NbN edge junction fabrication: edge profile control by reactive ion etching

In order to fabricate NbN edge junctions with low parasitic capacitance, an insulator with low dielectric constant, such as SiO₂, must isolate the base and counterelectrode except on the edge. Reactive ion etching was used to cut an edge in SiO₂/NbN bilayer films. For this process it is necessary that SiO₂ be etched more rapidly than NbN to form a suitably sloped edged profile. The authors investigated the influence of etching gas composition and other parameters on etching rates and edge profiles, using CF₄, CF₄/O₂, CF₄/CH ₄, CF₄/CHF₃, and CHF₃. It was found that CF₄ and CF₄/O₂ plasma etching generally yields poor, undercut edge profiles. However, satisfactory edge profiles were obtained with the other three gas combinations. The edge angle can be controlled by changing the proportions of the gases. Using this process, the authors have successfully fabricated NbN/oxide/PbBi edge junctions with <1 μm ^2- area by standard optical photolithography