Processing of Nb DC SQUIDs for RF amplification

DC SQUIDs amplifiers based on Nb/Al-Al2O3/Nb Josephson junctions technology have been designed, fabricated and characterized. To compare the RF perfor mances as a function of the device layout, several types of SQUIDs with var ious washer inductances and different input transformers have been fabricat ed on the same wafer. A seven mask process has been set up to fabricate the SQUID amplifiers without anodization steps. The geometric and electronic p arameters of the different device elements and the spread of the fabricatio n parameters have been studied. Critical steps such as the reliable fabrica tion of 4 and 9 mu m(2) area Josephson junctions, the control of their crit ical current density between 300 and 1000 A/cm(2), the SiO2 interlayers qua lity, and the step coverage have been characterized and optimized. Maximum voltage modulation Delta V and flux to voltage transfer function delta V/de lta Phi as high as 450 mu V and 1500 mu V/Phi(0), respectively, have been f ound. Delta V and delta V/delta Phi variation with the screening parameter beta(L) = 2LI(0)/Phi(0) and with the McCumber parameter beta(C) have been i nvestigated and the dependence of a typical device gain with frequency in t he 1-200 MHz range is presented. (C) 1998 Elsevier Science S.A. All rights reserved.