RADIATION DETECTION ELEMENT, RADIATION DETECTION APPARATUS, X-RAY CT APPARATUS, AND MANUFACTURING METHOD OF RADIATION DETECTION ELEMENT

A radiation detection element according to the present invention includes: a single-crystal semiconductor substrate configured to convert incident radiation into an electric charge; a first cathode electrode provided on a first main surface of the single-crystal semiconductor substrate, the first cathode electrode having a first thickness; a second cathode electrode provided so as to face a side surface of the single-crystal semiconductor substrate, the second cathode electrode having a second thickness that is smaller than the first thickness; and an anode electrode provided on a second main surface of the single-crystal semiconductor substrate, the second main surface being on an opposite side of the first main surface.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a radiation detection element, a radiation detection apparatus, an X-ray CT apparatus, and a manufacturing method of the radiation detection element.

Description of the Related Art

An X-ray detection method in which X-rays are indirectly detected has been proposed. In this method, X-rays are incident on a scintillator (phosphor) to be converted into visible light, and the converted visible light is incident on a single-crystal semiconductor substrate so as to detect the X-rays. In addition to the above method, there has been proposed another X-ray detection method in which X-rays are directly incident on a single-crystal semiconductor substrate to be detected. In the former method, the sensitivity of the X-ray detection is degraded due to the conversion from X-rays to visible light, whereas in the latter method, since such conversion is not performed, highly sensitive X-ray detection can be expected. Hereinafter, the latter method will be referred to as a “direct detection type”.

A direct-detection-type radiation detection element is provided with a cathode electrode on a first main surface of a single-crystal semiconductor substrate and an anode electrode on a second main surface (a surface on an opposite side of the first main surface) of the single-crystal semiconductor substrate. The single-crystal semiconductor substrate converts incident radiation (such as X-rays and gamma rays) into an electric charge. The electric charge generated in the single-crystal semiconductor substrate can be collected by applying a voltage between the cathode electrode and the anode electrode to form an electric field. When the electric field is formed, since the electric field is weakened at a side-surface portion of the single-crystal semiconductor substrate, charge collection efficiency decreases (charge loss increases).

U.S. Patent Application Publication No. 2007/0194243 (Specification) discloses a technique for improving charge collection efficiency by providing a cathode electrode also on a side surface of a s single-crystal semiconductor substrate.

SUMMARY OF THE INVENTION

A radiation detection element according to the present invention includes: a single-crystal semiconductor substrate configured to convert incident radiation into an electric charge; a first cathode electrode provided on a first main surface of the single-crystal semiconductor substrate, the first cathode electrode having a first thickness; a second cathode electrode provided so as to face a side surface of the single-crystal semiconductor substrate, the second cathode electrode having a second thickness that is smaller than the first thickness; and an anode electrode provided on a second main surface of the single-crystal semiconductor substrate, the second main surface being on an opposite side of the first main surface.

A first radiation detection apparatus according to the present invention includes the radiation detection element according to the present invention, the radiation detection element being provided in plurality and arranged in a planar manner. A second radiation detection apparatus according to the present invention includes: a plurality of radiation detection elements that include the radiation detection element according to the present invention and a radiation detection element not having the second cathode electrode, the plurality of radiation detection elements being arranged without intervals in a first direction and in a second direction perpendicular to the first direction, wherein, among the plurality of radiation detection elements, the radiation detection element according to the present invention is arranged as an outermost radiation detection element, and the radiation detection element according to the present invention does not have the second cathode electrode on a side surface adjacent to another radiation detection element among side surfaces of the single-crystal semiconductor substrate and has the second cathode electrode on a side surface not adjacent to another radiation detection element among the side surfaces of the single-crystal semiconductor substrate.

A manufacturing method of a radiation detection element according to the present invention includes: a step of forming a first cathode electrode on a first main surface of a single-crystal semiconductor substrate that converts incident radiation into an electric charge, the first cathode electrode having a first thickness; a step of forming a second cathode electrode so as to face a side surface of the single-crystal semiconductor substrate, the second cathode electrode having a second thickness that is smaller than the first thickness; and a step of forming an anode electrode on a second main surface of the single-crystal semiconductor substrate, the second main surface being on an opposite side of the first main surface.

An X-ray computed tomography apparatus according to the present invention includes: an X-ray generation unit; the radiation detection apparatus according to the present invention configured to detect X-rays emitted from the X-ray generation unit; and a signal processing unit configured to process a signal output from the radiation detection apparatus.

DESCRIPTION OF THE EMBODIMENTS

If the technique disclosed in U.S. Patent Application Publication No. 2007/0194243 (Specification) is used, the size of the radiation detection element increases, which results in decreasing resolving power (resolution) in a case where a plurality of radiation detection elements are arranged (tiled) (decreasing the number of radiation detection elements that can be arranged in a certain area).

