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SEMI MF1529-1104 © SEMI 2004 6 Figure 1 Schematic of Measurement Circuit Show ing Use of a Standard Re sistor and a Probe Wired fo r Configuration A NOTE 7: to semiconductor effect s occurring at higher measurement curre…
SEMI MF1529-1104 © SEMI 2004 5
parameter for the proper probe type in a given application.
The combination of probe radius and force that is chosen
affects both the likelihood of probe penetration and the
quality of electrical contact, which in turn affects the
measurement noise and accuracy. A use test is given to aid in
verifying appropriateness of a given probe for a specific layer
type. Controlled lowering of the probe pins so that contact is
made without lateral scrubbing of the probes against the wafer
surface has been found to be very important.
7.3 Microscope
7.3.1 The microscope for inspecting probe damage shall
have a magnification of at least 600×, and an eyepiece
magnification no greater than 15×. The microscope
shall be capable of dark-field, interference contrast, or
oblique illumination.
7.3.2 The microscope shall have a stage capable of
moving the specimen in order to examine a number of
adjacent damage marks made by each of the four probe
points.
7.4 Measurement Stage
7.4.1 Wafer Prober Stage — With a vacuum chuck or
comparable means of holding the wafer securely during
measurement. This vacuum chuck should be of
sufficient thermal mass to keep the wafer at a constant
temperature, within 1°C, during the time required for
all measurements. The stage should be provided with
stops, pins, engraved circles, or other means for
accurately and repeatedly positioning wafers. For
measurements on wafers where the deposited or
fabricated film may extend over the edges of the
substrate and make contact to the backside, a thin layer
of mica, or other electrical insulator must be used
between the wafer and the chuck.
7.4.2 Probe Assembly Support that allows the probes to
be lowered onto the wafer surface with no evidence of
lateral movement (probe skidding). This requirement
can be verified by lowering and raising the probes a
number of times onto a polished silicon surface with
steps of 50 to 100 m between these locations, and then
observing the probe damage marks for each of the
probe points with the required microscope.
NOTE 5: For a probe with blunt tips or well-conditioned
probe points, it is generally very difficult to view the probe
damage with bright field illumination; use of dark-field,
Nomarski, or oblique illumination is recommended. To aid in
locating the probe damage, the formation of a grid of
rectangles, by scribing, etching or other suitable process on a
polished wafer surface, has been found helpful. The
rectangles should be large enough to allow all four probe
points to be readily located within the boundaries and a
number of probe impressions to be made within the confines
of a single rectangle.
7.4.3 Wafer Probe Stage with a sufficient range of
motion to allow probing all desired locations on the
largest wafers to be measured. Except for restrictions
on the exclusion of three probe spacings at the
perimeter of the layer being measured, the accuracy of
sheet resistance measurements using this test method do
not require any particular accuracy on the position
coordinates.
NOTE 6: If this test method is used for referee
measurements, uncertainty in position coordinates may
produce accurate measurements at the locations measured but
may make comparison of data more difficult. It is
recommended that the wafer be centered on the stage with an
accuracy of 1 mm, or better, and that all positions measured,
have coordinates controlled with an accuracy of 10 m, or
better, with respect to the center of the stage.
7.4.4 The wafer stage shall be instrumented with a
temperature monitor to be used for any application
where the average sheet resistance of the layer is a
parameter to be reported. The temperature monitor
may be of any convenient type, but must be accurate to
0.3°C, or better.
7.5 Electrical Measuring Apparatus
7.5.1 The conceptual layout of the electronic circuitry is
shown in Figure 1 for the case where a standard resistor
is used to monitor the applied current. The standard
resistor can be omitted if the current value is set or
known directly.
7.5.1.1 Constant DC-Current Source — Having
sufficient compliance voltage to supply a constant
current that results in a measured voltage drop on the
specimen that is between 5 and 20 mV. Currents
between 10
6
and 10
2
A are required if the sheet
resistance range 1 to 20,000 is to be covered. The
output current must be stable to 0.01%, or better, during
the time required to take all data at each location; ripple
and other noise must be less than 0.1% of the dc-current
level. A compliance voltage in excess of 10 V is
generally not needed unless measurements must be
made through a significant layer of oxide or other
dielectric. A standard resistor (Section 7.5.1.2) is
needed to determine measurement current unless the
current supply is in calibration and known to output a
dc-current that is within 0.1% or better of the set-point
value. A wet or dry battery may be used for the current
source providing there is means for regulating the
output current.
SEMI MF1529-1104 © SEMI 2004 6
Figure 1
Schematic of Measurement Circuit Showing Use of a Standard Resistor
and a Probe Wired for Configuration A
NOTE 7: to semiconductor effects occurring at higher
measurement currents are avoided. However, for certain
applications of interest, such as metal films and very heavily
doped silicon substrates, it will generally be necessary to
accept measurements with fewer significant figures or else to
use a current supply with an output above the common
maximum of 100 mA, or else to use a voltmeter with sub-
microvolt resolution.
7.5.1.2 Standard Resistor — Used to monitor the value
of the measurement current if the current supply does
not meet the accuracy of settability given in Section
7.5.1.1. The standard resistor shall be selected to give a
potential difference of 0.5 to 5 times that measured
across the specimen. This requires the standard resistor
to have a value from 2.5 to 25 times the sheet resistance
of the layer. The value of the standard resistor must be
known at least to four significant digits.
