Plasma immersion ion implantation (PIII) with BF3 and SiF4 plasmas is used to fabricate shallow P+/N junctions in Si. Exposure to the plasma and accelerated ions can lead to simultaneous etching and deposition on the substrate during implantation. A simple mathematical model for this process is presented and applied to the case of shallow implantation of BF3. Etching rates of SiO2 are seen to vary with power and pressure of the process gas. Etching rates of Si, SiO2, and CoSi2 are studied by spectrophotometry and Rutherford backscattering spectrometry. The roughness of Si substrates and SiO2 and CoSi2 films before and after PIII is monitored by atomic force microscopy.