A W-Band Air-Cavity Filter Integrated on a Thin-Film Substrate

In this letter, a W-band air-cavity filter has been developed on a thin-film substrate using a lossy silicon substrate as a base plate, which is suitable for a mm-wave system-on-package. The lossy silicon suppresses a parasitic substrate mode excited in a thin-film substrate, while a coupling loss between a transmission line and a resonator is minimized by etching the backside of the lossy silicon substrate underneath the coupling area. In the backside etching process, 70 mum of silicon was left for mechanical support of the thin-film substrate. The resonator was fabricated using a low-cost silicon micromachining technique and was flip-chip integrated on a thin-film substrate. The fabricated air-cavity resonator showed an unloaded
Q
of 851 at a resonant frequency of 94.18 GHz. Improvement in the coupling loss by the backside etching process was verified with measurement results. The fabricated filter exhibited an insertion loss of 1.75 dB and a return loss better than 14.5 dB with a 1.3% 3 dB fractional bandwidth at a center frequency of 93.8 GHz.