Nuclear magnetic resonance probe coil

A solenoid-type probe coil wherein superconductive thin film is used, whose quality factor is high, and which is put in an uniform magnetic field and occupies a small space is provided. For that purpose, the coil is made by piling up, in generally parallel, two or more substrates on which superconductive film is formed and connecting superconductors and normal-metal thin films through capacitance or low contact resistance.

CLAIM OF PRIORITY

The present application claims priority from Japanese application JP2004-349462 filed on Dec. 2, 2004, the content of which is hereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to the construction of a nuclear magnetic resonance probe coil for an NMR (Nuclear Magnetic Resonance) apparatus comprising (i) a superconductive magnet to apply a static magnetic field to a sample and (ii) a probe whose tip is provided with the nuclear magnetic resonance probe coil, whose means of detecting output signals is formed out of superconductive thin film.

BACKGROUND OF THE INVENTION

The construction of a conventional nuclear magnetic resonance probe coil comprising superconductive thin film is discussed in the patent literature 1 (Japanese Patent Laid-Open No. H11(1999)-133127). The invention relates to a so-called birdcage-type probe coil, and the direction of the static magnetic field and the directions of insertion and drawing out of a sample are parallel.

To make a high-resolution, high-sensitivity NMR apparatus, it is effective to apply a strong static magnetic field to a sample or use a high-sensitivity probe coil. To generate an uniform, strong magnetic field, it is desirable to make the diameter of the coil to produce the magnetic field small and it is necessary to make small the space occupied by the probe coil disposed in the coil. The probe coil forms a resonance circuit. To make a high-sensitivity probe coil, it is effective to raise the quality factor of the probe coil. To achieve a high quality factor, it is necessary to reduce the resistance in the probe coil.

A trial to make a probe coil of superconductive thin film is discussed in the Japanese Patent Laid-Open No. H11(1999)-133127. As the direct-current resistance of a superconductor is zero and its high-frequency resistance is small, it is useful as a constituent of a probe coil. In this case, the resistance in the resonance circuit is expressed as a sum of the resistance of constituents and that of their connections, and the contribution of the constituent made of superconductive thin film can be ignored. Accordingly, the resistance in a resonance circuit which comprises superconductive thin film is lower than the resistance in a resonance circuit which does not comprise superconductive thin film.

However, although the above prior art relates to a probe coil made of superconductive thin film, it discusses the construction of a so-called birdcage-type probe coil. To make a high-sensitivity probe coil, it is effective to achieve a high quality factor, improve the uniformity of the magnetic field, reduce the space occupied by the probe coil, and use a solenoid-type probe coil, but the above prior art does not discuss the construction of a solenoid-type probe coil.

That is because the birdcage-type probe coil is used in the NMR device wherein the direction of the static magnetic field and the directions of insertion and drawing out of a sample are parallel, whereas the solenoid-type probe coil is used in the NMR device wherein the direction of the static magnetic field and the directions of insertion and drawing out of a sample are orthogonal. Namely, although the former device has been studied very much, the latter device has not been studied very much. As a result, the solenoid-type probe coil has not been studied very much.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a solenoid-type probe coil wherein superconductive thin film is used. More specifically, the object of the present invention is to provide a solenoid-type probe coil wherein superconductive thin film is used, whose quality factor is high, and which is put in an uniform magnetic field and occupies a small space.

The above object of the present invention is achieved by an NMR probe coil, whose means of receiving the signals outputted from a sample is a solenoidal coil, which comprises (i) two or more generally parallel substrates on which superconductive thin film is formed and (ii) wiring between or among the substrates.

