1. Field of the Invention
The present invention relates to a method for manufacturing a detector that can be applied to medical image diagnostic apparatuses, nondestructive inspection apparatuses and analyzers using radiation, and relates to a detector, a radiation detection apparatus and a radiation detection system.
2. Description of the Related Art
In recent years, thin-film semiconductor manufacturing techniques have been used for detectors and radiation detection apparatuses that use a pixel array including switching elements such as thin-film transistors (TFTs) and conversion elements such as photoelectric conversion elements.
In some of such detectors, the photoelectric conversion element and TFT of each pixel are formed on a substrate in a common process (see U.S. Pat. No. 6,682,960), and this type of detector hereinafter will be referred to as plane-type detector. U.S. Pat. No. 6,682,960 discloses the following techniques. It is performed through the same mask to form a metal layer such as Al (aluminum) layer that will be formed into source and drain electrodes of the TFT and to remove an impurity semiconductor layer from the region that will act as the channel of the TFT. Then, a metal layer such as an Al layer of the photoelectric conversion element is etched through another mask to form the upper electrodes of the photoelectric conversion element. In order to reduce the resistance of the metal layer that will be formed into the source and drain electrodes, a 1 μm thick Al film is used as the metal layer.
In U.S. Pat. No. 6,682,960, the metal layer is a 1 μm thick Al film. From the viewpoint of reducing the resistance, the metal layer can be formed of metals such as Al and Cu (copper), which are advantageously used as a wiring material in semiconductor devices and have specific resistances of less than 3.0 μΩ·cm at 300 K, to a thickness of 0.5 to 1 μm. Since these metals are not passive, they can be easily corroded by water or a remaining component of an etchant used in a manufacturing process. Accordingly, it becomes important that the source and drain electrodes are covered with a moisture-resistant passivation film with sufficient coverage. An inorganic insulating film formed by depositing silicon nitride (SiN) or the like by CVD is used as the moisture-resistant passivation film. Since the inorganic insulating film formed by CVD is hard, it can be cracked by thermal expansion and thermal contraction accompanying heat treatment performed in the manufacturing process if it is formed to a small thickness. Accordingly, in order to cover the source and drain electrodes with an inorganic insulating film with sufficient coverage, the inorganic insulating film is formed to a thickness of 0.5 to 1 μm, equal to the thickness of the source and drain electrodes. However, hard inorganic insulating films have high stresses, and may cause the substrate to warp. It is therefore undesirable to form the inorganic insulating film to a large thickness. In addition, since it takes a long time to form a thick inorganic insulating film by vapor deposition such as CVD, throughput is reduced. This is disadvantageous in manufacturing cost.
In the above-cited U.S. Pat. No. 6,682,960, the upper electrode of the photoelectric conversion element is made of a metal layer. In order to uniformly apply a bias to the entire photoelectric conversion element, the impurity semiconductor layer of the photoelectric conversion element is covered widely with a metal layer. However, if the impurity semiconductor layer of the photoelectric conversion element is widely covered with a metal layer, the aperture ratio, which is a ratio of the area of the semiconductor layer into which light can enter to the surface area of the photoelectric conversion element, is reduced.
Furthermore, if the upper electrode of the photoelectric conversion element and the source and drain electrodes of the TFT are formed in different steps, the number of masks is increased. Accordingly, the yield can be reduced and the cost can be increased.
Aspects of the present invention provide a method for manufacturing a detector including a photoelectric conversion element having a high aperture ratio and a corrosion-resistant TFT that are formed in a common process, without the increase in cost or decrease in yield accompanying the increase in the number of masks.
