Thin-film transistor and radiation sensor

Abstract

To improve characteristics of a thin-film transistor for use in a radiation sensor.SOLUTION: A thin-film transistor for use in a radiation sensor includes a gate electrode, an oxide semiconductor layer, and a gate insulator between the oxide semiconductor layer and the gate electrode. The gate insulator includes a silicon nitride layer, and a silicon oxide layer that forms a boundary surface with each of the silicon nitride layer and the oxide semiconductor layer, between the silicon nitride layer and the oxide semiconductor layer. The thickness of the silicon oxide layer is in the range of 1-4 nm.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to thin film transistor structures. [Background technology]

[0002] The technology of non-destructively inspecting the inside of a specimen using radiation, such as X-ray transmission images, has become an indispensable technology in fields such as industrial non-destructive testing. In particular, DR (Digital Radiography), which directly captures X-ray transmission images as electronic data, has become widely used due to its speed in interpretation and the use of image processing to assist interpretation. DR uses a device called an FPD (Flat Panel Detector).

[0003] FPDs used in X-ray sensors are generally classified into direct conversion and indirect conversion types. Direct conversion FPDs directly convert X-rays into electrical signals. Indirect conversion FPDs include an X-ray detection panel containing a phosphor (scintillator) that converts X-rays into light, such as visible light or ultraviolet light, and a photoelectric conversion element array that converts light into electrical signals. FPDs include pixels arranged in an array. Each pixel of the FPD includes a conversion element that converts X-rays or light into an electrical signal and a switch thin-film transistor for reading out the electrical signal. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Application Publication No. 2014 / 0374745 [Patent Document 2] U.S. Patent Application Publication No. 2012 / 0175618 Summary of the Invention [Problem to be solved by the invention]

[0005] According to the research of the inventors, it has been found that the switching thin film transistors used in FPDs can exhibit characteristic variations in response to radiation, and in particular, can exhibit large characteristic variations in response to strong X-rays. [Means for solving the problem]

[0006] One aspect of the present disclosure is a thin-film transistor for use in a radiation sensor, comprising: a gate electrode; an oxide semiconductor layer; and a gate insulating film between the oxide semiconductor layer and the gate electrode, wherein the gate insulating film comprises a silicon nitride layer; and a silicon oxide layer between the silicon nitride layer and the oxide semiconductor layer, forming interfaces with the silicon nitride layer and the oxide semiconductor layer, respectively, and wherein the silicon oxide layer has a thickness of 1 nm or more and 4 nm or less.

[0007] One aspect of the present disclosure is a thin-film transistor for use in a radiation sensor, comprising: a gate electrode; an oxide semiconductor layer; and a gate insulating film between the oxide semiconductor layer and the gate electrode, wherein the gate insulating film comprises a silicon nitride layer; and a silicon oxynitride layer between the silicon nitride layer and the oxide semiconductor layer, forming interfaces with the silicon nitride layer and the oxide semiconductor layer, respectively, and wherein the silicon oxynitride layer has a thickness of 1 nm or more and 3 nm or less.

[0008] One aspect of the present disclosure is a thin-film transistor for use in a radiation sensor, comprising: a gate electrode; an oxide semiconductor layer; and a gate insulating film between the oxide semiconductor layer and the gate electrode, wherein the gate insulating film comprises a silicon nitride layer; and a silicon oxynitride layer between the silicon nitride layer and the oxide semiconductor layer, the silicon oxynitride layer forming an interface with the silicon nitride layer and the oxide semiconductor layer, respectively, wherein a nitrogen atomic ratio in the silicon oxynitride layer is smaller than a silicon atomic ratio. [Effects of the Invention]

[0009] According to one aspect of the present disclosure, the characteristics of a thin film transistor used in a radiation sensor can be improved. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a block diagram showing an example of the configuration of an X-ray sensor. [Figure 2] FIG. 2 is a circuit diagram showing an example of an equivalent circuit configuration of a pixel. [Figure 3] 1 shows a cross-sectional structure of a pixel. [Figure 4] 1 shows a plan view of a pixel. [Figure 5] 1 is a cross-sectional view schematically illustrating a structure of a switch thin film transistor according to an embodiment of the present specification. [Figure 6] The figure shows multiple graphs showing the relationship between the change in current-voltage characteristics before and after application of gate voltage to an oxide semiconductor thin-film transistor and the film thickness of silicon oxide. [Figure 7] 1 shows a number of graphs illustrating the relationship between the change in current-voltage characteristics before and after X-ray irradiation of an oxide semiconductor thin film transistor and the thickness of a silicon oxide film. [Figure 8] Graphs summarizing the measurement results shown in FIGS. 6 and 7 are shown. [Figure 9] 10 shows a plurality of graphs illustrating the relationship between the change in current-voltage characteristics before and after application of a gate voltage to an oxide semiconductor thin film transistor and the film thickness of a silicon oxynitride film. [Figure 10] 1 shows a number of graphs illustrating the relationship between the change in current-voltage characteristics before and after X-ray irradiation of an oxide semiconductor thin film transistor and the film thickness of silicon oxynitride. [Figure 11] Graphs summarizing the measurement results shown in FIGS. 9 and 10 are shown. [Figure 12A] 1 shows the change in characteristics of an oxide semiconductor transistor including a silicon oxynitride film having a first composition ratio as an electron blocking layer before and after application of a gate voltage. [Figure 12B] 1 shows changes in characteristics of an oxide semiconductor transistor including a silicon oxynitride film having a first composition ratio as an electron blocking layer before and after X-ray irradiation. [Figure 13A] 10 shows the change in characteristics before and after gate voltage application of an oxide semiconductor transistor including a silicon oxynitride film having a second composition ratio as an electron blocking layer. [Figure 13B]10 shows the change in characteristics of an oxide semiconductor transistor including a silicon oxynitride film having a second composition ratio as an electron blocking layer before and after X-ray irradiation. [Figure 14] 1 shows the results of composition analysis of an oxide semiconductor transistor having a first composition ratio and the results of composition analysis of an oxide semiconductor transistor having a second composition ratio. [Figure 15] 1 is a cross-sectional view schematically illustrating an example of the structure of a switch thin-film transistor of an X-ray sensor according to an embodiment of the present specification. [Figure 16] FIG. 1 is a cross-sectional view showing an example of the structure of a switch thin-film transistor having a top gate structure. [Figure 17] FIG. 1 is a cross-sectional view showing an example of the structure of a switch thin-film transistor having a dual gate structure. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments will be described with reference to the accompanying drawings. The embodiments are merely examples for realizing the present disclosure and do not limit the technical scope of the present disclosure. For clarity of explanation, the dimensions and shapes of the objects shown in the drawings may be exaggerated.

