JP4088986B2 - Two-dimensional radiation detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a two-dimensional radiation detector suitable for use in, for example, a non-destructive inspection apparatus such as a food or a printed board, or a medical diagnostic apparatus that performs X-ray imaging.
[0002]
[Prior art]
2. Description of the Related Art A medical diagnostic apparatus or the like uses a solid scanning two-dimensional radiation detector that can detect radiation such as X-rays including its two-dimensional incident position.
[0003]
Conventionally, various proposals have been made regarding the solid-state scanning detector.
For example, in Japanese Patent Laid-Open No. 4-212458, as shown in FIG. 11, a bias electrode 102 and a scan switch layer 103 are laminated on both sides of a semiconductor layer 101 that generates electric charges upon incidence of radiation such as X-rays. A structured two-dimensional radiation detector is disclosed.
[0004]
In the two-dimensional radiation detector shown in FIG. 11, the common electrode 102 is formed on the entire surface in a uniform surface as shown in FIG. Further, as shown in FIGS. 12B to 12D, the scan switch layer 103 includes matrix pixel electrodes 131 and 131 that are in contact with the semiconductor layer 101, and stripes arranged corresponding to the rows of the matrix. Row electrodes 132... 132 and stripe pattern column electrodes 133... 133 arranged corresponding to the columns. Further, as shown in FIGS. 13 and 14, each pixel electrode 131. A plurality of switching elements (FETs) 134... 134 each having a drain D connected to each 131, a source S connected to each column electrode 133, and a gate G connected to each row electrode 132 are provided.
[0005]
As shown in FIG. 13, each row electrode 132 of the scan switch layer 103 is connected to the FET control line of the drive circuit 104, and each column electrode 133 is connected to the signal readout circuit 105 of the signal readout line. In addition, a bias power source 135 for applying a predetermined potential to each pixel is connected to the common electrode 102 (see FIG. 14).
[0006]
In the two-dimensional radiation detector having the above structure, when X-rays are incident on the semiconductor layer 101, electron-hole pairs are generated in the semiconductor layer 101 and charges are generated on the surface thereof. The charges on the semiconductor layer 101 are read out for each pixel corresponding to each of the pixel electrodes 131.. 131 by turning on the switching element (FET) 134.
[0007]
That is, by sending a drive signal to one row electrode 132 by the drive circuit 104, all the FETs 134 in one row (for example, i row) are turned on, and each pixel electrode in each column of i row passes through each of the column electrodes 133. 131.. 131 is used to simultaneously extract charges (currents) for each pixel in each column.
[0008]
[Problems to be solved by the invention]
However, in such a conventional radiation two-dimensional detector, there is a problem that the switching noise of the switching element is superimposed on the readout signal current and the image quality is deteriorated.
[0009]
The present invention has been made in view of such circumstances, and it is difficult for the switching noise of the switching element to be superimposed on the readout signal current, and the fluctuation range of the potential difference generated between both ends of the pixel capacitor is set within an appropriate range. Therefore, an object of the present invention is to provide a two-dimensional radiation detector that can obtain a high-quality X-ray image signal.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a two-dimensional radiation detector according to the present invention sandwiches a semiconductor layer 1 that converts an image of radiation into an image of charges, as illustrated in FIGS. In this way, the signal readout layer 2 and the scanning switch layer 3 provided on both surfaces thereof, and the drive circuit 4 connected to the scanning switch layer 3 are provided. Corresponding to the plurality of pixel electrodes 31... 31, a common electrode 33 for applying a common bias voltage to each pixel, and pixel electrodes 31. Switching elements (FETs) 34... 34 disposed between each of the switching elements 34 and the common electrode 33, and row power for supplying a driving signal from the driving circuit 4 to each switching element 34. 32. Further, the signal readout layer 2 is composed of a plurality of column electrodes 21... 21 divided corresponding to the columns of the pixel electrodes 31. Each of the electrodes 21... 21 is connected to a signal readout line. As illustrated in FIG. 7, high dielectric constant material layers 10 are respectively formed in part of the semiconductor layer corresponding to each pixel between the column electrode 21 and each pixel electrode 31. Characterized by.
