Optimized SU-8 UV-lithographical process for a Ka-band filter fabrication

Rapidly expanding of millimeter wave communication has made Ka-band filter fabrication to gain more and more attention from the researcher. Described in this paper is a high quality UV-lithographic process for making high aspect ratio parts of a coaxial Ka band dual mode filter using an ultra-thick SU-8 photoresist layer, which has a potential application in LMDS systems. Due to the strict requirements on the perpendicular geometry of the filter parts, the microfabrication research work has been concentrated on modifying the SU-8 UV-lithographical process to improve the vertical angle of sidewalls and high aspect ratio. Based on the study of the photoactive property of ultra-thick SU-8 layers, an optimized prebake time has been found for obtaining the minimum UV absorption by SU-8. The optimization principle has been tested using a series of experiments of UV-lithography on different prebake times, and proved effective. An optimized SU-8 UV-lithographical process has been developed for the fabrication of thick layer filter structures. During the test fabrication, microstructures with aspect ratio as high as 40 have been produced in 1000 gin ultra-thick SU-8 layers using the standard UV-lithography equipment. The sidewall angles are controlled between 85 similar to 90 degrees. The high quality SU-8 structures will then be used as positive moulds for producing copper structures using electroforming process. The microfabication process presented in this paper suits the proposed filter well. It also reveals a good potential for volume production of high quality RF devices.