Silicon Deep Reactive Ion Etching with aluminum hard mask

Silicon Deep Reactive Ion Etching (DRIE) process using a multi-layer mask containing a sputtered Aluminum thin film is studied. Aluminum is a candidate for very high aspect ratio DRIE, due to tis very low etch rate, but its adoption is hindered by the presence of micromasking. Effects of adding aluminum as the middle layer in a stack of silicon oxides are studied: heat dissipation and catalyst contribution to plasma chemistry are identified. The novel assumption that micromasking is generated only in the first part of the DRIE etching is demonstrated. Based on this, the effect of adding a protective coating on top of the mask is studied. The protective coating preserves aluminum in the first part of the etching, to reach a given depth before the aluminum mask is exposed to the etching plasma. A Safe aspect ratio value is obtained, beyond which micromasking does not take place. This aspect ratio is shown to be a function of the etching geometry and feature size.