[0011]
In the two-dimensional radiation detector of the present invention having the above structure, when X-rays are incident on the semiconductor layer 1, charges corresponding to the incident position and incident energy are generated on the surface of the semiconductor layer 1. When a drive signal is applied to the layer 3 for each row, the switching element (FET) 34 is turned on and a bias voltage is applied to the pixel electrode 31, the discharge current of the charge is the column electrode 21 of the signal readout layer 2. To the signal readout line. An image signal can be obtained by converting the signal current read in this way into a voltage signal by a signal readout circuit (see FIGS. 4 and 5) comprising a charge sensitive preamplifier and an integrating capacitor.
[0012]
Here, in the two-dimensional radiation detector of the present invention, as shown in the equivalent circuit diagrams of FIGS. 4 and 5, the semiconductor layer 1 is interposed between the switching element 34 and the signal readout line. Due to the distance between the two and the capacitance of the semiconductor layer 1, switching noise is difficult to ride on the signal readout line. Therefore, an image signal having an excellent S / N ratio can be obtained, and the image quality can be improved.
[0013]
In the two-dimensional radiation detector of the present invention, it is preferable to use an amplifier made of a crystalline semiconductor such as silicon as an element (charge-sensitive preamplifier) for amplifying a signal read by each column electrode. Using an amplifier made of a crystalline semiconductor has advantages in that signal noise and sensitivity are improved and the configuration of a signal readout line is simplified as compared with the case where a thin film amplifier is used.
[0014]
As the switching element (FET) used in the two-dimensional radiation detector of the present invention, a TFT (Thin Film Transistor) is preferable. Examples of the semiconductor layer used for radiation detection include a compound semiconductor such as CdTe or doped Se.
[0015]
Here, the two-dimensional radiation detector according to the present invention has a structure in which the capacitance of the semiconductor layer is interposed between the pixel electrode and the column electrode of the signal readout layer as described above. It becomes.
[0016]
That is, in this type of two-dimensional radiation detector, the semiconductor layer is usually about 500 μm thick in order to ensure the X-ray capture rate, but the equivalent capacitance of one pixel obtained with this film thickness is extremely small ( For example, when the structure of the present invention is adopted, there arises a problem that the fluctuation range associated with the generation of electric charges of the potential difference generated between both ends of the pixel capacitor becomes very large.
[0017]
In order to solve this problem, in the present invention, as shown in FIG. 7, a high dielectric constant material layer that is in contact with the column electrode 21 and the pixel electrode 31 is formed on a part of the semiconductor layer 1 constituting the pixel capacitor (an end portion of the pixel). 10 is employed. When such a high dielectric constant material layer 10 is formed, a high dielectric constant is added to the capacitance of the semiconductor layer 1 as shown in the equivalent circuit diagram of FIG. Since the additional capacitor by the material layer 10 is connected in parallel, the fluctuation range of the potential difference generated between the two ends of the pixel capacitor due to the generation of charge can be set to an appropriate range. Examples of the high dielectric constant material include barium titanate porcelain.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the two-dimensional radiation detector according to this embodiment includes a semiconductor layer 1 that converts an image of radiation into an image of electric charges, and signal readout provided on both sides of the semiconductor layer 1 so as to sandwich the semiconductor layer 1. It is mainly composed of the layer 2, the scan switch layer 3, and the drive circuit 4 connected to the scan switch layer 3.
[0020]
As shown in FIG. 2A, the signal readout layer 2 is composed of a plurality of column electrodes 21... 21 in a striped pattern divided into columns, and a signal readout circuit 5 is connected to each column electrode 21. Has been.
[0021]
As shown in FIG. 1 and FIGS. 2B to 2D, the scanning switch layer 3 includes pixel electrodes 31... 31 arranged in a matrix on the surface of the semiconductor layer 1 and the matrix pixel electrodes 31. A plurality of row electrodes 32... 32 in a stripe pattern divided corresponding to 31 rows, and a common electrode (uniform planar conductor) 33 to which a bias voltage is applied by a bias power source 35 are provided. FETs 34 are arranged between the pixel electrodes 31 and 31 and the common electrode 33, respectively.
[0022]
As shown in FIGS. 3 and 4, each FET 34 has a drain D connected to the pixel electrode 31 and a source S connected to the common electrode 33. The gate G of the FET 34 is connected to the row electrode 32, and the row electrode 32 connected to the gate G is connected to the FET control line of the drive circuit 4.
[0023]
As shown in FIG. 3, the drive circuit 4 includes a floating part 41, an optical isolation part 42, and a ground part 43.
