UHV-compatible spectroscopic scanning Kelvin probe for surface analysis

We have developed an ultra-high vacuum(UHV) compatible scanning Kelvin probe (SKP) to measure local work function (phi) differences between a conducting sample and a reference metal tip to less than 1 meV. The work function is an extremely sensitive indicator of surface condition and is affected by adsorption, evaporation, surface topography, etc. For example, the increase of phi due to oxidation of Si(111) and polycrystalline rhenium is 1.4 and 1.9 eV, respectively. We have performed SKP work function topographies of metal and semiconductor samples during various UHV cleaning processes to determine if changes in surface work function (Delta phi) can be attributed to chemical contamination, e.g., carbon, or surface structural changes due to thermal processing or ion sputtering. We can, for instance, see major changes in oxidation kinetics due to the type of cleaning mechanism, flash anneal or sputter-anneal, or through as little as 0.6% carbon contamination. The UHV SKP control loop utilises a never tracking system to maintain constant tip-to-sample spacing during scanning. Combined with the in-house 'Off-Null' detection method we have developed this allows a significantly higher signal-to-noise ratio than alternative detection methods including the lock-in amplifier. Using this system we have performed in situ surface photovoltage spectroscopy during the oxidation of Si(111), illustrating the capability of this technique to probe the local density of states, and surface barrier height spectroscopy during surface processing. (C) 1999 Elsevier Science B.V. All rights reserved.