Micromachining of Air-Bridges on SU-8 Substrates

This paper describes a procedure developed to fabricate air-bridges on coplanar transmission lines using a SU-8 substrate. SU-8 is attractive for micromachined multilayer circuit fabrication because it is photo-polymerizable resin, leading to safe, economical processing. The ability to fabricate air-bridges provides a means of suppressing the coupled slotline mode, thereby enabling single-mode design. The air-bridges also provide ground equalization at coplanar discontinuities. The properties of the air-bridge are investigated and compared to similar structures without air-bridges. As examples of possible device applications of this technology, an asymmetrical slot antenna and coplanar waveguide both operating at 35 GHz are presented. The return loss of both devices is 5 dB lower on structures that contain the air-bridge. This technology is promising for extending the use of SU-8 at millimeter-wave frequencies.