Diffusion tip shaping under vacuum breakdown

Summary form only given. Surface self-diffusion is an essential process initiating vacuum breakdown. It builds up microtips which leads to electric field and current density enhancement up to the threshold values. Using the Herring expression for chemical potential, tip surface point displacement may be calculated. When the maximum displacement point of the surface coincides with the maximum curvature point the drain condition is fulfilled. We find that for a needle-like tip stable build up is realized at a field about 1.5 higher then that given by the Dyke formula. This is just the condition for vacuum breakdown threshold to be achieved.