Thin film transistor and radiation sensor

Abstract

The invention relates to a thin film transistor and a radiation sensor. A thin film transistor for a radiation sensor is disclosed. The thin film transistor includes a gate electrode, an oxide semiconductor layer, and a gate insulating film between the oxide semiconductor layer and the gate electrode. The gate insulating film includes a silicon nitride layer, and a silicon oxide layer between the silicon nitride layer and the oxide semiconductor layer and having an interface with the silicon nitride layer and the oxide semiconductor layer. The thickness of the silicon oxide layer is larger than or equal to 1 nm and smaller than or equal to 4 nm.

Description

TECHNICAL FIELD

[0001] The present application relates to a structure of a thin film transistor. BACKGROUND

[0002] Techniques for non-destructive inspection of the inside of a sample using an X-ray image transmitted through the sample are essential in the field of industrial non-destructive testing. In particular, digital radiography (DR) that directly captures an image of transmitted X-rays as electronic data has been widely adopted because of the rapid image reading and image interpretation assistance that can be obtained through image processing. DR uses a device called a flat panel detector (FPD).

[0003] FPDs for X-ray sensors are generally classified into direct conversion type and indirect conversion type. A direct conversion type FPD directly converts X-rays into an electrical signal. An indirect conversion type FPD includes a luminescent material (scintillator) that converts X-rays into light (such as visible light or ultraviolet light) and an array of photoelectric conversion elements that convert the light into an electrical signal in its X-ray detection panel. The FPD includes array pixels each including a conversion element that converts X-rays or light into an electrical signal and a switching thin film transistor for taking out the electrical signal. SUMMARY

[0004] The inventors' studies have shown that the characteristics of the switching thin film transistor used in the FPD can change due to radioactive rays. In particular, strong X-rays can cause a large change in the characteristics.

[0005] One aspect of the present application is a thin film transistor for a radiation sensor. The thin film transistor includes a gate electrode, an oxide semiconductor layer, and a gate insulating film between the oxide semiconductor layer and the gate electrode. The gate insulating film includes a silicon nitride layer, and a silicon oxide layer between the silicon nitride layer and the oxide semiconductor layer and having interfaces with the silicon nitride layer and the oxide semiconductor layer. The thickness of the silicon oxide layer is greater than or equal to 1 nm and less than or equal to 4 nm.

[0006] One aspect of the present application is a thin film transistor for a radiation sensor. The thin film transistor includes a gate electrode, an oxide semiconductor layer, and a gate insulating film between the oxide semiconductor layer and the gate electrode. The gate insulating film includes a silicon nitride layer, and a silicon oxynitride layer between the silicon nitride layer and the oxide semiconductor layer and having interfaces with the silicon nitride layer and the oxide semiconductor layer. The thickness of the silicon oxynitride layer is greater than or equal to 1 nm and less than or equal to 3 nm.

[0007] An aspect of the present application is a thin film transistor for a radiation sensor. The thin film transistor includes a gate electrode, an oxide semiconductor layer, and a gate insulating film between the oxide semiconductor layer and the gate electrode. The gate insulating film includes a silicon nitride layer, and a silicon oxynitride layer between the silicon nitride layer and the oxide semiconductor layer and having interfaces with the silicon nitride layer and the oxide semiconductor layer. The silicon oxynitride layer has a proportion of nitrogen atoms lower than a proportion of silicon atoms.

[0008] An aspect of the present application improves characteristics of a thin film transistor to be used for a radiation sensor.

[0009] It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 is a block diagram showing a configuration example of an X-ray sensor.

[0011] Figure 2 is a circuit diagram showing a configuration example of an equivalent circuit of a pixel.

[0012] Figure 3 shows a cross-sectional structure of a pixel.

[0013] Figure 4 is a plan view of a pixel.

[0014] Figure 5 is a cross-sectional view schematically showing a structure of a thin film transistor in one embodiment of the present specification.

[0015] Figure 6 A plurality of graphs showing a relationship between a change in current-voltage characteristics caused by application of a gate voltage to an oxide semiconductor thin film transistor and a thickness of a silicon oxide film are provided.

[0016] Figure 7 A plurality of graphs showing a relationship between a change in current-voltage characteristics caused by irradiation of X-rays to an oxide semiconductor thin film transistor and a thickness of a silicon oxide film are provided.

[0017] Figure 8 is a graph in which measurement results in Figure 6 and Figure 7 are combined.

