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SEMI M46-1101 E © SEMI 2001 4 eyew ear, rubber gloves an d protective clothing. Prepare all electroly tes in a f um e hood. 9.2 Ens ure that all electroly tes are correctly labeled. 10 Specimen Prep aration 10.1 T he sam…
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7.3 Instrumentation
7.3.1 Capacitance Meter — Capable of measuring
values up to 300 nF, employing a test signal of
amplitude less than 100 mV RMS. The measurement
frequency should be typically between 1 and 100 kHz.
7.3.2 Equivalent Circuit Model — The meter should
assume a parallel circuit model for the
electrolyte/semiconductor interface and provision
should be made to compensate for the large series
resistance of the electrolyte and contacts.
7.3.3 Potentiostat — Capable of maintaining the
potential of the sample with respect to a reference
electrode constant to ±5mV during the measurement
and with a reference input impedance of > 10
13
Ohms.
7.3.4 Light Source — A source of uniform illumination
of above band gap energy for etching n-type materials.
7.4 Sample Holder
7.4.1 Electrochemical Cell — Used to hold the sample,
contain the electrolyte and support the sealing ring and
other electrodes, see Figure 1.
7.4.1.1 Sealing Ring — A corrosion resistant ring
which forms a seal between the electrolyte and the
sample. This ring defines the measurement area which
should be reproducible and well defined. The ring area
should not be less than 0.008 cm
2
and the
reproducibility between runs should be better than ±
3%. Large ring areas (typically ≥ 0.1 cm
2
) are preferred
for more accurate measurements.
7.4.1.2 Reference Electrode — Used to control the
sample bias via a potentiostat.
7.4.1.3 Counter Electrode — The bias and test signals
are applied between this electrode and the sample.
Figure 1
Electrochemical Cell
7.4.1.4 Sensing Electrode — Placed close to the
sample-electrolyte interface and used to measure the
admittance of the sample.
7.4.1.5 Sample Contacts — Ohmic contacts to the front
and/or back surface of the sample for the purpose of
applying bias.
8 Reagents and Materials
8.1 Purity of Reagents — All chemicals for which such
specifications exist shall conform to SEMI C1.
8.2 Purity of Water — Use either distilled or de-
ionized water having a resistivity greater than
2 MOhm•cm at 25
o
C as determined by the Non-Referee
Test of Test Methods D 1125.
8.3 Selection of Electrolyte — The choice of
electrolyte is not defined by this standard and is up to
the discretion of the user or by agreement between the
customer and the supplier.
8.3.1 InP, GaAs and Related Compounds — Preferred
electrolyte, Na
2
.EDTA (0.1 M) basified with
ethylenediamine to a pH of 10, although other
electrolytes can be satisfactorily used.
NOTE 1: Preferred electrolyte, see reference
“Electrochemical C-V Profiling of Heterojunction Device
Structures” A.C.Seabaugh et al. IEEE Trans on Electron Dev
36 No 2 (1989) 309-313.
9 Safety Precautions
9.1 The electrolytes used are potentially hazardous.
Read the Materials Safety Data Sheets before
attempting to prepare the electrolytes. Use safety
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eyewear, rubber gloves and protective clothing. Prepare
all electrolytes in a fume hood.
9.2 Ensure that all electrolytes are correctly labeled.
10 Specimen Preparation
10.1 The sample should be free from surface debris
and scratches. The sample should be cleaned with
deionized (DI) water and dried.
10.2 To measure a single layer or upper layer of a
multilayered structure, no additional sample preparation
is required.
10.3 For a buried layer in a multilayered structure,
measurement accuracy is improved by removing the
overlaying layers by either the use of selective or
controlled chemical etches.
10.4 If the upper layers cannot be removed by
chemical etching then the sample can be profiled
electrochemically but in some cases this may reduce the
accuracy of the method.
10.5 For conducting substrates Ohmic contacts should
be made to either the front or back of the wafer. For
insulating substrates an Ohmic contact should be made
to the front of the epilayer. Contacts are formed
electrically, by alloying or by any other suitable
method. The quality of the contact should be verified
electrically.
11 Calibration and Standardization
11.1 Sealing Ring — Calibration of the sealing ring
areas is performed using a “blue slice”, made by
growing an anodic oxide film on a polished n-type
substrate.
11.1.1 The “blue slice” is profiled according to this test
method. Etch only the minimum amount necessary to
see the illuminated area (typically 0.5 µm). Flush the
cell several times with DI water before removing the
sample. Wash and measure the sample as soon as
possible.
11.1.2 Wetted Area — The wetted area is observed due
to chemical attack of the oxide film and is measured
using a measuring microscope with cross hair eye-
pieces and micrometer XY stage or by using a zoom
camera and image processing system.
11.1.3 Illuminated Area — Etches in the light and
forms a well defined etch well, which is measured as
per the wetted area.
11.1.4 Calibration Interval — Regularly check the area
and condition of the sealing ring. For daily use, the ring
areas should be calibrated at least three times a week.
11.1.5 Replacement — Rings should be replaced if the
excess area exceeds 10% for rings of area ≤ 0.05 cm
2
or
5% for rings of area > 0.05 cm
2
or if the perimeter is
not uniform.
11.1.6 Alternative Calibration Method — The areas
can be measured using known n and p-type test
samples. This method is best suited for continual
monitoring of the ring area between “blue slice”
calibrations.
