Dependence of SiO2 etch rate on sidewall angle as affected by bottom materials in a high-density CHF3 plasma

Abstract

The dependence of SiO2 etch rate on sidewall angle, as affected by different bottom materials, in an inductively coupled CHF3 plasma was investigated. The bottom materials were SiO2, Si, and fluorocarbon polymer, and the bias voltage of the plasma was varied from −50to−400V. Differences in the angular dependency of the etch rate were observed for the different bottom materials at sidewall surface angles higher than 75°, in which case the amounts of bottom-emitted particles impinging on the sidewall surface were relatively large and the transfer of ion energy to the SiO2 substrate was relatively small due to the formation of a fluorocarbon passivation film. The extent that the bottom materials affected the dependence of etch rate on sidewall angle changed, depending on the bias voltage range. When the bias voltage was in the range between −50 and −200V, the effect of a SiO2 bottom was the smallest and those of Si and fluorocarbon polymer were essentially the same. However, at −400V, the effect was substantially larger for the fluorocarbon polymer bottom than for the other two materials, although it was slightly larger for Si than for SiO2. These differences, as a function of bias voltage range, can be attributed to two factors that determine the amounts of bottom-emitted particles: the thickness of the steady-state polymer film formed on the substrate surface during the etching process and the depth of ion-energy transfer to the polymer film. The thickness of the surface polymer film determines the amount of available source for particle sputtering and the depth of ion-energy transfer determines the extent to which incident ions contribute to the sputtering.