Method of operating a radiographic imaging device, radiographic imaging device and program

Abstract

To provide a method for operating a radiation imaging apparatus capable of correcting a pixel value and determining the service life of a sensor substrate by measuring the threshold voltage of a switching element included in a pixel, the radiation imaging apparatus, and a program.SOLUTION: The method for operating the radiation imaging apparatus includes a first process for supplying a first drive potential Voff2 to the control electrode of a switching element 101 in such a state that a reset switch 153 is turned on, after reset processing, to bring the switching element 101 into a weak non-conduction state, a second process for changing the bias potential of a conversion element 102 from a first bias potential Vs1 to a second bias potential Vs2, to accumulate an electric charge in the conversion element 102 and then, allowing an electric charge amount corresponding to a reference potential Vref, the first drive potential Voff2, and the threshold voltage of the switching element 101 to remain in the conversion element 102, a third process for reading the electric charge amount from the conversion element 102, and a fourth process for calculating the threshold voltage of the switching element 101 on the basis of the electric charge amount.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an operating method of a radiation imaging apparatus, a radiation imaging apparatus, and a program.

Background Art

[0002] There is an imaging apparatus in which a scintillator, a drive circuit, and a readout circuit are arranged on a sensor substrate in which pixels having a photoelectric conversion element such as a PIN diode and a switch element such as a thin film transistor (TFT) are formed in a two-dimensional matrix. Such an imaging apparatus is used not only for medical use but also for industrial use such as inspection of electronic components and inspection of piping in recent years. When the imaging apparatus is used for a long time, the threshold voltage of the switch element may fluctuate due to the influence of the voltage applied to the switch element or X-rays irradiated to the switch element, and the quality of the captured image may deteriorate.

[0003] Patent Document 1 describes an example in which an insulated gate field effect transistor is used as a switch element in an imaging apparatus. The document discloses a method for measuring the threshold voltage of an insulated field effect transistor. According to the document, it is described that the deterioration of the quality of the captured image is suppressed by setting the voltage applied to the switch element of the imaging apparatus according to the measured threshold voltage.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, it has been difficult to determine the threshold voltage of a switch element connected to a radiation conversion element in a radiation imaging device, particularly the rise voltage when the switch element conducts, and the amount of threshold voltage fluctuation due to aging. The present invention provides a technique advantageous for measuring the threshold voltage of a switch element in a radiation imaging device. [Means for solving the problem]

[0006] The radiation imaging apparatus of the present invention comprises a conversion element that converts radiation or light into electric charge and stores it, a switch element, a bias power supply that supplies a bias potential to the terminals of the conversion element, a drive circuit that supplies a drive potential to the control electrode of the switch element and controls the switch element, a readout circuit connected to the switch element that reads a signal from the conversion element through the switch element, and a control calculation unit. The first terminal of the switch element and the first terminal of the conversion element are connected, the readout circuit supplies a reference potential to the second terminal of the switch element, the bias power supply supplies a first bias potential to the second terminal of the conversion element, the drive circuit supplies a first drive potential to the control electrode to make the switch element non-conductive, the bias power supply changes the bias potential supplied to the second terminal of the conversion element from the first bias potential to the second bias potential to accumulate charge in the conversion element, and then leaves a charge amount in the conversion element corresponding to the reference potential, the first drive potential and the threshold voltage of the switch element. The control calculation unit is, The remaining The method is characterized by obtaining the threshold voltage of the switch element based on the amount of charge. [Effects of the Invention]

[0007] This technology can provide advantages for measuring the threshold voltage of a switch element in a radiation imaging device. [Brief explanation of the drawing]

[0008] [Figure 1] Block diagram of a radiation imaging system. [Figure 2] Part of the equivalent circuit of a radiation imaging device. [Figure 3] Pixel structure. [Figure 4] Explanation of the imaging operation. [Figure 5] Explanation of threshold voltage measurement operation. [Figure 6] Explanation of threshold voltage measurement operation. [Figure 7] Explanation of threshold voltage measurement operation. [Figure 8] Explanation of lifespan determination. [Figure 9] Explanation of characteristic variations. [Figure 10]Explanation of characteristic variations. [Figure 11] Explanation of image correction. [Figure 12] Explanation of the drive system changes. [Figure 13] Explanation of the drive system changes. [Modes for carrying out the invention]

[0009] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims. While the embodiments describe multiple features, not all of these features are essential to the invention, and the features may be combined in any way. Furthermore, in the attached drawings, identical or similar configurations are given the same reference numerals, and redundant descriptions are omitted.

[0010] (Example 1) An electronic component inspection system to which the present invention can be applied will be explained with reference to Figure 1. The radiation generator emits radiation in a conical shape towards the downward direction (-z direction) in the figure. A movable stage on which the object to be inspected (subject) is placed, and a movable stage on which a radiation imaging device is attached are each moved in a circular motion in the xy plane, while the radiation imaging device continuously acquires images and performs oblique CT scanning.

[0011] In this example, the subject and the radiation imaging device are moved by a movable stage, but other configurations are also acceptable as long as radiation irradiation and image acquisition can be performed while changing the relative positions of the radiation generator, subject, and radiation imaging device. The radiation control device supplies the radiation generator with the high voltage necessary for radiation generation, as well as signals to control radiation generation and stopping.

[0012] In addition, the electronic component inspection system may include a computer for controlling and processing the acquired information. The computer can control the radiation control device, the movable stage, and the radiation imaging device, and cause the radiation imaging device to perform various operations such as the imaging operation and the threshold voltage measurement operation described later. The computer can also save the images obtained by the radiation imaging device and reconstruct the oblique CT images, etc.

