JPH0738132A - Radiation detecting element, manufacture thereof, and radiation detector - Google Patents

Abstract

PURPOSE:To obtain a method of manufacturing a new radiation detecting element of CdTe crystal and a radiation detector formed of detecting elements and high in sensitivity. CONSTITUTION:A metal layer is provided to each of the front and rear of a radiation detecting element CdTe crystal of a resistivity of 10<7> to 5X10<9>OMEGA/cm, the CdTe crystal is cut into radiation detecting elements whose side faces are all covered with an ultrahigh resistive layer 10<10>OMEGA/cm or above in resistivity, and the radiation detecting elements are so arranged in one dimension or two dimensions as to come into contact with each other. By this setup, a two- dimensioanlly integrated radiation detector having a higher performance than a conventional one can be manufactured at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an integrated radiation detector for a non-destructive image inspection apparatus utilizing radiation such as X-rays and γ-rays, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Non-destructive image inspection methods such as radiographic image inspection are used, for example, for defect inspection of welded parts, computed tomography, baggage inspection, etc., and should be seen especially in recent years in expanding the range of use in the medical field. There is something. This method utilizes the fact that when radiation such as X-rays and γ-rays passes through an object to be inspected, the absorption capacity of the radiation differs depending on the structural material of the object.

Conventionally, there are various detectors for detecting this radiation, and an optimum one is selected and adopted according to the purpose and the use of the apparatus. Since a detector used in an image inspection apparatus is desired to be integrated, a semiconductor detector is optimum, and a CdTe crystal is used as a semiconductor. This semiconductor detector is a unit element in which electrodes are provided on two opposite surfaces of a high-resistance minute CdTe crystal, which are used individually or in an integrated manner, and a bias voltage is applied between the electrodes during use. When the radiation enters the CdTe crystal in this state, a part of the radiation is absorbed by the interaction between the radiation and the atoms in the crystal to generate electrons and holes, and the electrons and holes drift, A current pulse is generated. The height and the number of this current pulse per unit time are measured to know the energy and intensity of the radiation incident on the crystal.

There are three types of methods for measuring the radiation that has passed through an object using the unit element and forming an image. One is a method of two-dimensionally scanning the unit element itself in the XY direction. Yes, the time required for measurement is the longest. The second method is a method of stacking the unit elements in, for example, the X-axis direction and one-dimensionally scanning the obtained stacked body in the Y-axis direction. This method requires less time than the first method. The third method is one in which the unit elements are two-dimensionally stacked, and although the number of required unit elements is large, the time required for measurement is extremely short. Shortening the measurement time means that the irradiation dose of radiation is small, which is the most preferable in terms of safety. For this reason, it is most recommended to use a detector in which the unit elements are two-dimensionally stacked.

As an example of a detector in which unit elements are two-dimensionally stacked by using a semiconductor crystal plate, a metal layer is formed on the front surface of one side of a large semiconductor crystal plate, which is used as a common electrode and corresponds to each pixel on the opposite side. There is a type in which separated electrodes of the same size are two-dimensionally arranged and an independent amplifying device is provided for each of the arranged electrodes. In the case of this detector, an electric current will flow through the electrode immediately adjacent to which the radiation is incident, and the incident position of the radiation and the intensity of the radiation can be detected by measuring this current.

According to this method, the detector is integrally molded, so that it can be easily manufactured. However, since each element is not separated, a signal leak occurs between each element. This signal leakage depends on the distance between the electrodes and the thickness of the semiconductor crystal plate, and becomes more remarkable as the electrode distance becomes narrower and the semiconductor crystal plate becomes thicker. Therefore, the detector made by this method is not always suitable when the resolution is desired to be increased or high-energy radiation is used. In addition, if the radiation detection characteristic is defective even in one of the plurality of pixels, the entire detector becomes unusable.

