SUPERCONDUCTIVE JOSEPHSON JUNCTION, METHOD OF ITS PREPARATION, AND ITS USE IN SQUIDS

Abstract: A superconductive Josephson junction device comprising a substrate layer (3) having at least one layer of a superconductive ceramic material (2) comprising at least a first (2a) and a second (2b) superconductive crystal; said first and second crystals being joined in at least one grain boundary (1) which forms a Josephson junction; wherein the grain boundary has a reduced content of oxygen exhibiting a normal resistance which is larger than that of a fully oxidized grain boundary; a method of its preparation; use of such Josephson junction devices in a superconductive quantum interference device; a method of preparing a superconductive quantum interference device; and an integrated multilayered superconductive quantum interference device.

Description: SUPERCONDUCTIVE JOSEPHSON JUNCTION, METHOD OF ITS PREPARATION, AND ITS USE IN SQUIDS DESCRIPTION 1. BACKGROUND OF THE INVENTION The present invention relates to a Josephson junction device, a method of its preparation, its use in a superconductive quantum interference device (SQUID) , a method of preparing a SQUID, and an integrated multilayered SQUID. The Technical Field Superconducting Quantum Interference Devices (SQUIDs) are versatile sensitive devices comprising a superconductive ring having incorporated one or more Josephson junctions consisting of two superconductors separated by a thin insulating barrier through which electrons are able to tunnel . SQUIDs are able to measure any physical quantity that can be converted to a magnetic flux, e.g. a magnetic field, a magnetic field gradient, a current, a voltage, a displacement, a magnetic susceptibility, etc. Among various applications, SQUIDs have been applied as magnetometers for detection of biomagnetic signals such as bioelectromagnetic signals of the heart, the brain and the peripheral nerve system. The sensitivity of a SQUID to detect e.g. weak biomagnetic signals is limited by its noise properties, particularly those of the superconductive Josephson junction. Therefore, it is desirable to reduce its noise. Noise in a superconductive Josephson junction is generally measured by the spectral density of the current noise power by filtering the current response with a narrowband filter squaring the filtered response by a square law detector, and averaging the squared filtered response by an averaging circuit. Depending on the frequency f, the measured current noise power S(f) (in equilibrium excluding shot noise) can be devided into three major noise regions: (a) at low frequencies: "1/f-noise" (or flicker noise) S(f) = l/f α with α « 0.8-1.2; (b) at intermediate frequencies for k B T >> eV, hf in equilibrium: "white noise" (or Nyquist-Johnson thermal noise) S(f) = 4k B T/R; and (c) at very high frequencies: "quantum noise" S(f) « 2hf/R. In real systems, the quantum noise is suppressed because a system response is never instantaneous. The origin of 1/f-noise is not fully understood for neither low-T c nor high-T c superconductive Josephson junctions. In the RCSJ model, the white noise is proportional to the temperature T and proportional to the inverse of the shunt resistance R. Consequently, the white noise contributes significantly to the noise of high-T c superconductive Josephson junctions even when cooled at 77 K by liquid nitrogen. In real systems/ the shunt resistance R corresponds to the normal resistance R n of the superconductive Josephson junction, i.e. the resistance which an electron encounters during tunnelling through the junction. Prior Art Disclosures J. Bindslev Hansen reviews noise properties of superconductive Josephson junctions, i.e. superconductive tunnel junctions and superconductive metallic weak links such as point contacts and microbridges, see "Noise in Superconducting Josephson Junctions" in "Nonlinear Superconductive Electronic and Josephson Devices", Eds. G. Costabile et al., Plenum Press, New York, 1991, p. 161. Moeckly et al., Physical Review B, vol. 47, no. 1, 1993, 400-417, discloses electromigration studies of oxygen disorder and grain-boundary effects in YBa 2 Cu 3 0 7 -δ thin film microbridges consisting of grain-aligned and high- angle tilt grain boundary materials. Oxygen migration current densitites of > 1 MA/cm 2 applied at about 300 K create regions of strong oxygen disorder which increases the resistance above the critical temperature and drastically decreases the transition temperature, T c . After the oxygen migration, the normal resistance at a temperature below T c is increased. Shen et al . Appl . Phys . Lett. 59(11), 9 September 1991, 1365-1367 discloses laser writing and rewriting on YBa 2 Cu 3 0 7 films wherein semiconducting patterns can be formed by a reduction of the oxygen content due to local heating caused by the laser beam, and for which patterns superconductive properties can be restored when reapplying the same laser beam and rewriting the pattern on the film in ah oxygen atmosphere. It is mentioned that during in si tu four-probe resistance measurements, the resistance under the exposure was increased due to the loss of oxygen of the exposed parts. Nothing is indicated nor suggested about white noise, or about the normal resistance under superconducting conditions. Shen et al . , Appl . Phys . Lett. 67 (14), 2 October 1995, 2081-83, discloses an integrated high T c superconducting magnetometer with multiturn input coil and grain boundary junctions which is fabricated in a YBa 2 Cu 3 θ7-χ - SrTi0 3 - YBa 2 Cu 3 0 7 - x multilayer process. All three layers were deposited with a thickness of about 150 nm at 0.2 mbar oxygen partial pressure and at a substrate temperature of 800 °C. Above 10 HZ, the noise characteristics are limited by white noise at 4xl0 ~5 Φ 0 /Hz 1/2 . Nothing is indicated nor suggested neither about reduction of oxygen at the grain boundary nor about the grain boundary being covered by an oxygen barrier layer. Drung et al., Appl. Phys. Lett. 68 (10), 4 March 1996, 1421-23, discloses an integrated YBa 2 Cu 3 0 7 - x magnetometer for biomagnetic measurements comprising a small washer- type SQUID with 9 Ω bicrystal junctions. The magnetometer is fabricated in a YBa 2 Cu3θ 7 - x - SrTi0 3 - YBa 2 0 7 - x multilayer process as disclosed by Shen et al . , Appl. Phys. Lett. 67 (14), 2 October 1995, 2081-83. Nothing is indicated nor suggested about how the 9 Ω asymptotic resistance is obtained. 