Manufacturing Process for Hybrid Microcircuits Containing Vias

Hybrid microcircuits designed for use in radars require metallized vias to interconnect front-side tantalum-nitride chromium-gold thin-film networks with metallized back-side ground planes on 95 X 114-mm alumina substrates. Processes were developed for fabricating precision holes in alumina substrates, metallizing substrates on both sides and through the via, and dry-film lithographing 6- µm-thick gold to linewidths and spacings of 127 µm. Front- and back-side resistance measurements through the via were used to determine the surface finish and metallization thickness on the via wall. Hole fabrication methods investigated included ultrasonic drilling in already-fired alumina and punching or drilling holes in green alumina before the firing. Photolithography techniques were developed which protect both sides of the substrate and the vias from etchants and delineate a thin-film network consistent with critical hybrid-microelectronic-circuit (HMC) linewidth tolerances for RF circuits. A metallization technique was developed, using a planetary system, to evaporate chromium-gold on the front and back side (while simultaneously depositing the metallization on the via wall). Front-side-to-backside resistance measurements through the via showed that the manufacturing methods used were compatible with a stable low-resistance connection for RF circuitry in hybrid microcircuits.