semi合集-English.pdf - 第7429页
SEMI MF1529-1104 © SEMI 2004 10 diameter being scanned. Doing so would risk noisy measurements if probes were placed in previous measurement locations. This may be avoided by slightly misaligning the probe pins with resp…
SEMI MF1529-1104 © SEMI 2004 9
where:
r
fi
and r
ri
= the calculated values for the analog box
resistor, with current in the forward an
d
reverse directions, respectively, for the
i
th
measurement,
R
s
=
the value of the standard resistor, ,
V
af
and V
ar
= the potentials measured across the
analog box in the forward and reverse
directions, respectively,
I
af
and I
ar
= the values of the current in the forward
and reverse directions, and
V
sf
and V
sr
= the potentials measured across the
standard resistor in the forward an
d
reverse directions.
11.3.3.2 For each of the ten pairs, r
fi
and r
ri
, calculate
the average, r
mi
:
)(
2
1
rifimi
rrr (2)
11.3.3.3 Calculate the overall average,
m
r
v
of these ten
values of r
mi
:
10
1
10
1
i
mim
rr
v
(3)
11.3.3.4 Calculate the sample standard deviation, s
a
, of
the ten values as follows:
10
1
2
3
1
i
mmia
rrs
v
(4)
11.3.4 Requirements for the Analog Test Circuit — For
the electrical equipment to be suitable for referee
measurements at the sheet resistance value just
simulated, it must meet the following requirements:
11.3.4.1 The value of
m
r
v
must be within 0.25% of the
known value of the resistor, r, and
11.3.4.2 The value of the standard deviation, s
a
must be
no greater than 0.1% of
m
r
v
.
NOTE 9: If not previously known, the value of the analog
circuit resistor may be determined by using the procedure of
Sections 11.3.1 to 11.3.3.2 but measuring the potential across
the analog box using the connections V in Figure 2.
12 Procedure
12.1 Specimen Preparation
12.1.1 If the specimens have been kept in a clean, non-
contaminating atmosphere, or are to be measured within
3 h after fabrication, proceed to Section 12.2.
12.1.2 Remove possible organic contaminants that may
arise from the storage container as follows: Rinse the
specimen in acetone for 1 min. Remove. Immediately
immerse in 2-propanol for 1 min. Remove. Blow dry
with filtered dry nitrogen. Repeat if necessary until
specimen is free from visible stains, streaking or other
visual evidence of residue.
12.2 Mounting and Checking the Wafer
12.2.1 Using vacuum paddle, tweezers, or robotic
arrangement, mount the wafer on the stage, so that the
wafer center is within 1 mm of the stage center. Clamp
the wafer to the stage with vacuum or other means.
Lower the probes onto the wafer. If not known
measure the electrical resistance between any of the
probe pins and the stage to verify that the electrical
isolation is at least 10
9
.
12.2.2 Allow sufficient time for the wafer's temperature
to equilibrate with that of the stage. Thirty seconds is
sufficient if the wafer had been held at room
temperature prior to mounting. Longer times are
necessary if the wafer was recently removed from a
reactor or other elevated temperature process.
NOTE 10: The absolute value of the wafer temperature
during measurement affects the absolute values of sheet
resistance, particularly for lightly doped layers, but it should
not affect the measurement of uniformity of sheet resistance
values, as long as the temperature is constant within 0.2°C
during measurement of any wafer. However, for referee
measurements or process control applications, for which the
absolute sheet resistance values are likely to be an important
part of the measurement results, it is important to maintain the
stage temperature within a narrow range (e.g., 23 ± 1°C) or to
develop an empirical relation between sheet resistance and
measurement temperature for each layer fabrication process
of interest.
12.3 Measurement Site Selection — Using the
measurement site selection agreed to for a referee
measurement or that decided upon for process control
or other application, follow Sections 12.4 through 12.6
at each measurement site before raising the probe and
moving to the next site.
NOTE 11: Various site selection plans may be chosen
according to the application needs. These include, but are not
limited to: diameter scans with step sizes appropriate to the
spatial resolution needed, sparse sampling extensions of the 5-
point and 9-point plans described in SEMI MF81, and area
sampling plans with the size of the area and number of points
chosen by measurement time constraints and process
information requirements, such as the circular or rectangular
patterns of SEMI MF1618. This test method makes no
recommendation about choice of sampling plans, since user
needs and interests are widely varied.
NOTE 12: If wafer diameter scans are chosen, the probe
should not be oriented so that the pins lie exactly along the
SEMI MF1529-1104 © SEMI 2004 10
diameter being scanned. Doing so would risk noisy
measurements if probes were placed in previous measurement
locations. This may be avoided by slightly misaligning the
probe pins with respect to the diameter.
12.4 Measuring in Configuration A to Obtain R
A
—
Connect the probe to the electronics so that the current
will flow through the outer pins (designated Pins 1 and
4), and that the specimen voltage will be measured by
the digital voltmeter (DVM) using the inner pins (Pins
2 and 3), Figure 4. Lower the probe into the wafer.
Start with the current in either direction (called
“forward”), and adjust the current to obtain a specimen
voltage between 7 and 12 mV, inclusive. It is
recommended that the current value be increased from
low levels until the required voltage is obtained, rather
than lowering the current from large values. Measure
the current (using the proper standard resistor, or by
monitoring the set point of a calibrated current supply),
and the specimen voltage, each to a resolution of at
least 0.01% of the value. Record the current reading as
I
f (1-4)
, and the voltage reading as V
f (2-3)
. Reverse the
direction of the current and again measure the specimen
voltage and current to the same resolution. Record the
current readings as I
r (1-4)
and the voltage reading as
V
r (2-3)
. The subscripts used for current and voltage are
those of the pins that perform the current-carrying and
voltage-measuring functions (see Caution, Note 8).
