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In this article, we present an ultra-narrowband silicon-micromachined bandpass filter with a wide and high-rejection stopband. The proposed filter uses high- $Q$ factor TM330 mode resonators. To avoid near-passband spurious resonances typically associated with higher order modes, a novel method of arranging the positions of the coupling slots is carried out. A fourth-order filter with a center frequency of 183 GHz and a fractional bandwidth (FBW) of 0.5% has been fabricated by silicon micromachining for the first time. The prototype employs out-of-plane transitions on the input and output ports, which results in axial ports enabling a direct characterization with the device simply mounted between the two standard waveguide test ports. The unloaded $Q$ factor extracted from the measurements is 1000, which is unparalleled by any previously reported narrowband filter in any technology in this frequency range. A spurious free response with a high stopband rejection in the entire waveguide band is obtained. The measured worst-case insertion loss and return loss (RL) in the passband are 4.5 and 9 dB, respectively.

A new transition from coplanar waveguide probe to micromachined rectangular waveguide for on-wafer device characterization is presented in this article. The transition is fabricated in the same double H-plane split silicon micromachined waveguide technology as the devices under test (DUT), requiring no additional postprocessing or assembly steps. We outline the design and fabrication process of the transition for the frequency band of 220–230 GHz. A coplanar waveguide structure acts as the probing interface, with an E-field probe protruding in the waveguide cavity exciting the fundamental waveguide mode. Guard structures around the E-field probe increase the aspect ratio during deep reactive ion etching and secure its geometry. A full equivalent circuit model is provided by analyzing its working principle. RF characterization of fabricated devices is performed for both single-ended and back-to-back configurations. Measured $S$-parameters of the single-ended transition are obtained by applying a two-tiered calibration and are analyzed using the equivalent circuit model. The insertion loss of the single-ended transition lies between 0.3 and 1.5 dB over the whole band, with the return loss in excess of 8 dB. In addition to previously reported characterization of a range of DUT the viability of the transition for on-wafer device calibration is demonstrated by characterizing a straight waveguide line, achieving an insertion loss per unit length of 0.02–0.08 dB/mm in the frequency band of 220–230 GHz.

In this article, we report for the first time on a low-loss compact platform that enables the integration of H- and E-plane rectangular waveguide subsystems enabled by 90° polarization rotation of rectangular waveguide sections on a silicon-micromachined chip. The proposed platform offers unprecedented design flexibility for a 2.5D fabrication technology such as silicon micromachining, since orthogonal waveguide device sections with full design freedom in H-plane geometries can be cofabricated with sections with full design freedom in E-plane geometries, enabled by novel, integrated waveguide twists optimized for 2.5D fabrication. The platform is developed for use in broadband millimeter- and submillimeter-wave waveguide circuits and prototypes are implemented in the 220–325-GHz band. A prototype chip demonstrating the platform, implemented by bonding three stacked silicon chips, is fabricated. The measured results of the twist prototype exhibit a very low insertion loss of less than 0.2 dB and a return loss of 20 dB or better in most of the 220–325-GHz band. An integrated eighth-degree lowpass waveguide filter with axial ports having a cut-off frequency of 280 GHz is codesigned with the twist transition and fabricated on the platform to demonstrate its application. The filter shows 0.4-dB measured insertion loss and has a measured return loss in the passband of better than 14 dB.