Topology of Thin Film RC Circuits

Integrated RC circuits can be made by depositing exceedingly thin metallic and dielectric films in suitable patterns on an insulating substrate. A resistor is made by depositing a long, narrow strip of conductor (usually in a zag-zag for compactness); a capacitor is made by superimposing conducting, dielectric, and conducting layers. Because the dielectric is thin, the capacitance per unit area is high. Fig. 1 shows a typical thin film pattern. Ordinarily printed circuits are strictly planar; crossovers are made only by leading one of the conductors entirely out of the plane of the circuit. In the thin film technique, however, conductors can be separated by thin insulating layers within the plane of the circuit. Thus, crossovers can be permitted provided a nonzero capacitance between the crossing conductors is acceptable. If an RC circuit can be laid out so that conductors cross only if the circuit requires a nonzero capacitance between them, we will say the circuit is realizable in thin film or just realizable. An example of a realizable nonplanar circuit is shown in Fig. 2. In this case, the schematic thin film layout brings out intrinsic symmetries not displayed by the circuit diagram. 1639