O Dry etching characteristics of amorphous As2 S3 film in CHF3 plasma

The authors describe the dry etching characteristics of amorphous As2 S3 films in CHF3 plasma and the development of an optimized fabrication process for compact waveguides. The observed etching behavior is due to the relative densities of fluorine atoms, polymer precursors, and ions in the plasma which are controlled by the process parameters. In particular, the flow rate of the CHF3 gas has a significant influence on the etched profile and surface roughness as well as the etch rate of the As2 S3. The profile evolves from isotropic to vertical with the flow rate due to passivation by increasing polymer deposition on the sidewalls. Such passivation also helps achieve smooth sidewalls because it inhibits differential etching between the phases in the inherently phase-separated As2 S3 film, which otherwise results in a grainy and rough etched surface. At the highest flow rate, however, excessive polymer deposition occurs and this results in positive-sloped sidewall and grassy etched surface due to micromasking.