2.5D+ plasma etching for a continuously adjustable sidewall angle in SiO2

Abstract

We present a systematic investigation of an SiO2 etching process using a standard fluorocarbon chemistry ICP-RIE etch tool with a cryogenically cooled electrode. Our goal is to enable the control of the SiO2 feature morphology, i.e., the sidewall angle, in order to add a degree of freedom for the design of resonant micro-/nanooptical elements. For such elements as e.g., whispering gallery mode resonators with specific mode profiles, it is essential to maintain low surface roughness. To this end, we investigate a variety of gas compositions. For statistical evaluation, we use a surface response methodology for several parameters and investigate the influence of the substrate temperature on the sidewall angle. Different hypotheses from the literature for the cause of non-anisotropic etch behavior are discussed for our specific case. Various investigations based on the prior hypothesis are presented, which provide more information about the pseudo-isotropic etch profile. Finally, we present two use cases: firstly, a classical anisotropic etch with a high aspect ratio and very low roughness (<1 nm), and secondly, an etch process for the fabrication of whispering gallery mode resonators that confine the light at the bottom of the resonator.