2. DISCLOSURE OF ' THE INVENTION Object of the Invention It is an object of the present invention to provide a superconductive Josephson junction with reduced white noise, and a method of preparing such a junction. It is another object of the present invention to provide a superconductive quantum interference device (SQUID) with reduced white noise, and a method of its preparation. It is still another object of the present invention to provide an integrated multilayered SQUID with reduced white noise. Solution According to the Invention According to the invention these objects are fulfilled by providing a superconcductive Josephson junction device comprising a substrate layer having at least one layer of a superconductive ceramic material comprising at least a first and a second superconductive crystal; said first and second crystals being joined in at least one grain boundary which forms a Josephson junction wherein the grain boundary has a reduced content of oxygen exhibiting a normal resistance which is larger than that of a fully oxidized grain boundary. It has surprisingly turned out that a reduction of the oxygen content of the grain boundary decreases the white noise of the device compared to a fully oxidized grain boundary. The normal resistance R n of the superconductive Josephson junction is measured by a four-probe technique, i.e two probes for conducting current and two probes for measuring voltage, at a temperature which is below the critical temperature of the junction. The probe current is at least ten times higher than the critical current of the Josephson junction, but less than the critical current of the superconductive first and second crystals making up the grain boundary. In case of a YBCO grain boundary junction the probe current is typically in the range from 10 4 A/cm 2 to 10 6 A/cm 2 . However, the probe current can be lower or higher depending on the specific grain boundary. "Superconductive Josephson junction" Superconductive Josephson junctions consist of two superconductors separated by a thin insulating barrier through which electrons are able to tunnel. Particularly, the grain boundary between two superconductive crystals can make up a superconductive Josephson junction. For example, in superconductive ceramic materials, the grain boundary between two single crystals having different crystalline orientation can make up the Josephson junction. Such a grain boundary can be formed by epitaxially growing the superconductive ceramic material on a suitable substrate having such a grain boundary or having a grain boundary formed by biepitaxially growth of layers on the substrate, whereby the epitaxial growth process ensures that the two single crystals of the superconductive ceramic material will have the same crystal orientations as those of the substrate. Another suitable * grain boundary is a step-edge grain boundary wherein the crystal planes of the two single crystals are substantially parallel but non-coinciding, and optionally joined by a third superconductive crystal. The third superconductive crystal can by an integral part of one of the two crystals which crystal plane bends gradually out of the plane parallel with the substrate plane as the distance from the grain boundary increases . Such a grain boundary can be formed by epitaxially growing the superconductive ceramic material on a suitable substrate having such a step grain boundary. Suitable bicrystalline and step grain boundaries are known in the art, e.g. as disclosed by Dimos et al. Phys. Rev. Lett., 61 (1988), page 219, and Daly et al . Appl. Phys. Lett., 58 (1991), page 543. "Superconductive ceramic materials" According to the invention, superconductive ceramic materials can be any suitable ceramic material comprising superconducting copper-oxygen chains and planes within their structure. Suitable ceramic materials comprise superconductive perovskites of the formula R v Ba x Cu y O z -δ, wherein R v is selected among the rare earth elements Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb, or mixtures thereof; v is about 1; x is about 2; y is about 3; z is about 7; and the oxygen depletion δ is selected in the range from 0.0- 1.0; said stoichiometric coefficients v, x, y, and z being determined by the degree of substitution of the elements which is allowed for the superconductive perovskite to maintain superconductivity. Particularly, the ceramic material YBa 2 Cu 3 0 7 -δ, here abbreviated "YBCO", is suitable. Superconductivity depends on the oxygen depletion δ. Specifically for a ceramic material such as YBa 2 Cu 3 0 7 _ δ in the orthorhombic phase with oxygen depletion δ in the range from 0 to 0.5, and at a temperature below the critical temperature of about or below 92 K, it becomes superconducting, whereas in the tetragonal phase with depletion δ in the range from 0.5 to 1,0, it becomes a semiconductor. The oxygen depletion δ can be controlled by heat processing in a controlled atmosphere resulting in oxygen diffusion in or out of the material. Suitable YBCO materials can be prepared according to methods known in the art. For example thoroughly mixing appropriate amounts of BaC0 3 , Y 2 0 3 , and CuO powders, each of 99.999% purity. Then heating of the mixed powders to a temperature of 960 °C in air in 10 hours. Then maintaining of the heated mixture at this temperature for 72 hours. Then cooling in 2 hours to 300 °C, after which the cooled mixture is removed from the furnace. The resulting YBCO material is a crystallized, sparkling black powder. Suitable superconductive ceramic materials are disclosed for example by Tarascon et al . , "Oxygen and rare-earth doping of 90-K superconducting perovskite YBa 2 Cu 3 0 7 - x ", Physical Review B, Vol. 36, No. 1, 1987, 226-234. "Reduced content of oxygen" According to the invention, the content of oxygen in the grain boundary is