The NMR probe coil of the present invention is a low-resistance solenoidal coil which is made by piling up, in generally parallel, two or more substrates on which superconductive film is formed and connecting superconductors and normal-metal films through capacitance or low contact resistance. Capacitance connection is made by forming superconductive thin film on one side of a substrate and normal-metal film on the other side of the substrate. Low contact-resistance connection is made by forming superconductive thin film and normal-metal film continuously without exposing them to the atmosphere. Besides, by disposing the axis of the solenoid orthogonally to the direction of the static magnetic field, the superconductors in interlinkage with the lines of magnetic force are only the thin parts of the superconductive thin film.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An NMR apparatus comprises a means of applying a static magnetic field to a sample and a probe. The probe is provided with a probe coil at its tip. The probe coil comprises a transmitting probe coil to input a high-frequency signal into a sample and a receiving probe coil to receive the signal outputted from the sample. The receiving probe coil detects the component of the magnetic moment outputted from the sample orthogonal to the static magnetic field. The receiving probe coil can serves as a transmitting probe coil. Namely, the receiving probe coil transmits a high-frequency signal to a sample and receives after a certain time the signal outputted from the sample. Although the present invention discusses mainly the construction of receiving probe coils, it is applicable to transmitting-cum-receiving probe coils, too.

According to prior art for high-sensitivity NMR apparatuses, a super conductive magnet is used to apply a vertical static magnetic field to a sample and the sample is inserted and drawn out vertically. Because it is necessary for the receiving probe coil to detect the component of the magnetic moment orthogonal to the static magnetic field, the receiving probe coil of a saddle or birdcage type has been used so that horizontal magnetic moment can be detected.

Embodiments of the present invention will be described below by referring to the drawings.

FIRST EMBODIMENT

In general, there are probe coils of birdcage, saddle, and solenoid types, and solenoid-type probe coils are more sensitive than birdcage- and saddle-type ones. The first embodiment of the present invention is a receiving probe coil of a solenoid type to make a high-sensitivity NMR apparatus.

FIG. 1is a sectional view of an NMR apparatus comprising the probe coil of the first embodiment of the present invention. A solenoidal coil4for applying a static magnetic field to a sample is divided into two41and42which are disposed side by side. Divided into two51and52is another solenoidal coil5, which surrounds the solenoidal coil4and corrects the static magnetic field. The solenoidal coils4and5are fitted into an inner tank6, which is put in an outer tank7. The inner tank6is filled with liquid helium; the outer tank7, liquid nitrogen. The bore of the solenoidal coil4is partially hollow, and a probe1is disposed in the hollow part. A probe coil2is provided at the tip of the probe1where a static magnetic field is applied to a sample. The probe coil2is of a solenoid type. The probe coil2is so disposed that the directions of its axis are the same as the directions of movement of a sample and a sample tube3can be inserted into it. The sample tube3is vertically inserted into and drawn out of the space between the solenoidal coils41and42. Accordingly, the probe coil2detects the vertical component of the magnetic moment outputted from a sample. The directions of x, y, and z axes at the bottom ofFIG. 1are shown in the other figures by the same standard.

In the first embodiment, the receiving probe coil is not of a birdcage or saddle type, but of a solenoid type to raise its sensitivity; therefore, it is necessary to divide the superconductive magnet into two and dispose them. To make a high-sensitivity probe coil, it is also necessary to secure the uniformity of the magnetic field, make small the space occupied by the probe coil, and realize a high quality factor.

To obtain an uniform, strong magnetic field, it is desirable to make small the diameter of solenoid coil4and it is necessary to make small the space occupied by the probe coil2. It is necessary in accordance with the first embodiment to make the sample space smaller for an even magnetic field as compared with the prior art wherein an undivided superconductive magnet is used. To achieve a high quality factor, it is effective to form the solenoidal probe coil2out of a low-resistance or superconductive material.

FIG. 2is a schematic perspective view of the probe coil2of the first embodiment. The probe coil2comprises a transmitting probe coil to transmit a high-frequency signal to a sample and a receiving probe coil to detect the signal outputted from the sample. As the receiving probe coil requires higher sensitivity than the transmitting probe coil, the former is of a solenoid type and formed out of oxide superconductive thin film to achieve high sensitivity. On the other hand, the transmitting probe coil is made of a normal metal and of a saddle type, surrounding the receiving probe coil. A static magnetic field is applied horizontally to a sample and the receiving probe coil detects the vertical component of the magnetic moment outputted from the sample.

