DIELECTRIC SURFACE FLASHOVER IN VACUUM - EXPERIMENTAL-DESIGN ISSUES

In order to determine the influence of intrinsic material properties o f dielectric materials on high field surface flashover characteristics in vacuum, it is important that we minimize or eliminate the influenc e of the experimental test structure on the high field processes. In t his paper we shall examine the flashover characteristics of polycrysta lline alumina insulators, using a lateral test structure similar to th e one used in semiconductor devices. A significant improvement in surf ace flashover strengths (similar to 200%) was found using a metalized contact electrode system compared with a classical solid electrode sys tem. The results indicate that, using a metalized film cathode, the sc atter in the flashover data is significantly reduced compared with the solid electrode system, implying that the electron-dielectric interac tion processes that lead to flashover occur in a more predictable fash ion. The new metal contact electrode system reveals the dependence of flashover strength on the surface preparation and the grain size of th e dielectric material. The average flashover strength increases and th e scatter in the flashover data decrease with a decrease in the averag e grain size of the particles composing the alumina ceramic. Thus the surface flashover processes in vacuum are related to the dielectric su rface microstructure, specifically the surface and sub-surface microda mage and grain-boundary defects.