An embedded mask for application in high-frequency integrated circuits using UV positive resist technology

Using thick positive resist structures and electro-deposition techniques, it is now possible to realize the hollow metallic cavities needed for high-performance integrated terahertz waveguide components. Positive resist is preferred as it has many of the desirable characteristics of negative resist, but by contrast is easily removed. Until now, however, such positive resist techniques have been restricted to structures of approximately 100 mu m in height, which in turn has limited the lowest frequency attainable using this approach. The overall limiting factor for positive resist is the depth to which ultra-violet radiation can penetrate and hence expose the resist. The technique described here removes this restriction by making use of cyclic exposure and development. Realignment of the mask to a pattern between exposures is avoided with the inclusion of an embedded photomask within the resist itself. A high yield of 250 mu m test structures has been realized with near-vertical side walls and minimal undercut. The fabrication procedure presented in this paper is quick, highly reproducible and easily implemented using standard photolithography equipment.