Performance Analysis of Direct Gold Wire Bonding on Superconducting Thin Films and Devices

The performance analysis of direct gold (Au) wire bonding on YBa₂Cu₃O_7-x (YBCO) thin films was performed. In a number of applications, YBCO thin films and devices are wire bonded on chip carriers or printed circuit boards for further electronic connections via the metallization of the contact pads of YBCO thin films. Although metallization for contact wiring is generally performed by thermal evaporation or sputtering at room temperature, the surface of a film is generally exposed to high temperatures by evaporated material or sputtering plasma. Moreover, an additional lithographic process (i.e, the application of a photoresist, baking, developing, and dry or wet etching) is necessary after the metal layer deposition in order to define contact pads. Since the superconducting properties of YBCO thin films and Josephson junctions are very sensitive to temperature and a humid environment, the metallization and additional lithographic processes during the fabrication cause significant degradation in the performance of the devices. In order to eliminate these additional steps, we performed a direct wire bonding process on YBCO thin films. Films of various thicknesses were deposited onto (100) SrTiO₃ (STO) substrates by direct-current magnetron sputtering, and 20-μm-wide meander-type microbridges were patterned. The contact pads of the devices were wire bonded onto chip carriers by a thermosonic wedge bonder without using any metallization layer. The reliability and performance analyses of the bonds were performed under several aggressive thermal cycling conditions from liquid nitrogen (LN, 77 K) up to 450 K. The superconducting properties of the samples were then analyzed by means of resistance-temperature (R-T) and current-voltage (I-V ) measurements. A resistance model was developed for the bonding layout, and a specific contact resistivity was derived as ~3 × 10^-6 Ω · cm².