JP4156995B2 - X-ray flat panel detector - Google Patents

Description

  The present invention relates to an X-ray flat panel detector.

In the medical field, an X-ray imaging apparatus using an X-ray flat panel detector having an X-ray charge conversion film formed of amorphous selenium (hereinafter a-Se) on a thin film transistor (hereinafter TFT) array has been proposed. . (For example, Patent Document 1)
This has a structure in which an X-ray charge conversion film is sandwiched between a pair of electrodes.

  A voltage is applied to the common electrode by the power source, and a voltage is applied to the X-ray charge conversion film. When the detector is irradiated with X-rays in this state, charges are generated inside the X-ray charge conversion film by the incident X-rays. This electric charge is temporarily stored in the capacitor via the pixel electrode arranged for each pixel. The accumulated electric charge can be read out by turning on the TFT. An X-ray transmission image can be obtained from the planar distribution of the read charge amount.

In a direct-type detector using a-Se as an X-ray charge conversion film, when imaging with 60 keV X-rays used for chest imaging or the like, a conversion film necessary for absorbing 68% of incident X-rays is used. The thickness is 970 μm. (Non-Patent Document 1)
In order to take out the electric charge generated in the a-Se film having a thickness of about 1 mm to the outside, it is necessary to apply a very high voltage of about 10000 V to the conversion film.

Other direct conversion type photosensitive films include PbI2, HgI2, etc. in addition to Se.
JP 61-62283 A S. O. Kasap, et. al, Journal of Non-Crystalline Solids 299-302 (2002) pp. 988-992

  In the conventional detector, since a-Se is used for the conversion film, a thickness of about 1 mm or more is necessary to sufficiently absorb X-rays. For this reason, there is a problem that an expensive high-voltage power supply for applying a high voltage to the conversion film is necessary, and the price of the detector is expensive.

  Further, the conventional detection material has a large impact on the environment, and since the impact on the environment is large, it is necessary to take a measure such as recovery, which increases the cost.

An embodiment of the present invention includes an X-ray charge conversion film, a pixel electrode and a common electrode provided in contact with both surfaces of the X-ray charge conversion film, and the X-ray charge conversion film includes La, Ce , Pr, Nd, Pm, Sm , Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, X -ray flat panel detector which is a compound of the lanthanide and iodine selected from among Lu It is.

Here, the X-ray charge conversion film is made of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, lanthanoid, iodine, It may be a compound with oxygen .

The X-ray charge conversion film may be Ce, Pr, Eu, Gd, Tb, Yb iodide.

  According to the present invention, by using a lanthanoid compound having a large atomic number and a large X-ray absorption coefficient for the conversion film, it is possible to reduce the thickness of the conversion film, and an X-ray flat panel detector that is inexpensive to manufacture. realizable.

  In addition, since rare earth halides have little environmental impact, there is an advantage that the cost of recovering the X-ray charge conversion film is low and the possibility of influence on the human body is low.

  Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

  FIG. 1 is a plan circuit diagram of an X-ray flat panel detector.

  The pixel e1.1 includes an amorphous silicon (a-Si) TFT 102, an X-ray charge / electric conversion film 210, and a pixel capacitor 105. Each pixel e is in the form of an array in which several hundred to several thousand are arranged on each side in the vertical and horizontal directions.

  A negative bias voltage is applied to the X-ray charge-electric conversion film 210 by the power source 109. The a-Si TFT 102 is connected to the signal line 103 and the scanning line 606, and on / off is controlled by the scanning line driving circuit 607. The end of the signal line 103 is connected to a signal detection amplifier 610 through a changeover switch 609 controlled by a signal line control circuit 608.

  When X-rays enter, holes and electrons are generated in the X-ray charge conversion film, and the electrons are temporarily stored in the storage capacitor 105.

