Sidewall-angle effect on the bottom etch profile in SiO₂ etching using a CF₄ plasma

The dependence of the bottom etch profile on the sidewall angle in the CF₄ plasma etching of an SiO₂ film was investigated using a Faraday cage, which allowed ions to impinge on the sidewall at specified angles. The bottom etch profile obtained at -500 V was not affected by the sidewall when the angle between the sidewall and the bottom surface was 45° but showed microtrenching when the angle was greater than 60°. The microtrench depth increased until the angle reached 80°, beyond which the local etch rate was drastically reduced to allow the corner rounding of the bottom profile. As the sidewall angle increases, the etch rate of the bottom surface near the corner is controlled by two opposing factors. The decreasing number of incident ions on the sidewall surface and the increasing shadowing of the bottom surface from ions and neutrals by the sidewall contribute to the reduced etch rate, whereas the increasing kinetic energy and the narrower ion-angular distribution of the ions reflected from the sidewall contribute to the enhancement of the etch rate. In addition to these factors, the enhanced roughness of the sidewall surface due to the fluorocarbon film deposition plays the critical role of suppressing the microtrench formation. Microtrenching was not observed when the sidewall surface was covered with rough fluorocarbon film but was observed when the surface was smooth due to the energy transferred by ions higher than the threshold energy for the fluorocarbon film sputtering. © 2001 American Vacuum Society.