Study on Etching Methods for Highly Vertical Sidewall Profiles in Niobium Films

Niobium (Nb) coplanar waveguide (CPW) resonators are widely used in superconducting devices, and their higher quality factors are often associated with more vertical Nb sidewall. In this work, Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) was used to etch Nb. The etch parameters, including ICP power, platen power, chamber pressure, and gas chemistry, were studied to investigate their effects on the sidewall profile of etched Nb. Additionally, two optimized process recipes are proposed to attain a sidewall profile with high verticality: an SF$\_{6}$gas recipe and an SF$\_{6}$/CF$_{4}$mixed-gas recipe. Experimental results demonstrate that plasmas generated by SF$\_{6}$or SF$\_{6}$/CF$\_{4}$mixed gas, at relatively low chamber pressure and relatively high platen power, can produce a nearly vertical sidewall profile with an angle of$90^{\circ }$$\pm 2^{\circ }$.