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SEMI MF1392-1103 © SEMI 2003 4 SEMI MF1153 — Test Method for Ch aracterization of Metal-Oxide-Silicon (MOS) Structures by Capacitance- Voltage Measurements SEMI MF1241— Terminology of Silicon Technology 4.2 ASTM Stan dar…
SEMI MF1392-1103 © SEMI 2003 3
2.7 Warnings and precautionary notes regarding
potential safety hazards are provided throughout the
document.
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 practices and determine
the applicability of regulatory or other limitations prior
to use.
3 Limitations
3.1 A poor Schottky contact, which is generally
indicated by an excessively high leakage current (see
Section 11.5) is the most common problem in
capacitance-voltage measurements made with mercury
probe instruments. It must be emphasized that the use
of a poor Schottky contact does not actually prevent a
carrier density determination but produces an
erroneous result.
3.2 Improper determination of the compensation
capacitance, C
comp
, (see Section 10.3) can cause
significant errors in the capacitance measurement. In
homogeneous material, improper zeroing or use of an
improper value for C
comp
results in an apparent
monotonic increase or decrease of carrier density with
distance from the Schottky barrier. In some fixtures,
inherently large stray capacitances exist; in such cases,
the value of C
comp
may depend both on the diameter of
the wafer and on the position of the wafer on the chuck.
If these dependencies are observed, they may be
reduced or eliminated by shielding the mercury probe
column. If shielding is not practical, probe calibration
procedures should be carried out with wafers of the
same diameter as the wafers being tested and care
should be taken to ensure that the geometry of wafer
and probe is the same during calibration and
measurement.
3.3 Alternating frequency test signals greater than 0.05
V rms may lead to errors in the measured capacitance.
3.4 Excessive series resistance in the capacitance
measurement circuit can cause significant errors in the
measured capacitance values. Series resistance values
greater than 1 kΩ have been reported to cause
measurement error in some cases.
4,5
The primary
source of excessive series resistance is generally a high-
resistance return contact; other possible sources are
bulk resistance in the wafer and wiring defects in the
4 Wiley, J. D., and Miller, G. L., “Series Resistance Effects in
Semiconductor CV Profiling,” IEEE Trans. Electron Devices ED-22,
265–272 (1975).
5 Schroder, D. K., Semiconductor Material and Device
Characterization (Wiley-Interscience, New York, 1990) Section
2.5.2.
mercury probe fixture or the test cables (see Section
11.4).
3.5 When exposed to air, a scum tends to form on the
exposed surface of the mercury used to form the
mercury probe contact. When freed from the surface,
this scum floats to the top of the mercury column. It is
necessary to make certain that the mercury that contacts
the wafer surface is clean by changing the mercury
periodically or by otherwise removing the scum from
the exposed surface.
3.6 A dirty or damaged capillary tube containing the
mercury column may also result in unstable
measurements (see Section 10.4.2.2).
3.7 If the reference wafer is not sufficiently uniform
throughout its thickness, the value of net carrier density,
N
ref
, determined by the four-point probe measurement
(see Section 8.4.3) may differ from the value of net
carrier density at the surface where the mercury probe
measurement is made. Use of erroneous values of N
ref
results in incorrect values for effective probe contact
area (see Section 10.3). Further, if the resistivity profile
of the reference wafer is not uniform near the surface,
an incorrect value of C
comp
may be obtained (see
Section 10.2). Incorrect values of probe contact area
and C
comp
result in incorrect values for the average net
carrier density of the test specimen.
4 Referenced Standards
4.1 SEMI Standards
SEMI C28 — Specifications and Guidelines for
Hydrofluoric Acid
SEMI C29 — Specifications and Guideline for 4.9%
Hydrofluoric Acid (10:1 v/v)
SEMI C30 — Specifications and Guidelines for
Hydrogen Peroxide
SEMI MF26 — Test Methods for Determining the
Orientation of a Semiconductive Single Crystal
SEMI MF42 — Test Methods for Conductivity Type of
Extrinsic Semiconducting Materials
SEMI MF81 — Test Method for Measuring Radial
Resistivity Variation on Silicon Wafers
SEMI MF84 — Test Method for Measuring Resistivity
of Silicon Wafers with an In-Line Four-Point Probe
SEMI MF672 — Test Method for Measuring
Resistivity Profiles Perpendicular to the Surface of a
Silicon Wafer Using a Spreading Resistance Probe
SEMI MF723 — Practice for Conversion Between
Resistivity and Dopant Density for Boron-Doped,
Phosphorus-Doped, and Arsenic-Doped Silicon
SEMI MF1392-1103 © SEMI 2003 4
SEMI MF1153 — Test Method for Characterization of
Metal-Oxide-Silicon (MOS) Structures by Capacitance-
Voltage Measurements
SEMI MF1241— Terminology of Silicon Technology
4.2 ASTM Standards
6
D 5127 — Guide for Ultra Pure Water Used in the
Electronics and Semiconductor Industry
D 4356 — Practice for Establishing Consistent Test
Method Tolerances
E 691 — Practice for Conducting an Interlaboratory
Study to Determine the Precision of a Test Method
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Definitions
5.1.1 For definitions of terms used in silicon wafer
technology refer to SEMI MF1241.
