JPH0763859A - Two-dimensional detector of radiation - Google Patents

Abstract

PURPOSE:To obtain a two-dimensional detector of radiation of a solid scan system which makes it possible to obtain an image of good quality having high sensitivity and high S/N and to increase the number of collected images for a unit time and which is excellent in light responsiveness. CONSTITUTION:A scintillator 1 converting an image obtained by an X ray into an image of light, a light-transmitting electrode 2, a photoconductive film 3 and a switching element matrix 5 are laminated sequentially, and the photoconductive film 3 is made an amorphous semiconductor layer which is held by electrodes having a check type contact and has a charge multiplying action.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid scanning two-dimensional radiation detector for detecting radiation such as X-rays suitable for use in medical diagnostic equipment such as X-ray imaging equipment.

[0002]

2. Description of the Related Art As a two-dimensional X-ray detector for converting an X-ray image into an electric signal in the medical field, a combination of an image intensifier and a television image pickup tube is generally used. The pixels of the image intensifier have at least a pixel number corresponding to a [hundreds × hundreds] matrix to a [thousands × thousands] matrix.

This X-ray two-dimensional detector has a structure in which a visible light image image-multiplied by an image intensifier is imaged by electron beam scanning by a television image pickup tube which is optically coupled to the image intensifier. for,
By the time the final electrical image signal is obtained from the X-ray image,
X-ray-visible light-electron-visible light-optical system-visible light-
Since many conversion steps of electric signals are included, there are problems that the conversion efficiency is deteriorated, the S / N ratio of the final image is lowered, and the apparatus becomes complicated and large.

Instead of picking up a visible light image multiplied by an image intensifier by an electron beam scanning with a television image pickup tube, a sensor for detecting X-rays and a switching element matrix composed of thin film transistors and the like are integrated. , Solid-state scanning radiation 2 which two-dimensionally scans a switching element to obtain an image signal
Various dimension detectors have been proposed.

For example, in Japanese Patent Laid-Open No. 185865/1993, electron-hole pairs (charges) generated by absorption of X-rays by applying a reverse bias voltage to the amorphous silicon layer.
There is disclosed a radiation detector in which an amorphous silicon thin film transistor provided in a matrix form is used for scanning / imaging. Japanese Patent Application Laid-Open No. 4-206573 discloses a radiation detector in which X-rays are converted into light by a scintillator layer and then imaged by a thin film photodiode and a thin film transistor.

[0006]

However, in the solid-state scanning radiation two-dimensional detector as described above, since the signal current is small, the spike noise generated when switching the pixels by the switching element becomes pixel noise, which deteriorates the image quality. . In order to reduce this deterioration in image quality, it takes a sufficient integration time to extract the signal, and the charge accumulation function of the photoconductive film that converts a radiation image into an electric charge image is not used, and each pixel is integrated. Due to the structure, the integration time is still required. Therefore, there is a problem that the number of images collected per unit time cannot be increased, in other words, the photoresponsiveness is poor.

On the other hand, in the type disclosed in JP-A-3-185865, which directly detects X-rays without converting them into light and converts them into electric signals, the thickness of the semiconductor layer, which is a sensitive layer for X-ray absorption, is several. There is a problem that a large signal cannot be obtained unless the film is formed to a thickness of 100 μm or more. Furthermore, when the thickness of the semiconductor layer is increased, the time constant related to the mobility of electron-hole pairs increases, resulting in a large afterimage, which is a performance problem.

