A gap reduction and manufacturing technique for thick oxide mask layers with multiple-size sub-μm openings

This paper introduces a technique for the fabrication of thick oxide hard masks on top of a substrate with adjustable opening sizes in the sub-mu m regime, while the only lithography step involved has mu m-scale resolution. This thick oxide mask layer with sub-mu m openings is suitable for etching deep narrow trenches in silicon using deep reactive ion etching (DRIE) tools. Openings of less than 100 nm are realized in a 1.5-mu m-thick oxide layer, while the original lithographically defined feature sizes are larger than 1 mu m in width. This method, combined with modified high aspect ratio DRIE recipes, shows a great potential for single-mask batch-fabrication of high frequency low-impedance single crystalline resonators on silicon-on-insulator (SOI) substrates. Dry-etched trenches with aspect ratios as high as 60:1 are fabricated in silicon using the gap reduction technique to realize 200 nm opening sizes in an oxide mask layer. Various resonator structures with sub-mu m capacitive gaps are also fabricated on a SOI substrate using a single-mask process. Measurement results from high-frequency and high-quality factor (Q) all single crystal silicon resonators are presented.