Mechanism of highly selective SiO2 contact hole etching

Abstract

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 CFx 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.