Oxidation of Si beneath thin SiO2 layers during exposure to HBr/O-2 plasmas, investigated by vacuum transfer x-ray photoelectron spectroscopy

Thin SiO2 layers were exposed to an HBr/O-2 plasma for a variety of short p eriods, reproducing the over-etching process after polycrystalline Si gate electrodes have been etched and the gate oxide layer is exposed. Samples we re transferred under vacuum to an x-ray photoelectron spectrometer for anal ysis. After relatively thick (> 60 Angstrom) films were exposed to a 10% O- 2/HBr plasma at an average ion energy of similar to 150 eV, the near-surfac e region becomes brominated, and the thickness of the film decreases, indic ating an etching rate of similar to 1 - 2 Angstrom/s. When the starting fil m thickness is between 10 and 20 Angstrom, however, exposure to the plasma results in an increases in the thickness of the film, and is enhanced with the increasing addition of oxygen to the feed gas. At mean ion energies of 40 or 150 eV, the transition from etching to deposition occurs at oxygen ad ditions of similar to 1% or similar to 8%, respectively. The increase in Si O2 thickness is ascribed mainly to oxidation of the Si at the oxide-substra te interface, and not to deposition resulting from sputtering of reactor ma terials. Consumption of crystalline Si beneath a 12 Angstrom thick SiO2 gat e oxide, adjacent to a 600 Angstrom linewidth, polycrystalline gate electro de was also observed after etching of this transistor structure, as confirm ed by transmission electron microscopy. (C) 1999 American Institute of Phys ics. [S0003-6951(99)01509-0].