[0018] Figure 9 A plurality of graphs showing a relationship between a change in current-voltage characteristics caused by application of a gate voltage to an oxide semiconductor thin film transistor and a thickness of a silicon oxynitride film are provided.

[0019] Figure 10A plurality of graphs showing the relationship between the change in current-voltage characteristics caused by X-ray irradiation to an oxide semiconductor thin film transistor and the thickness of a silicon oxynitride film are provided.

[0020] Figure 11 is a graph in which the measurement results in Figure 9 and Figure 10 are combined.

[0021] Figure 12A shows the change in characteristics of an oxide semiconductor transistor including an electron-blocking layer made of a silicon oxynitride film having a first composition ratio before and after application of a gate voltage.

[0022] Figure 12B shows the change in characteristics of an oxide semiconductor transistor including an electron-blocking layer made of a silicon oxynitride film having a first composition ratio before and after X-ray irradiation.

[0023] Figure 13A shows the change in characteristics of an oxide semiconductor transistor including an electron-blocking layer made of a silicon oxynitride film having a second composition ratio before and after application of a gate voltage.

[0024] Figure 13B shows the change in characteristics of an oxide semiconductor transistor including an electron-blocking layer made of a silicon oxynitride film having a second composition ratio before and after X-ray irradiation.

[0025] Figure 14 A composition analysis result of an oxide semiconductor transistor including an electron-blocking layer having a first composition ratio and an oxide semiconductor transistor including an electron-blocking layer having a second composition ratio is provided.

[0026] Figure 15 is a cross-sectional view schematically showing a structure example of a switching thin film transistor in an X-ray sensor of one embodiment of the present specification.

[0027] Figure 16 is a cross-sectional view showing a structure example of a switching thin film transistor having a top gate structure.

[0028] Figure 17 is a cross-sectional view showing a structure example of a switching thin film transistor having a dual gate structure. DETAILED DESCRIPTION

[0029] Hereinafter, embodiments will be described with reference to the drawings. The embodiments are merely examples of implementing the present application and do not limit the technical scope of the present application. In order to clearly understand the description, the size or the shape of some elements in the drawings can be exaggerated.

[0030] An embodiment of the present specification discloses a structure of an oxide semiconductor thin film transistor suitable for a radiation sensor. A radiation sensor including an oxide semiconductor thin film transistor having high driving capability has been actively developed. However, when the oxide semiconductor thin film transistor is irradiated with radioactive rays, particularly X-rays, its threshold voltage can significantly change and work improperly. This problem is significant in an industrial field in which a transistor is irradiated with a high dose of X-rays, compared to a medical field in which a low dose of X-rays is used.

[0031] To improve the resistance of the oxide semiconductor thin film transistor to radioactive rays, the inventors found a solution of using silicon nitride (SiNx) as a gate insulating film. However, another problem was found that the oxide semiconductor thin film transistor having a silicon nitride gate insulating film shows threshold voltage drift when electrons are induced in a channel by applying a gate voltage.

[0032] An embodiment of the present specification provides a gate insulating film composed of multiple layers of different materials, in which an electron blocking layer is provided between a silicon nitride film and an oxide semiconductor film. The electron blocking layer is in direct contact with and has an interface with each of the silicon nitride film and the oxide semiconductor film. The electron blocking layer can be a silicon oxide film or a silicon oxynitride film. The electron blocking layer reduces the threshold voltage shift caused by the induction of electrons in a channel.

[0033] Hereinafter, an X-ray sensor will be described as an example; however, the features disclosed herein are applicable to other radioactive ray sensors other than X-ray sensors.

[0034] First Embodiment

[0035] Figure 1 is a block diagram showing a configuration example of an X-ray sensor. The X-ray sensor 10 is an image sensor for imaging X-rays transmitted through an object. The X-ray sensor 10 includes a pixel matrix 101, a scanning circuit 170, and a detector circuit 150. The pixel matrix 101 includes pixels 102 arranged in a matrix. The pixel matrix 101 is manufactured on a sensor substrate 100. The sensor substrate 100 is a substrate having insulating properties (for example, a glass substrate).

