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IEEE Transactions on Applied Superconductivity

Rapid single flux quantum T-flip flop operating up to 770 GHz

W. Chen, Alexander V. Rylyakov, Vijay Patel, James E. Lukens, K. K. Likharev

Published Jun 1, 1999• 277 citations
Publisher

Abstract

Rapid Single Flux Quantum (RSFQ) T-flip flops (TFFs) operating up to 770 GHz have been demonstrated at 4.2 K. The devices, consisting of either resistively shunted or unshunted Josephson junctions, are fabricated using a planarized Nb/AlO/sub x//Nb trilayer process. Electron beam lithography is used to pattern all levels with a minimum junction area less than 0.1 /spl mu/m/sup 2/. Critical current densities of 0.5 mA//spl mu/m/sup 2/ and 2.5 mA//spl mu/m/sup 2/ are used for the shunted (tested at 1.8 K) and unshunted devices (tested at 4.2 K) respectively. The input and output frequencies of the TFFs are obtained from the input and output voltages by the Josephson relation. The output voltage is exactly half of the input voltage when the divide-by-two operation is correct.

DOI
10.1109/77.783712

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Rapid Single Flux Quantum (RSFQ) T-flip flops (TFFs) operating up to 770 GHz have been demonstrated at 4.2 K. The devices, consisting of either resistively shunted or unshunted Josephson junctions, are fabricated using a planarized Nb/AlO/sub x//Nb trilayer process. Electron beam lithography is used to pattern all levels with a minimum junction area less than 0.1 /spl mu/m/sup 2/. Critical current densities of 0.5 mA//spl mu/m/sup 2/ and 2.5 mA//spl mu/m/sup 2/ are used for the shunted (tested at 1.8 K) and unshunted devices (tested at 4.2 K) respectively. The input and output frequencies of the TFFs are obtained from the input and output voltages by the Josephson relation. The output voltage is exactly half of the input voltage when the divide-by-two operation is correct.
The paper presents the determination of the high-frequency performance of Rapid Single Flux Quantum (RSFQ) circuits fabricated in superconductor-insulator-normal metal-insulator-superconductor (SINIS) technology. Circuits of different designs have been realized and were experimentally investigated, including specially designed T flip-flop (TFF) structures and circuits consisting of a switch terminated in Josephson transmission lines. For TFF circuits, the operational functionality was investigated in the frequency range from dc up to nearly 200 GHz. Two frequency domains were found. In the first domain, ranging up to frequencies of 80 GHz (f/sub C//spl cong/Y/sub C///spl Phi//sub 0/), correct digital operation of the device was ascertained. Beyond this range a second domain was found, ranging to nearly 200 GHz, for which quasi-analog frequency dividing is assumed. The bias current margins are /spl plusmn/30% at 40 GHz and /spl plusmn/15% between 130 GHz and 200 GHz. For circuits consisting of a switch, the operational functionality was proved in the frequency range from dc up to 50 GHz, with bias current margins of more than /spl plusmn/20%. The critical current density of the circuits is j/sub C//spl cong/750 A/cm/sup 2/. The smallest junction area is about A=12 /spl mu/m/sup 2/, and the characteristic voltage is V/sub C/=170 /spl mu/V.
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.
A simple test circuit of the rapid single-flux-quantum (RSFQ) logic family has been implemented in a planarized all-refractory technology using only two superconducting layers and small Nb/AlOx/Nb Josephson junctions (nominally 1.5×1.5 μm2) with critical current density jc≊6 kA/cm2. A special layout design of the integrated circuit (including a SFQ pulse generator, Josephson transmission line, and T flip–flop) was used to reduce 3D inductances resulting from absence of the ground plane. For samples with various junction parameters, the maximum frequency of the SFQ pulse train, which could be divided by 2 by the flip–flop, ranged from 200 to 370 GHz.
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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.
The operational functionality of rapid-single-flux-quantum (RSFQ) T-flipflop circuits has been experimentally investigated in the frequency range from dc up to nearly three times the characteristic frequency . The operational range found is divided into two domains different in nature. In the first domain, ranging up to frequencies of 120 GHz , correct digital operation of the device has been found. Beyond this range a second domain has been found, ranging from about 200 GHz to nearly 350 GHz, in which quasi-analogous operation of frequency dividing is suggested. Typical bias current margins are in the first domain and in the second. The circuits have been fabricated using - Al/Nb trilayer technology with externally shunted Josephson tunnel junctions of critical current densities of and smallest areas of about . The characteristic voltage is and the McCumber parameter .