The reference numerals111to114are the components of the receiving probe coil11. Each receiving-probe-coil component is a one-turn coil with an opening formed out of oxide superconductive thin film on one side of a substrate12. In the first embodiment, the four one-turn components111to114are disposed in parallel with one another. Two normal-metal thin films13are formed on the side of each substrate12opposite to the side on which a receiving-probe-coil component is formed. The positions of the two normal-metal thin films13on each substrate12correspond to the positions of the two ends of the receiving-probe-coil component on said substrate12to constitute two capacitors. Normal-metal connecting wires15are connected to the normal-metal thin films13by resistive connection. Thus, the normal-metal connecting wires15and the receiving-probe-coil components111to114are connected through capacitance, forming a necessary circuit. The components111and112are connected in series to form a two-turn coil, the components113and114are connected in series to form another two-turn coil, and these two-turn coils are connected in parallel. Thus, a two-turn two-parallel circuit receiving probe coil is formed. A detecting circuit10is connected to the receiving probe coil11through normal-metal extracting wires17. A sample tube3is inserted into the inside of the receiving probe coil11.

The reference numerals181to1810are the components of the transmitting probe coil18, the components181to1810assembled to form a saddle-type transmitting probe coil18. The transmitting probe coil18comprises a one-turn coil made of the components181,183,185, and188and another one-turn coil made of the components182,184,186, and187, and the two one-turn coils are connected in parallel to surround the receiving probe coil11. A transmitting circuit20applies a large pulse current to the transmitting probe coil18through normal-metal extracting wires17′, which are connected to the component189and1810, so as to cause the sample to generate a magnetic moment containing a component orthogonal to the static magnetic field. When the component of the magnetic moment orthogonal to the static magnetic field attenuates gradually, the receiving probe coil11receives a signal outputted from the sample.

The receiving probe coil11constitutes an inductance (L)-capacitance (C) resonance circuit, the inductance being of a trimmer capacitor (not shown) in the probe1and the receiving probe coil11, the capacitance being between the normal-metal connecting wires15and the oxide superconductive thin films111to114. To increase the sensitivity of detection, it is necessary to increase the quality factor of the LC resonance circuit. To increase the quality factor of the LC resonance circuit, it is necessary to decrease the parasitic, or incidental, resistance in the LC resonance circuit; accordingly, the solenoidal coil of the present invention is made of a superconductive material.

To achieve a high sensitivity, it is necessary to enhance the uniformity of the magnetic field. A superconductor is diamagnetic and has a large magnetic susceptibility of −¼π. In the first embodiment of the present invention, superconductors are so disposed that the interlinkage between them and the lines of magnetic force occurs as little as possible in order not to disturb the distribution of the magnetic field. Namely, a one-turn coil was formed out of a superconductive thin film11ion a substrate12and four substrates were piled up to constitute a solenoid-type probe coil11. The substrates12are so arranged that their normal lines are orthogonal to the direction of the magnetic field and only the thin parts of the superconductive thin films111to114are in interlinkage with the lines of magnetic force.

To make a more-than-one-turn solenoidal receiving probe coil, it is necessary to connect one-turn coils on substrates with wire. In the first embodiment of the present invention, thin film of the oxide superconductive material YBa2Cu3O7is used, but the problem in connecting the superconductive thin films formed on substrates is that the contact resistance is large if normal metal-oxide superconductor connection is merely made by using an ordinary manufacturing process. Namely, if an oxide superconductive thin film is formed on a substrate and the film is processed by lithography into a circular one-turn coil and then a normal metal such as Au is connected to the oxide superconductive thin film by using an ordinary manufacturing process, the contact resistance between Au and the oxide superconductive thin film is large and the parasitic, or incidental, resistance in the LC resonance circuit is large, reducing the quality factor. To solve this problem, in the first embodiment of the present invention, insulators are put between the oxide superconductive thin films111to114and the normal-metal thin films13to form capacitors, and the normal-metal thin films13and the normal-metal connecting wires15are connected by resistive connection. Thus, because the normal-metal connecting wires15and the oxide superconductive thin films111to114are connected through the capacitance, the problem of high contact resistance can be avoided. The normal-metal connecting wires15are connected to the detecting circuit10through the normal-metal extracting wires17.