[0013] The radiation imaging device includes a sensor substrate for detecting radiation, a readout circuit for reading information from the sensor substrate, a drive circuit for controlling the drive of the sensor substrate, and a power supply unit for supplying voltage to these components. The radiation imaging device also has a control arithmetic unit for controlling the sensor substrate, the readout circuit, the drive circuit, and the power supply unit. The control arithmetic unit can control operations such as the imaging operation and the threshold voltage measurement operation described later, and cause the radiation imaging device to perform various operations.

[0014] The control arithmetic unit also performs necessary image processing such as forming image information based on the output information from the readout circuit. The radiation imaging device may further have a memory for storing two-dimensional map information (defect map) of defective pixels and two-dimensional map information (V0 map) of threshold voltages, which will be described later.

[0015] Figure 2 is a diagram showing the schematic of the circuit of the radiation imaging device. The radiation imaging device has a drive circuit 114, a sensor substrate 112, a readout circuit 113, an output buffer amplifier 109, and an analog / digital (A / D) converter 110.

[0016] A plurality of pixels 100 for detecting radiation are arranged in a matrix on the sensor substrate 112. In Figure 2, for simplicity of explanation, only some of the pixels 100 are shown on the sensor substrate 112, but the actual sensor substrate has more pixels. For example, in the case of a 17×17 inch sensor substrate, about 2800 rows × about 2800 columns of pixels can be arranged on the sensor substrate.

[0017] Each pixel 100 includes a conversion element 102 that converts radiation or light into electric charge, and a switch element 101 connected to the conversion element. An electrical signal corresponding to the charge accumulated in the conversion element 102 is output through the switch element 101. The switch element 101 is a transistor such as a TFT and has a gate electrode 101b, a source electrode 101c, a drain electrode 101a, and a channel layer (not shown). From the viewpoint of increasing the speed and resolution of the radiation imaging device, an oxide semiconductor, such as amorphous oxide semiconductors like IGZO or IZO, can be used for the channel layer.

[0018] The conversion element 102 is either an indirect or direct conversion element that converts irradiated radiation into electric charge. An indirect conversion element has a wavelength converter that converts radiation into light and a photoelectric conversion element that converts that light into electric charge. A direct conversion element is an element that can directly convert radiation into electric charge. Here, as an example of an indirect conversion element, we will explain using a PIN-type diode made mainly of amorphous silicon (a-Si).

[0019] The conversion element 102 has individual electrodes 102a for extracting signals, a common electrode 102c to which a bias potential is supplied, and a photoelectric conversion layer 102b made mainly of a-Si sandwiched between them. The photoelectric conversion layer 102b is a PIN type diode such that the side closer to the individual electrodes 102a is n+ type and the side closer to the common electrode 102c is p+ type. The individual electrodes 102a are connected to the source electrode 101c of the switch element 101, and the common electrode 102c is electrically connected to the bias power supply via a common bias line Bs. The bias line Bs extends in the column direction, but may also extend in the row direction.

[0020] The gate electrode 102b of each switch element in the k-th row (k=0 to Y-1) is commonly connected to the drive line Vg(k) corresponding to the row of the drive circuit. The drain electrode 101a of each switch element in the j-th column (j=0 to X-1) is commonly connected to the signal line Sig(j) corresponding to the column of the readout circuit 113. The source electrode 101c of each switch element is connected to the individual electrode 102c of the conversion element 102 of the pixel where the switch element is located. The drive circuit 114 is, for example, a shift register, and controls the conduction state of the switch elements 101 by supplying drive signals to the switch elements 101 via the drive lines Vg(0), Vg(1), ...

[0021] The readout circuit 113 is provided with an amplification circuit 106 corresponding to the signal line Sig(k), which amplifies the electrical signals of each signal line. The amplification circuit 106 includes an integrating amplifier 105, a variable gain amplifier 104, and a sample-and-hold circuit 107. The integrating amplifier 105 amplifies the electrical signals of the signal lines. The variable gain amplifier 104 amplifies the electrical signals from the integrating amplifier 105 with a variable gain.

[0022] The integrating amplifier 105 includes an operational amplifier 151 that amplifies and outputs the electrical signal of the signal line, an integrating capacitor 152, and a reset switch 153. The integrating amplifier 105 can change its gain (amplification ratio) by changing the value of the integrating capacitor 152. The sample-and-hold circuit 107 samples and holds the electrical signal amplified by the variable gain amplifier 104.

[0023] Furthermore, the readout circuit 113 is provided with a switch 126 corresponding to each row, and the switches 126 are connected to the multiplexer 108. The multiplexer 108 sequentially activates the switches 126 in each row, thereby sequentially outputting the electrical signals output in parallel from each amplification circuit 106 as serial signals to the output buffer amplifier 109.

[0024] The output buffer amplifier 109 converts the electrical signal to an impedance and outputs it. The analog-to-digital (A / D) converter 110 converts the analog electrical signal output from the output buffer amplifier 109 into a digital electrical signal and outputs it to the control calculation unit.

[0025] The drive circuit 114 outputs drive signals from the drive power supply 115 to each drive line Vg(0), Vg(1), etc., which have a conduction potential Von that makes the switch element conduct and a non-conduction potential Voff1 or Voff2 that makes it non-conductive. The reference power supply 111 supplies the reference potential Vref that is supplied to the forward input terminal of the operational amplifier 151. The bias power supply 103 supplies the bias potential Vs1 or Vs2. For the sake of explanation, this example shows the drive power supply 115, reference power supply 111, and bias power supply 103 arranged in a specific configuration, but the power supplies for each application may be combined.

[0026] The bias potential Vs1 or Vs2 from the bias power supply 103 can be changed by the control calculation unit. The drive potential from the drive circuit, including the conduction potential Von and the non-conduction potential Voff1 or Voff2, can also be switched by the control calculation unit. The reference potential Vref is usually set to a potential close to the ground potential GND (Vref = approximately -3 to +3V).