As described above, when an attempt is made to make a detector in which the CdTe crystal plate generally used as a semiconductor crystal is two-dimensionally stacked, the CdTe crystal plate is limited to a maximum of 1 due to crystal manufacturing restrictions. Only a ~ 2 cm square object can be obtained, and since it is an extremely brittle material, a large-area detector cannot be obtained.

Another method of stacking unit elements in two dimensions is as follows:
This is a method in which independent minute unit elements are created and each element is arranged with a constant interval. In this method, since there is a space between each element, they are completely electrically separated, and the signal leakage does not occur even when a thick semiconductor crystal plate is used. However, complicated assembly is inevitable. In addition, the element is made smaller to increase the resolution,
If the distance between the elements is narrowed, there is a drawback that the sensitivity is lowered. That is, the sensitivity of the entire detector obtained by two-dimensional integration is: Sensitivity = A · L / (L + D) ^{2 where} L is the size of the element, D is the distance between the elements, and A is the proportional coefficient. As is clear from this equation, the sensitivity decreases rapidly as L approaches D.

[0009]

DISCLOSURE OF THE INVENTION The present invention provides a novel radiation detecting element using a CdTe crystal, a method for producing the same, and a highly sensitive radiation detecting obtained by laminating the radiation detecting element one-dimensionally or two-dimensionally. The purpose is to provide dexterity.

[0010]

The radiation detecting element of the present invention for solving the above-mentioned problems has a specific resistance of 10 ^{7 to} 5 × 10 ⁹ ohm / cm.
Of the CdTe crystal for a radiation detecting element, wherein a metal layer is provided on the front surface and the back surface, and an ultrahigh resistance layer having a specific resistance of 10 ¹⁰ ohm / cm or more is provided on all side surfaces thereof. The method is to polish the front surface and the back surface of the CdTe crystal plate, remove the processing strained layers on the front surface and the back surface, and then provide a metal layer on the front surface and the back surface, and then to obtain a desired material by mechanical means such as wire sawing or dicing. It is cut into pieces. The radiation detector of the present invention is a radiation detector using a radiation detector element in which unit elements are laminated one-dimensionally or two-dimensionally, and the radiation detection element of the present invention is used as a unit element as a radiation detector detection element. Are laminated one-dimensionally or two-dimensionally so as to be in contact with each other, or one-dimensionally or two-dimensionally arranged with one of the front surface electrode and the rear surface electrode as a common electrode.

[0011]

[Operation] CdTe which is usually used as a radiation detecting element
As the crystal, one having a specific resistance of 10 ^{7 to} 5 × 10 ⁹ ohm / cm grown by the Bridgman method or THM method is used. In this respect, the present invention does not disclose anything novel. The invention which can be made the present invention has a specific resistance of 10 ¹⁰ ohm / cm as the radiation detection element on the entire side surface except the front surface and the back surface.
The above CdTe crystal covered with the ultra-high resistance layer is provided with a metal layer on the front surface and the back surface thereof, a method of manufacturing the element, and a radiation detector using the element.

Incidentally, the specific resistance is 10 ^{7 to} 5 × 10 ⁹ ohm / cm.
The reason for using the above item is that if the specific resistance is lower than this, a current will flow when a bias voltage is applied to the electrodes of the element, and noise will be generated. Also, if the specific resistance is higher than this, the mobility and life of carriers in the crystal become too small, and the current due to radiation incidence cannot be detected. CdTe for detecting radiation having such a specific resistance
The crystal can be obtained by adding a trace amount of impurities such as chlorine, indium and gallium, or by purifying it.

The reason why an ultra-high resistance layer having a specific resistance of 10 ¹⁰ ohm / cm or more is not provided on the front surface and the back surface of the crystal used in the device of the present invention is that electrodes are provided on the front surface and the back surface, and radiation passes through the crystal. This is because the current generated at that time is detected efficiently.
An ultra-high resistance layer having a specific resistance of 10 ¹⁰ ohm / cm or more is provided on all side surfaces except the front surface and the back surface when the unit elements made of the crystal are arranged in one-dimensional or two-dimensional contact with each other. This is because the ultra-high resistance layer acts as an insulating layer to prevent signal leakage. It is necessary that at least one of the front surface electrode and the back surface electrode be electrically isolated from each other, but the other electrode may be integrated as a common electrode without any problem.