A birdcage-type probe coil of superconductive thin film is disclosed in the Japanese Patent Laid-Open No. H11(1999)-133127. In this case too, connecting wires and oxide superconductive thin films are connected through capacitance. However, superconductive thin film is formed on only one side of a substrate and the substrate is pressed into a connecting ring with a press. The distance between the two conductors constituting a capacitor and hence its capacitance depend on the pressing force; therefore, it is difficult to make capacitors with the exactly desired characteristics. Besides, the above prior art discusses a method of wiring applicable to birdcage-type probe coils and not a method of wiring applicable to solenoid-type probe coils.

The first embodiment of the present invention is a solenoidal coil of a superconductive material whose quality factor is high when a static magnetic field is applied horizontally and which is put in an uniform magnetic field and occupies a small space.

In the first embodiment of the present invention, oxide superconductive thin films of YBa2Cu3O7are used as superconductors. Substrates on which superconductive thin films are formed have to be made of a non-magnetic material to secure the uniformity of the magnetic field. Besides, it is necessary to use a material of high thermal conductivity in order to secure the cooling of superconductive thin films. To satisfy both the requirements, substrates made of sapphire are used in the first embodiment.

FIGS. 3A to 3Care illustrations of a one-turn coil of superconductive thin film11formed on a substrate12in accordance with the first embodiment of the present invention.FIG. 3Ais a plan view of one side of the substrate12. The substrate12is made of sapphire (Al2O3). Formed on one side of the substrate12is a circular coil made of superconductive thin film11of the oxide superconductive material YBa2Cu3O7. The two ends of the superconductive thin film11are opened and extended outward. The reference numeral101is a hole, in which a sample tube3is inserted. The reference numerals102's are screw holes at the four corners, which are used to join piled substrates12, each having a one-turn coil, to make a probe coil. The reference numerals104to107are holes, which let the transmitting-probe-coil components184to187through.FIG. 3Bis a plan view of the other side of the substrate12. The reference numerals13's are normal-metal thin films of Au, whose positions correspond to the positions of the two ends and two outward extensions of the superconductive thin film11.FIG. 3Cis a sectional view taken along the arrowed line A—A ofFIG. 3A, showing that the superconductive thin film11is formed on one side of the substrate12, the normal-metal thin films13are formed on the opposite side, and the positions of the normal-metal thin films13correspond to the positions of the two ends and two outward extensions of the superconductive thin film11.

A method of making the one-turn coil of superconductive thin film11ofFIGS. 3A to 3Cwill be described below.

First, a film of CeO2of the thickness of 100 nm is formed as a buffer layer on one side of a sapphire (Al2O2) substrate12. Then, a superconductive thin film11is formed out of the oxide superconductive material YBa2Cu3O7. The thickness of the superconductive thin film11is larger than the length, 100 nm, of penetration of a magnetic field. If the thickness of thin film of YBa2Cu3O7goes beyond one micrometer, its surface becomes rough; therefore, the appropriate thickness of thin film of YBa2Cu3O7is over 100 nm and below one micrometer. The superconductive thin film11of the first embodiment is 150-nm thick. Next, the superconductive thin film11is processed into a circular pattern by the ordinary process of application of a resist, photolithography, and Ar etching.

Next, an Nb film is formed as a backing film on the other side of the substrate12. Then, normal-metal thin films13of Au are formed. The thickness of the Au films has to be larger than the depth of penetration and is 10 μm. If an Au film is formed directly on the substrate12, it is liable to peel off; therefore, an Nb film is formed as a backing film. A Ti film or a film consisting of a Pt layer and a Ti layer may be used as a backing film. Next, the Au films are processed into desired patterns by the ordinary process of application of a resist, photolithography, and Ar etching.

Thus, Au—Al2O3—YBa2Cu3O7capacitors are formed. Because the thin films of YBa2Cu3O7and Au stick fast to the substrate12, the distances between the metals constituting the capacitors do not change; therefore, capacitors of a desired capacitance can be made.

