JPH05190920A - Fine working method of oxide superconducting thin film and planar type superconducting element using the same - Google Patents

Abstract

PURPOSE:To form a planar type superconducting element which can not be formed by using a lamination type, by using material whose thermal expansion coefficient is remarkably different from a oxide superconducting thin film, as a substrate. CONSTITUTION:As an oxide superconducting thin film, e.g. YBa2Cu3 O7-x(YBCO) is used, and, as substrate material, SrTi03 (STO) is used. The thermal expansion coefficients of them are remarkably different, and its feature is shown by the change of length of a crystal axis of each material (C axis in the case of YBCO) versus the temperature. Since the YBCO film sharply expands at 400 deg.C or higher, internal stress is generated in the YBCO film during a cooling process from the forming temperature at 550-700 deg.C, when the YBCO thin film is formed. The absolute value of the stress increases in proportion to the YBCO film thickness, so that cracks of about 10nm width are generated to relieve the stress for the thickness larger than the critical film thickness. By using said cracks, various kinds of planar type superconducting elements can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for finely processing an oxide superconducting thin film and a planar type superconducting element using the oxide superconducting thin film having fine cracks formed by this method.

[0002]

2. Description of the Related Art FIG. 15 is a schematic sectional view showing the structure of a conventionally known laminated proximity effect element, and a measuring system is also shown. In FIG. 15, this element is composed of a noble metal paraelectric body 4 having a thickness of 10 nm between an oxide superconductor 2 formed on a substrate 1 and two superconductors of a metal superconductor 3.
It is a Josephson element that has a structure sandwiching the above and uses the exudation of electron pairs from the superconductors 2 and 3 to the normal electric body 4. 5 is an insulator of a spacer, 6 is an oscillator for measurement, and 7 is a voltmeter for measurement.

FIG. 16 is a schematic sectional view showing the structure of a field effect element using a metal superconductor. In FIG. 16, this device has a structure in which a compound semiconductor 9 is sandwiched between two metal superconductors 8, and a gate electrode 11 is provided on the upper surface of the compound semiconductor 9 via an insulator 10. A high carrier concentration channel region is formed in the compound semiconductor 9 by applying a voltage. Since the bleeding of electron pairs from the superconductor 8 to the compound semiconductor 9 due to the electric field effect depends on the carrier concentration in the compound semiconductor 9, the superconducting current can be controlled by the presence or absence of the voltage applied to the gate electrode 11. In the case of this element using the metal superconductor 8, since the coherence length of the metal superconductor 8 is 10 times or more longer than that of the oxide superconductor, the gap between the two metal superconductors 8, that is, the compound semiconductor 9 is sandwiched. The part that operates is 100 nm. However, when the oxide superconductor is used, it is necessary to set the thickness to about 10 nm as in the proximity effect element described in FIG.

[0004]

A Josephson device using an oxide superconductor, for example, a three-terminal device such as a field effect device, is easier to take out the gate electrode when formed in a planar type. Is advantageous. However, since this material has a short coherence length, fine processing of about 10 nm is required. For example, in order to form the Josephson device shown in FIG.
It is necessary to etch with a width of nm. That is, in the laminated type shown in FIG. 13, 10 nm, which corresponds to the film thickness of the paraelectric body 4, has many difficulties in that the planar type has to be finely processed as an etching width.

Conventionally known fine processing techniques of 1 μm or less are an electron beam exposure method, an X-ray exposure method, a focused ion beam method and the like. However, these methods have the following drawbacks. In addition, conventionally, it has been impossible to manufacture a planar Josephson device. 1 Can not be processed to a width of about 10 nm,
m is the limit. 2 The machined surface is deteriorated or damaged.

The present invention has been made in view of the above points, and an object thereof is a method capable of finely processing an oxide superconducting thin film to a thickness of about 10 nm, and a planar type superconducting element formed by using this method. To provide.

[0007]

In order to solve the above-mentioned problems, the present invention provides a material of a substrate having a coefficient of thermal expansion significantly different from that of the oxide superconducting thin film when forming the oxide superconducting thin film on the substrate. The micro stress is applied to the oxide superconducting thin film due to the internal stress caused by the difference in thermal expansion between the oxide superconducting thin film and the substrate during the cooling process from the thin film forming temperature. There is a proximity effect type planar superconducting element in which a noble metal is formed in the crack, a planar superconducting element for receiving light in which a semiconductor layer is formed in the crack,
Alternatively, a field effect type planar superconducting element in which a semiconductor, an insulator, and a metal are formed in this order in a crack portion can be obtained.

