First in situ Optical Emission Spectroscopy measurements during cesium low-energy depth profiling

Studying ex situ the fundamental processes involved during cesium low-energy molecular depth profiling of polymers is a hard task owing to the well-known cesium reactivity. Lately, we developed in-situ Optical Emission Spectroscopy (OES) on our ToF-SIMS IV instrument. Photons emitted from the sputtered area are monitored in the wavelength range 200 nm to 950 nm. Surprisingly, intense cesium atomic lines are detected throughout the depth profile of a polymer (polystyrene) deposited on a silicon substrate, at 852 nm and 894 nm. However, no optical signals were detected from 200 nm to 950 nm when xenon or oxygen primary ions were used to depth profile polymers. This efficient infrared channel of energy dissipation is unique to the cesium sputtering process. We assume it can play an important role in the way the local energy density is transferred to the material contrary to the use of other primary ions for sputtering. We will show that ToF-SIMS and OES depth profiles can be recorded simultaneously. Finally, cesium-xenon cosputtering experiments on silicon will correlate the 852-nm signal intensity to the cesium concentration in the primary ion beam. The importance of inelastic collisions and electron promotion leading to light emission is often shadowed in our understanding of the sputtering process. These experiments are a first insight of this fundamental process during cesium depth profiling.