Fabrication of reproducible superconducting YBa 2 Cu 3 O 7−x (YBCO) thin films with T c values above 85 K on Si wafers has been realized by optimizing the thin film deposition process. Prior to the deposition of YBCO thin film on (100) p-type Si wafer, YSZ and CeO 2 thin films were deposited as buffer layers by RF magnetron sputtering, and subsequently, YBCO thin film was deposited by dc magnetron sputtering. The deposition parameters such as substrate temperature, process gas pressure, Ar/O 2 ratio, and power density were optimized for all layers in order to enhance the whole structure to prevent microcrack formation caused by misfits in crystal lattice parameters and thermal expansion coefficients between Si/YSZ/CeO 2 and YBCO. Structural analyses were performed on YSZ and CeO 2 layers, and electrical and magnetic measurements were carried out on a YBCO layer by employing XRD, SEM, resistance vs. temperature, and AC magnetic susceptibility vs. temperature measurements, respectively. The YBCO layer was also patterned as microbridges in order to test the durability of the whole Si/YSZ/CeO 2 /YBCO structure during the standard photolithography and wet etching process commonly used in device applications.The work has been done in Faculty
The performance analysis of direct gold (Au) wire bonding on YBa 2 Cu 3 O 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) SrTiO 3 (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 · cm 2 .
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