Near-surface residue formation in CF4/H2 reactive ion etching of silicon

Surface residues and near-surface damage present in the surface and near-surface regions of Si after reactive ion etching with CF4, CF4+H2, and CHF3 were investigated with secondary ion mass spectrometry, laser ellipsometry, x-ray photoelectron spectroscopy, and cross-sectional transmission electron microscopy. Specifically, the change in the thickness and composition of the deposited residues with changes in the backfill gas chemistry was investigated. A fluorocarbon (FC) residue was found to develop on all etched samples, with thicknesses ranging from a few angstroms to microns. Two regimes of FC deposition were identified, where (a) a steady-state FC layer quickly develops during etching, and the net FC deposition rate drops to zero thereafter, and (b) the FC deposition continues at a constant rate throughout the etch. F was found to diffuse to the Si surface to form a fluorinated Si layer between the deposited FC layer and the bulk Si. This layer was taken to be the reaction layer resulting from diffusion-limited kinetics in this etch system. The diffusion of F or C into the near-surface region of Si was not observed to change with increasing H2 additions. A simple quantitative model was developed based on rate-limiting Fickian diffusion through the deposited FC layer. The model was found to agree well the available etch rate data. Si etching with CHF3 was found to behave similiarly to CF4+H2 etching in the steady-state FC layer regime.