Secondary electron emission and surface potential of SiO 2 film surface by negative ion bombardment

Abstract

Secondary electron emission (SEE) yield and surface potential of SiO 2 film induced by carbon-negative-ion bombardment were studied in relation to ion velocity in the order of 10 5 m s −1. By using SiO 2 thin films of 120 and 360 nm thick and decreasing current density of ion beam from 200 to 0.1 nA cm −2, the charging effect was compensated by a leakage current through the insulating film. True SEE yield was evaluated as the saturated value of apparent SEE yield obtained along decreasing current density. The SEE yield from SiO 2 for C − varied from 1 to 4 in proportion to the ion velocity in a range of 1–6 × 10 5 m s −1. At the same momentum, both SEE yields for diatomic and monatomic ions, C 2 − and C −, were almost same value. SEE yield for C − was larger than that for C + by about one unit. As a result, SEE mechanism from SiO 2 by negative-ion bombardment is considered to have two mechanisms of a kinetic emission as well as positive ions and an additional emission of electron due to negative-ion. The surface potential of 1 μm-thick SiO 2 under C − and C 2 − bombardments were measured by the energy shift of secondary electron energy distribution. Obtained surface potential was several volts negative and gradually decreased from −3 to −5 V with increase in ion velocity. This slight surface potential means a high surface barrier for electron is formed in the state under negative-ion bombardment, where true SEE yield is suppressed to one unit. Here, based on this consideration with results of SEE yield and surface potential, an electric double layer model was investigated in order to increase surface barrier for the charging mechanism of insulator under negative-ion bombardment.