Silicon etch by fluorocarbon and argon plasmas in the presence of fluorocarbon films

Abstract

Molecular dynamics simulations have been conducted to study the mechanisms of silicon etch in the presence of fluorocarbon species (CF and C4F4), F atoms and Ar+ ions. The specific goal of the study was to find conditions in which steady Si etching occurs in the presence of a fluorocarbon (FC) film. Results indicate that if incident species are not properly chosen for the simulation, either steady etching is observed with no FC film present, or a FC film is present (often continuously growing in thickness) with no steady etching of the underlying film. With the proper set of incident species, C∕F ratio, neutral/ion flux ratio, and ion energy, we observed steady Si etching in the presence of a steady FC film. We also observed that the thicker the FC film, the lower the etch yield. A sufficiently thick film results in no etching and a continuous deposition. Simulation results are in qualitative agreement with analogous experimental measurements. The key is to find FC species that will stick with a high probability, forming a relatively open and porous film. In addition, our results suggest that this film will fluctuate in thickness from impact to impact, resulting in better transport of incident F and SiFx species, to and from the underlying Si, respectively. Steady state etching appears unlikely if the overlying FC film has the hard, dense, cross-linked character of films deposited from energetic fluorocarbon species. The basic mechanisms of etching, and the composition and depth of the underlying layers appear to be largely unaffected by the presence of the FC film. We have found ion energy deposition at a range of depths to be crucially important in the creation and transport of etch products. Ion energy deposition, ion-induced mixing, and reaction promotion are the key processes in all of the ion-assisted processes simulated, including the present case of etching in the presence of FC film. The FC film retards etching by slowing the rate at which etchants (e.g., F) reach the underlying film and the rate at which etch products leave. If the film is sufficiently thick or dense, etching will cease.