Processing of DC SQUIDs for radiofrequency amplification

Abstract

The goal of this studies is to use dc Squids as low noise radiofrequency amplifier (Tn≃1K) in the frequency range 1-150 Mhz and to integrate them in NMR probe. DC Squids have been built using planar technology and with two types of superconducting materials: low Tc(Nb) and High Tc(YBa 2 Cu 3 O 7 ). Contrary to the traditional set up used in magnetometry (flux locked loop) the dc Squid is here fluxed biased near Oz=(2n+1)O o /4 and no flux locked loop is connected. Small variations of flux (O s <<O o ) introduced from signal source via the input coil are amplified following the transfer characteristic V O =dV/dO. The low Tc devices are set up to now the best, concerning the reproducibility and the qualities of the Josephson Junctions obtained with a three layer process Nb/Al-Al 2 O 3 /Nb. Such junctions have been realized following the so called SNOP process, without anodization. The main characteristic obtained at 4.2K are R J /R N =10, 2Δ=2.8mev and a Josephson current density J≃10 3 A/cm 2 . The high Tc devices have been built with bi-epitaxial YBa 2 Cu 3 O 7 junctions and with a 30×30μm 2 area loop. I(V) and V(O) characteristics curves obtained are promizing for applications as RF detection in the temperature range: 4-77K. Modelization studies are currently in progress for the optimization of those devices (low Tc and high Tc) as RF amplifiers (minimization of parasitic capacitance between the input coil and the Squid loop) in order to increase their upper frequency limit