A D-band waveguide diplexer, implemented by silicon micromachining using releasable filling structure (RFS) technique to obtain high-precision geometries, is presented here for the first time. Prototype devices using this RFS technique are compared with devices using the conventional microfabrication process. The RFS technique allows etching large waveguide structures with nearly 90 • sidewall angles for the 400-µm-tall waveguides. The diplexer consists of two direct-coupled cavity six-pole bandpass filters, with the lower and the upper band at 130-134 and 141-148.5 GHz, respectively. The measured insertion loss of the two bands is 1.2 and 0.8 dB, respectively, and the measured return loss is 20 and 18 dB, respectively, across 85% of the passbands. The worst case adjacent channel rejection is better than 59 dB. The unloaded quality factors of a single cavity resonator are estimated from the measurements to reach 1400. Furthermore, for the RFS-based micromachined diplexer, an excellent agreement between measured and simulated data was observed, with a center frequency shift of only 0.8% and a bandwidth deviation of only 8%. In contrast to that, for the conventionally micromachined diplexer of this high complexity, the filter poles are not well controllable, resulting in a large center frequency shift of 3.5%, a huge bandwidth expanding of over 60%, a poor return loss of 6 and 10 dB for the lower and the upper band, respectively, and an adjacent channel rejection of only 22 dB.

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In this article, we present a new technology platform for creating compact and loss-efficient wafer-scale integrated micromachined substrate-integrated waveguides with silicon-core (Si-SIWs) for the 230-330-GHz frequency range. The silicon dielectric core enables highly integrated sub-millimeter-wave systems, since it allows for downscaling the waveguide's cross section by a factor of 11.6, and the volume of components by a factor of 39.3, as compared to an air-filled waveguide. Moreover, geometrical control during fabrication of this type of waveguides is significantly better as compared to micromachined hollow waveguides. The measured waveguide's insertion loss (IL) is 0.43 dB/mm at 330 GHz (0.14 dB/λ g , normalized to the guided wavelength). A low-loss ultrawideband coplanar-waveguide (CPW) transition is implemented to enable direct measurements of devices and circuits in this waveguide platform, and this is also the very first CPW-to-SIW transition in this frequency range. The measured IL of the transition is better than 0.5 dB (average 0.43 dB above 250 GHz), which is lower than for previously reported CPW-to-SIW transitions even at 3 times lower frequencies; the return loss is better than 14 dB for 75% of the band. As devices examples implemented in this platform, a filter and H-plane waveguide bends are shown. The waveguides and the devices are manufactured by deep-silicon etching using a cost-efficient two-mask micromachining process.

In this paper, we present for the first time on, to the best of our knowledge, the first silicon-core micromachined substrate-integrated waveguide (SIW) in the 220-325 GHz frequency range. In contrast to the fabrication methods used for conventional SIW known from substantially lower frequencies, micromachining allows for a full-height waveguide and near-ideal and arbitrarily shaped sidewalls. The silicon dielectric core allows for downscaling the waveguide and components by a factor of 3.4 as compared to an air-filled waveguide. At 330 GHz, the measured waveguide insertion loss is as low as 0.43 dB/mm (0.14 dB/λg, normalized to the guided wavelength). Devices were manufactured using a two-mask micromachining process. Furthermore, a low-loss ultra-wideband coplanar-waveguide (CPW) transition was successfully implemented, which comprises the very first CPW-to-SIW transitions in this frequency range. The measured transition performance is better than 0.5 dB insertion loss (average of 0.43 dB in the band above 15% above the waveguide-cutoff frequency), which is lower than previously reported CPW-to-SIW transitions even at 3 times lower frequencies, and the return loss is better than 14 dB for 75% of the waveguide band.