[0012] One embodiment of the present specification discloses the structure of an oxide semiconductor thin film transistor that can be used in a radiation sensor. Radiation sensors using oxide semiconductor thin film transistors with high driving capabilities have been actively developed. However, when oxide semiconductor thin film transistors are irradiated with radiation, particularly X-rays, their threshold voltage fluctuates significantly, causing them to malfunction. This problem is more pronounced in the industrial field, where X-ray exposure is high, than in the medical field, where X-ray exposure is low.

[0013] The inventors discovered a method for improving the radiation resistance of oxide semiconductor thin-film transistors by using silicon nitride (SiNx) for the gate insulating film. However, it was found that oxide semiconductor thin-film transistors with silicon nitride gate insulating films suffer from a new problem: the threshold voltage fluctuates when electrons are induced in the channel by applying a gate voltage.

[0014] In one embodiment of the present specification, a gate insulating film is composed of multiple layers of different materials, and an electron blocking layer is disposed between a silicon nitride film and an oxide semiconductor film. The electron blocking layer is in direct contact with the silicon nitride film and the oxide semiconductor film, respectively, and forms an interface therewith. The electron blocking layer may be a silicon oxide film or a silicon oxynitride film. The electron blocking layer can suppress a change in threshold voltage caused by inducing electrons in the channel.

[0015] In the following, an X-ray sensor will be described as an example, but the features disclosed below can be used in sensors for radiation other than X-rays. First Embodiment

[0016] 1 is a block diagram showing an example of the configuration of an X-ray sensor. The X-ray sensor 10 is, for example, an image sensor used to capture an X-ray transmission image. The X-ray sensor 10 includes a pixel matrix 101, a scanning circuit 170, and a detection circuit 150. The pixel matrix 101 includes pixels 102 arranged in a matrix. The pixel matrix 101 is formed on a sensor substrate 100. The sensor substrate 100 is an insulating substrate (for example, a glass substrate).

[0017] Pixels 102 are arranged at each intersection of a plurality of signal lines 106 extending vertically and arranged horizontally in Fig. 1 and a plurality of gate lines (scanning lines) 105 extending horizontally and arranged vertically. The pixels 102 are each connected to a bias line 107 extending vertically and arranged horizontally in Fig. 1. In Fig. 1, only one pixel, one signal line, one gate line, and one bias line are indicated by the reference symbols 102, 106, 105, and 107, respectively.

[0018] The signal lines 106 are connected to different pixel columns. The gate lines 105 are connected to different pixel rows. The signal lines 106 are connected to a detection circuit 150, and the gate lines are connected to a scanning circuit 170. The bias lines 107 are connected to a common bias line 108. A bias potential is applied to a pad 109 of the common bias line 108.

[0019] 2 is a circuit diagram showing an example of an equivalent circuit configuration of a pixel 102. The pixel 102 includes a photodiode 103, which is a photoelectric conversion element, and a thin film transistor (TFT) 104, which is a switching element. The gate of the thin film transistor 104 is connected to a gate line 105, one of the source and the drain is connected to a signal line 106, and the other of the source and the drain is connected to the cathode of the photodiode 103. In the example of FIG. 2, the anode of the photodiode 103 is connected to a bias line 107.

[0020] The thin film transistor 104 is, for example, an oxide semiconductor thin film transistor. In the configuration example shown in Figure 2, the thin film transistor 104 has an n-type conductivity. The thin film transistor may have other conductivity types. The oxide semiconductor thin film transistor exhibits excellent switching characteristics.

[0021] The X-ray sensor 10 reads out a signal by externally extracting the signal charge stored in the photodiode 103 in accordance with the X-ray irradiation dose by turning on the thin film transistor 104 arranged in the pixel 102. Specifically, when light is incident on the photodiode 103, a signal charge is generated and accumulated in the photodiode 103.

[0022] The scanning circuit 170 sequentially selects the gate lines 105 and applies a pulse that turns on the thin film transistor 104. The anode terminal of the photodiode 103 is connected to the bias line 107, and a reference potential is applied to the signal line 106 by the detection circuit 150. Therefore, the photodiode 103 is charged with a differential voltage between the bias potential of the bias line 107 and the reference potential. This differential voltage is set to a reverse bias voltage in which the cathode potential is higher than the anode potential.

[0023] The charge required to recharge the photodiode 103 to this reverse bias voltage depends on the amount of light incident on the photodiode 103. The detection circuit 150 reads the signal charge by integrating the current that flows as the photodiode 103 is recharged to the reverse bias.

[0024] The charge stored in the photodiode 103 inevitably decreases due to the reduction in charge caused by irradiated light and due to a dark leakage current that flows even when the photodiode 103 is not irradiated with light, and therefore, in reading out the signal charge, the voltage of the terminal of the thin film transistor 104 connected to the signal line 106 is equal to or higher than the voltage of the terminal connected to the photodiode 103. In other words, in detecting the signal charge, the terminal of the thin film transistor 104 connected to the signal line 106 is the drain, and the terminal connected to the photodiode 103 is the source.

[0025] Fig. 3 shows the cross-sectional structure of a pixel. In the following description, in the example of Fig. 3, the side opposite the sensor substrate 100 to the photodiode 103 is the front side. In addition, in terms of the positional relationship of the components of the pixel, the side facing the sensor substrate 100 is called the lower side, and the opposite side is called the upper side.