When obtaining a crystal used in the radiation detecting element of the present invention, a crystal plate having a desired thickness is cut out from a CdTe crystal, and the front and back surfaces of the obtained crystal plate are polished to remove mechanical strain and then mechanically removed. It is cut into a desired size and shape according to the cutting method. Polishing the front and back surfaces of the CdTe crystal plate,
The reason for removing the processing strain is to provide electrodes on both surfaces. The reason why the cut surface is not polished but remains rough is that the layer having a processing strain has an extremely high resistance value. The magnitude of this resistance value is not irrelevant to the magnitude of the processing strain, and although the quantitative relationship is not clear, the resistance value increases as the processing strain increases. Specific resistance is 10 ^{7 to} 5 × 10 ⁹
When a CdTe crystal of ohm / cm is used, if the strained layer on the side surface has a specific resistance of 10 ¹⁰ ohm / cm or more, the strained layer substantially functions as an insulating layer. Therefore, the method that can be adopted as the mechanical cutting method must be a method that causes a processing strain that causes a specific resistance of 10 ¹⁰ ohm / cm or more. This degree of processing strain can be sufficiently obtained by ordinary mechanical cutting methods such as wire sawing and dicing.

The radiation detecting element of the present invention is provided with electrodes for applying a bias to the front and back surfaces of the element crystal obtained by the above method. In order to prepare the radiation detecting element of the present invention, a metal layer may be provided on the polished surface of the crystal for the radiation detecting element, the front surface and the back surface of the CdTe crystal plate are polished, and the processing strain layer on the surface is removed. A metal layer may be provided on the back surface and the back surface, and thereafter, cutting may be performed by a normal mechanical means such as wire sawing or dicing, and the same result can be obtained even by this method. In the case of the latter method, since the periphery of the metal part which becomes the electrode of each radiation detection element after cutting is slightly chamfered when cutting, even if the obtained radiation detection elements are arranged in contact with each other, the distance between the electrodes is about 50.
A gap of μm is secured, and there is no problem in producing a radiation detector detection element for a radiation detector using the element.

The metal used as the electrode has good adhesion to the crystal,
Any material can be used as long as it can induce an electric field in the crystal, and it is not particularly limited, but gold and platinum are usually most widely used.
Further, as a method for forming the electrodes, various general film forming methods such as electroless plating, vacuum deposition, and sputtering can be used.

The radiation detector of the present invention may be a radiation detector element obtained by laminating the radiation detection elements of the present invention obtained by the above-mentioned method in a one-dimensional or two-dimensional manner, or by using a front surface electrode and a back surface. One of the electrodes is used as a common electrode, and the detection element for the radiation detector is arranged one-dimensionally or two-dimensionally. Laminating the radiation detecting elements of the present invention so as to be in contact with each other when creating a detection element for a radiation detector, eliminating the interval between the elements, making the dead zone in radiation detection as narrow as possible,
It is intended to suppress the decrease in sensitivity as much as possible when the radiation detecting element is made small. By the way, the dead zone of the detection element for the radiation detector of the present invention is only the width of the processing strained layer caused by the cutting. For example, when it is cut by wire sawing, it is at most 50 μm, and the sensitivity is lowered even if the element is made small. Can be significantly reduced.

Further, when the radiation detecting element obtained by the method of the present invention is laminated, it becomes possible to laminate while preliminarily removing the element having the defective characteristic, and it is possible to manufacture a good and high quality detecting element for a radiation detector. Not only is it possible, but the assembly work is extremely simple and the manufacturing cost can be greatly reduced.

