Reliable low noise DC-SQUID

A novel type of multiloop DC SQUID (superconducting quantum interference device) has been fabricated. The device is made of six different layers deposited on a 2-in silicon wafer; the layers are patterned by means of optical lithography. Every chip is a square 6.35 mm on a side and contains two SQUIDs and their input coils; the typical coil inductance is about 1 μH. The junctions are 2 μm² planar windows on SiO made of Nb-NbO_x-PbAuIn with a current density of 500 A/cm² and V_m≈20 mV. The device was repeatedly cooled and stored in air for six months; after these thermal cycles no variations in critical current were observed. The measured mutual inductance between the input coil and SQUID is 2.1 nH, and the SQUID inductance is about 28 pH. The energy sensitivity referred to the input and in the flux-locked loop is approximately 1400 h in the white region with a 1/f corner frequency at 10 Hz. These SQUIDS are already operating in the gravitational wave experiment of the Rome group in Geneva