[0026] The thin film transistor 104 and the photodiode 103 included in the pixel each have a stacked structure. The thin film transistor 104 includes a gate electrode 302 formed on an insulating sensor substrate 100, a gate insulating film 303 on the gate electrode 302, and an oxide semiconductor layer 304 on the gate insulating film 303.

[0027] 3 has a bottom-gate structure, and a gate electrode 302 is located below an oxide semiconductor layer 304. The thin film transistor 104 further includes a source / drain electrode 305 and a source / drain electrode 306 on a gate insulating film 303. The source / drain electrodes 305 and 306 are each connected to the oxide semiconductor layer 304. The source / drain electrodes 305 and 306 are formed so as to contact the side surfaces and part of the top surface of the island-shaped oxide semiconductor layer 304.

[0028] In detecting the charge of the photodiode 103, the electrode 305 is the drain electrode and the electrode 306 is the source electrode.

[0029] The gate insulating film 303 is formed to cover the entire surface of the gate electrode 302. The gate insulating film 303 is formed between the gate electrode 302 and the oxide semiconductor layer 304, and between the gate electrode 302 and each of the source / drain electrodes 305 and 306.

[0030] The first interlayer insulating film 307 covers the entire thin film transistor 104. Specifically, the first interlayer insulating film 307 covers the upper surface of the oxide semiconductor layer 304 and the upper surfaces of the source / drain electrodes 305 and 306.

[0031] The sensor substrate 100 is made of, for example, glass or resin. The gate electrode 302 is a conductor and can be made of metal or impurity-doped silicon. The gate insulating film 303 has a multi-layer structure. Each layer of the gate insulating film 303 may be, for example, a silicon oxide (SiOx) layer, a silicon nitride (SiNx) layer, or a silicon oxynitride (SiOxNy) layer. The gate insulating film 303 will be described in detail later.

[0032] The oxide semiconductor constituting the oxide semiconductor layer 304 is an oxide semiconductor containing at least one of In, Ga, and Zn, and an example of such an oxide semiconductor is IGZO. Examples of IGZO include amorphous InGaZnO (a-InGaZnO) and microcrystalline InGaZnO. Other oxide semiconductors that can be used include a-InSnZnO and a-InGaZnSnO. In the examples described below, amorphous or microcrystalline InGaZnO (hereinafter, these will also be simply referred to as IGZO) will be mainly used.

[0033] The source / drain electrodes 305 and 306 are each a conductor and can be formed of, for example, a metal such as Mo, Ti, Al, or Cr, an alloy thereof, or a laminate thereof. The first interlayer insulating film 307 is an inorganic or organic insulator. The thin film transistor 104 shown in FIG. 3 has a bottom-gate structure, but the thin film transistor 104 may alternatively have a top-gate structure or a dual-gate structure. A dual-gate structure has both a top gate and a bottom gate, a bottom-gate structure has only a bottom gate, and a top-gate structure has only a top gate.

[0034] The photodiode 103 is formed on the first interlayer insulating film 307. The example of the photodiode 103 shown in FIG. 3 is a PIN diode. The PIN diode can detect light efficiently due to the formation of a wide depletion layer in the film thickness direction. The photodiode 103 includes a semiconductor stack sandwiched between a lower electrode 308 and an upper electrode 312 on the first interlayer insulating film 307. The lower electrode 308 is connected to the source / drain electrodes 306 of the thin film transistor 104 through an interconnection portion of a via hole 321 in the first interlayer insulating film 307.

[0035] The lower electrode 308 is a conductor and can be formed of, for example, a metal such as Cr, Mo, or Al, an alloy thereof, or a laminate thereof. The upper electrode 312 is an electrode that is transparent to the light from the scintillator 316 and is, for example, ITO.

[0036] Photodiode 103 includes an n-type amorphous silicon layer 309 on a lower electrode 308, an intrinsic amorphous silicon layer 310 on n-type amorphous silicon layer 309, and a p-type amorphous silicon layer 311 on intrinsic amorphous silicon layer 310. An upper electrode 312 is formed on p-type amorphous silicon layer 311. Light to be detected enters photodiode 103 from the upper electrode 312 side (p-type amorphous silicon layer 311 side).

[0037] A second interlayer insulating film 313 is formed to cover the photodiode 103. Specifically, the second interlayer insulating film 313 is formed on the first interlayer insulating film 307, a part of the lower electrode 308, and the upper electrode 312. The second interlayer insulating film 313 is an inorganic or organic insulator.

[0038] The bias line 107 is formed on the second interlayer insulating film 313. The bias line 107 is connected to the upper electrode 312 by an interconnection formed in a via hole 322 of the second interlayer insulating film 313. The bias line 107 is a conductor and can be formed of, for example, a metal such as Mo, Ti, or Al, an alloy thereof, or a laminate thereof.

[0039] A passivation layer 315 is formed so as to cover the bias line 107 and the second interlayer insulating film 313. The passivation layer 315 covers the entire area of ​​the pixel matrix 101. The passivation layer 315 is an inorganic or organic insulator. A scintillator 316 is disposed on the passivation layer 315.

[0040] The scintillator 316 covers the entire area of ​​the pixel matrix 101. The scintillator 316 converts incident X-rays into light of a wavelength that is detected by the photodiode 103. The photodiode 103 accumulates signal charges in response to the light from the scintillator 316.

[0041] 4 shows a plan view of a pixel 102. As shown in FIG. 4, the gate line 105 extends in the left-right direction in the figure, and the signal line 106 extends in the up-down direction in the figure. The gate electrode 302 is continuous with the gate line 105, and they are part of a continuous metal film. The gate electrode 302 protrudes from the gate line 105 in a direction perpendicular to the direction in which the gate line 105 extends.

[0042] The source / drain electrodes 305 of the thin film transistor are continuous with the signal line 106 and are part of a continuous metal film. The source / drain electrodes 305 protrude from the signal line 106 in a direction perpendicular to the direction in which the signal line 106 extends. The source / drain electrodes 306 are island-shaped electrodes and are spaced apart from the source / drain electrodes 305.

