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SEMI MF2166-0304 © SEMI 2004 4 parameters that cover the typical param eter ranges in Section 7.4 . 6.2.5 Short-cycle Time Modificati o n of Method B — The short-cycle time modification is performed in the same manner ex…

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theoretical Q
bias
vs. SPV curve or equivalent “Q-Q”
plot, in the band gap region.
5.2.6 depletion condition — the condition that exists in
the semiconductor surface region when the free carrier
density is insufficient to neutralize the fixed dopant
charge density of donors and acceptors.
5.2.7 dielectric (oxide) electrical thickness, (T
ox
) — the
silicon dioxide equivalent thickness (when the silicon
dioxide dielectric constant is assumed) calculated from
electrical measurement of Q
bias
vs V
surf
(or V
cpd
) for the
capacitor.
5.2.8 effective charge, (Q
eff
) — the oxide charge/cm
2
in
the capacitor that gives the measured flatband voltage.
5.2.9 flatband condition — the point at which an
external applied voltage causes there to be no internal
potential difference across the MOS structure.
5.2.9.1 Discussion — In the flatband condition, the
SPV is zero.
5.2.10 flatband voltage, (V
fb
) —- the applied voltage
necessary to produce the flatband condition.
5.2.11 inversion condition — the condition in which a
minority carrier layer is formed at the semiconductor
surface separated from the bulk by a depletion region.
5.2.12 Kelvin probe — a reference electrode that
vibrates perpendicular to the wafer surface and
generates an AC signal by varying the electrode-wafer
capacitance.
5.2.13 mobile charge density, (Q
m
) — the calculated
charge/cm
2
that moves in the oxide under temperature
and electrical field stress, creating shifts in SPV, V
fb
,
and V
cpd
(or V
surf
).
5.2.14 Monroe probe — a reference electrode that is
stationary and generates an AC signal by the horizontal
vibration of a grounded fork which shields the probe
from the wafer.
5.2.15 surface photovoltage, (SPV) — the change of
the electrostatic potential of the silicon surface caused
by illumination.
5.2.16 surface voltage, (V
surf
) — the potential measured
by the Kelvin or Monroe probe that results from the
potential difference between two metals or between a
semiconductor and a metal, due to their difference in
work function. Also called contact potential difference
(V
cpd
); see Section 5.2.3 .
6 Summary of Practices
6.1 Method A
6.1.1 A special reference wafer is loaded into the
NCDCS.
6.1.2 A series of tests, which must include CTS, is
executed to position mobile ions into a consistent pre-
test condition. All CTS times must be at least 4
minutes to allow full movement of the mobile ions.
6.1.3 Three additional identical test sequences are
executed to collect data to evaluate performance of the
instrument.
NOTE 1: The bias, temperature and time used for the last
CTS prior to carrying out the above tests should be employed
for these test sequences.
6.1.4 Performance of the NCDCS and its selected
components is checked by measuring the following
characteristics: initial surface voltage (V
surf
), flatband
voltage (V
fb
), effective charge (Q
eff
), density of interface
traps (D
it
), mobile charge (Q
m
), and electrical dielectric
thickness (T
ox
).
6.1.5 Measurements are made at three points on the
surface of the wafer and the mean, standard deviation
and relative standard deviation of the results obtained in
the three identical sequences (Section 6.1.3 ) are
computed and recorded.
6.1.6 The results are compared with the values
supplied with the special reference wafer to evaluate the
NCDCS performance.
6.1.7 If it is desired to check the system over its full
range of operation, this procedure is repeated using two
or more additional special reference wafers with
parameters that cover the typical parameter ranges in
Section 7.4 .
6.1.8 Short-cycle Time Modification of Method A —
The short-cycle time modification is performed in the
same manner except that the measurement of Q
m
is
carried out first, in order to assure consistent alignment
of the mobile ions prior to the other measurements.
The Q
m
measurement is followed by the measurement
of the other parameters. No repetitions are required; the
test sequence in this modification is only run once.
6.2 Method B
6.2.1 First any remaining residual charge on a special
reference wafer is neutralized by immersing it in DI
water.
6.2.2 Then the special reference wafer is loaded into
the NSCDS and the surface voltage (contact potential
difference) is zeroed to within ± 0.3 V.
6.2.3 Then the steps in Sections 6.1.2 –6.1.4 and 6.1.6
are followed.
