JP3459983B2 - Superconducting tunnel junction detector - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting tunnel junction detector for detecting radiation, and more particularly to a superconducting junction detection device in which a superconducting junction element has a normal distribution shape. About the vessel. 2. Description of the Related Art A radiation detector using a superconducting tunnel junction has a high energy resolution (about several eV) and a wide measurement area (infrared, It has features such as visible, X-ray, α-ray, high energy particle beam) and high counting rate (about 10 kCPS). It is expected to be applied to analysis (fluorescence X-ray method, electron beam excitation X-ray method, ion beam impact X-ray method, etc.). In addition, since the detection of very weak photons and energy analysis can be performed at the same time, it is planned to be mounted on a satellite as a high-performance eye for solving the mysteries of the universe. This radiation detector is used in a cryogenic environment (about 1
K) and a superconductive junction (hereinafter referred to as STJ, Superconductive Tu).
nnel Junction) Josephson Current and Fisketep
(FiskeStep) It is necessary to operate under conditions where the current is suppressed. As one of the suppression methods, the present inventors have filed a patent application for Japanese Patent Application No. 11-219286 (Japanese Patent Application Laid-open No. 2000-15).
No. 0975) on the substrate on which the ground plane is provided.
We applied for an STJ element having a structure in which a superconducting coil of a microstrip line was directly coupled on a TJ. This structure has a feature that it can be easily integrated on one chip by the conventional superconducting integrated circuit technology. [0004] By using this element structure, the applied magnetic field can be concentrated between the microstrip line and the ground plane and around the microstrip line. Due to this magnetic field concentration effect, a Josephson current suppression effect equivalent to the applied magnetic field (10 mT or more) by a large external electromagnet used in the radiation detector is applied to the microstrip coil integrated on the chip by a small current of several tens mA. Can be obtained by injecting. In addition, since the applied magnetic field can be concentrated around the radiation detector, the radiation detector and the Josephson integrated circuit that processes the data of the radiation detection signal can be arranged at a relatively close position. It has become possible to construct a high-performance radiation detection system. [0005] However, even when the magnetic field applying method of the above-mentioned patent application is used, a STJ having a normal shape such as a square, a diamond, or a circle has a magnetic field applied with a Josephson current. Since the characteristics have a periodic structure (Fraunhofer pattern), it has been difficult to set a condition in which Josephson current is suppressed even when a magnetic field effect equivalent to 10 mT is obtained by a microstrip coil. That is, in the Fraunhofer pattern, the value of the externally applied magnetic field at which the Josephson current becomes 0 appears at a period that is an integral multiple of the flux quantum penetrating the STJ, but the range is extremely narrow. It was difficult to set this value. In addition, when the direction of the magnetic field and the current is not orthogonal to the junction surface of the STJ, the Josephson current does not become 0. Therefore, in the conventional device, the absolute value of the maximum Josephson current is reduced in the magnetic field region where the periodic fluctuation is small. Until then, an external magnetic field was applied. In order to further suppress the Josephson current,
It is considered that when the magnetic field application is further increased, the superconductivity is deteriorated by the magnetic field, and the radiation detection performance is reduced. Therefore, it is required that the intensity of the applied magnetic field used for suppressing the Josephson current be as small as possible. When the quality of the STJ, such as superconductivity of the material and flatness of the junction interface, is improved with the aim of improving the detection performance, the Q value in the STJ increases, and a resonance current called a Fiske step is increased. Many beaks occur (see the characteristic curve drawn by the dotted line in FIG. 2). In the detection of radiation by the STJ, electrons excited by the incidence of radiation are output as a signal to the outside through a tunnel barrier. Therefore, if a current caused by these electrons is trapped in a current step, the output voltage decreases, and furthermore, It has caused a situation of extinction, which has been a major factor in lowering radiation detection performance. This will be described with reference to FIG. Figure 3 shows the STJ
5 shows the current-voltage characteristics. The horizontal axis in the figure is ST
The voltage V between J and the vertical axis is the current I flowing through the bias line. In the drawing, L0 indicates a current-voltage characteristic curve when radiation does not enter the STJ, and Lm indicates a current-voltage characteristic when radiation enters the STJ. When radiation enters the STJ, the curve of the current-voltage characteristic of the STJ varies from L0 to Lm as shown in FIG. 3 in accordance with the amount of incident energy. When the bias current I and I _B of FIG flowing in STJ, the voltage between the STJ varies toward the Vm from the operating point voltage V0 of FIG. But,
Since there is a Fiske step between V0 and Vm, electrons are trapped in the Fiske step, the voltage does not increase any more, and the detected voltage signal lacks a portion of Vm 'or more as shown in the figure. Even if the step is passed, it is difficult to detect the energy accurately because the portion is lost and a part of the detected voltage waveform is lost. Since there are many Fiske steps as shown in FIG. 2, the detection signal is limited to the voltage range between the Fiske steps. In order to avoid the influence of the Fiske step, the drive method of the bias current of the STJ is set to the voltage type or the bias current value is precisely controlled. In order to solve the above problems, a superconducting tunnel junction element having a normal distribution shape is formed by being separated from a superconducting ground plane by an insulating film. Provided is a superconducting detector that magnetically couples to a superconducting microstrip line on the opposite side, thereby suppressing a Fiske step of the STJ and concentrating a magnetic field applied to the STJ. The left-right symmetry axis passing through the vertex of the normal distribution shape STJ is aligned with the applied magnetic field direction, and the relative
A bias current terminal is provided at each of the apexes, and
The bias current is set to the voltage at which the Fiske step appears.