[0043] The oxide semiconductor layer 304 is disposed so as to overlap the gate electrode 302 in plan view, with a source / drain electrode 305 disposed on one side and a source / drain electrode 306 disposed on the opposite side. The source / drain electrode 306 partially overlaps the lower electrode 308 of the photodiode 103 and is connected to it via a via hole 321.

[0044] 4, the entire area of ​​the upper electrode 312 of the photodiode 103 is present within the area of ​​the lower electrode 308 in a plan view. The bias line 107 extends in the vertical direction of the figure. A portion of the bias line 107 overlaps with the upper electrode 312 and is connected to the upper electrode 312 through a via hole 322.

[0045] 1 to 4 show a photodiode as the element that converts radiation into an electrical signal, but other types of elements may be used. For example, an element that can directly convert X-rays into an electrical signal without using a scintillator may be used.

[0046] One of the features of an embodiment of the present specification is the structure of a switch thin-film transistor in a pixel of a radiation sensor. The structure of a thin-film transistor according to an embodiment of the present specification is described below. Research by the inventors has revealed that the structure of the gate insulating film of the switch thin-film transistor affects the characteristics of the switch thin-film transistor under radiation exposure, particularly under X-ray exposure.

[0047] Specifically, when the gate insulating film is composed of a single silicon oxide (SiOx) layer, X-ray irradiation can cause the threshold voltage Vth of the thin-film transistor to shift in the negative direction, making it difficult for the transistor to function normally. This is presumably because holes generated by X-ray irradiation are trapped in the silicon oxide layer, causing the threshold voltage Vth to shift in the negative direction.

[0048] On the other hand, when the gate insulating film is composed of a single silicon nitride (SiNx) layer, the application of a positive voltage to the gate electrode of the thin film transistor shifts the threshold voltage Vth of the thin film transistor in the positive direction. This is presumably because, when a voltage is applied to the gate electrode, electrons are induced in the oxide semiconductor layer, and the electrons move to the silicon nitride layer and are trapped in the silicon nitride layer.

[0049] In one embodiment of the present specification, a gate insulating film is configured with multiple layers, and a silicon oxide film (silicon oxide layer) or a silicon oxynitride (SiOxNy) film (silicon oxynitride layer) is disposed between a silicon nitride film (silicon nitride layer) and a gate electrode. The silicon oxide film or the silicon oxynitride film is in direct contact with the silicon nitride layer and the gate electrode, respectively, and forms an interface therewith.

[0050] 5 is a cross-sectional view schematically illustrating the structure of a switch thin-film transistor according to one embodiment of the present specification. The switch thin-film transistor 430 includes a gate electrode 432, a gate insulating film 433 on the gate electrode 432, and an oxide semiconductor layer 434 on the gate insulating film 433. The oxide semiconductor layer 434 is made of IGZO. An interlayer insulating layer 437 covers the entire thin-film transistor 430.

[0051] The gate insulating film 433 has a two-layer structure and is composed of a silicon nitride film (silicon nitride layer) 438 made of silicon nitride (SiNx) and an insulating film 439 made of silicon oxide (SiOx) on the silicon nitride film 438 .

[0052] The thin film transistor 430 has a bottom gate structure, and the gate electrode 432 is present below (on the substrate side of) the oxide semiconductor layer 434. The thin film transistor 430 further includes a source / drain electrode 435 and a source / drain electrode 436 on a gate insulating film 433. The source / drain electrodes 435 and 436 are each connected to the oxide semiconductor layer 434. Although this embodiment is a channel-etched type, the thin film transistor 430 may also be a channel-protected type in which the channel portion of the oxide semiconductor layer 434 is protected by an insulating film.

[0053] The silicon oxide film 439 is thinner than the silicon nitride film 438. In one embodiment of this specification, the thickness of the silicon oxide film 439 is 1 nm or more and 4 nm or less. The thickness of the silicon nitride film 438 is, for example, several hundred nm, and may be 100 nm to 900 nm.

[0054] When a voltage is applied to the gate electrode, electrons can be injected from the oxide semiconductor layer to the silicon nitride film. The silicon oxide film 439 between the silicon nitride film 438 and the oxide semiconductor layer 434 is an electron blocking layer that inhibits (suppresses) the movement of electrons from the oxide semiconductor layer 434 to the silicon nitride film 438.

[0055] By disposing a silicon oxide film between the oxide semiconductor layer and the silicon nitride film, it is possible to suppress changes in the characteristics of the oxide semiconductor thin film transistor caused by carriers trapped in the silicon nitride film due to the operation of the oxide semiconductor thin film transistor (application of gate voltage).

[0056] The silicon oxide film having a thickness equal to or greater than a specific value can effectively inhibit the movement of electrons from the oxide semiconductor layer to the silicon nitride film. According to the inventors' research, a silicon oxide film having a thickness of 1 nm or more can effectively stabilize the characteristics of an oxide semiconductor thin film transistor.

[0057] On the other hand, the silicon oxide film, which is a component of the electron blocking layer, can have a significant effect on the threshold voltage Vth of the oxide semiconductor layer directly above it due to carriers generated and trapped by irradiation with radiation, particularly X-rays. By reducing the thickness of the silicon oxide film, the effect on the threshold voltage Vth can be effectively suppressed.

[0058] According to the inventors' research, a silicon oxide film having a thickness of 4 nm or less effectively suppresses the influence on the threshold voltage Vth of an oxide semiconductor thin-film transistor, enabling stable operation of the switch thin-film transistor. In particular, when a strong dose of X-rays is irradiated, for example, at a dose of 500 Gy or more, the stability of the switch thin-film transistor can be maintained.

[0059] As described above, by disposing a silicon oxide film between the silicon nitride film and the oxide semiconductor layer, it is possible to prevent electrons from moving from the oxide semiconductor layer to the silicon nitride film, and by reducing the thickness of the silicon oxide film, it is possible to avoid the influence of trapped holes in the silicon oxide film, thereby providing a thin film transistor that is radiation-resistant and has stable characteristics.

