Shadow evaporation for refractory materials like niobium
The use of PMMA based masks leads to outgassing due to heating of the mask, if refractory materials, e. g. niobium, are evaporated, and therefore the deposited layer is contaminated. In particular the superconducting properties of niobium are deteriorated dramatically.
This contamination can be prevented to a large extent, if the polymer polyethersulfone (PES) is used for the bottom layer of the mask. PES remains tough and shows very low outgassing if it is heated.
The preparation of the shadow mask is likewise to the standard shadow evaporation technique. While the undercut in the PES layer is created in an oxygen-plasma, the top PMMA layer is removed. The resulting shadow mask consists of PES and Ge, only. Evaporation under different angels now allows for the fabrication of niobium based SET devices.
