Aspect Ratio Independent Etching: Fact or Fantasy?

Abstract

The scaling of anisotropic plasma etch rates with feature dimensions is an important issue when fabricating microelectronic and photonic devices. For a wide rage of conditions, etch rates are found to scale primarily with the feature depth/width or aspect ratio. Features with large aspect ratios etch more slowly than those with low aspect ratios regardless of feature width. However, in the presence of etch inhibitors, the scaling of etching rates with feature dimensions becomes more complicated. Smaller aspect ratio features may etch more slowly than larger aspect ratio features suggesting that etch-inhibiting chemistry could be exploited in achieving the here-to-fore elusive goal of aspect ratio independent etching (ARIE). Using a well-established etching model, based on Langmuir adsorption kinetics, we derive the etch inhibitor flux and surface adsorption parameters required to achieve ARIE. Although the required parameters are non-physical when the range of aspect ratios is unconstrained, it is possible to achieve ARIE over a restricted range of aspect ratios. However, the balance required between ion, etchant, and inhibitor fluxes is a fragile one. The best approach to minimizing the variation in etching rate with aspect ratio is to operate a high density plasma at low pressure with high neutral flow rates.