According to an aspect of the present invention, a method is provided for manufacturing a detector including a photoelectric conversion element that includes on a substrate, in this order from the substrate, a first electrode, an insulating layer, a semiconductor layer, an impurity semiconductor layer, and a second electrode to which a common electrode wire is electrically connected, and a thin film transistor that includes on the substrate, in this order from the substrate, a control electrode, an insulating layer, a semiconductor layer, an impurity semiconductor layer, and a first and a second main electrode including a first electroconductive member and a second electroconductive member. The method includes the first step of depositing a second electroconductive film containing a non-passive metal over the substrate so as to cover an impurity semiconductor film, and forming the first electroconductive member of the first and second main electrodes and the electrode wire from the second electroconductive film. The method also includes the second step of depositing a transparent electroconductive oxide film over the substrate so as to cover the impurity semiconductor film, the electrode wire and the first electroconductive member, forming the second electroconductive member of the first and second main electrodes and the second electrode from the transparent electroconductive oxide film, and forming the impurity semiconductor layer of the thin film transistor and the impurity semiconductor layer of the photoelectric conversion element from the impurity semiconductor film. The second electroconductive member, the second electrode, the impurity semiconductor layer of the thin film transistor, and the impurity semiconductor layer of the photoelectric conversion element are formed with the same mask in the second step, and the first electroconductive member and the electrode wire are formed with another mask in the first step.
Aspects of the present invention can provide a plane-type detector including a photoelectric conversion element having a high aperture ratio and a corrosion-resistant TFT that are formed in a common process, without increasing the cost or reducing the yield.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Some embodiments of the present invention will be described in detail with reference to the drawings. The radiation mentioned herein includes beams produced from particles (including photons) emitted by radioactive decay, such as α rays, β rays, and γ rays, and beams having the same energy or more, such as X rays, corpuscular beams, and cosmic rays.
The structure of the pixel of a detector according to a first embodiment of the invention will first be described with reference to
Each pixel 11 of the detector of an embodiment of the invention includes a photoelectric conversion element 12 that converts radiation or light into a charge, and a thin film transistor (TFT) 13, or a switching element, that outputs electrical signals according to the charge of the photoelectric conversion element 12. The photoelectric conversion element 12 has an MIS structure, which is the same layered structure as the TFT 13. The photoelectric conversion element 12 and TFT 13 are arranged side by side in the same plane on an insulating substrate 100, such as a glass substrate. The photoelectric conversion element 12 and TFT 13 are formed on the substrate 100 in a common process.
The photoelectric conversion element 12 includes on the substrate 100, in this order from the substrate, a first electrode 121, an insulating layer 122, a semiconductor layer 123, and an impurity semiconductor layer 124 having a higher impurity concentration than the semiconductor layer 123, and a second electrode 125. An electrode wire 14 of a metal such as Al is electrically connected to the second electrode 125 of the photoelectric conversion element 12. The second electrode 125 is made of a transparent electroconductive oxide such as ITO, and covers the entire surfaces of the impurity semiconductor layer 124 and the electrode wire 14, in the region of the photoelectric conversion element 12 in which the semiconductor layer 123 and the impurity semiconductor layer 124 are disposed. The second electrode 125 helps apply a uniform bias to the entirety of the photoelectric conversion element 12, and allows the photoelectric conversion element 12 to have a high aperture ratio.
The TFT 13 includes on the substrate 100, in this order from the substrate, a control electrode 131, an insulating layer 132, a semiconductor layer 133, and an impurity semiconductor layer 134 having a higher impurity concentration than the semiconductor layer 133, and a first and a second main electrode 135. The impurity semiconductor layer 134 is partially in contact with the first and second main electrodes 135, and the channel region of the TFT is defined between the portions of the semiconductor layer 133 in contact with the portions of the impurity semiconductor layer 134 in contact with the first and second main electrodes 135. The control electrode 131 is electrically connected to a control line 15. One of the first and second main electrodes 135 is electrically connected to the first electrode 121 of the photoelectric conversion element 12, and the other is electrically connected to a signal line 16. In the present embodiment, this electrode of the first and second main electrodes 135 is integrated with the signal line 16 using the same electroconductive layer, and serves as a part of the signal line 16. The signal line 16 and the first and second main electrodes 135 include a first electroconductive member 136 made of a metal such as Al and a second electroconductive member 137 made of a transparent electroconductive oxide such as ITO. The first electroconductive member 136 is covered with the second electroconductive member 137 and disposed between the second electroconductive member 137 and the impurity semiconductor layer 134.