In view of the above, the present invention is capable of obtaining a high-quality image with high sensitivity and a high S / N ratio, and is excellent in light responsivity capable of obtaining a large number of images collected per unit time, and has a structure. It is an object of the present invention to provide a two-dimensional radiation detector which is simple and has a large field of view. Also,
Under optimal conditions, a photoconductive film made of an amorphous semiconductor layer having a charge multiplying effect, sandwiched between electrodes having a charge multiplying effect, which causes a charge multiplying effect, and a scintillator for converting X-rays into light are independently provided under optimum conditions. It is also an object to provide a radiation two-dimensional detector that can be formed into

[0009]

In order to solve the above problems and to achieve the above objects, a two-dimensional radiation detector of the first invention is a scintillator for converting an image of radiation into an image of light, and A target film made of a photoconductive film that is in close contact with a photoconductive electrode, and the target film is made of an amorphous semiconductor layer having a charge multiplying effect, sandwiched between electrodes having blocking contacts to which a voltage is applied. A switching element matrix composed of a photoconductive film, which is two-dimensionally arranged in contact with the photoconductive film for scanning the target film and extracting a signal, a driving of the switching element matrix, and a signal extraction circuit are provided. , Is characterized.

A two-dimensional radiation detector according to a second aspect of the invention is a photoconductive film (including a translucent electrode) formed of an amorphous semiconductor layer having a charge multiplication effect, which is sandwiched between electrodes having a blocking contact with a scintillator. ), A transparent flat plate having a flat surface is interposed.

[0011]

The scintillator converts the input radiation into light. The converted light passes through the translucent electrode and enters the photoconductive film to generate electron-hole pairs (charges) in the film.
The generated electron-hole pairs move in the film due to the electric field in the photoconductive film, and at this time multiply the electron-hole pairs. As a result, the potential of the target film is largely changed according to the amount of incident light. Since the target film has no large electric field in the surface direction, a potential distribution corresponding to the incident image appears on the target film.

The charges multiplied by the charge multiplication effect inside the photoconductive film are sequentially driven by the drive circuits of the switching elements arranged in a matrix in contact with the photoconductive film, and the signal extraction circuit operates. It is read out sequentially. The electric signal read out by the signal extraction circuit is a current flowing at a potential corresponding to each pixel in the target film, and is an image signal corresponding to a radiation image. This image signal is very large due to the charge multiplication effect inside the photoconductive film and has a large dynamic range. Therefore, a high-quality image with high sensitivity and high S / N ratio can be obtained. Radiation two-dimensional detector can be provided.

Further, since the blocking target film has a small afterimage, a two-dimensional radiation detector excellent in photoresponsiveness can be obtained in which a large number of images can be collected per unit time. Moreover, since the conversion of radiation into light is performed by a scintillator such as CsI, by thickening the scintillator,
Radiation can be efficiently converted into light, and the photoconductive film can be thinned.

That is, since a large image signal is obtained in combination with the highly efficient conversion of radiation into light by the scintillator and the charge multiplication action inside the blocking target film, a two-dimensional radiation with high sensitivity is obtained. A detector can be provided. Amorphous semiconductor layers such as amorphous selenium are vulnerable to heat.
If it exceeds ℃, it will crystallize and destroy. Further, when CsI is used as the scintillator, the crystal state changes greatly with the substrate temperature (deposition temperature), and a substrate temperature of about 200 to 300 ° C. is preferable to obtain an effective CsI scintillator.
Thus, the formation conditions of the amorphous semiconductor layer and the CsI scintillator are different, and the conditions are contradictory.

In the second invention, a transparent flat plate having a flat surface is interposed between the scintillator and a photoconductive film made of an amorphous semiconductor layer having a charge multiplication effect, which is sandwiched between electrodes having blocking contact. As a result, it becomes possible to form the scintillator on the transparent flat plate under the optimum temperature condition separately from the formation of the amorphous semiconductor layer, and thermal damage at the time of forming the scintillator can be given to the amorphous semiconductor layer. Absent. The transparent flat plate on which the scintillator is formed is joined and integrated with a photoconductive film made of an amorphous semiconductor layer formed on another substrate by an optical adhesive.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing the basic configuration of a two-dimensional radiation detector of the present invention. In the case of X-rays having energy of 70 KeV as a typical radiation, the scintillator 1 absorbs the X-rays sufficiently and emits light.
It is formed to a thickness of 00 to 400 μm. When cesium iodide (CsI) is used for the scintillator, it absorbs 50 to 70% of 70 KeV X-rays and emits light by this X-ray absorption. A transparent electrode 2 is provided in optical contact with the scintillator 1 and is made of an indium tin oxide (ITO) film or the like, and is formed as thin as possible in order to reduce the influence of light scattering. There is.