The present disclose provides a technique for improving resolution in a case where a plurality of radiation detection elements are arranged, while preventing a decrease in charge collection efficiency.

Hereinafter, an embodiment of the present invention will be described. A radiation detection element according to the present embodiment is an element (chip) that employs a method in which radiation such as X-rays or gamma rays is directly incident on a single-crystal semiconductor substrate to be detected. Hereinafter, this method will be referred to as a “direct detection type”. The single-crystal semiconductor substrate of the direct-detection-type radiation detection element is formed of, for example, a single crystal of a cadmium zinc telluride (CdZnTe: Cd1-xZnxTe (x is, for example, 0.5 or less)) semiconductor, which is an alloy of cadmium telluride CdTe and zinc telluride ZnTe. A Cd1-xZnxTe semiconductor is also referred to as CZT. In the present embodiment, CZT will be mainly described. However, the present invention is not limited to this embodiment and can be applied to any single-crystal semiconductor substrate capable of directly detecting X-rays. For example, the present invention can be applied to a single-crystal semiconductor substrate that includes cadmium telluride CdTe, cadmium tungstate CdWO4, sodium iodide Nal, cesium iodide CsI, or the like.

Radiation Detection Element

FIG.1Ais a sectional view illustrating a configuration example of a direct-detection-type radiation detection element, taken along a plane perpendicular to a main surface (a first main surface or a second main surface, which will be described below) of a single-crystal semiconductor substrate. As illustrated inFIG.1A, the direct-detection-type radiation detection element is provided with a cathode electrode2on a first main surface of a single-crystal semiconductor substrate1and an anode electrode3on a second main surface (a surface on an opposite side of the first main surface) of the single-crystal semiconductor substrate1. The anode electrode3is provided for each pixel, and inFIG.1A, a plurality of anode electrodes3are provided in a single radiation detection element so that a plurality of pixels are arranged in the single radiation detection element. The single-crystal semiconductor substrate1converts incident radiation into an electric charge. The electric charge generated in the single-crystal semiconductor substrate1can be collected by applying a voltage between the cathode electrode2and the anode electrode3to form an electric field. When the electric field is formed, since the electric field is weakened at a side-surface portion of the single-crystal semiconductor substrate1, charge collection efficiency decreases (charge loss increases).

In view of the above-described problem, a technique for improving the charge collection efficiency has been proposed. In this technique, as illustrated inFIG.1B, a cathode electrode4having the same thickness as that of the cathode electrode2on the first main surface is provided on an individual side surface of the single-crystal semiconductor substrate1. The potential of the cathode electrode4is set to, for example, the same potential as that of the cathode electrode2.

However, the configuration illustrated inFIG.1Bincreases the size of the radiation detection element, which results in decreasing resolving power (resolution) in a case where a plurality of radiation detection elements are arranged (tiled) (decreasing the number of radiation detection elements that can be arranged in a certain area).

Therefore, in the present embodiment, as illustrated inFIG.1C, the cathode electrode4on the side surface is made thinner than the cathode electrode2on the first main surface. With this configuration, the increase in size of the radiation detection element can be prevented. Consequently, it is possible to increase the resolution in a case where a plurality of radiation detection elements are arranged, while preventing a decrease in charge collection efficiency.

Manufacturing Method of Radiation Detection Element

The radiation detection element according to the present embodiment is produced by a manufacturing method including the following three steps, for example. In the first step, the cathode electrode2having a first thickness is formed on the first main surface of the single-crystal semiconductor substrate1. In the second step, the cathode electrode4having a second thickness is formed on a side surface of the single-crystal semiconductor substrate1. The second thickness is at least smaller than the first thickness. In the third step, the anode electrode3is formed on the second main surface of the single-crystal semiconductor substrate1.

Each of the above-described three steps includes, for example, a step of forming an electrode layer on the single-crystal semiconductor substrate1by sputtering, a step of masking the electrode layer with a resist, a step of etching the electrode layer, and a step of removing the resist. Nickel, gold, platinum, indium, nickel/gold alloy, titanium/tungsten alloy, platinum/gold alloy, and the like can be used for the various electrodes. The thickness of each electrode is determined by, for example, the etching time and the etchant. For example, the cathode electrode2on the first main surface is formed to be approximately 20 nm to 250 nm thick, and the cathode electrode4on the side surface is formed to be thinner than the cathode electrode2. The anode electrode3on the second main surface is formed to have the same thickness as that of the cathode electrode2on the first main surface, for example.

Modifications of Radiation Detection Element

The configuration of the radiation detection element according to the present embodiment may be modified from the configuration illustrated inFIG.1Cto configurations illustrated inFIGS.2A to2C. In the configurations illustrated inFIGS.2A to2C, too, the cathode electrode4on the side surface is thinner than the cathode electrode2on the first main surface.

