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

To measure electrical conductivity of materials in scales ranging from nano meter 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 sc ales. The SEM was used for observing the tips for positioning, as well as t he sample surface together with scanning reflection-high-energy electron di ffraction capability. This four-point probe system has two kinds of special devices. One is octapole tube-type scanners for tip scanning parallel to t he sample surface with negligible displacements normal to the surface. Anot her is a pre-amplifier which can be switched in current measurement mode be tween tunnel contact for scanning tunneling microscopy and direct contact f or four-point probe method. The electrical resistance of a silicon crystal with a Si(1 1 1)-7 x 7 clean surface was measured with this machine as a fu nction of probe spacing between I mm and I un. The result clearly showed an enhancement of surface sensitivity in resistance measurement by reducing t he probe spacing. (C) 2001 Elsevier Science B.V. All rights reserved.