JP4026377B2 - Radiation detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a radiation detection apparatus used in a medical radiation imaging apparatus or an industrial nondestructive inspection apparatus, and more particularly to a radiation detection apparatus in which a radiation conversion film is connected to a thin film transistor (TFT) array.
[0002]
[Prior art]
Conventionally, a combination of a film screen or a scintillator and an image pickup tube has been used as a means for collecting an X-ray image in a medical X-ray apparatus or the like. There were problems in terms of storage and data processing, and there were also problems in terms of performance such as exposure dose and concentration resolution.
Recently, as an alternative, a two-dimensional radiation (X-ray) detection device using a semiconductor film has been developed.
[0003]
A conventional two-dimensional X-ray detection apparatus using such a semiconductor film will be described with reference to FIGS. FIG. 5 is a diagram schematically showing the configuration of the X-ray detection apparatus, FIG. 6 is a perspective view in which a part of the X-ray conversion film of the X-ray detection apparatus is removed, and FIG. 7 is one sensor of the X-ray detection apparatus. FIG. 8 is a diagram illustrating a detailed structure of a pixel, and FIG. 8 is a diagram schematically illustrating a configuration of a sensor pixel.
As shown in FIG. 5, this X-ray detection apparatus has 25 sensor pixels (radiation detectors) 11, 12,..., 55 arranged in a matrix of 5 rows × 5 columns. The gate lines 6 (61, 62,..., 65) input in common to the rows and the readout signal lines 7 (71, 72,..., 75) common to the columns are wired.
[0004]
The gate lines 61, 62,..., 65 are sequentially and selectively driven by the gate driver circuit 101, and outputs from the sensor pixels belonging to the driven rows are transmitted through the readout signal lines 71, 72,. It is taken out to the outside and inputted to the amplifier array circuit 102. The control circuit 103 is provided for overall control of the operations of the gate driver circuit 101 and the amplifier array circuit 102. By sequentially driving the gate line 6 as 61, 62, 63. Output data from the sensor pixels 11, 12,... 55 can be collected, and finally a two-dimensional image can be constructed.
[0005]
As shown in FIGS. 6 to 8, the sensor pixels are common for applying a bias voltage to an X-ray conversion film (for example, a-Se) 1 that converts incident X-rays into a charge signal, and the X-ray conversion film 1. An electrode 2, a pixel electrode 3 for collecting charge signals generated in the X-ray conversion film 1, a charge storage capacitor electrode 10 for forming a storage capacitor 5 for storing charges with the pixel electrode 3, A thin film transistor (TFT) 4 for outputting charges accumulated in the storage capacitor 5 to an external circuit, and these electrodes and TFT are manufactured on a glass substrate 8 by using thin film technology.
The TFT 4 includes a drain electrode 4a, a source electrode 4b, and a gate electrode 4c. The drain electrode 4a is connected to the storage capacitor 5, the source electrode 4b is connected to the read signal line 7, and the gate electrode 4c is connected to the gate line 6. Yes. The conduction / non-conduction between the drain electrode 4a and the source electrode 4b is controlled by the potential (control voltage) of the gate electrode 4c. When the TFT is n-type, the conduction state is obtained by shifting the control voltage in the positive direction. A transition to the negative direction results in a non-conduction state. In general, the control potential when conducting is normally a positive potential (about + 10V), and the control potential when non-conducting is usually a negative potential (about -10V).
[0006]
By the way, in a radiation (X-ray) detector using a conversion film of a type that directly converts X-rays such as amorphous selenium (a-Se) into an electric signal, a high voltage (a-Se) is applied to the conversion film. In this case, it is necessary to apply 1000 V or more). Therefore, when excessive X-ray irradiation is applied to the conversion film, a high voltage is applied to the connected TFT, and the TFT may be broken beyond the withstand voltage limit (usually about 50 V).
[0007]
In order to solve this problem, a method using a dual gate TFT, a method of inserting a protection diode, and the like have been proposed. However, when a dual gate TFT or a protection diode is used, there is a problem that the TFT structure becomes complicated. In a medical X-ray surface sensor, normally, about 3000 pixels × 3000 pixels = 9 million pixels must be integrated on a single substrate. If the basic structure of one pixel becomes a little complicated, the overall manufacturing yield Therefore, it is not preferable to provide each sensor pixel with a high voltage protection mechanism such as a dual gate TFT or a protection diode.
