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A Bulk Micromachined MEMS Digital Distributed Phase Shifter Based on Metal-Air-Metal Capacitors Formed by Intrinsic Stress

In this paper, the authors propose a butterfly-like multilayer microbridge, with heavily boron doped Si layer acting as the skeleton. Both microbridge sidewings are curved by intrinsic stress. The elasticity and deformation of the microbridge and its folded beam suspensions are theoretically analysed. When actuated by DC voltage, the sidewings snap down onto the coplanar waveguide (CPW) on the substrate first. With the further increase of the DC voltage, the central beam of the microbridge snaps onto the CPW signal plane, while the sidewings keep their shape. As a result, two metal-air-metal capacitors of nearly fixed value, between the microbridge and CPW ground planes, are formed. The ups and downs of 12 microbridges distributed along the CPW realize a binary variation of its insertion phase ( i.e. phase shift). The bulk Si fabrication process is shown. Optical test results confirm the anticipation of the deformation and action of the microbridges. The RF measurement shows a phase shift of 186° @ 20GHz can be achieved for a 6.3mm long phase shifter with a return loss under -8dB.