A Broadband, Micromachined Rectangular Waveguide to Cavity-Backed Coplanar Waveguide Transition Using Impedance-Taper Technique

This paper presents a broadband fully micromachined transition from rectangular waveguide to cavity-backed CPW line for submillimeter-wave (sub-MMW) and terahertz applications. A cavity-backed CPW line (CBCBW) is a planar transmission line that is designed and optimized for minimum loss while providing 50- Ω characteristic impedance. This line is shown to provide less than 0.12 dB/mm loss over the entire J-band (220-325 GHz). The transition from CBCPW to waveguide is realized in three steps to achieve a broadband response with a topology amenable to silicon micromachining. The first step is a tapered transition from the CBCPW line to a 50- Ω reduced-height waveguide. The next two steps utilize a novel in-plane impedance tapering technique to transition from the reduced-height waveguide to the on-wafer regular height waveguide. The full transition has less than 0.9 dB of insertion loss and more than 13 dB of return loss over the entire J-band (39% bandwidth). Silicon micromachining technology is used to fabricate prototypes of back-to-back CBCPW line-to-waveguide transitions. A novel waveguide-probe measurement setup is introduced and utilized to evaluate the performance of the transitions and the CBCPW lines.