11.2 Instrumentation — The bias voltage,
measurement frequency and capacitance measurement
should be checked annually.
11.2.1 Bias — The bias voltage should be checked
with a DVM with an input impedance > 1 MOhm.
11.2.2 Capacitance — The capacitance measurement
should be checked using calibrated fixed value
capacitors covering the expected sample capacitance
range or from 1–100 nF.
11.2.3 Frequency — The measurement frequency
should be checked with a frequency counter.
12 Measurement Procedure
12.1 Choice of Electrolyte — An electrolyte should be
selected that forms a rectifying barrier with the sample,
gives a flat, mirror finish etch well and generates only a
small excess area.
12.2 Sample Mounting — A region of the sample
should be defined which will form the rectifying
contact with the electrolyte. This area can be defined by
some form of sealing ring or mask, but it is important
that the area of contact between the electrolyte and the
sample be precisely known and remain constant during
the measurement.
12.2.1 Sealing Ring — If the sample is pressed against
a sealing ring, the seal must be cleaned with DI water
and dried before positioning the sample. If the sample
needs repositioning, the sample and sealing ring should
be recleaned and dried.
12.2.2 Ring Loading — As the mounting pressure may
effect the reproducibility of the measurement and the
lifetime of the sealing ring, it must be controlled (e.g.
by the reproducible use of a suitable compression
spring).
12.3 Cell Filling and Debubbling — Fill the cell,
which holds the sealing ring and other measurement
electrodes, with the electrolyte. The process of filling
the cell frequently traps bubbles of air on the surface of
the sample. These bubbles must be removed before a
measurement can be made.
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12.4 Contact Evaluation — Verify that the contacts,
for both directions of current flow have low and
approximately equal resistance.
12.5 Rest Potential — Measure the sample’s rest
potential to verify that the reference electrode and
sample are electrochemically stable. Its value usually
lies between 0 and −1 V (respective to a SCE reference
electrode). Values outside this range can indicate a
problem with the sample or reference electrode.
12.6 Cable Compensation — Determine any stray
capacitance or additional resistance associated with the
cell and test leads that would impact the measurement.
12.7 Current vs. Voltage (I-V) — Used to select the
bias for measurement and etching and as a general test
of material quality.
12.7.1 Measurement Bias — is set in the reverse bias,
low dark current region.
12.7.2 Etching Bias (p-type material) — is set in the
forward bias region. The abruptness of the forward bias
breakdown is indicative of the ohmic nature of the
contacts. No or shallow forward breakdown often
indicates unsuitable ohmic contacts.
12.7.3 Etching Bias (n-type material) — is set in the
reverse bias, low dark current region. Illumination
should result in a significant increase in current (photo-
current).
12.8 Capacitance vs. Voltage (C-V) — Used to
evaluate the quality of the electrochemical diode and to
select a range of possible measurement voltages. The
measured flatband potential (respective to a SCE
reference electrode) should lie between −0.5 and
−2.5 V for n-type semiconductors and between –1 and
+1 V for p-type semiconductors.
12.8.1 Measurement Bias — is set in a region of low
dissipation, where the 1/C
2
vs V plot is linear. For
layers grown with concentration gradients, the latter
condition cannot be strictly adhered to and in such
cases the amplitude of the test signal, used for
capacitance measurement, should be kept as small as
possible.
12.8.2 Dissipation — The normally accepted safe level
is < 0.4.
12.8.3 Excess Area Capacitance — For n-type surface
layers the capacitance of the electrolyte/semiconductor
interface is measured prior to etching and the
capacitance associated with the excess area is computed
by multiplying the measured capacitance by the ratio of
the excess area to the wetted area.
12.9 Carrier Concentration Profiling — Profile the
sample using the etching and measurement conditions
determined from the I-V and C-V data.
12.9.1 Carrier Concentration — The carrier
concentration is determined from the C-V data.
12.9.2 Excess Area Correction — For n-type material
the capacitance associated with material in the excess
area should be subtracted from the measured
capacitance.
12.9.3 Etching — The sample is anodically etched at
the etching bias. The depth of the etch well is
determined from the time integral of the current using
Faraday’s law of electrolysis.
12.9.4 Profiled Depth — The profile assumes that the
measured carrier concentration lies at a depth equal to
the sum of the etch and the depletion depths.
12.10 Ring Area — It is advised to measure the etch
well area after profiling and to use this value to
recalculate the carrier concentration vs. depth profile.
13 Calculations and Interpretation of Results
13.1 Carrier Concentration, N
The principle of this method is based on measuring the
slope of 1/C
2
vs V.
()
1
2
2
0
/1
.
2
−
ú
û
ù
ê
ë
é
−
=
dV
Cd
Aq
N
r
εε
[cm
-3
]
where
C is the measured capacitance (for n-type
material the capacitance associated with the
excess area should be subtracted from the
measured capacitance),
V is the applied bias,
q is the electronic charge,
ε
r
is the relative permittivity of the sample,
ε
o
is the permittivity of free space
(= 8.854 × 10
-12
Fm
-1
)
A is the area of the etch well
It is also possible to measure dC/dV by modulating the
bias with a low frequency (secondary signal). The
d(1/C
2
)/dV of the upper equation may be evaluated to
result in the following formula.
dV
dC
C
Aq
N
r
3
2
0
.
1
εε
=
[cm
-3
]
13.1.1 Excess Area Capacitance, excessC
w
iw
excess
A
AA
CC
−
= .
[F]