The homebuilt sheet resistance measurement station consists of an in-line four point probe that is connected to a current source and a Keithley voltmeter.
A. Turn on power to the voltmeter and current source.
B. Set the voltmeter to the 200 mV scale.
C. Set the current source to 0.453 mA.
D. At these settings the measured voltage in mV multiplied by 10 is equal to
the sheet resistance in ohms/square. For different samples, it may be necessary
to adjust the voltage or current ranges. In general, as long as the probe spacing
is larger than the wafer thickness, Rs=4.53V/I.
E. There are two knobs on the current source, 'Voltage Compliance' and 'Output Selector'.
Never touch the former and make sure the Output Selector is in Standby mode.
F. Place the standard sample (a piece of TCO-coated glass) under the 4 point probe.
G. When the sample is not in contact you should see the voltage jumping around. Put the
four-point probe in contact with the TCO - the voltmeter should read essentially zero.
H. While maintaining contact, switch the Output Selector to either + or -. The
absolute value of the voltage (in mV) is 1/10 the sheet resistance which should
be ~ 10 ohms/square for the TCO. Switching from +/- just changes the direction
of the current flow so the induced voltage changes polarity, however the absolute
values should be similar. There will be differences which give you an error
bar (why might there be differences?)
I. Remove the TCO sample and repeat the above steps with your sample.
J. Turn off both the current source and voltmeter when finished.
K. Report any deviations or anomolous behavior to your instructor.