Manufacturing tolerances for UV LIGA using SU-8 resist

Abstract UV LIGA involves the exposure of SU-8 negative resist, using a UV mask aligner, to produce high aspect ratio pillars or microchannels as part of the manufacturing process for microsystems. This has been made possible by the widespread use of a UV sensitive resist SU-8. Many papers have been written on the Fresnel diffraction theory of exposure, some key properties of SU-8 and prototype applications, but little attention has been paid to the variation in parameters that affect the tolerances and hence the reproducibility with which component dimensions can be maintained. It will be shown that the accuracy and precision, with which a feature on a mask can be reproduced throughout a thick resist structure, will depend on key parameters in the set-up of the mask aligner, the material properties of the SU-8 resist and the thickness of the resist structure. A simplified optical exposure model is used to predict the dimensions and wall taper angles of resist pillars or microchannels and their variation with changes in these parameters.