We have developed a fabrication technology for superconducting integrated circuits with Nb-based Josephson junctions. The standard fabrication process with 10 mask levels includes 3 Nb superconducting layers and a Mo resistor layer. The influence of deposition parameters on film stress, electrical properties, and surface roughness were studied systematically. High quality Nb, Al, Mo, and SiO ₂ films were successfully deposited for the subsequent fabrication of circuits. The circuit fabrication started with the fabrication of Mo resistors with a target sheet resistance R _sh of 2 Ω, followed by the deposition of Nb/Al-AlO _x /Nb trilayer Josephson-junction. The target critical current density J _c was set at 6 kA/cm ² . Small-scale circuits such as our standard library cells have been successfully fabricated and tested, confirming the capability of our fabrication technology for superconducting integrated circuits.

Recently, Bi ₂ O ₂ Se was revealed as a promising two-dimensional (2D) semiconductor for next generation electronics, due to its moderate bandgap size, high electron mobility and pronounced ambient stability. Meanwhile, it has been predicted that high-quality Bi ₂ O ₂ Se-related heterostructures may possess exotic physical phenomena, such as piezoelectricity and topological superconductivity. Herein, we report the first successful heteroepitaxial growth of Bi ₂ O ₂ Se films on SrTiO ₃ substrates via pulsed laser deposition (PLD) method. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate. The growth mode transition to three-dimensional (3D) island from quasi-2D layer of the heteroepitaxial Bi ₂ O ₂ Se films on SrTiO ₃ (001) substrates is observed as prolonging deposition time of films. The maximum value of electron mobility reaches 160 cm ² V ⁻¹ s ⁻¹ at room temperature in a 70 nm thick film. The thickness dependent mobility provides evidence that interface-scattering is likely to be the limiting factor for the relatively low electron mobility at low temperature, implying that the interface engineering as an effective method to tune the low temperature electron mobility. Our work suggests the epitaxial Bi ₂ O ₂ Se films grown by PLD are promising for both fundamental study and practical applications.

$\rm Nb/Al-AlO_x/Nb$ Josephson junctions are widely used in SQUIDs and Rapid Single Flux Quantum (RSFQ) circuits. Very large-scale RSFQ integration often has a strict requirement on wafer-scale junction qualities. In particular, Nb film stress in $\RM Nb/Al-AlO_x/Nb$ trilayer based Josephson junctions is a critical factor af-fecting their qualities. In this work, we develop a multi-step sput-tering process to prepare Nb films with different combinations of stress and roughness for investigating the correlation between Nb film stress and junction quality. In particular, this method allows us to prepare Nb films with almost the same roughness but dif-ferent stress, a key factor to study solely the influence of stress on junction qualities. Our measurements indicate that within a very tight range of film roughness (less than 2 nm), tensile film stress is found to decrease the quality of $\rm Nb/Al-AlO_x/Nb$ Josephson junctions and larger film roughness has a positive effect on junc-tion quality. By decreasing the total tensile stress and increasing the film roughness properly, we fabricated $\rm Nb/Al-AlO_x/Nb$ Jo-sephson junctions with a high quality factor characterized by $R_{sg}/R_N$ (30 averagely). The minimum size of junctions was 1 μm.

We have developed a 6-kA/cm superconducting integrated circuit fabrication process toward large-scale single flux quantum digital circuits. To evaluate the process reliability and controllability, a set of process control monitors (PCMs) for monitoring fabrication and design parameters was designed and tested. Fabrication parameters, for example, film thicknesses and etched depths of the layers, were measured by testing specific patterns during fabrication. Design parameters such as the critical current density J _c , normal-state resistance R _n and actual sizes of Josephson junctions, the sheet resistance of Mo resistor R _sh , and the penetration depth of Nb films were evaluated by electrically characterizing PCM circuits after fabrication. We also monitored superconducting critical currents for vias and Nb lines, and insulation properties between the metal layers systematically.