[0036] The pixel 102 is provided at an intersection between a plurality of signal lines 106 and a plurality of gate lines (scanning lines) 105. In the present embodiment, the signal lines 106 are provided to extend vertically and be horizontally spaced apart from each other, and the gate lines 105 are provided to extend horizontally and be vertically spaced apart from each other. Figure 1 In the present embodiment, the signal lines 106 are provided to extend vertically and be horizontally spaced apart from each other, and the gate lines 105 are provided to extend horizontally and be vertically spaced apart from each other. Each pixel 102 is connected to a bias line 107. In the present embodiment, a plurality of bias lines 107 are provided to extend vertically and be horizontally spaced apart from each other. Figure 1 In the present embodiment, the signal lines 106 are provided to extend vertically and be horizontally spaced apart from each other, and the gate lines 105 are provided to extend horizontally and be vertically spaced apart from each other. Each pixel 102 is connected to a bias line 107. In the present embodiment, a plurality of bias lines 107 are provided to extend vertically and be horizontally spaced apart from each other. Figure 1In the present embodiment, only one pixel, one signal line, one gate line, and one bias line are provided with reference numerals 102, 106, 105, and 107, respectively.

[0037] Each signal line 106 is connected to a different column of pixels. Each gate line 105 is connected to a different row of pixels. The signal lines 106 are connected to a detector circuit 150, and the gate lines 105 are connected to a scan circuit 170. Each bias line 107 is connected to a common bias line 108. A bias potential is supplied to a pad 109 of the common bias line 108.

[0038] Figure 2 is a circuit diagram showing a configuration example of an equivalent circuit of the pixel 102. The pixel 102 includes a photodiode 103 as a photoelectric conversion element and a thin film transistor (TFT) 104 as a switching element. In the thin film transistor 104, a gate is connected to the gate line 105; one of a source / drain is connected to the signal line 106; and the other of the source / drain is connected to a cathode of the photodiode 103. In the photodiode 103, an anode is connected to the bias line 107. Figure 2 In an example of the photodiode 103, the anode is connected to the bias line 107.

[0039] The thin film transistor 104 can be an oxide semiconductor thin film transistor. Figure 2 The thin film transistor 104 in the configuration example has n-type conductivity. The thin film transistor 104 can have different conductivity. An oxide semiconductor thin film transistor exhibits good switching characteristics.

[0040] The X-ray sensor 10 reads a signal of the pixel 102 by taking out signal charges stored in proportion to the amount of X-ray irradiation from the photodiode 103 to the outside. The signal charges can be taken out by turning on the thin film transistor 104 in the pixel 102. Specifically, when light enters the photodiode 103, signal charges are generated and stored in the photodiode 103.

[0041] The scan circuit 170 selects the gate lines 105 one by one to apply a pulse, thereby turning on the thin film transistor 104. The anode terminal of the photodiode 103 is connected to the bias line 107, and the signal line 106 is supplied with a reference potential by the detector circuit 150. Thus, the photodiode 103 is charged with a difference voltage between the bias potential of the bias line 107 and the reference potential. The difference voltage is determined so that the cathode potential is higher than the anode potential to reverse bias the photodiode 103.

[0042] The charge required to recharge the photodiode 103 to the reverse bias depends on the amount of light incident on the photodiode 103. The detector circuit 150 reads the signal charges by integrating the current that flows before the photodiode 103 is recharged to the reverse bias.

[0043] The charge stored in the photodiode 103 also inevitably decreases due to incident light and a dark leakage current that flows even when the photodiode 103 is not irradiated with light. Therefore, in the thin-film transistor 104 under the signal charge reading operation, the voltage at the terminal connected to the signal line 106 is equal to or higher than the voltage at the terminal connected to the photodiode 103. That is, in detecting the signal charge, the terminal connected to the signal line 106 is the drain, and the terminal connected to the photodiode 103 is the source.

[0044] Figure 3 A cross-sectional structure of the pixel is shown. In the following description, the side on which the photodiode 103 faces the sensor substrate 100 is defined as the front side. In the positional relationship of the components of the pixel, the side closer to the sensor substrate 100 is referred to as the lower side, and the opposite side is referred to as the upper side. Figure 3

[0045] The thin-film transistor 104 and the photodiode 103 included in the pixel each have a layered structure. The thin-film transistor 104 includes a gate electrode 302 provided above the sensor substrate 100 having insulating properties, a gate insulating film 303 above the gate electrode 302, and an oxide semiconductor layer 304 above the gate insulating film 303.