reduced to a content of oxygen exhibiting a normal resistance which is larger than that of a fully oxidized grain boundary. This can be obtained by any suitable method which reduces the oxygen content in the grain boundary, but which does not reduce the oxygen content of the bulk of the superconductive ceramic material so that the superconducting property of the superconductive layer is maintained. A method comprises heating and cooling of the superconductive ceramic material in a controlled oxygen atmosphere. Heating of the superconductive ceramic material at the grain boundary includes local and global heating. Local heating includes focused laser beam irradiation at the grain boundary and diffusion of oxygen out into a more oxygen deficient atmosphere wherein the ceramic material is placed. Global heating includes large area substrate heating by light irradiation. Thus for both methods, by controlling the oxygen content of the atmosphere and the temperature, the normal resistance across the grain boundary can be increased in a controlled manner without destroying the superconductive properties of Josephson junction. Another method comprises deposition of an oxygen barrier layer on the superconductive layer whereby reoxygenation of the superconductive layer can be controlled. Thus, by heating of the YBCO material, e.g. to a temperature above 400 °C, oxygen will diffuse out of the grain boundary and bulk of the material. Then, by the depositing of an oxygen barrier layer of a suitable kind and thickness over the grain boundary, the amount of oxygen which can diffuse through the oxygen barrier and reoxygenate the grain boundary and the bulk YBCO material in a subsequent cooling step can be controlled. Since the YBCO bulk material can be fully reoxygenated, it restores its superconductive properties. However, since the grain boundary becomes depleted of oxygen, the normal resistance across the grain boundary can be increased. Still another method comprises a combination of these two methods, i.e. both controlling the oxygen content of the atmosphere and the temperature during a heating and cooling process and controlling the reoxygenation by an oxygen diffusion barrier. The degree of oxygen reduction can be measured directly or indirectly by any suitable method known in the art. A direct method comprises mesuring oxygen directly, provided it is sufficiently sensitive. An indirect method comprises measurement of the normal resistance R n across the Josephson junction e.g. by the four-probe technique, mentioned earlier, carried out on the device at 77 K, see e.g. Vase et al . , Appl. Surface Science 46, (1990), 61- 62. Preferred Embodiments In a preferred embodiment, the device further comprises at least one oxygen barrier layer deposited on the superconductive ceramic material covering the grain boundary whereby it is obtained that reoxygenation of the oxygen content of the superconductive ceramic material and thereby the white noise properties can be particularly effectively controlled. The device may comprise more superconductive layers. In another preferred embodiment, the device comprises a multilayer structure of two or more superconductive ceramic materials separated by one or more oxygen barrier layers whereby * a particularly compact device having incorporated a number of structural components such as multiturn coil can be obtained. The ability of the oxygen barrier layer to function as an oxygen barrier according to the invention depends, in addition to the oxygen barrier material itself, on its thickness. In a preferred embodiment, the at least one oxygen barrier layer has a thickness larger than 50 nm, preferably larger than 150 nm, more preferred in the range from 150 to 300 nm, most preferred about 200 nm whereby it is obtained that an effective barrier against oxygen diffusion into the superconductive ceramic material is obtained. Also, depending on the oxygen barrier material, a good electric insulating layer can be obtained. The oxygen barrier layer can be of any suitable material known in the art which can provide a barrier to oxygen at the relevant temperatures, for example at the temperature of manufacture, e.g. in the range from 600 to 900 °C, and at the temperature of operation, e.g. at 77 K, at which the oxygen barrier must have a sufficient oxygen barrier capacity to ensure low white noise at long operation times. In a preferred embodiment, the oxygen barrier layer comprises SrTi0 3 , which allows epitaxial deposition of an additional ceramic layer, e.g. an YBCO film. Further, the SrTi0 3 -layer is electrically insulating. The two superconductive ceramic crystals have different crystal orientations which can be of any suitable orientation which ensures that the formation of the grain boundary forms a Josephson junction. In a preferred embodiment, the orientations of the crystal planes of the first and second crystals of the superconductive ceramic material are substantially parallel with the substrate surface . In another preferred embodiment, one of the crystal planes is not parallel with the substrate surface, or at least not parallel with the substrate surface throughout the whole of its extension. The one crystal plane may bend gradually out of the plane parallel with the substrate plane as the distance from the grain boundary increases. Also, the grain boundary can consist of a step-edge boundary in which the first and second crystals can have any crystal orientations, but are located in different planes and joined by a third superconductive crystal providing two Josephson junctions. In another preferred embodiment, the orientations of the crystal planes of the first and second crystals of the superconductive ceramic material are non-parallel with the substrate surface and joined by a third superconductive crystal. In a preferred embodiment, the first and second crystals of the superconductive ceramic material have different crystal orientations forming an angle in the range from 10 to 80 degrees, preferably from 20 to 60 degrees, more preferred from 20 to 30 degrees, most preferred about 24 or 36 degrees. The superconductive material can be deposited