JPH08213662A - Vortex flow transistor and its manufacture - Google Patents

Abstract

PURPOSE: To improve characteristics of a vortex flow transistor, by applying the part where a superconducting thin film is thin as compared with other parts, to a channel through which vortexes transit, and arranging a superconducting control line for controlling the number of vortexes, in the vicinity of the channel. CONSTITUTION: On a (100) face substrate 1 of Si, TiO3 , a first superconducting thin film 2 of Bi 2212 is deposited to be 200nm thick, at a substrate temperature 650 deg.C by a high frequency sputtering method. A superconducting wiring pattern is obtained by photolithography and ion milling. On a patterned superconducting wiring, Bi 2212 is grown to be 50nm under the same condition as the Bi 2212 of the first superconducting thin film 2. A second superconducting thin film 3 is grown in a pattern having a channel part which is nearly identical with the pattern forming the first superconducting thin film 2, and heat-treated at 600 deg.C in oxygen for 8 hours. Thereby a vortex flow transistor is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vortex flow transistor which is a superconducting circuit element such as a superconducting computer, a superconducting digital circuit or a superconducting quantum device, and a manufacturing method thereof.

[0002]

2. Description of the Related Art A vortex flow transistor or magnetic flux quantum flow device is placed in a lower critical magnetic field of the II
The quantized magnetic flux (flux quantum or vortex) existing in the seed superconductor or superconducting thin film is accelerated in the bias current and moves in the channel. As a result, according to Josephson's relation, the moving vortex It is an amplification type superconducting three-terminal device that utilizes the fact that a voltage proportional to the speed is generated. The output voltage is proportional to the number of vortexes, that is, the externally applied magnetic field, and thus to the current of the control line placed near the channel. The vortex flow transistor is a current input voltage output type transistor.

Conventionally, there has been an element called a Josephson vortex flow transistor. This device utilized the running of vortex existing in the Josephson junction line. This is called the Josephson vortex in distinction from the former vortex, and the vortex used in the present invention is called Abrikosov vortex.
In addition, the output of the above element is as small as several 100 μV, and there is a problem in impedance matching with a semiconductor circuit or the like.

A vortex flow transistor utilizing Abrikosov vortex in a high-temperature superconductor has an advantage that the output voltage can be as large as several tens of mV, but has a narrow proportional region with respect to the control current of the output voltage, and controls rising. There were drawbacks such as large current. Furthermore, since the channel was formed by chemically or physically removing a part of the upper part of the superconducting thin film, the film thickness of the channel part varies from device to device, and the reproducibility of characteristics is poor. There was a flaw.

[0005]

In order to improve the characteristics of the vortex flow transistor, a wide linear region in the relationship between the control current and the output voltage and a large output voltage with a small control current ( Large gain)
In addition, the control current when the output voltage rises is required to be as small as possible. For this purpose, a channel format with good vortex flow characteristics is essential. In order to improve the vortex flow characteristics, it is necessary that the vortex viscosity coefficient η is small and that the vortex can easily penetrate into the channel portion even in a weak magnetic field.
The upper critical magnetic field is H _C2 , the magnetic flux quantum is Φ ₀ , and the normal resistance is ρ _n
Then, η = Φ ₀ H _C2 / ρ _n c ² approximately, so H
_It suffices that _C2 is small and ρ _n is large, and since H _C2 and ρ _n are values peculiar to the substance, the characteristics are roughly determined by the superconductor used if the structures are the same. However, in the case of a very thin thin film, H _C2 decreases and ρ _n equivalently increases as the film thickness decreases. Therefore, in order to form a channel having good vortex flow characteristics, the thickness of the channel portion should be made extremely thin. Further, if the channel portion is formed of a superconductor thin film having a very small thickness and a small H _C2 and a large ρ _n , it is expected that the flow characteristics of the channel will be further improved. It

The channel of the conventional vortex flow transistor forms continuous holes in the superconducting thin film,
Alternatively, a groove is formed in the thin film by a chemical method or a physical method such as ion milling, so that the superconducting characteristic of the channel portion is deteriorated, the channel thickness is different for each element, or the characteristic is different. There was a drawback.

The present invention provides a vortex transistor composed of a channel having excellent vortex flow characteristics as described above, and a method of manufacturing the channel with good controllability, thereby improving the characteristics of the vortex flow transistor. With the goal.

[0008]

[Means for Solving the Problems] The above-mentioned object is to use a portion of the heat-conducting thin film having a thin thickness as compared with other portions, as a channel through which vortex (flux quantum) travels, and to determine the number of vortexes. Achieved by placing a controlling superconducting control line near the channel, and
This is achieved by the fact that the portion other than the above-mentioned channel is composed of two layers of a superconductor forming the channel and a superconductor other than that.