Set to be larger . The inventors of the present invention have found that an STJ having a normal distribution shape does not exhibit a periodic change in the effect of suppressing a Josephson current due to an external magnetic field, which is seen in conventional devices having various shapes. Have just found out. That is, assuming that the axis perpendicular to the magnetic field applied to the STJ is the X axis, and the axis passing through the vertex of the normal distribution shape of the STJ in the direction of the applied magnetic field is the Y axis, the normal distribution shape f (x) becomes In the equation (1), the Josephson current of the normal distribution shape STJ (see below-described STJ in FIG. 1) in which the vertices are inverted and the bases are connected is expressed by the equation (2). In the equation, A represents the area of the junction, σ represents the dispersion coefficient of a normal distribution shape, d represents the thickness of magnetic penetration parallel to the tunnel barrier of the STJ, Jc represents the critical current density, and B represents the externally applied magnetic field. ## EQU1 ## When the integration of the equation (2) is executed, the current flowing through the STJ is obtained by the following equation (3). (3) where I
(0) = Jc × A, 1 = square root of A × σ. (Equation 2) FIG. 4 is a calculation example of the dependence of the STJ on the externally applied magnetic field with respect to the Josephson current. The solid line shows the characteristic curve of the normal distribution shape STJ of the present invention, and the dotted line shows the characteristic curve of the conventional rectangular shape STJ. The characteristic curve of the normal distribution shape STJ has such a characteristic that the periodic structure disappears. By utilizing this characteristic, the effect of suppressing the Josephson current can be stably increased up to about several times as compared with the related art, and the microstrip coil current can be reduced accordingly. As a result, the magnetic field around the STJ can be further reduced as compared with the above-described conventional element. This makes it possible to increase the compatibility between the radiation detector that operates while suppressing the Josephson current and the Josephson integrated circuit that performs data processing of the radiation detection signal using the Josephson current. A high-performance radiation detection system can be configured. Further, it has been confirmed by the present inventors that the Fiske step does not appear in the normal distribution shape STJ except for the first term. FIG. 5 is a current-voltage characteristic diagram schematically drawn on the same scale as FIG. 2 to compare with the conventional characteristics of FIG. 2 based on the experimental example. FIG. 5 shows that only the first Fiske step appears in a range smaller than about 0.1 mV. By applying a large bias current in a voltage range larger than this voltage range,
Radiation detection performance can be greatly improved. In the specification and the drawings, the same elements are denoted by the same reference numerals, and the description is omitted. An embodiment of the present invention will be described with reference to FIG. FIG. 1 schematically shows the detection principle of the radiation detector. In the figure, 1 is a STJ having a normal distribution shape, 2 and 3 are superconductor thin films, 4 is an insulator thin film, 5 is a bias line connected to the superconductor thin film 2, and 6 is connected to the superconductor thin film 3. A bias line, 7 indicates a superconducting microstrip coil for generating a magnetic field applied to suppress Josephson current, 8 indicates a superconducting ground plane, and 9 and 10 indicate voltage terminals for extracting a voltage between STJ1. When a current Im is applied to the microstrip coil 7 in the direction shown in the figure, the magnetic flux Φ induced by the coil disposed directly above the STJ is all between the microstrip coil 7 and the ground plane 8 and at the joint surface. Pass in a direction parallel to STJ1 has a normal distribution shape in which the vertices are inverted and the bases are connected, and the bias lines 5 and 6 are connected to the vertices of the shape on the right. The STJ 1 is arranged between the microstrip coil 7 and the ground plane 8 such that the axis of symmetry passing through the STJ junction surface and the vertex of the normal distribution shape is parallel to the applied magnetic field. FIG. 6 is a diagram for explaining the operation of the radiation detector using the normally distributed STJ of the present invention. The horizontal axis in the figure is the voltage V between the terminals 9 and 10 of the STJ, and the vertical axis is the current I flowing through the bias lines 5 and 6. In the figure, L0 indicates that the radiation is S
A current-voltage characteristic curve when light does not enter the TJ, and Lm indicates a current-voltage characteristic when radiation enters the STJ. A bias current (in the example of FIG. 4 about 0.1 mV) emerging voltage Fi scale steps of paragraph is set to a bias current I _B from. When radiation is incident, no Fiske step other than the first term appears, so that the voltage between the terminals 9 and 10 of the STJ changes from V0 to Vm according to the amount of incident energy. In the figure, the detection output signal (the energy amount is the integral value of the energy amount) due to the voltage change is a detection signal corresponding to the radiation amount. As described above, the range of the voltage change for obtaining the detection output signal can use the range between the Fiske step of the first term and the gap voltage Vg of the STJ, which is the most part of the characteristic range available for detection. It is. The normal distribution shape STJ of the present invention can be alternately fitted and used as a cell of a radiation detector. An array detector that detects a radiation image by reading output signals of these cells in parallel or in series can be configured. According to the present invention, the magnetic field applied to the STJ for suppressing the Josephson current can be minimized and the influence of the Fiske step can be easily avoided.
