Micromachined cavity-based bandpass filter and suspended planar slow-wave structure for vacuum-microelectronic millimeter-wave/THz microsystem embedded in LTCC packaging substrates

Micromachining of ceramic packaging interposers was demonstrated by the authors in the 62nd and 63rd ECTC as one of the prospective paths leading to high-density and heterogeneous 3D integration, by showing 3 individual micromachined modules, i.e. micro-accelerometers, cooling microchannels and RF passive structures, which were fabricated on a unified platform with both LTCC SIP integration and micromachining capability. As follow ups, in this paper, initial advancements in exploring the potential of the unified platform in vacuum-microelectronic millimeter-wave/THz microsystem integration are reported, that is, the development of two 3-D micromachined millimeter-wave/THz SIP modules. Firstly, the designing, fabricating and testing of a W-band (millimeter wave) bandpass filter based on 3D micro-cavity structures is revealed, as a representative of passive millimeter wave/THz devices, which can be entirely embedded into a LTCC packaging substrate (including interposers). The test results show a mid-band (central) frequency at 88GHz and 2GHz pass-band width, a low passband attenuation of better than -3dB and a stop-band rejection of -8dB @ 5GHz off mid-band frequency (i.e. 4GHz above and below passband edges respectively) and -14dB@10GHz off mid-band frequency. Secondly, interdigitated lines screen-printed on LTCC interposer are proposed as feasible slow-wave structures that can be embedded into SIP interposers, which are the essential parts for vacuum microelectronic active devices like backward wave oscillators and travelling wave amplifiers. Full-wave electromagnetic analysis proved the effectiveness of these planar designs as slow wave micro-functional structures, showing low and smooth transmission loss and dispersion characters, as well as promising coupling impedance characters, in 5~45GHz bands. In addition, two slow-wave structures consisting of interdigitated lines suspended on LTCC membrane are proposed as improvements for higher frequency use (up to 110GHz), and full-wave analysis proves their enhanced performance and potential capability in extending the operation frequency range, maybe into the THz range. In the future, these two modules may find applications into highly integrated wireless telecommunication and radar transceivers which are now being the research and development hotspot, facilitating advanced air-borne, space-borne and high-end automotive/consumer electronics.