5.1.2 Definitions of the statistical terms repeatability
and reproducibility are given in ASTM Practice E 691.
5.2 Definitions of Terms Specific to This Standard:
5.2.1 compensation capacitance, C
comp
— the sum of
the stray capacitance of the measurement system and
the peripheral capacitance of the mercury probe contact
(see Section 10.3 ).
5.2.2 low-resistance contact — an electrically and
mechanically stable contact
7
in which the resistance
across the contact does not result in excessive series
resistance as determined in Section 11.4 (see also
Section 3.4 ).
5.2.2.1 Discussion — a low-resistance contact may
usually be achieved by using a metal-semiconductor
contact with an area much larger than that of the
mercury probe contact.
5.2.3 mercury probe contact — a Schottky barrier
diode formed by bringing a column of mercury into
contact with an appropriately prepared polished or
epitaxial silicon surface.
6 Available from ASTM International, 100 Barr Harbor Drive, West
Conshohoken, PA 19428-2959, USA. Telephone: 610.832.9585,
Fax: 610.832-9555, Website: www.astm.org. ASTM D 5127 is
published in Volume 11.01 of Annual Book of ASTM Standards while
ASTM D 4356 and ASTM E 691 are published in Volume 14.02.
7 Rhoderick, E. H., and Williams, R. H., Metal-Semiconductor
Contacts, 2nd Edition (Clarendon Press, Oxford, 1988).
6 Summary of Test Method
6.1 The compensation capacitance and effective
mercury probe contact area are determined using a
reference wafer.
6.2 The test specimen is placed in the mercury probe
fixture. A column of mercury is brought into contact
with the epitaxial or polished surface of the specimen
by a pressure differential between the mercury and
ambient to form a Schottky barrier diode (mercury
probe contact).
6.3 A low-resistance return contact is also made to
either the front or back surface of the wafer. This
contact may be either a metal plate or a second
mercury-silicon contact with an area much larger than
the mercury probe contact.
6.4 The quality of the Schottky barrier diode formed by
the mercury probe contact is evaluated by measuring its
series resistance and its reverse current characteristics.
6.5 The small-signal, high frequency capacitance of the
mercury probe contact is measured as a function of the
voltage applied between the mercury probe column and
the return contact. The polarity of the applied voltage
is such that the mercury probe contact is reverse biased
and the low-resistance return contact is forward biased.
6.6 The net carrier density profile (net carrier density
as a function of depth from the surface) is calculated
from the measured values of capacitance and applied
voltage by one of two equivalent methods.
NOTE 2: Net carrier density values obtained by this test
method are often converted to resistivity, which is generally a
more familiar parameter in the industry. If this is done, the
conversion should be made in accordance with the
computational methods given in Section 7.2 of SEMI MF723.
Note that in applying this conversion procedure in either
direction it is assumed that the net carrier density is equal to
the dopant density.
7 Apparatus
7.1 Mercury Probe Fixture — One of the following
fixtures depending on the type of test specimen to be
measured. Warning: Mercury is a toxic material.
Refer to the appropriate Material Safety Data Sheet
prior to use. Avoid physical contact with mercury and
breathing of its vapor.
7.1.1 Back-Surface-Return-Contact Fixture, for use in
measuring polished wafers or epitaxial layers deposited
on substrates of the same conductivity type. A probe
fixture that holds the treated wafer and provides a
single mercury column contained in a capillary tube
with nominal inside diameter of 0.4 to 2.0 mm. The
fixture shall be capable of forming a mercury probe
contact area on the front polished or epitaxial surface of
SEMI MF1392-1103 © SEMI 2003 5
the wafer with a repeatability of ± 1% or better. The
fixture must also provide a low-resistance return
contact to the back surface of the wafer.
7.1.2 Front-Surface-Return-Contact Fixture, for use in
measuring epitaxial wafers deposited on substrates of
the opposite conductivity type or on substrates with
insulating back surface films. A probe fixture that
holds the treated wafer and provides two contacts to the
front polished or epitaxial surface of the wafer. One
contact is the mercury probe contact as described in
7.1.1, and the other is a low-resistance return contact.
