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

The sensor performance of galvanically coupled Y1Ba2Cu3O7-x (YBCO) de SQUID gradiometers on 24 degrees bicrystal substrates has been improved by thick ness reduction in the region of the grain boundary Josephson junctions usin g ion beam etching, The prepared etching mask allows the reduction of the c ritical current by more than one order of magnitude while the SQUID inducta nce is slightly increased. This treatment shifts the SQUID parameter beta(L ) from values above 10 to the proposed optimum around 1, The authors observ ed with decreasing critical current and increasing normal resistance a redu ced ICRN product with values between 300 and 400 mu V at 150-nm film thickn ess changing to values near 150 mu V at 50-nm film thickness. Despite this fact, the white flux noise level as well as the low-frequency noise is redu ced, With their galvanically coupled 4 x 8 mm(2) de SQUID gradiometer the a uthors obtained a white noise level of 4.2 mu(Phi)0/root Hz corresponding t o a field gradient sensitivity of 430 ft/cm root Hz at 77 K after the trimm ing process.