Transient effects in ultra shallow depth profiling of silicon by secondary ion mass spectrometry

摘要

The significant and often unpredictable variations, or transient effects, observed in the secondary ion intensities of 0:!: and Si :!: during the initial stages of depth profiling with Cs + have been studied. These were found to be priJ::narily due to two competing eff~cts: (a) the steady accumulation of Cs in the substrate as a function of sputtering time and (b) the varying 9xygen content fr~m the native oxide as a function of depth. These effects prevail over depths approximated ~y -2Rnonn , where Rnonn is the primary ion range normal to the surface. The Cs+ induced effects are consi&tent with a work function controlled resonance charge transfer. process. A method forco~trolling these effects, namely the prior evapbration of Cs and use of an O2 leak during analysis is described. Doped (As and Sb) and undoped Si wafers with -0.9 nm thick native oxides were analyzed using 0.75 and .1 keY Cs+ beams incident at 60°. The more intense polyato~c AsSi- and SbSi- emissions did not exhibit these effects, although other relatively minor intensity fluctuations were still noted over the first -0.5 nm.