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IEEE Transactions on Magnetics

A high performance Josephson binary counter implemented in Nb and NbN technology

Stephen R. Whiteley, F. Kuo

Published Mar 1, 1991• 0 citations
Publisher

Abstract

A Josephson binary counter with nondestructive readout has been implemented and tested in both niobium and niobium nitride technology. Successful operation of the Nb version has been observed. The design incorporates an additional tapered-edge SiO/sub 2/ level in the Nb processing sequence, which increases interferometer inductance, decreases capacitance, and ensures that geometric resonances are as high in frequency as possible. This level has the added advantage of providing mask compatibility with the NbN process, as this level is skipped in the NbN flow, thereby compensating in part for the larger penetration depth of NbN. The counter cell is designed to be as compact as possible to minimize stray inductance and maximize top count rate and high count rate bias margins. A novel readout SQUID coupling layout that allows low read SQUID inductance and requires no holes in the groundplane is used. Coupling to the adjacent count SQUID is provided by a common control line level metallization, which directs count SQUID flux through the read SQUID loop. Nb versions of the circuit functioned as expected, but low device yield limited the longest chain of functioning stages to five. The NbN circuits did not function due to higher than predicted inductance values.

DOI
10.1109/20.133812

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A Josephson binary counter with nondestructive readout has been implemented and tested in both niobium and niobium nitride technology. Successful operation of the Nb version has been observed. The design incorporates an additional tapered-edge SiO/sub 2/ level in the Nb processing sequence, which increases interferometer inductance, decreases capacitance, and ensures that geometric resonances are as high in frequency as possible. This level has the added advantage of providing mask compatibility with the NbN process, as this level is skipped in the NbN flow, thereby compensating in part for the larger penetration depth of NbN. The counter cell is designed to be as compact as possible to minimize stray inductance and maximize top count rate and high count rate bias margins. A novel readout SQUID coupling layout that allows low read SQUID inductance and requires no holes in the groundplane is used. Coupling to the adjacent count SQUID is provided by a common control line level metallization, which directs count SQUID flux through the read SQUID loop. Nb versions of the circuit functioned as expected, but low device yield limited the longest chain of functioning stages to five. The NbN circuits did not function due to higher than predicted inductance values.
A Josephson binary counter using single-flux quanta transitions of DC SQUIDs (superconducting quantum interference devices) has been fabricated using an eight-level NbN-based process. High-speed binary division has been demonstrated at 4.2 K, with single-cell counting observed at 60 GHz using the Josephson voltage-to-frequency relationship. Count rate was primarily limited by conservative process and design rules. The counter was designed for operation at 4.2 K. At 8-10 K, the beta /sub L/ of the SQUIDs would not allow operation, though the junction characteristics were good.>
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A fast Josephson circuit using a threshold logic is developed for application to a multiplier and a binary counter. The former is a typical combinational circuit and the latter is a typical sequential circuit. The junction and barrier materials used were Nb-AlO/SUB X/-Nb. An optimized asymmetric two-junction interferometer maximized the operating margin of the threshold gate. A speed-up junction was introduced to decrease the switching delay without sacrificing the operating margin. A dumping resistor, which was inserted parallel to the input signal line of the threshold gate between its two terminals, decreased the reflection of the input signal caused by the gate inductance, thereby ensuring the margin and speed. To demonstrate the high-speed possibility of the Josephson threshold logic, a high-speed experiment for the circuits was performed. The multiplier demonstrated 210-ps operation.
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A new structure of the base-electrode for Josephson devices have been developed in which niobium (Nb) and niobium nitride (NbN) films were sequentially sputtered to form a double-layered structure. Owing to this structure, it is possible not only to reduce the kinetic inductance of the base-electrode due to the small penetration depth of Nb film, but also to obtain good tunneling characteristics with a small subgap conductance due to the surface stability of NbN film. Single-flux-quantum (SFQ) memory cells with this structure have been fabricated and it has been observed that the kinetic inductance is much decreased comparing with that of the SFQ cell with a base-electrode of NbN single layer.