Next, a hole101for the sample tube, screw holes102, and holes104to107for the transmitting coil are made in the substrate12.

FIGS. 4A to 4Cshow a probe coil comprising piled one-turn coils of superconductive thin films11in accordance with the first embodiment of the present invention.

FIG. 4Ais a schematic top plan view of the probe coil of the first embodiment. The reference numeral141is an upper protecting board of aluminum nitride disposed at the top of the probe coil. Although the thermal conductivity of aluminum nitride is high, it is an electric insulator; therefore, while insulating the superconductive thin film, the upper protecting board cools it efficiently. The upper protecting board141has an aperture21which corresponds to the position of the normal-metal connecting wires15and the normal-metal extracting wires17and17′. Thus, the top sapphire substrate121and the transmitting and receiving probe coils are covered and invisible, but they are shown for the sake of explanation inFIG. 4A. As the receiving-coil component111is formed on the bottom side of the sapphire substrate121, the former is shown by broken lines inFIG. 4A. The reference numerals131's are normal-metal thin films whose positions correspond to the positions of the two ends and outward extensions of the receiving-coil component111. Normal-metal connecting wires15are connected to the normal-metal thin films131, and connected to the normal-metal thin films131are normal-metal extracting wires17and17′ which appear in the aperture21. The transmitting-coil components181and182go down through the corresponding holes, and the transmitting-coil components183,184,185and186go down through the corresponding holes. The reference numeral19's are aluminum-nitride screws to join the piled substrates12.

FIG. 4Bis a schematic sectional view taken along the arrowed line B—B ofFIG. 4A. An aluminum-nitride spacer142is disposed under the upper protecting board141, and the transmitting-coil components181, and182go up through the spacer142. The shape of the spacer142is almost the same as the shape of the upper protecting board141, but the aperture21of the spacer142is widened for the connections of normal-metal connecting wires15. The spacer142has holes101and104to107which are the same as the holes101and104to107of each sapphire substrate12. The same is true of spacers143to146. Then, the sapphire substrate121is disposed under the spacer142, the normal-metal thin films131being on the top side of the sapphire substrate121, the receiving-coil component111being on the bottom side of the sapphire substrate121. Arranged below the sapphire substrate121are a spacer143, a sapphire substrate122, a spacer144, a sapphire substrate123, a spacer145, a sapphire substrate124, and a spacer146. The transmitting-coil components183,184,185, and186go down through the corresponding holes of the sapphire substrates and spacers and appear under the spacer146, and the transmitting-coil components187,188, and189are arranged, transmitting-coil components183and184connected to those187and188. Then, disposed is a transmitting-coil component1810to which the components185and186are connected. Lastly, a lower protecting board147is disposed under the spacer146. After the completion of alternating piling up of sapphire substrates and spacers, they are fixed by the aluminum-nitride screws19.

FIG. 4Cis a sectional view of two coils between substrates12. The two coils between substrates12are capacitance-connected by a normal-metal connecting wire15which is connected to normal-metal thin films13by ultrasonic bonding. The normal-metal thin films131and132are connected by the normal-metal connecting wire15. The aperture21of the spacer143is widened to avoid the interference with the normal-metal thin films131and132. As already explained by referring toFIG. 2, a normal-metal extracting wire17of Cu is connected to a normal-metal connecting wire15between the receiving-coil components112and113by ultrasonic bonding, and another normal-metal extracting wire17of Cu is connected to a normal-metal connecting wire15between the receiving-coil components111and114by ultrasonic bonding. These normal-metal extracting wires17are used to take signals out of the receiving probe coil11. A normal-metal extracting wire17′ of Cu is connected to the transmitting-coil component189by ultrasonic bonding, and another normal-metal extracting wire17′ of Cu is connected to the transmitting-coil component189by ultrasonic bonding. These normal-metal extracting wires17′ are used to send signals into the transmitting probe coil18.

