RF Channel-Select Micromechanical Disk Filters, Part II: Demonstration.

This part II of a two-paper sequence presents fabri-cation and measurement results for a micromechanical disk-based RF channel-select filter designed using the theory and procedure of Part I. Successful demonstration of an actual filter required sev-eral practical additions to an ideal design, including the introduc-tion of a 39nm-gap capacitive transducer, voltage-controlled fre-quency tuning electrodes, and a stress relieving coupled array de-sign, all of which combine to enable a 0.1%-bandwidth 223.4-MHz channel-select filter with only 2.7dB of in-band insertion loss and 50dB rejection of out-of-band interferers. This amount of rejection is more than 23dB better than a previous capacitive-gap trans-duced filter design that did not benefit from sub-50nm gaps. It also comes in tandem with a 20dB shape factor of 2.7 realized by a hi-erarchical mechanical circuit design utilizing 206 micromechani-cal circuit elements, all contained in an area footprint of only 600μm×420μm. The key to such low insertion loss for this tiny per-cent bandwidth is Q's >8,800 supplied by polysilicon disk resona-tors employing for the first time capacitive transducer gaps small enough to generate coupling strengths of Cx/Co ~0.1%, which is a 6.1× improvement over previous efforts. The filter structure uti-lizes electrical tuning to correct frequency mismatches due to pro-cess variations, where a dc tuning voltage of 12.1V improves the filter insertion loss by 1.8dB and yields the desired equiripple pass-band shape. Measured filter performance, both in- and out-of-channel, compare well with predictions of an electrical equivalent circuit that captures not only the ideal filter response, but also par-asitic non-idealities that distort somewhat the filter response.