., hvitedpaper al fhe Second Symposium on Low Temperature Electronics and High Temperature Superconductivity, Electrochemical Society Meeting, Honolulu, Hawaii, May 17-21,1993 HIGH TEMPERATURE SUPERCONDUCTOR JOSEPHSON WEAK LINKS B. D. HunG J. B. Bamer, M. C. Foote, and R. P. Vasquez Center for Space Microelectronics Technology, Jet Propulsion Laboratory, California Institute of Technology, Pasadena,CA91109 High Tc edge-geometry SNS microbridges have been fabricated using ion- damaged YBa2Cuq07.X (YBCO) and a nonsuperconducting phase of YBCO (N-YBCO) as normal metals. Optimization of the ion milling process used for YBCO edge formation and cleaning has resulted in ion- damage barrier devices which exhibit I-V characteristics consistent with the Resistively-Shunted-Junction (RSJ) model, with typical current densities (J c ) of= 5 x I@ A/cm2 at 4.2 K. Characterization of N-YBCO films suggests that N-YBCO is the orthorhombic YBCO phase with oxygen disorder suppressing T c . Weak links using N-YBCO as the normal metal show RSJ I-V characteristics and exponential scaling of J c , with a normal metal coherence length of = 23 & In behavior similar to that reported for grain boundary junctions, the N-YBCO IcRn products scale as JcO”~ for current densities below 105 A/cm2. For both types of devices, typical IcRn products at 4.2 K are limited to 1 -2 mV at the highest current densities, possibly due to self-shielding effects. INTRODUCTION High temperature superconductor (FITS) Josephson devices are potentially useful for a variety of applications including high speed digital logic, Thz frequency sources and detectors, and sensitive magnetometers. One promising approach to HTS Josephson device fabrication is the use of superconductor/normal-metal/superconductor (SNS) microbridges. Such weak links generally possess nonhysteretic current-voltage (I-V) characteristics, which are well-suited for high speed logic and magnetometer applications. In addition, the utilization of a normal metal bridge can relax the severe bridge length constraints of an all-superconducting microbndge. This approach also allows control of J c and R~ over a broad range simply by varying the normal metal bridge length. There are a number of possible device geometries for fabrication of HTS SNS weak links including planar SNS microbridges (1,2,3,4), step-edge SNS weak links (5,6,7), sandwich-geometry SNS trilayers (8,9,10,1 1,12), and edge-geometry SNS weak links (13,14,15,16,17,18,19,20). This work focuses on epitaxial edge-geometry SNS weak links due to advantages associated with this approach. A schematic diagram of an edge geometry SNS weak link is shown in Figure 1. The basic device structure consists of a c- axis-oriented YBCO base electrode with an exposed edge. An cpitaxial normal metal is deposited on the YBCO edge, followed by deposition of the YBCO counterelectrode. Because the top surface of the base electrode is covered by a thick insulator, electrical contact between the YBCO electrodes is confined to the edge of the lower YBCO fdm. The principal advantages of the edge geometry include the facts that the critical N/S interfaces are located on the longer coherence length YBCO surfaces, current flow is along the high J c direction parallel to the a-b planes throughout the device, and very small device areas can be produced using conventional photolithography. The edge geometry also enables very short microbridge lengths to be achieved and controlled, because the bridge length is determined by the deposited normal metal thickness. Edge junction