EFFECT OF PARASITIC CAPACITANCE AND INDUCTANCE ON THE DYNAMICS AND NOISE OF DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

Practical thin-film dc superconducting quantum interference devices (SQUIDs) are often built with an integrated input coil which leads to the formation of various parasitic elements. We have designed and fabricated several square washer-shaped dc SQUID test devices with various amounts of parasitic capacitance formed by covering the washer slits with superconducting coverplates of various widths. In the simplest approximation, the parasitic capacitance C(p) appears entirely across the Josephson junctions; in a better approximation, C(p) divides the total inductance into two parts, an effective SQUID loop inductance L and a small parasitic inductance L(p). For both models, noise-free and noise-rounded current-voltage and voltage-flux characteristics are calculated for various values of the ratios L/L(p) and C(p)/C, where C is the Josephson junction capacitance. The SQUID dynamics are significantly affected by C(p) and L(p). The energy resolution epsilon at first deteriorates with increasing C(p) for C(p)/C less than or similar to 2, but as C(p)/C becomes large, epsilon improves to less than twice the value for C(p)/C almost-equal-to 0. The optimal points of operation for large C(p), however, are found at voltages above the low-frequency resonance determined by L and C(p) where the flux-to-voltage transfer function is low. Furthermore, for devices with C(p)/C much greater than 1 and L(p) not-equal 0, the double-loop geometry leads to an additional resonance determined by L(p) and C that appears at high frequencies. The energy resolution inside this high-frequency resonance is nearly independent of C(p) and comparable to the energy resolution of a device having the same total inductance but with C(p)/C much less than 1. Extensive noise mappings on several different test devices have been carried out using a computer-controlled measurement system. The measured characteristics and noise performance are shown to be in good agreement with the simulations.