Residence time effects on SiO2/Si selective etching employing high density fluorocarbon plasma

Abstract

The residence time effects on SiO2 etching characteristics using inductively coupled plasma of C4F8 alone were first studied in the range from 6 to 300 ms. It was then found that SiO2 and Si etch rates were minimum at a residence time of 25 ms, at which the CF1+ ion density and the fluorocarbon polymer deposition rate measured at 130 °C became maximum. From this good correspondence, the SiO2 etching was considered to follow a reaction model where the CF1+ ions might contribute to the polymer deposition, thus suppressing the SiO2 etching, and where the dominantly observed CF3+ ions could etch SiO2 on the assumption of elevated SiO2 surface temperature due to the ion bombardment. Next, in the condition of short residence times (<25 ms), Ar was added to C4F8 in order to allow Ar+ ions to remove the fluorocarbon polymer film that is responsible for the reduction of the SiO2 etch rate. For a residence time of 10 ms the SiO2 etch rate continuously increased with the Ar concentration up to a maximum etch rate of 0.4 μm/min for 90% Ar addition. Various plasma diagnostics demonstrated that the 90% Ar addition resulted in an increase of the CFx+ (x=1–3), C+, and Ar+ ion densities, in contrast to a decrease of the CF3 radical density. Metastable Ar atoms as well as highly elevated electron temperature are considered to be responsible for the increasing ion species. Consequently SiO2/Si contact hole features with 0.18 μm opening and 2 μm depth were successfully fabricated employing the 90% Ar/C4F8 mixture at a residence time of 10 ms. The He addition has also been investigated and showed similar changes in plasma characteristics to those observed for Ar addition, but the “etch stop” occurred. This probably resulted from the poor sputtering effect of He+ ions due to their light mass.