on the substrate by any suitable method known in the art. In a preferred embodiment, the superconducting ceramic material is epitaxially grown on the substrate layer whereby it is obtained that e.g. the grain boundary of the substrate can be reproduced. In a preferred embodiment, the superconductive ceramic material comprises the ceramic material YBa 2 Cu 3 0 7 _ δ , wherein the oxygen depletion δ is in the range from 0 to 0.5. The substrate layer can be any suitable substrate known in the art. Particularly bicrystalline substrates are preferred whereby the grain boundary of the substrate can be reproduced between the two superconductive crystals through epitaxially deposition thereof. In a preferred embodiment, the substrate layer comprises a SrTi0 3 bicrystal or an MgO bicrystal. The oxygen content in the grain boundary is reduced to a suitable level. In a preferred embodiment the oxygen content is reduced to exhibit a normal resistance times grain boundary interface area which is larger than 10 nΩ cm 2 , preferably larger than 30 nΩ cm 2 , more preferred in the range from 40 to 60 nΩ cm 2 , most preferred about 50 nΩ cm 2 , whereby it is ensured that the white noise is reduced. In addition, the I c is also reduced. Other Aspects of the Invention According to the invention, the object is further fullfilled by providing a method of preparing a superconductive Josephson junction device comprising the steps of: a) providing a substrate layer; b) depositing on the substrate layer of a superconductive ceramic material comprising at least a first and a second superconductive crystal; said first and second crystals being joined in at least one grain boundary which forms a Josephson junction; and c) reducing the oxygen content in the grain boundary so that the device exhibits a normal resistance which is larger than that of a fully oxidized grain boundary. In a preferred embodiment the method further comprises deposition of at least one oxygen barrier layer on the superconductive material covering the grain boundary whereby a production effective method is obtained, in particular for the production of multilayered, integrated structures . The oxygen barrier material may be depositied on the substrate by any suitable method. In a preferred embodiment, the deposition of the oxygen barrier layer comprises epitaxially growing of the oxygen barrier material on the superconductive material at a sufficient temperature and oxygen partial pressure so that the device will have a transition temperature in the range from 80 to 95 K, preferably in the range from 83 to 91 K, most preferred about 86 K. The oxygen barrier is deposited in a suitable oxygen atmosphere which ensures that the oxygen content in the grain boundary is reduced. It is preferred that the oxygen barrier is deposited in an atmosphere with an oxygen partial pressure in the range from 10 "4 to 10 mbar, preferably in the range 10 ~4 to 1 mbar, most preferred about 0.2 mbar. The oxygen barrier is deposited at a suitable rate which ensures that a homogenous oxygen barrier is formed. It is preferred that the oxygen barrier is deposited at a rate in the range from 0.03 to 10 nm/s, preferably in the range 0.1 to 3 nm/s, most preferred about 1 nm/s. The temperatures at which the oxygen barrier is deposited are chosen so that the oxygen achieves a suitable diffusibility out of the grain boundary. It is preferred that the oxygen barrier is deposited at a temperature higher than 600 °C, preferaby in the range from 700 to 900 °C, most preferred about 800 °C . The superconductive material is deposited in a suitable oxygen atmosphere so that superconductivity is obtained. In a preferred embodiment, the superconductive ceramic material is deposited in an atmosphere with an oxygen partial pressure in the range from 0.01 to 10 mbar, preferably in the range from 0.1 to 10 mbar, most preferred about 0.2 mbar. The rate of deposition of the superconductive ceramic material is chosen to ensure a suitable deposition time and thickness of the deposited layers . In a preferred method, the superconductive ceramic material is deposited at a rate in the range from 0.03 to 10 nm/s, preferably in the range from 0.1 to 10 nm/s, most preferred about 1 nm/s . The deposition temperature of the superconductive ceramic material is chosen so that oxygen can diffuse into the bulk material allowing a superconductive crystal to be formed. In a preferred embodiment the superconductive ceramic material is deposited at a temperature higher than 600 °C, preferably in the range from 700 to 900 °C, most preferred about 800 °C. The temperature at which the oxygen content is reduced is chosen so that the oxygen in the grain boundary can diffuse out in a suitable period of time. In a preferred embodiment the oxygen content is reduced by heating the grain boundary to a temperature higher than 600 °C, preferably in the range from 700 to 900 °C, most preferred about 800 °C . The atmosphere in which the heated grain boundary is cooled to room temperature is chosen so that the reduced content of oxygen is maintained. In a preferred embodiment, the heated grain boundary is cooled to room temperature in an oxygen atmosphere of 10 ~J to 1 atm, preferably 0.1 to 1 atm, most preferred about 1 atm. It is preferred that the rate of cooling is in the range from 2 to 30 °C/min, preferably from 10 to 30 °C/min, most preferred about 20 °C/min. The preparation of the two superconductive crystals on the substrate can be achieved by any method known in the art. It is preferred that the superconducting ceramic material is epitaxially grown on the substrate layer whereby an effective preparation is obtained which ensures that the grain boundary between the two superconductive crystals forms a superconductive Josephson junction. It is preferred that the oxygen barrier layer comprises SrTi0 3 . It is preferred that the superconductive ceramic material comprises the ceramic material YBa 2 Cu 3 0 7 -s, wherein the oxygen depletion δ is in the range from 0 to 0.5. It is preferred * that the substrate layer comprises a SrTi0 3 bicrystal or an MgO bicrystal. "Use of superconductive quantum