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SEMI MF1392-1103 © SEMI 2003 6 7.10.5 Spin Dryer, for drying the wafers i n an air atmosphere. 7.10.6 Hot Plate, for p -type wafers or other m eans for baking the wa fer at 120 ± 10° C in air may also be required. 7.10.7…
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.
SEMI MF1392-1103 © SEMI 2003 6
7.10.5 Spin Dryer, for drying the wafers in an air
atmosphere.
7.10.6 Hot Plate, for p-type wafers or other means for
baking the wafer at 120 ± 10° C in air may also be
required.
7.10.7 Interval Time, for controlling treatment times.
7.10.8 Nonmetallic Tweezers or Vacuum Wand, for
holding and manipulating wafers.
8 Reagents and Materials
8.1 Purity of Reagents — All chemicals for which such
specifications exist shall adhere to Grade 1 SEMI
specifications for those specific chemicals. Other
grades may be used, provided it is first determined that
the chemical is of sufficiently high purity to permit its
use without lessening the accuracy of the test.
8.2 Mercury shall be triple distilled and conform to
reagent grade, as specified in Reagent Chemicals.
8
It
shall be changed regularly or otherwise maintained in a
clean state to avoid interference from surface scum (see
Section 3.5) (Warning—see Section 7.1).
8.3 Purity of Water — Reference to water shall be
understood to mean Type E-3 or better water as
described in ASTM Guide D 5127.
8.4 Reference Wafers — One or more polished bulk
silicon wafers of the same conductivity type as the layer
or wafer to be tested. If one reference wafer is used, its
net carrier density shall lie between one-half and two
times the net carrier density of the layer or wafer to be
tested. If several reference wafers are used, their net
carrier densities shall cover the range of net carrier
densities of the layers or wafers to be tested. Bulk
reference wafers shall have the following
characteristics:
8.4.1 Flat spreading resistance profile over a depth
equal to or greater than that to be profiled in this test
method. The flat profile is required to ensure correct
determination of C
comp
(see Section 3.7, Section 10.2).
Determine that the spreading resistance profile is flat in
the following manner:
8.4.1.1 Measure the spreading resistance profile in
accordance with SEMI MF672 on a small chip from a
portion of the wafer. Use a minimum of 10 spreading
resistance values in constructing the spreading
resistance profile.
8.4.1.2 Fit the spreading resistance data to a straight
line by a least-squares method.
8 “Reagent Chemicals, American Chemical Society Specifications,”
Am. Chemical Soc., Washington, DC.
8.4.1.3 For the profile to be considered flat, the fitted
values at the beginning and end of the profile shall be
equal to within ± 2% and the maximum deviation of
any measured value from the fitted line shall not exceed
5%.
8.4.2 Resistivity variation over the central region of the
wafer ≤ 5%. Resistivity variation over this region
should be as small as possible to obtain maximum
accuracy of the determination of the net carrier density
of the reference wafer; accurate determination of the net
carrier density is required for accurate determination of
the probe contact area (see Section 10.3).
8.4.2.1 Determine radial unformity from resistivity
measurements taken at 2.0 mm intervals along two
perpendicular diameters for a distance of 6 mm from
the center of the wafer in each direction. Analyze the
data in accordance with the maximum/minimum
convention of Sample Plan D of SEMI MF81.
8.4.2.2 Establish axial uniformity (see Section 3.7) by
spreading resistance measurements across a cleaved
portion of the wafer or by some other method agreed
upon by the parties to the test.
8.4.3 Known Net Carrier Density — Determine the net
carrier density as follows:
8.4.3.1 Measure the resistivity at the center of the
wafer and correct it to 23° C in accordance with SEMI
MF84.
8.4.3.2 Convert the resistivity value to net carrier
density using the computational methods given in
Section 7.2 of SEMI MF723.
NOTE 7: In applying this conversion procedure in either
direction it is assumed that the net carrier density is equal to
the dopant density. The appropriate equation given in this
section for resistivity must be solved iteratively for the net
carrier density. It is necessary to use the same equation for
conversion from net carrier density to resistivity and vice
versa in order to eliminate the self-consistency errors in SEMI
MF723. The choice of conversion direction in this test
method was made so that the more laborious, iterative
procedure is applied to the less frequently measured reference
wafers and the direct conversion procedure is applied to
material being evaluated by this test method.
8.4.3.3 Record the net carrier density just obtained, as
N
ref
, in cm
−3
.