12.5 Measuring in Configuration B to Obtain R
B
—
Without raising the probe from the measurement site,
connect the probe to the current supply so the current
flows through one of the outer probes, and the
nonadjacent inner probe, Figure 5. (There are two ways
of doing this that should provide equivalent results.)
Connect the remaining two probes to the voltmeter,
making sure that the wiring polarity corresponds to that
used for the connections to the current supply. With the
current in the forward direction, measure and record the
current as I
f (1-3)
and the voltage as V
f (2-4)
), with the same
resolution as in Section 12.4. Reverse the direction of
the current; measure and record the current I
r (1-3)
and
the voltage as V
r (2-4)
.
NOTE 13: The notation in Section 12.5 assumes Pins 1 and 3
were used for current, and Pins 2 and 4 were used for
specimen voltage measurement. If the other choice of wiring
the probe for Configuration B were used, the subscript
notation in Section 12.5 (and Section 13.1.2) would be
changed accordingly.
12.6 Moving to the Next Measurement Site
12.6.1 Turn off, or short circuit, the output of the
current supply; raise the probe and move the stage to
the next measurement site. Follow the procedure of
Sections 12.4 and 12.5 at this and all other sites.
Figure 4
Schematic Wiring of Four-point Probe for
Measurement in Traditional Configuration A
Figure 5
Schematic Wiring of Four-point Probe in One of the
Two Choices for Configuration B
SEMI MF1529-1104 © SEMI 2004 11
12.6.2 The current at each measurement site may be
kept at the value used for the first site, or it may be
readjusted for each site to give a specimen voltage
across the specimen sites that is within very tight limits.
Whenever the probes are lowered or the current is
changed, make sure that there is a short pause to ensure
that the probes, the current supply, and the
measurement voltage lines have stabilized before taking
data. Depending on the wafer type and the design of
the instrumentation, this may be from a fraction of a
second to several seconds.
12.6.3 Measure and record the temperature of the stage
to the nearest 0.1°C at least at the beginning and the
end of the measurements. Actual measurement of
temperature for each measurement site is preferred if
experience indicates that noticeable drift or fluctuation
(greater than 1°C) is likely to occur during the course of
the measurements.
13 Calculations
13.1 Calculate the Resistances from Each
Configuration for Each Site
13.1.1 Calculate the average of the forward voltage-
current ratio and the reverse voltage-current ratio for
Configuration A as follows to obtain R
A
:
)41(
)32(
)41(
)32(
2
1
r
r
f
f
A
I
V
I
V
R
(5)
13.1.2 Calculate the average of the forward voltage-
current ratio and the reverse voltage-current ratio for
Configuration B as follows to obtain R
B
(see Note 15):
)31(
)42(
)31(
)42(
2
1
r
r
f
f
B
I
V
I
V
R
(6)
13.1.3 The forward and reverse voltage-current ratios
must agree within 5% of the larger of these ratios both
for R
A
and for R
B
in order to be acceptable for use in
referee measurements.
13.2 Calculate Sheet Resistance at Each Site — For
each measurement site, calculate the sheet resistance,
R
s
, as follows:
Aas
RKR (7)
where:
2
2
872.7173.25696.14
B
A
B
A
a
R
R
R
R
K (8)
13.3 Reviewing the Data — Many applications of
uniformity testing involve acquisition of large amounts
of data. It is generally beneficial to review these data
before interpreting them simply as manifestations of
sheet resistance non-uniformity. A useful technique is
to plot the values obtained for R
s
as a time sequence,
that is, in the order the data for the site were taken for
these sites. Examination of this plot may be correlated
with measurement site coordinates to reveal unexpected
edge proximity effects or regions of the wafer that have
unusual non-uniformity that may need to be re-probed
with a different site-selection plan. Examination may
also reveal one or more points that are highly
inconsistent with adjacent points, possibly indicating
problems due to vibrations or spots of surface
contamination. Note such observations, but do not
delete or edit the data, unless agreed upon by all parties
to the test.
13.4 Analysis and Summary of Data — The sheet
resistance data may be analyzed and summarized in a
number of ways as agreed upon by parties to referee
test, or as appropriate for process control or other
applications. These may include, but are not limited to:
listing of all data with site coordinates, contour plots of
the deviations from average value, and distributional
statistics of the data.
14 Report
14.1 The report shall include the following information:
14.1.1 Operator's name, date and time of
measurements,
14.1.2 Wafer identification number and description,
14.1.3 Identification of instrument used, by
manufacturer, serial number, and model number,
14.1.4 Identification of probe used, by manufacturer,
serial number and probe spacing, probe tip radius and
material, and probe force specifications,
14.1.5 Statement of site location plan used,
14.1.6 Initial and final temperature at wafer stage for
the measurement sequence,
14.1.7 Summary of sheet resistance data as chosen in
connection with Section 13.4,
14.1.8 Notations about aberrant measurement sites as
identified by any data screening procedures employed,
and
14.1.9 Data from probe qualification tests.
15 Precision and Bias
15.1 Precision — Single laboratory values for
repeatibility of the measurement of wafer uniformity
were obtained from a series of tests run in 1991. For
these tests, five types of wafers with average sheet
resistance values from about 15 to about 400 were
tested, as detailed in Related Information 1.