Electronic digital logic circuitry – Superconductor – Tunneling device
Reexamination Certificate
2001-08-22
2002-11-19
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Superconductor
Tunneling device
C326S007000, C327S368000
Reexamination Certificate
active
06483339
ABSTRACT:
REFERENCE TO PRIOR APPLICATIONS
This application includes certain subject matter contained in an application of Herr, Ser. No. 09/934,493, filed Aug. 22, 2001, concurrently herewith entitled, “Double Flux Quantum Superconductor Driver, ” and the prior application of Herr, Abelson &, Kerber, Ser. No. 09/882,979 filed Jun. 15, 2001, entitled “Capacitor for Signal Propagation Across Ground Plane Boundaries in Superconductor Integrated Circuits,” both of which are copending herewith, and which are assigned to the assignee of the present application.
FIELD OF THE INVENTION
This invention relates to superconductor devices and, more particularly, reduction of bias current demand required in superconductor integrated circuits to power large numbers of Josephson Junctions contained within the integrated circuits, and to coupling circuits for superconductor single flux quantum pulses.
BACKGROUND
Metals, metal alloys and ceramics found to exhibit zero electrical resistance are commonly referred to as superconductors. Typically, those superconductors don't attain the superconductive state unless cooled to extremely low temperatures, referred to as cryogenic temperatures. Each such superconductor material possesses a unique cryogenic temperature, referred to as the transition temperature (“Tc”), at which the respective metal and metal alloy becomes superconducting, changing in electrical resistance from a measurable or relatively high value of resistance to zero. One known superconductor is niobium, a refractory metal, which transitions to a superconducting state at a temperature of 9.2 Kelvin.
Superconductor digital electronic devices have previously been constructed of superconductor metals and the functionality of such devices demonstrated. As example, with a zero-resistance characteristic during superconductivity, electrical current induced into a loop formed of the superconductor metal, refrigerated below the transition temperature of the metal, persists indefinitely. With appropriate drivers and sensors, the foregoing loop may serve as a digital memory. When the direction of the current induced in the loop is in a clockwise direction the memory state may represent a “
1
” digital bit; when the direction of induced current is counterclockwise, the memory state may represent the bit “
0
”.
Superconductor digital electronics devices have been fabricated as integrated circuits on a silicon wafer using the photo-lithographic mask and etch techniques or other known techniques most familiar to those in the semiconductor industry. Such superconductor integrated circuit devices provide the desired functionality in a very small package or chip. Superconductor devices operate at very high speeds, as example, 100 GHz to 770 GHz, and very low power, which is unattainable with present semiconductor devices. Because of the high speeds of operation and low power requirement, superconductor electronic devices remain attractive for many applications.
A principal element to the construction of a superconductor digital electronic device is the Josephson junction. The Josephson junction is formed, as example, of two layers of superconductors, such as niobium, separated by a very thin layer of electrical insulation, such as aluminum oxide. When cooled to the transition temperature and biased with DC current below a certain “critical current”, (“I
c
”) the Josephson junction is superconducting and the junction conducts current without developing a voltage drop there across and without dissipation of energy, exhibiting no electrical resistance. Consequently, the junction does not produce heat, which is a significant advantage for integrated circuits. If biased above the critical current, the Josephson junction produces an RF signal, consisting of a series of pulses at RF frequencies. Thus, the critical current is a boundary at which the electrical properties of the junction changes as described.
Superconductor circuits utilize the foregoing property of the Josephson junction to regenerate single flux quantum (“SFQ”) pulses. The time integral of the voltage of a single flux quantum pulse is a physical constant approximately equal to 2.07 millivolt picoseconds or, in alternate terms, 2.07 milliamp picohenry. When an SFQ pulse is applied to a Josephson junction that is properly DC biased below the critical current, the current produced by the SFQ pulse when added to the DC bias current may cause the Josephson junction to briefly exceed the critical current. The Josephson junction then undergoes a 360 degree shift in quantum phase or, as otherwise termed, electronically “flips-over”. In undergoing that shift the Josephson junction generates an SFQ pulse in response to the applied SFQ pulse.
In superconducting integrated circuit (“IC”) devices containing multiple Josephson junctions, the junctions are formed on a common superconductor metal layer, referred to as a ground plane, deposited over an insulator substrate, such as silicon, a readily available and inexpensive material. The multiple Josephson junction devices may be logically divided into groups of two or more junctions, the groups referred to as “SQUIDS” (an acronym for superconducting quantum interference device). For example, a single flux quantum pulse transmission line, referred to as a Josephson transmission line, may be formed of a number of SQUIDS arranged in serial order, each SQUID containing two Josephson junctions connected electrically in parallel in a superconducting loop, the latter also sometimes referred to as a Josephson loop.
A single flux quantum pulse applied to the input of the Josephson transmission line (“JTL”), may be said to propagate along the transmission line to the output, moving from SQUID to SQUID in that line, and thence to the electrical load connected to the output of the transmission line. In fact, the SFQ pulse is regenerated at each Josephson junction (stage), which can produce current and power gain. The transmission line may in total contain two or more Josephson junctions, the number of Josephson junctions (and SQUIDS) that form the transmission line can be increased to traverse the desired distance.
At present, powering (e.g. biasing) superconducting single flux quantum circuits requires very low DC voltage, but appreciable current. Typically, the DC bias supply must supply about 0.1 mA to each Josephson junction contained within a superconducting IC. With many such junctions (or SQUIDS) in a superconductor device, the total bias current is cumulative and in total is very large. Existing techniques for powering Josephson Junctions in superconductor circuits (e.g. SFQ circuits) are based on a parallel bias, that is, bias current supplied to the circuits in parallel, in which all the superconductor digital gates and functional blocks thereto have a common circuit ground. Increasing the number of gates increases the current demand required of the power bus and the DC bias power supply that supplies the power to that bus.
For superconductor circuits of several thousand gates (e.g. SQUIDS) or larger, more than one ampere of total current is required, which is relatively large for integrated circuit devices. Requiring large current at low voltage, even at power levels as low as one milliwatt, presents at least two disadvantages. First, semiconductor power converters presently available do not deliver an ampere of current at one millivolt in voltage as efficiently as they deliver one milliamp current at one volt of voltage. Secondly, the transmission of larger currents from an external current supply, positioned in the ambient temperature, to the cryogenic package containing the superconductor circuits implies electrical conductors for the power bus that are large in cross-section.
The large cross section of the power bus conductors, in addition to conducting current, provides a thermal path from the ambient into the cryogenic package that is of greater thermal conductivity than with bus's of small cross-section. Due to the greater thermal conductivity, more heat could be conducted into the cryogenic package.
Durand Dale J.
Herr Quentin P.
Johnson Mark W.
Goldman Ronald M.
TRW Inc.
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