The latter may be either a second mercury column or a
metal plate. Its area shall be such that its capacitance is
not less than 32 times the capacitance of the smaller
mercury column. In addition, it is recommended that
this fixture also provide a low-resistance return contact
to the back surface of the wafer to permit the apparatus
also to be used in the back-surface-return-contact
configuration (see Section 7.1.1).
7.2 Equipment for Handling Mercury — Hypodermic
needle or other means for transferring mercury from a
storage bottle to the mercury column and equipment for
neutralizing and picking up spilled mercury
(Warning—see Section 7.1).
7.3 Capacitance Bridge or Meter, with ranges from 1
to 1000 pF full scale or greater, in range multiples of 10
or less. The accuracy shall be 1.0% of full scale or
better for each range, and the rated reproducibility shall
be 0.25% of full scale or better. The internal a-c
voltage signal shall not exceed 0.05 V rms. The
measurement frequency shall be in the range from 0.9
to 1.1 MHz inclusive. The instrument shall be capable
of sustaining an external d-c bias of up to 200 V.
Provision shall be made to compensate a compensation
capacitance of up to 10 pF.
NOTE 3: Capacitance meters or bridges capable of
measuring the phase angle, equivalent series resistance,
conductance or total impedance in addition to the capacitance
may be used.
NOTE 4: Capacitance meters with nominal frequencies from
100 kHz to 1 MHz have been used for measurements of the
type covered by this test method. If an instrument with a
nominal frequency other than 1 MHz is employed, the user
shall demonstrate that it obtains results equivalent to the
specified instrument.
7.4 DC Power Supply, continuously variable from 0 V
to the maximum expected reverse bias or 200 V,
whichever is less, capable of supplying voltages of
either polarity with a peak-to-peak noise of 25 mV or
less. This power supply may be integrated with the
capacitance meter, if desired.
NOTE 5: The maximum reverse bias depends on the net
carrier density in the wafer under test, see Figure 1.
7.5 Digital Voltmeter, with a minimum of four digits,
ranges from ± 1 to ± 200 V full scale or greater in range
multiples of 10 or less, a sensitivity of 1 mV or less,
and an accuracy of 0.5% of full scale or better, a rated
reproducibility of 0.25% of full scale or better, an input
impedance of 100 MΩ or more, and a common-mode
rejection ratio of 100 dB or greater at 60 Hz. This
voltmeter may be integrated with the capacitance meter
and power supply, if desired.
7.6 Curve Tracer, or other apparatus, capable of
monitoring the reverse and forward current-voltage
characteristics of the mercury probe contact. It shall be
capable of applying 200 V at 0.1 mA in the reverse
direction and 1.1 V at 1 mA in the forward direction
and have a sensitivity of 10 µA/division or better.
7.7 Shielded Cables, as required, for making electrical
connections between the probe fixture, power supply,
capacitance bridge or meter, and digital voltmeter.
7.8 Precision Capacitors, with an accuracy of 0.25%
or better at 1 MHz, the measurement frequency,
required only for capacitance meter adjustment and
verification (see Section 10.5). If used, at least two
precision capacitors with values differing by at least a
factor of 10 and lying within the expected capacitance
ranges to be measured are required.
7.9 Precision Voltage Source, capable of providing
output voltages from 0 to ± 200 V with an accuracy of
0.1% of the output voltage or better, required only for
voltmeter verification (see Section 10.6).
7.10 Facilities for Wafer Surface Treatment, if required
(see Sections 8.5, 10.3.5, 10.4.3.1, and 11.5.6):
7.10.1 Fume Hood, equipped with tanks to hold
hydrofluoric acid at room temperature and, for n-type
wafers only, hydrogen peroxide at 70° to 90° C. The
tanks shall be of sufficient size to allow complete
immersion of the cassettes holding the desired size
wafers.
7.10.2 Cassettes, for holding the wafers in hydrofluoric
acid.
7.10.3 Additional Cassettes, for holding n-type wafers
in hydrogen peroxide at 70° C. Warning: When using
a cassette that has been previously used in hydrofluoric
acid in hydrogen peroxide without prior cleaning make
sure that the cassette is clean because of the likelihood
of contamination of the hydrogen peroxide bath.
NOTE 6: A cassette that has been previously used in
hydrofluoric acid may be cleaned for the hydrogen peroxide
treatment by boiling in water for 1 h.
7.10.4 Dump Rinser, with air atmosphere.