[0046] Figure 3 The thin-film transistor 104 in the pixel has a bottom gate structure; the gate electrode 302 is located below the oxide semiconductor layer 304. The thin-film transistor 104 further includes one of a source / drain electrode 305 and the other 306 above the gate insulating film 303. The source / drain electrodes 305 and 306 are connected to the oxide semiconductor layer 304, respectively. Each of the source / drain electrodes 305 and 306 is in contact with a part of the side surface and the top surface of the island-shaped oxide semiconductor layer 304.

[0047] In detecting the charge of the photodiode 103, the electrode 305 is the drain electrode, and the electrode 306 is the source electrode.

[0048] The gate insulating film 303 is provided so as to cover the entire gate electrode 302. The gate insulating film 303 is provided between the gate electrode 302 and the oxide semiconductor layer 304, between the gate electrode 302 and the source / drain electrode 305, and between the gate electrode 302 and the source / drain electrode 306.

[0049] A first interlayer insulating film 307 is provided so as to cover the entire thin-film transistor 104. Specifically, the first interlayer insulating film 307 covers the top surface of the oxide semiconductor layer 304 and the top surfaces of the source / drain electrodes 305 and 306.

[0050] ​The sensor substrate 100 can be made of glass or resin. The gate electrode 302 is a conductor and can be made of a metal or silicon doped with impurities. The gate insulating film 303 has a multi-layer structure. Each layer of the gate insulating film 303 can be made of silicon oxide (SiOx), silicon nitride (SiNx), or silicon oxynitride (SiOxNy). Details of the gate insulating film 303 will be described later.

[0051] The oxide semiconductor used for the oxide semiconductor layer 304 is an oxide semiconductor including at least one of In, Ga, and Zn, such as IGZO. Examples of IGZO include amorphous InGaZnO (a-InGaZnO) and microcrystalline InGaZnO. Other oxide semiconductors, such as a-InSnZnO and a-InGaZnSnO, can also be used. The examples described below mainly use amorphous or microcrystalline InGaZnO (hereinafter can also be simply denoted as IGZO).

[0052] The source / drain electrodes 305 and 306 are conductors and can be made of a metal such as Mo, Ti, Al, or Cr, an alloy thereof, or a laminate of these metals or alloys. The first interlayer insulating film 307 is an inorganic or organic insulator. Although Figure 3 The thin film transistor 104 in FIG. 1A has a bottom-gate structure, but the thin film transistor 104 can have a top-gate structure or a dual-gate structure. The dual-gate structure has a top gate and a bottom gate; the bottom-gate structure has only a bottom gate; and the top-gate structure has only a top gate.

[0053] The photodiode 103 is fabricated over the first interlayer insulating film 307. Figure 3 An example of the photodiode 103 in FIG. 1A is a PIN diode. The PIN diode has a thick depletion layer in the film thickness to achieve efficient light detection. The photodiode 103 includes a layered semiconductor sandwiched between a lower electrode 308 and an upper electrode 312 over the first interlayer insulating film 307. The lower electrode 308 is connected to the source / drain electrode 306 of the thin film transistor 104 through an interconnection region in a through-hole 321 of the first interlayer insulating film 307.

[0054] The lower electrode 308 is a conductor and can be made of a metal such as Cr, Mo, or Al, an alloy thereof, or a laminate of these metals or alloys. The upper electrode 312 is a transparent electrode that transmits light from the scintillator 316 and can be made of, for example, ITO.

[0055] The photodiode 103 includes an n-type amorphous silicon layer 309 over the lower electrode 308, an intrinsic amorphous silicon layer 310 over the n-type amorphous silicon layer 309, and a p-type amorphous silicon layer 311 over the intrinsic amorphous silicon layer 310. An upper electrode 312 is provided over the p-type amorphous silicon layer 311. Light to be detected enters the photodiode 103 from above the upper electrode 312 (p-type amorphous silicon layer 311).

[0056] A second interlayer insulating film 313 is provided to cover the photodiode 103. Specifically, the second interlayer insulating film 313 is provided over the first interlayer insulating film 307, a portion of the lower electrode 308, and the upper electrode 312. The second interlayer insulating film 313 is an inorganic or organic insulator.

[0057] A bias line 107 is provided over the second interlayer insulating film 313. The bias line 107 is connected to the upper electrode 312 through an interconnection region provided in a through-hole 322 of the second interlayer insulating film 313. The bias line 107 is a conductor, and can be made of a metal such as Mo, Ti, or Al, an alloy thereof, or a laminate of these metals or alloys.

