A new method is presented,which can obtain high aspect ratio in SU8 structures.Instead that the top of the photo resist layers are exposed to UV light through masks in conventional lithography,the new method utilizes a mask-back exposure technique,i.e.the SU8 resist layer coated on a mask surface (metal patterns on a glass plate),is irradiated by UV light through the back of the mask.So a desired exposure dose on the bottom of the resist layer can be easily achieved without over-exposing from its top.This has a two-fold effect,i.e.proper dose on the bottom of the resist and less internal stress.Initial experimental results show that compared to an aspect ratio of 18 obtained by conventional method,a higher aspect ratio of 32 in the SU8 structures can be achieved by this new method.
