Cryogenic Etching of High Aspect Ratio 400 nm Pitch Silicon Gratings
Houxun Miao, Lei Chen, Mona Mirzaeimoghri, Richard J. Kasica, Han Wen
The cryogenic process and Bosch process are two widely used processes for reactive ion etching of high aspect ratio silicon structures. This paper focuses on the cryogenic deep etching of 400.0 nm pitch silicon gratings. By creating a metal hard mask on top of the polymer grating lines, the limitation on the etching depth imposed by the selectivity of resist is eliminated, which allows free tuning of all the etching parameters. At a moderate etching depth of around 3.5 μm, the etching profile is studied with respect to the ratio of the etching gases, the chamber pressure, the inductively coupled plasma (ICP) power and the radio frequency (RF) bias power. By optimizing the etching parameters, we etched a 400.0 nm pitch grating to 10.0 μm in depth, corresponding to an aspect ratio of 50, which is more than 50% beyond the nominal aspect ratio of 30 for cryogenic process. Factors limiting the achievable etching depth are discussed.