As already explained by referring toFIG. 2, the receiving probe coil11is a two-turn two-parallel circuit coil and each coil component11iis formed out of a superconductive material on one side of a substrate12i. Formed on the other side of the substrate12iis a normal-metal thin films13iof Au whose positions correspond to the positions of the two ends and outward extensions of the coil component11i. The thickness of the Au ribbon lines of the first embodiment is 50 μm to reduce high-frequency resistance, but they may be thicker.

As described above, in the first embodiment of the present invention, capacitors are formed by forming an oxide superconductive thin film on one side of a sapphire substrate and normal-metal films on the other side of the sapphire substrate, the substrate serving as an insulator between the superconductive thin film and the normal-metal films. Thus achieved is metal/superconductor connection through exactly desired capacitance. Besides, the axis of the solenoid is orthogonal to the static magnetic field's direction which is horizontal, and the superconductors are so disposed that the interlinkage between the superconductors and the lines of magnetic force is minimized, the superconductors in interlinkage with the lines of magnetic force being only the thin parts of the superconductive thin films. Thus achieved is a probe coil in an uniform magnetic field.

SECOND EMBODIMENT

The second embodiment of the present invention will be described below by referring toFIG. 5. To raise the quality factor of a probe coil, it is necessary to reduce the resistance in the LC resonance circuit. As described in the section of “Problems to be Solved by the Invention,” however, if an ordinary manufacturing process is used, normal metal/oxide superconductor junction of high contact resistance is made. Accordingly, in the first embodiment, an oxide superconductive thin film and a normal-metal film are connected through capacitance. In the second embodiment of the present invention, an improved manufacturing process is used to achieve normal metal/oxide superconductor junction of low contact resistance. By using this, oxide superconductive thin films and normal-metal wires are connected to make a solenoidal coil of superconductive thin films whose quality factor is high.

The construction of the probe coil of the second embodiment is the same as the construction of the probe coil of the first embodiment ofFIG. 2in that both the probe coils comprises a receiving coil of superconductive thin films and a transmitting coil of a saddle type. The second embodiment is different from the first embodiment in that the superconductive thin films of the receiving coil and the normal-metal films for sending signals out of the second embodiment are connected by resistance connection. InFIG. 5, the same components and components of the same functions as in the first embodiment are tagged with the same reference numerals as in the first embodiment.

FIG. 5Ais an illustration of a one-turn coil of a superconductive thin film11and two normal-metal thin films13of Au formed on a substrate12in accordance with the second embodiment of the present invention.FIG. 5Bis a sectional view taken along the arrowed line D—D ofFIG. 5A. It can be seen that the two normal-metal thin films13of Au is connected to the two open ends of the superconductive thin film11.FIG. 5Cis a sectional view of two coils between substrates12. The two coils between substrates12are connected by a normal-metal connecting wire15which is connected to a normal-metal thin film13of one coil and a normal-metal thin film13of the other coil by ultrasonic bonding.

As shown inFIG. 5A, a film of CeO2of the thickness of 100 nm is first formed as a buffer layer on one side of a sapphire substrate12. Then, a superconductive thin film11of the thickness of 150 nm is formed out of the oxide superconductive material YBa2Cu3O7. Next, the sample is conveyed into another chamber in a vacuum device without exposing the sample to the atmosphere and then transferred to a substrate holder so as to form two films out of Au at a desired area where normal metal-oxide superconductor connection is to be made. Then, two normal-metal films13of the thickness of one micrometer are formed out of Au. Next, the superconductive thin film11is processed into a circular pattern by application of a resist, photolithography, and Ar etching. Then, the Au films are processed into desired patterns by application of a resist, photolithography, and Ar etching. Next, the substrate12with the films was annealed at 500° C. in an oxygen atmosphere to increase the oxygen in the YBa2Cu3O7thin film for better superconductivity and reduce the contact resistance between Au and YBa2Cu3O7. Thus formed on the substrate12is a one-turn coil which is made of a superconductive thin film of YBa2Cu3O7and to which the normal-metal thin films13of Au are connected.