[0058] A passivation layer 315 is provided to cover the bias line 107 and the second interlayer insulating film 313. The passivation layer 315 covers the entire pixel matrix 101. The passivation layer 315 is an inorganic or organic insulator. A scintillator 316 is provided over the passivation layer 315.

[0059] The scintillator 316 covers the entire pixel matrix 101. The scintillator 316 converts received X-rays into light having a wavelength that can be detected by the photodiode 103. The photodiode 103 stores signal charges depending on the amount of light from the scintillator 316.

[0060] Figure 4 is a plan view of the pixel 102. As shown in Figure 4 , the gate line 105 extends from the left side to the right side of Figure 4 , and the signal line 106 extends from the top to the bottom of Figure 4 . The gate electrode 302 is not separated from the gate line 105; these are part of an unseparated metal film. The gate electrode 302 protrudes from the gate line 105 in a direction perpendicular to the gate line 105.

[0061] The source / drain electrode 305 of the thin film transistor is not separated from the signal line 106; these are part of an unseparated metal film. The source / drain electrode 305 protrudes from the signal line 106 in a direction perpendicular to the signal line 106. The source / drain electrode 306 is an island-shaped electrode, and is away from the source / drain electrode 305.

[0062] When viewed from a planar perspective, the oxide semiconductor layer 304 is arranged to overlap with the gate electrode 302. The source / drain electrode 305 is provided on one side of the oxide semiconductor layer 304, and the source / drain electrode 306 is provided on the opposite side. The source / drain electrode 306 is partially covered by the lower electrode 308 of the photodiode 103 and is connected to the lower electrode 308 through the through hole 321.

[0063] When viewed from a flat surface Figure 4 In the example of FIG. 1 , the entire upper electrode 312 of the photodiode 103 is within the region of the lower electrode 308. The bias line 107 is connected from Figure 4 The bias line 107 overlaps with the upper electrode 312 and is connected to the upper electrode 312 through the through hole 322.

[0064] Although the reference Figures 1 to 4 The configuration example described includes a photodiode as an element for converting radioactive rays into an electrical signal, but other types of elements may also be employed. For example, an element that can directly convert X-rays into an electrical signal without a scintillator may be employed.

[0065] One embodiment of the present disclosure features the structure of a switching thin-film transistor in each pixel of a radiation sensor. The following describes the structure of thin-film transistors related to some embodiments of the present disclosure. The inventors' research has shown that the structure of the gate insulating film in a switching thin-film transistor affects the characteristics of the switching thin-film transistor when exposed to radioactive radiation, particularly X-rays.

[0066] Specifically, when the gate insulating film is made of a single silicon oxide (SiOx) layer, the threshold voltage (Vth) of the thin-film transistor shifts negatively in response to X-ray irradiation, potentially making proper operation difficult. This is presumably because holes generated in response to X-ray irradiation are trapped in the silicon oxide layer, causing the threshold voltage (Vth) to shift negatively.

[0067] When the gate insulating film is made of a single silicon nitride (SiNx) layer, the threshold voltage Vth of the thin film transistor shifts in the positive direction in response to the application of a positive voltage to the gate electrode of the thin film transistor. This is presumably because electrons in the oxide semiconductor layer, caused by the voltage applied to the gate electrode, move to the silicon nitride layer and are trapped in the silicon nitride layer.

[0068] One embodiment of the present specification includes a gate insulating film composed of multiple layers, in which a silicon oxide film (silicon oxide layer) or a silicon oxynitride (SiOxNy) film (silicon oxynitride layer) is provided between a silicon nitride film (silicon nitride layer) and a gate electrode. The silicon oxide film or silicon oxynitride film is in direct contact with the silicon nitride layer and the gate electrode, and has an interface therewith.

[0069] Figure 5 is a cross-sectional view schematically showing a structure of a switching thin film transistor in one embodiment of the present specification. The switching thin film transistor 430 includes a gate electrode 432, a gate insulating film 433 over the gate electrode 432, and an oxide semiconductor layer 434 over the gate insulating film 433. The oxide semiconductor layer 434 is made of IGZO. The entire thin film transistor 430 is covered with an interlayer insulating layer 437.

[0070] The gate insulating film 433 has a two-layer structure of a silicon nitride film (a silicon nitride layer) 438 made of silicon nitride (SiNx) and an insulating film 439 made of silicon oxide (SiOx) over the silicon nitride film 438.

