IEEE Journal of Solid-State Circuits

A 60-GHz NbN single-flux quantum counter circuit

J.W. Spargo, JD Cooper, G.E. Kerber, G.R. King, Richard Morris, A.G. Toth

Published Jun 1, 1991• 3 citations

Abstract

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.>

DOI: 10.1109/4.78279

Related papers

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

Stephen R. Whiteley, F. Kuo

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.

IEEE Transactions on MagneticsMar 1, 19910 citations

A fast Josephson SFQ shift register

F. Kuo, Stephen R. Whiteley, S. M. Faris

A Josephson SFQ (single flux quantum) shift register circuit operating under a two-phase power was designed and tested. The test cell was fabricated using the Nb/AlO/sub x//Nb junction SNEAP process. The main data latch is DC biased; the data storage is accomplished by the transfer of a single flux quantum in or out of a SQUID (superconducting quantum interference device) superconducting loop. A two-phase sinusoidal clock with offset is used to operate the circuit. Simulations verify that the operating frequency can go beyond 70 GHz, with operating margins exceeding 20%. Preliminary measurements indicate that the circuit operates as intended.>

IEEE Transactions on MagneticsMar 1, 19894 citations

High speed non-latching SQUID binary ripple counter

Arnold H. Silver, R. Phillips, Robert D. Sandell

High speed, single flux quantum (SFQ) binary scalers are important components in superconducting analog-to-digital converters (ADC). This paper reviews the concept for a SQUID ADC and the design of an SFQ binary ripple counter, and reports the simulation of key components, and fabrication and performance of non-latching SQUID scalers and SFQ binary ripple counters. The SQUIDs were fabricated with Nb/Nb2O5/PbIn junctions and interconnected by monolithic-superconducting transmission lines and isolation resistors. Each SQUID functioned as a bistable flip-flop with the input connected to the center of the device and the output across one junction. All junctions were critically damped to optimize the pulse response. Operation was verified by observing the do I-V curves of successive SQUIDs driven by a cw pulse train generated on the same chip. Each SQUID exhibited constant-voltage current steps at 1/2 the voltage of the preceding device as expected from the Josephson voltage-to-frequency relation. Steps were observed only for the same voltage polarity of successive devices and for proper phase bias of the SQUID. Binary frequency division was recorded up to 40GHz for devices designed to operate to 28GHz.

IEEE Transactions on MagneticsMar 1, 198529 citations

100 GHz Binary counter based on DC SQUID's

C.A. Hamilton, Frances L. Lloyd

A binary counter using bistable dc SQUID's as flip flop circuits is demonstrated. All of the functions: LOAD, COUNT, STORE, READ, and CLEAR can be performed. The use of single flux quantum logic results in high sensitivity (10-18J input pulse energy), high speed (100 GHz count rate) and low power (10-7W at 100 GHz count rate).

IEEE Electron Device LettersNov 1, 198264 citations

Experimental evaluation of some rapid single flux quantum cells

Ying Ki Kwong, V. Nandakumar

Some basic cells in the rapid single-flux quantum (RSFQ) family of superconductive logic circuits were studied. An input stage, a buffer, and an OR gate were simulated and laid out for a standard niobium-based fabrication process. The resulting circuits perform properly for clock speeds up to 1 GHz. For the simpler circuits tested, the measured margins are wide, consistent with simulations, and not very dependent on clock speed. However, margin decreases with increased circuit complexity. The input stage converts a rising edge into an SFQ pulse, which has a small amplitude and narrow width in time. On the present circuits, these pulses are about 200 mu V in amplitude and 10 ps in width. To facilitate measurement, the authors have chosen as the output an asymmetrical superconducting quantum interference device (SQUID) consisting of two Josephson junctions. It converts SFQ pulses into 2.5 mV latching output levels on chip. By inductively coupling SFQ pulses into the SQUID loop, DC current isolation between the RSFQ circuit-under-test and the output stage is provided. The SQUID was optimized for 2 GHz operations.>

IEEE Transactions on Applied SuperconductivityMar 1, 19937 citations

Temperature-dependent performance of all-NbN DC-SQUID magnetometers

Quansheng Liu, Huiwu Wang, Qiyu Zhang +3 more

Integrated NbN direct current superconducting quantum interference device (DC-SQUID) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications operating at high temperatures. We report the current–voltage and voltage–flux characteristics and the noise performance of the NbN DC-SQUIDs for temperatures ranging from 4.2 to 9 K. The critical current and voltage swing of the DC-SQUIDs decreased by 15% and 25%, respectively, as the temperature was increased from 4.2 to 9 K. The white flux noise of the DC-SQUID magnetometer at 1 kHz increased from 3.9 μΦ0/Hz1/2 at 4.2 K to 4.8 μΦ0/Hz1/2 at 9 K with 23% increase, corresponding to the magnetic field noise of 6.6 and 8.1 fT/Hz1/2, respectively. The results show that NbN DC-SQUIDs improve the tolerance of the operating temperatures and temperature fluctuations in SQUID applications.

Applied Physics LettersMay 29, 201717 citations