Transmission Of Ultra-high-bandwidth Pulses On A Low-distortion Coplanar Stripline

In recent years the desire to increase the bandwidth of guided propagation along planar transmission lines [1] and to create submillimeter-wave integrated circuits with low-radiation and dispersion characteristics [2] has led to the development of a stmcture using a thin membrane as a substrate. This so-called "microshield" technology uses a 1.5-µm-thick membrane comprised of silicon dioxide and silicon nitride to suppo1t metal lines and other patterned circuit structures. The use of the membrane, which is rugged enough to withstand standard processing techniques, eliminates the frequency-dependent radiation into the substrate material and the frequency dependent phase velocity caused by modal dispersion, both of which increase with the relative pennittivity of the substrate. These improvements are realized because the elect1ic fields from the conductors essentially do not experience any dielectric inhomogeneity, i.e., they arc completely surrounded by air. To demonstrate the benefits provided by such a guiding geometry for millimeter- and submillimeter-wave propagation, we have experimentally launched a singlepicosecond-duration electrical pulse on a coplanar stripline (CPS) fabricated on a membrane substrate and measured the transmitted waveform at several distances from the source. Results indicate nearly negligible distortion of the signals even after 5 mm of propagation, with only a small attenuation attributed to the ohmic losses of the conductors.