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SEMI MF1392-1103 © SEMI 2003 10 10.5.2 Connect the cables to one of the precision capacitors. Measure and record the capacitance in pF to three significant figures. Disconnect the capacitor. Repeat for the other precisio…
SEMI MF1392-1103 © SEMI 2003 9
∑∑
==
=
5
11
5
1
j
n
i
ijavg
N
n
N (4)
where:
N
ij
=
net carrier density, cm
−3
, for the i
th
depletion
depth in the j
th
measurement set, as calculated in
accordance with Calculations (Section 12), and
n = number of capacitance-voltage pairs measured
in each of the five measurement sets.
10.3.4 Calculate the effective mercury probe contact
area, A
eff
, in cm
2
, for each reference wafer as follows:
kref
kavg
keff
N
N
AA
)(
)(
)( = (5)
where:
A = nominal capillary area, cm
2
,
(N
ref
)
k
= known net carrier density of reference wafe
r
k, cm
−3
, and
(N
avg
)
k
= the calculated average net carrier density o
f
reference wafer k, cm
−3
.
10.3.4.1 If one reference wafer was measured, record
this value as the effective mercury probe contact area,
A
eff
.
10.3.4.2 If more than one reference wafer was
measured, determine and record the effective mercury
probe contact area, A
eff
, as the average of the effective
contact areas for each of the reference wafers.
10.3.5 For control charting purposes, repeat Sections
10.3.2 and 10.3.3 using A
eff
as the area of the mercury
probe contact. If the value of average net carrier
density for any reference wafer differs from the known
value by more than 2%, condition the surfaces of the
appropriate reference wafer or wafers (see Related
Information 1) and repeat the calibration procedure
(Sections 10.2 and 10.3). If it is impossible to achieve
deviations of 2% or less for all wafers, it may be
necessary to subdivide the net carrier density range and
assign effective probe contact areas to each subgroup.
NOTE 13: Recommended chemical surface treatment
procedures are described in Related Information 1.
10.4 Control Procedures — Periodically carry out tests
and maintain control charts to ensure proper operation
of the electronics (Section 10.4.1), the electronics plus
the mechanical functionality of the probe station
(Section 10.4.2), and the entire system (Section 10.4.3).
10.4.1 Monitor the performance of the electronic
components with a moving range control chart of the
net carrier density of a packaged Schottky diode.
10.4.1.1 Determine the net carrier density as follows:
Attach the diode to the capacitance meter terminals and
carry out the procedure, beginning with Section 11.6.4,
and Calculations, Section 12.
10.4.1.2 If out-of-control conditions are noted, or if the
control limits are wider than is desirable for the
application, verify the performance of the capacitance
meter and voltmeter independently in accordance with
Sections 10.5 and 10.6, respectively, to establish which
requires adjustment or recalibration.
10.4.2 Monitor the performance of the electronic
components together with the mechanical functionality
of the probe station with a moving range control chart
of the oxide capacitance, C
ox
, of an oxidized reference
wafer.
10.4.2.1 Determine C
ox
of an oxidized reference wafer
in accordance with SEMI MF1153, with the mercury
probe serving as the metal capacitor electrode.
10.4.2.2 If out-of-control conditions are noted, or if the
control limits are wider than is desirable for the
application, and the electronics are known to be
performing as expected, inspect the mercury capillary
carefully for dirt or damage. If it is dirty, clean it
thoroughly and refill with clean mercury. If it appears
to be damaged, repair or replace the capillary and refill
with clean mercury.
10.4.3 Monitor the overall system performance with
moving range control charts of net carrier density of the
reference wafers (see Sections 8.4 and 10.3) or test
specimens used as controls.
10.4.3.1 Determine the net carrier density of these
specimens in accordance with this test method. Two or
more controls are required to ensure that the control
wafers are stable; if one control becomes unstable,
retreat the surface (see Related Information 1) and
redetermine its net carrier density. If this does not
bring the specimen into control, discard it and replace
with a fresh control.
10.4.3.2 If system out-of-control conditions are noted,
or if the control limits are wider than is desirable for the
application, and they cannot be attributed to control
wafer instability, mechanical malfunctions (Section
10.4.2) or electronic malfunction (Section 10.4.1),
redetermine C
comp
(see Section 10.2) and the effective
area of the mercury probe contact (see Section 10.3).
