A method of forming interconnections or crossovers in MMICs (monolithic microwave integrated circuits). The bridge consists of a layer of plated metal with no material other than air between it and the slice. Advantages include a low parasitic capacitance, immunity to edge profile problems, and the ability to carry substantial current. The major steps in the formation of an air bridge are shown in the diagram. A layer of resist is deposited on the slice and processed to produce the required pattern of interconnections. This is then covered by a very thin layer of metal, usually using sputtering techniques (Fig. a). A second layer of resist is added, and processed to leave the thin metal layer exposed in those areas where the plating is to take place (Fig. b). The slice is then plated (Fig. c). The presence of the thin metal layer ensures that the plating current is carried to all parts of the slice. Finally the layers of photoresist and thin metal are removed to leave the required interconnections (Fig. d).

Steps in the formation of an air bridge