SU-8 as an electron beam lithography resist

SU-8 resist is an epoxy resin dissolved with a photoinitiator in an organic solvent. The result is a negative resist originally developed for high aspect ratio MEMS applications. For some applications SU-8 has several advantages over the most commonly used e-beam resist, PMMA, which include a much higher sensitivity and increased chemical and mechanical robustness. We have used our Raith 150 electron beam lithography tool to investigate SU-8 in two different applications. First, we investigated the properties of a specially formulated SU-8 which can be spun as thin as ∼100 nm. This is much thinner than normal formulations but necessary for high-resolution lithography. We then exposed an array of single pixel lines with a pitch of 200 nm. At a dose of 30 pC/cm we obtained a line width of ∼60 nm. Second, using standard formulations of SU-8, we discovered that films as thick as 8 microns can be exposed with a 30 kV electron beam, the maximum of our system. Using the contrast curve as a calibration reference, we were able to make analog three-dimensional structures by spatially varying the dose as the feature is being written. With this technique we fabricated a 3×3 array of f/9 spherical lenses.