Proton beam writing for minimum step lithography in multilayer patterning

We report the use of proton beam lithography as a convenient procedure consisting of minimum steps for micro-patterning in polymer multilayers comprising of a metallic intermediate layer - a structure which can facilitate many application including vertical organic field effect transistors. We have used focused micro-beam of 2 MeV protons to pattern array of micro-holes in a 1 mu m thick tri-layer film. A tri-layer film was prepared by spin coating 400 nm poly(methyl methacrylate) (PMMA) film on a glass substrate followed by thermal evaporation of 200 nm thick aluminium film and spin coating of a second PMMA layer of thickness 400 nm on the top. MeV protons suffer very little scattering inside the tri-layer film and pass through the tri-layer following almost straight path. Thus MeV proton beam causes molecular chain scissions in the top and the bottom PMMA layer in desired patterns in a single exposure which is not possible with other techniques such as e-beam lithography (EBL) or photolithography. By following a step by step development/etching of the three layers, we achieved an array of through micro-holes in the tri-layer film on the glass substrate. Potential application of such micro-patterns is discussed. We also report optimized parameters of proton beam writing for arrayed micro-structures fabrication in single layer of two different common polymer resists, SU-8 and PMMA. The surface morphology and side wall broadening of the fabricated structures were characterized using SEM and profilometer study respectively.