[0060] The following describes measurement results of some oxide semiconductor thin film transistors having silicon oxide films of different thicknesses. The oxide semiconductor thin film transistors to be measured had the structure shown in FIG. 5, in which the insulating film 439 was made of silicon oxide and the oxide semiconductor layer 434 was made of IGZO.

[0061] First, we will explain the effect of inhibiting electron migration from the oxide semiconductor layer to the silicon nitride film. Figure 6 shows several graphs illustrating the relationship between the change in IV (current-voltage) characteristics before and after application of a gate voltage to an oxide semiconductor thin-film transistor and the silicon oxide (SiOx) film thickness. Specifically, the graphs show the measurement results for oxide thin-film transistors with silicon oxide film thicknesses of 0 nm (no SiOx), 1 nm, 2 nm, 3 nm, 4 nm, and 5 nm. Each graph shows the measurement results of drain current versus gate voltage sweep. No radiation was irradiated during the measurements. In each graph, the horizontal axis represents gate voltage, and the vertical axis represents drain current. In each graph, the solid line represents the first measurement result, and the dashed line represents the second measurement result.

[0062] As can be seen from the graph in Figure 6, in the oxide semiconductor thin film transistor without silicon oxide (no SiOx), a large change in threshold voltage is observed between the first and second measurement results. On the other hand, in each of the oxide semiconductor thin film transistors with a silicon oxide film of 1 nm or more (SiOx: 1 nm, SiOx: 2 nm, SiOx: 3 nm, SiOx: 4 nm, SiOx: 5 nm), the change in threshold voltage is suppressed and falls within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0063] Next, we will explain the effect of X-ray irradiation on the IV characteristics of the oxide semiconductor thin-film transistor from the electron blocking layer. Figure 7 shows several graphs showing the relationship between the change in IV (current-voltage) characteristics of the oxide semiconductor thin-film transistor before and after X-ray irradiation and the silicon oxide (SiOx) film thickness.

[0064] Specifically, the graphs show the measurement results of oxide thin-film transistors with silicon oxide film thicknesses of 0 nm (no SiOx), 1 nm, 2 nm, 3 nm, 4 nm, and 5 nm, respectively. Each graph shows the measurement results of drain current versus gate voltage sweep. In each graph, the horizontal axis represents gate voltage, and the vertical axis represents drain current. In each graph, the solid line represents the measurement results before X-ray irradiation, and the dashed line represents the measurement results after X-ray irradiation. The X-ray dose was 660 Gy.

[0065] As can be seen from the graph in Figure 7, a large change in threshold voltage is observed between the measurement results before and after X-ray irradiation in the oxide semiconductor thin film transistor having a silicon oxide film of 5 nm (SiOx: 5 nm). On the other hand, in each of the oxide semiconductor thin film transistors having a silicon oxide film of 4 nm or less (no SiOx, SiOx: 1 nm, SiOx: 2 nm, SiOx: 3 nm, SiOx: 4 nm), the change in threshold voltage is significantly suppressed, falling within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0066] FIG. 8 shows a graph summarizing the measurement results shown in FIGS. 6 and 7. The horizontal axis represents the film thickness of the silicon oxide film. The left vertical axis represents the amount of threshold voltage shift before and after gate voltage application, as described with reference to FIG. 6. The right vertical axis represents the amount of threshold voltage shift before and after X-ray irradiation, as described with reference to FIG. 7. A dashed line 61 represents the change in the amount of threshold voltage shift before and after gate voltage application. A solid line 62 represents the change in the amount of threshold voltage shift before and after X-ray irradiation.

[0067] As can be seen from the measurement results in FIGS. 6 to 8, when the thickness of the silicon oxide film is in the range of 1 nm to 4 nm, the change in threshold voltage before and after gate voltage application and before and after X-ray irradiation can be suppressed to a level that does not substantially affect the operation of the oxide semiconductor transistor.

[0068] Next, the film thickness of a structure using a silicon oxynitride film instead of the silicon oxide film 439 will be described. Below, measurement results of some oxide semiconductor thin film transistors having silicon oxynitride films of different thicknesses will be described. The oxide semiconductor thin film transistors to be measured had the structure shown in FIG. 5, in which the insulating film 439 was formed of silicon oxynitride and the oxide semiconductor layer 434 was formed of IGZO.

[0069] First, we will explain the effect of inhibiting electron migration from the oxide semiconductor layer to the silicon nitride film. Figure 9 shows several graphs showing the relationship between the change in IV (current-voltage) characteristics before and after application of a gate voltage to an oxide semiconductor thin-film transistor and the film thickness of the silicon oxynitride film. Specifically, the graphs show the measurement results for oxide thin-film transistors with silicon oxynitride film thicknesses of 0 nm (no SiOxNy), 1 nm, 2 nm, 3 nm, and 4 nm. Each graph shows the measurement results of the drain current as the gate voltage is swept. No radiation was irradiated during the measurements. In each graph, the horizontal axis represents the gate voltage, and the vertical axis represents the drain current. In each graph, the solid line represents the first measurement result, and the dashed line represents the second measurement result.

[0070] 9, in the oxide semiconductor thin film transistor without silicon oxynitride (no SiOxNy), a large change in threshold voltage is observed between the first and second measurement results. On the other hand, in each of the oxide semiconductor thin film transistors with a silicon oxynitride film of 1 nm or more (SiOxNy: 1 nm, SiOxNy: 2 nm, SiOxNy: 3 nm, SiOxNy: 4 nm), the change in threshold voltage is suppressed and falls within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0071] Next, we will explain the effect of X-ray irradiation on the IV characteristics of the oxide semiconductor thin-film transistor from the electron blocking layer. Figure 10 shows several graphs showing the relationship between the change in IV (current-voltage) characteristics of the oxide semiconductor thin-film transistor before and after X-ray irradiation and the film thickness of the silicon oxynitride (SiOxNy).

[0072] Specifically, the graphs show the measurement results for oxide thin-film transistors with silicon oxynitride film thicknesses of 0 nm (no SiOxNy), 1 nm, 2 nm, 3 nm, and 4 nm, respectively. Each graph shows the measurement results of drain current versus gate voltage sweep. In each graph, the horizontal axis represents gate voltage, and the vertical axis represents drain current. In each graph, the solid line represents the measurement results before X-ray irradiation, and the dashed line represents the measurement results after X-ray irradiation. The X-ray dose was 660 Gy.

[0073] As can be seen from the graph in Figure 10, a large change in threshold voltage is observed between the measurement results before and after X-ray irradiation in the oxide semiconductor thin film transistor having a 4 nm silicon oxynitride film (SiOxNy: 4 nm). On the other hand, in each of the oxide semiconductor thin film transistors having a silicon oxynitride film of 3 nm or less (no SiOxNy, SiOxNy: 1 nm, SiOxNy: 2 nm, SiOxNy: 3 nm), the change in threshold voltage is significantly suppressed, falling within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0074] FIG. 11 shows a graph summarizing the measurement results shown in FIGS. 9 and 10. The horizontal axis represents the film thickness of the silicon oxynitride film. The left vertical axis represents the amount of threshold voltage shift before and after gate voltage application, as described with reference to FIG. 9. The right vertical axis represents the amount of threshold voltage shift before and after X-ray irradiation, as described with reference to FIG. 10. A dashed line 65 represents the change in the amount of threshold voltage shift before and after gate voltage application. A solid line 66 represents the change in the amount of threshold voltage shift before and after X-ray irradiation.

[0075] As can be seen from the measurement results in FIGS. 9 to 11, when the thickness of the silicon oxynitride film is in the range of 1 nm to 3 nm, the changes in threshold voltage before and after gate voltage application and before and after X-ray irradiation can be suppressed to a level that does not substantially affect the operation of the oxide semiconductor transistor. <Second embodiment>

[0076] Our research has revealed that the composition of the silicon oxynitride film used as the electron blocking layer is important. Having a specific composition, the silicon oxynitride film can prevent electrons from migrating from the oxide semiconductor layer to the silicon nitride film while minimizing the effect on the threshold voltage after radiation exposure. The structure of the switch thin-film transistor can be explained with reference to Figures 4 and 5, except that the electron blocking layer is made of silicon oxynitride instead of silicon oxide.

[0077] 12A and 12B show changes in characteristics of an oxide semiconductor transistor including a silicon oxynitride film having a first composition ratio as an electron blocking layer. The stacked structure of the oxide semiconductor transistor is as shown in FIGS.

[0078] Figure 12A shows the measurement results of the change in IV characteristics before and after gate voltage application. The horizontal axis represents gate voltage, and the vertical axis represents drain current. The solid line represents the change in drain current due to the first gate voltage sweep. The dashed line represents the change in drain current due to the second gate voltage sweep. As shown in Figure 12A, the second measurement result is significantly different from the first measurement result. In other words, the threshold voltage changes significantly due to the application of gate voltage. The explanation for this change can be applied to silicon nitride.

[0079] Figure 12B shows the measurement results of the change in IV characteristics before and after X-ray irradiation. The horizontal axis represents gate voltage, and the vertical axis represents drain current. The solid line represents the measurement results before X-ray irradiation, and the dashed line represents the measurement results after X-ray irradiation. As can be seen from the measurement results in Figure 12B, no significant change in characteristics was observed before and after X-ray irradiation.

[0080] Next, the characteristic changes of an oxide semiconductor transistor including a silicon oxynitride film with a second composition ratio different from the first composition ratio as an electron blocking layer are shown. Figures 13A and 13B show the characteristic changes of an oxide semiconductor transistor including a silicon oxynitride film with the second composition ratio as an electron blocking layer. The stacked structure of the oxide semiconductor transistor is as shown in Figures 4 and 5.

[0081] Figure 13A shows the measurement results of the change in IV characteristics before and after gate voltage application. The horizontal axis represents gate voltage, and the vertical axis represents drain current. The solid line represents the change in drain current due to the first gate voltage sweep. The dashed line represents the change in drain current due to the second gate voltage sweep. As shown in Figure 13A, no significant change in characteristics is observed between the first and second measurement results.

[0082] Figure 13B shows the measurement results of the change in IV characteristics before and after X-ray irradiation. The horizontal axis represents gate voltage, and the vertical axis represents drain current. The solid line represents the measurement results before X-ray irradiation, and the dashed line represents the measurement results after X-ray irradiation. As can be seen from the measurement results in Figure 13B, no significant change in characteristics was observed before and after X-ray irradiation.

[0083] From the above explanation, it is understood that the silicon oxynitride film having the second composition ratio functions effectively as an electron blocking layer, and also suppresses the change in threshold voltage of the oxide semiconductor transistor caused by X-ray irradiation.

[0084] 14 shows the composition analysis results of an oxide semiconductor transistor having a first composition ratio and an oxide semiconductor transistor having a second composition ratio. The measurements were performed using electron energy loss spectroscopy (EELS). FIG. 14 shows a graph 71 of the nitrogen concentration ratio, a graph 72 of the oxygen concentration ratio, and a graph 73 of the silicon concentration ratio.

[0085] In each graph, the horizontal axis represents the distance from the top surface of the oxide semiconductor layer (IGZO), and the vertical axis represents the concentration ratio (atomic ratio) of the corresponding element. In each graph, the solid line represents the measurement result of the oxide semiconductor transistor including the silicon oxynitride film with the first composition ratio, and the dashed line represents the measurement result of the oxide semiconductor transistor including the silicon oxynitride film with the second composition ratio.

[0086] As shown in Figure 14, the thickness of the silicon oxynitride (SiOxNy) film was 3 nm. In the silicon oxynitride film with the first composition ratio, the nitrogen concentration ratio (29 at%) was higher than the silicon concentration ratio (28 at%). On the other hand, in the silicon oxynitride film with the second composition ratio, the nitrogen concentration ratio (24 at%) was lower than the silicon concentration ratio (28 at%). In this way, by forming the electron blocking layer with a silicon oxynitride film in which the nitrogen atomic ratio is lower than the silicon atomic ratio, it is possible to effectively suppress changes in the characteristics of the oxide semiconductor transistor before and after application of a gate voltage and before and after X-ray irradiation.