[0027] The bias potential Vs1 and the conduction potential Von and non-conduction potential Voff1 of the drive potential supplied to the drive circuit are potentials used in the imaging operation described later. The bias potential Vs1 is set to be sufficiently negative relative to the reference potential Vref in order to apply a reverse bias to the conversion element 102 in the photoelectric conversion layer 102b and sufficiently deplete the photoelectric conversion layer 102b to perform photoelectric conversion. For example, Vs1-Vref = -2 to -10V.

[0028] Furthermore, the conduction potential Von is set to a sufficiently large positive potential (Von-Vref = +20 to +5V) so that the switch element is in a fully conduction state in order to quickly transfer signal charge from the conversion element. The non-conduction potential Voff1 is set to a sufficiently large negative potential (Voff1-Vref = -5 to -20V) so that the drain-source leakage current in the switch element is negligibly small (for example, 10^-14A or less). Also, Voff1 is set to a negative potential than Vs1 (Voff1-Vs1 = -1 to -3V). On the other hand, the bias potential Vs2 and non-conduction potential Voff2 are used in the threshold voltage measurement operation S100, which will be described later. Examples of the setting values ​​for these potentials will be described later.

[0029] Figure 3 is an example of a schematic cross-sectional view of the sensor substrate 112. The channel layer 133 of the switch element is an oxide semiconductor, such as an amorphous oxide semiconductor like IGZO or IZO. The conversion element 12 includes, in order from the interlayer insulating layer side, individual electrodes 122, an n+ type impurity semiconductor layer 123, an intrinsic semiconductor layer 124, a p+ type impurity semiconductor layer 125, and a common electrode 126 on top of the interlayer insulating layer 120.

[0030] The common electrode 126 is electrically connected to the bias wire 14 via a contact hole (not shown). A phosphor 300 is positioned above the conversion element 12 (on the opposite side of the substrate 100), separated by a light-transmitting underlayer 301. When radiation is applied, visible light emitted by the phosphor 300 enters the semiconductor layer 124 of the conversion element.

[0031] (Imaging operation) The operation of the radiation imaging device when performing the imaging operation S009 is outlined in the flowchart in Figure 4(A) and the timing chart in Figure 4(B). The radiation imaging device performs the following steps S001 to S004 in order.

[0032] Signal charge accumulation process S001: The switch element 101 is deactivated, and signal charges corresponding to the radiation exposure amount are accumulated in each conversion element 102 on the sensor substrate 112.

[0033] Signal charge readout process S002: In the sensor substrate 112, conduction potentials Von are sequentially applied to the drive lines Vg(0), Vg(1), ..., and the signal charges accumulated in the conversion elements 102 of each row are sequentially transferred to the readout circuit 113. The readout circuit 113 measures the amount of charge transferred from the conversion elements 102, performs A / D conversion, and outputs to the control calculation unit. Two-dimensional image information can be generated in the control calculation unit.

[0034] Image Correction Process S003: The control calculation unit performs sensitivity correction and missing image correction on the 2D image. Sensitivity correction is an image processing that corrects the sensitivity variations for each pixel or column of the radiation imaging device. By multiplying the value of each pixel in the 2D image by a coefficient (sensitivity map) that has been determined for each pixel through prior calibration, the sensitivity variations of the scintillator and conversion element, as well as the gain variations of the amplification circuit, are corrected.

[0035] On the other hand, defect correction is an image processing technique that removes the effects of defective pixels from a two-dimensional image. For example, a pixel that does not produce the desired signal charge output due to open or short circuits originating from the manufacturing process of the sensor substrate is defined as a defective pixel. The coordinates of the defective pixels are stored in memory beforehand. This is called a defect map. In defect correction, the pixel value of the defective pixel is corrected by replacing it with another value, such as the average value of the pixel values ​​of several neighboring normal pixels. In addition to sensitivity correction and defect correction, this process may also include adjustment of the offset level for each pixel of the radiation imaging device (offset correction).

[0036] Image transmission process S004: The corrected image information is sent from the control calculation unit to an external device (such as a computer) outside the radiation imaging device.

[0037] Figure 4(B) shows the timing chart for the periodic repetition of the signal charge accumulation process S001 and the signal charge reading process S002 on the sensor substrate during imaging. In parallel with these processes, the control calculation unit can also repeatedly perform the image correction process S003 and the image transmission process S004 on the pixel values ​​of the previous frame read out in the previous cycle while the signal charge is being read out.

[0038] (How to operate) Next, as an example of how the radiation imaging device operates, the threshold voltage measurement operation S100 of the switch element connected to the conversion element will be explained using the flowchart in Figure 5(A) and the timing chart in Figure 5(B).

[0039] Reset process S010: The reset switch 153, located between the input and output of the operational amplifier 151, is turned on, and the reference potential Vref is supplied to the drain 101a of the switch element 101 via the signal line Sig(0)... The potential of the drive line is sequentially set to the conduction potential Von, and any unnecessary charges accumulated in the conversion element (such as dark charge due to the dark current of the conversion element) are discharged to the readout circuit, thereby resetting the conversion element.

[0040] If the individual electrode potential is Vx and the capacitance of the depleted conversion element 102 is C1, then in the pixels of a row where the drive line potential becomes the conduction potential Von, the individual electrode potential 102a becomes approximately equal to the reference potential Vref. The amount of charge Q accumulated in the conversion element becomes Q = C1 × (Vx - Vref) ≈ 0, and the conversion element 102 is reset. Note that since the purpose of this process is to discharge unnecessary charge, 2D image formation in the control calculation unit is not required. After the reset process, the drive line potential is set to the drive potential Voff1, which makes the switch non-conductive. Here, an example of each potential is Vref = 1V, Von = 15V, Voff1 = -6.5V.

[0041] First step S011: With the reset switch turned on, under the control from the control arithmetic unit, the driving line potential of all rows is switched from Voff1 to Voff2. Voff2 is a negative potential weaker than Voff1 (Voff1 < Voff2 < Vref), but the switch is maintained in the non-conductive state. As a result, in each switching element, the gate-source voltage becomes smaller compared to when the switch in the accumulation step S001 is off. For this reason, although it is in the non-conductive state, it can be in a weaker non-conductive state than when a normal switch is off. Hereinafter, this is referred to as the "weak non-conductive state". As an example, Vgs = -4V.

[0042] Second step (1) S012: Under the control from the control arithmetic unit, the bias potential is changed from Vs1 to Vs2 (Vs2 < Vs1). At this time, a potential difference of Vs2 - Vs1 transiently occurs in the pixel electrode, and a charge Q = C1×(Vs2 - Vs1) is applied to the conversion element 102. Thereafter, the individual electrode potential Vx also changes from Vref to Vref+(Vs2 - Vs1). The potential of each terminal at this time is shown in Fig. 6(A). The gate-source voltage Vgs of the switching element 101 is as shown in the following formula 1.

[0043] Vgs = Voff2 - Vx = Voff2 - Vref - (Vs2 - Vs1) (Formula 1) Here, the values of Vs1, Vs2, Voff2, and Vref are appropriately set so that Vgs > V0. For example, when Vref = +1V, Vs1 = -2V, Vs2 = -9V, and Voff2 = -3V, Vgs = +3V. If the threshold voltage V0 of the switching element is +3V or less, the switching element becomes conductive. The Vgs in this step is set to a smaller value compared to the signal charge readout step S002 (Vgs = Von - Vref = +20~+5V). The conductive state becomes a weaker conductive state than the conductive state of the switch in the signal charge readout step S002. This state is referred to as the switching element being in the "weak conductive state". As an example, it is a state where Vgs is about -1~+3V or Ids is about 10^-7A~10^-12A.

[0044] Next, in the second step (2) S013, a weak conduction state is maintained. At this time, as shown in Figure 6(B), a current Ids flows between the drain and source. The current Ids is a finite-sized leakage current (Ileak) as shown in the following equation 2.

[0045] Ileak = κ(Vx - (Voff² - V0)) 2 (Formula 2) Here, κ is the proportionality constant.

[0046] Because the value of Vgs is small, a portion of the charge Q applied to the conversion element due to the change in bias potential gradually flows out into the readout circuit as leakage current. Subsequently, the state is maintained without changing the drive line potential or bias potential for the amount of time it takes for the leakage current of the switch element generated in the second step to settle to almost zero. As shown in Equation 1, the leakage current Ileak becomes almost zero when the individual electrode potential Vx = Voff2 - V0. This state is Ids = 0 when Vx = Voff2 - V0, as shown in Figure 6(C). At this time, the amount of residual charge Q' remaining in the conversion element is given by Equation 3.

[0047] Q'=C1×(Vref-Voff2+V0)...(Formula 3) If the threshold voltage V0 differs for each pixel, the value of the residual charge Q' will also differ for each pixel, so the charge Q' is calculated for each pixel.

[0048] Third step S014: Under control from the control calculation unit, the drive line potential of all rows is returned from Voff2 to Voff1. Then, similar to the signal charge readout step S002, conduction potentials Von are sequentially applied to the drive lines Vg(0), Vg(1), ..., and the switches are turned on to sequentially transfer the charge Q' remaining in the conversion elements of each row to the readout circuit 113. The readout circuit 113 outputs a voltage based on the amount of charge transferred from the conversion element 102, performs A / D conversion, and outputs it to the control calculation unit as digital data.

[0049] The control calculation unit acquires the value of Q' based on digital data. Next, two-dimensional information of the charge Q' is generated based on the pixel position and the corresponding charge Q' value. Note that the magnitude of the charge Q' read out in this step may be larger than the magnitude of the signal charge read out in the signal charge readout step S002, so it is advisable to set the integral capacitance 152 of the integral amplifier 105 in the readout circuit 113 to be sufficiently large in advance and reduce the gain. After this step is completed, the bias potential is returned to Vs1.

[0050] Fourth step S015: The control calculation unit calculates a threshold voltage V0 corresponding to each pixel based on the above equation 3, and stores two-dimensional information of V0 corresponding to the pixel position (V0 map) in memory. The V0 map may record the value of V0 for each pixel, or the effective area of ​​the sensor board may be divided into several areas, and representative values ​​of V0 (average, maximum, minimum, median, etc.) and the variation of V0 values ​​within each area may be recorded for each area.

[0051] Figure 5(B) is a timing chart for the reset process S010 to the third process S014 performed on the sensor board. Following the third process, the control calculation unit performs the fourth process S015 to calculate the threshold voltage.

[0052] Note that the reset process may be included in the first step. In that case, instead of performing it prior to the first step as shown in Figure 5(B), it may be performed simultaneously with the first step for each row as shown in Figure 7(A). Figure 7(B) is a timing chart corresponding to when the reset process and the first step are performed simultaneously. As shown here, it is also possible to reset the conversion element by sequentially setting the drive lines to the conduction potential Von while the non-conducting potential is switched from Voff1 to Voff2. After that, the process may be carried out up to the third step S014 with the non-conducting potential at Voff2, and then the non-conducting potential may be returned to Voff1.

[0053] Next, we will describe an example of determining the lifespan of a radiation imaging device based on the measured threshold voltage or its fluctuation. Figure 8 is an example of a flowchart of the operation for determining the lifespan of a radiation imaging device. After power is turned on to the radiation imaging device S201, the radiation imaging device enters threshold voltage measurement operation S100 and creates a V0 map in memory, recording the threshold voltage V0 value for each pixel.

[0054] Next, the process enters the defect count calculation operation S203, where pixels whose threshold voltage V0 falls outside a predetermined range are designated as defective pixels, and the total number of defective pixels within the effective area is defined as the defect count D. The predetermined range can be a specified range, such as approximately +1V to -3V, depending on the characteristics of the switch element.

[0055] Next, the system proceeds to the judgment operation S204, where it is determined whether the number of defects D exceeds a predetermined standard value, the allowable value Dmax. If D > Dmax, the system proceeds to the notification operation S205, where the system notifies the user by any means that the radiation imaging device has reached the end of its lifespan and that it should be replaced. This notification may be given directly to the user via an LED lamp or by displaying it on a display device of the radiation imaging system.

[0056] Subsequently, the radiation imaging device may repeat the imaging operation S009 for any period of time, regardless of the result of the determination operation S204. The radiation imaging device may perform the threshold voltage measurement operation S100 again under any conditions. For example, this may be done after a predetermined period of time has elapsed since the previous threshold voltage measurement operation S100, or before or after calibration (updating the sensitivity map) in the radiation imaging device.

[0057] The threshold voltage measurement operation S100 may be performed automatically by the radiation imaging device according to any conditions, without being instructed from outside the radiation imaging device, as described above. It may also be performed at a timing controlled by the computer of the radiation imaging system or at a timing instructed by the user.

[0058] Furthermore, in the defect count calculation operation S203, instead of determining defective pixels based on the threshold voltage V0 value as described above, defective pixels may be determined based on the amount of variation from the initial value of the threshold voltage V0 of each pixel (for example, the threshold voltage V0 value at the time of factory shipment). Even when the V0 map is created for each region containing multiple pixels, rather than for each pixel, the radiation imaging device can perform defect determination in the same manner as described above. When performed for each region, the defect count calculation operation S203 can determine regions where the representative value or variation of V0 in the region exceeds a predetermined range as defective regions, and determine the total number of defective regions as the defect count D. In this case, the tolerance value Dmax used in the determination operation S204 should be the maximum allowable value determined for the total number of defective regions.

[0059] (Definition of threshold, etc.) The following provides a definition of the threshold voltage measured in this invention, and supplements the significance of measuring the threshold voltage in oxide TFTs. The characteristic changes that occur when IGZO-TFTs, a typical oxide TFT, are irradiated with X-rays have the following two characteristics. Feature 1: The amount of characteristic shift differs between the linear region and the rising edge region of conduction. Feature 2: The magnitude of characteristic variation is random for each element on the same sensor substrate.

[0060] First, let's explain Feature 1. Figures 9(A) and (B) show examples of the current-voltage characteristics (Ids-Vgs curve, Ids = drain-source current, Vgs = gate-source voltage) when an IGZO-TFT is irradiated with X-rays. Figures 9(A) and (B) show the same data with the vertical axis = linear and vertical axis = logarithmic, respectively. Now, we define the following, for example, as characteristic values ​​for the linear region of conduction (the region where Ids is approximately proportional to Vgs) and the rise region (the region where Ids changes by several orders of magnitude with respect to Vgs; this can also be called the nonlinear region of conduction). • Threshold voltage Vth in the linear region: The X-intercept obtained by linearly approximating the Ids-Vgs characteristic at Vgs = +15V in the linear graph Figure 9(A) and extrapolating it to the X-axis. • Threshold voltage V0 in the rising region: In the logarithmic graph Figure 9(B), Ids=10-13 Vgs becomes A.

[0061] Table 1 shows the initial characteristics and Vth and V0 after irradiation with a cumulative dose of 500 Gy. Both the linear region and the rise region are shifted in the negative direction, but the amount of shift differs between the two.

[0062] [Table 1]

[0063] During normal imaging, when the switch element of the radiation imaging device transfers the signal charge from the conversion element to the readout circuit, the switch element operates in the linear region. Therefore, the threshold voltage that can be measured by the method disclosed in Patent Document 1 is similar to the threshold voltage Vth in the linear region. On the other hand, the threshold voltage measurement operation S100 in this disclosure leaks the charge applied to the conversion element through the switch element and detects the Vgs of the switch element when the leakage stops. The threshold voltage obtained in this disclosure is similar to the threshold voltage V0 in the rising edge region.

[0064] As shown in Figure 9(B), even if the negative shift amount in the linear region is small, if the negative shift in the rising region is large, signal charge may unintentionally flow out in the signal charge accumulation process S001 of the radiation imaging device. Due to this outflow of signal charge, the amount of charge measured in the signal charge readout process S002 may decrease. In other words, the threshold voltage measurement method disclosed in Patent Document 1 could not correctly detect the characteristic fluctuations of the oxide TFT.

[0065] Next, we will explain Feature 2. Figure 10 is a conceptual diagram showing an example of degradation of the output image of a radiation imaging device that has a sensor substrate structure similar to that of Figure 2 and uses an IGZO-TFT as the switch element. In the figure, the "initial image" is an image corresponding to the signal charge obtained by uniformly irradiating with X-rays, and the "after X-ray irradiation" is an image corresponding to the signal charge obtained by uniformly irradiating with X-rays after using the radiation imaging device for a long time while irradiating with X-rays. In the image after long-term use, even though the X-rays are irradiated uniformly, the pixel values ​​differ from pixel to pixel. For example, there is a mixture of pixels whose pixel values ​​are almost the same as in the initial image (pixel A) and pixels whose pixel values ​​have decreased significantly compared to the initial image (pixel B).

[0066] In pixel A, the threshold voltage V0 of the switch element did not change much, so it is thought that there was no effect on the pixel value. On the other hand, in pixel B, the threshold voltage V0 of the switch element shifted significantly to the negative, so the switch element could not be made non-conductive, and it can be understood that some of the signal charge leaked out and was lost during the storage period.

[0067] In other words, the figure shows that the magnitude of the characteristic variation of the switch element due to X-ray irradiation (negative shift of V0) is random for each switch element in each pixel. The magnitude of the characteristic variation of oxide TFTs under X-ray irradiation will also differ among transistors, assuming that multiple transistors are contained in the same pixel. In some cases, such as pixel B, the conduction / non-conductivity operation as a switch may be virtually lost. Therefore, it can be said that simply determining the threshold in the linear region in the conventional method was insufficient to address image quality degradation.

[0068] (Example 2) Figure 11 illustrates an example of operation in a radiation imaging device that performs image correction according to the measured threshold voltage or its fluctuation.

[0069] The power-on operation S201 and threshold voltage measurement operation S100 are the same as in Example 1. Next, the missing map update operation S303 is performed. The control calculation unit identifies pixels containing switches whose threshold voltage V0 falls outside a predetermined range (for example, 0V to -3V) as defective pixels and adds their positions to the missing map as new missing pixels. After that, the radiation imaging system may repeat the imaging operation S009 for any period of time. In each imaging operation S009, image correction is performed using the updated missing map, thereby reducing the impact of defective pixels.

[0070] When updating the missing image map, the conditions under which the radiation imaging device performs the threshold voltage measurement operation S100 again can be any conditions, similar to the threshold measurement operation. However, if the number of defective pixels increases with the operating time of the radiation imaging device, it is advisable to update the missing image map periodically.

[0071] Even when V0 maps are created for each region rather than for each pixel, the radiation imaging system can similarly create a missing image map. In this case, for example, the missing image map update operation S303 can determine regions where the representative value or variation of V0 exceeds a predetermined range as defective regions, and add the pixels corresponding to the defective regions to the missing image map as missing pixels.

[0072] Furthermore, by measuring the threshold voltage V0 of the switch element for each pixel or region and preparing a lookup table of the Ids-Vgs characteristics of the switch element for each pixel, the leakage current generated in each defective pixel or region can be estimated. The lookup table can be prepared on the radiation imaging device or an external computer. The amount of signal charge Qleak lost due to leakage current in the defective pixel or region during the imaging operation S009 can be estimated from the threshold voltage V0, the Ids-Vgs characteristics, and the voltage supplied to the control electrode. Instead of interpolating from the surrounding pixel values ​​as the pixel value of the defective pixel or region in the image correction process S003, it is also possible to use a value corrected by adding back the lost signal charge Qleak to the pixel value of the defective pixel or region obtained in the imaging operation S009.

[0073] (Example 3) The following shows an example in which the driving of the radiation imaging device is controlled according to the threshold voltage measured by the radiation imaging device or the amount of change thereof, and the life of the radiation imaging device can be extended. FIG. 12(A) shows a part of the equivalent circuit of the radiation imaging device in this example. It is different from FIG. 2 in that the values of the non-conducting potential supplied from the power supply unit to the driving circuit are increased to three types, Voff1, Voff1', and Voff2, and can be switched by the control from the control arithmetic unit.

[0074] FIG. 12(B) shows an example of a flowchart in the radiation imaging device. The power-on S201 and the threshold voltage measurement operation S100 are the same as the life determination operation. In the subsequent defective pixel count calculation operation S203, pixels whose threshold voltage has shifted significantly in the negative direction are regarded as defective pixels. That is, pixels whose difference from the initial value of the threshold voltage V0 is less than a predetermined lower limit value (for example, -3V, etc.) are regarded as defective pixels, and the total number of defective pixels in the effective area is obtained and set as the number of defects D.

[0075] Subsequently, the determination operation S204 is entered, and it is determined whether the number of defects D exceeds a predetermined allowable value Dmax. If D > Dmax, the radiation imaging system determines that the image quality of the radiation imaging device is deteriorating, and switches the non-conducting potential from Voff1 to Voff1' (Voff1' < Voff1). In the subsequent imaging operation S009, since Voff1' is more negative than Voff1, the leakage current can be reduced in the pixels determined to be defective in the determination operation S204, and the deterioration of the image quality can be prevented as compared with the case where the non-conducting potential is not switched.

[0076] The condition for the radiation imaging device to perform the threshold voltage measurement operation S100 again can be any condition, for example, according to the elapsed time of use, similar to the threshold measurement operation. Note that the number of non-conducting potential values prepared may be more than three. In this case, as the deterioration of the radiation imaging device progresses, the life of the radiation imaging device can be further extended by gradually increasing the non-conducting voltage negatively.

[0077] Below is another embodiment of a radiation imaging system using the radiation imaging device of the present invention, in which the drive of the radiation imaging device is controlled according to the measured threshold voltage or its fluctuation, thereby extending the lifespan of the radiation imaging device. Figure 13 shows a part of the equivalent circuit of the radiation imaging device. It differs from Figure 12A in that the non-conductive potential values ​​supplied from the power supply unit to the drive circuit are the same as in Figure 2, Voff1 and Voff2, but the conductive potential values ​​have increased to two types, Von1 and Von1', and can be switched by control from the control calculation unit.

