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SEMI MF525-0705 © SEMI 2003, 2005 7 11.4.1 Measure the sprea d ing resistance of the uniform p -type silicon specimen (see ¶8 .2) in accordance with ¶¶13.4 throug h 14.2. Consider the probes t o be satisfactor y if the m…
SEMI MF525-0705 © SEMI 2003, 2005 6
NOTE 3: It is desirable to use at least three resistivity values per decade of resistivity. For applications where spreading
resistance measurements are being used to sort material within a narrow resistivity range (one decade or less) it may be necessary
to have 8 or 10 calibration specimens spanning the decade of interest to compensate for the resistivity averaging effect of a four
probe on nonhomogeneous calibration specimens. This is particularly important for resistivity ranges where calibration material
is found to have significant fine-scale resistivity variations (striations).
8.1.2 The standards shall match the conductivity type and orientation of the test specimens. The surface finish of
the standards shall match the surface finish of the test specimens as closely as possible. For use in connection with
epitaxial layer test specimens, standards shall have a polished surface.
8.2 A p-type silicon specimen of uniform resistivity with a (111) orientation, for which the typical spreading
resistance per probe tip is between 300 and 1000 , shall be used to ensure that the probe tips are properly
conditioned (see ¶11.4.1).
NOTE 4: An epitaxial layer of 0.1 to 1 ·cm, 10 m or thicker is acceptable.
8.3 A substrate, lapped or ground with 5 m grit slurry or a tape or cloth coated with an abrasive grit of 8,000 to
12,000 grade, shall be used for conditioning the probe tips (see ¶11.4).
8.4 Specimen Preparation Materials — Consisting of polishing materials and pads or other backing plates, as
follows:
8.4.1 Silica Powder — Colloidal or non-colloidal in aqueous suspension with pH from 7.0 to 11.5 and either hard
plastic such as methyl methacrylate (for small area specimens) or a poromeric polishing pad of urethane or
polyurethane foam with a napped or sueded surface and attached to a rigid backing plate (for large area specimens).
8.4.2 Diamond Suspension — In fluid or paste carrier with diamond in the range from 0.1 to 1 m, together with
(1) a glass plate lapped with aluminum oxide in the range from 5 to 12 m (and thoroughly removed subsequent to
lapping), for use with small area specimens, or (2) a nonwoven chemotextile polishing pad attached to a rigid
backing plate, for use with large or small area specimens.
8.4.3 Aluminum Oxide — 0.3 m particle size in water together with a non-woven chemotextile polishing pad
bonded to a rigid backing plate.
8.4.4 Aluminum Oxide — 5 m particle size, together with a frosted glass or cast iron lapping plate.
9 Safety Precautions
9.1 Use normal safety precautions in operating the electrical equipment.
10 Sampling
10.1 Since the most suitable sampling plan depends upon individual conditions, no general sampling procedure is
included here. For referee purposes a sampling plan shall be agreed upon by the parties to the test before the tests
are started.
11 Preparation of Apparatus
11.1 Adjust the probe separation to the desired value if a multiple-probe arrangement is employed. Record the
probe separation.
11.2 Choose a loading in the range from 10 to 100 gf (98 to 980 mN), inclusive, to be applied to the probes. In a
multiple-probe arrangement, use the same loading for each probe.
11.3 Once the loading on the probes has been chosen, determine an appropriate descent rate of the probes on the
specimen to minimize damage to the points and avoid fracturing the specimen.
NOTE 5: The descent rate specified in ¶7.1.1.3 is adequate for a load of 40 gf (390 mN).
11.4 Condition probe tips by stepping them at least 500 times on a silicon substrate that has been ground with 5 m
grit slurry (see ¶8.3) or very lightly abrade the individual probe tips using the abrasive cloth or tape.
NOTE 6: It is permissible to leave the slurry on the substrate during conditioning, but it must be removed from probe tips before
measuring test specimens.
SEMI MF525-0705 © SEMI 2003, 2005 7
11.4.1 Measure the spreading resistance of the uniform p-type silicon specimen (see ¶8.2) in accordance with
¶¶13.4 through 14.2. Consider the probes to be satisfactory if the measured spreading resistance is within ±10% of
the usual value that has been found to be consistent over a long period of time.
11.4.2 If the probes are not satisfactory, repeat the conditioning step or use new probe tips.
11.5 Using the microscope at 400×, examine the probe marks for reproducibility (Note 7). If the probe marks from
a given probe do not appear similar, recondition (see ¶11.4) or replace the probe.
NOTE 7: All the marks from a specific probe should appear to be similar. Probe imprints from different probes need not be
identical.
11.6 If the two-probe arrangement is being employed, verify that the spreading resistances of the two probes are
equal to within 10% when measured on the uniform p-type specimen (see ¶8.2) using the one-probe configuration.
If they are not, recheck or adjust the loading (see ¶11.2) and descent rate (see ¶11.3) to be equal on both probes. If
satisfactory results are not achieved with equal probe loading and descent rate, recondition (see ¶11.4) or replace the
probes.
11.7 Connect the appropriate electrical circuit (see Figures 1 through 3). If a voltage source is used (constant-
voltage or log comparator methods), adjust the potential to 20 mV or less. If a current source is used (constant-
current method), short circuit the output.
