Fabricating microstructures by MeV proton beam micromachining

Proton beam micromachining (PBM) can be used to produce high aspect ratio microstructures in PMMA, similar to LIGA. For example, the range of 3-MeV protons in PMMA is approximately 120 microns, the beam can be focused to approximately 1 micron spot size, and structures with aspect ratios around 100 are possible. Patterns are created by directly writing with a focused ion beam that is scanned over the resist. We have found that achieving smooth walls is closely related to uniform dose and beam spot size. This paper describes our electrostatic rastering system and a method of controlling pixel-to-pixel exposure by integrating the sample current. We present our experimental results and process improvements for achieving high quality microstructures using PBM. We also show examples of complex (multilayered and tilted) parts created by direct writing and microstructures formed by alternative proton beam lithographies (shadow mask).