That is, making the channel portion of the vortex flow transistor thinner than other portions,
And, instead of a method such as chemical etching or ion milling, which has been conventionally used as a channel forming method, or focused ion beam milling, a thin film of a superconductor excellent in vortex flow characteristics of the same kind or different kind in the above channel part, A vortex flow channel is formed by newly depositing in another process to form a vortex flow transistor with excellent characteristics.

[0010]

[Function] In order to improve the characteristics of the vortex transistor, it is sufficient to use a superconductor having a small H _{C2 and a} large ρ _n as described above, but if the H _C2 is too small, the vortex interval becomes large, and the vortex travels. A large output cannot be obtained because the number of On the other hand, in a high-temperature superconductor, H _C2 is not small, but ρ _n is large, so that η is small and it is excellent as a superconductor for forming a channel. In particular, Bi ₂ Sr ₂ CaCu ₂ O ₈ or (Bi _1-x P
b _x ) ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ (x <0.35) (hereinafter B
Since the i-type superconductor) has a large two-dimensionality, the flux lattice is melted in a wide temperature range and the effect of collective pinning is lost, so that the vortex travel becomes easy and it is a superconductor suitable for a channel.

A superconductor excellent for forming the above channel can be used in the same manner as the above superconductor even if Bi, Sr, and Ca each change about 10%.

On the other hand, when the thickness of the channel is made thin, the demagnetizing factor becomes large and the apparent lower critical magnetic field H _C1 becomes small. Therefore, with a weak externally applied magnetic field, the vortex can be introduced into the channel portion with a small control current. A Bi-based superconductor having a layered structure and high anisotropy, which has a particularly large anisotropy, has such a property even if H _C1 is small. Therefore, the same effect can be expected even if it is not extremely thin. . Therefore, there is an advantage that it is easy to process.

In the method of cutting the upper part of the thin film when reducing the thickness of the channel part, it is difficult to control the cutting depth, and the part left after the cutting is damaged and the superconducting property is deteriorated. Also, if there is a variation in the thickness of the thin film, there is a variation in the thickness of the channel, and there are many drawbacks such that the reproducibility of the channel characteristics cannot be obtained. On the other hand, leaving the portion other than the channel (2 in FIG. 1), first removing all the portions other than the above including the channel, and then again having a thickness sufficient for the thickness of the channel on the whole superconductivity. After depositing the thin film (3 in FIG. 1), the entire element including the channel (6 in FIG. 1) is left and the other part is removed, so that the channel part is not exposed to damages caused by the process such as etching. Moreover, by controlling the film thickness when depositing the second thin film, it is possible to always obtain a constant thickness. Also, the second
It is also possible to use a different type of superconducting film from the first superconducting film. In this way, a channel having excellent vortex flow characteristics can be formed, and the characteristics of the vortex flow transistor can be improved.

When depositing the second superconducting thin film, the first superconducting thin film is exposed to vacuum at a high temperature, so that the surface characteristics are deteriorated to some extent due to oxygen deficiency.
Therefore, the electric contact with the second superconductor may be insufficient. In such a case, by performing heat treatment in an oxygen atmosphere after depositing the second thin film, the deficient oxygen is compensated, the interface characteristics are restored, and good electrical contact can be obtained. The heat treatment after depositing the second thin film is performed in an oxygen atmosphere at 500 ° C. or higher. In this case,
Even if it heat-processes at the temperature below 500 degreeC, the remarkable effect cannot be acquired with respect to the recovery of an oxygen deficiency, Therefore Therefore,
The interface resistance cannot be reduced. By the method of the present invention described above, it is possible to solve various problems of the vortex flow transistor, which has been a problem in the past.

[0015]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a perspective view showing the concept of an embodiment of a vortex flow transistor according to the present invention.

First Embodiment _On a (100) plane substrate (hereinafter referred to as STO substrate) 1 of SrTiO ₃ , Bi ₂ Sr ₂ CaCu ₂ O ₈ (hereinafter, Bi 22) was formed by a high frequency sputtering method at a substrate temperature of 650 ° C.
12) was deposited to a thickness of 200 nm. After that, by photolithography technology and ion milling technology,
The superconducting wiring pattern shown in No. 2 was obtained. The superconducting wiring pattern does not include a channel portion. The length of the channel, that is, the width of the superconducting wiring was 50 μm, and the width of the channel was 5 μm. Then, the Bi2212 having a thickness of 50 n is grown under the same conditions as when the first layer of Bi2212 was grown on the thus obtained patterned superconducting wiring.
It was made to grow. After that, the second layer was processed into a pattern having almost the same channel portion as the pattern on which the first layer was formed to obtain a vortex flow transistor as shown in FIG.

