Micromachined microwave transmission lines by commercial CMOS fabrication

Coplanar waveguides were designed and fabricated in standard CMOS with post-processing micromachining. Transmission line layouts were designed with commercial CAD tools, ICs were fabricated through a commercial CMOS foundry, and subsequently suspended by maskless top-side etching. Absence of the lossy silicon substrate after etching results in significantly improved insertion loss and dispersion characteristics, and phase velocity. Two types of layout are presented for different ranges of characteristic impedance. Measurements were performed at frequencies from 1 GHz to 40 GHz, before and after micromachining. These show improvement of loss characteristics of orders of magnitude. For the entire range of frequencies, for the 50 /spl Omega/ layout, losses do not exceed 4 dB/cm. Before etching, losses are as high as 38 dB/cm. Phase velocity in the micromachined transmission lines is close to that in free space.