[0087] A silicon oxynitride film can be formed, for example, by annealing a silicon nitride film. For example, after forming and patterning an oxide semiconductor layer (IGZO layer) on a silicon nitride film, annealing is performed at 400°C for 1 hour. The annealing can be performed in the atmosphere. Oxygen from the oxide semiconductor layer can form a thin silicon oxynitride film within the silicon nitride film. <Third embodiment>

[0088] Other configuration examples of the oxide semiconductor transistor according to the embodiments of this specification will be described below.

[0089] 15 is a cross-sectional view schematically illustrating an example of the structure of a switch thin-film transistor of an X-ray sensor according to one embodiment of the present specification. The switch thin-film transistor 520 includes a gate electrode 522, a gate insulating film 523 on the gate electrode 522, and an oxide semiconductor layer 524 on the gate insulating film 523. An interlayer insulating film 527 covers the entire switch thin-film transistor 520. Although this embodiment is a channel etch type, it may be a channel protection type in which the channel portion of the oxide semiconductor layer 434 is protected by an insulating film.

[0090] The gate insulating film 523 has a four-layer structure and is composed of a lower silicon nitride (SiNx) film 531, a silicon oxide (SiOx) film 532 on the lower silicon nitride film 531, an upper silicon nitride film 533 on the silicon oxide film 532, and a silicon oxide film 534 on the upper silicon nitride film 533.

[0091] The silicon oxide film 534 is in direct contact with the upper silicon nitride film 533 and the oxide semiconductor layer 524 to form an interface. The silicon oxide film 534 has a thickness of 1 nm to 4 nm and functions as an electron blocking layer for the oxide semiconductor layer 524 to the upper silicon nitride film 533. The thickness of the silicon oxide film 534 is thinner than the thickness of each of the lower silicon nitride film 531, the silicon oxide film 532, and the upper silicon nitride film 533, for example. The silicon oxide film 534 may be replaced with the silicon oxynitride film of the first or second embodiment as the electron blocking layer. The description of the first or second embodiment regarding the electron blocking layer is applicable to this configuration example.

[0092] The silicon oxide film 532 forms interfaces by being in direct contact with the lower silicon nitride film 531 and the upper silicon nitride film 533. The lower silicon nitride film 531 forms an interface with the gate electrode 522. The upper and lower silicon nitride films 531 and 533 sandwiching the silicon oxide film 532 can reduce the influence of the silicon oxide film 532 on the threshold voltage due to X-ray irradiation.

[0093] The switch thin-film transistor 520 has a bottom-gate structure, and a gate electrode 522 is located below a semiconductor layer 524. The switch thin-film transistor 520 further includes a source / drain electrode 525 and a source / drain electrode 526 on a gate insulating film 523. The source / drain electrodes 525 and 526 are each connected to the oxide semiconductor layer 524. The components described with reference to FIG. 3 or 4 can be applied to the gate electrode 522, the source / drain electrodes 525 and 526, and the interlayer insulating film 527.

[0094] 16 is a cross-sectional view showing an example of the structure of a switch thin-film transistor having a top-gate structure. The switch thin-film transistor 700 includes an oxide semiconductor layer 704, a gate insulating film 703 on the oxide semiconductor layer 704, and a gate electrode 702 on the gate insulating film 703. The gate insulating film 703 has a two-layer structure and is composed of a silicon oxide film (SiOx) 711 and a silicon nitride (SiNx) film 712 on the silicon oxide film 711.

[0095] The silicon nitride film 712 is in direct contact with each of the gate electrode 702 and the silicon oxide film 711 to form an interface. The silicon oxide film 711 is in direct contact with each of the oxide semiconductor layer 704 and the silicon nitride film 712 to form an interface.

[0096] The silicon oxide film 711 has a thickness of 1 nm to 4 nm, and functions as an electron blocking layer for the layer from the oxide semiconductor layer 704 to the silicon nitride film 712. The thickness of the silicon oxide film 711 is thinner than the thickness of the silicon nitride film 712. As the electron blocking layer, the silicon oxynitride film of the first or second embodiment may be disposed instead of the silicon oxide film 711. The description of the electron blocking layer in the first or second embodiment is applicable to this configuration example.

[0097] 15, an insulating film having a three-layer structure consisting of upper and lower silicon nitride films and a silicon oxide film between them may be used instead of the silicon nitride film 712. The thickness of the silicon oxide film 711 may be thinner than the thickness of each layer constituting the insulating film having a three-layer structure, for example.

[0098] The switch thin-film transistor 700 has a top-gate structure, and a gate electrode 702 is located above an oxide semiconductor layer 704. The switch thin-film transistor 700 further includes a source / drain electrode 705 and a source / drain electrode 706. The source / drain electrodes 705 and 706 are connected to the semiconductor layer 704 through an interlayer insulating film 707 and a gate insulating film 703, respectively. The above description with reference to the thin-film transistor having a bottom-gate structure can be applied to the gate insulating film 703. The description with reference to FIG. 3 or 4 can be applied to the gate electrode 702, the source / drain electrodes 705 and 706, and the interlayer insulating film 707.

[0099] 17 is a cross-sectional view showing an example of the structure of a switch thin-film transistor having a dual-gate structure. The switch thin-film transistor 720 includes a gate electrode 722, a gate insulating film 723 on the gate electrode 722, and an oxide semiconductor layer 724 on the gate insulating film 723. The switch thin-film transistor 720 further includes a gate insulating film 743 on the oxide semiconductor layer 724, and a gate electrode 762 on the gate insulating film 743.

[0100] The gate insulating film 723 has a two-layer structure and is composed of a silicon nitride film 731 and a silicon oxide layer film on the silicon nitride film 731. The silicon oxide film 732 is in direct contact with the silicon nitride film 731 and the oxide semiconductor layer 724 to form interfaces. The silicon nitride film 731 forms interfaces with the gate electrode 722 and the silicon oxide film 732.

