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IEEE International Superconductive Electronics Conference (ISEC)

Uniformity and Reproducibility of Submicron 20kA/cm² Nb/AlOx/Nb Josephson Junction Process

Shuichi Nagasawa, T. Satoh, Mutsuo Hidaka

Published Jul 1, 2015• 12 citations
Publisher

Abstract

Submicron Nb/AlOx/Nb junctions having high critical current density of more than 20 kA/cm2are required for realizing single-flux-quantum (SFQ) circuits operating at a high speed of more than 100 GHz. We have developed a 20 kA/cm2process having submicron circular junctions. 1000-serially-connected circular shape junctions having six different diameters of 0.5 μm, 0.564 μm, 0.7 μm, 0.9 μm, 1.1 μm, and 1.3 μm have been designed. The 0.564 μm junction is designed to be 50 μA with critical current density of 20 kA/cm2, which is a minimum current value of a target SFQ circuit design. We totally fabricated 12 wafers with the 20 kA/cm2process. The 1σ spread of critical currents (Ic) depending on junction sizes and the run-to-run reproducibility of critical current densities are reported as a summary of these fabrications. We obtained Icscattering of 1σ≤ 2% even in the minimum circular junction of 0.25 μm2in several fabrication runs.

DOI
10.1109/isec.2015.7383488

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Submicron Nb/AlOx/Nb junctions having high critical current density of more than 20 kA/cm2are required for realizing single-flux-quantum (SFQ) circuits operating at a high speed of more than 100 GHz. We have developed a 20 kA/cm2process having submicron circular junctions. 1000-serially-connected circular shape junctions having six different diameters of 0.5 μm, 0.564 μm, 0.7 μm, 0.9 μm, 1.1 μm, and 1.3 μm have been designed. The 0.564 μm junction is designed to be 50 μA with critical current density of 20 kA/cm2, which is a minimum current value of a target SFQ circuit design. We totally fabricated 12 wafers with the 20 kA/cm2process. The 1σ spread of critical currents (Ic) depending on junction sizes and the run-to-run reproducibility of critical current densities are reported as a summary of these fabrications. We obtained Icscattering of 1σ≤ 2% even in the minimum circular junction of 0.25 μm2in several fabrication runs.
We are investigating device yield in our Nb-nine-layer fabrication process for single-flux-quantum (SFQ) circuits, in which the critical current density of the Nb/AlOx/ Nb Josephson junction (JJ) is 10 kA/cm2and the minimum JJ size is 1 μm2. We fabricated several shift registers (SRs) with different sizes that exhibit bit capacities from 16 to 2560 bits on a chip on many wafers. The SR yields correlate roughly with process defect test results using diagnostic chips, which are included in all wafers and consist of various diagnostic patterns such as for interlayer and intralayer insulations. When the defect number was above a certain level, the SR yield drastically degraded. The other reason of SR yield degradation is poor size uniformity of JJ patterns. The JJ pattern resolutions using our i-line stepper depend on the several-μm-level flatness of wafers. All SRs on a chip consisting of 68 990 JJs operated correctly. We obtained both “good” and “poor” chips without intentional changes of process conditions. The “poor” chip was strongly affected by magnetic flux traps. We believe that determining the cause of this difference is key to fabricating SFQ circuits consisting of 100 000 or more JJs.
Rapid Single-Flux-Quantum Circuits Fabricated Using 20-kA/cm2\hbox{20}\hbox{-}\hbox{kA}/\hbox{cm}^{2}20-kA/cm2 Nb/AlOx/Nb\hbox{Nb}/\hbox{AlO}_{x}/\hbox{Nb}Nb/AlOx​/Nb Process
We designed and fabricated rapid single-flux-quantum (RSFQ) circuits using a new 20-kA/cm2process based on conventional Nb/AlOx/Nb Josephson junction (JJ) technology. Circular JJs were fabricated to improve their uniformity. They had sufficiently low spread of the critical currents for large-scale RSFQ circuits. We selected the circuit design parameters carefully, including the bias voltage and McCumber-Stewart parameter, balancing the energy consumption, switching speeds, margins, and integration density. We demonstrated several logic gates and shift registers at a clock frequency of 154 GHz with the designed bias voltage of 2.5 mV, and demonstrated energy-efficient shift registers using the low-voltage RSFQ (LV-RSFQ) technique at 83 GHz.
The Superconductivity Research Laboratory has successfully fabricated large quantities of single flux quantum (SFQ) large scale integrated circuits, including several thousands of Josephson junctions (JJs). Using a Jc = 2.5 kA cm−2 process in which the number of Nb layers was four and the minimum JJ size was 2 µm square. We developed a new advanced fabrication process that produced a Jc = 10 kA cm−2, nine Nb layers and a minimum JJ size of 1 µm square. The increase in the number of Nb layers was achieved by using a planarization technique. The target of our next generation process is a Jc = 40 kA cm−2 with a 0.5 µm square for the minimum junction size. This specification will be achieved by using advanced semiconductor technologies. This process will enable SFQ circuits to be produced with one million JJs on a chip and achieve a clock frequency greater than 100 GHz.
In NEC's standard process, the minimum junction size is 2 µm and the critical current density ( J C ) is 2.5 kA cm -2 . In the process, i-line stepper lithography and reactive ion etching with SF 6 gas are used and the standard deviation ( ) of the critical current ( I C ) was 0.9% for the 2 µm junctions. This junction uniformity enables integration of more than 10M junctions if an I C variation of ±10% permits correct circuit operation. A 512-bit shift register was designed and fabricated by our standard process. Correct 512-bit delay operation was obtained. These results are promising for the large-scale integration of single flux quantum circuits.
Superconducting digital circuits based on Josephson junctions with amorphous niobium-silicon (a-NbSi) barriers have been designed, fabricated, and tested. Single-flux-quantum (SFQ) shift registers operated with ±30% bias margins, confirming junction reproducibility and uniformity. Static digital dividers operated up to 165 GHz for a single value of bias current, which was only marginally slower than circuits fabricated with externally shunted AlOx-barrier junctions having a comparable critical current density of 4.5 kA/cm2. In comparison, self-shunted a-NbSi junctions enabled a doubling in circuit density. This and their relatively thick 10 nm barriers could increase the yield of complex SFQ circuits.
We present characteristics of overdamped Josephson junctions consisting of Nb/AlOx/Al/ AlOx/Nb structures. The junctions were fabricated using a well-developed Nb/AlOx/Nb-junction technology and showed well-defined Josephson characteristics at 4.2 K. The characteristic voltage Vc [the product of the critical current Ic and the effective normal resistance Rn(eff)] of junctions, which determines high-frequency performance of the junction, was in the range of 0.1–0.5 mV, and the critical current density Jc in the range of 102–104 A/cm2. Maximum-to-minimum variations in Ic over a wafer were ±4% for junctions with Vc=0.15 mV and ±13% for junctions with Vc=0.5 mV.