SU-8 Ka-band filter and its microfabrication

This paper presents the design and microfabrication of a coaxial dual model filter for applications in LMDS systems. The coaxial structure is formed by five conductive layers, each of which is of 700 m thickness. The filter uses an air filled coaxial transmission line. It is compact with low dispersion and low loss. The design has been extensively tested using a prototype filter micromachined using laser drilling on a copper sheet and the results show a good agreement with the theoretical calculations. The laser fabrication has exposed weakness in suitability to volume production, uneven edges and oxide residuals on the edges, which affects the filter performance. A process for fabrication of such a filter in SU-8 has been developed which is based on a UV lithographical process. In order to fabricate such thick SU-8 layers, the SU-8 process has been optimized in terms of UV radiation and post exposure baking. During the test fabrication, the optimized SU-8 process has produced microstructures with an aspect ratio of 40:1 and a sidewall of 90 0.1 . The high quality SU-8 structures can be then either coated with a conductive metal or used as moulds for producing copper structures using an electroforming process. The microfabrication process presented in this paper suits the proposed filter well. It also reveals a good potential for volume production of high quality RF devices.