Toshiki Furutani

Interface-Treated Josephson Junctions in Trilayer Structures

We have demonstrated interface-treated Josephson junctions without deposited barriers in a trilayer structure. In the junctions, barriers were formed through an etching process and an annealing process for base YBa2Cu3O7-x (YBCO) films. The junctions showed resistively-shunted-junction-type characteristics over the entire temperature range below 70 K. A magnetic field modulation of more than 80% was observed throughout the operating temperature range. The obtained IcRn product was 2.1 mV at 4.2 K. The 1σ spreads in the junction parameters of 10% or better were observed for eight out of nine junctions in a chip.

Planarization of the YBa2Cu3O7-x Base Electrodes in Trilayer Josephson Junctions

We have applied the chemical-mechanical polishing (CMP) planarization technique to YBa2Cu3O7-x (YBCO) films for the base electrodes in trilayer Josephson junctions. In our planarization process, YBCO films were directly polished using a diamond-based slurry and cleaned by low-angle ion etching. The planarized films showed extremely flat surfaces with the average roughness (Ra) less than 0.5 nm. Moreover, the c-axis-oriented YBCO/PrBa2Cu3O7-y (PBCO)/YBCO trilayer junctions fabricated using the planarized films as the base electrodes showed resistively shunted junction-like characteristics even when the PBCO barriers were as thin as 10 nm. The IcRn products of the junctions were more than 1 mV at 4.2 K. Our results indicate that CMP is an effective method for obtaining flat YBCO films and for improving the properties of trilayer Josephson junctions.

Study on fabrication conditions of the interface-treated trilayer junctions

We have investigated the effects of fabrication conditions on the properties of the interface-treated trilayer Josephson junctions. In the junctions, barriers are formed by ion milling, followed by annealing. We controlled the accelerating voltage for the milling process and the gas pressure for the annealing process. Josephson currents were observed in the junctions fabricated under various conditions. It was found that higher accelerating voltage contributes to the reduction of leakage paths in the barriers. However, clear dependence of the Josephson currents on the conditions was not observed in contrast to the results for the ramp-edge junctions.

Effects of etching conditions on interface-treated trilayer junctions

Abstract The effects of an etching condition on the trilayer junctions fabricated by the interface treatment have been studied. The junctions have no deposited barriers and are made through an etching process and an annealing process at the surface of YBa 2 Cu 3 O 7− x (YBCO) films. We controlled the incidence angle of the ion beam in the etching process. The atomic-force microscope observations of the YBCO surfaces etched with different incidence angles suggested that the incidence angle affects the formation of amorphous layer. Moreover, it was found that the critical current of the junctions changed by the incidence angle. These results indicate that we can control the junction parameters of the interface-treated trilayer junctions.

Mechanism Verification of Electrochemical Migration of Fine Cu Wiring

The mechanisms of electrochemical migration and consequent insulation failure in fine-pitch Cu wiring (line/space = 3 µm/3 µm) are investigated. A model in which the migration takes place in the presence of water in a phenol-based resin–resin interface between biased Cu traces is proposed. Replacing the bottom resin layer by a SiN layer is found to be an effective method of considerably enhancing the leakage characteristics.

Fabrication and Evaluation of Sandwich-Type Interface-Treated Josephson Junctions

We have fabricated interface-treated sandwich-type Josephson junctions without depositing an interlayer or a barrier layer. The barrier is formed during an etching process and subsequent annealing process after the base electrode is deposited. The etching conditions are various accelerating voltages and microwave power of 200W. The annealing conditions are at 680°C in various pressures for 1hour. In case that the base electrode was etched for 15 minutes with 900V and annealed in 1Pa of an Ar+50%-O2 mixture gas, the junctions displayed resistively-shunted-junction (RSJ) current-voltage (I-V) characteristics with hysteresis at 4.2K. The characteristic voltage (IcRn), the critical current density (Jc) and the normalized resistance (RnA) of the junction were 2.6mV, 1.5 × 103A/cm2 and 1.7 × 10-6 Ω cm2 at 4.2K, respectively. The junction showed Fraunhofer-like magnetic field modulation of the critical current (Ic), and the modulation depth of Ic is 83% at 4.2K.