Evaluation of optical qualities of a LIGA-spectrometer in SU-8

In the past few years, SU-8 negative resist has been used in addition to PMMA positive resist for MEMS applications using deep X-ray lithography [1]. The advantage of SU-8 compared to PMMA is a higher sensitivity and a higher chemical stability. However, it is not yet as well analyzed in terms of microstructure quality. In this work SU-8 was examined with regards to its suitability to be used for high-resolution MOEMS. This is done exemplarily for a LIGA microspectrometer as this device can be thoroughly analyzed evaluating the side wall quality and the optical gratings as single structures, as well as the whole optical system [2]. In order to eliminate the high damping of visible light inside solid SU-8 material, a hollow wave guide design has been chosen. SU-8 was detected to reproduce structures in the nanometer-regime, combined with an averaged peak-to-valley profile better than PMMA. Although the measured roughness of SU-8 is worse than that of PMMA, there is still a comparable damping of the signal in MOEMS for both resists.