Passivation, transition width, and noise for YBCO bolometers on silicon

We are developing YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thermometers for large area bolometers that include a heater for calibration by the electrical substitution of power. Because YBCO on buffered Si is under mechanical stress and must be very thin to avoid cracking, we find it is electrically sensitive to its passivation layer. For example, passivation by SrTiO/sub 3/ raised the noise in our films by about a factor of 100. An alternative is to first cap the YBCO with Au for passivation, and then add an insulator for electrical isolation. Such devices have a narrower transition width, by a factor of 3.9. A model with a shunt resistor across the superconductor predicts a narrowing of the transition, but by less than the observed amount. The noise equivalent temperatures of our thermometers, about 4 nK Hz/sup -1/2/, are not degraded by the normal metal shunt, although the resistance is decreased. The resistance is decreased. The resistance can be raised using AgAu alloy in place of Au, with equivalent noise performance.