High aspect ratio CPW fabricated using silicon bulk micromachining with substrate removal

A silicon micromachining processconductors has been developed to fabricate high aspect ratio CPW (HARC). The micromachining process combines Si DRIE, thermal oxidation, electroplating, and planarization to create tall CPW with Au conductors and SiO₂ dielectrics. Si is removed underneath the transmission lines to minimize the field interaction with the substrate which virtually eliminates substrate loss. It is shown that HARC has better loss and isolation compared to microstrip and conventional CPW. Transmission lines with characteristic impedances of 18 - 25 ¿ have been fabricated on high resistivity Si. Attenuation was measured to be 2.4 - 3.4 dB/cm at 30 GHz before Si removal and 1.7 - 2.4 dB/cm at 30 GHz after Si removal, demonstrating a substantial improvement in loss due to Si removal. Applications of HARC include CMOS MMICs and hybrid microwave circuits.