LITHOGRAPHIC PROCESSING OF YBA2CU3O7 THIN-FILMS - IMPACT ON ELECTRICAL CONTACTS

Silver contact pads to c-axis-oriented thin films of YBa2Cu3O7 (YBCO) were defined using three different techniques: (1) deposition through stencil masks; (2) liftoff; and (3) Ar plasma etching of in situ deposited Ag. A specific contact resistance of rho(c)(77 K) = 1 X 10(-7)-OMEGA-cm2 was found for the in situ deposited contacts. Contacts defined with stencil masks and liftoff showed a rho(c)(77 K) approximately 10(-5) and approximately 10(-2)-OMEGA-cm2, respectively. Core-level photoelectron spectroscopy measurements indicate that the resist and acetone applied to the YBCO thin film during patterning do not introduce surface impurity phases in addition to those formed upon air exposure. Distinct chemical shifts originating from process-induced surface impurity phases were observed in photoelectron spectra taken on films prepared for contact definition by liftoff, i.e., exposed to the lithographic developer and deionized water. These findings correlate with the measured increase in contact resistance. Cross-sectional transmission electron microscopy images of an in situ deposited Ag contact unveil a 20-30-angstrom-thick interface layer, in which the 12 A periodicity along the YBCO c axis vanishes. The presence of this layer could explain the absence of Josephson effects in YBCO/noble-metal/low-T(c) superconductor junctions where the currents are forced to flow perpendicular to the a-b planes.