Creating sub-surface channels in PMMA with ion beam lithography in only one step

Ion beam lithography is a very powerful technique for 3D direct and maskless writing of resist materials. Light high-energy ions, like protons, can be used to structure these materials. For positive photoresist, like PMMA, this is possible because the protons cause chain scissions in the molecules. Afterwards the irradiated area can be developed in a chemical bath. In this paper, we show the realization of sub-surface channels, or microtunnels, which have been fabricated in only one exposure and without cutting or resurfacing the material. Using our Van-de-Graaff accelerator, the resist (PMMA) has been exposed with high-energy protons (2.7 MeV). The range of charged particles in matter is well-defined and depends on the energy. In addition the linear energy transfer is highest at the end of the ion path. Therefore, it is possible to obtain a dose which is sufficient to develop the bottom part of the ion paths but not the top part. Thus, by selecting the energy and the exposure time, a big variety of microtunnels can be realized.