[0008]
Therefore, as a TFT 4 for introducing the charge accumulated in the storage capacitor 5 into an external circuit in order to prevent permanent destruction of the transistor switch due to excessive potential rise without adding a dual gate TFT, a protective diode, etc. Using an n-type transistor, the transition polarity (negative voltage) of the control voltage when the TFT 4 is turned from ON to OFF, and the transition polarity of the storage capacitor potential that is displaced by the accumulation of the output charge from the X-ray conversion film 1 (ie, X It has been proposed that the polarity of the bias voltage applied to the line conversion film 1 is the same polarity (negative voltage).
If the bias voltage Vhv applied to the common electrode 2 has the same polarity (negative voltage) as the transition direction of the control voltage when the TFT 4 is turned from ON to OFF, the X-ray is incident on the X-ray conversion film 1. 9, the potential Vp of the storage capacitor 5 is Vp = −Q / C (Q is the stored charge amount, and C is the storage capacitor 5) as shown in the switching characteristics of the TFT in FIG. 9 (Vp is the pixel voltage and Id is the drain current). It drops (shifts to the negative potential side) with the relational expression of (capacitance). That is, in the initial state, the TFT is off (a), but when X-rays are irradiated, the potential Vp decreases (b) and eventually falls below the gate potential Vg of the TFT 4. If Vp <Vg, the TFT 4 turns ON (c), and the charge stored in the storage capacitor 5 is discharged through the channel of the TFT 4, and the potential does not drop any further, so that the TFT 4 is prevented from being destroyed. it can.
[0009]
[Problems to be solved by the invention]
The conventional radiation detection apparatus is configured as described above. However, when the structure is such that charges generated by excessive X-ray irradiation are trapped in the immediate vicinity of the TFT, the TFT is not necessarily turned on normally. Therefore, there was a problem that it did not play the role of high-pressure protection.
Electrons generated by X-ray irradiation are usually collected by the pixel electrode 3, but a part of the electrons is trapped on the TFT as shown in FIG. This trapped charge affects the IV characteristics of the TFT. That is, as shown in FIG. 10, if there is charge trapping in the vicinity of the TFT, X-rays are irradiated and the potential Vp of the storage capacitor 5 is lowered, and the charge trapped on the upper portion of the TFT is also increased. The characteristics change as shown in FIG. 11, and even if Vp <Vg (c), the TFT 4 does not turn on as expected. In this case, since the excessive charge in the storage capacitor 5 is not discharged, the value of Vp continues to decrease, and eventually the TFT is destroyed.
[0010]
The present invention has been made in view of such circumstances, and provides a radiation detection apparatus capable of removing the influence of trap charges of an X-ray conversion film and realizing a stable high-voltage protection operation of a TFT. The purpose is that.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a radiation conversion film that outputs a charge signal in response to radiation, a common electrode provided on the radiation incident side of the radiation conversion film, and an output charge from the radiation conversion film. A pixel electrode that collects the charge collected in the pixel electrode, a thin film transistor (TFT) array that outputs the signal charge accumulated in the storage capacitor to an external circuit, the radiation conversion film, and the Rutotomoni provided between the TFT, and a biased shield electrode to a fixed potential, the pixel electrode includes a lower portion which is also used as a drain electrode of the TFT, the lower and the radiation converting film It consists of a formed upper portion between the portion and the upper portion does not extend over the TFT, the electric potential at the time of turning off the TFT to the common electrode It is characterized in that to operate by applying a bias voltage polarity.
[0012]
The radiation detection apparatus of the present invention is configured as described above, and a shield electrode that is always set to a constant potential is provided between the TFT and the X-ray conversion film, so that the influence of trap charges on the X-ray conversion film is completely eliminated. Can be blocked. Accordingly, a constant IV characteristic of the TFT is guaranteed, so that a stable high voltage protection operation of the TFT can be realized.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the radiation detector of the present invention will be described with reference to the drawings. The schematic diagram of the radiation detection apparatus of the present invention is the same as the schematic diagram of the conventional radiation detection apparatus shown in FIG. 5, and includes 25 sensor pixels (radiation) arranged in a matrix of 5 rows × 5 columns inside. Detectors 11, 12,..., 55, and gate lines 6 (61, 62,..., 65) that are input in common to the rows and readout signal lines 7 (71, 72) that are common to the columns. .., 75) are wired.
