Low-T, ramp-type Josephson junctions for SQUIDS

The Josephson tunnel junction is the basic element of a superconducting quantum interference device (SQUID). Amongst other parameters, the junction capacitance determines the characteristics of a (digital) SQUID. In a conventional dc SQUID, reducing the junction capacitance decreases the flux noise of the sensor, whereas in digital SQUIDs, the operating frequency can be increased when reducing the junction capacitance. For digital SQUIDs, this means that not only the flux noise decreases, but also the flux slew rate increases. Slew rates up to lo8 @ds can be achieved by reducing the junction size to the sub-pm2 level. Using a ramp-type structure allows sub-pm2 Josephson junctions sizes using standard lithography. In this paper we present the first results on low-T, ramp-type Josephson junctions and dc SQUIDs based on these junctions. The first junctions and SQUIDs showed nonhysteretic behavior at 4.2 K caused by the A1 bottom layer in the design.