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IEEE Transactions on Magnetics

An all-NbN time domain reflectometer chip functional above 8 K

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

Published Mar 1, 1989• 19 citations
Publisher

Abstract

Niobium nitride has a superconducting transition temperature nearly twice that of niobium. A 5-ps time-domain reflectometer chip based on NbN technology has been designed, fabricated, and tested. The circuit is operable up to 9 K. The NbN process and limitations are discussed, and present drawbacks in the junction fabrication method are pointed out. Electrical properties are discussed, the circuit operation is described, and simulations are presented that are based on model parameters extracted from device measurements. The actual output of the circuit is presented as evidence of basic functionality. This is the first demonstration of a functional high-speed circuit based entirely on a compound superconductor technology and operable at temperatures above 8 K.>

DOI
10.1109/20.92402

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IEEE Transactions on Applied Superconductivity•Aug 17, 2020•9 citations
Niobium nitride has a superconducting transition temperature nearly twice that of niobium. A 5-ps time-domain reflectometer chip based on NbN technology has been designed, fabricated, and tested. The circuit is operable up to 9 K. The NbN process and limitations are discussed, and present drawbacks in the junction fabrication method are pointed out. Electrical properties are discussed, the circuit operation is described, and simulations are presented that are based on model parameters extracted from device measurements. The actual output of the circuit is presented as evidence of basic functionality. This is the first demonstration of a functional high-speed circuit based entirely on a compound superconductor technology and operable at temperatures above 8 K.>
A novel process suitable for fabricating superconducting circuitry based on the all refractory material NbN is described. In this process, an in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 5mV have been achieved for high quality junctions. In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators.
An analog to digital converter (ADC) using the rapid single flux quantum (RSFQ) logic family implemented in niobium nitride (NbN) technology is described. The circuit was originally developed and demonstrated in niobium technology. An identical circuit was then laid out, fabricated and demonstrated in NbN technology. The chips were fabricated using an eight-layer NbN-based process with Josephson junction critical current density of 500 A/cm/sup 2/. In this paper, we report on the measurement results for a 6-bit flux quantizing ADC which exhibited proper operation and good DC bias margins. We also demonstrate results from an ADC chip operating up to 9.5 K.>
A novel process suitable for fabrication superconducting circuitry based on the all refactory material NbN is described. In this process, and in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 3mV have been achieved for high quality junctions (Vm 20 mV). In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators. (Author)
HYPRES, Inc. has introduced to the commercial marketplace a Sampling Oscilloscope/Time Domain Reflectometer (TDR) based on Josephson junction technology. The unit offers measurement performance commensurate with the inherent high speed of the Josephson elements, e.g., bandwidths in excess of 70 GHz and rise times on the order of 5 ps. A Josephson {rigger recognizer and delay circuit allows triggering of the sampling oscilloscope from the signal itself, with full view of the trigger point. Input modules offering different sensitivities or TDR capability are quickly interchangeable, and operating temperature is achieved in less than one minute. The technical details of the cooling technique and the chip circuitry will be described in this paper .
The high kinetic inductance of niobium nitride (NbN) thin films can be used for an implementation of compact on-chip inductances in cryoelectronic circuits. Here, for the first time, we demonstrate the implementation of a hybrid superconducting technology that includes the fabrication of standard aluminum submicron Josephson junctions and the NbN atomic layer deposition process. As an example, we fabricated and characterized a single and array of Al Josephson junctions together with NbN interconnections. The main Al Josephson junction parameters as well as NbN superconducting properties are in a good agreement with the values obtained by our standard fabrication process. The combination of technological processes for the NbN layers with Al Josephson junction allows implementing a new generation of innovative superconducting devices for different applications.
In recent years, rapid single flux quantum (RSFQ) circuits, which are promising for the development of high-speed digital electronics, have attracted widespread interest. Niobium nitride (NbN) films have also attracted significant attention for high-frequency applications in superconducting electronics because of their superior material parameters, including their high critical temperature Tc and large gap voltage Vg. To enable the development of all-NbN SFQ circuits, we have developed a fabrication process based on NbN junctions and inductors made of NbN strip lines. It is important to evaluate the inductance because it can affect the performance of an RSFQ circuit and is essential for the RSFQ design. We measured the inductance properties of both epitaxial and polycrystalline NbN strip lines using a superconducting quantum interference device modulation technique at 4.2 K and obtained the width dependence on the inductance of the NbN films. The penetration depth was then calculated from these measurements. The NbN inductance values were calculated numerically with respect to the penetration depth and were then compared with the measured values from the experiments. It was demonstrated that the differences between these values were within 5%. The results reported here will be useful for the design and fabrication of all-NbN RSFQ circuits.