Fabrication of vertically coupled InP microdisk resonators by using smooth, CH4-based reactive ion etching methods

Obtaining smooth sidewalls is crucial to suppress the scattering loss of whispering gallery modes along the disk edges and to achieve a high quality factor in microdisk resonators. CH₄-based reactive ion etching (RIE) is a preferred choice for InP disk formation, due to the sidewall protection afforded by polymers formed in the etching process and the excellent etching selectivity achieved with common dielectric masking materials. We developed a multi-step process RIE involving high-pressure (75 mTorr) RIE conditions for 10 min., followed by a lower pressure (15 mTorr) etch to the completion of the structure. The higher pressure etch yields higher etch rate with less sensitivity to surface conditions but results in severe undercutting during extended etching. The overall etching rate of this process is approximately 0.03 μm/min