Analysis and Design of Monolithic Rectangular Coaxial Lines for Minimum Coupling

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Coupling through holes etched in a common wall between the neighboring surface micromachined rectangular coaxial lines is studied in this paper. Analytical models for single- and multiple-hole structures are developed using Bethe's small aperture theory, and excellent agreement with finite-element and finite-integration simulations is obtained. These models are used in a circuit-level simulator for efficient optimization of the selected multihole structure. An isolation test structure composed of 200-<formula><tex>μ</tex> </formula>m center-to-center separated 300-<formula><tex>μ</tex></formula>m-tall 50-W rectangular coaxial lines is designed, built, and measured. Coupling levels below <formula formulatype="inline"><tex>-</tex> </formula>55 dB over 1-cm-long coupling lengths are achieved throughout the <formula><tex>$Ka$</tex></formula>-band. It is found that the probe radiation is more than 10 dB higher than the coupling through the common wall holes. This radiation severely affects the accuracy of the measurements and has to be considered when the coupling through the common walls is analyzed. </para>