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IEEE Transactions on Applied Superconductivity

Improved $\hbox{All-MgB}_{2}$ Toggle Flip-Flop Circuit With Increased Operating Speed and Temperature Range

Elias Galán, Daniel Cunnane, Ke Chen, X. X. Xi

Published Nov 4, 2014• 0 citations
Publisher

Abstract

We have fabricated and tested rapid single flux quantum toggle flip-flop circuits using self-shunted MgB2/ MgO/MgB2Josephson junctions with a single ground layer. The MgB2films were deposited using HPCVD, and the MgO barrier was deposited using DC reactive sputtering. We highlight the circuit's voltage divider operation dependence on current bias and temperature which show operation from 7 to 30 K. The highest attained operating speed is 335 GHz at 7 K. These results demonstrate the versatile temperature range and speed of MgB2digital circuits.

DOI
10.1109/tasc.2014.2366912

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Improved All-MgB2\hbox{All-MgB}_{2}All-MgB2​ Toggle Flip-Flop Circuit With Increased Operating Speed and Temperature Range
We have fabricated and tested rapid single flux quantum toggle flip-flop circuits using self-shunted MgB2/ MgO/MgB2Josephson junctions with a single ground layer. The MgB2films were deposited using HPCVD, and the MgO barrier was deposited using DC reactive sputtering. We highlight the circuit's voltage divider operation dependence on current bias and temperature which show operation from 7 to 30 K. The highest attained operating speed is 335 GHz at 7 K. These results demonstrate the versatile temperature range and speed of MgB2digital circuits.
Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.
We report a developed single-flux-quantum (SFQ) circuit fabrication process for vertically-stacked interface-treated Josephson junctions by using high-temperature superconductor (HTS) and also report the 500 GHz operation of a toggle flip-flop (TFF). As the first trial of the circuit operation, we fabricated TFF circuits, because TFF is used as a benchmark circuit to indicate potential of the circuit operating speed. To develop the circuit fabrication process, we utilized the counter YBCO layer as a wiring layer. We obtained resistively-shunted-junction behavior of single junction fabricated by new process. In addition, we observed current-voltage characteristics at the input port and the output port of the TFF, and confirmed that the input voltage coincided with twice the output voltage up to 1.1 mV at 4.2 K, which corresponds to the correct operation up to 500 GHz. This frequency is the highest value in HTS circuits ever observed, which suggests the high potential of the vertically-stacked JJs for the circuit operation.
A rapid-single-flux-quantum toggle-flip-flop logic gate was fabricated using high-temperature superconducting interface-engineered Josephson junctions. It was shown that the gate can operate as a voltage divider up to 155 GHz at 15 K and 19 GHz at 27 K. At the same time, the temperature dependence of the IcRn product and the maximum divided voltage was compared. As a result, it was found that the ratio of these values is 0.4–0.1 for 15 K>T>27 K. Circuit simulation with noise sources reveals this peculiar temperature dependence of the maximum divided voltage.
We fabricated and tested high-temperature superconducting (HTS) rapid-single-flux-quantum (RSFQ) circuits using interface-engineered Josephson junction (IEJ). The characteristics of high-speed operation were examined by voltage-divider operation based on the toggle-flip-flop circuit. An operation at 155 GHz was obtained at 15 K and the proper functioning of the voltage divider was observed up to 27 K. Temperature dependence of the maximum operation speed was analyzed by circuit simulation with thermal-noise sources. To achieve higher speed operation using a Josephson junction with a higher I/sub c/R/sub n/ product, we also fabricated an RSFQ balanced comparator using IEJ; proper switching characteristics of the circuit was confirmed up to 190 GHz at 30 K. The bias condition dependencies were studied based on the circuit simulations in the relationship of the I/sub c/R/sub n/ product. The results of circuit simulation indicate that a Josephson junction with an I/sub c/R/sub n/ product of more than 0.7 mV is required for high-speed operation (100 GHz) in high-temperature range (30-40 K). We believe that the IEJ is a promising junction candidate for realizing high-speed operation in a high-temperature range.
We designed and fabricated a rapid-single-flux-quantum T flip-flop (TFF) with high- T c interface-engineered Josephson junctions. Y 1 Ba 2 Cu 3 O 7− d and Sr 2 AlTaO 6 were deposited for the superconducting layer and the insulating layer, respectively. The Josephson junction was formed through an interface treatment process using Ar ion milling and vacuum annealing. We simulated a TFF circuit and designed a physical layout using WRspice and Xic. The fabricated TFF has a minimum junction width of 3 μm. Through the measurement of the voltage divider operation, the maximum operation frequency was estimated to be 53 GHz at 22 K and 106 GHz at 12 K.
Toggle-flip-flop (T-FF) is one of the most important high-Tcsuperconducting single-flux quantum (HTS SFQ) circuit components and has been designed and fabricated by using YBa2Cu3O7-deltaramp-edge junction technology. The circuit layout of the T-FF was improved to suppress the junction critical current (Ic) spread in the circuit. Test circuits, which include a T-FF with a single output for evaluating the logic operation and measuring the operating frequency, were fabricated and their operation characteristics were investigated. The T-FF circuit with a single output was successfully operated and finite direct current (dc) supply current margins were obtained at temperatures from 27 to 34 K. Moreover, the maximum operating frequency of the T-FF was estimated to be 360 GHz at 4.2 K and 114 GHz at 41 K. In addition, reduction of dc supply current margins due to thermal noise was also investigated. According to the numerical simulation in which parasitic inductances were taken into account, the narrowest margin in the T-FF circuit wider than plusmn10% was maintained with a bit-error rate (BER) of 10-6up to 40 K