Cosputtered Nb-Si barrier Josephson junctions for superconductive electronics applications

David I. Olaya, Manuel A. Castellanos-Beltran, Javier F. Pulecio +6 more

We describe our process for fabricating single-flux-quantum circuits for applications in communications metrology, advanced computing, and quantum information. The process flow includes four superconducting metal layers, one junction layer, one palladium-gold bias resistor layer, and a contact pad layer. Planarization using chemical-mechanical polishing is performed on the two lower insulating layers. This process is based on Josephson junctions with niobium superconducting electrodes and self-shunted, amorphous niobium-doped silicon barriers on the insulating side near the metal-insulator transition. The barrier material is deposited by cosputtering silicon and niobium, with the junction properties controlled by the relative sputtering powers and the duration of the barrier deposition step. The profiles of the deposition rates across 3-in (76.2 mm) wafers give a corresponding radial profile of the critical current density Jcthat increases by ~46% from the center to the edge of the wafer. To characterize our process, we have measured and show data for niobium metal quality and its magnetic penetration depth, as well as for wire and via inductances, and for via critical currents. The uniformity of the junction critical current Icacross a wafer was measured on test chips with 2.7-μm-diameter junctions. Within a 5 mm × 5 mm area, the standard deviation of Icwas ~2% for chips near the center of the wafer and up to ~7% for chips near the edge. We also describe process improvements and innovations under development aimed at increasing circuit density and high-speed performance.

Daiki Hasegawa, Yuto Takeshita, Kyosuke Sano +3 more

S. Pagano, Sandro Barbanera, Vincenzo Boffa +6 more

Himmel, Nico

Josephson junctions are used as active devices in superconducting electronics and quantum information technology. Outstanding properties are their distinct non-linear electrical characteristics and a usually sinusoidal relation between the current and the superconducting phase difference across the junction. In general the insertion of ferromagnetic material in the barrier of a Josephson junction is associated with a suppression of superconducting correlations. But also new phenomena can arise which may allow new circuit layouts and enhance the performance of applications. This thesis presents a systematic investigation for two concepts to fabricate Josephson junctions with a rather uncommon negative critical current. Such devices exhibit an intrinsic phase slip of Pi between the electrodes, so they are also known as Pi-junctions. Both studies go well beyond existing experiments and in one system a Pi-junction is shown for the first time. All the thin film junctions are based on superconducting Nb electrodes. In a first approach, barriers made from Si and Fe were investigated with respect to the realisation of Pi-junctions by spin-flip processes. The distribution of Fe in the Si matrix was varied from pure layers to disperse compounds. The systematic fabrication of alloy barriers was facilitated by the development of a novel timing-based combinatorial sputtering technique for planetary deposition systems. An orthogonal gradient approach allowed to create binary layer libraries with independent variations of thickness and composition. Second, Nb | AlOx | Nb | Ni60Cu40 | Nb (SIsFS) double barrier junctions were experimentally studied for the occurrence of proximity effect induced order parameter oscillations. Detailed dependencies of the critical current density on the thickness of s-layer and F-layer were acquired and show a remarkable agreement to existing theoretical predictions. Especially a variation of jc and IcRn by the s-layer thickness up to the value of nonmagnetic SIS junctions is notable. Additionally information on the emergence of superconductivity with the s-layer thickness was acquired.

M. Yu. Kupriyanov, Alexander Brinkman, Alexander A. Golubov +2 more

Meng Xiaofan, Wang Xiewen

This paper reports the fabrication and properties of small Nb-(a-Si)-Nb-(a-Si)-Nb Josephson tunnel junctions. The smallest junction area is about 0.3μm 2 . A thin a-Si film deposited by electron beam evaporation was oxidized and used for the tunnel barriers. TEM and XPS investigations indicate that the barrier layers consist of SiO x and Si. The junctions have been used for RF SQUID with an energy resolution of 1.9 × 10 -28 J/Hz.