Josephson junction" Josephson junctions according to the invention are useful in a large number of applications, particularly a magnetometer . In a preferred use, the superconductive Josephson junction device is used in a superconductive quantum interference device, particularly in a sensor of magnetic flux, such as a magnetometer. "A method of preparaing a SQUID" According to the invention, the object is further fulfilled by providing a method of preparing a superconductive quantum interference device comprising a superconductive ceramic material with at least one Josephson junction grain boundary; said method comprising: (a) deposition of a superconductive layer comprising a ceramic material, preferably YBa 2 Cu 3 0 7 -δ, wherein δ is in the range from 0 to 0.5, and having a thickness in the range from 100 to 500 nm, preferably 200 nm, on a substrate, preferably of a bicrystalline or a step- edge substrate material, most preferred a SrTi0 3 bicrystalline material, preferably epitaxially by using laser ablation, in an atmosphere having an oxygen partial pressure in the range from 0.01 to 10 mbar, preferably 0.2 mbar, and at a temperature in the range from 600 to 900 °C, preferably 800 °C; (b) cooling of the deposited superconductive ceramic material and substrate to room temperature in an atmosphere having an oxygen partial pressure in the range from 10 ~3 to 1 atm, preferably 1 atm; said cooling being performed at a rate in the range from 2 to 30 °C, preferably at 20 °C; (c) patterning of the superconductive layer device by removing parts thereof, preferably by using electron beam lithography and Ar-ion milling; (d) depositing an oxygen barrier layer preferably SrTi0 3 , having a thickness in the range from 100 to 500 nm, preferably 200 nm, on the superconductive layer, preferably epitaxially by using laser ablation, in an atmosphere having an oxygen partial pressure in the range from 10 "4 to 1 mbar, preferably 0.2 mbai , and at a temperature in the range from 600 to 900 °C, preferably 800 °C; (e) patterning of the oxygen barrier layer by removing parts thereof maintaining the oxygen barrier layer covering the Josephson junction grain boundary; (f) heating the substrate, patterned superconductive layer and patterned oxygen barrier layer to a temperature in the range from 600 to 900 °C, and (g) cooling the heated substrate, patterned superconductive layer, and patterned oxygen barrier to room temperature in an atmosphere having an oxygen partial pressure in the range from 10 "J to 1 atm, preferably 1 atm, said cooling being performed at a rate in the range from 2 to 30 °C, preferably at 20 °C, whereby a SQUID exhibiting reduced white noise is obtained. "Integrated multilayer ed SQUID' According to the invention, the object is further fulfilled by providing an integrated multilayered superconductive quantum interference device comprising a superconductive ceramic material with at least one Josephson junction grain boundary, said device prepared by a method comprising providing an oxygen barrier layer on the superconductive ceramic material covering the grain boundary which is sufficiently effective to avoid reoxygenation of the grain boundary whereby a particularly compact SQUID exhibiting reduced noise is achieved. 3. BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention is further disclosed with detailed description of preferred embodiments, reference being made to the drawings in which Figs. 1A and IB * show a top view sketch of a preferred embodiment of a superconductive quantum interference device comprising a superconductive Josephson junction according to the invention; Fig. 2A shows a top view sketch of a preferred embodiment of one of the two Josephson junction grain boundaries shown in Figs. 1A and IB; Fig. 2B shows a cross-sectional sketch of the preferred Josephson junction grain boundary shown in fig. 2A illustrating a bicrystalline substrate; Figs. 3A and 3B show similar sketches as in Figs. 2A and 2B of another preferred embodiment of one of the two Josephson junction grain boundaries; and Fig. 4 shows an illustration of a preferred sequence of preparing an integrated multilayered superconductive quantum interference device according to the invention. 4. DETAILED DESCRIPTION Figs. 1A and IB show a top view sketch of a preferred embodiment of a superconductive quantum interference device comprising a superconductive Josephson junction according to the invention; said device comprising on a substrate (not seen) a superconductive ring 4 around an air gab core 5 with superconductive electrodes 6a, 6b, supplying currents to and from the device. The superconductive ring 4 and electrode 6b comprise a first 2a and a second 2b superconductive YBCO film crystal having different crystal orientations, and being joined in two Josephson junction grain boundaries 1 (see Fig. IB) ; said boundaries being reduced in oxygen according to the invention. Fig. 2A shows a top view sketch of a preferred embodiment of one of the two Josephson junction grain boundaries 1 shown in Figs. 1A and IB. Two superconductive YBCO film crystals 2a and 2b have different crystal orientations which in a preferred embodiment is obtained by epitaxially growing the YBCO film crystals on a bicrystalline substrate 3, more clearly shown in Fig. 2B. Figs. 3A and 3B show similar sketches of another preferred embodiment of one of the two Josephson junction grain boundaries 1 shown in Figs. 1A and IB; said preferred embodiment being a step-edge grain boundary in which the two superconductive YBCO film crystals 2a and 2b are displaced in different planes with respect to the substrate. The crystal grown on the ramp 2c joining the crystals 2a and 2b has a different crystal orientation than those of the crystals 2a and 2b, respectively. In another embodiment, the ramp and the second crystal 2b are one crystal bending gradually in different planes with respect to the substrate. Figs. 4A-4D show an illustration of a preferred sequence of steps in preparing an integrated multilayered superconductive quantum interference device according to the invention; said steps comprising: 1) providing a bicrystalline