Neutral transport during etching of high aspect ratio features

Abstract

This paper studies the transport of neutral etch species in cylindrical holes, which are of interest for advanced memory devices. The etching of these devices utilizes ions and neutral reactive species, which must travel to the etch front deep inside the feature. For gas pressures in the millitorr and feature sizes in the nanometer range, neutrals reach the bottom of an etching feature via the Knudsen transport1,2. For an aspect ratio of depth to diameter of 100:1, the flux at the bottom of the feature is only 1.3% of the incoming flux. This is a challenge for etching of advanced memory devices with ever increasing aspect ratios. We present computational results for the neutral transport in high aspect ratio features as a function of aspect ratio, profile shape, and surface processes such as adsorption, desorption, and diffusion of neutral species. Pertinent parameters are varied over a wide range to identify salient trends. When available, we include values for the case of fluorine radicals on silicon and silicon oxide in the parameter scans. The results predict that steady state transmission probability increases meaningfully in the presence of surface diffusion. Spontaneous and collision induced desorption of adsorbed neutrals on their own does not change steady state transmission probability, but they affect the time to reach it. In the presence of surface diffusion, however, spontaneous desorption increases the transmission probability, while desorption due to collisions with co-flowing nonreactive gas reduces it. These results indicate an enhancement of neutral transport at low surface temperatures that facilitate physisorption and surface diffusion.