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 100 nm, 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) bilayer. The advantage of using PMGI as the support layer is that it develops in aqueous base solutions, while PMMA is insensitive to aqueous solutions and only develops in certain organic solvents. This allows the two layers to be developed independently, ensuring that the imaging layer is not biased during the development of the support layer and allowing the process to achieve the full resolution of the PMMA imaging layer, which can be extremely high. Additionally, the extent of the undercut in the support layer can be precisely controlled by defining it lithographically, rather than simply varying the PMGI development time as in previous processes. Although PMGI is sold as a "liftoff resist" and widely assumed to be electron insensitive, their experiments have shown that this is not the case. Instead, when dilute developer and low electron doses are used, PMGI behaves very much like a conventional photoresist. By exploiting this behavior, as well as its high electron sensitivity with respect to PMMA, the authors were able to define undercuts by defining low-dose regions adjacent to their features, exposing the underlying PMGI separately. In this manner, it is possible to create well-controlled undercut regions as large as 600 nm. Extensive modeling of both the exposure and development processes was used to verify their results. By using a Monte Carlo simulation of electron scattering to simulate the electron exposure and mass-transfer relationships to simulate the process of developing the undercut region, the authors were able to produce a model that closely matches experimental results. With the process fully characterized, it is possible to produce nearly any linewidth/undercut combination, limited only by PMMA resolution and the mechanical stability of large overhang structures. This robustness, combined with the high resolution of the PMMA imaging layer, will allow the reliable fabrication of many interesting devices and circuits based on nanoscale Josephson junctions.