The Influence of Electrode Surface Conditions on the Bouncing Behaviour of Low-Velocity Charged Microspheres in a High Field Gap

Abstract

This study presents an experimental and theoretical investigation into the influence of electrode surface conditions on the bouncing impact behaviour of charged microparticles in a high voltage gap, It uses a simulation technique based on a microparticle gun that can inject positively charged, low velocity microparticles into a planar UHV gap. Experiments have been performed on different combinations of particle/ target materials (e.g. carbonyl - iron, gold, titanium, copper and nickel ) and a variety of surface states (e.g. mechanically polished, argon ion etched, electron bombarded and electropolished) which are monitored ellipsometrically. It is found firstly, that the mean mechanical behaviour as characterized by the coefficient of restitution or e-values exhibits general trends which are similar to those found with corresponding macro systems, The observed differences in the critical velocity for elastic impact for the micro and macro systems and the large scatter in the measured e-values are interpreted in terms of the microtopography of a surface, Secondly, the electrical behaviour, as characterized by the ratio reversed/initial charge or Q-values, is found i) to exhibit overall trends that are similar for all particle/ electrode combinations and ii) to vary in magnitude for individual surfaces, This is interpreted in terms of the electrical properties of the contaminating films, in particular the tunnelling resistivity and relative work functions of the junction oxide layers which are shown to be influential in determining the magnitude of charge transferred (a) and hence the Q-values. Theoretically, the evaluation and assessment of an existing model based on quantum mechanical tunnelling across the oxide junction has resulted in a modified expression for which predicts theoretical values that are in close agreement with experimental values. Finally, in considering the technological implications of these findings it has been shown that although both the mechanical and electrical properties determine the breakdown criterion for a material it is the electrical properties which are the more dominant.