Micromachined Bandpass Filters with Enhanced Stopband Performance and Q-factor of 950 at 700 GHz

In this paper, we present two bandpass filters at 687.5 and 700 GHz with fractional bandwidths (FBW) of 3.64% and 1% respectively. Both 4th-order all-pole filters utilize a pair of dual-mode cavities: the first filter uses elliptic cavities with quasi-TMno degenerate modes, while the second one uses rectangular cavities with TM410-TM140 modes. In the latter, the coupling slots between the cavities are arranged to enhance the stopband performance by suppressing spurious resonances in the stopband. The filters are fabricated using silicon micromachining with gold metallization. The measured average insertion loss in the passband of the 3.64% FBW filter is 1.45 dB, and 2.5 dB for the 1% FBW filter. The experimentally extracted unloaded quality-factors are 450 for the elliptic cavities and 950 for the rectangular cavities. The measured filter performance of the first prototypes agrees very well with the simulation results, exhibiting a frequency shift of less than 0.7%. These are the best insertion loss and quality factors ever published in this frequency range for narrow-band filters, and the first time that a 1%-FBW microwave filter is demonstrated above 500 GHz.