Further, a target film which is the photoconductive film 3 is provided in a thickness of 1 to several tens of μm in close contact with the electrode 2. The transparent electrode 2 has a small resistance and is provided to apply a constant potential to the photoconductive film 3 having a low electric conductivity. The photoconductive film 3 has a structure in which an amorphous semiconductor 31 containing selenium as a main component is sandwiched between an electrode 32 made of cesium oxide and an electrode made of antimony sulfide 33. Each of the electrodes has a so-called blocking type contact for blocking charge injection of holes / electrons, and the transparent electrode 2 is provided by a power supply device 4 provided outside so that a strong electric field is created in the photoconductive film layer 3. A voltage is applied through.

The light emitted from the scintillator 1 is absorbed by the photoconductive film 3 to generate electron-hole pairs (charges), and when these charges are moved by the electric field, a charge multiplication action is caused to cause a photoconductive film 3. Change the potential of. In these processes, electron-hole pairs are generated with a gain of 1 or more with respect to the number of incident photons, so that the potential change appearing in the photoconductive film is much larger than that in the conventional photoconductive film, and the sensitivity is high. Also, this potential is
Since there is no large electric field in the plane direction, the blur of the image due to the relaxation phenomenon of the potential can be ignored during the normal pixel scanning time, and the potential distribution corresponding to the incident X-ray transmission image appears on the film.

The reading of the potential appearing on the photoconductive film 3 is 2
This is performed by the switching elements 5 arranged in a dimension. This switching element 5 is a so-called thin film transistor (TF).
T) or a thin film diode (TFD) device, the former is a transistor composed of a silicon semiconductor layer, an insulating layer and an electrode, and the latter is generally composed of an amorphous silicon diode or a tantalum electrode-tantalum oxide-chromium electrode. A MIM element type diode can be considered.

The switching elements arranged in a matrix are sequentially driven by the drive circuit 6, and a current flowing for resetting the potential appearing on the photoconductive film 3 to the initial potential is a signal extraction circuit for each pixel. Read by 7. The transparent electrode 2 may be omitted.

[Embodiment 1] An embodiment of the first invention using a silicon semiconductor TFT element as the switching element 5 will be described. First, semiconductor elements are provided in a matrix on an insulating substrate (not shown) whose surface is sufficiently smooth by a vapor phase growth method such as plasma CVD or a sputtering method, or a photolithography technique. An electrode is provided on. A glass substrate is most preferable for the insulating substrate, and so-called alkali-free glass having a very low sodium content is most suitable for avoiding deterioration of TFT characteristics. Further, the smoothness is a so-called, in which the curvature within a range of several cm is 200 μm or less and within a range of about 1 mm.
It is desirable that the swelling is several nm or less. Amorphous silicon or polycrystalline silicon can be used for the semiconductor element.

If amorphous silicon is used, elements can be formed at a low temperature, and thus inexpensive glass can be used, which is economically preferable. Further, since polycrystalline silicon has a higher carrier mobility in the TFT element than amorphous silicon, polycrystalline silicon is preferable in terms of element characteristics. On the semiconductor element, the photoconductive film 3 having the above-mentioned blocking contact electrode and the transparent electrode 2 are formed by the vapor deposition method or the sputtering method. Furthermore, the scintillator 1 is provided thereon by a vapor deposition method. Scintillator 1
When cesium iodide (CsI) is used as the material, the cesium iodide has a deliquescent property and must be sealed or housed in a vacuum container in order to avoid the deterioration of its emission characteristics due to humidity. Is preferred.

