Use of SU-8 photoresist for very high aspect ratio x-ray lithography

This paper describes the process of deep x-ray lithography using epoxy negative photoresist SU-8. Coating, softbake, exposure, post exposure bake, and development of the resist is characterised. Influence of x-ray source spectrum on the lithographic image contrast is calculated and optimal x-ray mask layers compositions for the spectrum in use are proposed. Method for resist film thickness control during casting step is reported. Temperature limit for the post exposure bake was found to ensure safe post bake to obtain maximum resist sensitivity. Optimised development and rinsing process is presented. Resist structures with aspect ratio as high as 100:1 (height:width) are demonstrated.