6.2.4 If it is desired to check the system over its full
range of operation, this procedure is repeated using two
or more additional special reference wafers with
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parameters that cover the typical parameter ranges in
Section 7.4 .
6.2.5 Short-cycle Time Modification of Method B —
The short-cycle time modification is performed in the
same manner except that the measurement of Q
m
is
carried out first, in order to assure consistent alignment
of the mobile ions prior to the other measurements.
The Q
m
measurement is followed by the measurement
of the other parameters. No repetitions are required; the
test sequence in this modification is only run once.
6.3 Method C
6.3.1 The steps in Sections 6.2.1 and 6.2.2 are
followed.
6.3.2 Then a dummy mobile ion run is performed to
position the mobile ions into a consistent pre-test
condition.
6.3.3 Then three additional mobile ion runs are made
and the average, standard deviation, and relative
standard deviation of the mobile ion density are
determined.
6.3.4 The results are compared with the mobile ion
density data supplied with the special reference wafer to
evaluate the system performance.
6.3.5 If it is desired to check the system over its full
range of operation, this procedure is repeated using two
or more additional special reference wafers with
parameters that cover the typical parameter ranges in
Section 7.4 .
6.3.6 Short-cycle Time Modification of Method C —
The short-cycle time modification is performed in the
same manner except that the first CTS time is 4 min
and the second is 8 min. Only a single mobile ion run
is carried out.
7 Special Reference Wafers
7.1 Fabricate the special reference wafers (SRW) from
p-type silicon with a multi-layer dielectric including an
embedded layer of mobile ions confined to the top
portion of the dielectric. These mobile ions drift during
temperature-bias stressing. Since the drift occurs only
in the top of the dielectric, the Kelvin (Monroe) probe
easily senses a shift in V
surf
(V
cpd
) for the mobile ion
concentration calculation. Confining the mobile ions to
the top of the dielectric also ensures that measurements
associated with the silicon-dielectric interface are left
unchanged after the mobile ion test.
7.2 Limit the thickness of the dielectric on the special
reference wafers to between 30 and 70 nm, to provide
sufficient distance for the mobile ions to drift while
remaining clear of the interface.
7.3 Ensure the dielectric is rugged enough so that
repeated use, for example, once a day for six months, of
the same test sites does not result in shifts in electrical
parameters.
7.3.1 Check parameter drift every 6 months. Parameter
drift over time should be less than 10% in six months.
If drift exceeds this value, replace the special reference
wafers.
7.3.2 Test wafers must be re-calibrated and
documented semi-annually by testing the wafers on two
documented NCDCSs that have been maintained using
this Practice. The results must match within 10%. If
the results exceed this value, replace the special
reference wafers.
7.4 Test Parameter Ranges — p-type special reference
wafers should have parameters appropriate for the test
application. Parameters in the following ranges cover
all anticipated applications of the SRWs.
7.4.1 V
fb = −0.1 to −5.0 V,
7.4.2 Qeff = 0.5 to 10 × 10
11
charge/cm
2
,
7.4.3 Qm = 0.7 to 20 × 10
10
charge/cm
2
,
7.4.4 Dit = 0.5 to 50 × 10
11
charge/cm
2
, and
7.4.5 Tox = 30 to 200 nm.
7.5 Supporting Data
7.5.1 Data collected using the appropriate method shall
be supplied with each special reference wafer.
7.5.2 Provide appropriate reference data for each wafer
on a data sheet similar to that in Figure 1 (Methods A or
B) or Figure 2 (Method C).
7.5.3 These values are the targets against which the
system is evaluated.
7.6 To determine the suitability of the system over its
range of operation, select a set of at least three special
reference wafers with values of each parameter that
span the ranges given in Section 7.4 .
8 Calibration Data Sheet
8.1 A test plan shall be agreed upon among all parties
to the test. Data shall be recorded on a Test
Certification Sheet similar to that supplied with the
special reference wafers. Examples are shown in
Figures 1 and 2.
9 Procedure
9.1 Observe and record the temperature and relative
humidity of the room where the testing is taking place.
9.2 Method A — for NCDCDs that employ a point
source corona:
9.2.1 Place a special reference wafer in the NCDCS.
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9.2.2 Program the NCDCS to measure the following
parameters of p-type wafers: initial V
surf
, T
ox
, Q
eff
, V
fb
,
D
it
, and Q
m
.