[0078] The flowchart for the radiation imaging system is the same as in Figure 12(B). In the determination operation S204, if the number of defects D is D > Dmax, the radiation imaging system determines that the image quality of the radiation imaging device is deteriorating and switches the conduction potential from Von1 to Von1' (Von1' > Von1). In the subsequent imaging operation S009, since Von1' is positively greater than Von1, a shift in the opposite direction to the threshold voltage shift caused by radiation occurs in all pixels, including the pixels determined to be defective in the determination operation S204. This reverse shift cancels out the threshold voltage shift caused by radiation. As a result, the deterioration of the radiation imaging device can be slowed down and its lifespan extended compared to when the conduction potential is not switched.

[0079] Alternatively, instead of increasing the conduction potential value using the circuit configuration in Figure 13, the circuit configuration may remain the same as in Figure 2, but the duty cycle of the switch element in the signal charge readout process S002 and the reset process S010 may be changed. The duty cycle is changed to lengthen the period during which the conduction potential is applied. In this case as well, the threshold voltage shift caused by radiation can be canceled out by generating a shift in the opposite direction to the threshold voltage shift caused by radiation.

[0080] (Other examples) The above embodiments 1 to 3 can be implemented not only individually but also in combination. For example, it is possible to configure the radiation imaging device so that only the image correction shown in Embodiment 2 is performed at the time of factory shipment, and as the degradation of the radiation imaging device progresses, the life extension by drive control as shown in Embodiment 3 is performed. Furthermore, when further life extension is no longer possible even after switching between multiple conduction and non-conduction potentials as shown in Embodiment 3, it is also possible to provide a life-extension notification to the user as in Embodiment 1.

[0081] Furthermore, in the above-described embodiment, the channel layer of the switch element 101 was an n-type semiconductor, and the photoelectric conversion layer 102b was a PIN-type diode such that the side closer to the individual electrode 102a was n+-type and the side closer to the common electrode 102c was p+-type. However, the channel layer of the switch element may be a p-type semiconductor, and the photoelectric conversion layer 102b may be a NIP-type diode such that the side closer to the individual electrode 102a was p+-type and the side closer to the common electrode 102c was n+-type. In this case, the polarity of the bias potentials Vs1, Vs2, conduction potential Von, and non-conducting potentials Voff1, Voff2 should all be reversed.

[0082] Furthermore, the applications of the radiation imaging device of the present invention are not limited to the radiation imaging system for inspecting electronic components and the like shown in this embodiment. It can also be applied to other industrial radiation imaging systems, such as those for pipe inspection, and medical radiation imaging systems for diagnostic imaging.

[0083] The present invention can also be realized by supplying a program that implements one or more of the functions of the above-described embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that implements one or more functions.

[0084] The invention is not limited to the embodiments described above, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, claims are attached to disclose the scope of the invention. [Explanation of Symbols]

[0085] 14 Bias line, 100 Pixel, 101 Switch element, 102 Conversion element, 103 Bias power supply, 104 Variable gain amplifier, 105 Integrating amplifier, 106 Amplification circuit, 107 Sample-and-hold circuit, 108 Multiplexer, 109 Output buffer amplifier, 110 A / D converter, 111 Amplifier reference power supply, 112 Sensor board, 113 Readout circuit, 114 Drive circuit, 115 Drive power supply

Claims

1. A radiation imaging apparatus comprising: a conversion element that converts radiation or light into electric charge and stores it; a switch element; a bias power supply that supplies a bias potential to the terminals of the conversion element; a drive circuit that supplies a drive potential to the control electrode of the switch element and controls the switch element; a readout circuit connected to the switch element that reads a signal from the conversion element through the switch element; and a control calculation unit, The first terminal of the switch element and the first terminal of the conversion element are connected. The readout circuit supplies a reference potential to the second terminal of the switch element, the bias power supply supplies a first bias potential to the second terminal of the conversion element, and the drive circuit supplies a first drive potential to the control electrode that causes the switch element to be in a non-conductive state. The bias power supply changes the bias potential supplied to the second terminal of the conversion element from the first bias potential to the second bias potential, thereby accumulating charge in the conversion element, and then leaves a charge amount in the conversion element corresponding to the reference potential, the first drive potential, and the threshold voltage of the switch element. The radiation imaging apparatus is characterized in that the control calculation unit acquires the threshold voltage of the switch element based on the amount of remaining charge.

2. The radiation imaging apparatus according to claim 1, wherein the control calculation unit calculates the threshold voltage by the following formula, Q'=C1×(Vref-Voff+V0) Herein, Q' is the amount of charge accumulated in the conversion element by changing the bias potential, Vref is the reference potential, Voff is the drive potential applied to the control electrode when the switch element is in a non-conductive state, C1 is the capacitance of the conversion element, and V0 is the threshold voltage, in a radiation imaging apparatus.

3. A method for operating a radiation imaging apparatus comprising: a sensor substrate having a conversion element that converts radiation or light into electric charge and stores it, and a switch element, wherein a plurality of pixels are arranged in a matrix, the first terminal of the conversion element and the first terminal of the switch element being connected; a bias power supply that supplies a bias potential to the second terminal of the conversion element; a drive circuit that supplies a drive potential to the control electrode of the switch element to control the switch element; a readout circuit connected to the second terminal of the switch element that reads a signal from the conversion element through the switch element; and a control calculation unit, wherein The first step involves the readout circuit supplying a reference potential to the second terminal of the switch element, the bias power supply supplying a first bias potential to the second terminal of the conversion element, and the drive circuit supplying a first drive potential to the control electrode, thereby causing the switch element to become non-conductive. The bias power supply performs a second step of changing the bias potential supplied to the second terminal of the conversion element from the first bias potential to the second bias potential to accumulate charge in the conversion element, and then leaving a charge amount in the conversion element corresponding to the reference potential, the first drive potential, and the threshold voltage of the switch element. A third step involves supplying a second drive potential from the drive circuit to the control electrode and reading the charge amount from the conversion element through the switch element, A method for operating a radiation imaging apparatus, characterized by including a fourth step of calculating a threshold voltage of the switch element using the control calculation unit based on the amount of charge.