[0008]

Function: The oxide superconducting thin film is generally 550 to 700 ° C.
Formed at high temperature. Therefore, by using a material whose coefficient of thermal expansion is significantly different from that of the oxide superconducting thin film as the substrate, a large internal stress is generated in the oxide superconducting thin film during the cooling process from the formation temperature, and in order to alleviate this, a fine film is formed in the thin film. It appears as a crack. Since the crack width obtained at this time is about 10 nm, this can be adopted as a microfabrication method to manufacture the above-mentioned various planar superconducting elements which cannot be obtained by the laminated type.

[0009]

EXAMPLES The present invention will be described below based on examples. As a material for the oxide superconducting thin film, LnBa ₂ Cu ₃ O is used.
_7-X (Ln is Y, Nb, La, Sm, Eu, Gd, D
y, Ho, Er, Tm, Yb, Lu, any one element, and X is an oxygen deficiency), but here, YBa
_{A case where 2} Cu ₃ O _7-x (hereinafter referred to as YBCO) is used and SrTiO ₃ (hereinafter referred to as STO) is used as a substrate material will be described. The thermal expansion coefficients of these materials are remarkably different, and the state is shown in Fig. 1. The crystal axes (YBCO is C
(Axis) is shown in a diagram represented by the change in length with respect to temperature.
According to FIG. 1, the YBCO film expands sharply at about 400 ° C. or higher. Therefore, when the YBCO thin film is formed on the STO substrate by the sputtering method, the CVD method, or the like, the thermal expansion coefficients of the two become different in the cooling process from the formation temperature of 550 to 700 ° C. Internal stress occurs. The absolute value of this stress is YBCO
Since it increases in proportion to the film thickness, it is expected that fine cracks will occur in the YBCO film in order to relieve stress above the critical film thickness. In order to confirm this, by using a magnetron sputtering method, a crystal plane direction (110), (013), or (10) is formed on the STO substrate.
The YBCO thin film of 3) was formed, and the surface thereof was observed by a scanning electron microscope (SEM).

FIG. 2 shows an SEM photograph of the surface of the YBCO film formed on the STO (110) substrate, magnified 50,000 times. As shown in FIG. 2, it was confirmed that a crack of about 10 nm enters in the direction perpendicular to the C axis of the YBCO film. However, such a crack depends on the formation temperature and the film thickness of the YBCO film, and when the film thickness is small, the stress inside the film does not reach the critical value, so that the stress is accumulated inside the film. , It does not occur as a crack. Even when the film formation temperature is low, there is no substantial difference in thermal expansion, so cracks are not introduced. FIG. 3 is a diagram showing the relationship between the YBCO film thickness and the substrate temperature. Depending on these conditions, the YBCO film has a cracked region and a crackless region, which are shown separately.

FIG. 4 is a measurement sample prepared by patterning the cracked YBCO thin film obtained as described above to examine the temperature dependence of electric resistance and whether or not the crack penetrates the film. It is the schematic diagram which looked at from the front. In FIG. 4, Y formed on the STO substrate 12
Patterning is performed so that the crack 14 introduced into the BCO film 13 is located at the center, and the measuring terminals 15 are provided at the four corners.
The terminal 15 is connected to the measuring voltmeter 16 and the measuring current source 17. The reason why the region of the YBCO film 13 where the crack 14 is inserted is made narrower because the crack cannot be formed if there is a thin portion of the film 13 so that the influence of the variation in the film thickness is avoided.

FIG. 5 is a temperature-resistance diagram showing the measurement results of the sample of FIG. From the shape of the curve in FIG.
Penetrates the YBCO film 13 and the crack 14 is perpendicular to the current path shown by the arrow (FIG. 4).
The flowing current is due to single electron tunneling tunneling through the crack 14. On the other hand, the decrease in the resistance R at the temperature T = 80 K or less is due to the superconducting transition of the YBCO film 13. Therefore, this crack 14, that is,
It can be seen that it is possible to form a planar structure type superconducting element by utilizing fine cracks caused by the difference in thermal expansion between the YBCO film and the substrate. Hereinafter, those plane type superconducting elements will be described.

FIG. 6 is a schematic sectional view showing the structure of the planar superconducting element. In this device, a YBCO film 19 having the shape shown in FIG. 4 is formed on the STO substrate 18, cracks 20 are formed, and then noble metal such as Au 21 is deposited on the cracks 20 by a magnetron sputtering method.
It has a planar structure of CO / Au / YBCO.

A current-voltage characteristic diagram at 4.5K of this device is shown in FIG. By cooling this element below the critical temperature of the YBCO film 19, the electron pair of the YBCO film 19 exudes to Au21. As a result, Au21 has weak superconductivity, and a proximity effect type Josephson element is formed.

The coherence length (ξ) in a semiconductor is a function of the carrier concentration (n) and the mobility (μ) of the semiconductor.
It is given by the following formula. ξ = (h ³ μ / 6πKT) ^1/2 (3π ² n) ^1/3

When the carrier concentration and the mobility are high, the amount of exudation increases. Therefore, by forming an oxide superconductor on a substrate having a high carrier concentration or a substrate having oxygen defects, a planar type Josephson device is obtained. Obtainable. By adding Nb to the STO (110) substrate, the carrier concentration can be increased up to about 10 ²⁰ cm ⁻³ , and using this substrate, the oxide superconductor-STO-oxide superconductivity at the crack portion with a width of 10 nm is increased. A body structure is formed and a proximity effect occurs between the oxide superconductor and the STO substrate.

Therefore, an STO single crystal substrate in which YBCO was used as the oxide superconductor and Nb was added was prepared by the pulling method. On the other hand, for the conductive substrate using oxygen defects, STO produced by the pulling method was heated to 1000 ° C. or higher in an ultrahigh vacuum containing a Ti getter to form oxygen defects. Since Nb or oxygen defects in STO act as a donor impurity, the carrier concentration was determined by measuring the Hall effect. In an STO substrate with Nb added to have a carrier concentration of 4 × 10 ²⁰ cm ⁻³ , the conductivity increases, so that a current flows to the substrate side. Then, since the proximity effect occurs with the superconducting transition of the YBCO film, the STO substrate acts as a weak superconductor. Therefore, when an STO substrate with impurities replaced is used, Y
Regarding the temperature dependence of the resistance of the BCO film, the resistance becomes 0 below the critical temperature of the YBCO film.

The obtained measurement sample is shown in FIG. 8 in accordance with FIG. 4, and portions common to FIG. 4 are designated by the same reference numerals. The difference between FIG. 8 and FIG. 4 is that the STO substrate 12a added with impurities is used. A current-voltage characteristic diagram at 4.2 K for the sample of FIG. 8 is shown in FIG. 9 in comparison with the case of using an STO substrate (insulating substrate) to which impurities are not added. It can be seen from FIG. 9 that the superconducting current flows because the coherence length is increased by using the impurity-added substrate.

Similarly, using a BaTiO ₃ (110) single crystal substrate or a LaCuO ₄ (110) single crystal substrate,
By forming an oxide superconductor on these substrates, cracks can be formed in the oxide superconductor due to the difference in thermal stress. However, La is added to BaTiO _{3 and} Ba is added to LaCuO _4. By doing so, it is possible to increase the carrier concentration to 10 ²⁰ cm ⁻³ and to make it a semiconductor. At this time, the conductivity of the substrate increases, so that the current will flow to the substrate side instead of the crack portion,
Therefore, a planar type Josephson device can be obtained by forming the oxide superconductor on the LaTiO ₃ -doped BaTiO ₃ (110) substrate or the Ba-doped LaCuO ₄ (110) substrate.

When a semiconductor is irradiated with light, electrons and holes are generated inside the semiconductor. Therefore, the conductivity of the semiconductor increases and the coherence length also increases. This coherence length means the length of the oozing of the electron pair from the superconductor to the semiconductor. When the oozing of the electron pair from the two superconductors overlaps inside the semiconductor, a superconducting current (Josephson current) flows. .. Therefore, by using a semiconductor having photoconductivity in place of Au21 which is the noble metal of the device shown in FIG. 6, the superconducting current flowing through the device is changed depending on the presence or absence of light irradiation to obtain a planar type light receiving device. You can

In this embodiment, for example, C is used as a semiconductor.
FIG. 10 shows a schematic cross-sectional view of an element in which the YBCO film 19 is processed into the shape shown in FIG. 4 using dS to form the crack 20, and then the CdS 22 is formed in the crack 20 by the vacuum evaporation method. In FIG. 10, the same parts as those in FIG. 6 are designated by the same reference numerals. That is, this element uses CdS22 instead of Au21 in FIG. 6, and the other configurations are the same as those in FIG. FIG. 11 shows the current of the device shown in FIG.
It is a voltage characteristic diagram, and this diagram shows the light response characteristics when the element is irradiated with light and when it is not irradiated with light. From FIG. 11, it can be seen that the element operates as a planar light receiving element.

Since the coherence length in the semiconductor is given by the above equation, it is possible to control the superconducting current flowing in this element by intentionally modulating the carrier concentration in the semiconductor. The method of changing the carrier concentration in the semiconductor can be performed by an electric field in addition to the light irradiation. FIG. 12 shows a schematic cross-sectional view of a field effect type planar superconducting element, and the same parts as those in FIGS. 6 and 10 are represented by the same reference numerals. The difference between FIG. 12 and FIGS. 6 and 10 is that a semiconductor such as p-type GaA is present in the crack 20.
s23 is deposited and an insulating layer 24 of 100 nm is deposited on the s23,
For example, silicon oxide is formed, an aluminum layer is finally formed, and this is used as the gate electrode 25. In this device, when a negative voltage is applied to the gate electrode 25, a layer having a high carrier concentration, that is, a channel is formed in the GaAs 23. As a result, a proximity effect occurs and a superconducting current of superconductor / GaAs / superconductor flows. And the carrier concentration induced by the gate voltage is
Since it depends on the magnitude of the gate voltage, the superconducting current flowing through the device can be controlled by the gate voltage.

Further, the minute cracks of about 10 nm width obtained by the present invention can be applied to a high sensitivity magnetic sensor as an oxide superconductor bridge type Josephson junction element. When manufacturing a high sensitivity magnetic sensor (SQUID) using a superconductor, it is necessary to connect two Josephson junction elements in parallel. According to the fine processing method of the present invention, this parallel connection is performed. You can For example, a YBCO film on an STO (110) substrate,
250 at 650 ° C by RF magnetron sputtering method
The YBCO film is cracked in the course of being deposited on the YBCO film in the process of cooling to room temperature, and then a pattern of a high-sensitivity magnetic sensor is prepared by ordinary microfabrication, and finally an electrode is attached to complete the device. FIG. 13 is a schematic diagram showing the shape of the high-sensitivity magnetic sensor, where 26 is a YBCO film, 27 is a crack, and 2 is a crack.
Reference numeral 8 represents a loop portion, and 29 represents a gold electrode for measurement. Here, as the loop portion, two types of loop areas in the bonding surface, 100 μm ² and 400 μm ² , were prepared, and the characteristics of both were measured. In this device, cracks 27 on the YBCO film 26 are generated in the YBCO film 26 in a constant direction and at equal intervals. Therefore, using this pattern facilitates parallel connection of two Josephson junctions. Can be done.

FIG. 14 shows the liquid nitrogen temperature (7
It is a diagram showing a magnetic field response characteristics in 7K), (a)
Represents a device having a loop area of 100 μm ² , and (b) represents a device having a loop area of 400 μm ² . It is understood from FIG. 14 that the loop area and the response sensitivity are inversely proportional, and the magnetic field response characteristic of the element of (a) is four times that of the element of (b). This result shows that the high-sensitivity magnetic sensor obtained according to the present invention is operating well.

[0025]

As described above, according to the present invention, the oxide superconducting thin film is formed on the substrate having a different thermal expansion coefficient. During the cooling process from the formation temperature of the thin film, minute cracks with a width of about 10 nm are generated in this thin film. Therefore, by utilizing these cracks, various planar types using oxide superconductors, which have been impossible so far, are used. A superconducting element can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing changes in length of crystal axes of a YBCO film and an STO substrate with respect to temperature.

FIG. 2 is an electron micrograph showing the structure of particles on the surface of the YBCO film.

FIG. 3 is a diagram showing a relationship between YBCO film thickness and substrate temperature, which causes cracks.

FIG. 4 is a schematic diagram of an electric resistance measurement sample of a YBCO film having a crack.

5 is a temperature-resistance diagram showing the measurement results of the sample of FIG.