9.2.2.1 Program the CTS portion of the Q
m
mea-
surement to run for at least 4 min at a temperature from
170 to 200°C.
9.2.3 Align the point probe over position A, then
execute a dummy measurement to stabilize the mobile
ions. Repeat this procedure at two other locations. An
example would be the locations (0,0; −40,40; and 40,
−40), where the x and y locations are expressed in
millimeters, relative to the wafer center, using a
Cartesian wafer coordinate system as defined in SEMI
M20. (See Figure R1-1.)
9.2.4 Execute three additional, identical runs.
Determine and record the parameters listed in Section
9.2.2 .
9.2.5 Calculate and record the average, standard
deviation, and % standard deviation for each parameter
measured at each location in runs 2 through 4. Enter
the data on a data sheet such as the example in Figure 1.
9.2.6 To check the NCDCS over its full range of
operation, repeat Sections 9.2.1 –9.2.5 using at least
two additional special reference wafers with parameter
values that span the typical parameter ranges in Section
7.4 .
9.3 Method A, Short-cycle Modification
9.3.1 Execute the procedure in Section 9.2.1 .
9.3.2 Next, execure the procedure in Section 9.2.2 ,
except program the NCDCS to measure Q
m
first,
followed by the other listed measurements.
9.3.2.1 Run the CTS portion of the measurements for
at least 4 minutes at a temperature from 170–200°C.
9.3.3 Then, execute the procedure in Section 9.2.3 .
9.3.4 Enter the data on a data sheet such as the example
in Figure 1. Use the “Mean” columns, leaving the “Std
Dev” and “% Std Dev” columns blank.
9.4 Method B — for NCDCSs that employ a line
source corona and can be programmed to obtain a full
set of parameters:
9.4.1 Rinse a special reference wafer in a DI water bath
for 10 min then dry for 10 min to naturalize any
residual charge.
9.4.2 Place the special reference wafer in the NCDCS.
9.4.3 Measure the voltage on the wafer surface (V
cpd
).
9.4.4 Zero the voltage on the wafer (V
cpd
should be
between −0.3 V and +0.3 V). Use positive or negative
corona sweeps as appropriate to erase the wafer
voltage. Re-measure and record the voltage on the
wafer surface to verify it has been adjusted to zero.
NOTE 2: To determine the voltage change for one corona
sweep, measure the voltage on the wafer. Then sweep the
wafer with one positive sweep. Re-measure the voltage on
the wafer. The difference in voltage on the wafer is the
voltage change per corona sweep.
9.4.5 Program the NCDCS to measure the following
parameters of p-type wafers: T
ox,
Q
eff,
V
fb,
D
it,
and Q
m
.
9.4.5.1 Program the CTS portion of the Q
m
mea-
surement to run for at least 4 min at a temperature from
170 to 200°C.
9.4.6 Choose three test locations such as (0,0; −40,40;
and 40,−40). Execute a dummy measurement to
stabilize the mobile ions.
9.4.7 Execute three additional, identical runs.
Determine and record the parameters listed in Section
9.4.5 .
9.4.8 Calculate and record the average, standard
deviation, and % standard deviation for each parameter
measured at each location in runs 2 through 4. Enter
the data on a data sheet such as the example in Figure 1.
9.4.9 To check the NCDCS over its full range of
operation, repeat Sections 9.3.1–9.3.7 using at least two
additional special reference wafers with parameter
values that span the typical parameter ranges in Section
7.4 .
9.5 Method B, Short-cycle Modification
9.5.1 Execute the procedures in Sections 9.2.1 –9.4.4 .
9.5.2 Next, execute the the procedure in Section 9.4.5 ,
except program the NCDCS to measure Q
m
first,
followed by the other listed measurements.
9.5.2.1 Run the CTS portion of the measurement for at
least 4 minutes at a temperature from 170–200°C.
9.5.3 Execute the procedure in Section 9.4.6 .
9.5.4 Enter the data on a data sheet such as the example
in Figure 1. Use the “Mean” columns, leaving the “Std
Dev” and “% Std Dev” columns blank.
9.6 Method C — for NCDCSs that employ a line
source corona but can only determine mobile ion
density.
9.6.1 Execute the procedures in Sections 9.4.1 through
9.4.4 .
9.6.2 Perform four mobile ion runs as follows: