Light detection device

Abstract

To provide a photodetector capable of achieving high definition.SOLUTION: A photodetector (100) includes a first 1TFT (10) that converts light into an electric signal, the first 1TFT including a first source electrode (14), a first drain electrode (16), a first semiconductor oxide film (12) extending between the first source electrode and the first drain electrode, and a first gate electrode (18) that controls a current in the first semiconductor oxide film, the first gate electrode and the first source electrode being electrically connected to each other.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a photodetector device having a thin film transistor. [Background technology]

[0002] Flat panel type photodetectors, in which photodiodes that convert light into electric charges and thin film transistors (TFTs) that function as switching elements are arranged in a matrix, are widely used as image sensors, photosensors, etc. An example of a radiation imaging device equipped with such a photodetector is disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-156119 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 discloses that the semiconductor layer of a TFT may be made of an oxide semiconductor such as indium gallium zinc oxide (InGaZnO) or zinc oxide (ZnO). Compared to other semiconductors, oxide semiconductors have the advantage of low leakage current.

[0005] However, the photodiode used in the photodetector disclosed in Patent Document 1 needs to have a large area to obtain the required sensitivity, which makes it difficult to achieve high-definition photodetectors.

[0006] An object of one embodiment of the present invention is to provide a photodetector that can achieve high definition. [Means for solving the problem]

[0007] In order to solve the above problem, a photodetector according to one embodiment of the present invention includes a first TFT (Thin Film Transistor) that converts light into an electrical signal, the first TFT having a first gate electrode, a first source electrode, a first drain electrode, and a first oxide semiconductor film spanning between the first source electrode and the first drain electrode, and the first gate electrode and the first source electrode are electrically connected to each other. [Effects of the Invention]

[0008] According to one aspect of the present invention, a photodetector capable of achieving high definition can be realized. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view of a photodetector according to one embodiment of the present invention. [Figure 2] 10 is a graph showing changes in the relationship between voltage and current when a TFT of a comparative example and a first TFT included in a photodetector according to one embodiment of the present invention are irradiated with X-rays. [Figure 3] FIG. 2 is a diagram illustrating a first TFT and a second TFT in a plan view. [Figure 4] 4 is a graph showing the relationship between gate voltage and drain current in the first TFT and the second TFT illustrated in FIG. 3. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, one embodiment of the present invention will be described in detail.

[0011] FIG. 1 is a schematic cross-sectional view of a photodetector 100 according to one embodiment of the present invention. The photodetector 100 is, for example, a flat panel type, and may be, for example, a photosensor, an image sensor, or a radiation detection device (X-ray imaging display device), but is not limited to these. As shown in FIG. 1, the photodetector 100 includes a first TFT (Thin Film Transistor) 10 and a second TFT 20. The first TFT 10 and the second TFT 20 are formed on a substrate (not shown).

[0012] The first TFT 10 converts light into an electrical signal. The first TFT 10 includes a first source electrode 14, a first drain electrode 16, a first oxide semiconductor film 12 extending between the first source electrode 14 and the first drain electrode 16, and a first gate electrode 18 for controlling a current in the first oxide semiconductor film 12. The first TFT 10 further includes a wiring layer 19 for applying a bias to the first drain electrode 16.

[0013] The first gate electrode 18 and the first source electrode 14 are electrically connected to each other. Therefore, the first TFT 10 functions as a two-terminal diode. Specifically, the first gate electrode 18 and the first source electrode 14 function as the cathode terminal of the two-terminal diode. Furthermore, the first drain electrode 16 functions as the anode terminal of the two-terminal diode.

[0014] FIG. 2 is a graph showing the change in the relationship between voltage and current when a comparative example TFT and a first TFT 10 are irradiated with X-rays. The comparative example TFT has the same configuration as the first TFT 10, except that the first gate electrode 18 and the first source electrode 14 are not electrically connected to each other. In FIG. 2, reference numeral 201 indicates the relationship between gate voltage Vg (V) and drain current Id (A) in the comparative example TFT. Reference numeral 202 indicates the relationship between voltage V and current I in the first TFT 10. In the first TFT 10 in which the first gate electrode 18 and the first source electrode 14 are electrically connected to each other, the terms gate voltage and drain current are inappropriate, and are therefore simply referred to as voltage and current. In both reference numerals 201 and 202, the graphs before X-ray irradiation are indicated by dashed lines, and the graphs after X-ray irradiation are indicated by solid lines.

[0015] As shown by the reference numeral 201, when the TFT of the comparative example is irradiated with X-rays, the threshold voltage shifts in the negative direction. The threshold voltage here is the gate voltage Vg when the drain current Id reaches a predetermined value. The predetermined value for the drain current Id is, for example, 1 nA. This shift in threshold voltage means that the leakage current increases.

[0016] As indicated by the reference numeral 202, even in the first TFT 10 in which the first gate electrode 18 and the first source electrode 14 are electrically connected to each other, the relationship between V and I changes when X-rays are irradiated. In the example indicated by the reference numeral 202, an increase in leakage current can be seen more clearly. That is, it can be said that the first TFT 10 converts light (X-rays) into an electrical signal (leakage current). It is believed that the detection of such leakage current amount in the first TFT 10 is based on the same principle as the detection of light leakage amount in a photodiode.

[0017] When the first TFT 10 or the second TFT 20 is irradiated with ionizing radiation including X-rays, electron-hole pairs are generated in the gate insulating film 61. Of these, the electrons are released from the gate insulating film 61 in a short time. On the other hand, the holes have lower mobility than the electrons. Therefore, some of the holes in the gate insulating film 61 are trapped near the interfaces with the first oxide semiconductor film 12 and the second oxide semiconductor film 22 and become fixed positive charges. It is believed that these fixed positive charges in the gate insulating film 61 cause a change in Vth.

[0018] The second TFT 20 detects the electrical signal converted by the first TFT 10. Specifically, the second TFT 20 is a switching element that is switched in response to the electrical signal converted by the first TFT 10. The second TFT 20 includes a second source electrode 24, a second drain electrode 26, a second oxide semiconductor film 22 that spans between the second source electrode 24 and the second drain electrode 26, and a second gate electrode 28 that controls the current in the second oxide semiconductor film 22.

[0019] The first source electrode 14 and the second drain electrode 26 are electrically connected to each other. This allows the second TFT 20 to detect the electrical signal converted by the first TFT 10. In the example shown in FIG. 1, the first source electrode 14 and the second drain electrode 26 are integrally formed. As described above, the first gate electrode 18 and the first source electrode 14 are electrically connected to each other. Therefore, the first gate electrode 18 and the second drain electrode 26 are electrically connected to each other via the first source electrode 14.

[0020] The first gate electrode 18 and the second gate electrode 28 may be formed in the same layer and made of the same material. The first oxide semiconductor film 12 and the second oxide semiconductor film 22 may be formed in the same layer and made of the same oxide semiconductor material. The first source electrode 14, the first drain electrode 16, the second source electrode 24, and the second drain electrode 26 may be formed in the same layer and made of the same material. This allows the first TFT 10 and the second TFT 20 to be formed in the same process, as described below. Therefore, damage to the second TFT 20 during the process of forming the first TFT 10 is reduced compared to, for example, forming the first TFT 10 after forming the second TFT 20.

[0021] The oxide semiconductor material forming the first oxide semiconductor film 12 and the second oxide semiconductor film 22 may contain at least one element selected from In, Ga, and Zn. This allows the photodetector 100 to have higher sensitivity and be smaller in size than when the oxide semiconductor material is formed from other oxide semiconductor materials. However, in the photodetector 100, the oxide semiconductor material does not have to contain any of In, Ga, and Zn.

[0022] The photodetector 100 further includes a gate insulating film 61. The gate insulating film 61 is an insulating film that covers the first gate electrode 18 and the second gate electrode 28. The gate insulating film 61 has a contact hole 61a. The first gate electrode 18 and the first source electrode 14 are electrically connected to each other via the contact hole 61a.

[0023] The photo-detecting device 100 further includes a first passivation film 62 and a second passivation film 63. The first passivation film 62 is an insulating film that covers the first TFT 10 and the second TFT 20. The second passivation film 63 is an insulating film that covers the first passivation film 62.

[0024] The first passivation film 62 has a contact hole 62a. The second passivation film 63 has a contact hole 63a. The contact holes 62a and 63a overlap each other in a plan view. The wiring layer 19 is exposed to the outside of the photodetector 100 through the contact holes 62a and 63a.

[0025] 3 is a diagram illustrating the first TFT 10 and the second TFT 20 in a plan view. In this figure, reference numeral 301 denotes the first TFT 10 in a plan view perpendicular to the first oxide semiconductor film 12 and the second oxide semiconductor film 22, and reference numeral 302 denotes the second TFT 20 in the plan view.

[0026] As shown in FIG. 3 , the size of the first oxide semiconductor film 12 may be larger than the size of the second oxide semiconductor film 22 in a planar view. For example, the size of the first oxide semiconductor film 12 may be 8 μm in both length and width. On the other hand, the size of the second oxide semiconductor film 22 may be 8 μm in length and 4 μm in width. Here, "length" refers to the direction from the first source electrode 14 to the first drain electrode 16 and the direction from the second source electrode 24 to the second drain electrode 26 in a planar view. Furthermore, "width" refers to the direction perpendicular to the length in a planar view. However, the sizes of the first oxide semiconductor film 12 and the second oxide semiconductor film 22 are not limited to this.

[0027] Fig. 4 is a graph showing the relationship between gate voltage Vg and drain current Id in the first TFT 10 and the second TFT 20 shown in Fig. 3. In Fig. 4, reference numeral 401 indicates the relationship between Vg and Id in the first TFT 10, and reference numeral 402 indicates the relationship between Vg and Id in the second TFT 20. The relationship between Vg and Id shown in reference numeral 401 is that when the first source electrode 14 and the first gate electrode 18 in the first TFT 10 are not electrically connected to each other. In both reference numerals 401 and 402, the graphs before X-ray irradiation are shown by dashed lines, and the graphs after X-ray irradiation are shown by solid lines.

[0028] 4, the first TFT 10 experiences a larger change in threshold voltage due to X-ray irradiation than the second TFT 20. This is because the size of the first oxide semiconductor film 12 is larger than the size of the second oxide semiconductor film 22 in a plan view. In a TFT including an oxide semiconductor film, the larger the size of the oxide semiconductor film in a plan view, the larger the area of ​​the interface between the oxide semiconductor film and the gate insulating film, and the greater the effect of X-ray irradiation on the TFT. Therefore, by specifying the sizes of the first oxide semiconductor film 12 and the second oxide semiconductor film 22 as described above, the sensitivity of the first TFT 10 to X-rays can be improved and the effect of X-rays on the second TFT 20 can be reduced.

[0029] Furthermore, the photodiode must receive X-rays and convert them into electrons, so it needs a certain area for receiving light. Therefore, in a conventional photodetector that uses a photodiode to detect light, the size of the photodiode is, for example, 150 μm in length and width. In contrast, the first TFT 10 can directly detect X-rays, so it can be, for example, 8 μm in length and width, as described above. Therefore, by providing the photodetector 100 with the first TFT 10 instead of a photodiode, the light-detecting element can be made smaller.

[0030] In particular, in the photodetector 100, the first gate electrode 18 and the first source electrode 14 are electrically connected to each other, eliminating the need for wiring to apply a voltage to the first gate electrode 18. This also eliminates the need for space for such wiring, allowing the photodetector 100 to have a high resolution.

[0031] (Manufacturing method) The following describes a method for manufacturing the photodetector 100. First, a conductive film that will become the first gate electrode 18 and the second gate electrode 28 is formed on a substrate to a film thickness of 50 nm to 500 nm.

[0032] Examples of the substrate include a glass substrate, a silicon substrate, and a heat-resistant plastic substrate. In particular, the plastic substrate may be made of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), acrylic, polyimide, or the like.

[0033] The conductive film may be a film of a metal such as aluminum (Al), tungsten (W), molybdenum (Mo), tantalum (Ta), chromium (Cr), titanium (Ti), or copper (Cu), or an alloy or metal nitride thereof. The conductive film may also be a laminate of two or more of these. For example, a W film is formed on the substrate to a thickness of 370 nm, followed by a TaN film to a thickness of 50 nm, to form the first gate electrode 18 and the second gate electrode 28 having a laminated structure of W and TaN (W / TaN=370 nm / 50 nm). Specifically, W and TaN are deposited on the substrate by sputtering, and then the first gate electrode 18 and the second gate electrode 28 are formed into the desired shapes by photolithography using dry etching.

[0034] Next, a gate insulating film 61 is formed on the first gate electrode 18 and the second gate electrode 28. The gate insulating film 61 may have a two-layer structure. The gate insulating film 61 may be made of silicon oxide (SiO x ), silicon nitride (SiN x ), silicon oxynitride (SiO x N y ) (x>y), silicon oxynitride (SiN x O y ) (x>y) or the like can be used as appropriate. When the gate insulating film 61 has a two-layer structure, the lower gate insulating film located on the first gate electrode 18 and second gate electrode 28 side is made of SiN to prevent the diffusion of impurities and the like from the substrate. x or SiN x O y (x>y), etc. The upper gate insulating film located on the opposite side to the first gate electrode 18 and the second gate electrode 28 may be formed of SiO xor silicon oxynitride SiO x N y (x>y) and so on.

[0035] A dense insulating film can be formed at a relatively low temperature by mixing a rare gas such as argon into the reactive gas used to form the gate insulating film 61 and mixing the rare gas into the gate insulating film 61. By making the gate insulating film 61 a dense insulating film, leakage current can be reduced.

[0036] For example, a CVD (Chemical Vapor Deposition) apparatus is used to deposit a 325 nm thick SiN film as a lower layer. Furthermore, a 10 nm thick SiO2 film is continuously deposited on top of that as an upper layer, thereby forming a two-layer insulating film that will become the gate insulating film 61. At this point, the gate insulating film 61 does not have a contact hole 61a, and therefore the gate insulating film 61 is not complete. However, for simplicity, in the following explanation, the insulating film without the contact hole 61a will also be referred to as the gate insulating film 61.

[0037] The first oxide semiconductor film 12 and the second oxide semiconductor film 22 are formed on the gate insulating film 61 to a thickness of 30 nm to 100 nm. As described above, the first oxide semiconductor film 12 and the second oxide semiconductor film 22 may be formed of an InGaZnO semiconductor, but are not limited to this. Specifically, the first oxide semiconductor film 12 and the second oxide semiconductor film 22 may be formed of InGaO3(ZnO)5, magnesium zinc oxide (Mg x Zn 1-x O), cadmium zinc oxide (Cd x Zn 1-xThe first oxide semiconductor film 12 and the second oxide semiconductor film 22 may be made of ZnO doped with one or more impurity elements selected from the group 1, 13, 14, 15, and 17 elements. In this case, the ZnO may be in an amorphous state, a polycrystalline state, or a microcrystalline state in which the amorphous and polycrystalline states are mixed. Furthermore, the first oxide semiconductor film 12 and the second oxide semiconductor film 22 may be made of ZnO doped with no impurity elements.

[0038] For example, the oxide semiconductor films that will become the first oxide semiconductor film 12 and the second oxide semiconductor film 22 are formed by a sputtering method. Then, the first oxide semiconductor film 12 and the second oxide semiconductor film 22 are formed into desired shapes by a photolithography method using dry etching.

[0039] After the first oxide semiconductor film 12 and the second oxide semiconductor film 22 are formed, a contact hole 61a is provided in the gate insulating film 61. Specifically, by photolithography using dry etching, the contact hole 61a is provided in a part of the gate insulating film 61 that overlaps with the first gate electrode 18. In this way, the gate insulating film 61 is completed.

[0040] A first source electrode 14, a first drain electrode 16, a second source electrode 24, and a second drain electrode 26 are formed on the first oxide semiconductor film 12 and the second oxide semiconductor film 22. Specifically, a conductive film is formed on the gate insulating film 61, the first oxide semiconductor film 12, and the second oxide semiconductor film 22. Furthermore, the conductive film is processed into a desired shape using a resist mask in a photolithography process to form the first drain electrode 16, the second source electrode 24, and the second drain electrode 26. As the conductive film, metals such as aluminum (Al), tungsten (W), molybdenum (Mo), tantalum (Ta), copper (Cu), chromium (Cr), titanium (Ti), or alloys thereof, or metal nitrides thereof can be appropriately used. Here, a film of Ti / Al / Ti=100 nm / 300 nm / 30 nm was formed by sputtering, and then a first source electrode 14, a first drain electrode 16, a second source electrode 24, and a second drain electrode 26 of desired shapes were formed by photolithography using dry etching. In this way, a first TFT 10 and a second TFT 20 were formed.

[0041] A first passivation film 62 is formed to a thickness of 200 nm to 300 nm so as to cover the first TFT 10 and the second TFT 20. The first passivation film 62 may be formed using a thin film formation method such as plasma CVD or sputtering. The first passivation film 62 may be made of an insulating material such as silicon nitride, silicon oxide, silicon nitride oxide, or silicon oxynitride.

[0042] Specifically, a thin film of an insulating material that will become the first passivation film 62 is formed so as to cover the first TFT 10 and the second TFT 20. Thereafter, contact holes 62a are formed by photolithography using dry etching, thereby completing the first passivation film 62.

[0043] Furthermore, a second passivation film 63 is formed on the first passivation film 62 to a thickness of 200 nm to 300 nm. Like the first passivation film 62, the second passivation film 63 may be formed using a thin film formation method such as plasma CVD or sputtering. Like the material of the first passivation film 62, the second passivation film 63 may be made of an insulating material such as silicon nitride, silicon oxide, silicon nitride oxide, or silicon oxynitride. The material of the second passivation film 63 may be the same as or different from the material of the first passivation film 62.

[0044] Specifically, a thin film of an insulating material that will become the second passivation film 63 is formed so as to cover the first passivation film 62. Thereafter, contact holes 63a are formed by photolithography using dry etching, thereby forming the second passivation film 63.

[0045] It is not necessary to form the contact hole 62a immediately after forming the thin film of insulating material that will become the first passivation film 62. In this case, after forming a thin film of insulating material that will become the second passivation film 63, the contact holes 62a and 63a may be formed in succession.

[0046] 1, the photodetector 100 has two passivation films, a first passivation film 62 and a second passivation film 63. However, the photodetector 100 may have only the first passivation film 62, or may further have a passivation film other than the first passivation film 62 and the second passivation film 63. After the first passivation film 62 and the second passivation film 63 are formed, a process of heating the entire surface of the substrate may be further performed. Heating is performed until the substrate reaches 350°C, for example.

[0047] The wiring layer 19 is formed on the first passivation film 62 and the second passivation film 63 by, for example, sputtering or photolithography. The wiring layer 19 is made of, for example, Mo or Ti. Through the above steps, the photodetector 100 can be manufactured.

[0048] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. [Explanation of symbols]

[0049] 10. 1st TFT 12 First oxide semiconductor film 14 First source electrode 16 First drain electrode 18 First gate electrode 20 2nd TFT 22 Second oxide semiconductor film 24 Second source electrode 26 Second drain electrode

Claims

1. a first TFT (Thin Film Transistor) that converts light into an electrical signal; the first TFT includes a first gate electrode, a first source electrode, a first drain electrode, and a first oxide semiconductor film extending between the first source electrode and the first drain electrode; The photodetector device, wherein the first gate electrode and the first source electrode are electrically connected to each other.

2. further comprising a second TFT for detecting the electrical signal; the second TFT includes a second gate electrode, a second source electrode, a second drain electrode, and a second oxide semiconductor film extending between the second source electrode and the second drain electrode; The photodetector device according to claim 1 , wherein the first gate electrode, the first source electrode, and the second drain electrode are electrically connected to one another.

3. the first gate electrode and the second gate electrode are formed in the same layer and made of the same material, the first oxide semiconductor film and the second oxide semiconductor film are formed in the same layer from the same oxide semiconductor material, 3. The photodetector device according to claim 2, wherein the first source electrode, the first drain electrode, the second source electrode, and the second drain electrode are formed in the same layer and made of the same material.

4. The photodetector device according to claim 3 , wherein the oxide semiconductor material contains at least one element selected from the group consisting of In, Ga, and Zn.

5. The photodetector according to claim 2 , wherein the first oxide semiconductor film is larger in size than the second oxide semiconductor film in a plan view.

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