4. The method for operating a radiation imaging apparatus according to claim 3, characterized in that the readout circuit supplies the reference potential to the second terminal of the switch element, the bias power supply supplies the first bias potential to the second terminal of the conversion element, the drive circuit performs a reset process by supplying the second drive potential to the control electrode, and after the reset process, the drive circuit supplies the first drive potential to the control electrode to bring it into a non-conductive state.

5. The threshold voltage calculated by the control calculation unit is calculated from the following formula: Q'=C1×(Vref-Voff+V0) The method for operating a radiation imaging apparatus according to claim 3 or 4, characterized in that, here, Q' is the amount of charge read from the conversion element in the third step, Vref is the reference potential, Voff1 is the first drive potential, C1 is the capacitance of the conversion element, and V0 is the threshold voltage.

6. A method for operating a radiation imaging apparatus comprising: a sensor substrate having a conversion element that converts radiation or light into electric charge and stores it, and a switch element, wherein a plurality of pixels are arranged in a matrix, the first terminal of the conversion element and the first terminal of the switch element being connected; a bias power supply that supplies a bias potential to the second terminal of the conversion element; a drive circuit that supplies a drive potential to the control electrode of the switch element to control the switch element; a readout circuit connected to the second terminal of the switch element that reads a signal from the conversion element through the switch element; and a control calculation unit, the method for operating a radiation imaging apparatus, The readout circuit supplies a reference potential to the second terminal of the switch element, the bias power supply supplies a first bias potential to the second terminal of the conversion element, and the drive circuit supplies a first drive potential to the control electrode. The bias power supply changes the bias potential supplied to the second terminal of the conversion element from the first bias potential to the second bias potential, thereby accumulating charge in the conversion element. A method for operating a radiation imaging device, comprising supplying a second drive potential from the drive circuit to the control electrode and reading the charge from the conversion element through the switch element.

7. A method for operating a radiation imaging apparatus according to any one of claims 3 to 6, characterized in that the channel layer of the switch element is an oxide semiconductor.

8. A sensor substrate having a plurality of pixels arranged in a matrix, each having a conversion element that converts radiation or light into an electric charge and stores it, and a switch element connected to the conversion element. The switching element is a thin-film transistor having a source electrode, a drain electrode, and a gate electrode. The conversion element is a diode having a pixel electrode, a common electrode, and a photoelectric conversion layer sandwiched between them. The sensor substrate further comprises a plurality of drive lines extending along the row of the pixel, a plurality of signal lines extending along the column of the pixel, and a plurality of bias lines extending along the row and column. The radiation imaging apparatus according to claim 1 or 2, characterized in that the gate electrode of the switch element arranged in the same row is connected to the drive circuit via the drive line of the same row, the drain electrode of the switch element arranged in the same column is connected to the readout circuit via the signal line of the same column, the source electrode of the switch element arranged in the pixel is connected to the pixel electrode of the conversion element arranged in the same pixel, and the common electrode of the conversion element is commonly connected to the bias power supply via the plurality of corresponding bias lines.

9. The radiation imaging apparatus according to claim 8, characterized in that the control calculation unit determines whether the threshold voltage of the switch element for each pixel, or the representative value of the threshold voltage of the switch element for each region including a plurality of pixels, or the variation of the threshold voltage exceeds a predetermined range.

10. The radiation imaging apparatus according to claim 9, characterized in that the control calculation unit determines the lifespan of the sensor substrate by comparing the number of pixels or regions exceeding the predetermined range with a reference value.

11. The radiation imaging apparatus according to claim 9, characterized in that the control calculation unit interpolates the pixel value of a pixel or a pixel value in a region that exceeds the predetermined range using the pixel values ​​of surrounding pixels or regions.

12. The radiation imaging apparatus according to claim 9, characterized in that the control calculation unit estimates the leakage current of the switch element in the pixel or region, and corrects the pixel value by adding back the amount of charge lost due to the leakage current to the pixel value of the pixel in the pixel or region.

13. The radiation imaging apparatus according to claim 9, characterized in that the first drive potential output from the drive circuit is adjusted according to the number of pixels or the number of regions that exceed the predetermined range.

14. The radiation imaging apparatus according to claim 9, characterized in that, depending on the number of pixels or regions exceeding the predetermined range, at least one of the value of the first drive potential output from the drive circuit or the second drive potential used to read the charge amount from the conversion element through the switch element, or the duty cycle, is adjusted.

15. A radiation imaging apparatus comprising: a conversion element for converting radiation or light into electric charge and storing it; a switch element; a bias power supply for supplying a bias potential to the terminals of the conversion element; a drive circuit for supplying a drive potential to the control electrode of the switch element and controlling the switch element; a read circuit connected to the switch element for reading a signal from the conversion element through the switch element; and a control calculation unit, The first terminal of the switch element and the first terminal of the conversion element are connected, the readout circuit supplies a reference potential to the second terminal of the switch element, the bias power supply supplies a first bias potential to the second terminal of the conversion element, and the drive circuit supplies a first drive potential to the control electrode. The bias power supply changes the bias potential supplied to the second terminal of the conversion element from the first bias potential to the second bias potential, thereby accumulating charge in the conversion element. A radiation imaging device that supplies a second drive potential from the drive circuit to the control electrode and reads the charge from the conversion element through the switch element.

16. The radiation imaging apparatus according to any one of claims 1, 2, 8 to 15, characterized in that the channel layer of the switch element is an oxide semiconductor.

17. A program for causing a computer to perform the operation method of a radiation imaging apparatus described in any one of claims 3 to 7.

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