The Veeco four point probe is used for measuring
resistivity and calculating sheet resistance of samples.
See Theory of operation.
The Veeco can penetrate thin insulating
surface layers by applying a short 170v pulse
to the probe tips when the penetrate function
is selected. Then, after making the resistivity
measurement, the unit can determine the type of
the semiconductor. For low resistivity samples
the type is determined by applying an ac signal
between two probes and monitoring the dc bias
of the wafer with a third probe. Lightly
doped semiconductors will form rectifying
contacts with the probe tips causing the
wafer bias to go up (positive) for N type
material and negative for P type material.
Highly doped semiconductors will not develop
a conclusive wafer bias, but the applied ac
signal can heat such material sufficiently
for observation of the thermoelectric effect
(as in the hot-point probe). If neither
method is conclusive, both N and P indicators
will flash.
NOTE: The electronics do not always know
when the tests for conductivity type are
inconclusive. If it disagrees with the hot
point probe, disregard it.
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