A Specific Thin-Film Technology for Microwave Integrated Circuits

This paper collects together in one place detailed procedures for making microwave integrated circuits in high conductivity copper on sapphire, alumina, or quartz. The relative merits of the "plate-through" and "etch-back" techniques, where the processing uses only one layer of photoresist, have been examined by sectioning and lapping the stripline conductors that result from each method. From photographic evidence the first method is seen to yield linewidths slightly greater than the mask size due to the edge being sloped, or cambered. Thus, for example, a coupling gap between two adjacent conductors turns out smaller than it should. The opposite happens with the etch-hack method; the etched edge recedes under the photoresist image, so a coupling gap is enlarged. In contrast to the usual concept of "undercutting" this edge shows no slope but remains square. Finally, the metal deposit with plate-up sometimes is less uniform, depending upon the layout of components in the circuit pattern.