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

The significant and often unpredictable variations, or transient effects, o bserved in the secondary ion intensities of O+/- and Si+/- during the initi al stages of depth profiling with Cs+ have been studied. These were found t o be primarily due to two competing effects: (a) the steady accumulation of Cs in the substrate as a function of sputtering time and (b) the varying o xygen content from the native oxide as a function of depth. These effects p revail over depths approximated by similar to 2R(norm), where R-norm is the primary ion range normal to the surface. The Cs+ induced effects are consi stent with a work function controlled resonance charge transfer process. A method for controlling these effects, namely the prior evaporation of Cs an d use of an O-2 leak during analysis is described. Doped (As and Sb) and un doped Si wafers with similar to0.9 nm thick native oxides were analyzed usi ng 0.75 and 1 keV Cs+ beams incident at 60 degrees. The more intense polyat omic AsSi- and SbSi- emissions did not exhibit these effects, although othe r relatively minor intensity fluctuations were still noted over the first s imilar to0.5 nm. (C) 2000 American Institute of Physics. [S0021-9606(00)702 43-1].