The reason why the floating part 41 and the FET control line are electrically separated from the ground part 43 by the optical isolation part 42 in this way is that a bias voltage is applied to the source S of the FET 34 via the common electrode 33 in this embodiment. This is because it is necessary to float the FET control line connected to the gate G of the FET 34 from the ground. A configuration for electrically separating the FET control line from the ground is conceivable in addition to the use of the optical isolation unit 42 as described above.
[0024]
In the above structure, a support substrate 22 (see FIG. 7) such as glass is provided on the front side of the signal readout layer 2, and the support substrate (see FIG. 7) is also provided on the back side of the scan switch layer 3. Not shown).
[0025]
Next, the operation of the embodiment of the present invention will be described.
When X-rays enter the semiconductor layer 1, charges corresponding to the incident position and incident energy are generated on the surface of the semiconductor layer 1. The charge on the surface of the semiconductor layer 1 is read out for each pixel corresponding to each of the pixel electrodes 31... 31 by turning on the FET 34. In the present embodiment, the pixel signal current is These are taken out through each of the column electrodes 21 of the signal readout layer 2, stored in the integrating capacitor 52 of the charge sensitive preamplifier 51, outputted as a voltage signal, and sent to an A / D converter (not shown).
[0026]
For example, if a drive signal is given to the i-th FET control line and all i-th FETs 34 are turned on, the signals of the pixels corresponding to the i-row pixel electrodes 31. It will be.
[0027]
The integration switch 53 of the signal readout circuit 5 is for short-circuiting the integration capacitor 52 and discharging the capacitor 52 in preparation for the next scanning after the above A / D conversion is completed.
[0028]
Here, in the present embodiment, since the signal readout circuit 5 is connected to the column electrode 21 disposed on the surface of the semiconductor layer 1 opposite to the scan switch layer 3, the signal readout circuit 5, the FET 34, Since the semiconductor layer 1 is interposed between the FET 34 and the capacitance effect of the semiconductor layer 1 and the distance effect that the distance between the gate G of the FET 34 and the column electrode 21 of the signal readout layer 2 is increased, the FET 34 As a result, it becomes difficult for the gate leakage current and other switching noise from the scanning switch layer 3 to ride on the signal readout line and the S / N ratio of the signal current is improved, so that a high-quality image signal can be obtained. Further, since the common electrode 33 connected to the source S of the FET 34 of the scan switch layer 3 is a uniform planar conductor, there is an advantage that a shielding effect is generated and it is difficult to be influenced by external noise.
[0029]
Next, the operation of the present embodiment will be described more specifically.
FIG. 6 shows the two-dimensional radiation detector having the structure shown in FIG. 1, in which pixel electrodes 31... 31 are formed in a matrix of 1000 rows × 1000 columns, and all of the pixel electrodes 31. A time chart in the case of scanning 30 times per second by the switch layer 3 and outputting an image signal at 30 frames per second is shown.
[0030]
FIG. 6A shows the potential based on the bias voltage of the FET control line. The FET control line is sequentially set to a higher voltage by the drive circuit 4 for each row, and the FET 34 in that row (for example, i row) is turned on. Become.
[0031]
For example, paying attention to the pixel electrodes 31... 31 located in the i rows and j columns of the matrix pixel electrodes 31... 31, the potential of the semiconductor layer 1 corresponding to the pixels is accumulated as shown in FIG. However, when the i-th FET 34 is turned on, the voltage drops to the level of the bias voltage (FIG. 6B is expressed with reference to the bias voltage).
[0032]
Correspondingly, a signal read current for (i, j) pixels flows in the signal read line of the j column as shown in FIG. This signal current is stored in the integrating capacitor 52 of the charge-sensitive preamplifier 51, and is output as an integrated voltage signal as shown in FIG. 6D, and during the hold period until the integrating switch 53 is turned on. / D converted.
[0033]
FIG. 7 is a diagram schematically showing a main part structure of another embodiment of the present invention.
In this embodiment, a slit-like layer extending along the column electrode 21 is in contact with both the column electrode 21 and the pixel electrode 31 in a part of the semiconductor layer 1 (pixel end) constituting the pixel capacitor. The high dielectric constant material layer 10 is characteristically formed. When the high dielectric constant material layer 10 is formed in this way, in addition to the capacitance of the semiconductor layer 1, as shown in the equivalent circuit diagram of FIG. Since the additional capacitance due to the high dielectric constant material layer 10 is connected in parallel, the fluctuation range of the potential difference generated between both ends of the pixel capacitance due to the generation of charge can be set within an appropriate range.
