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2.6.3 Oxidation
Oxidation is probably the biggest source of poor contact resistance in a well
maintained probe station. Oxidation builds up on the probe and pad metals over time
to form an electrically insulating layer that prevents metal to metal contact. The light
oxidation that forms between routine uses can normally be wiped clean when the
probe is landed. If the probe or pad is allowed to form a thick oxide film (tarnish), more
aggressive action is necessary. Pre-cleaning or over-travel of the probe tip
(scratching) may be required to create forces large enough to break the film. The
larger, softer tips that help increase surface area may not be as good at scratching
through oxidation; therefore, a compromise is often necessary. Controlled electrical
current can also be used to break through any remaining insulating barrier.
2.6.4 Four-Lead
Measurement
A four-lead measurement technique is frequently used during resistance
measurements to eliminate the effect of unwanted contact and lead resistance.
In this technique, the two excitation current leads are separated from the two voltage
measurement leads all the way down to the probe tips. A reasonable amount of
contact resistance and small changes in contact resistance will not appear in the
voltage measurement because there is no current flowing through the voltage
contact. However, this technique will not overcome contacts that have too much
resistance or are non-ohmic.
2.6.5 Ohmic versus
Non-ohmic Contacts
Ohmic contacts result from a good interface between two conductive surfaces.
They are called ohmic because they exhibit a linear relationship between current and
voltage, like a resistor. Non-ohmic contacts are typically formed when oxides or other
contamination is present between the conductive surfaces. They exhibit a non-linear
relationship between current and voltage more closely resembling a diode. This is
undesirable because signals resulting from the contact cannot easily be subtracted
from the desired signal of the sample.
2.6.6 Measuring
Contact Quality
Measuring contact quality is always recommended for critical measurements to
make sure the contact resistance is low and the contact is ohmic. Both contact
resistance and ohmic behavior can be checked at the same time. The most common
DC technique is to excite two probe contacts at a time with different positive and
negative currents and plot the measured voltage (IV curve). A linear curve with a low
slope indicates a good contact. Non-linearity in the curve indicates a non-ohmic
contact. If the test cannot be performed on the actual device, probing technique can
be verified by landing two probes on one sample pad and making an IV curve prior to
probing the device.
In the case of a microwave measurement, contact resistance increases the series
resistance of the microwave circuit. However, the film between the probe and
measurement pad can form a capacitor. This capacitance will change the
S-parameters of the device as measured by a network analyzer. A level response on
the network analyzer is typically the best measure of contact quality.
2.6.7 Lab Protocol
Lake Shore recommends developing a lab protocol to ensure consistent contacts. The
protocol should include probe handling, routine cleaning, landing probes and
measuring contacts. The probe landing instructions given in section 4.6.2 describe
the action of skating the probe tip on the sample pad. The amount of skate is one of
the most important parts of the protocol. More skate provides more wiping to clean
away oxidation and more pressure to increase actual contact area. Too much skate
will damage the probe tip.
