Independently driven four-tip probes for conductivity measurements in ultrahigh vacuum

To measure electrical conductivity of materials in scales ranging from nanometer to millimeter, a four-point probe system was developed and installed in an ultrahigh-vacuum scanning electron microscope (UHV-SEM). Each probe, made of a W tip, was independently driven with piezoelectric actuators and a scanner in XYZ directions to achieve precise positioning in nanometer scales. The SEM was used for observing the tips for positioning, as well as the sample surface together with scanning reflection-high-energy electron diffraction capability. This four-point probe system has two kinds of special devices. One is octapole tube-type scanners for tip scanning parallel to the sample surface with negligible displacements normal to the surface. Another is a pre-amplifier which can be switched in current measurement mode between tunnel contact for scanning tunneling microscopy and direct contact for four-point probe method. The electrical resistance of a silicon crystal with a Si(1 1 1)-7×7 clean surface was measured with this machine as a function of probe spacing between 1 mm and 1 μm. The result clearly showed an enhancement of surface sensitivity in resistance measurement by reducing the probe spacing.