The increasing complexity of modern single flux quantum (SFQ) circuits has increased the importance of flux trapping and trapped magnetic fields within SFQ systems. This trapped flux reduces margins while damaging the operability of superconductive circuits. In this paper, an area efficient stripline topology is introduced to prevent flux from being trapped within striplines. The topology is composed of coupled narrow lines rather than wide striplines. The proposed topology uses a fingered narrow line configuration. The fingered narrow line topology enhances the scalability of SFQ systems while not requiring additional area. The proposed topology decreases the length of the striplines by exploiting the mutual inductance between narrow parallel lines. The topology requires less area while preventing flux from being trapped within wide superconductive striplines. Due to the configuration of the proposed stripline, residual current is eliminated in VLSI complexity SFQ circuits. The fingered narrow line topology also reduces coupling capacitance between striplines. The proposed topology is compatible with automated routing of large scale SFQ integrated circuits.
The increasing complexity of modern single flux quantum (SFQ) circuits has increased the importance of flux trapping and trapped magnetic fields within SFQ systems. This trapped flux reduces margins while damaging the operability of superconductive circuits. In this paper, an area efficient stripline topology is introduced to prevent flux from being trapped within striplines. The topology is composed of coupled narrow lines rather than wide striplines. The proposed topology uses a fingered narrow line configuration. The fingered narrow line topology enhances the scalability of SFQ systems while not requiring additional area. The proposed topology decreases the length of the striplines by exploiting the mutual inductance between narrow parallel lines. The topology requires less area while preventing flux from being trapped within wide superconductive striplines. Due to the configuration of the proposed stripline, residual current is eliminated in VLSI complexity SFQ circuits. The fingered narrow line topology also reduces coupling capacitance between striplines. The proposed topology is compatible with automated routing of large scale SFQ integrated circuits.
The increasing complexity of modern superconductive circuits, and single flux quantum (SFQ) circuits in particular has made the issue of flux trapping of growing importance. The use of wide superconductive striplines for signal routing has exacerbated this issue. Trapping residual magnetic fields in these striplines degrades performance while reducing margins, damaging the operability of superconductive circuits. In this article, an area efficient topology for striplines is introduced to manage flux trapping in large scale SFQ circuits. This topology is composed of narrow parallel lines in series with small resistors. The proposed topology decreases the length of the striplines by exploiting the mutual inductance between the narrow parallel lines. The topology requires significantly less area while preventing flux trapping within wide superconductive striplines. The narrow parallel line topology also reduces coupling capacitance between striplines. The proposed approach is compatible with automated routing of large scale SFQ integrated circuits.
Superconductive electronics based on Josephson junctions (JJ) is a promising cryogenic alternative technology to complementary metal oxide semiconductor (CMOS) technology for ultra-low energy, high speed stationary applications. For complex superconductive systems, the automated routing process determines the topology and methodology to connect cells while satisfying design constraints. On-chip signal routing has become an issue of growing importance in modern superconductive technologies; particularly, single flux quantum (SFQ) systems. Specialized routing methods for these systems are required. These routing methods include passive transmission lines (PTLs) and Josephson transmission lines (JTLs) behaving as interconnects. A primary issue within a long SFQ interconnect is the effects of resonance due to the imperfect match between the PTLs and Josephson junctions. A repeater insertion methodology to reduce and manage these resonance effects is required for driving long and short interconnect in VLSI complexity SFQ systems. Permissible interconnect lengths are suggested to ensure the reflections do not affect the returning signals. The microwave behavior of the interconnect striplines is also considered to accurately estimate the surface inductance of the lines. A closed-form expression describing the dependence of the surface inductance of a stripline on the line thickness, magnetic field, and current density is discussed. Another primary issue within SFQ circuits is coupling noise between transmission striplines, degrading performance and decreasing margins. Inductive and capacitive coupling noise between the routing layers, for the MIT LL SFQ5ee process, is described. An analysis of inductive and capacitive coupling noise can determine the minimum physical spacing between lines to enhance the automated routing process in large scale systems. The increasing complexity of modern SFQ circuits has also made the issue of flux trapping of growing importance. The use of wide striplines for signal routing has exacerbated this issue. Trapped residual magnetic fields within the striplines damage the operability of superconductive circuits. Area efficient topologies for striplines are introduced to manage flux trapping in large scale SFQ circuits. These topologies are composed of several narrow lines rather than a single wide stripline. The first approach is a narrow parallel line topology in series with small resistors where each narrow line is connected to a single small resistor and via. The resistors in the parallel line topology remove any trapped fluxons and break any loops while requiring additional vias. The second topology is a fingered narrow line topology. The fingered narrow line topology enhances the scalability of SFQ systems while not requiring additional area and vias. These proposed topologies require significantly less area while preventing flux from being trapped within wide superconductive striplines and reducing coupling noise between striplines. These methodologies and techniques are intended as guidelines to enable robust routing with superconductive interconnects. With these and other advances in design methodologies for superconductive electronics, the complexity of SFQ circuits is expected to greatly increase.
We propose an inductance-load-biasing method in order to reduce power consumption of rapid single flux quantum (SFQ) logic circuits. The main idea arises from the fact that a current source can be made of a large inductor accompanied by a large flux. In our proposal, the current source is composed of a large inductor L b , a small resistor R b and a small voltage source V b . Computer simulations of inductance-load-biased Josephson transmission lines (JTLs) show that an SFQ pulse propagates correctly when L b is large enough even if R b is very small. In order to implement the inductance-load-biased JTL, we have made two different layouts: one uses a large bias inductance L b of a typical stripline structure on a ground plane which occupies a rather large area; the other uses L b in the shape of a coplanar stripline, which costs a smaller area.
High current requirements in single flux quantum (SFQ) circuits pose scalability challenges, contributing to issues such as high thermal loads, static power dissipation, and increased sensitivity to flux trapping under non-ideal shielding or cool down conditions. AC biasing, successfully used in other superconductive logic families, has been integrated into rapid single flux quantum (RSFQ) circuits and typically includes AC-to-DC converters, introducing area and complexity overhead. This work proposes an alternative method for applying AC biasing to existing RSFQ circuits using inductively coupled bias junctions, eliminating the need for AC-to-DC converters. This approach also combines resistive DC biasing with inductive AC biasing, reducing static power dissipation. The proposed approach targets clock splitters, which are significant contributors to on-chip current demand, achieving an average 94% reduction in splitter current and a 62% reduction in overall system current with a Josephson junction overhead of 6.5%. The technique is compatible with conventional RSFQ system architectures and logic families while significantly enhancing energy efficiency and scalability.
The increasing complexity of modern superconductive circuits, and single flux quantum (SFQ) circuits in particular, has made the issue of coupling noise of growing importance. Limited metal resources within superconductive circuits have exacerbated this issue. In this article, the different sources of coupling noise within SFQ circuits are described. Coupling noise among inductors, routing striplines, and bias microstriplines within SFQ circuits degrade performance while decreasing margins. In this article, inductive and capacitive coupling between the different layers are characterized. Inductive coupling models between different layers in the MIT LL SFQ5ee process match experimental data within 3%. The dependence of inductive coupling on the thickness of the oxide and metal layer is also discussed. An understanding of inductive and capacitive coupling can determine the minimum physical distance between lines. In addition, tradeoffs exist among inductive coupling, capacitive coupling, layout complexity, and the vias between ground layers. The different coupling sources are characterized, and guidelines are provided to enhance the automated routing process.
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.