Robust shadow-mask evaporation via lithographically controlled undercut

Suspended shadow-mask evaporation is a simple, robust technique for fabricating Josephson-junction structures using scanning electron-beam lithography. The basic process entails the fabrication of an undercut structure in a resist bilayer to form a suspended “bridge,” followed by two angle evaporations of superconducting material with a brief oxidation step in between. The result is two overlapping wires separated by a thin layer of oxide. Josephson junctions with sub-50-nm diameters are of particular interest in quantum computing research. Unfortunately, standard shadow-mask fabrication techniques are highly variable at linewidths below 100nm, due to the difficulty of simultaneously fabricating a narrow line and a large undercut region. While most previous processes used poly(methylmethacrylate) (PMMA) for the top (imaging)layer and either lower-molecular-weight PMMA or a PMMA/methacrylic acid copolymer for the bottom (support) layer, the authors’ process uses a PMMA/poly(methylglutarimide) (PMGI) bilaye...