  When the scanning line driving circuit 607 drives the scanning line 606 and turns on one column of a-Si TFTs 601 connected to one scanning line 606, the accumulated charge is transferred to the amplifier 610 side through the signal line 103. Is done. The changeover switch 609 inputs the charge for each pixel to the amplifier 610 and converts it into a dot sequential signal that can be displayed on a CRT or the like. The amount of charge varies depending on the amount of light incident on the pixel e, and the output amplitude of the amplifier 610 changes, which corresponds to the luminance.

  FIG. 2 is a cross-sectional view of one pixel.

An a-Si layer 204 is provided over the substrate 201 with the gate electrode 104 and the insulating layer 202 interposed therebetween. A stopper layer 206 is formed on the a-Si layer 204 with high positional accuracy with respect to the gate electrode 104. A drain electrode 2051 and a source electrode 2052 are provided on the a-Si layer 204 outside the gate electrode 104. In this way, the TFT 102 is formed. Further, a signal line electrode 103 and a source electrode 205 are provided on the drain electrode 2051.
2 is provided with a connection electrode 2071.

  A lower capacitor electrode 203 and an insulating layer 202 are provided on the substrate 201. The lower capacitor electrode 203 is opposed to the upper capacitor electrode 207 with the insulating layer 202 interposed therebetween, and forms a storage capacitor 105. Here, the upper capacitor electrode 207 and the connection electrode 2071 may be a common electrode or two electrodes that are electrically connected.

  A resin layer 208 is provided on the TFT 102 and the upper capacitor electrode 207, and a pixel electrode 209 is further provided thereon. The pixel electrode 209 and the connection electrode 2071 are electrically connected.

  An X-ray charge conversion film 210 is provided on the pixel electrode 209.

  A common electrode 108 is provided on the X-ray charge conversion film 210.

  FIG. 3 shows a schematic circuit diagram of FIG. 2, and the same reference numerals are given to portions common to FIG. 2.

  As shown in FIG. 3, the X-ray charge conversion film 210 is sandwiched between the common electrode 108 and the pixel electrode 209, and the common electrode 209 is connected to the power source 109. On the other hand, the pixel electrode 209 is connected to the TFT 102 and the storage capacitor 105. The TFT 102 is controlled by the gate line 104 and has an on / off function of the pixel electrode 209 and the signal line 103.

  Here, the configuration of the X-ray charge conversion film 210 will be described.

  In general, the X-ray absorption coefficient of a substance varies depending on the energy of X-rays and the absorption shell, but the larger the atomic number, the larger. For this reason, as the X-ray detector material, the atoms after the periodic table can obtain a larger absorption with a smaller film thickness. For this reason, iodides of heavy elements such as PbI2 and HgI2 exhibit good characteristics.

  Lanthanoids belong to the same cycle as Pb and Hg, and show similar absorption coefficients. The atomic number Z of the lanthanoid is La (Z = 57), Ce (Z = 58), Pr (Z = 59), Nd (Z = 60), Pm (Z = 61), Sm (Z = 62), Eu ( Z = 63), Gd (Z = 64), Tb (Z = 65), Dy (Z = 66), Ho (Z = 67), Er (Z = 68), Tm (Z = 69), Yb (Z = 70) and Lu (Z = 71), which is larger than Se (Z = 34), and these elements have a larger X-ray absorption coefficient than Se.

  However, since lanthanoids have a low resistivity by themselves, it is difficult to use them as X-ray charge conversion film materials. Therefore, in the present invention, the X-ray charge conversion film is formed using a compound of lanthanoid, which is a substance having high resistivity, and iodine, bromine, and chlorine. An appropriate amount of F may be included for the purpose of controlling the chemical stability of the material.

  The atomic numbers of these halogens are F (Z = 9), Cl (Z = 17), Br (Z = 35), and I (Z = 53), and Br and I with larger atomic numbers are preferable.

  The X-ray absorption coefficient can be defined by the product of the material density and the absorption coefficient of the constituent elements. Lanthanoid iodide has a specific gravity of 4 to 6 and a specific gravity close to that of PbI2 or HgI2.