[0101] The gate insulating film 743 has a two-layer structure and is composed of a silicon oxide film 751 and a silicon nitride film 752 on the silicon oxide film 751. The silicon oxide film 751 is in direct contact with the oxide semiconductor layer 724 and the silicon nitride film 752 to form an interface. The silicon nitride film 752 also forms an interface with the silicon oxide film 751 and the gate electrode 762.

[0102] The switch thin-film transistor 720 has a dual-gate structure, and an oxide semiconductor layer 724 is disposed between a top gate electrode 762 and a bottom gate electrode 722. The switch thin-film transistor 720 further includes a source / drain electrode 725 and a source / drain electrode 726. The source / drain electrodes 725 and 726 are connected to the oxide semiconductor layer 724 through an interlayer insulating film 727 and a gate insulating film 743, respectively.

[0103] The description of the gate insulating film in the bottom gate structure described with reference to FIG. 5 or FIG. 15 can be applied to the gate insulating film 723. Specifically, the silicon oxide film 732 has a thickness of 1 nm to 4 nm, and functions as an electron blocking layer for the oxide semiconductor layer 724 to the silicon nitride film 731. The thickness of the silicon oxide film 732 is thinner than that of the silicon nitride film 731. The silicon oxynitride film of the first or second embodiment may be disposed as the electron blocking layer instead of the silicon oxide film 732. The description of the electron blocking layer in the first or second embodiment can be applied to this configuration example.

[0104] 15, a three-layer insulating film consisting of upper and lower silicon nitride films and a silicon oxide film between them may be used instead of the silicon nitride film 731. The thickness of the silicon oxide film 732 may be thinner than the thickness of each layer constituting the three-layer insulating film, for example.

[0105] The description of the gate insulating film in the top-gate structure described with reference to FIG. 16 can be applied to the gate insulating film 743. Specifically, the silicon oxide film 751 has a thickness of 1 nm to 4 nm, and functions as an electron blocking layer for the oxide semiconductor layer 724 to the silicon nitride film 752. The thickness of the silicon oxide film 751 is thinner than that of the silicon nitride film 752. As the electron blocking layer, the silicon oxynitride film of the first or second embodiment may be disposed instead of the silicon oxide film 751. The description of the electron blocking layer in the first or second embodiment can be applied to this configuration example.

[0106] 15, an insulating film having a three-layer structure consisting of upper and lower silicon nitride films and a silicon oxide film between them may be used instead of the silicon nitride film 752. The thickness of the silicon oxide film 751 may be thinner than the thickness of each layer constituting the insulating film having a three-layer structure, for example.

[0107] The gate insulating films 723 and 743 may have the same or different structures. The gate electrodes 722 and 762, the source / drain electrodes 725 and 726, and the interlayer insulating film 727 can be described with reference to FIGS. 3 and 4.

[0108] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above embodiments. Those skilled in the art can easily modify, add, or convert each element of the above embodiments within the scope of the present disclosure. It is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. [Explanation of symbols]

[0109] 10 X-ray sensor 102 pixels 103 Photodiode 104 Thin-film transistor 302, 432, 522, 722, 762 Gate electrode 303, 433, 523, 703, 723, 743 Gate insulating film 304, 434, 524 Oxide semiconductor layer 438, 531, 533, 712, 731, 752 Silicon nitride film 439, 534, 711, 732, 751 Silicon oxide film or silicon oxynitride film 532 Silicon oxide film

Claims

1. A thin film transistor for use in a radiation sensor, comprising: a gate electrode; an oxide semiconductor layer; a gate insulating film between the oxide semiconductor layer and the gate electrode; Including, The gate insulating film is a silicon nitride layer; a silicon oxide layer between the silicon nitride layer and the oxide semiconductor layer, the silicon oxide layer forming an interface with the silicon nitride layer and the oxide semiconductor layer; Including, The thickness of the silicon oxide layer is 1 nm or more and 4 nm or less. Thin film transistor.

2. A thin film transistor for use in a radiation sensor, comprising: a gate electrode; an oxide semiconductor layer; a gate insulating film between the oxide semiconductor layer and the gate electrode; Including, The gate insulating film is a silicon nitride layer; a silicon oxynitride layer between the silicon nitride layer and the oxide semiconductor layer, the silicon oxynitride layer forming an interface with the silicon nitride layer and the oxide semiconductor layer; Including, The thickness of the silicon oxynitride layer is 1 nm or more and 3 nm or less. Thin film transistor.

3. A thin film transistor for use in a radiation sensor, comprising: a gate electrode; an oxide semiconductor layer; a gate insulating film between the oxide semiconductor layer and the gate electrode; Including, The gate insulating film is Silicon nitride layer and a silicon oxynitride layer between the silicon nitride layer and the oxide semiconductor layer, the silicon oxynitride layer forming an interface with the silicon nitride layer and the oxide semiconductor layer; Including, In the silicon oxynitride layer, the nitrogen atomic ratio is smaller than the silicon atomic ratio; Thin film transistor.

4. 4. A thin film transistor according to claim 1, 2 or 3, the silicon nitride layer is a first silicon nitride layer; The gate insulating film is a second silicon nitride layer between the gate electrode and the first silicon nitride layer; a second silicon oxide layer between the second silicon nitride layer and the first silicon nitride layer; Including, the second silicon oxide layer forms an interface with each of the second silicon nitride layer and the first silicon nitride layer; the second silicon oxide layer forms an interface with the gate electrode; Thin film transistor.

5. 4. A thin film transistor according to claim 1, 2 or 3, the gate electrode is located between a substrate and the oxide semiconductor layer; Thin film transistor.

6. A radiation sensor comprising: A substrate; a photoelectric conversion element on the substrate; a thin film transistor according to claim 1 , 2 or 3 present between the substrate and the photoelectric conversion element; A signal line and Including, the thin film transistor switches a connection between the signal line and the photoelectric conversion element; Radiation sensor.

Images