[0019]

EXAMPLES Next, examples of the present invention will be described. Example 1 A 4-inch diameter CdTe crystal having a specific resistance of 10 ⁹ ohm / cm by addition of chlorine was cut to obtain a single crystal plate having a thickness of 2 mm. Both sides of this single crystal plate were lapping-polished using an abrasive of # 1000 to remove the work strain layers on both sides. Then, a methanol solution containing about 1% of bromine was used as a polishing liquid, and the surface-processed strained layer caused by lapping was completely removed by mechanochemical polishing on both surfaces. The obtained crystal plate had a thickness of about 1.2 mm.

Next, the crystal plate obtained above was immersed in a gold plating solution prepared by dissolving 1 g of gold chloride in 0.1 liter of water for several seconds, then washed with water and dried to form a crystal layer having a thickness of about 1 μm on the entire surface of the crystal. Gold plated. Then, the gold-plated crystal plate was cut in two directions in the vertical and horizontal directions using a multi-wire saw to prepare a square radiation detection element having a side length of 2 mm.

When the radiation detection characteristics of the 100 elements thus produced were measured using the radiation of 60 Kev emitted from Am-241, the variation of the sensitivity was (maximum value-
The minimum value / average value was 0.08. One electrode surface of these 100 detection elements is placed in series on the printed wiring board using a conductive adhesive, and a thin layer of conductive paint is applied to the entire electrode surface of the detection element that appears on the surface to form a common electrode. This electrode is connected to the ground terminal and the width is 2 mm and the length is 200 mm.
A dimensional array detection terminal was created. The common electrode side of the detection terminal serves as a radiation incident surface.

Lead wires are provided on the back surface of the printed wiring board at positions corresponding to the respective elements, and the lead wires are connected to the electrodes through the conductive adhesive. A radiation detector was constructed by connecting each element to an amplifier.

A bias voltage of +50 V was applied to the back surface side through a 100 M ohm resistor, and radiation collimated to 1.8 mm □ was irradiated to any one element in the array, and adjacent two
The signal detected by each element was observed and the signal leakage between elements was investigated. As a result, it was confirmed that there was no signal leakage and each element was electrically separated completely.

(Embodiment 2) A detector element for a two-dimensional array detector of 6 mm square was prepared by using the nine radiation detector elements used in Embodiment 1, and a radiation detector was prepared by using this detector element. Signal leakage between each radiation detection element was examined in the same manner as in Example 1 except that the element was irradiated with radiation. as a result,
It was confirmed that there was no signal leakage and each element was electrically completely separated.

[0025]

The ultra-high resistance layer provided on the side surface of the radiation detecting element of the present invention completes the electrical insulation between the respective elements, so that the detecting element for the radiation detector is prepared by using this element. If a radiation detector is created by using, it becomes possible to manufacture a two-dimensional integrated radiation detector having higher performance than ever before at low cost.

Claims (4)

[Claims] 1. A CdTe crystal for a radiation detecting element having a specific resistance of 10 ^{7 to} 5 × 10 ⁹ ohm / cm, a metal layer is provided on the front and back surfaces, and the specific resistance is 10 ¹⁰ ohm / cm on all side surfaces.
A radiation detecting element comprising the above-mentioned ultra-high resistance layer. 2. A Cd having a specific resistance of 10 ^{7 to} 5 × 10 ⁹ ohm / cm.
After polishing the front and back surfaces of the Te crystal plate to remove the processing strain layers on the front and back surfaces, a metal layer is provided on the front and back surfaces, and then a desired size is obtained by mechanical means such as wire sawing or dicing. Cutting is performed by cutting.
A method for manufacturing the radiation detection element described. 3. A radiation detector using a detection element for a radiation detector in which unit elements are laminated one-dimensionally or two-dimensionally, and the radiation detection element according to claim 1 is used as the unit element so that the elements are in contact with each other. A radiation detector characterized by being laminated one-dimensionally or two-dimensionally. 4. The radiation detector according to claim 3, wherein one of the front surface electrode and the rear surface electrode is a common electrode.