NOTE 8: The advantage of using bulk reference wafers is
that the net carrier density can be related to that of resistivity
standard reference materials issued by the National Institute
of Standards and Technology. However, material
inhomogeneity may make it difficult to obtain accurate values
of net carrier density from bulk reference wafers (see Section
3.7). As an alternative, epitaxial wafers may be used as
reference wafers. In this case, the epitaxial layer thickness
should be large enough that the resistivity profile in the near-
SEMI MF1392-1103 © SEMI 2003 7
surface region may be obtained from spreading resistance
measurements through the use of a calibration curve only,
without the need for using correction factors. Since the
procedures for characterizing such reference wafers have not
been standardized, epitaxial wafers should be used as
reference wafers only if agreed upon between the parties to
the test. If used, the epitaxial reference wafer should meet the
requirements of Section 8.41 for a flat spreading resistance
profile. Net carrier density should be determined using an
metal oxide silicon (MOS) capacitor structure in accordance
with SEMI MF1153 or by another mutually acceptable
method.
8.5 Reagents for Surface Treatment — If surface
treatment is required, the following chemicals may be
needed. Grade 1 chemicals are preferred (see Related
Information 1).
8.5.1 Hydrofluoric Acid, HF, concentrated, 49.00 ±
0.25%, in accordance with grade 1 of SEMI C28 or
dilute, 4.9 ± 0.1%, in accordance with grade 1 of SEMI
C29.
8.5.1.1 Once a week, fill the hydrofluoric acid tank
with fresh HF, either concentrated or dilute.
8.5.2 Hydrogen Peroxide, H
2
O
2
, unstabilized, 30%, in
accordance with grade 1 of SEMI C30.
8.5.2.1 Every 8 h, fill the hydrogen peroxide tank with
a fresh 15% H
2
O
2
solution by mixing equal volumes of
H
2
O
2
(30%) and water. Wait until the freshly poured
bath reaches 70 ± 5° C before using to treat wafer
surfaces.
8.5.2.2 During use, approximately every 2 h (or when
the H
2
O
2
level falls below the wafers being treated),
replenish the tank by adding 30% H
2
O
2
.
9 Sampling
9.1 It is generally impractical to measure every wafer
in a particular lot owing to the potential for
contamination from the handling and chemical
treatments involved. A wafer sampling plan shall
therefore be agreed upon between the parties to the test.
9.2 Locations on the wafer where measurements are to
be made shall also be agreed upon between the parties
to the test.
10 System Calibration and Control
10.1 Frequency of Calibration and Control Procedures
10.1.1 Calibrate the capacitance meter and voltmeter in
accordance with manufacturer’s instructions on initial
installation and following hardware or software
modifications. Calibration of the capacitance meter and
voltmeter may be carried out in-house or by a qualified
testing laboratory.
10.1.2 Determine the compensation capacitance and
effective area of the mercury probe in accordance with
the procedures of this section on initial installation and
after any corrective action has been carried out to bring
the system back into control.
10.1.3 Conduct periodic tests and maintain control
charts in accordance with the procedures in this section
to demonstrate that the instruments are in control and
that the variability is within the requirements of this test
method.
10.2 Determination of Compensation Capacitance,
C
comp
10.2.1 Measure a series of capacitance-voltage pairs on
the reference wafer carrier density of 1 × 10
−14
cm
−3
or
less or, if such a reference wafer is not available, on the
reference wafer with the lowest value of net carrier
density (see Section 8.4) in accordance with Procedure
(Section 11), using either the front- or back-surface-
return-contact configuration depending on the type of
wafer to be tested (see Section 7.2). Be certain that the
series resistance of the diode circuit formed with the
reference wafer meets the requirements of Section
11.4.1.7 or Section 11.4.2.7 before proceeding with the
measurement of the capacitance-voltage pairs. Record
each measured capacitance, C
i
, and its associated
voltage, V
i
. Determine the value of C
comp
either from
the net carrier density profile (Section 10.2.2) or from
V
i
as a function of C
i
−2
(Section 10.2.3).
10.2.2 Net Carrier Density Profile
10.2.2.1 Measure a total of n pairs, sufficient to
calculate at least five values of net carrier density, N
i
,
and depth, W
i
.
NOTE 9: If the incremental method of calculation (see
Section 12.3) is used, n ≥ k + 5, where k is an integer chosen
such that C
i + k
is between 80 and 85% of C
i
; if the curve-
fitting method of calculation (see Section 12.4) is used, n ≥ 5.
10.2.2.2 Calculate the net carrier density profile in
accordance with the incremental method (see Section
12.3) or the curve-fitting method (see Section 12.4)
taking C
comp
= 0 (see Section 12.1).
NOTE 10: If an approximate value of C
comp
is known, this
procedure may be shortened by starting with this approximate
value. In this case, if the slope of the net carrier density
profile is positive, increase C
comp
as directed in Section
10.2.2.3 and Section 10.2.2.4 and decrease it after the first
slope change as directed in Section 10.2.2.5; if the slope is
negative, decrease C
comp
initially and increase it after the first
slope change.
10.2.2.3 Increase C
comp
by 0.1 pF and recalculate the
net carrier density profile (see Note 10). For the
recalculation, correct each of the C
i
to C
′
i
as follows: