JP3167768B2 - Grain boundary Josephson device and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Josephson device for forming a thin-film Josephson device using a Josephson junction, for example, a squid, a mixer, a Josephson FET, and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Y which exhibits superconductivity above the temperature of liquid nitrogen
Oxide, high-temperature superconductors, such as Bi-based and Tl-based, have been discovered, along with research on thin-film fabrication techniques by various methods such as laser abrasion, CVD, sputtering, and vapor deposition, and Josephson using this thin-film. Research and development of technology for practical use of devices is also being pursued by many research institutions. However, high-temperature superconductors have a shorter coherence length than metal superconductors, which are conventional materials. For this reason, it is technically difficult to form both a stacked tunnel barrier and a thin-film weak junction. .

[0003] Up to now, the observation of the Josephson effect has been reported only to point contact type, grain boundary junction type, and recently, edge junction type devices. Among them, in the point contact type, a combination of an Nb needle and an oxide superconductor has been reported so far, but in this case, the operation is limited to 4.2K, and the oxidation is not performed. When a superconductor is used as a needle, a sharp shape is required at the tip of the needle, so there is a problem in mechanical strength with ceramic materials, the contact part is brittle, and there are problems in reproducibility and stability. . Next, most of the reported bridge type using grain boundary junctions use naturally formed grain boundaries, and the crystal grain size and bridge size are uniform depending on the film forming conditions. Due to differences in various grain boundary structures
Similar to the contact type, there was a problem in reproducibility and stability. Even in the case of the edge junction type, it is considered that a crystal grain boundary formed in a thin film on the edge portion of the substrate is used, and it is considered that there is a problem similar to that of the naturally formed grain boundary type. Further, in the grain boundary junction type, since the crystal grain boundary exists over the entire thin film, the energy resolution is poor when operating at the temperature of liquid nitrogen due to noise caused by the fluctuation of the magnetic flux captured by the grain boundary. There was a problem.

Further, as one of bridge type devices using a grain boundary junction, an inclined grain boundary used as a Josephson barrier (a crystal grain boundary having an in-plane rotation at a certain angle).
Attempts to artificially form have been studied by the IBM. In the case of IBM, for a Josephson device using a YBCO thin film, there is only one artificial tilt grain boundary made of SrTiO ₃ crystal having relatively good lattice matching with the base surface of YBCO at a predetermined portion in the substrate plane. Such a substrate is manufactured.

Here, the surface of the SrTiO ₃ bicrystal substrate has a (001) plane, and the crystal grain boundaries that divide the substrate into two are the above-mentioned tilt boundaries. According to reported papers, bicrystals are made by hot pressing two SrTiO ₃ single crystals or by growing them together from two seed crystals. And, on this substrate, YBC
When an O thin film is deposited, YBCO is deposited on each single crystal.
Grows epitaxially, so that there is no tilt grain boundary on the single crystal, but the same tilt grain boundary as the substrate is formed only at this position on the bonding interface of the substrate. IBM
Now, the dc-SQ using the YBCO thin film on this SrTiO ₃ substrate
A UID (squid) has been fabricated to improve reproducibility and noise characteristics. In addition, SQUID operation up to 87 K has been confirmed, reflecting the high critical temperature (Tc) of YBCO on SrTiO ₃ .

However, also in this dc-SQUID (squid), the 1 / f noise level in the 77K, 1-10 Hz frequency band is expressed in units of energy resolution [J / Hz] in units of Mg.
Compared to the natural grain boundary type on O ₂ , the YBC on SrTiO ₃
It is thought that the orientation of the O thin film was improved, but although the improvement was about two orders of magnitude, it was still more than two orders of magnitude higher than the dc-SQUID of the conventional material and the rf-SQUID of the conventional material.
(Squid) shows a value that is about one digit higher than that of (Squid).
For practical use at 77K, future improvement for reducing 1 / f noise is required.

In the case of a bicrystal substrate using SrTiO ₃ as a substrate material as described above, the reproducibility of a Josephson junction is improved, but when a microwave application such as a mixer or a voltage standard is considered, Due to the high dielectric constant of SrTiO ₃ and the loss tangent (dielectric constant is 1875 at 80K, loss tangent is 80K, 100 at 1 GHz), the dielectric loss becomes too large for a substrate material, which is not practical.

[0008] In addition, in the above-described method in which two single crystals are diffusion-bonded by hot pressing or the like and then cut into a substrate shape, the manufacturing process is complicated and the processing accuracy is poor. There was a drawback that it easily occurred.
Therefore, it has been desired to develop a bicrystal substrate and a thin-film bridge-type Josephson device on this substrate using a new material and a new manufacturing method for an artificial grain boundary junction Josephson device.

[0009]

[Problems to be Solved by the Invention] As described above, in the present artificial grain boundary junction type device using the SrTiO ₃ bicrystal substrate, the reproducibility and the stability of the characteristics are stable compared to the natural grain boundary junction type device. Although it can be said that the technology has been improved, it is stated that the problem of large dielectric loss of the substrate material remains for microwave application, which is one of the major application fields of Josephson devices, and that the fabrication process is complicated. Was. Accordingly, an object of the present invention is to provide a grapho-epitaxial substrate by photolithography technology conventionally used in semiconductor device fabrication, an artificial grain boundary junction Josephson device suitable for microwave applications, and a method of manufacturing the same. And

[0010]

[Means for Solving the Problems and Their Functions]
Regarding the above-mentioned problem of the dielectric loss, as a substrate material, a dielectric material at room temperature and 1 MHz has a dielectric constant of about 12, which is equivalent to that of MgO.
The use of i-crystals corresponds. A bicrystal substrate made of a Si crystal has been disclosed by Henry I. Smith and others in "Appl. Phys. Lett. 35, 71 (1979)", "J.
Vac. Sci. Technol. 16, 1640 (1979) ",
37, 454 (1980) and the like, using a graphoepitaxy method proposed in a paper such as “Appl. Phys. Lett. 37, 454 (1980)”. With the graphoepitaxy method, when a microcrystal is formed in a thermal equilibrium state, since the surface energy differs depending on the crystal plane, it is crystallized to form more crystal planes with lower surface energy, and the shape is
It is based on the principle that it follows the wulf theorem.

That is, in the case of a Si crystal, $ 10
Since the surface energy of {0} is the lowest, a regular hexahedron surrounded by {100} is expected. When crystal growth on an amorphous substrate is considered, one surface of the regular hexahedron comes into contact with the substrate surface. However, since it is uniaxially oriented, it grows into a polycrystalline film having a random orientation in the plane because there is no other restriction. On the other hand, when there is a groove having a rectangular cross section as shown in FIG. 1A on the amorphous substrate, crystal growth is restricted also on the side surface of the groove. Is uniquely determined by the direction. Here, as shown in FIG. 1A, when the groove is cut with an angle θ, a Si crystal formed thereon, that is, a Si thin film also grows in two regions having an angle θ, respectively. At the interface, an inclined grain boundary having an angle θ is formed.

Here, as the amorphous substrate for the Si thin film, an amorphous SiO ₂ , so-called quartz glass substrate may be used. A superconducting thin film, for example, YB
When a CO thin film is used, a zirconia layer for preventing mutual diffusion, an yttria layer for promoting the epitaxial growth of the YBCO thin film, and the like are sequentially formed. These zirconia layers, yttria layers, and the like are formed by a usual laser abrasion method, a CVD method, a sputtering method, an evaporation method, or the like.

In manufacturing the Si grapho-epitaxial substrate, a conventional grapho-epitaxial method is used. That is, first, an amorphous Si which has been subjected to submicron or micron order groove processing as shown in FIG.
After similarly forming an amorphous Si layer on an O ₂ substrate, an oriented thin film is produced through a two-step process of crystallizing the amorphous Si layer by heat treatment such as laser annealing or heater annealing. Here, if the formation of the Si layer is performed at a temperature sufficiently high for crystallization, the oriented thin film can be obtained in-situ (at that location) without going through the subsequent crystallization step. However, the surface shape of the thin film is affected by the unevenness of the groove, and in this state, it is not suitable as a substrate layer of a superconducting thin film. After a two-step process like this
An iGrafo epitaxial substrate will be produced.

The conventional grapho-epitaxial method is for obtaining a single-oriented Si layer. However, in the case of the present invention, an amorphous layer having two-directional grooves intersecting at an angle θ as shown in FIG. An amorphous Si layer 3 is formed on one layer of a porous SiO ₂ substrate, and is then crystallized to form a Si substrate layer 5 having a tilt boundary 4 at an angle θ (see FIG. 2). In the present invention, this is called a Si grapho epitaxial substrate 5. On the Si grapho-epitaxial substrate 5 manufactured as described above, the YBa ₂ Cu ₃ O _7-
The thin film 7 is formed by a usual laser abrasion method, a sputtering method, an evaporation method, or the like. The thin film used in the present invention is preferably a C-axis oriented film whose in-plane orientation is the same as that of the substrate or oriented in a certain relationship. That is, in each region on the grapho-epitaxial substrate 5, a single-crystal thin film is ideal, and even in a polycrystalline thin film, it is desirable that all in-plane orientations of crystal grains are constant. Furthermore, a high-quality film without deposits, holes, etc. is desirable, and the film formation method and conditions are selected so as to obtain such a film.

The YBCO thin film 7 is patterned into a required bridge shape by ordinary dry etching, wet etching or the like. Here, YBCO in the bridge
The tilt boundaries of the thin film 7 must exist. And
Preferably, the bridge is patterned so that the tilt boundaries are orthogonal. The width of the bridge part is 10μm or less,
The length is 10 μm or less and the thickness is 1 μm or less.

FIG. 2 is a schematic diagram of the artificial grain boundary Josephson device manufactured as described above.

[0017]

As described above, according to the present invention, a rectangular cross section having a predetermined angle is formed on an amorphous SiO ₂ substrate without joining two single crystals and machining a bicrystal block. By simply forming a groove, a Si film is formed by a normal film forming method, and a crystallization heat treatment is performed if necessary.
An iGrafo epitaxial substrate can be produced. Using this substrate, an artificial grain boundary Josephson device of a superconducting thin film can be produced with good reproducibility.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. Here, the production of the Si grapho-epitaxial substrate by the grapho-epitaxial method was carried out by partially improving the method of the above-mentioned document reported by Smith et al. In 1979-1980.

The surface of the Si wafer is oxidized to a depth of about 1 μm to form an SiO ₂ layer, and two-way grooves intersecting at an angle θ are dug into this surface as shown in FIG. The trenches are dug at intervals of about 0.5-2 μm to a depth of about 100 nm,
It is preferable that the cross-sectional shape is rectangular, the side surface is perpendicular to the surface of the substrate 1, and the corner at the bottom of the groove is 5 nmR or less. The groove is formed by a reactive ion etching method, a chemical photolithography method, or the like. Next, as shown in FIG.
Is formed to a thickness of about 0.5 μm by the CVD method, and the amorphous Si layer 3 is formed at a temperature of 1100 to 1300 ° C. in the air or an oxidizing atmosphere by a laser annealing method, a heater annealing method, or the like. Crystallized at temperature. At this time, the orientation of the Si crystal thin film is determined by the grooves on the amorphous substrate 1, and the boundary surface of the Si crystal thin film divided by the two regions as shown in FIG. Grain boundaries 4 will be formed.

On the (001) Si grapho-epitaxial substrate 5 as described above, there is an SiO ₂ layer formed at the time of forming an amorphous Si layer.
The Z layer 6 and the Y ₂ O ₃ layer 7 are sequentially formed. First, YBCO
The YSZ layer 6 prevents mutual diffusion with the YSZ. ZrO ₂ +9 mol% Y ₂ O ₃ is used as the YSZ to form a film having a thickness of 80 nm, and then the Y ₂ O ₃ layer 7 is formed thereon. Was formed into a film having a thickness of 20 nm. Through the above steps, a Si grapho-epitaxial substrate 5 for a YBCO Josephson device and a YSZ layer 6 as a buffer layer were produced.

A laser abrasion method, a CVD method, and the like are formed on the Si grapho-epitaxial substrate 5 obtained by this method.
A YBCO thin film 8 having a thickness of 1 μm or less is formed by a sputtering method, a sputtering method, an evaporation method, or the like.
As shown in FIG. 2, 10 μm is sandwiched between the inclined grain boundaries 4 as shown in FIG.
An artificial grain boundary junction bridge-type Josephson device can be manufactured by patterning a bridge structure having a width of m × 10 μm and a length of 0.1 μm by laser etching. This device is essentially a mechanism for forming an artificial tilt grain boundary 4 of a YBCO thin film 8 using a bicrystal substrate made of SrTiO ₃ or Si produced by diffusion bonding of a single crystal block using a hot press. The same,
The reproducibility and the stability of the characteristics of the Josephson junction are expected to provide a good Josephson response to microwave irradiation, which is the same as that obtained in the case of a Si bicrystal substrate by diffusion bonding.

[0022]

According to the present invention, the Si grapho-epitaxial substrate manufactured according to the present invention is useful as a simple method for obtaining a substrate for artificially forming only one tilt grain boundary of a YBCO thin film at a predetermined position. By using a substrate, a Josephson junction device having good reproducibility and good control of characteristics can be manufactured.
In addition to the conventional SQUID application, the use of Si or SiO ₂ with low dielectric loss as the substrate material enables the provision of a Josephson device suitable for microwave applications.

[Brief description of the drawings]

FIG. 1 is a view showing a process of manufacturing a Si grapho epitaxial substrate according to the present invention. FIG. 2A is a diagram showing a state in which a groove is dug on an amorphous SiO ₂ substrate. b: A view showing a state in which a Si layer is formed on the amorphous SiO ₂ substrate indicated by a. c: A diagram showing a Si grapho-epitaxial substrate on which a tilt boundary at an angle θ is formed.

FIG. 2 is a schematic diagram of an artificial grain boundary Josephson device on a Si grapho epitaxial substrate according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Amorphous SiO ₂ substrate 2 Microcrystals oriented and vapor-deposited on an amorphous substrate 3 Amorphous Si layer 4 Artificial tilt grain boundary 5 Si substrate 6 YSZ layer 7 Y ₂ O ₃ layer 8 YBCO thin film

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-121220 (JP, A) JP-A-59-162195 (JP, A) JP-A-64-30117 (JP, A) 6873 (JP, A) Suzuki et. al. Superconductor Science and Technology, vol. 4, no. 9, September 1991, p. 479-481 (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 39/22-39/24 H01L 39/00

Claims (5)

(57) [Claims] 1. Intersecting at a predetermined angle θ on an amorphous substrate
An artificial grain boundary Josephson device using a Si grapho-epitaxial substrate manufactured by providing grooves in two directions . 2. The artificial grain according to claim 1, wherein a zirconia layer and a yttria layer are sequentially formed on a Si grapho- epitaxial substrate, and a superconducting thin film is formed thereon. World Josephson element. 3. The Si grapho-epitaxial substrate and the zirconia layer and the yttria layer and the superconducting layer thereon are each epitaxial or have a certain surface in relation to the in-plane orientation of the respective regions of the grapho-epitaxial substrate. 2. The artificial grain boundary Josephson device according to claim 1, wherein the Josephson device has an internal orientation. 4. A method for producing a Si bicrystal thin film using a grapho-epitaxial method, wherein grooves are formed on an amorphous substrate in two directions intersecting at a predetermined angle θ. 5. After forming the amorphous Si films in the amorphous SiO ₂ substrate, Si bicrystal method of manufacturing請<br/> Motomeko 4 of crystallizing heat treatment of amorphous Si.