10.5 Capacitance Meter Adjustment and Verification
10.5.1 Connect to the capacitance bridge or meter
shielded cables of a length suitable for measuring the
precision capacitors. Zero the capacitance bridge or
meter with the cables attached only to the bridge or
meter, not to a precision capacitor.
SEMI MF1392-1103 © SEMI 2003 10
10.5.2 Connect the cables to one of the precision
capacitors. Measure and record the capacitance in pF to
three significant figures. Disconnect the capacitor.
Repeat for the other precision capacitors.
10.5.3 If the measured capacitance values are not
within 1% of the known values, make necessary
adjustments consistent with the appropriate instrument
instruction manuals to bring the instrument to within
specifications before proceeding with measurements of
test specimens.
10.6 Voltmeter Verification
10.6.1 Measure the precision voltage source at five or
more voltages within the range from 0 to ± 200 V,
inclusive to verify that the digital voltmeter is within
specification over this range.
10.6.2 If any measured voltage value is not within
0.5% of the known value, make necessary adjustments
consistent with the appropriate instrument instruction
manuals to bring the instrument to within specifications
before proceeding with measurements of test
specimens.
11 Procedure
11.1 Refer to Related Information 2 for suggested data
sheet formats for recording the data if the data
collection and calculations are carried out manually or
off-line.
NOTE 14: The following procedures are given in sufficient
detail for manual data collection and calculations to be carried
out. However, it is strongly recommended that both data
collection and analysis be carried out using computer con-
trolled equipment, with data storage and display capabilities.
In such cases, the procedures and algorithms employed must
be equivalent to those given in this test method.
11.2 If not known, determine the conductivity type and
surface orientation of the test wafers in accordance with
SEMI MF42 and SEMI MF26, respectively.
11.3 Estimate the reverse bias voltage range over
which the measurements are to be made based on the
curve in Figure 1, an estimate of the value for the
dopant density of the test specimen, and the range of
depth over which the profile is desired. Do not exceed
200 V or 80% of the breakdown voltage, whichever is
lower.
11.4 Determination of the Series Resistance of the
Diode Circuit — Determine the series resistance of the
diode circuit in one of the following ways:
11.4.1 Measurement of Forward Resistance — If the
capacitance bridge or meter measures capacitance only,
measure the diode forward resistance, R, in ohms, at 1
V forward bias as follows:
11.4.1.1 Connect the curve tracer to the mercury probe
column and to the return contact of the probe fixture.
11.4.1.2 Place the wafer to be tested onto the mercury
probe fixture set up in the configuration used to
determine C
comp
(see Section 10.3) in such a way that
the mercury column(s) will contact the polished or
epitaxial surface of the wafer. If the back-surface-
return-contact configuration is used, make a suitable
return contact to the substrate or back surface of the
wafer.
11.4.1.3 Bring the mercury column(s) into contact with
the surface of the wafer.
11.4.1.4 Measure and record as I
1
the current through
the diode at 0.9 V forward bias, in mA, to two
significant figures.
11.4.1.5 Measure and record as I
2
the current through
the diode at 1.1 V forward bias, in mA, to two
significant figures.
11.4.1.6 Calculate the forward resistance, R, in kΩ, as
follows:
12
2.0
II
R
−
=
(6)
where:
I
1
= current at 0.9 V forward bias, mA, and
I
2
= current at 1.1 V forward bias, mA.
11.4.1.7 If the forward resistance is 1 kΩ or less,
proceed to Section 11.5. If the forward resistance
exceeds 1 kΩ, improve the return contact, and repeat
Section 11.4.1.
NOTE 15: The diode forward resistance at 1 V, R,
determined in this way is a measure of the total series
resistance of the test circuit that includes the bulk, cable, and
return contact resistances.
11.4.2 Direct Determination of Equivalent Series
Resistance — For capacitance meters or bridges
capable of measurement of phase angle, conductance,
or total impedance, determine the equivalent series
resistance directly as follows:
11.4.2.1 Place the wafer to be tested onto the mercury
probe fixture set up in the configuration used to
determine C
comp
(see Section 10.3) in such a way that
the mercury column(s) are just above but do not contact
the polished or epitaxial surface of the wafer. If the
back-surface-return-contact configuration is used, make
a suitable return contact to the substrate or back surface
of the wafer.
