Development-less deep ultraviolet positive tone photolithography with polymethyl methacrylate

The authors describe a new lithography technique that relies on spatially thinning down a polymethyl methacrylate (PMMA) film through ultraviolet (UV) radiation exposure. Patterns on chrome-on-quartz mask plates or shadow masks can be transferred to an underlying PMMA film as UV light at 254 nm is projected through the mask. This work made use of cheap and easily available low pressure hot filament mercury discharge tubes as the UV radiation source. UV irradiation causes chain scission in PMMA followed by the removal of chain fragments. The process is synergistically aided by heating the PMMA-covered sample. This process thins down the PMMA wherever it receives UV irradiation, creating a topographic pattern in the polymer film. With sufficient irradiation dose, PMMA can be completely removed, all the way down to the substrate. The UV-induced decomposition of PMMA is shown to be aided by a secondary exposure from photoelectrically generated electrons ejected from the substrate. Subsequently, both additive and subtractive processes can be performed on the patterned resist to fabricate desired structures. This novel process works with even several microns thick PMMA films and benefits from the unparalleled transparency and high resolution capability of PMMA. The authors also show some example structures fabricated using this technique.