Photolithographic structuring of a thin metal film coil on a Zerodur cylinder

Abstract

Photolithographic structuring is a common method for generating micro structures (<approximately 20 μm) on flat substrates, especially on silicon in the mass production of integrated circuits. After applying a thin, homogeneous, UV-sensitive photoresist film to the substrate, a mask (generally a transparent film) with the desired structure is positioned in the beam path. After a photochemical reaction has taken place in the photoresist, a chemical base is used to remove the exposed (photo positive process) or unexposed (photo negative process) layer. Subsequently, a PVD (Physical Vapour Deposition) process based on cathode sputtering is used to apply the metal layer. In the last step, the metal layer on the photoresist structure as well as the photoresist structure itself are removed by a chemical process. Applying a metal layer (50–200 μm wide and 800 nm thick) to a Zerodur cylinder, the conventional photolithographic process leads to an unacceptably high reject rate in the fabrication process on cylindrical structures. The weak point of this process is the area in which the two sides of the transparent film overlap, as these have to be adjusted very precisely to the Zerodur substrate. An alternative method is to write the structure directly on the photoresist using a UV-beam, or to ablate the photoresist and to proceed then with the same fabrication steps as in the previously described process. A direct-writing photolithographic process in combination with a cathode-sputtering PVD method has made it possible to fabricate an accurate pair of thin niobium film coils on a Zerodur cylinder. Superconductive properties of the niobium film were obtained by the use of a low-power PVD process. Final tests confirmed the superconductive behaviour of the niobium thin film structure.