[0014]
On the other hand, the structure of one sensor pixel of the radiation detection apparatus of the present invention is different from that of the conventional FIG. 7 and has the structure shown in FIG.
As shown in the detailed structural diagram of FIG. 1, the sensor pixel of the radiation detection apparatus of the present invention includes an X-ray conversion film (for example, a-Se) 1 that converts incident X-rays into a charge signal, and an X-ray conversion film 1. A storage electrode 5 for accumulating charges is formed between the common electrode 2 for applying a bias voltage, the pixel electrode 3 for collecting charge signals generated in the X-ray conversion film 1, and the pixel electrode 3. The charge storage capacitor electrode 10 for this purpose, the TFT 4 for outputting the charge stored in the storage capacitor 5 to an external circuit, and the shield electrode 9 formed on the TFT 4 are constituted. The TFT 4 includes a drain electrode 4a, a source electrode 4b, and a gate electrode 4c, as in FIG. 7, and the drain electrode 4a serves as a storage capacitor 5, the source electrode 4b serves as a read signal line 7, and the gate electrode 4c serves as a gate. Connected to line 6.
[0015]
A method for manufacturing this radiation detection apparatus will be described. First, a non-alkali glass substrate is used as the glass substrate 8, and a gate electrode 4c made of a metal film such as Ta (tantalum), Al (aluminum), Mo (molybdenum) is formed thereon. The gate electrode 4c is formed by forming the metal film by sputtering deposition and then patterning it into a desired shape. At the same time, the charge storage capacitor electrode 10 constituting the storage capacitor 5 is also formed.
[0016]
Next, SiNx (silicon nitride) or SiOx (silicon oxide) is formed by a CVD (Chemical Vapor Deposition) method to form an insulating layer. This insulating layer acts as a dielectric for the gate insulating film and the charge storage capacitor 5. Next, an a-Si layer 4d to be a channel portion of the TFT 4 is formed by a CVD method and then patterned into a desired shape.
[0017]
Next, a source electrode 4b made of a metal film such as Ta, Al, Ti (titanium) and a lower portion of the pixel electrode 3 that is also used as the drain electrode 4a are formed. The lower portions of the source electrode 4b and the pixel electrode 3 are formed by forming the metal film by sputtering deposition and then patterning it into a desired shape. Thereafter, an insulating protective film is formed for the purpose of insulating and protecting the region other than the connection portion with the upper portion of the pixel electrode 3. This insulating protective film is formed by forming an insulating film of SiNx or SiOx by a CVD method and then patterning it into a desired shape.
[0018]
Next, the shield electrode 9 made of a metal film such as Ta, Al, Ti, and the upper portion of the pixel electrode 3 are formed. The shield electrode 9 and the upper part of the pixel electrode 3 are formed by forming the metal film by sputtering deposition and then patterning it into a desired shape. Thereafter, an insulating protective film is formed for the purpose of insulating and protecting the region other than the opening in the upper portion of the pixel electrode 3. This insulating protective film is formed by forming an insulating film of SiNx or SiOx by a CVD method and then patterning it into a desired shape.
[0019]
Then, after forming the X-ray conversion film 1 by forming an a-Se film by vacuum evaporation, Au (gold) is formed on the X-ray conversion film 1 by vacuum evaporation. The common electrode 2 is formed.
[0020]
Next, an operation when the X-ray detection apparatus of FIG. 1 is irradiated with X-rays will be described. The X-rays incident on the X-ray conversion film 1 are converted into a charge signal in the X-ray conversion film 1 and are driven by the applied internal electric field to reach the pixel electrode 3, where the charge storage capacitor 5 is used as a charge signal. Will be stored. When a negative bias (usually about −1 kV to −10 kV) is applied to the common electrode 2, electrons move to the pixel electrode, and as a result, the potential of the pixel electrode 3 becomes the amount of incident X-rays. It shifts to negative according to. At this time, as shown in FIG. 2, space charges may be formed on the upper part of the TFT 4 by trapped electrons. The effect of this is caused by the shield electrode 9 biased to a fixed potential (usually 0 V). Since it is cut off, the IV characteristic of the TFT is not subjected to modulation as shown in FIG. Accordingly, the switching characteristics as shown in FIG. 9 are maintained, and even when excessive X-ray incidence occurs, the TFT 4 can be stably turned on, and discharge breakdown can be prevented.
