Oxidation of Si Beneath Thin SiO sub 2 Layers During Exposure to HBr/O sub 2 Plasmas, Investigated by Vacuum Transfer X-Ray Photoelectron Spectroscopy

Thin SiO sub 2 layers were exposed to an HBr/O sub 2 plasma for a variety of short periods, reproducing the over-etching process after polycrystalline Si gate electrodes have been etched and the gate oxide layer is exposed. Samples were transferred under vacuum to an x-ray photoelectron spectrometer (XPS) for analysis. After relatively thick (>60 angstroms) films were exposed to a 10% O sub 2/HBr plasma at an average ion energy of ~150 eV (obtained by applying an RF bias that resulted in a DC bias of - 110 V), the near-surface region becomes brominated, and the thickness of the film decreases, indicating an etching rate of ~1-2 angstroms/min. When the starting film thickness is between 10 and 20 angstroms, however, exposure to the plasma results in an increase in the thickness of the film. This increase in thickness is enhanced with the increasing addition of oxygen to the feed gas. At a mean ion energies of 40eV or 150eV, the transition from etching to deposition occurs at oxygen additions of ~1% or ~8%, respectively. The increase in SiO sub 2 thickness is ascribed mainly to oxidation of the Si at the oxide-substrate interface, and not to deposition resulting from sputtering of reactor materials. Consumption of crystalline Si beneath a 12 angstroms thick SiO sub 2 gate oxide, adjacent to a 600 angstrom linewidth, polycrystalline gate electrode was also observed after etching of this transistor structure, as confirmed by transmission electron microscopy.