. Significance and Use
treated wafer and provides a single mercury column contained
5.1 This test method can be used for research and develop-
in a capillary tube with nominal inside diameter of 0.4 to 2.0
ment, process control, and materials specification, evaluation,
mm. The fixture shall be capable of forming a mercury probe
and acceptance purposes. However, in the absence of interlabo-
contact area on the front polished or epitaxial surface of the
ratory test data to establish its precision, this test method
wafer with a repeatability of+1% or better (one standard
should be used for materials specifications and acceptance only
deviation). The fixture must also provide a low-resistance
after the parties to the test have established repeatability,
return contact to the back surface of the wafer.
reproducibility, and correlation.
7.2.2 Front-Surface-Return-Contact Fixture, for use in mea-
suring epitaxial wafers deposited on substrates of the opposite
6. Interferences
conductivity type or on substrates with high resistivity or
6.1 A poor Schottky contact, which is generally indicated by
insulating back surface films, a probe fixture that holds the
an excessively high leakage current (greater than 100 μA) (see
treated wafer and provides two contacts to the front polished or
11.5) is the most common problem in measurements made with
epitaxial surface of the wafer. One contact is the mercury probe
mercury probe instruments. It must be emphasized that the use
contact as described in 7.2.1, and the other is a low-resistance
of a poor Schottky contact will not actually prevent a carrier
return contact. The latter may be either a second mercury
density determination but will produce an erroneous result.
column or a metal plate. Its area shall be such that its
6.2 Excessive series resistance in the measurement circuit
capacitance is not less than 32 times the capacitance of the
can cause significant errors in the measured values. Series
smaller mercury column. In addition, it is recommended that
resistance values greater than 1 kV have been reported to cause
this fixture also provide a low-resistance return contact to the
measurement error in some cases (4, 5). The primary source of
back surface of the wafer to permit the apparatus also to be
excessive series resistance is generally a high-resistance return
used in the back-surface-return-contact configuration (see
contact; other possible sources are bulk resistance in the wafer
7.2.1).
and wiring defects in the mercury probe fixture or the test
7.3 Equipment, for handling mercury-hypodermic needle or
cables and excessive spacing between mercury Schottky and
other means for transferring mercury from a storage bottle to
mercury return contact or using a backside return contact when
the mercury column and equipment for neutralizing and
using higher resistivity substrates (see 11.4).
picking up spilled mercury (Warning: see Note 4).
6.3 When exposed to air, a scum tends to form on the
7.4 Miller Feedback Profiler Electronics (6), having a fre-
exposed surface of the mercury used to form the mercury probe
quency of 1 MHz nominal and an input capacitance range from
contact. When freed from the surface, this scum floats to the
5 pF to 700 pF (see Appendix X3). Provision should be made
top of the mercury column. It is necessary to make certain that
for calibration of stray capacitance of up to 10 pF.
the mercury that contacts the wafer surface is clean by
7.5 D-
...