In the vortex flow transistor obtained as described above, the output voltage rises with a small control current, and due to the relation of the output voltage with respect to the bias current and the control current, the linear region is remarkably larger than that of the conventional vortex flow transistor. Expanded.

In FIG. 1, after the second-layer superconducting thin film 3 is grown, it is subjected to a heat treatment in oxygen at 600 ° C. for 8 hours, and then the vortex flow transistor obtained through the same steps as above is 0 The magnitude of the bias current, which is output by the control current, doubled. This is because the interface between the first layer and the second layer is improved to improve the electrical contact, which is advantageous when the vortex flow transistor is miniaturized and integrated.

Note that Bi ₂ Sr ₂ C is used instead of Bi2212.
aCu ₂ O ₈ and (Bi _0.8 Pb _0.2 ) ₂ Sr ₂ Ca ₂ Cu ₃
Similar effects were obtained when O ₁₀ was used.

Second Embodiment YBa ₂ Cu ₃ O ₇ (hereinafter referred to as YBCO) is used for the STO substrate at a substrate temperature of 700 ° C. by a high frequency sputtering method.
Was grown to a thickness of 200 nm. Then, the patterned superconducting wiring 2 was obtained by the same method as in the first embodiment.
Next, Bi2212 was grown to a thickness of 50 nm on the superconducting wiring 2 at a substrate temperature of 650 ° C. Next, heat treatment was performed in oxygen at 600 ° C. for 8 hours, and the vortex flow transistor as shown in FIG. 1 was obtained through the same steps as in the first embodiment.

In the vortex flow transistor obtained as described above, as in the first embodiment, the output voltage rises with a small control current, and the linear region is the same as the conventional one because of the relationship between the bias current and the output voltage with respect to the control current. Significantly expanded compared to the vortex flow transistor.

[0022]

As described above, in the vortex flow transistor and the method of manufacturing the same according to the present invention, a portion of the superconducting thin film having a thin film thickness as compared with other portions is used as a channel through which the vortex (flux quantum) travels. And a superconducting control line for controlling the number of the vortex is arranged near the channel, and the manufacturing of the vortex flow transistor removes the first superconducting thin film except the portion other than the channel. And a second step of depositing a second superconducting thin film of the same kind or of a different kind on the remaining pattern and substrate, and a second step leaving a portion including a channel. By including the third step of removing the superconducting thin film, the channel can be manufactured with good controllability, and the linear region is remarkably expanded in the relationship between the control current and the output voltage. Can, raises the output voltage with a small control current, it is possible to obtain a vortex flow transistor having a large output voltage is obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing the concept of an embodiment of a vortex flow transistor according to the present invention.

[Explanation of symbols]

 2 First superconducting thin film 3 Second superconducting thin film 5 Superconducting control line 6 Channel

Front page continued (72) Inventor ▲ Tsuru ▼ Koji 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Inventor Shubo Kubo 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corp. (72) Inventor Keiichi Tanabe 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corp.

Claims (5)

[Claims] 1. A superconducting thin film in which a thin film is thinner than other parts is used as a channel through which a vortex (flux quantum) travels, and a superconducting control line for controlling the number of the vortex is provided. A vortex flow transistor placed near the channel. 2. The vortex flow transistor according to claim 1, wherein the superconductor portion other than the channel is
A vortex flow transistor comprising two layers of a superconductor forming a channel and another superconductor. 3. The vortex flow transistor according to claim 1, wherein the channel-forming superconductor is Bi ₂ Sr ₂ CaCu ₂ O ₈ or (Bi _1-x).
A vortex flow transistor comprising Pb _x ) ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ (x <0.35). 4. A vortex flow transistor comprising a superconducting thin film having a channel through which a vortex travels, and a superconducting control line for controlling the number of vortex travels, the first ultra-thin film except for the above-mentioned channel. A first step of patterning and removing the conductive thin film, a second step of depositing a second superconducting thin film of the same type or a different type on the remaining pattern and substrate, and a super step including a channel. And a third step of removing the superconducting thin film deposited in the second step while leaving the conductor portion, and a method of manufacturing a vortex flow transistor. 5. The method of manufacturing a vortex flow transistor according to claim 4, wherein after depositing the second superconducting thin film in the second step, the second superconducting thin film is subjected to 500 in an oxidizing atmosphere.
A method for manufacturing a vortex flow transistor, which comprises performing heat treatment at a temperature of ℃ or higher.