  A trivalent iodide of a lanthanoid element selected from La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu is used for the X-ray charge conversion film. it can. These trivalent lanthanoid iodides have a larger X-ray absorption coefficient than Se.

  In addition, a lanthanoid divalent iodide selected from Sm, Eu, Dy, Yb, and Lu may be used for the X-ray charge conversion film. These divalent lanthanoid iodides also have a larger X-ray absorption coefficient than Se.

  Further, a lanthanoid oxyiodide lanthanoid which is a compound of lanthanoid, oxygen and iodine may be used for the X-ray charge conversion film. Oxidized iodinated lanthanoids are chemically more stable to moisture and air than iodinated lanthanoids. Therefore, the manufacturing process and storage equipment after film formation can be simplified. Further, since the characteristics of the X-ray charge conversion film hardly deteriorate during use of the detector, a detector with high long-term reliability can be realized.

  In these detectors using an X-ray charge conversion film made of divalent and trivalent lanthanoid iodides and lanthanoid oxide lanthanoids, when imaging is performed with 60 keV X-rays used for chest imaging, 68 X The thickness of the conversion layer necessary for absorbing% is about 900 μm or less, which can be made thinner than the conversion film using Se.

  By using these materials having large X-ray absorption for the conversion film, X-rays can be sufficiently absorbed even if the film thickness is reduced. By using such a conversion film, it is possible to take out charges generated by X-rays with a lower applied voltage to the outside with a lower applied voltage. Therefore, there is an advantage that a power source having a small capacity and a low price can be used.

  In addition, since the environmental impact is small, there is a merit that the cost of collecting the photosensitive film can be reduced.

  Next, an example of the manufacturing method of this embodiment will be described.

  An Al alloy or MoW film having a thickness of about 300 nm is formed on the glass substrate 201 by sputtering and then patterned to form the gate electrode 104 and the storage capacitor electrode 203.

  A silicon oxide film having a thickness of about 300 nm is formed thereon as the insulating layer 202 by CVD.

  On this, the a-Si layer 204 is patterned.

  Further, a SiNx film is formed thereon, backside exposure is performed, and the stopper layer 206 is formed with high positional accuracy with respect to the gate electrode 104.

  An n + Si layer is formed by a CVD method and patterned to form a drain electrode 2051 and a source electrode 2052.

  Next, an Al alloy layer is patterned to form the signal line electrode 103, the connection electrode 2071, and the upper capacitor electrode 207.

  On this, an acrylic resin layer 208 is pattern-formed with a thickness of about 3 to 4 μm.

  Further, an ITO layer is patterned on the resin layer 208 to form a pixel electrode 209.

  As described above, the TFT array substrate 101 is formed.

  On top of this, compounds of iodine, bromine and chlorine, which are lanthanoid elements selected from La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu by vacuum deposition or the like The film is formed.

    Further, Cr having a thickness of about 200 nm was formed as the common electrode 108 by sputtering. Other metals such as Pt and Pd may be used for the common electrode 108.

  Although illustration is omitted, passivation is formed at the end portion between the common electrode 108 and the TFT array substrate 101 to prevent creeping discharge.

  A detection amplifier, an AD converter, an image processing circuit, etc. are connected to complete an X-ray flat panel detector.

  Further, a protective TFT can be disposed under the pixel electrode 209. This is connected between the pixel electrode and the ground line or the like in order to prevent the TFT 102 and the storage capacitor 105 from being destroyed when the electrons reaching the pixel electrode 209 become excessive. When the potential of the pixel electrode 209 exceeds a certain value, the diode-connected protection TFT is opened, and excess charge of the pixel electrode is discharged to the ground line or the like.

  The X-ray charge conversion film 210 is selected from La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu on the surface of the TFT array substrate 101 by vacuum deposition. The trivalent iodide of the lanthanoid element is deposited in the range of about 100 μm to about 900 μm.

