The work presented here has been a collaborative effort between Dartmouth
College and IBM Microelectronics.  Our force-based Kelvin Probe is based upon
a non-contact atomic force microscope designed and built at IBM -- Yorktown.
It consists of two control loops.  One control loop is used to maintain the
probe/sample spacing such that the van der Waals (vdW) force gradient remains
constant, yielding an estimate of the surface topography of the sample under
study.  The second control loop is used to minimize the deflection of the
probe's cantilever due to electrostatic forces.  From this feedback we obtain
estimates of the work function difference (EPD) and capacitance (dC/dz).
The system is able to detect lateral features as small as 25 nm.  The vdW loop
has a sensitivity of 0.5 angstroms per square root Hertz and the EPD loop has
a sensitivity of 5 mV per square root Hertz.