Next, reading of the potential appearing on the photoconductive film 3 will be described with reference to the electrical equivalent circuit of FIG. Although the photoconductive film is structurally an integrated film in the entire detection portion, it is electrically represented by a parallel circuit composed of a capacitance and a resistance for each pixel, and the light-transmitting electrode side at one end is a power supply device. 4 keeps the same potential. When X-rays enter the scintillator 1, scintillation light emission is generated, and the light is guided to the photoconductive film 3 via the transparent electrode 2. When light enters each pixel of the photoconductive film 3, the potential charged in the capacitance changes according to the amount of light incident.

If, for example, the [i] row is selected by the drive circuit 6, the TFT elements on the [i] row are turned on, and a current for recharging each pixel flows,
Each pixel is reset to the initial potential. The current amount at this time becomes a signal and is passed through the signal extraction circuit 7 to each column [j-
1, j, j + 1 ...]. The signal output from the signal extraction circuit 7 is a sample hold circuit (not shown), a multiplexer circuit (not shown), A /
It can be recorded as a digital image signal through a D conversion circuit (not shown) or the like, or can be combined with an appropriate timing circuit (not shown) to form an analog image signal such as a television signal. [Embodiment 2] Another embodiment of the first invention using an amorphous silicon semiconductor element as a switching element will be described. It should be noted that this embodiment and the following third embodiment are different from the first embodiment only in the structure of the switching element portion, and the other structures are the same as the structure shown in the first embodiment. First, amorphous silicon is deposited on a glass substrate having a sufficiently smooth surface by a vapor phase growth method or the like, and a diode element is formed in a matrix by a photolithography technique. Each diode element is provided with driving electrodes and signal reading electrodes in a matrix. A photoconductive film 3 and a translucent electrode 2 are formed thereon, as in Example 1, and a scintillator 1 is further provided. The potential appearing on the photoconductive film 3 is
The drive circuit turns on the diode element of a certain selected row, whereby the signal of each pixel is read out through the signal extraction circuit. [Embodiment 3] Still another embodiment of the first invention will be described in which an MIM element is used as a switching element. After depositing tantalum metal on a glass substrate having a sufficiently smooth surface by a sputtering method, an unnecessary portion is removed by using a photolithography technique. Next, the surface of this tantalum metal is oxidized by an anodic oxidation method to form a tantalum oxide. Further, chromium metal is deposited by sputtering similarly to tantalum metal to remove unnecessary portions.

In this way, the MIM diode is manufactured,
A photoconductive film 3 and a translucent electrode 2 are formed on this element, and a scintillator 1 is further provided. Photoconductive film 3
The potential that appears on the top depends on the MI of a selected row by the drive circuit.
When the M element is turned on, the signal of each pixel is read out through the signal extraction circuit. This method has a small number of steps for producing an element, is simple, and is economically advantageous.

In Examples 1 to 3 described above, the scintillator 1 was vapor-deposited on the translucent electrode 2 provided on the photoconductive film 3, and the scintillator 1 and the photoconductive film 3 were closely formed. As an example, next, a transparent flat plate having a flat surface is interposed between the scintillator and the photoconductive film, so that the scintillator and the photoconductive film can be separately manufactured so that the surface between them can be formed separately. The second invention with a flat transparent plate interposed will be described with reference to the drawings.

[Embodiment 4] In the embodiment shown in FIG. 3, thin glass is used as a transparent flat plate, and a scintillator is formed on thin glass 9. Separately, the glass substrate 8 on which the switching element 5, the photoconductive film 3 and the translucent electrode 2 are provided are bonded to each other with an optical adhesive 91. Although not shown, it is also effective to form a polyimide layer on the transparent electrode 2 for smoothing the joint surface and protecting the element. For example, when CsI is selected as the scintillator, it is known that the crystal structure of CsI formed by the temperature of the surface to be vapor-deposited (so-called substrate temperature) greatly changes, and the substrate temperature is 200 to 300 ° C.
Is suitable.

According to this method, since the thin glass plate 9 is used, it is possible to separate the formation of the photoconductive film or the switching element portion from the formation of the scintillator portion. Since it is not necessary to consider the characteristic deterioration due to CsI, the optimum temperature control for CsI vapor deposition becomes possible. In addition, it is not necessary to consider the temperature rise of the surface to be vapor-deposited when the vapor deposition rate is high, and the C
sI vapor deposition becomes possible, a scintillator can be produced under optimum conditions, and the photoconductive film and the like are not deteriorated by heat.

