A bulk micromachined distributed digital phase shifter for microwave phased array applications

A novel distributed digital MEMS (micro-electro-mechanical system) microwave phase shifter, fabricated with bulk Si micromachining techniques, is presented. Using a distributed nonlinear transmission line technique, with single crystal silicon based multilayer microbridge structures as loading varactors, this phase shifter delivers true time delay and wideband performance. A micromachining process flow, combining anodic bonding and single crystal silicon etching with heavy boron doping, was developed to fabricate the movable Si-based microbridges over a coplanar waveguide (CPW). An electrostatic voltage, applied between the Si layer of the microbridges (varactors) and CPW ground plane, pulls down the microbridges, effecting a phase shift varying linearly with. frequency. The principle, process and RF performance characterization are described. The actuated phase shifter has achieved 90.2° phase shift at 20.5 GHz, with good reflection loss (-12 dB) in the frequency range of 5-20.5 GHz. The insertion loss is a little high, 3.87 dB, however, due to the lossy substrate of Pyrex 7740 glass and the thin metalization of the CPW.