High density fabrication process for single flux quantum circuits

Abstract

We implemented, optimized, and fully tested a superconducting Josephson junction fabrication process over multiple runs tailored for integrated digital circuits that are used for control and readout of superconducting qubits operating at millikelvin temperatures. This process was optimized for highly energy efficient rapid single flux quantum (ERSFQ) circuits with critical currents reduced by a factor of ∼10 as compared to those operated at 4.2 K. Specifically, it implemented Josephson junctions with 10 μA unit critical current fabricated with 10 μA/μm2 critical current density. In order to circumvent the substantial size increase in the SFQ circuit inductors, we employed an NbN high kinetic inductance layer with 8.5 pH/sq sheet inductance. Similarly, to maintain the small size of junction resistive shunts, we used a non-superconducting PdAu alloy with 4.0 Ω/sq sheet resistance. For integration with quantum circuits in a multi-chip module, 5 and 10 μm height bump processes were also optimized. To keep the fabrication process in check, we developed and thoroughly tested a comprehensive process control monitor chip set.

Energy efficient half-flux-quantum circuit aiming at milli-kelvin stage operation

“…Due to our experimental conditions, a 50 nm thick PdNi film is patterned as the bias resistors, despite it may cause flux trapping in the adjacent superconducting loop and affect the circuit operation. PdAu or Pd resistors are desired in much more compacted circuits for milli-kelvin stage demonstrations [22,25]. The microphotograph of the HFQ TFF in figure 5 shows multiple π-π-π SQUIDs connected in serial or parallel, where the middle JJ acts as the π phase shifter while the upper and bottom JJs serve as the switching JJs.…”

“…Since information is stored as a magnetic flux in SFQ circuit, the small critical current (I c ) inevitably leads to a larger inductance (L) according to LI c = 0.4 ∼ 1.2Φ 0 , where Φ 0 is the magnetic flux quantum. The kinetic inductance of the superconductor stripe [24,25] and nonconducting soft magnetic films [26] are adopted or proposed to increase the sheet inductance. Further increase in circuit density requires two or more Josephson junction (JJ) layers [27], where not only the parasitic inductance of the stacked JJs can be used to replace the geometric one [28], but also to provide intrinsic π phase shifters with ferromagnetic JJs [29][30][31].…”