Electron beam induced conductivity in polymethyl methacrylate, polyimide, and SiO2 thin films

Abstract

Electron beam induced conductivity (EBIC) is one sensitive parameter that controls the charging of insulating materials under electron beam irradiation. In an earlier work [J. Vac. Sci. Technol. B 21, 2638 (2003)], we reported the measurement of EBIC in polymethyl methacrylate (PMMA) and thermal SiO2 thin films using an external bias method. The thin films under test were sandwiched between a silicon substrate and a metal electrode. One important observation is that the exposed region in the PMMA resist is ohmic regardless of the bias polarity. However, the EBIC in the metal-thermal oxide-silicon structure is highly asymmetric under opposite bias polarities. A model involving the internal emission of secondary electrons was proposed to interpret the asymmetric EBIC in thermal oxide. In this study, we extend the EBIC measurements to deposited SiO2 thin film and another polymeric material, polyimide. By putting the deposited oxide between symmetric metal electrodes, we validated the conjecture of the internal secondary electron emission from silicon substrate into thermal oxide when silicon is used as the bottom electrode. The results on deposited oxide also implicate the close dependence of EBIC in oxide on the property of Si∕SiO2 interface. On the other hand, under the same exposure condition, polyimide thin film is 50 times more conductive than PMMA, probably as a result of their different molecular structures.