  For example, LuI3 is deposited to a thickness of about 500 μm. In this case, the X-ray absorption amount of the X-ray charge conversion film 210 and the charge extraction amount from the X-ray charge conversion film 210 can be set to preferable values.

  As the X-ray charge conversion film 210, a divalent iodide of a lanthanoid selected from Sm, Eu, Dy, Yb, and Lu is used.

  The X-ray charge conversion film 210 is formed in a range of about 90 μm to about 850 μm by vacuum deposition.

  For example, LuI2 is deposited to a thickness of about 400 μm. When the thickness is about 400 μm, the amount of X-ray absorption of the X-ray charge conversion film and the amount of charge extracted from the X-ray charge conversion film can be set to preferable values.

  The detector using a divalent lanthanoid iodide has a larger ratio of lanthanoid to iodine and a larger X-ray absorption coefficient than a detector using a trivalent lanthanoid iodide. Can be thinned.

  The X-ray charge conversion film 210 is a compound of a lanthanoid element selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and iodine and oxygen. Use lanthanoid oxyiodide.

  A lanthanoid oxide oxide compound is deposited in a thickness range of about 100 μm to about 900 μm by vacuum deposition.

  For example, about 450 μm of LuOI is deposited to form the X-ray charge conversion film 210.

  When the thickness is about 450 μm, the X-ray absorption amount of the X-ray charge conversion film 210 and the amount of charge taken out from the X-ray charge conversion film 210 can be set to preferable values.

  The lanthanoid oxyiodide thus formed is more stable than lanthanoid halides with respect to moisture and air, and the manufacturing process and storage equipment after film formation can be simplified. Further, since the characteristics of the conversion film hardly deteriorate during use of the detector, long-term reliability is high.

  Ce, Pr, Eu, Gd, Tb, and Yb iodide are used as the X-ray charge conversion film 210.

  For example, TbI2 having a thickness of about 100 μm to about 1000 μm is formed on the pixel electrode 209. The film thickness is preferably about 500 μm.

  When the thickness is about 500 μm, the amount of X-ray absorption of the X-ray charge conversion film 210 and the amount of charge taken out from the X-ray charge conversion film 210 can be set to preferable values.

  When TbI2 is used, Tb emits light by X-rays, and thus light emission reaches an adjacent pixel. As shown in FIG. 4, the light is absorbed by TbI2 of the adjacent pixel and generates an electron-hole pair to generate a weak signal charge. Therefore, the resolution is slightly deteriorated. As a result, a soft image is obtained and shot noise can be reduced. This is particularly effective for moving images used with weak X-rays.

  It is also possible to further reduce the resolution by reducing the light emission by the adjacent pixels into charges, thereby reducing shot noise and the like. For this purpose, the pixel electrode 209 is made of a material transparent to visible light, and a photodiode for sensing light is formed on the substrate. This photodiode converts the light emission of the strong light emitting group halide into an electric charge.

  Such an effect can be obtained by Ce, Pr, Eu, Gd, Tb, and Yb, which emit strong light in the non-closed shell transition.

  Rare earths emit light at a transition in the level of 4f, which is a non-perfectly closed shell, so that the light emission characteristics are almost maintained even when compounds with other elements are formed. Depending on the state of the incompletely closed shell level, it can be classified into a strong emission group of Ce, Pr, Eu, Gd, Tb, and Yb and a weak emission group of Nd, Pm, Sm, Dy, Ho, Er, Tm, and Lu.

  In the strong emission group, the number of levels in the incomplete closed shell is small, the energy between levels is large, and visible light emission is strong. On the other hand, in the weak emission group, there are many levels in the incompletely closed shell, the energy between levels is small, and the emission of visible light is weak. Lu emits light weakly because there is no order in the incompletely closed shell.

  For this reason, the resolution deteriorates when the halide of the strong light emission group is used.

