Polymer layers formed on silicon wafer during silicon oxide overetching using C4F8 helicon wave plasmas and their properties were investigated using spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry. Thr degree of overetching and d.c. self-bias voltage were varied to investigate the effects on the characteristics of the polymers remaining on the overetched silicon surface. The increase of bias voltage from - 80 V to -120 V increased the CIF ratio and carbon bonds such as C-C, C-CFx, and C-Si in the polymer while reducing the thickness of the polymer layer. However, prolongation of the overetch time from 50% to 100% did not change the chemical composition of the polymer layer and the carbon binding states in the polymer layer remained the same even though the polymer thickness was increased. The polymer layer formed at the higher d.c. self-bias voltage was more difficult to remove by the subsequent post-etch treatments compared to that formed at longer overetch time.