Sperry Corporation

Meir Gershenson

David L Fleming

Mark F Sweeny

Dale J Fixsen

Robert E Barta

Dennis D Long

A two-junction DC SQUID is supplied a single square-wave frequency signal that both biases the SQUID and provides modulation with a square-wave train that is 90.degree. phase displaced with respect to the bias square-wave train in order to eliminate low frequency noise with cross-modulation frequency products.

Single frequency noise reduction circuit for squids

A thin, nominally 150 angstroms, normal metal layer, nominally the noble normal metal palladium, sandwiched intermediate between two superconducting layers, nominally niobium, supports totally superconducting current flow therethrough such normal metal layer by the quantum mechanical tunneling effect. Such a thin normal metal layer is useful as a stop etch layer for fabrication process control, including especially the selective niobium anodization process, as an oxidation resistant or immune layer to which totally superconducting electrical contact may be reliably made such as by soldering, and as a patterned resistor in those regions of the substrate plane where such thin normal metal layer is not sandwiched by superconducting layers. Particularly concerning such stop etch function, a thin normal layer of palladium will serve as a stop etch layer to the etching of niobium while not precluding totally superconducting contact therethrough such thin palladium layer. Particularly concerning the electrical contact function, a superconducting wire, nominally niobium, may be soldered with lead-tin or indium-tin to a thin normal metal layer of palladium in the formation of a totally superconducting connection therethrough such thin palladium layer. Particularly concerning such patterned resistor, terminus superconducting sandwiches of niobium-thin palladium-niobium may be electrically resistively interconnected by such thin palladium layer precisely patterned in the region(s) wherein it is not sandwiched, only such unsandwiched region(s) of such thin palladium layer contributing controllably determinable resistance to the interconnection(s).

Intermediate normal metal layers in superconducting circuitry

In an improvement to the Selective Non-Anodizing Process (SNAP) an anodizable layer, nominally 100 nm of aluminum, is deposited on top of a 300 nm niobium--6 nm silicon--30 nm niobium tri-layer upon a substrate of oxidized silicon. The structure is then masked with photoresist and etched with an (aluminum) etchant, nominally phosphoric plus acetic plus nitric acid, which is selective to etch aluminum but not niobium. The structure, now containing a hard layer of aluminum plus an uppermost layer of photoresist over the regions where Josephson junctions will be formed, is then anodized by voltage ramping from 0 to 50 volts each 10 seconds in a saturate solution of ammonium penta borate in an equal solution of ethylene glycol and water. Both the uppermost niobium of the tri-layer and the aluminum are anodized save where protected by the photoresist. A protective insulator layer, nominally silicon mono oxide, is deposited by sputtering after which the mask of photoresist over aluminum is removed by the lift-off technique etching both anodized and remaining non-anodized aluminum. To the well-defined islands of niobium in a plane of anodized niobium, upper contact electrodes are formed by conventional deposition, photo lithographic, and etching techniques. This method so fabricating Josephson junction devices well-protects and defines the junctions under development while simultaneously reducing and simplifying the number of steps required in the patterning of the insulator layer.

Aluminum mask anodization with lift-off for patterning Josephson

A type of Superconducting Quantum Interference Device (SQUID) requires that relatively large, circular, toroidally wound wire coils within a first cavity of a superconducting canister should be inductively coupled to a relatively small SQUID created as an annular ring plus Josephson junctions upon a substrate within a second cavity of the superconducting canister. The required inductive coupling is through a dielectric filled cavity called a coupling cavity which is conical in the shape of a dunce's cap. The conically shaped coupling cavity within the superconducting canister minimizes the parasitic stray inductance which is coupled to the SQUID, and thusly improves the noise performance of the SQUID.

SQUID Canister with conical coupling cavity

A superconducting quantum interference device (SQUID) is direct current biased through physical connections asymmetric to, and preferably maximally asymmetric to, the two Josephson junctions. The asymmetric SQUID so created is, responsively to such physical asymmetry, biased for operation in the linear region of the input magnetic flux/output (voltage or current) device response curve. A resistance of specified value is connected in parallel, or shunt, to the parasitic bridge capacitance of the asymmetric SQUID in order to minimize hysteresis. Two asymmetric SQUIDS of opposite asymmetry are serially connected as a push-pull linear amplifier stage which exhibits zero output (voltage or current) at zero input magnetic flux, and which is specifiable in parameters of construction so as to exhibit optimum linearity of response about such point. Plural successive such linear amplifier stages are connected by an LC filter, which filter is lowpass to the Josephson oscillation frequency, in order to form a linear amplifier entirely with the cyrogenic environment and with a bandwidth of the order to D.C. to 10.sup.9 Hertz.

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Saint Paul, MN, United States of America

Meir Gershenson