substrate A consisting of two single crystals 3 having different crystalline orientations and forming the grain boundary 1, see Fig. 4A; 2) depositing a superconductive layer B on the substrate A the grain boundary 1 of which is extended into the superconductive layer B by epitaxial growth thereon, see Fig. 4B; 3) depositing an oxygen barrier layer C of e.g. SrTi0 3 epitaxially grown on the superconductive layer B. The oxygen barrier layer may be patterned by removing parts thereof (see arrow) , but without removing the barrier layer C covering the grain boundary, see Fig. 4C; and 4) heat treating the thus prepared substrate with oxygen barrier layer and superconductive layer in a controlled, reduced oxygen atmosphere and at a sufficient temperature for the normal resistance R n of the grain boundary to reach a desired value larger than that of a non-heat treated, but similarly prepared substrate; and 5) optionally depositing and patterning a second layer of superconductive layer D thereon. 5. EXAMPLES The invention is further illustrated by the preparation of SQUIDs in examples A, B, and C using different methods for reducing the oxygen content. Example A - "Preparation of a SQUID" "Materials" The following materials were used: The YBCO material consisted of a laser ablation poly- crystalline target having a composition of YBa 2 Cu 2 . 825 0 x with a purity larger than 99 % and supplied from Jupiter Technologies Inc., Ithaca, N.Y., USA. The SrTi0 3 oxygen barrier material was supplied from Cerac Incorporated, Milwaukee, Wisconsin, USA. The SrTi0 3 bicrystal substrate was supplied from Wako Bussan Co., Tokyo, Japan. "Deposition of YBCO layer by laser ablation" A 200 nm thick YBCO layer was deposited by laser ablation on a SrTi0 3 bicrystal substrate having a 36 degrees misalignment between the orientations of the two crystals. The deposition technique by laser ablation consisted in directing a high effect laser beam onto the target material whereby surface atoms of the target were evaporated. In front of the target, a substrate was placed on a substrate holder. The evaporated target atoms were trapped on the substrate forming a thin film of target material. For further details of the technique, see Vase et al . Physica C, Vol.235-240, (1994), p. 641- 642; and Appl. Surf. Science, Vol. 46, (1990), p. 61-66. "Deposition conditions of the YBCO layer' The YBCO layer was deposited at 800 °C in 0.2 mbar oxygen partial pressure at a deposition rate of 1 nm/s. After the deposition, the chamber was filled with oxygen at a partial oxygen pressure of 1 atm. Then the YBCO layer and substrate were slowly cooled to room temperature at a rate of 20 °C/min oxygen. "YBCO layer characteristics" The YBCO layer was highly epitaxial. The two YBCO crystals each had a superconductive transition temperature T c at 89 - 90 K. Their critical current densities j c were larger than 2 MA/cm 2 at 77 K, and their densities of surface particles were less than 7000 per mm 2 . A sample of this material constituting a Josephson junction of 4 μm width, typically had a normal resistance of 77 K at about 0.5 Ω and a white noise of 2xl0 -5 Φ 0 /Hz 1/2 . "Surface particle density" A method of measuring surface particle density and methods of reducing surface particles is disclosed by Humphreys et al . IEEE Transactions on Magnetics, Vol. 27 (2), (1991), p. 1357 - 1360. "Patterning of the SQUID" The pattern of SQUID as shown in Fig. 1 was provided by electron beam lithography and Ar-ion milling. At this stage of the preparation, the Josephson junctions of 4 μm width typically had a normal resistance of 77 K at about 0.5 Ω. Example B - "Laser Writing" The SQUID as prepared in Example A was mounted on a set of motor controlled translation stages and electrically connected to a four-probe Ohm meter. A laser beam was provided by a mul " tiwavelength (488/515.5 nm) Ar-ion laser and focused into a 2μm diameter spot by a 20 x 0.35 diameter microscope lens. The objective was placed affront of the SQUID so that the focus point was at the YBCO plane. Scanning of the beam was carried out by displacing the translation stages at a speed of 5 mm/s. The atmosphere surrounding the SQUID was 100% nitrogen. After two scans along the grain boundary, using a laser power of 16 mW, the normal resistance of the junction was increased from 0.5 Ω to 4 Ω. Example C - "Deposition of oxygen barrier layer" A 200 nm thick SrTi0 3 layer was deposited epitaxially on the YBCO layer of the SQUID as prepared in Example A using laser ablation. "Deposition conditions for the oxygen barrier layer" The SrTi0 3 layer is deposited at 800 °C at a rate of 1 nm/s at a partial oxygen pressure of 0.2 mbar. The deposition of the oxygen barrier layer is optimatized so that the superconducting transition temperature is about 86 K. "Patterning of the oxygen barrier layer" The SrTi0 3 layer is patterned by electron beam lithography and Ar-ion milling, leaving the SrTi0 3 layer on the grain boundary. "Final annealing" * The oxygen barrier, YBCO layer, and substrate were heated to 800 °C in an atmosphere of 0.2 mbar partial oxygen pressure, and then cooled to room temperature in an atmosphere of 1 atm partial oxygen pressure and at a rate of 20 °C/min. Josephson junctions of 4 μm width typically had a normal resistance of 77 K at about 9 Ω. A white noise of 8xl0 ~6 Φ 0 /Hz 1/2 was obtained. This shows a significant reduction of the white noise. 'Additional YBCO layer' Before the final annealing, an additional layer of YBCO material can optionally be deposited on the oxygen barrier layer by heating the oxygen barrier, YBCO layer, and substrate to 800 °C in an atmosphere of 0.2 mbar partial oxygen and depositing the additional YBCO layer at a rate of 1 nm/s similar to the deposition conditions for the first YBCO layer mentioned above. It is advantageous to have the additional YBCO layer deposited on the oxygen barrier layer instead of directly on the first YBCO layer because the deposition of the additional layers will not affect the high asymptotic resistance, R n , in the Josephson junction of the first YBCO layer if there is an oxygen barrier layer in between.

