In Plasma ion beam analysis of polymer layer and adsorbed H monolayer etching

Abstract

We present two experiments where a layer is plasma-etched while monitoring its evolution by in plasma ion beam analysis. First, we etch a photoresist with a diffuse O${}_2$ plasma at low pressure. Using a 4.335 MeV He beam, Rutherford Backscattering Spectrometry and Elastic Recoil Detection spectra are acquired every minute during 8 h. Etching of most elements follows a linear trend, but H desorbs faster at the beginning of the plasma process, which we ascribe to the ion beam-induced desorption. In addition, we observe a thin Mo layer building up at the surface, likely due to the sputtering of an electrode in the plasma source. Secondly, we etch in HF a crystalline Si (c-Si) sample with $<100>$ surface orientation, which should leave 14 H/nm${}^2$ bonded to the c-Si surface. The sample is then introduced in the chamber and exposed to a diffuse Ar plasma at low pressure. During plasma processing, the H surface concentration is monitored using a resonant nuclear reaction with a ¹⁵N beam at 6.385 MeV. The initial H concentration is $11.7\pm 1.1$ H/nm${}^2$, and it decreases over a 3-minute timescale to an equilibrium concentration of $6.0\pm 0.8$ H/nm${}^2$. Over the range of experimental conditions investigated, the diffuse Ar plasma is therefore not able to entirely sputter the H from the c-Si surface.