InFIG.2A, the cathode electrode2on the first main surface and the cathode electrode4on the side surface are integrated. With this configuration, the cathode electrode4faces the side surface of the single-crystal semiconductor substrate1over a wider area than that in the configuration illustrated inFIG.1C. Consequently, the configuration illustrated inFIG.2Acan further prevent the decrease in charge collection efficiency when compared to the configuration illustrated inFIG.1C. The configuration illustrate inFIG.2Acan also be regarded as a configuration in which the cathode electrode2on the first surface is extended to the side surface of the single-crystal semiconductor substrate1.

InFIG.2B, an insulating layer5is provided between the side surface of the single-crystal semiconductor substrate1and the cathode electrode4. The other portions in the configuration illustrated inFIG.2Bare the same as those illustrated inFIG.2A. With this configuration, the withstand voltage between the anode electrode3on the second main surface and the cathode electrode4on the side surface can be improved to reduce leakage current. Consequently, the configuration illustrated inFIG.2Bcan further prevent the decrease in charge collection efficiency when compared to the configuration illustrated inFIG.2A. Alternatively, the insulating layer5may be added to the configuration illustrated inFIG.1C.

InFIG.2C, while a sum of the thickness of the cathode electrode4and the thickness of the insulating layer5remains the same as the configuration (a predetermined value) inFIG.2B, the cathode electrode4is made thinner than the configuration inFIG.2B, and the insulating layer5is made thicker than the configuration inFIG.2B. For example, the insulating layer5is made thicker than the cathode electrode4. The other portions in the configuration illustrated inFIG.2Care the same as those illustrated inFIG.2B. With this configuration, the withstand voltage between the anode electrode3on the second main surface and the cathode electrode4on the side surface can be further improved to further reduce leakage current. Consequently, the configuration illustrated inFIG.2Ccan further prevent the decrease in charge collection efficiency when compared to the configuration illustrated inFIG.2B.

Radiation Detection Apparatus

When a radiation detection apparatus is configured by using the radiation detection element according to the present embodiment, the radiation detection element provided in plurality are arranged in a planar manner (a plurality of radiation detection elements are tiled).FIG.3Ais a plan view illustrating an example of a radiation detection apparatus (a plurality of tiled radiation detection elements) viewed from a direction perpendicular to the main surfaces of the plurality of radiation detection elements. InFIG.3A, only the single-crystal semiconductor substrates1and the cathode electrodes4on the side surfaces are illustrated. The arrangement pattern of the plurality of radiation detection elements is not particularly limited. InFIG.3A, the plurality of radiation detection elements are arranged in a matrix (in a row direction and a column direction). InFIG.3A, the plurality of radiation detection elements are arranged at intervals in both the row direction and the column direction.

Modifications of Tiling

The configuration of the radiation detection apparatus according to the present embodiment may be modified from the configuration illustrated inFIG.3Ato configurations illustrated inFIGS.3B to3G.

InFIG.3B, the plurality of radiation detection elements are arranged without intervals in the row direction and arranged at intervals in the column direction. InFIG.3C, the plurality of radiation detection elements are arranged without intervals in the column direction and arranged at intervals in the row direction. With these configurations, it is possible to further increase the resolution in the case of tiling, when compared to the configuration (configuration illustrated inFIG.3A) in which the plurality of radiation detection elements are arranged at intervals from each other.

InFIG.3D, the plurality of radiation detection elements are arranged without intervals in both the row direction and the column direction. With this configuration, it is possible to further increase the resolution in the case of tiling, when compared to the configurations illustrated inFIGS.3B and3C.

The single-crystal semiconductor substrate1has two first side surfaces, which are side surfaces perpendicular to the row direction (side surfaces parallel to the column direction) and two second side surfaces, which are side surfaces perpendicular to the column direction (side surfaces parallel to the row direction). InFIG.3E, while the cathode electrodes4are provided on the second side surfaces, the cathode electrodes4are not provided on the first side surfaces. The other portions in the configuration illustrated inFIG.3Eare the same as those illustrated inFIG.3B. By not providing the cathode electrodes4on the first side surfaces and having no (zero) intervals between the plurality of radiation detection elements in the row direction, the configuration illustrated inFIG.3Ecan further increase the resolution in the case of tiling, when compared to the configuration illustrated inFIG.3B. InFIG.3E, the cathode electrodes4are not provided on the outermost first side surfaces, and this results in weakening the electric fields of these portions. Thus, the cathode electrodes4may be provided on the outermost first side surfaces. The electric fields of the other first-side-surface portions are not easily weakened even if the cathode electrodes4are not provided thereto.

