A Planar High- $Q$ Micromachined Monolithic Half-Coaxial Transmission-Line Filter

In this paper, a surface micromachined high-Q planar transmission line is presented. The proposed structure is a half-coaxial transmission line on a quartz substrate. The half-coaxial transmission line has a 100-mum air gap between the half-coaxial center conductor and ground plane. The unloaded quality factor extracted from an experimental resonator was 153 at 32 GHz. The proposed structure can be easily integrated with other components on a chip and can be used to partially isolate components, e.g., a microwave filter. A three-pole 5% fractional bandwidth filter centered at 31.75 GHz with a Chebyshev response has been designed with the proposed half-coaxial transmission line, where a measured insertion loss of 2.83 dB has been obtained. Variations in filter response due to different etch hole positions on the suspended ground plane is discussed