11.4.2.2 Zero the meter in accordance with the manu-
facturer' s instructions.
SEMI MF1392-1103 © SEMI 2003 11
11.4.2.3 Bring the mercury column(s) into contact with
the surface of the wafer.
11.4.2.4 Set the bias to a nominal value of 0 V, and
read the capacitance, C, and the phase angle,
θ
, or the
conductance, G
m
.
11.4.2.5 If the phase angle and capacitance were
measured, calculate the series resistance, R
s
, in kΩ, as
follows:
θπ
tan2
10
3
Cf
R
s
=
(7)
where:
θ
= measured phase angle, degrees,
C = measured capacitance, pF, and
f = measurement frequency, MHz.
11.4.2.6 If the capacitance and conductance were
measured, calculate the series resistance, R
s
, in kΩ, and
the phase angle,
θ
, in degrees, as follows:
232
)102(
−
×+
=
CfG
G
R
m
m
s
π
(8)
and
−=
−
CRf
s
π
θ
2
10
tan
3
1
(9)
where:
G
m
= measured conductance, S,
f = measurement frequency, MHz, and
C = measured capacitance, pF.
11.4.2.7 If the series resistance is 1 kΩ or less and the
phase angle is between −87° and −90° , proceed to
Section 11.5. If either of these conditions is not met,
improve the return contact, and repeat Section 11.4.2.
If improvement of the return contact decreases the
series resistance but does not bring the phase angle
within the desired range, qualify the Schottky contact in
accordance with Section 11.5; when the Schottky
contact is satisfactory, repeat Section 11.4.2 to verify
that the phase angle is within the desired range.
11.5 Qualification of Schottky Contact — To qualify
the Schottky contact, determine the reverse current
characteristics of the mercury probe contact as follows:
11.5.1 If a curve tracer was used to determine the diode
forward resistance, do not disconnect it. If the series
resistance was measured directly, connect a curve tracer
or other apparatus for monitoring the current-voltage
characteristics of the mercury probe contact (see
Section 7.7). Apply a reverse bias voltage of about 1 V
to the mercury column. Warning: Avoid physical
contact with the probe fixture when bias is applied.
11.5.2 Measure and record this voltage as V
1
, and
measure the current that exists at this voltage.
Calculate the current density at this value of reverse
bias voltage, J
r1
, in mA·cm
−2
, as follows:
eff
r
r
A
I
J
1
1
= (10)
where:
I
r1
= current, mA, at the reverse bias voltage V
1
, and
A
eff
= mercury probe contact area, cm
2
, see Section
10.3.4.
11.5.3 Increase the magnitude of the reverse voltage at
intervals until the maximum reverse bias voltage that is
to be applied during the test (see Section 11.3) is
reached. Measure each current and calculate the
current density, J
r
, at each value of voltage. In
addition, calculate the rate of increase of the reverse
current density with voltage, in mA·V
−1
·cm
−2
, as
follows:
ii
riir
r
VV
JJ
V
J
−
−
=
∆
∆
+
+
1
)1(
(11)
where:
∆
J
r
/
∆
V
= rate of increase of the reverse current
density with voltage,
J
ri
= current density at voltage, V
i
, and
J
r(i+1)
= current density at voltage, V
i+ 1
.
11.5.4 Also observe whether or not the reverse current
density is stable with time.
11.5.5 If the reverse current density, J
r
, equals or
exceeds 3 mA/cm
2
at any voltage up to the maximum
value applied, first determine if this is due to carrier
density variations in the structure. In this case, reduce
the maximum applied reverse bias voltage to be used in
the test to the highest value for which the reverse
current density is less than 3 mA·cm
−2
.
NOTE 16: If the depletion depth extends to a region with a
rapidly increasing doping density, the breakdown voltage may
be significantly lower than estimated from the expected net
carrier density. For example, if the test specimen consists of a
lightly doped epitaxial layer on a heavily doped substrate and
if the profile extends deeper into the structure than the flat
region of the layer, the breakdown voltage would be lower
than that estimated from the expected net carrier density in the
flat region.
11.5.6 Otherwise, if J
r
> 3 mA·cm
−2
, or if ∆J
r
/∆V > 0.3
mA·V
−1
·cm
−2
, or if the reverse current is unstable in
time, treat the wafer surface with an acceptable