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European Microwave Conference (EuMC)

Fullwave frequency dependent inductance estimation for RSFQ applications in NbN multilayer technology

Junwu Tao, Agnès Roussy, Pascal Febvre, Jean-Claude Villégier, N. Hadacek

Published Oct 1, 2000• 1 citation
Publisher

Abstract

A frequency dependent inductance estimation has been carried out for Rapid-Single-Flux Quantum (RSFQ) applications in NbN multilayer technology up to frequencies above the NbN gap frequency. For this purpose an extension of the fullwave modified transverse resonance method (MTRM) has been used, in which each superconducting layer is characterized by its complex conductivity according to the BCS derived model of Mattis and Bardeen. Our estimation shows significant influence of the gap frequency on the NbN multilayer structure inductance at high frequencies.

DOI
10.1109/euma.2000.338576

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A frequency dependent inductance estimation has been carried out for Rapid-Single-Flux Quantum (RSFQ) applications in NbN multilayer technology up to frequencies above the NbN gap frequency. For this purpose an extension of the fullwave modified transverse resonance method (MTRM) has been used, in which each superconducting layer is characterized by its complex conductivity according to the BCS derived model of Mattis and Bardeen. Our estimation shows significant influence of the gap frequency on the NbN multilayer structure inductance at high frequencies.
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.
The Rapid Single Flux Quantum (RSFQ) technique is regarded as one of the most promising electronics due to its extremely high operating speed and low power consumption. Middle-scale RSFQ applications operating at multigigahertz frequency have recently been reported on. At such high operating frequencies, the on-chip interconnects start to play a limiting role for the performance of the densely packaged digital circuits. Our previous studies have theoretically investigated the parasitic coupling effects between superconductive microstrip transmission lines and conclusions have been drawn about the critical coupling levels of several typical microstrip geometries. Here, basic RSFQ structures are investigated experimentally, which contain a Josephson transmission line stage with large inductance and can either trap or pass SFQ pulses. These structures are incorporated in critical microstrip layouts, thus being subjected to parasitic coupling. The bias levels are monitored at which the structures start to pass the SFQ pulses with and without coupling influences. Thus, the conclusions of our previous theoretical investigations are verified experimentally.
We report on the fabrication and the operation of all-NbN rapid single flux quantum (RSFQ) circuits. The NbN integrated circuit consists of NbN/AlN/NbN tunnel junctions, Mo resistors, sputtered SiO/sub 2/ insulating layers, and 400-nm-thick NbN ground plane and wiring layer. The circuits were fabricated with the minimum junction size of 2 /spl mu/m/spl times/2 /spl mu/m and the alignment margin of +/-0.5 /spl mu/m. We designed RSFQ logic cells under critical current density of 2.5 kA/cm/sup 2/, sheet resistance of 2 /spl Omega/ and sheet inductance of 1.4 pH. We experimentally investigated dc bias margin of the fabricated RSFQ cells at 9 K. We observed relatively large dc bias margin of more than +/-25% for a pulse splitter, a confluence buffer and an RS flip-flop, while the dc bias margin of a T flip-flop was less than +/-5%. We succeeded to demonstrate the operation of 16-bit concurrent flow shift register at 9 K.
We present the planar microwave filters developed as the front selective and impedance matching circuit for the RSFQ based (Rapid Single Flux Quantum) A/D converters, operating at cryogenic temperature (T = 9 K). The structures are to be patterned in thin NbN (niobium nitride) superconducting films with critical temperature T ap 16 K. The filter design is experimentally verified at lower frequency (f0= 5 GHz, microstrip) on the Ro3006 substrate. During the design, we aim to reach high stopband attenuation and matching of different input - output impedances. The filters use high impedance level in order to obtain suitable dimensions of the lines, important for the final accuracy. A layout of 1 cm2NbN filter designed for f0= 30 GHz and CPW is presented. This NbN filter is intended for the extraction of parameters such as kinetic inductance or London penetration depth, in order to improve the simulation accuracy of our NbN process.
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.
The frequency dependence of the coupling of niobium superconducting transmission lines is measured for frequencies up to and above the superconducting gap frequency. For this purpose superconducting microstrips are integrated in log-periodic planar antennas and the spectral response is measured by means of a Fourier transform spectrometer with 1 μm2 Nb/Al2O3/Nb SIS junctions as detectors. Resonances of microstrips are observed up to 650 GHz, the superconducting gap frequency of niobium, but above that frequency no resonances have been detected. The measured resonance frequencies are in good agreement with a dispersive model for superconducting transmission lines based on the Mattis–Bardeen theory. The radiation coupling is lower than calculated, indicating significant dielectric losses.