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Effect of geometry and film thickness on self-cooling of SIN junctions intended for particle detector applications

An SINIS structure should self-cool significantly if it is biased properly because the tunneling current removes the more energetic electrons from the normal electrode. However, the junction may be reheated if these hot quasiparticles do not escape the junction region before they either backtunnel or emit recombination phonons which may be absorbed in the normal electrode. We are studying this effect by using our full-wafer Al/Al2O3/Cu photolithographic process to produce SINIS structures with geometries which eliminate quasiparticle bottlenecks. Fabrication quality is monitored by analyzing IV data at low bias voltage. Self-heating is evaluated by exposing the devices to 6 keV X-rays and using a SQUID array to measure the variation of current pulse height with bias voltage. For devices having the same junction area, narrow junctions on thick superconducting base layers self-heat less than square junctions on thin base layers. For devices having the same shape, junctions with smaller dimensions self-heat less.