[0071] The thin film transistor 430 has a bottom-gate structure; the gate electrode 432 is positioned below (closer to the substrate side of) the oxide semiconductor layer 434. The thin film transistor 430 further includes one of source / drain electrodes 435 and the other of source / drain electrodes 436 over the gate insulating film 433. The source / drain electrodes 435 and 436 are connected to the oxide semiconductor layer 434, respectively. Although the thin film transistor 430 in this embodiment is of a channel-etched type, it can be of a channel-protective type so that a channel region of the oxide semiconductor layer 434 is protected with an insulating film.

[0072] The silicon oxide film 439 is thinner than the silicon nitride film 438. The silicon oxide film 439 in one embodiment of the present specification has a thickness greater than or equal to 1 nm and less than or equal to 4 nm. The silicon nitride film 438 can have a thickness of several hundred nanometers, or a thickness between 100 nm and 900 nm.

[0073] Application of a voltage to the gate electrode can cause electrons to be injected from the oxide semiconductor layer into the silicon nitride film. The silicon oxide film 439 between the silicon nitride film 438 and the oxide semiconductor layer 434 is an electron barrier layer that blocks (suppresses) movement of electrons from the oxide semiconductor layer 434 to the silicon nitride film 438.

[0074] The provision of the silicon oxide film between the oxide semiconductor layer and the silicon nitride film reduces changes in characteristics of the oxide semiconductor thin film transistor due to carriers trapped in the silicon nitride film caused by operation (application of a gate voltage) of the oxide semiconductor thin film transistor.

[0075] The silicon oxide film can effectively block movement of electrons from the oxide semiconductor layer to the silicon nitride film by having a thickness greater than a certain value. The inventors' studies have shown that a silicon oxide film with a thickness greater than or equal to 1 nm can effectively stabilize characteristics of the oxide semiconductor thin film transistor.

[0076] However, carriers generated and trapped in the silicon oxide film that is a component of the electron-blocking layer by irradiation with a radioactive ray, particularly an X-ray, can significantly affect the threshold voltage Vth of the oxide semiconductor layer directly above the silicon oxide film. This effect on the threshold voltage Vth can be effectively reduced by thinning the silicon oxide film.

[0077] The inventors' studies have shown that a silicon oxide film with a thickness of less than or equal to 4 nm can effectively reduce the effect on the threshold voltage Vth, allowing the oxide semiconductor thin film transistor to stably operate as a switching thin film transistor. In particular, the stability of the switching thin film transistor can be maintained under irradiation with strong X-rays (e.g., X-rays of 500 Gy or more).

[0078] As described above, the provision of the silicon oxide film between the silicon nitride film and the oxide semiconductor layer prevents the movement of electrons from the oxide semiconductor layer to the silicon nitride film, and the thinning of the silicon oxide film eliminates the effect of the trapped holes in the silicon oxide film. Thus, a thin film transistor with radiation resistance and stable characteristics can be provided.

[0079] Hereinafter, some measurement results of oxide semiconductor thin film transistors including silicon oxide films with different thicknesses are described. The oxide semiconductor thin film transistors used in the measurements had the structure shown in FIG. 1; the insulating films 439 of these oxide semiconductor thin film transistors were made of silicon oxide, and the oxide semiconductor layers 434 thereof were made of IGZO. Figure 5 The structure shown in FIG. 1; the insulating films 439 of these oxide semiconductor thin film transistors were made of silicon oxide, and the oxide semiconductor layers 434 thereof were made of IGZO.

[0080] First, the effect of blocking the movement of electrons from the oxide semiconductor layer to the silicon nitride film is described. Figure 6 A plurality of graphs showing the relationship between the change in the IV (current-voltage) characteristics caused by the application of a gate voltage to an oxide semiconductor thin film transistor and the thickness of a silicon oxide (SiOx) film is provided. Specifically, these graphs are measurement results of oxide semiconductor thin film transistors including silicon oxide films with thicknesses of 0 nm (no SiOx), 1 nm, 2 nm, 3 nm, 4 nm, and 5 nm. Each graph is a result of measuring the drain current in response to a scan of the gate voltage. In the measurements, no radioactive ray was emitted. 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 a first measurement result, and the broken line represents a second measurement result.

