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.
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.
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.
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 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.
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.
Superconductive passive transmission lines (PTL) are widely used for signal routing in large-scale rapid single flux quantum (RSFQ) circuits. Due to the imperfect matching of the transmission lines between the driver and receiver, single flux quantum (SFQ) pulses are partially reflected. The round trip propagation time of these reflections can coincide with the following SFQ pulse, resulting in a decrease in bias margins or incorrect circuit behavior. This resonant effect depends upon the length of the PTL and the clock frequency of the signal. A methodology to reduce and manage this effect is the focus of this article. A closed-form expression describing the dependence of the resonance frequency on the length of the PTL is presented. This expression describes a set of forbidden lengths for PTL interconnect segments in RSFQ circuits. The proposed methodology and algorithm insert active PTL-based repeaters into long superconductive interconnect while ensuring the length of the line segment is outside the forbidden region and increasing bias margins.
The introduction of passive-transmission-line (PTL) wiring to large-scale SFQ logic circuits offers several advantages over traditional Josephson-transmission-line (JTL) wiring, such as smaller wiring area, less delay, lower power consumption, and more freedom in routing. PTL wiring can drastically change the style of designing SFQ logic circuits. We propose a new methodology for designing large-scale SFQ circuits using PTL wiring. Aiming at generalizing SFQ circuit design, we chose synchronous clocking. We developed a placer that was suitable for SFQ circuit design to assist us with the new design methodology. The placer first synthesizes an H-tree clock distribution network, placing SFQ pulse splitter cells at each branch. It then places logic cells to minimize the maximum length of data PTL wires, aiming at higher operating speeds. Finally, a router completes the PTL wiring. To evaluate the design methodology we designed an 8-bit general purpose RISC processor. Within twelve hours, we obtained a 15-mm-square SFQ circuit that could operate up to 27.6 GHz. We adopted an advanced Nb process, which was characterized by a critical current density of 10 kA/cm2, two 5-mum-wide PTL wiring layers, and a minimum cell size of 30 mum times 30 mum. The circuit consisted of about 20,000 logic cells, which approximated more than 400,000 Josephson junctions including splitters to distribute the clock signal.