If a YBa2Cu3O7thin film is put in contact with water or exposed to the atmosphere during an ordinary manufacturing process such as photolithography, its surface deteriorates and changes into a high-resistance layer. Accordingly, if an Au film is formed after the ordinary manufacturing process such as photolithography, normal metal-oxide superconductor connection of high contact resistance is made. In this embodiment, the Au films are formed without exposing the YBa2Cu3O7thin film to the atmosphere after the formation of the YBa2Cu3O7thin film; therefore, the surface of the YBa2Cu3O7thin film to be put in contact with Au does not deteriorate and hence normal metal/oxide superconductor junction of low contact resistance is made.

It can be seen fromFIGS. 5B and 3Cthat the second embodiment differs from the first embodiment in that the superconductive film11and the normal-metal thin films13of the second embodiment are formed on one and the same side of a substrate12and directly connected.

It can be seen fromFIGS. 5C and 4Cthat the second embodiment differs from the first embodiment in that the superconductive film11is formed on the top side of a substrate12in the second embodiment, whereas the superconductive film11is formed on the bottom side of a substrate12in the first embodiment. The second embodiment is the same as the first embodiment in that the normal-metal thin films13are connected by normal-metal connecting wires15in both the embodiments.

Then, as in the case of the first embodiment, holes are made in the sapphire substrates12, the substrates12are piled up, a transmitting coil18is built in, and the normal-metal thin films13on the sapphire substrates12and the normal-metal connecting wires15of Au ribbon lines are connected by ultrasonic bonding to complete the probe coil.

Thus, in the second embodiment, the manufacturing process is improved so that the coil is formed by using an oxide superconductor and the normal-metal films are connected without exposing them to the atmosphere. Therefore, normal metal/oxide superconductor junction with low contact resistance was achieved. Besides, as in the first embodiment, the axis of the solenoid is orthogonal to the static magnetic field's direction which is horizontal and the superconductors are so disposed that the interlinkage between the superconductor and the lines of magnetic force is minimized, the superconductors in interlinkage with the lines of force being only the thin parts of the superconductive thin film. Thus achieved is a probe coil in an uniform magnetic field.

THIRD EMBODIMENT

Now, referring toFIG. 6, the third embodiment of the present invention will be described. In order to increase the quality factor of the probe coil, as in the first embodiment, an oxide superconductive thin film and a normal-metal connecting wire are connected through capacitance, forming a solenoidal coil of a superconductive thin film whose quality factor is high. However, the third embodiment is different from the first embodiment in that the capacitor is provided on a side where the oxide superconductive thin film (coil11) is formed. The construction of the probe coil of the third embodiment is the same as the construction of the probe coil of the first embodiment ofFIG. 2in that the probe coil comprises a receiving coil of superconductive thin films and a transmitting coil of a saddle type. InFIG. 6, the same components and components of the same functions as in the first and second embodiments are tagged with the same reference numerals.

FIG. 6Ais an illustration of a substrate12with a one-turn coil11, normal-metal thin films13, and an insulating layer311in accordance with the third embodiment of the present invention. The insulating layer311is formed between the one-turn coil11and the normal-metal thin films13.FIG. 6Bis a sectional view taken along the arrowed line E—E ofFIG. 6A. It can be seen that there are formed ends of the superconductive thin film11opened and extended outwardly of the substrate and the insulating layer311between normal-metal thin films13of Au.FIG. 6Cis a sectional view of two coils between substrates12. The two coils between substrates12are capacitance-connected by a normal-metal connection wire15which is connected to normal-metal thin films13by ultrasonic bonding.

As shown inFIG. 6A, a film of CeO2of the thickness of 100 nm is first formed as a buffer layer on one side of a sapphire substrate12. Then, a superconductive thin film11of the thickness of 150 nm is formed out of the oxide superconductive material YBa2Cu3O7. Next, the YBa2Cu3O7thin film is processed into a circular pattern by application of a resist, photolithography, and Ar etching. Next, the substrate12with the films was annealed at 500° C. in an oxygen atmosphere to increase the oxygen in the YBa2Cu3O7thin film for better superconductivity. Photosensitive polyimide of the thickness of 20 micrometer is applied all over the substrate. Then, a desired pattern is formed on the substrate by using lithography, and an insulating layer311which is a constituent element of a capacitor is formed. Further, on the insulating layer311, normal-metal thin films13of the thickness of one micrometer are formed out of Au.