Claims: SUPERCONDUCTIVE JOSEPHSON JUNCTION, METHOD OF ITS PREPARATION, AND ITS USE IN SQUIDS CLAIMS 1. A superconductive Josephson junction device comprising a substrate layer (3) having at least one layer of a superconductive ceramic material (2) comprising at least a first (2a) and a second (2b) superconductive crystal; said first and second crystals being joined in at least one grain boundary (1) which forms a Josephson junction; CHARACTERIZED in that the grain boundary has a reduced content of oxygen exhibiting a normal resistance which is larger than that of a fully oxidized grain boundary. 2. A device according to claim 1, CHARACTERIZED in that it further comprises at least one oxygen barrier layer (4) deposited on the superconductive ceramic material covering the grain boundary. 3. A device according to claim 2, CHARACTERIZED in that it comprises a multilayer structure of two or more superconductive ceramic materials separated by one or more oxygen barrier layers. 4. A device according to claims 2 or 3, CHARACTERIZED in that the at least one oxygen barrier layer has a thickness larger than 50 nm, preferably larger than 150 nm, more preferred in the range from 150 to 300 nm, most preferred about 200 nm. 5. A device according to claims 2,3 or 4, CHARACTERIZED in that the oxygen barrier layer comprises SrTi0 3 . 6. A device according to claim 1, CHARACTERIZED in that the orientations of the crystal planes of the first and second crystals of the superconductive ceramic material are substantially parallel with the substrate surface. 7. A device according to claim 1, CHARACTERIZED in that the orientations of the crystal planes of the first and second crystals of the superconductive ceramic material are non-parallel with the substrate surface and joined by a third superconductive crystal. 8. A device according to claim 6, CHARACTERIZED in that the first and second crystals of the superconductive ceramic material have different crystal orientations forming an angle in the range from 10 to 80 degrees, preferably from 20 to 60 degrees, more preferred from 20 to 30 degrees, most preferred about 24 degrees or 36 degrees . 9. A device according to claim 1, CHARACTERIZED in that the superconducting ceramic material is epitaxially grown on the substrate layer. 10. A device according to claim 1, CHARACTERIZED in that the superconductive ceramic material comprises the ceramic material YBa 2 Cu 3 0 7 - δ , wherein the oxygen depletion δ is in the range from 0 to 0.5. 11. A device according to claims 1 or 9, CHARACTERIZED in that the substrate layer comprises a SrTi0 3 bicrystal or an MgO bicrystal. 12. A device according to any of claims 1-11, CHARACTERIZED in that the oxygen content is reduced to exhibit a normal resistance times grain boundary interface area which is larger than 10 nΩ cm 2 , preferably larger than 30 nΩ cm 2 , more preferred in the range from 40 to 60 nΩ cm 2 , most preferred about 50 nΩ cm 2, 13. A method of preparing a superconductive Josephson junction device comprising the steps of: a) providing a substrate layer (3] b) depositing on the substrate layer of a superconductive ceramic material (2) comprising at least a first (2a) and a second (2b) superconductive crystal; said first and second crystals being joined in at least one grain boundary (1) which forms a Josephson junction; and c) reducing the oxygen content in the grain boundary so that the device exhibits a normal resistance which is larger than that of a fully oxidized grain boundary. 14. A method according to claim 13, CHARACTERIZED in that it further comprises deposition of at least one oxygen barrier layer on the superconductive material covering the grain boundary. 15. A method according to claim 14, CHARACTERIZED in that the deposition of the oxygen barrier layer comprises epitaxially growing of the oxygen barrier material on the superconductive material at a sufficient temperature and oxygen partial pressure so that the device will have a transition temperature in the range from 80 to 95 K, preferably in the range from 83 to 91 K, most preferred about 86 K. 16. A method according to claims 14 or 15, CHARACTERIZED in that the oxygen barrier is deposited in an atmosphere with an oxygen partial pressure in the range from 10 ~4 to 10 mbar, preferably in the range 10 ~4 to 1 mbar, most preferred about 0.2 mbar. 17. A method according to claims 14-16, CHARACTERIZED in that the oxygen barrier is deposited at a rate in the range from 0.03 to 10 nm/s, preferably in the range 0.1 to 3 nm/s, most preferred about 1 nm/s. 18. A method according to claims 14-17, CHARACTERIZED in that the oxygen barrier is deposited at a temperature higher than 600 °C, preferaby in the range from 700 to 900 °C, most preferred about 800 °C . 