The electrode wire 14 and the first electroconductive member 136 are made of an Al film having a thickness of about 1 μm from the viewpoint of reducing the resistance. Other materials that can be used for the first electroconductive member 136 include metals having a specific resistance of less than 3.0 μΩ·cm at 300 K, such as Cu, and alloys mainly containing such a metal. In the description herein, metals having a specific resistance of less than 3.0 μΩ·cm and alloys mainly containing such a metal are referred to as low-resistance metals. Since low-resistance metals are not passive, they can be easily corroded by water or a remaining component of an etchant used in the manufacturing process. A passive metal refers to a metal in a state where the metal does not corrode even though it is under corroding conditions in a thermodynamic sense, and the corrosion of a metal means that the metal reacts with the environment in use and turns into a non-metal state from the surface, and is thus gradually lost. The low-resistance metal member may be provided with films of a metal such as Mo, Cr or Ti having a higher specific resistance than the low-resistance metal on and under the low-resistance metal member. These metal films are intended to prevent the resistive contact of Al or the like with other members and the diffusion of Al or the like, and are referred to as barrier layers or ohmic contact layers. Even in this structure, a non-passive metal is exposed at the side surfaces of the electrode wire 14 and first electroconductive member 136 that have been formed by etching. The electrode wire 14 and the first electroconductive member 136 can have a thickness of 0.5 to 1 μm in view of electric resistivity and the precision of film forming (depositing). The second electrode 125 and the second electroconductive member 137 are made of a transparent electroconductive oxide, such as ITO. Exemplary transparent electroconductive oxides include ZnO, SnO2, and CuAlO2, in addition to ITO. Transparent electroconductive oxides are passive, and therefore have higher corrosion resistances than the above-described low-resistance metals. Transparent electroconductive oxides can be deposited to form a film with a low hardness by sputtering, and this film can cover the first electroconductive member 136 with a higher coverage than an inorganic film deposited by CVD. By covering the first electroconductive member 136 made of a non-passive low-resistance metal with the second electroconductive member 137 made of a passive transparent electroconductive oxide, a first and a second main electrode 135 highly resistant to corrosion can be formed for the TFT 13. In order to reduce the amount of retreat of the transparent electroconductive oxide film by etching (side etching amount), the thickness of the transparent electroconductive oxide film is set to about 50 nm. In view of the aperture ratio of the photoelectric conversion element and the S/N ratio according to the aperture ratio, a plane-type detector requires that the photoelectric conversion element have an electrode widely covering the impurity semiconductor layer and having a high light transmittance, and that the TFT be as small as possible and have a high operation speed. In order to prepare a TFT having a high operation speed, it is important to increase the ratio of the channel width (W) to the channel length (L) (W/L ratio). For a small TFT having a high operation speed, accordingly, the channel length of the TFT is reduced. Thus, the thickness of the transparent electroconductive oxide film can be 100 nm or less depending on the W/L ratio to be provided in view of the operation speed provided by the TFT, the aperture ratio of the photoelectric conversion element. In addition, in view of the electric resistivity to be provided by the second electrode 125 of the photoelectric conversion element, the thickness of the transparent electroconductive oxide film can be 50 nm or more. Furthermore, the thicknesses of the second electrode 125 and the second electroconductive member 137 can be smaller than and 0.02 to 0.1 times those of the common electrode wire 14 and the first electroconductive member 136. By covering the first electroconductive member 136 with the second electroconductive member 137, the second electroconductive member 137 defines the end faces of the first and second main electrodes 135. Thus, the channel length of the TFT 13 is determined by the second electroconductive member 137 that has been etched with a reduced retreat amount, and hence the channel length of the TFT 13 can be reduced.
The photoelectric conversion element 12 and TFT 13 are covered with a protective layer 147.
Turning now to
In the first step shown in
Subsequently, in the second step shown in
Subsequently, in the third step shown in
Subsequently, in the fourth step shown in
Subsequently, in the fifth step shown in
Then, a protective layer 147 is formed so as to cover the photoelectric conversion element 12 and the TFT 13. Thus, the structure shown in
The second electroconductive layer 145 formed in the above process is completely covered with the third electroconductive layer 146. Since the third electroconductive layer 146 is made of a corrosion-resistant transparent electroconductive oxide, such as ITO, the protective layer 147 need not cover the entire surfaces of the photoelectric conversion element 12 and the TFT 13. The protective layer 147 may be formed of an inorganic insulating film by CVD to such a thickness as can cover the side walls of the semiconductor layer 143 and impurity semiconductor layer 144 and the region of the semiconductor layer 143 that will act as the channel, for example, a thickness of 200 nm, smaller than the thickness of the second electroconductive layer 145. Alternatively, an organic insulating film that has a lower corrosion resistance but can be formed to a larger thickness, than the inorganic insulating film may be used for the protective layer 147, instead of the inorganic insulating film.
