Fabrication and Characterization of Microshield Circuits

Microshield is a partially shielded, planar transmission line in which the center conductor and upper ground planes are surrounded by air and supported by a 1.5 pm thick dielectric membrane. This configuration allows single-mode, TEM wave propagation over a broad bandwidth with very low dispersion and zero dielectric loss. A lower shielding cavity is used to minimize radiation loss, vary the characteristic impedance, and prevent power loss into substrate modes. These properties highlight the potential of microshield as a high performance medium for submillimeter wave transmission. Here we present an overview of the fabrication of microshield lines, and report on the recent progress in the characterization of various circuit structures. Theoretical and experimental results are shown for lowpass filters, transitions to grounded coplanar waveguide, and different resonant stub designs. A description of an ultrafast electro-optic sampling ex eriment using membrane-based coplanar strip transmission lines is also included, and preliminary data is presented for line losses up to 500 GHz.