Kelvin Probe Actuator

A variety of ingenious mechanisms have been used to generate the required spacing modulation including rotation, piezo-electric and voice-coil drivers- even piano wire has been attempted! In terms of vacuum applications piezo-electric and voice coil drivers remain the easiest to implement. The figure above shows an example of an UHV compatible head stage using a voice coil driver. The majority of the probe construction is stainless steel including the suspension system which consists of three laser-cut diaphragm springs. The vibration frequency (30 - 300 Hz) and amplitude of oscillation (0.01 - 1.00 mm) and probe mean spacing (± 50 nm) are controlled by a digital oscillator located in the host PC.

The voice-coil driving system is deliberately well removed (250 mm) from the tip ensuring that no traces actuator signal can be detected on the input signal, i.e. driver talk-over. The stainless steel tip customarily has a diameter of 2-3 mm and can be gold plated if desired. The suspension system can be easily adapted to multiple tip geometry allowing both high and low spatial resolution on one head stage.
Alternative piezo-electric actuator designs such as Besocke's are customarily located in close proximity (10-20) mm from the sample. My experience of such drivers indicates that capacitive coupling in the form of talkover noise can pose considerable problems particularly if a LIA based self-nulling detection system is used. In such cases it can be difficult, in the vacuum environment, to distinguish real changes in Vc from stray capacity effects- a 'clean' Kelvin signal is essential for high resolution measurements.