Thin-Film Microwave Integrated Circuits

There has been an increase in the use of thin-film technology for the preparation of microwave circuits since it permits the realization of very complex circuitry with precision, reliability, and economy. The requirements for the realization of such circuits have made it necessary to extend the state of the art of the hybrid technology in many areas. Two new substrate types (fine-grain alumina and fused silica) and low-loss conductor systems that are both solderable and thermocompression (TC) bondable were introduced. Further complications which were overcome included bilevel patterns with laser-drilled plated via-holes, the control and measurement of the alumina substrates dielectric constant, and the use of laser trimming to adjust small geometries of tantalum nitride (Ta
2
N) termination resistors. Precise pattern delineation of conductors by laser machining, sputter etching, and selective plating was evaluated and the latter process was found to be the most economical and reliable, since the selective plating technique depends on good photoresist delineation, resist properties were evaluated with special attention given to the variation of linewidth as a function of exposure conditions and resist adhesion during the plating of the gold conductors to prevent underplating. The line shape of the resist was also determined as a function of exposure, development, mask-to-substrate distance, and postbake conditions. Linewidth tolerances such as ± 2.5 µm on fused silica and ±5.0 µm on alumina were routinely achieved. Microwave integrated circuits (an 18-GHz downconverter, a 1.7-GHz amplifier, a filter, and a demodulator) prepared by this technique are illustrated and their performances are discussed. These circuits are used in a digital transmission system currently under development.