Further, since the thin glass plate 9 is weak in strength and difficult to handle, a so-called FOP (fiber optical plate) formed by bundling glass fibers having a diameter of several μm is optically polished. If used, the resolution is not deteriorated due to the dispersion of light, the FOP can be used as a supporting substrate to have a thickness, and the handling is easy.

[Fifth Embodiment] FIG. 4 shows another embodiment using the FOP. The transparent electrode 2 and the photoconductive film 3 are sputtered on one surface of the FOP 92 that has been sufficiently optically polished.
It is formed using a vapor deposition method. The scintillator 1 is formed on the other surface. Since these are one film that is not divided for each pixel, a high-level alignment technique such as a photolithography method is not required, and thus it can be produced relatively easily. On the other hand, the matrix-shaped switching element 5 is formed on another glass substrate 8 and combined with the photoconductive film 3 formed on the FOP using the conductive adhesive 93.

As the conductive adhesive 93, a commercially available one containing silver as a main component may be used. However, a hemispherical solder bump is provided for each pixel for low-temperature adhesion, or a mercury-based compound is used. It can also be used and bonded. In this embodiment, since the scintillator and the photoconductive film can be formed separately, the scintillator is first formed on one of the FOPs by elevating the temperature of the substrate, and then the photoconductive film is formed on the other surface. In this case, the photoconductive film is not deteriorated in characteristics due to heat when forming the scintillator.

[0032]

According to the present invention, radiation can be efficiently converted into light by a scintillator, and this converted light has a charge multiplication effect sandwiched between electrodes having a blocking contact to which a strong electric field is applied. Is guided to the photoconductive film made of an amorphous semiconductor layer,
The amount of signal obtained in combination with the charge multiplication action therein and the highly efficient conversion of radiation into light by the scintillator becomes extremely large, and a high quality image with a good S / N ratio can be obtained and the unit time The number of images collected per hit can be increased.

Further, it is possible to form a scintillator such as CsI in a large area in the conventional image intensifier manufacturing technology, and it is easy to increase the size of the photoconductive film of amorphous selenium. Similar to the tensifier, it is possible to realize a economical two-dimensional detector with a large area field of view and a thin radiation two-dimensional detector having a large number of pixels of [hundreds × hundreds] to [thousands × thousands] matrix.
Moreover, its structure and manufacture are simple.

[Brief description of drawings]

FIG. 1 is a schematic view showing a structure of a two-dimensional radiation detector according to an embodiment of the first invention.

FIG. 2 is an equivalent circuit diagram of FIG.

FIG. 3 is a schematic view showing an embodiment of the second invention in relation to a manufacturing method.

FIG. 4 is a schematic view showing another embodiment of the second invention in relation to the manufacturing method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Scintillator 2 ... Translucent electrode 3 ... Photoconductive film 31: Amorphous semiconductor 32: Antimony sulfide 3
3: Cesium oxide 4 ... Power supply device 5 ... Switching element 6 ... Driving circuit 7 ... Signal extraction circuit 8 ... Insulating substrate (glass substrate) 9 ... Thin glass 91: Optical adhesive 92: FOP 93: Conductive adhesive

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H01L 27/14 31/09 7630-4M H01L 31/00 A

Claims (2)

[Claims] 1. A scintillator for converting a radiation image into a light image, a target film for converting a light image from the scintillator into a charge image, and the target film.
In a radiation two-dimensional detector formed by stacking a switching element matrix for two-dimensionally scanning and reading out charges accumulated in a target film, the target film has a charge multiplication function sandwiched between electrodes having blocking contact. A two-dimensional radiation detector characterized in that it is a photoconductive film made of an amorphous semiconductor layer. 2. The two-dimensional radiation detector according to claim 1, wherein a transparent flat plate having a flat surface is interposed between the scintillator and the photoconductive film.