It can be seen fromFIGS. 6B and 5Bthat the third embodiment is the same as the second embodiment in that formed on the same side of the substrate12is the coil11made of a superconductive thin film of YBa2Cu3O7and to which the normal-metal thin films13of Au are connected. However, they are different in that the open ends of the coil11are extended outwardly of the substrate12, and the insulating layer311is formed between the open ends and the normal-metal thin films13of Au.

As shown inFIG. 6C, according to the third embodiment, the coil11and the normal-metal thin films13of Au are formed on one and the same side of the substrate12as in the second embodiment. When compared with the second embodiment shown inFIG. 5C, the third embodiment is the same as the second embodiment in that the normal-metal thin films13of Au are connected by normal-metal connecting wires15in both the embodiments.

Then, as in the cases of the first and second embodiments, holes are made in the sapphire substrates12, the substrates12are piled up, a transmitting coil18is built in, and the normal-metal thin films13of Au on the sapphire substrates12and the normal-metal connecting wires15of Au ribbon lines are connected by ultrasonic bonding to complete the probe coil.

Thus, in the third embodiment, the solenoidal coil is made wherein the connection wires and an oxide superconductive thin film are connected through a capacitor made from Cu/polyimide/YBa2Cu3O7. In this way, metal/superconductor connection through exactly desired capacitance is achieved by using the capacitor of Cu/polyimide/YBa2Cu3O7. Besides, the axis of the solenoid is orthogonal to the static magnetic field's direction which is horizontal, and the superconductors are so disposed that the interlikage between the superconductors and the lines of magnetic force is minimized, the superconductors in interlinkage with the lines of magnetic force being only the thin parts of the superconductive thin films. Thus achieved is a probe coil in an uniform magnetic field.

OTHER EMBODIMENT

In each of the first to third embodiments, the receiving probe coil is a two-turn two-parallel circuit coil. However, the receiving probe coil may be a four-turn one-parallel circuit coil.FIG. 7is a sectional view taken along the arrowed line B—B ofFIG. 4A. The probe coil is converted into a four-turn type. Like reference numerals are given to like parts. In the case of a four-turn one-parallel circuit, the beginning and end of the wire of each coil are connected in sequence. Therefore, the coils of the substrates are connected by the normal-metal connecting wires15in sequence. A normal-metal thin film131′ corresponding to a tip of the beginning of the coil111is connected to one of the normal-metal extracting wires17. Further, a normal-metal thin film131corresponding to a tip of the end of the coil111and a normal-metal thin film132′ corresponding to a tip of the beginning of the coil112are connected to the normal-metal connecting wires15. In the same way, tips of the ending and beginning of the coils are sequentially connected, and a normal-metal thin film134corresponding to a tip of the ending of the last coil114is connected to the other normal-metal extracting wire17. The transmitting coil18may be the same as the one described in the first embodiment by referring toFIG. 4B.

Also, in the third embodiment, photosensitive polyimide was used as an insulating material constituting the capacitor. However, the same effect can be produced by using, instead of the photosensitive polyimide, a fluoride-resin film or an insulating film such as the ones made of CeO2or Y2O3formed by a thin-film manufacturing process.

Further, in either of the embodiments, the normal-metal thin film may be of Au, Cu, Ag, or Al.

Though not described specifically, it is needless to say that the probe coil1is connected to a lead connected to a cooling source and is cooled.

According to the present invention, a low-resistance solenoidal coil can be provided. Besides, the superconductors in interlinkage with the lines of magnetic force are only the thin parts of the superconductive thin films. Therefore, the present invention can provide a nuclear magnetic resonance probe coil of a solenoid type wherein superconductive thin film is used, whose quality factor is high, and which is put in an even magnetic field and occupies a small space.