InFIG.3F, while the cathode electrodes4are provided on the first side surfaces, the cathode electrodes4are not provided on the second side surfaces. The other portions in the configuration illustrated inFIG.3Fare the same as those illustrated inFIG.3C. By not providing the cathode electrodes4on the second side surfaces and having no (zero) intervals between the plurality of radiation detection elements in the column direction, the configuration illustrated inFIG.3Fcan further increase the resolution in the case of tiling, when compared to the configuration illustrated inFIG.3C. InFIG.3F, the cathode electrodes4are not provided on the outermost second side surfaces, and this results in weakening the electric fields of these portions. Thus, the cathode electrodes4may be provided on the outermost second side surfaces. The electric fields of the other second-side-surface portions are not easily weakened even if the cathode electrodes4are not provided thereto.

InFIG.3G, no cathode electrodes4are basically provided on the side surfaces of the single-crystal semiconductor substrate1. The other portions in the configuration illustrated inFIG.3Gare the same as those illustrated inFIG.3D. By not providing the cathode electrodes4on the side surfaces of the single-crystal semiconductor substrate1and having no (zero) intervals between the plurality of radiation detection elements in both the row direction and the column direction, it is possible to further increase the resolution in the case of tiling, when compared to the configuration illustrated inFIG.3D. However, if the cathode electrodes4are not provided on any of the side surfaces, the electric fields are weakened at the portions of the outermost side surfaces (side surfaces not adjacent to other radiation detection elements) among the plurality of side surfaces of the plurality of single-crystal semiconductor substrates1. Thus, inFIG.3G, the cathode electrodes4are provided on the outermost side surfaces to prevent the electric fields from being weakened (to prevent the decrease in charge collection efficiency). The cathode electrodes4may be provided on all the outermost side surfaces or may be provided on only a part of the outermost side surfaces.

The radiation detection apparatuses according to the present embodiment can be applied to a detector of an X-ray CT apparatus.FIG.4is a block diagram illustrating an X-ray CT apparatus according to the present embodiment. An X-ray CT apparatus30according to the present embodiment includes an X-ray generation unit310, a wedge311, a collimator312, an X-ray detection unit320, a top plate330, a rotating frame340, a high-voltage generation apparatus350, a data acquisition system (DAS)351, a signal processing unit352, a display unit353, and a control unit354.

The X-ray generation unit310includes, for example, a vacuum tube that generates X-rays. A high voltage and a filament current are supplied from the high-voltage generation apparatus350to the vacuum tube of the X-ray generation unit310. X-rays are generated by irradiation of thermal electrons from a cathode (filament) toward an anode (target).

The wedge311is a filter that adjusts the amount of X-rays emitted from the X-ray generation unit310. The wedge311attenuates the amount of X-rays so that the X-rays emitted from the X-ray generation unit310to an object have a predetermined distribution. The collimator312includes a lead plate or the like that narrows the irradiation range of the X-rays that have passed through the wedge311. The X-rays generated by the X-ray generation unit310are shaped into a cone beam shape via the collimator312and reach the object on the top plate330.

The X-ray detection unit320is configured using the radiation detection apparatus according to the present embodiment. The X-ray detection unit320detects X-rays that have been emitted from the X-ray generation unit310and passed through the object and outputs a signal corresponding to the amount of the X-rays to the DAS351.

The rotating frame340has an annular shape and is configured to be rotatable. The X-ray generation unit310(the wedge311, the collimator312) and the X-ray detection unit320are arranged to face each other inside the rotating frame340. The X-ray generation unit310and the X-ray detection unit320are rotatable together with the rotating frame340.

The high-voltage generation apparatus350includes a booster circuit and outputs a high voltage to the X-ray generation unit310. The DAS351includes an amplifier circuit and an A/D conversion circuit and outputs a signal from the X-ray detection unit320to the signal processing unit352as a digital signal.

The signal processing unit352includes a central processing unit (CPU), a read-only memory (ROM), and a random access memory (RAM) and is capable of performing image processing and the like on digital data. The display unit353includes a flat display device or the like and can display an X-ray image. The control unit354includes a CPU, a ROM, a RAM, and the like and controls the entire operation of the X-ray CT apparatus30.

The embodiments (including the modifications) described above are merely examples, and configurations obtained by appropriately modifying or changing the above-described configurations within the scope of the gist of the present invention are also included in the present invention. Configurations obtained by appropriately combining the above-described configurations are also included in the present invention.

According to the present embodiment, it is possible to increase the resolution in a case where a plurality of radiation detection elements are arranged, while preventing a decrease in charge collection efficiency.

This application claims the benefit of Japanese Patent Application No. 2023-034571, filed on Mar. 7, 2023, which is hereby incorporated by reference herein in its entirety.