Processing of Nb DC SQUIDs for RF amplification

Abstract

DC SQUIDs amplifiers based on Nb/Al–Al 2O 3/Nb Josephson junctions technology have been designed, fabricated and characterized. To compare the RF performances as a function of the device layout, several types of SQUIDs with various washer inductances and different input transformers have been fabricated on the same wafer. A seven mask process has been set up to fabricate the SQUID amplifiers without anodization steps. The geometric and electronic parameters of the different device elements and the spread of the fabrication parameters have been studied. Critical steps such as the reliable fabrication of 4 and 9 μm 2 area Josephson junctions, the control of their critical current density between 300 and 1000 A/cm 2, the SiO 2 interlayers quality, and the step coverage have been characterized and optimized. Maximum voltage modulation Δ V and flux to voltage transfer function δV/ δΦ as high as 450 μV and 1500 μV/Φ 0, respectively, have been found. Δ V and δV/ δΦ variation with the screening parameter β L =2 LI 0/Φ 0 and with the McCumber parameter β C have been investigated and the dependence of a typical device gain with frequency in the 1–200 MHz range is presented.