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Superconductor Science and Technology

Optimization of superconducting microstrip interconnects for rapid single-flux-quantum circuits

M R Rafique, Irina Kataeva, Henrik Engseth, Michael A. Tarasov, Anna Kidiyarova-Shevchenko

Published Jun 20, 2005• 13 citations
Publisher

Abstract

In this paper, issues related to the optimization of superconducting passive interconnects are discussed. Results of the microwave optimization of bends, via connections and crossings of superconducting microstrip lines (SMSLs) are reported. The optimum design of the SMSL cross gives more than 95% of transmission and can be well used in a two-bus cross design with up to 14 signal wires. The results have been confirmed by time-domain simulations and measurements.

DOI
10.1088/0953-2048/18/8/007

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In this paper, issues related to the optimization of superconducting passive interconnects are discussed. Results of the microwave optimization of bends, via connections and crossings of superconducting microstrip lines (SMSLs) are reported. The optimum design of the SMSL cross gives more than 95% of transmission and can be well used in a two-bus cross design with up to 14 signal wires. The results have been confirmed by time-domain simulations and measurements.
The increasing complexity of modern rapid single flux quantum (RSFQ) circuits has made on-chip signal routing, an issue of growing importance. In this paper, several methods for routing large-scale RSFQ circuits are described, and a process is presented for determining when to use passive microstrip transmission lines (PTL) and active Josephson transmission lines (JTL). The effect of the size of the JTL inductor and Josephson junctions on the length of a JTL chain for a target delay is also discussed. The dependence of the JTL inductance on the physical layout is evaluated, and the effects of the primary PTL parameters on delay are characterized. A novel PTL driver and receiver configuration is also proposed. Tradeoffs among the number of JJs, inductance, and length of a PTL stripline in the receiver and driver circuits are reported. The energy dissipation is evaluated for two different interconnects. A tradeoff between the proposed PTL circuits and an optimized JTL in terms of energy dissipation and delay is discussed. Guidelines for choosing the optimal element values are determined, and a simulated bias margin of ±29% for the bias current of the proposed receiver operating at 20 GHz in a 10 kA/cm2technology for a 1-mm transmission line is achieved. Summarizing, guidelines and design tradeoffs appropriate for automated layout and synthesis are provided for driving long interconnect in SFQ VLSI circuits.
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 developed a very flexible wiring technique of passive transmission lines (PTLs) in single-flux-quantum (SFQ) circuits. For high flexibility, we introduced two types of wiring structures of strip lines (SLs) and microstrip lines (MSLs) using the fabrication process which has four metal layers. The SLs and MSLs are realized using the second and fourth metal layers as signal lines, respectively, from the bottom (the other two metal layers are used as ground planes). We also introduced via-holes for transmission between the SL and MSL. For solving the problem of impedance mismatch on complicated discontinuous sections such as joints [between the driver(receiver) and the end of SLs(MSLs)] and via-holes, we optimized the structures of these discontinuous sections and evaluated propagation properties to ensure high-speed transmission of SFQ pulses. We tested the transmission of SFQ pulses through SLs/MSLs including more than 50 via-holes, one of the most complicated lines in actual designs. We confirmed the good propagation properties of SFQ pulses up to 50 Gb/s with sufficiently wide bias margins of ±20% to be used in practical designs.
We developed a design approach for interface circuits to connect Single Flux Quantum (SFQ) cells by using passive transmission lines (PTL's). In the approach, an interface circuit between a PTL and JTL is optimized to obtain a standard interface circuit, and then, modifications are made to previously designed SFQ cells and the standard interface circuit to connect the SFQ cells by using PTL's. The key point is the use of approximately the same interface circuit with every SFQ cell to maintain the matching condition between the interface circuit and the PTL's. Based on this approach, we designed an interface circuit and a test circuit composed of two D-flip-flops connected using 2-mm-long PTL's via the interface circuits. The impedance of the PTL was 2 /spl Omega/. We achieved high-speed operation of the test circuit up to 35 GHz with a bias margin of -15/+30%.
We have developed a method of designing single-flux-quantum (SFQ) logic circuits with passive gate-to-gate interconnections. Based on our method, we designed a 2/spl times/2 switch in which all the interconnections are implemented with passive transmission lines (PTLs) while short Josephson transmission line (JTL) segments are used only to adjust the signal timings. Compared with an identical switch using JTL interconnections, the switch using PTL interconnections has 45% fewer wiring junctions and requires 48% less wiring power current. The switch operated at 40 GHz with a bias margin of /spl plusmn/9.5%.
To enable the use of passive transmission lines (PTLs) for the interconnection of single-flux-quantum (SFQ) circuits, we have implemented a driver and a receiver and have developed a method for designing SFQ circuits with passive interconnections. Basic components and properties of passive interconnections, such as the frequency characteristics of the driver and receiver, the PTL delay, and the crosstalk between PTLs, have been experimentally verified. Our developed components and design method have been applied to actual SFQ circuits, such as a 4 x 4 switch having block-to-block passive interconnections and a 2 x 2 switch having gate-to-gate passive interconnections. We have also shown the advantages of PTLs over Josephson transmission lines (JTLs). We also discuss the prospects of SFO circuits having passive interconnections.