[0081] From the results shown in FIGS. 2A to 2F, it is clear that the effect on the threshold voltage Vth can be effectively reduced by thinning the silicon oxide film. Figure 6As can be understood from the graph in FIG1 , the oxide semiconductor thin film transistor without silicon oxide (no SiOx) shows a large change in threshold voltage between the first measurement result and the second measurement result. However, the oxide semiconductor thin film transistor having a silicon oxide film with a thickness greater than or equal to 1 nm (SiOx: 1 nm, SiOx: 2 nm, SiOx: 3 nm, SiOx: 4 nm, and SiOx: 5 nm) shows a small change in threshold voltage, which is within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0082] Next, the influence of the electron blocking layer on the IV characteristics of the oxide semiconductor thin film transistor irradiated with X-rays is described. Figure 7 Graphs are provided showing the relationship between changes in IV (current-voltage) characteristics caused by X-ray irradiation to an oxide semiconductor thin film transistor and the thickness of a silicon oxide (SiOx) film.

[0083] Specifically, these graphs are measurement results of oxide semiconductor thin film transistors including silicon oxide films with thicknesses of 0 nm (no SiOx), 1 nm, 2 nm, 3 nm, 4 nm, and 5 nm. Each graph is the result of measuring the drain current in response to a sweep of the gate voltage. 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 measurement result before irradiation with X-rays, and the dotted line represents the measurement result after irradiation with X-rays. The dose of X-rays is 660 Gy.

[0084] from Figure 7 As can be understood from the graph in , the oxide semiconductor thin film transistor including a silicon oxide film with a thickness of 5 nm (SiOx: 5 nm) shows a large change in threshold voltage between the measurement results before and after X-ray irradiation. However, the oxide semiconductor thin film transistor including a silicon oxide film with a thickness of less than or equal to 4 nm (no SiOx, SiOx: 1 nm, SiOx: 2 nm, SiOx: 3 nm, and SiOx: 4 nm) shows a small change in threshold voltage, which is within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0085] Figure 8 It's merged Figure 6 and Figure 7 The horizontal axis represents the thickness of the silicon oxide film. The left vertical axis represents the thickness of the silicon oxide film. Figure 6 The right vertical axis represents the threshold voltage shift caused by the application of the gate voltage. Figure 7 The threshold voltage shift due to X-ray irradiation is shown in FIG. 6. A dotted line 61 shows the change in threshold voltage shift due to gate voltage application, and a solid line 62 shows the change in threshold voltage shift due to X-ray irradiation.

[0086] from Figures 6 to 8 From the measurement results in , it can be understood that a silicon oxide film with a thickness in the range of 1 nm to 4 nm can suppress the threshold voltage drift caused by gate voltage application and X-ray irradiation to a level that basically does not affect the operation of the oxide semiconductor transistor.

[0087] Next, the thickness of the silicon nitride oxide film used instead of the silicon oxide film 439 is discussed. Hereinafter, some measurement results of oxide semiconductor thin film transistors including silicon nitride oxide films having different thicknesses are described. The oxide semiconductor thin film transistors used in the measurement have Figure 5 The structure shown; the insulating film 439 of these oxide semiconductor thin film transistors is made of silicon oxynitride, and their oxide semiconductor layer 434 is made of IGZO.

[0088] First, the effect of blocking the movement of electrons from the oxide semiconductor layer to the silicon nitride film is described. Figure 9 A plurality of graphs are provided showing the relationship between the change in IV (current-voltage) characteristics caused by the application of a gate voltage to an oxide semiconductor thin film transistor and the thickness of the silicon oxynitride film. Specifically, these graphs are measurement results of oxide semiconductor thin film transistors including silicon oxynitride films having thicknesses of 0 nm (no SiOxNy), 1 nm, 2 nm, 3 nm, and 4 nm. Each graph is the result of measuring the drain current in response to a scan of the gate voltage. No radioactive rays are emitted in the measurement. 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 dotted line represents the second measurement result.

[0089] from Figure 9 As can be understood from the graph in FIG1 , the oxide semiconductor thin film transistor without silicon oxynitride (no SiOxNy) shows a large change in threshold voltage between the first measurement result and the second measurement result. However, the oxide semiconductor thin film transistor having a silicon oxynitride film with a thickness greater than or equal to 1 nm (SiOxNy: 1 nm, SiOxNy: 2 nm, SiOxNy: 3 nm, and SiOxNy: 4 nm) shows a small change in threshold voltage, which is within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0090] Next, the influence of the electron blocking layer on the IV characteristics of the oxide semiconductor thin film transistor irradiated with X-rays is described. Figure 10 Graphs are provided showing the relationship between changes in IV (current-voltage) characteristics caused by X-ray irradiation to an oxide semiconductor thin film transistor and the thickness of a silicon oxynitride (SiOxNy) film.

