Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
2001-06-15
2003-02-11
Ho, Hoai (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S662000, C257S663000, C257S664000, C257S037000, C257S039000
Reexamination Certificate
active
06518673
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to superconductor integrated circuits (“ICs”) and, more particularly, to reduction of electrical current demand and electronic noise in superconductor ICs, to a single-flux-quantum Josephson transmission line (JTL) formed of a superconductor IC, to reduction of self-inductance of superconductor leads, to a capacitor design that permits coupling single flux quantum pulses without interference caused by self inductance of the capacitor leads that is useful in the foregoing transmission line, and to the method of fabricating that capacitor and the single flux quantum pulse transmission line.
BACKGROUND
Metals and metal alloys found to exhibit zero electrical resistance at some temperature are commonly referred to as superconductors. Each such superconductor metal or metal alloy possesses a particular cryogenic temperature, referred to as the transition temperature (Tc), at which the respective metal and metal alloy becomes superconducting and changes in electrical resistance from a measurable or relatively high value of resistance to a value of zero.
At room temperatures those metals and metal alloys possess a measurable value of electrical resistance and are not superconducting. The metals and alloys do not attain the superconductive state unless cooled, typically, to extremely low temperatures, cryogenic temperatures. As a consequence of the zero-resistance characteristic of the superconductor in the superconducting state, electrical current induced, as example, into a loop formed of the superconductor cooled below the respective transition temperature persists indefinitely. One well known superconductor or, as alternately referred to, superconductor metal is niobium, a refractory metal, which transitions to a superconducting state at a temperature of 9.2 Kelvin.
Digital electronic devices have previously been constructed of superconductor metals and the functionality of such devices demonstrated. A principal element to the construction of a superconductor digital electronic device is the Josephson junction, discovered in the early '60's. A 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 “critical current”, the Josephson junction conducts current without developing a voltage drop across the junction and without dissipation of the current. Consequently the junction does not produce heat, which is a significant advantage for electronic circuits or 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.
Another interesting property is that current or energy introduced to the junction by a single flux quantum pulse is sufficient, when added to the appropriate DC bias current, to cause the Josephson junction to momentarily exceed the critical current for the junction and undergo a 360 degree shift in quantum phase or, as otherwise termed, electronically “flip-over”. The single flux quantum pulse is a physical constant and comprises 2.07 millivolts per picosecond or, in alternate terms, 2.07 milliamps per picohenry. In undergoing that shift the junction reproduces the single flux quantum pulse.
Superconductor digital electronic devices typically require cryogenic temperatures, below the transition temperature of the superconductor. Hence a necessary component of the electronic device is an appropriate refrigeration or other cooling apparatus. The device further requires a relatively large DC bias current. Thus, another necessary component is the inclusion of DC bias current supplies, each typically required to supply about 0.1 mA to each Josephson junction within the superconducting ICs. Despite such unwelcome appendages, such 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.
In superconducting integrated circuit 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. The multiple Josephson junction devices may be logically divided into groups of two or more junctions, the groups referred to as “SQUIDs” (the acronym for superconducting quantum interference device). For example, a single flux quantum pulse transmission line may contain a number of SQUIDs arranged in serial order, each SQUID containing two Josephson junctions connected electrically in parallel in a superconducting loop, also referred to herein as a Josephson loop (See, as example, Josephson junctions
1
and
3
in FIG.
5
).
A single flux quantum pulse, introduced at the input to the Josephson transmission line (JTL), propagates along the transmission line to the output, effectively transferring the single flux quantum pulse from SQUID to SQUID in that line. In addition, the pulse is regenerated at each 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 is proportional to the distance the SFQ pulse is to traverse.
For operation, each Josephson junction in the SQUID is required to be biased with a certain level of DC current. Because the Josephson junctions are connected, directly or indirectly, to a common superconductor metal that serves as the ground plane and, hence, as a connection point for the ground polarity lead of the bias power supply, the DC bias currents required by the individual junction devices is additive. That is, the DC bias current is supplied from the current source, the power supply, in parallel to each Josephson junction. More complex superconductor devices, such as superconductor very large scale integrated circuits (“VLSI”) may contain even greater numbers of Josephson junctions, and, hence, in accordance with existing design, requires a power supply capable of supplying even larger levels of DC current. The bias current demand of a superconductor VLSI with one million junctions could easily require one-hundred or more amperes from the power supply at a very low voltage.
A large DC current requirement is undesirable, since the feed lines for that current will generate large magnetic fields, that may interfere with circuit operation. Moreover, delivery of the current to the cryogenic system requires heavy-gauge wires that have a high thermal conductivity and forms a path over which external heat could be introduced, increasing the load on the cryogenic system. Thus, both the thermal load and total system power are increased, which is undesirable.
An approach one might take to lower the DC current demand on the bias source is to place the various Josephson junctions in an electrical series circuit and employ a bias power supply of higher voltage than before to provide the DC bias current through each of the Josephson junctions in series. Each junction then receives the same requisite bias current required for operation. To form such a series circuit, the various SQUIDs (or Josephson junctions) cannot be connected to a common ground plane (superconductor metal layer), as in the existing design, described earlier. Instead, each SQUID (or junction) must contain a separate ground plane and the individual ground planes must be DC isolated from one another.
Although the foregoing approach would appear to solve the bias supply problem by eliminating high current draw while providing the requisite electrical isolation, such a solution fails to take into account the functioning and purpose of the circuit. In the exam
Abelson Lynn A.
Herr Quentin P.
Kerber George L.
Goldman Ronald M.
Ho Hoai
Pham Ly Duy
TRW Inc.
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