FIG. 6 is a schematic cross-sectional view showing the configuration of a proximity effect type planar superconducting element according to the present invention.

FIG. 7 is a current-voltage characteristic diagram of the device shown in FIG.

FIG. 8 is a schematic diagram of a measurement sample using an impurity-added substrate

FIG. 9 shows currents shown in comparison between the sample of FIG. 8 and an insulating substrate.
Voltage characteristic diagram

FIG. 10 is a schematic cross-sectional view showing the structure of a field effect type planar superconducting element according to the present invention.

FIG. 11 is a schematic cross-sectional view showing the configuration of a planar superconducting element for receiving light according to the present invention.

12 is a current-voltage characteristic diagram of the device shown in FIG.

FIG. 13 is a schematic diagram showing the shape of a high-sensitivity magnetic sensor according to the present invention.

14 is a magnetic field response characteristic diagram of the device shown in FIG.

FIG. 15 is a schematic cross-sectional view showing the configuration of a conventional proximity effect type superconducting element.

FIG. 16 is a schematic cross-sectional view showing the structure of a conventional field effect element.

[Explanation of symbols]

 1 Substrate 2 Oxide Superconductor 3 Metal Superconductor 4 Normal Conductor 5 Insulator 6 Measurement Oscillator 7 Measurement Voltmeter 8 Metal Superconductor 9 Compound Semiconductor 10 Insulator 11 Gate Electrode 12 STO Substrate 12a Impurity-Added Substrate 13 YBCO Film 14 crack 15 measurement terminal 16 measurement voltmeter 17 measurement current source 18 STO substrate 19 YBCO film 20 crack 21 Au 22 CdS 23 GaAs 24 insulating layer 25 gate electrode 26 YBCO film 27 crack 28 loop part 29 measurement gold electrode

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Takashi Ishii 1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Fuji Electric Co., Ltd. No. 1 inside Fuji Electric Co., Ltd. (72) Inventor Kazuro Mue 1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa Inside Fuji Electric Co., Ltd.

Claims (12)

[Claims] 1. When forming an oxide superconducting thin film on a substrate, a material having a coefficient of thermal expansion significantly different from that of the thin film is used as the substrate, and the thin film and the substrate are separated from each other in a cooling process from the formation temperature of the thin film. A method for microfabrication an oxide superconducting thin film, characterized in that cracks are generated in the thin film due to internal stress caused by the difference in thermal expansion. 2. The method according to claim 1, wherein the oxide superconducting thin film is LnBa ₂ Cu ₃ O _7-X (Ln is Y, Nb, L
a, Sm, Eu, Gd, Dy, Ho, Er, Tm, Y
b, any one element of Lu, X is oxygen deficiency)
The substrate is made of SrTiO ₃ and is a microfabrication method for an oxide superconducting thin film. 3. The method for microfabrication of an oxide superconducting thin film according to claim 1, wherein the crack is perpendicular to the C axis of the oxide superconducting thin film. 4. The method for microfabrication of an oxide superconducting thin film according to claim 1, wherein the crack width is about 10 nm. 5. A proximity effect type planar superconducting element, characterized in that a noble metal is formed in a crack obtained by the method according to any one of claims 1 to 4. 6. A planar superconducting device for receiving light, characterized in that a semiconductor is formed in a crack obtained by the method according to any one of claims 1 to 4. 7. A field effect type planar superconducting element, characterized in that a semiconductor, an insulator and a metal are formed in this order in a crack obtained by the method according to claim 1. 8. A plane type superconducting device comprising a high sensitivity magnetic sensor in which two bridge type Josephson junction elements are connected in parallel by using the crack portion obtained by the method according to any one of claims 1 to 4. element. 9. The method for microfabrication of an oxide superconducting thin film according to claim 1, wherein SrTiO ₃ having a donor introduced therein is used as the substrate. 10. The method according to claim 1, wherein
A fine processing method for an oxide superconducting thin film, wherein the introduction of a donor is performed by adding an Nb element or forming an oxygen defect. 11. The method for microfabrication of an oxide superconducting thin film according to claim 1, wherein a BaTiO ₃ (110) single crystal containing La is used as a substrate. 12. A method for finely processing an oxide superconducting thin film according to claim 1, wherein a LaCuO ₄ (110) single crystal containing Ba is used as the substrate.