NOISE AND THE KELVIN METHOD

The Kelvin probe, used to measure changes in work function (wf), has proved particularly useful as a UHV surface analysis technique, having applications in the study of thin films, adsorption kinetics, surface photovoltage spectroscopy, surface topographies, etc. However the Kelvin circuit is plagued by noise problems, which are considerably enhanced by long connecting wires and the nonideal UHV environment. Careful shielding is essential in order to improve the signal-to-noise (S/N) ratio, especially at backing potentials close to the null condition. Although lock-in-amplifer (LIA) based automatic compensation systems are available they operate on low signal levels and are prone to electromagnetic pickup from the driving element and mechanical instabilities of the vibrating capacitor. We discuss the general noise spectrum and describe an alternative "off-null" approach utilizing a data-acquisition-system (DAS) based measuring system that features an active suspension system (ASS), automatic set-up, and software elimination of driver talkover. This system has applications where very high resolution ( < 0.1 meV) is required, or if low S/N ratios caused by reduced probe dimensions or a high noise background would otherwise prejudice the measurement.