[0021]
In the above embodiment, the shield electrode and the pixel electrode are separately formed. However, as shown in FIG. 3, the upper portion of the pixel electrode may be extended to serve as the shield electrode. In the radiation detection apparatus having the structure shown in FIG. 3, the switching characteristics of the TFT are slightly affected by the potential change of the pixel electrode as shown in FIG. By setting the thickness to 1 μm or more, this influence can be minimized. In this case, a configuration for applying a constant potential to the shield electrode is not necessary, and the configuration can be simplified.
[0022]
In the above-described embodiment, the pixel electrode is divided into an upper portion and a lower portion and formed separately. However, the configuration can be simplified by omitting the upper portion of the pixel electrode.
Furthermore, in the above embodiment, as the material of the X-ray conversion film, but using a-Se, it is also possible to use CdTe, _CdZnTe, a HgI 2, PbI _2, and the like. In the above embodiment, for convenience of explanation, the X-ray detection apparatus has been described as having 5 rows × 5 columns and 25 sensor pixels, but in actuality, it has many sensor pixels such as 3000 pixels × 3000 pixels. is doing.
[0023]
【The invention's effect】
Since the radiation detection apparatus of the present invention is provided with the shield electrode that is always set to a constant potential between the TFT and the X-ray conversion film as described above, the influence of the trap charge on the X-ray conversion film is affected by this shield electrode. Can be completely blocked. Accordingly, constant switching characteristics of the TFT are always maintained, and even when excessive X-ray incidence occurs, the TFT can be stably turned on, and discharge breakdown can be prevented.
In addition, the upper part of the pixel electrode can be extended to serve as a shield electrode. In this case, a configuration for applying a constant potential to the shield electrode is not necessary, and the configuration can be simplified.
[Brief description of the drawings]
FIG. 1 is a diagram showing a detailed structure of a sensor pixel of a radiation detection apparatus according to the present invention.
2 is a diagram showing a state of the sensor pixel of FIG. 1 at the time of X-ray irradiation.
FIG. 3 is a diagram showing another embodiment of the sensor pixel of the radiation detection apparatus of the present invention.
4 is a diagram showing a change in switching characteristics of TFTs of the radiation detection apparatus of FIG. 3;
FIG. 5 is a diagram schematically showing a configuration of a conventional radiation detection apparatus.
FIG. 6 is a perspective view in which a part of an X-ray conversion film of a conventional radiation detection apparatus is removed.
FIG. 7 is a diagram illustrating a structure of a sensor pixel of a conventional radiation detection apparatus.
FIG. 8 is a diagram schematically illustrating a sensor pixel of a conventional radiation detection apparatus.
FIG. 9 is a diagram showing switching characteristics of TFTs of the radiation detection apparatus.
FIG. 10 is a diagram showing a state at the time of X-ray irradiation of a sensor pixel of a conventional radiation detection apparatus.
FIG. 11 is a diagram showing a change in switching characteristics of a TFT of a conventional radiation detection apparatus.
[Explanation of symbols]
1 X-ray conversion film 2 Common electrode 3 Pixel electrode 4 TFT
5 Storage Capacitance 6 Gate Line 7 Signal Line 8 Glass Substrate 9 Shield Electrode 10 Charge Storage Capacitance Electrode

Claims (1)

A radiation conversion film that responds to radiation and outputs a charge signal, a common electrode provided on the radiation incident side of the radiation conversion film, a pixel electrode that collects output charges from the radiation conversion film, and a collection on the pixel electrode A storage capacitor for storing the generated charge, a thin film transistor (TFT) array for outputting the signal charge stored in the storage capacitor to an external circuit, and between the radiation conversion film and the TFT, and at a fixed potential. A biased shield electrode, and the pixel electrode includes a lower portion also used as a drain electrode of the TFT and an upper portion formed between the radiation conversion film and the lower portion. The upper portion does not extend on the TFT, and is operated by applying a bias voltage having the same polarity as the potential when the TFT is turned off to the common electrode. The radiation detecting apparatus according to claim Rukoto.

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