Mechanism of highly selective SiO₂ contact hole etching

To investigate the mechanism of highly selective SiO₂ contact hole etching, we analysed surfaces exposed to C₄F₈/Ar/O₂ plasma in a dual-frequency parallel-plate etching system. The thickness and composition of the fluorocarbon layer on large areas and at the bottom of contact holes were quantitatively analysed, in conjunction with the flux of CF_x chemical species and the etch rates. In a highly selective etch processes, the thickness of the fluorocarbon layer on the SiO₂ surface is less than 1 nm, while that on the Si surface is 4–6 nm. We found that the etch rate is strongly affected by the thickness of the fluorocarbon film on the SiO₂ and Si surfaces at the bottom of the contact holes, as well as on the large areas. However, a small increase of C₄F₈ gas flow causes a larger increase in the fluorocarbon film thickness at an aspect ratio of 4 than that on the large flat area surface. Furthermore, we observed a slightly thinner and C-rich film at an aspect ratio of 10. This was probably caused by the decrease in the radical flux passing through the small hole. This aspect-ratio dependence is the cause that the process window for highly selective hole etching tends to be narrow.