Improvement of sensor performance of high-T/sub C/ thin film planar SQUID gradiometers by ion beam etching

The sensor performance of galvanically coupled Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) dc SQUID gradiometers on 24/spl deg/ bicrystal substrates has been improved by thickness reduction in the region of the grain boundary Josephson junctions using ion beam etching. The prepared etching mask allows the reduction of the critical current by more than one order of magnitude while the SQUID inductance is slightly increased. This treatment shifts the SQUID parameter /spl beta//sub L/ from values above 10 to the proposed optimum around 1. The authors observed with decreasing critical current and increasing normal resistance a reduced I/sub C/R/sub N/ product with values between 300 and 400 /spl mu/V at 150-nm film thickness changing to values near 150 /spl mu/V at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is reduced. With their galvanically coupled 4/spl times/8 mm/sup 2/ dc SQUID gradiometer the authors obtained a white noise level of 4.2 /spl mu//spl Phi//sub 0///spl radic/Hz corresponding to a field gradient sensitivity of 430 fT/cm/spl radic/Hz at 77 K after the trimming process.