19. A method according to claims 13-19, CHARACTERIZED in that the superconductive ceramic material is deposited in an atmosphere with an oxygen partial pressure in the range from 0.01 to 10 mbar, preferably in the range from 0.1 to 10 mbar, most preferred about 0.2 mbar. 20. A method according to claims 13-19, CHARACTERIZED in that the superconductive ceramic material is deposited at a rate in the range from 0.03 to 10 nm/s, preferably in the range from 0.1 to 10 nm/s, most preferred about 1 nm/s . 21. A method according to claims 13-20, CHARACTERIZED in that the superconductive ceramic material is deposited at a temperature higher than 600 °C, preferably in the range from 700 to 900 °C, most preferred about 800 °C . 22. A method according to claims 13-18, CHARACTERIZED in that the oxygen content is reduced by heating the grain boundary to a temperature higher than 600 °C, preferably in the range from 700 to 900 °C, most preferred about 800 °C. 23. A method according to claim 20, CHARACTERIZED in that the heated grain boundary is cooled to room temperature in an oxygen atmosphere of 10 "3 to 1 atm, preferably 0.1 to 1 atm, most preferred about 1 atm. 24. A method according to claims 20-21, CHARACTERIZED in that the rate of cooling is in the range from 2 to 30 °C/min, preferably from 10 to 30 °C/min, most preferred about 20 °C/min. 25. A method according to claims 13-24, CHARACTERIZED in that the superconducting ceramic material is epitaxially grown on the substrate layer. 26. A method according to claims 13-25, CHARACTERIZED in that the oxygen barrier layer comprises SrTi0 3 . 27. A method according to claims 13-24, CHARACTERIZED in that the superconductive ceramic material comprises the ceramic material YBa 2 Cu 3 0 7 -δ, wherein the oxygen depletion δ is in the range from 0 to 0.5. 28. A method according to claim 18, CHARACTERIZED in that the substrate layer comprises a SrTi0 3 bicrystal or an MgO bicrystal. 29. Use of a superconductive Josephson junction device as claimed in claims 1-12, or prepared as claimed in claims 13-28, in a superconductive quantum interference device. 30. Use as claimed in claim 20 wherein the superconductive quantum interference device is a sensor of magnetic flux. 31. A method of preparing a superconductive quantum interference device comprising a superconductive ceramic material with at least one Josephson junction grain boundary; said method comprising: (a) deposition of a superconductive layer comprising a ceramic material, preferably YBa 2 Cu 3 0 7 -s, wherein δ is in the range from 0 to 0.5, and having a thickness in the range from 100 to 500 nm, preferably 200 nm, on a substrate, preferably of a bicrystallic or a step- edge substrate material, most preferred a SrTi0 3 bicrystalline material, preferably epitaxially by using laser ablation, in an atmosphere having an oxygen partial pressure in the range from 0.01 to 10 mbar, preferably 0.2 mbar, and at a temperature in the range from 600 to 900 °C, preferably 800 °C; (b) cooling of the deposited superconductive ceramic material and substrate to room temperature in an atmosphere having an oxygen partial pressure in the range from 10 -3 to 1 atm, preferably 1 atm; said cooling being performed at a rate in the range from 2 to 30 °C, preferably at 20 °C; (c) patterning of the superconductive layer device by removing parts thereof, preferably by using electron beam lithography and Ar-ion milling; (d) depositing an oxygen barrier layer preferably SrTi0 3 , having a thickness in the range from 100 to 500 nm, preferably 200 nm, on the superconductive layer, preferably epitaxially by using laser ablation, in an atmosphere having an oxygen partial pressure in the range from 10 "4 to 1 mbar, preferably 0.2 mbar, and at a temperature in the range from 600 to 900 °C, prefeably 800 °C; (e) patterning of the oxygen barrier layer by removing parts thereof maintaining the oxygen barrier layer covering the Josephson junction grain boundary; (f) heating the substrate, patterned superconductive layer and patterned oxygen barrier layer to a temperature in the range from 600 to 900 °C, and (g) cooling the heated substrate, patterned superconductive layer, and patterned oxygen barrier to room temperature in an atmosphere having an oxygen partial pressure in the range from 10 ~3 to 1 atm, preferably 1 atm, said cooling being performed at a rate in the range from 2 to 30 °C, preferably at 20 °C . 32. An integrated multilayered superconductive quantum interference device comprising a superconductive ceramic material with at least one Josephson junction grain boundary, said device prepared by a method comprising providing an oxygen barrier layer on the superconductive ceramic material covering the grain boundary which is sufficiently effective to avoid reoxygenation of the grain boundary.

Inventors: SHEN YUEQIANG, SHEN, YUEQIANG
Applicants: NKT RES CENTER AS, SHEN YUEQIANG, NKT RESEARCH CENTER A/S, SHEN, YUEQIANG
Application year: 1997
Main classification: H01L39
IPC codes: G01R33, H01L39