7.5.1.3 Switches — Double pole, double throw switch
for reversing the direction of the current, and four pole,
double throw switch for changing the probe
configuration. The switching functions may be
accomplished by wafer switches or relays. Isolation
between all switch poles or relays must be 10
6
times the
sheet resistance of the layer being measured; isolation
of 10
9
, or greater is recommended.
7.5.1.4 Electronic Voltmeter — To read the potential
difference across the specimen and standard resistor, or
if calibrated in conjunction with the current source, to
read the voltage-current ratio directly. The voltmeter
shall be capable of measuring dc voltages between 1
and 100 mV full scale, and be able to resolve the
measured voltages to 0.01%, or better. The meter must
have an input impedance of at least 10
9
.
7.5.2 Analog Test Circuit — Five resistors connected as
shown in Figure 2 shall be used according to the
measurement electronics qualification procedure of
Section 11.3 for evaluating the accuracy and precision
of the electronics in the presence of large series
resistors simulating the probe contact resistances.
Several circuits of this type may be needed with the
resistance of the central resistor, r, of each being,
selected according to the expected sheet resistance of
the layer to be measured, as listed in Table 2.
Figure 2
Analog Test Circuit, Simulating the Contact
Resistances in a Four-Probe Measurement
7.5.3 Conductivity-Type Instrument — Apparatus in
accordance with Method A of SEMI MF42.
7.6 Computer Control — It is intended that this test
method will be under control of a computer for
positioning the sample at each of the intended
measurement sites, lowering the probes, and performing
all necessary control of circuit switching, setting of
current values, and measuring and logging voltages. It
is beyond the scope of this test method to specify
details of the computer-based automation.
SEMI MF1529-1104 © SEMI 2004 7
Table 2 Nominal Values of the Standard Resistor
and of the Center-leg Resistor, r, for the Analog
Circuit Appropriate to Various Sheet Resistance
Range
#1
The resistance shall be within a range from one-half to twice the value listed
and its value shall be known to 0.05%.
8 Reagents and Materials
8.1 Purity of Reagents — All chemicals for which such
specifications exist shall conform to the assay and
impurity levels of Grade 1 SEMI specifications for
these 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 Purity of Water — Reference to water shall be
understood to mean Type E-3 or better deionized (DI)
water as described in ASTM Guide D 5127.
8.3 Qualification Wafers
8.3.1 Polished Silicon Wafers — Of any convenient
diameter for making probe impressions to be inspected
for general probe related damage and contact size and
shape. It is useful to divide the surface into rectangular
regions by use of scribe lines or similar technique to aid
in locating a particular series of impressions under the
inspection microscope.
8.3.2 One, or more, wafers from each of the processes
to be evaluated, for testing the electrical suitability of a
given probe for the intended evaluation.
8.4 Reagents for Surface Treatment — If surface
treatment is required, the following chemicals may be
needed:
8.4.1 Buffered HF — 10:1 or more dilute, in
accordance with SEMI C23.
8.4.2 2-Propanol — In accordance with SEMI C41.
8.4.3 Acetone — In accordance with SEMI C19.
8.4.4 Filtered dry nitrogen.
9 Sampling
9.1 In the case of referee measurements, it is left to the
parties to the test to agree upon the number of wafers
from a batch, and their selection procedure, as well as
the number and location of test positions on each wafer.
9.2 In the case of non-referee measurements, for
example, process control or research applications, it is
left to the user of this test method to determine the
number and location of test positions on each wafer.
10 Suitability of Test Specimen
10.1 The front and back surfaces of the wafer to be
measured should be tested for conductivity type using
Method A of SEMI MF42. If they are of the same
conductivity type, a thin sheet of insulating material,
such as mica, should be placed between the wafer and
the stage. This test is not necessary if the front and
back surfaces are known to be of opposite conductivity
type or if the layer is fully isolated from the substrate
by a dielectric layer.
10.2 If the wafer to be measured was fabricated by a
process that uses “finger” clamps or other types of
clamping that intrude into the top surface area of the
water, the wafer is unsuitable for use with sampling
plans that require measurements within several probe
spacings from the wafer perimeter.
11 Preparation of Apparatus
11.1 Visual Inspection of Probe Impressions — This
inspection should be performed when a new, rebuilt or
reconditioned probe is first installed to get an initial
indication of the mechanical performance of the probes.
Once a probe is installed, meets the visual inspection
criteria, and is left mounted, visual inspection of probe
impressions is generally not needed; functional probe
performance tests detailed in Section 11.2 generally
suffice to qualify the probe for continued use. Further
visual inspection of probe impressions is advised,
however, when a probe has trouble meeting the
requirements of Section 11.2.1. More routine
inspection of probe impressions is also advised if
probes are interchanged routinely for special
applications, thus increasing the risk of changes in the
alignment or rigidity of the probe mounting.
11.1.1 After selecting a probe for the intended
application, make a series of at least 10 probe
impressions on a polished silicon surface in steps of 50
to 125 m (0.002 to 0.005 in.). Examine the
impressions from each of the pins to determine that
there is no probe skidding, no cracks or fracture lines
surrounding any of the impressions, and that the
impressions are generally compact in nature (see Figure
3).
11.1.2 If fracture lines are seen, the probe must be
replaced, or conditioned on surfaces of ceramic,
sapphire, lapped silicon or other suitable, non-
Sheet Resistance,
Analog and Standard
Resistor,
#1
<2.5 1
2–25 10
20–250 100
200–2,500 1,000
2,000–25,000 10,000