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IEEE Transactions on Applied Superconductivity

High-frequency crosstalk in superconducting microstrip waveguide interconnects

Marc Currie, Roman Sobolewski, T.Y. Hsiang

Published Jun 1, 1999• 6 citations
Publisher

Abstract

A low-temperature electro-optic sampling system was implemented to study the crosstalk of picosecond pulses between niobium microstrip interconnects. This system has been used to perform noninvasive, nodal testing on superconducting integrated circuits. We have characterized the crosstalk arising from the crossing of two microstrip waveguides. This is representative of high-speed interconnects in VLSI technology in which one signal line must cross above another. The measured crosstalk signal showed the capacitive nature of the coupling, thereby providing the material's dielectric permittivity at high frequencies. Our results have provided feedback for improving computer simulations of superconducting electronic circuits.

DOI
10.1109/77.783809

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A low-temperature electro-optic sampling system was implemented to study the crosstalk of picosecond pulses between niobium microstrip interconnects. This system has been used to perform noninvasive, nodal testing on superconducting integrated circuits. We have characterized the crosstalk arising from the crossing of two microstrip waveguides. This is representative of high-speed interconnects in VLSI technology in which one signal line must cross above another. The measured crosstalk signal showed the capacitive nature of the coupling, thereby providing the material's dielectric permittivity at high frequencies. Our results have provided feedback for improving computer simulations of superconducting electronic circuits.
We have implemented a low-temperature electro-optic sampling system for non-invasive, nodal testing of superconducting Nb integrated circuits. With submillivolt sensitivity and a subpicosecond temporal response, this system has been used to perform nodal analysis on rapid-single-flux quantum (RSFQ) devices and superconducting microstrip interconnects. Here we demonstrate that by measuring the propagation of 6-ps-wide pulses at various test nodes, we are able to fully characterize a superconducting microstrip waveguide the size of an entire chip. The transmission line was selected not only to perform the first complete characterization of a superconducting microstrip, but also to demonstrate full nodal testing of a foundry-fabricated RSFQ integrated circuit. Finally, our results provided much-needed feedback for improving computer simulations of superconducting digital circuits.
Abstract Interconnects are a major discriminator for superconducting digital technology, enabling energy-efficient data transfer and high-bandwidth heterogeneous integration. We report a method to simulate propagation of picosecond pulses in superconducting passive transmission lines (PTLs). A frequency-domain propagator model obtained from the Ansys high-frequency structure simulator (HFSS) field solver is incorporated in a Cadence Spectre circuit model, so that the particular PTL geometry can be simulated in the time-domain. The Mattis Bardeen complex conductivity of the superconductor is encoded in the HFSS field solver as a complex-conductivity insulator. Experimental and simulation results show that Nb 20 Ω microstrip PTLs with 200 nm interlayer dielectric thickness can support propagation of a single-flux-quantum pulse up to 7 mm and a double-flux-quantum pulse up to 28 mm.
Superconducting single flux quantum (SFQ) circuits can process information at extremely high speeds, in the range of hundreds of GHz. SFQ circuits are based on Josephson junction cells for switching logic and ballistic transmission for transferring SFQ pulses. Multi-chip modules (MCMs) are often used to implement larger complex designs, which cannot be fit onto a single chip. We have optimized the design of wideband interconnects for transferring signals and SFQ pulses between chips in flip-chip MCMs and evaluated the importance of several design parameters such as the geometry of bump pads on chips, length of passive microstrip lines (MSLs) and number of corners in MSLs as well as flux trapping and fabrication effects on the operating margins of the MCMs. Several test circuits have been designed to evaluate the above mentioned features and fabricated in the framework of a 4.5 kA cm −2 HYPRES process. The MCM bumps for electrical connections have been deposited using a wafer-level electroplating process. We have found that, at the optimized configuration, the maximum operating frequency of the MCM test circuit, a ring oscillator with chip-to-chip connections, approaches 100 GHz and is not noticeably affected by the presence of MCM interconnects, decreasing by only about 3% with respect to the same circuit with no inter-chip connections.
We have investigated and tried to improve the packaging technologies for a bit-error-rate (BER) measurement system for high-T/sub c/ superconducting circuits. Signal cables, test fixture, and magnetic shields were specially designed and assembled for the BER measurement system, taking into account electrical losses, signal reflection, heat inflow, and power consumption of a cryocooler. BER was further improved by changing the way of connection between the semiconductor amplifiers outside the sample vacuum chamber. These improvements led to a BER less than 10/sup -12/ at an output voltage (V/sub out/) of 1.7 and 2.3 mV for a 50-/spl Omega/ standard microstrip transmission line and a superconducting microstrip-line to coplanar-waveguide transmission-line converter, respectively. The temperature rise of the test fixture was as low as about 2 K. These values are approximately one order of magnitude smaller than those in the previous measurement system and small enough to be achieved by using superconducting interface circuits. Furthermore, 1-Gb/s operation of a superconducting quantum interference device array-type interface circuit was demonstrated in the BER measurement system at 40 K.
Superconducting microstrip, with phase velocities of about 0.01c, employing kinetic inductance have been fabricated using niobium nitride and a silicon dielectric. Delay lines using this phenomenon have several advantages for analog signal processing, including low loss and very compact size. Measurements of kinetic inductance microstrip delay lines were made in the frequency domain and the time domain. The results were compared to theoretical predictions and an accurate circuit model was developed. The circuit model was used to determine the frequency- and temperature-dependent losses of the delay line.