[0034]
Further, as shown in the equivalent circuit diagram of FIG. 9, a method of connecting the additional capacitor 11 between the pixel electrode 31 and the predetermined common potential Va so as to be parallel to the semiconductor layer 1 is adopted. In the same manner as described above, the fluctuation range of the potential difference between the two ends of the pixel capacitor due to the generation of charges can be set within an appropriate range.
[0035]
Here, in the above embodiment, the bias power source 35 is connected to the common electrode 33 of the scan switch layer 3 to apply a bias voltage to each pixel. In addition, the equivalent circuit diagram of FIG. As described above, the bias power source 35 may be connected to the signal readout circuit 5 side.
[0036]
Further, in the above embodiment, the common electrode 33 is a uniform planar conductor, but the present invention can be implemented by adopting a configuration such as wiring pattern (bias line) routing instead. is there.
[0037]
【The invention's effect】
As described above, according to the two-dimensional radiation detector of the present invention, the signal readout layer is arranged on one surface of the semiconductor layer, the scanning switch layer is arranged on the opposite surface, and the switching element, the signal readout line, Therefore, the S / N ratio of the signal current is improved without picking up switching noise. As a result, a high quality X-ray image signal can be obtained.
[0038]
In addition, since the gate line and the data line (signal readout line) are allocated to the front and back of the semiconductor layer, the crossing distance between the gate line and the data line is more than 100 times that of the conventional one, and the crosstalk is reduced. And lowering the resistance and capacity of the data line.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing the structure of an embodiment of the present invention. FIG. 2 is a diagram schematically showing the structure of each layer of the embodiment. FIG. 3 is a connection of FETs in the embodiment of the present invention. FIG. 4 is a diagram schematically showing the relationship. FIG. 4 is an equivalent circuit diagram of the embodiment of the present invention. FIG. 5 is an equivalent circuit diagram of one pixel of the embodiment of the present invention. FIG. 7 is a diagram schematically showing the main structure of another embodiment of the present invention. FIG. 8 is an equivalent circuit diagram for one pixel of the embodiment shown in FIG. FIG. 9 is an equivalent circuit diagram for one pixel according to another embodiment of the present invention. FIG. 10 is an equivalent circuit diagram for one pixel according to still another embodiment of the present invention. FIG. 12 schematically shows the structure of each layer of the two-dimensional radiation detector shown in FIG. 11. FIG. Figure 14 is an equivalent circuit diagram of a two-dimensional radiation detector shown in FIG. 11 schematically showing an FET of the connections of the two-dimensional radiation detector shown in [Description of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor layer 2 Signal read-out layer 21 Column electrode 3 Scan switch layer 31 Pixel electrode 32 Row electrode 33 Common electrode 34 FET (switching element)
35 Bias power supply 4 Drive circuit 5 Signal readout circuit 51 Charge sensitive preamplifier 52 Integration capacitor 53 Integration switch 10 High dielectric constant material layer 11 Additional capacitance

Claims (2)

A semiconductor layer for converting an image of radiation into an image of electric charge, a signal readout layer and a scan switch layer provided on both sides of the semiconductor layer so as to sandwich the semiconductor layer, and a drive circuit connected to the scan switch layer,
The scan switch layer includes a plurality of pixel electrodes arranged in a matrix to correspond to each pixel on the surface of the semiconductor layer, a common electrode for applying a common bias voltage to each pixel, and a matrix A switching element disposed between each of the disposed pixel electrodes and the common electrode, and a row electrode for supplying a driving signal from the driving circuit to each switching element for each row,
The signal readout layer has a plurality of column electrodes divided corresponding to the columns of pixel electrodes arranged in a matrix, and each of the column electrodes is connected to a signal readout line,
A two-dimensional radiation detector, wherein a high dielectric constant material layer is formed on a part of a semiconductor layer corresponding to each pixel between the column electrode and each pixel electrode.   2. The two-dimensional radiation detector according to claim 1, wherein an amplifier made of a crystalline semiconductor is connected to the signal readout line as an element for amplifying a signal read out by each column electrode.

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