2.5D+ etching of silicon oxide and nitride for optical and photonic systems

Plasma etching is a well-established technological approach for the precise structuring of optical and photonic elements. Tapered sidewalls have been documented numerous times for the etching of dielectrics and were deemed undesirable. However, the sidewall angle can be used as an additional degree of freedom. For these elements we coined the term 2.5D+ structures. We established the ability to control the sidewall angle for the plasma etching of SiO2 and Si3N4 using binary masks while maintaining extremely low roughness (Ra<0.6nm) using cryogenic sample temperatures. A methodology for investigating and narrowing the enormous parameter space was developed and used until the established design of experiments method could be used. In this case, a surface response methodology was used to optimize the process. We found a strong correlation between the sidewall angle and sample temperature, enabling control of the sidewall angle from 50°-90°, and found a curved etch profile named pseudo-isotropic. This new capability enables control over a new degree of freedom for fabricating optical and photonic systems, as demonstrated by the whispering gallery modes.