Exposure characteristics and three-dimensional profiling of SU8C resist using electron beam lithography

The properties of a new type of chemically amplified resist, SU8C, are evaluated for electron beam lithography. This resist is a modification of the ultraviolet sensitive negative epoxy SU8. Experimental results show that the sensitivity of SU8C is one of the highest among the different kinds of commercially available resists. At 50 keV energy exposure, the saturation dosage of SU8C is ∼3.8 μC/cm2 while that of PMMA is ∼500 μC/cm2. This implies that SU8C resist is suitable for large area exposure. The contrast γ can also be adjusted to near unity by adjusting the postannealing time, and this is essential for multilevel profiling. 16 level steps have been exposed in one electron beam exposure by changing the clock frequency in sequence, and a vertical profile resolution of 20 nm has been achieved. Lines as narrow as 100 nm have been produced, showing that the resist has high resolution down to the nanoscale. Hardness measurements have also been carried out, and SU8C is an order of magnitude harder than epoxy and PMMA. SU8C resist has high refractive index, high resolution, high sensitivity, and large hardness, making it suitable for microscale binary optics fabrication. To demonstrate the suitability of SU8C resist in microscale optical device fabrication, an array of Fresnel lenses has been designed and demonstrated.