Low-noise superconducting quantum interference device with a high dV/d/spl Phi/ optimized by thermally controlling critical current

The transfer function dV/d/spl Phi/ and the noise characteristics of a low-T/sub c/ superconducting quantum interference device (SQUID) have been improved by annealing to adjust the critical current, thereby optimizing the McCumber parameter /spl beta//sub c/. A SQUID structure suitable for annealing and using surface nitridation and planarization has also been developed. By annealing a SQUID that initially has a /spl beta//sub c/ greater than 1, the I-V characteristics can be changed from characteristics with hysteresis to characteristics without hysteresis. A SQUID with /spl beta//sub c/=0.81, close to the hysteresis limitation, had a large dV/d/spl Phi/ (3.7 mV//spl Phi/) and good noise characteristics (1.5 /spl mu//spl Phi//sub 0///spl radic/(Hz) at 100 Hz and 3.0 /spl mu//spl Phi//sub 0///spl radic/(Hz) at 1 Hz). These characteristics can be attained even when using a simple measuring circuit connected directly to preamplifier without adaptive components such as flux modulation or additional positive feedback (APF).<>