On the low temperature limits for cryogenic etching: A quasi in situ XPS study
Résumé
The cryogenic plasma etching of silicon for nano- and microelectromechanical devices is known to show an optimal operating temperature around -100 degrees C. The physicochemical mechanisms occurring beyond this limit, however, are not often discussed. Thus, to explore the adsorption of residual gases on the uppermost surface of a Si/SiO2 wafer at -100 <= T <= -147 degrees C, we performed a comprehensive, quasi in situ X-ray photoelectron spectroscopy (XPS) study. Precisely, the cooling down of the sample was performed in a typical plasma-etching reactor at a residual pressure of 10(-4) Pa, being afterwards transferred to the XPS chamber at a constant temperature, under high vacuum. We report that water build-up on the surface becomes a major issue for T < -120 degrees C, as detected in the O 1 s spectra and modelled by QUASES-Generate. The thickness of the H2O layer increases exponentially as the surface temperature decreases, following an anti-Arrhenius behavior. At -147 degrees C, the overlayer thickness is estimated to similar to 125 angstrom, thus, the silicon is barely probed by XPS. Adsorbed fluorine can also be detected at low temperatures, although less markedly. Finally, the effect of the temperature-dependent Fermi level change on the Si 2p peak binding energy is discussed.
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