  However, when the resolution is too high, aliasing noise is emphasized or shot noise is conspicuous, but it is convenient for moving picture diagnosis used with weak X-ray illuminance.

On the contrary, in the case of a still image in which resolution is important, it is preferable to use a halide of weak emission group as the X-ray charge conversion film.

  A cross-sectional view of this example is shown in FIG. The same parts as those in FIG.

  Here, a mixed layer of the lanthanoid halide particles 212 and the organic polymer 211 is formed on the surface of the TFT array 101 manufactured by the same process as in the first embodiment.

  For example, YbCl3 is used as a lanthanoid halide by processing into particles having a diameter of about 50 μm.

  As the organic polymer 211, polycarbonate is used as a base polymer. Here, the ratio of the weight of the polycarbonate to the lanthanoid halide used in the mixed layer is 1 for the lanthanoid halide to 1 for the polycarbonate.

  In advance, the base polymer is dissolved in cyclohexanone as an organic solvent. In addition to this, toluene, DMF, or the like may be used as the organic solvent.

A butadiene derivative that is a charge transport agent is dissolved in the organic solvent. It is preferable that the amount of the charge transfer agent is about 15 wt% or less in the completed X-ray charge conversion film 210. As the organic transport material, Alq ₃ , TPD, polyphenylene vinylene, polyalkylthiophene, polyvinyl carbazole, triphenylene, liquid crystal molecules, and metal phthalocyanine can be used.

  A dispersion solution in which lanthanoid halide particles are dispersed in a base polymer is applied onto the TFT array substrate 101 and heated at about 80 ° C. for about 1 hour or longer to evaporate the solvent. Finally, the amount of dispersion liquid dropped on the TFT array 101 is adjusted in advance so that the film thickness of the X-ray charge conversion film 210 is about 200 μm or more.

  For example, when the weight ratio of the lanthanoid halide to the polycarbonate is 5 for the lanthanoid halide to 1 for the polycarbonate, the film thickness is preferably 1000 μm.

  Next, Cr or the like is deposited as a common electrode 108 with a film thickness of about 200 nm by sputtering deposition.

  In this way, a dispersion film can be formed without using an expensive vacuum deposition apparatus by dispersing a mixture of a lanthanoid halide and an organic polymer in a solvent and applying this dispersion solution on the TFT array. Can be made inexpensively.

The plane schematic diagram explaining an embodiment. 1 is a schematic cross-sectional view illustrating an embodiment. Cross-sectional explanatory drawing explaining embodiment. Explanatory drawing explaining Example 4. FIG. FIG. 6 is a schematic cross-sectional view illustrating Example 5;

Explanation of symbols

101 ... TFT array substrate 102 ... TFT
104 ... Gate electrode 105 ... Storage capacitor 108 ... Common electrode 202 ... Insulating layer 204 ... a-Si layer 206 ... Stopper layer 2051 ... Drain electrode 2052 ... Source electrode 203 ... Lower capacitance electrode 207 ... Upper capacitance electrode 2071 ... Connection electrode 209 ... Pixel electrode 210 ... X-ray charge conversion film 211 ... Organic polymer layer 212 ... Lanthanoid halide particles

Claims (3)

An X-ray charge conversion film; and a pixel electrode and a common electrode provided on both sides of the X-ray charge conversion film. The X-ray charge conversion film includes La, Ce, Pr, Nd, Pm, and Sm. , Eu, Gd, Tb, Dy , Ho, Er, Tm, Yb, Lu, X -ray flat panel detector which is a compound of the lanthanide and iodine selected from among. The X-ray charge conversion film is composed of a lanthanoid selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, iodine, and oxygen. The X-ray flat panel detector according to claim 1, which is a compound. The X-ray charge conversion film, Ce, Pr, Eu, Gd , Tb, X -ray flat panel detector according to claim 1, characterized in that a compound of Yb and iodine.