The equivalent circuit of a radiation detection apparatus according to the first embodiment of the invention will now be described with reference to the schematic diagram shown in
The operation of the radiation detection apparatus of the present embodiment will be described below. A reference potential Vref is applied to the first electrode 121 of the photoelectric conversion element 12 through the TFT 13, and a bias potential Vs is applied to the second electrode 125. Thus, a bias that can deplete the semiconductor layer 123 is applied to the photoelectric conversion element 12. In this state, the radiation emitted to a test subject is transmitted through the subject while being attenuated and is converted into visible light by the scintillator. The visible light enters the photoelectric conversion element 12 and is converted into a charge. When the TFT 13 is brought into electrical continuity by driving pulses applied to the control line 15 from the driving circuit 2, an electrical signal according to the charge is outputted to the signal line 16, and read outside as digital data by the read circuit 4. Then, positive carriers generated and remaining in the photoelectric conversion element 12 are removed by converting the potential of the common electrode wire 14 from a bias potential Vs to an initialization potential Vr and bring the TFT 13 into electrical continuity. Then, the photoelectric conversion element 12 is initialized by converting the potential of the common electrode wire 14 from an initialization potential Vr to a bias potential Vs and bringing the TFT 13 into electrical continuity.
Although the present embodiment has described a structure in which the control electrode 131 is electrically connected to the control line 15 and one of the first and second main electrodes 135 is electrically connected to the first electrode 121 of the photoelectric conversion element 12, the invention is not limited to this structure. For example, one of the first and second main electrodes 135 may be electrically connected to the electrode wire 14 in each pixel, and the first electrode 121 may be common to the photoelectric conversion elements 121. In this instance, the contact hole described with reference to
The structure of the pixel of a detector according to a second embodiment of the invention will now be described with reference to
The detector of the present embodiment includes an interlayer insulating layer 148 covering the side walls of the semiconductor layer 123 of the photoelectric conversion element 12 and the semiconductor layer 133 of the TFT 13, and an etch stop layer 149 covering the region of the semiconductor layer 133 that will act as the channel of the TFT 13, in addition to the structure of the first embodiment. This structure enhances the water resistance of the side walls of the photoelectric conversion element 12 and TFT 13. In addition, since two insulating layers are provided between the control line 15 and the signal line 16, the parasitic capacitance applied to the signal line 16 can be reduced, and thus noise can be reduced.
Turning now to
In the fourth step shown in
Subsequently, in the fifth step shown in
Subsequently, in the sixth step shown in
Subsequently, in the seventh step shown in
Then, a protective layer 147 is formed so as to cover the photoelectric conversion element 12 and the TFT 13. Thus, the structure shown in
A radiation detection system including the detector of an embodiment of the invention will now be described with reference to
An X ray 6060 generated from an X-ray tube 6050, or radiation source, penetrates the chest 6062 of a patient or test subject 6061 and enters the radiation detection apparatus 6040 in which a scintillator is disposed above the photoelectric conversion elements 12 in the photoelectric conversion portion 3. The incident X ray includes information of the interior of the patient's body. The scintillator emits light corresponding to the incidence of the X ray. The light is converted into electrical signals in the photoelectric conversion portion 3, and thus electrical information is produced. This information is converted into digital signals, and is then image-processed by an image processor 6070, which is a signal processing device. Thus, the information can be observed on a display 6080 that is a display unit in a control room.
In addition, the patient's information can be transmitted to a remote place through a transmission device, such as a telephone line 6090, and thus can be displayed on a display 6081 that is a display unit or stored in a recording device such as an optical disk, in a doctor room or the like in another place. Thus, the system allows doctors in remote places to diagnose. The information can be stored in a film 6110 that is a recording medium by a film processor 6100 used as a recording device.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2011-092151 filed Apr. 18, 2011, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
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2011-092151 | Apr 2011 | JP | national |