There is no need to change the drive method of the bias current of the STJ to a voltage type or to precisely control the bias current value, so that the radiation detection performance can be greatly improved. In addition, compatibility between the radiation detector and a Josephson integrated circuit that performs data processing of radiation detection signals is increased, and a high-performance radiation detection system can be configured. As a result, it is possible to drastically improve both the detection performance and the stable operation of the radiation detector using the STJ, so that a high-performance radiation detector can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically showing a superconducting junction detector of the present invention. FIG. 2 is a diagram showing a Fiske step appearing in a conventional STJ. FIG. 3 is a diagram illustrating the operation of a conventional superconducting junction detector. FIG. 4 is a diagram illustrating suppression of a Fraunhofer pattern. FIG. 5 is a diagram illustrating suppression of a Fiske step according to the present invention. FIG. 6 is a diagram illustrating the operation of the superconducting junction detector of the present invention. [Description of Signs] 1 Normal distribution shape STJ 2,3 Superconducting thin film 4 Insulating thin film 5,6 Bias line 7 Microstrip coil 8 Ground plane Φ Applied magnetic flux Vg Gap voltage

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Susumu Takada 255, Shimo-Okubo, Urawa City, Saitama Prefecture (72) Inventor Hiroaki Mesuren 255, Shimo-Okubo, Urawa City, Saitama Prefecture (72) Inventor, Department of Engineering, Saitama University Person Takeshi Iizuka 255, Shimo-Okubo, Urawa-shi, Saitama Prefecture (72) Within the Faculty of Engineering, Saitama University (72) Katsuya Kikuchi 255, Shimo-Okubo, Urawa-shi, Saitama Prefecture Within the Faculty of Engineering, Saitama University (56) References Katsuya, Hiroaki Akira, Takeshi Iizuka, Susumu Takada, Fabrication of normally distributed superconducting tunnel junction, Proc. Of the 46th Joint Lecture Meeting of the Japan Society of Applied Physics, Japan Society of Applied Physics, 1999 March 28, 1st volume, p. 278, 28a-ZG-7 Katsuya Kikuchi, Hiroaki Akeren, Takeshi Iizuka, Susumu Takada, Superconducting Tunnel Junctions with Normally Distributed Shapes, Proc. Of the 60th Annual Meeting of the Japan Society of Applied Physics, Japan, The Physical Society of Japan, September 1, 1999, first volume, p. 188, 3a-ZV-8 Katsuya Kikuchi, Hiroaki Akira, Takeshi Iizuka, Susumu Takada, Applied magnetic field angle dependence of the Fraunhofer pattern of a normally distributed STJ device, Proceedings of the 61st Annual Conference of the Japan Society of Applied Physics, Japan, Japan Society of Applied Physics, September 3, 2000, 1st volume, p. 202, 3a-ZM-3 (58) Fields investigated (Int. Cl. ⁷ , DB name) G01T 1/00-7/12 H01L 31/00 H01L 39/22

Claims (1)

(57) [Claims 1] The X-axis value is defined as the side corresponding to the X-axis value as the base.
Defined by the normal distribution function in the Y-axis direction generated by
Defined by the normal distribution function
Vertices are inverted, and the bottom side is connected.
A normally distributed superconducting tunnel junction element to be formed, and formed by being separated from the superconducting tunnel junction element by an insulating film.
And the ground plane, the superconducting tunnel contact on the opposite side to the superconducting ground plane
Superconducting microstrip line magnetically coupled to a composite device
A superconducting image detector having the following two vertices of the superconducting tunnel junction element:
And the applied magnetic field, and a bias current terminal is provided at each of the opposing vertices.
And make the bias current larger than the voltage at which the Fiske step appears.
Superconducting image characterized by setting
Di detector .