Plasma cleaned Si analyzed in situ by x-ray photoelectron spectroscopy, secondary ion mass spectrometry, and actinometry

Abstract

Silicon surfaces are cleaned in an electron cyclotron resonance excited hydrogen plasma and characterized by in situ x-ray photoelectron spectroscopy and in situ static secondary ion-mass spectrometry. Emission spectroscopy and actinometry are used to characterize the hydrogen plasma. Exposure to the plasma for 3 to 4 minutes without applying heat or bias to the substrate completely removes the native silicon oxide resulting in a hydrogen terminated surface that is resistant to reoxidation. Adventitious hydrocarbon, when present on the surface, is also completely removed by the plasma. A shift in the isotope ratios of silicon suggests that a clean 〈100〉 silicon surface is monohydride terminated, whereas a 〈111〉 silicon surface appears largely dihydride terminated. A depth profile of the silicon isotope ratios shows a temporal instability, which with the assignment of a H 1s state in the valence-band spectra provides evidence that the hydrogen is concentrated at the surface and has not diffused deep into the silicon lattice. The oxygen removal rate has the following characteristics: two distinct microwave operating regimes separated by a discontinuity in power around 600 W; a singularity corresponding to rapid oxygen removal at 2.5 mTorr; an abrupt and near monotonic decrease in oxygen removal above 14 mTorr; and an invariance of the removal rate to ion-energy from about 10 to 100 eV. The density of hydrogen excited species and the ground state hydrogen atom density are correlated with the oxygen removal rate under all conditions except high pressure, where the density of hydrogen ions is low. This suggests an ion-induced etching mechanism whereby the native silicon oxide removal is enhanced with low-energy hydrogen ion bombardment.