11.8 Note and record the voltage to be applied in millivolts as V (constant-voltage method), the current to be
applied in milliamperes as I (constant-current method), or the value of the standard resistor in ohms as R
0
(log
comparator method).
12 Calibration
12.1 Measure the resistivity of the specimens to be used as standards (see ¶8.1) at 23 ± 3°C in accordance with
SEMI MF84, if not previously measured. Record the results or the previously determined value.
12.2 Prepare the surface of the calibration specimens using the materials of ¶¶8.4.1, 8.4.2, 8.4.3, or 8.4.4 using
manual or machine procedures, the choice of materials and procedure being governed only by the requirement that
the calibration specimens must be prepared in the same manner as the specimens to be tested. If the specimen to be
tested is epitaxial, the use of a lapped surface is not to be used for the calibration specimens.
12.2.1 Clean the polishing material from the calibration specimens using water, or solvent in the case of solvent-
soluble vehicles used for certain diamond polishes. Place the specimen on a hot plate at 150 ± 20°C for 10 to 15
min if calibration specimens were prepared with silica or aluminum oxide polishing or lapping compounds.
NOTE 8: If the chosen calibration specimen preparation is anything other than lapping with 5 m aluminum oxide, it is
advisable to remove a minimum of 25 m of specimen surface the first time the specimens are prepared, subsequent to resistivity
measurement by SEMI MF84 in order to remove any subsurface damage left by the 5 m lapping process required in SEMI
MF84.
NOTE 9: The frequency with which calibration specimen surfaces should be re-prepared is a function both of the inherent
stability of the surface for the chosen preparation procedure, and of possible exposure of the calibration specimens to
environments such as acid fumes. The stability of the specimen surfaces for the chosen procedure should be established by each
user of this method.
12.3 Make a minimum of 20 spreading resistance measurements on each of the proposed standards. Make the
measurements as closely as possible to the region where the four-probe measurements (see ¶12.1) were made. Make
these measurements over a length approximately equal to the separation of the outer probe tips of the four-probe
array used for the resistivity measurements. Record the results.
12.4 Compute the mean of the measurements made on each of the proposed standards and calculate the standard
deviation for each set of measurements. If the standard deviation of the spreading resistance measurements is
greater than 10% of the mean, do not use the specimen as a calibration standard.
12.5 Using the resistivity value and the corresponding spreading resistance mean for each suitable calibration
standard, fit an appropriate polynomial, piecewise linear, or spline curve to the calibration data for the groups of
specimens of each conductivity type and orientation. Plot the data and calibration curve on graph paper.
SEMI MF525-0705 © SEMI 2003, 2005 8
13 Procedure
13.1 Handle the specimen carefully to avoid contamination or damage to the surface.
13.2 Make all measurements with an ambient temperature of 23 ± 3°C. Record the ambient temperature.
13.3 Determine the orientation of the specimen surface in accordance with SEMI MF26, determine the
conductivity type of the substrate and, if present, the epitaxial layer in accordance with SEMI MF42. For epitaxial
layers, determine the thickness in accordance with either SEMI MF95 or SEMI MF110. Record the results.
13.4 Prepare all bulk specimens to be tested using the same procedure as was used for the calibration specimens in
¶12.2. Do not prepare the surface of any epitaxial specimens. Thoroughly clean polishing material residues from
the specimen using water or solvent as necessary. If the materials of ¶8.4.1 or ¶8.4.3 were used for specimen
preparation, proceed to ¶13.5, otherwise proceed to ¶13.6.
13.5 Place the specimen on a hot plate with the surface to be measured upwards, not in contact with the hot plate.
Heat the specimen in air at 140 ± 20°C for 10 to 15 min. Remove the specimen from the hot plate. Allow the
specimen to return to 23 ± 3°C before continuing with the measurements.
13.6 Rigidly mount specimen on stage under probes. Use of wax mounting to base block used to hold specimen
during surface preparation or use of vacuum clamping is acceptable. Position the specimen on the specimen holder
so that the probe or probes will be applied at the desired measurement location.
13.7 Lower the points to make contact with the specimen surface, and adjust to within 0.1% (unless the comparator
method is being used) the voltage or current source to the desired value (see ¶11.8).
13.8 After a stabilization period of 1 s or longer, measure and record the current in milliamperes as I (constant-
voltage method), the voltage in millivolts as V (constant-current method), or the output of the log comparator as
log(i
1
/i
2
) (log comparator method).
13.9 Adjust the voltage source to a value of 20 mV or less or short circuit the current source and lift the probes.
13.10 Move the specimen to the next position, making sure that the step spacing is larger than the diameter of the
specimen area damaged by the probes.
13.11 Note and record the step spacing.
13.12 Repeat ¶¶13.6 through 13.11 until the desired number of measurements has been made.
14 Calculations
14.1 Calculate the spreading resistance, R
s
, in ohms, for each measurement as follows:
14.1.1 Constant-Voltage Method
I
V
R
s
(3)
where:
V
= applied voltage, mV, and
I = measured current, mA.
14.1.2 Constant-Current Method
I
V
R
s
(4)
where:
V
= measured voltage, mV, and
I = applied current, mA.
14.1.3 Comparator Method
)/log(
210
iiRR
s
(5)