[0091] Specifically, these graphs are measurement results of oxide semiconductor thin film transistors including silicon oxynitride films having thicknesses of 0 nm (no SiOxNy), 1 nm, 2 nm, 3 nm, and 4 nm. Each graph is the result of measuring the drain current in response to a sweep of the gate voltage. 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 measurement result before irradiation with X-rays, and the dotted line represents the measurement result after irradiation with X-rays. The X-ray dose is 660 Gy.

[0092] from Figure 10 As can be understood from the graph in FIG, the oxide semiconductor thin film transistor including a silicon oxynitride film having a thickness of 4 nm (SiOxNy: 4 nm) shows a large change in threshold voltage between the measurement results before and after X-ray irradiation. However, the oxide semiconductor thin film transistor including a silicon oxynitride film having a thickness of less than or equal to 3 nm (no SiOxNy, SiOxNy: 1 nm, SiOxNy: 2 nm, and SiOxNy: 3 nm) shows a small change in threshold voltage, which is within a range that does not substantially affect the operation of the oxide semiconductor transistor.

[0093] Figure 11 It's merged Figure 9 and Figure 10 The horizontal axis represents the thickness of the silicon oxynitride film. The left vertical axis represents the thickness of the silicon oxynitride film. Figure 9 The right vertical axis represents the threshold voltage shift caused by the application of the gate voltage. Figure 10 The dashed line 65 shows the change in the threshold voltage shift due to the application of the gate voltage, and the solid line 66 shows the change in the threshold voltage shift due to the X-ray irradiation.

[0094] from Figures 9 to 11 From the measurement results in , it can be understood that a silicon oxynitride film with a thickness in the range of 1 nm to 3 nm can suppress the threshold voltage drift caused by gate voltage application and X-ray irradiation to a level that basically does not affect the operation of the oxide semiconductor transistor.

[0095] Second embodiment

[0096] The inventors' research has shown that an important factor in the operation of the silicon oxynitride film as an electron blocking layer is its composition. A silicon oxynitride film with a specific composition can reduce the effect of radioactive rays on the threshold voltage while preventing electrons from moving from the oxide semiconductor layer to the silicon nitride film. Regarding the structure of the switching thin film transistor, except that the electron blocking layer is made of silicon oxynitride instead of silicon oxide, reference is made to Figure 4 and Figure 5 The description provided is applicable.

[0097] Figure 12A and Figure 12B Changes in characteristics of an oxide semiconductor transistor including an electron-blocking layer made of a silicon oxynitride film having a first composition ratio are shown. The layered structure of the oxide semiconductor transistor is shown in Figure 4 and Figure 5 .

[0098] Figure 12A Measurement results of changes in IV characteristics before and after application of a gate voltage are shown. The horizontal axis represents the gate voltage, and the vertical axis represents the drain current. The solid line represents changes in the drain current caused by a first scan of the gate voltage, and the broken line represents changes in the drain current caused by a second scan of the gate voltage. As Figure 12A indicated, the second measurement results are significantly different from the first measurement results. Specifically, the gate voltage application causes a large threshold voltage shift. This shift is explained by the description on silicon nitride.

[0099] Figure 12B Measurement results of changes in IV characteristics before and after X-ray irradiation are shown. The horizontal axis represents the gate voltage, and the vertical axis represents the drain current. The solid line represents measurement results before the X-ray irradiation, and the broken line represents measurement results after the X-ray irradiation. From Figure 12B the measurement results in FIG. 6B, it is understood that there is no significant change in characteristics before and after the X-ray irradiation.

[0100] Next, changes in characteristics of an oxide semiconductor transistor including an electron-blocking layer made of a silicon oxynitride film having a second composition ratio different from the first composition ratio are shown. Figure 13A and Figure 13B Changes in characteristics of an oxide semiconductor transistor including a silicon oxynitride film having the second composition ratio are shown. The layered structure of the oxide semiconductor transistor is shown in Figure 4 and Figure 5 .

[0101] Figure 13A Measurement results of changes in IV characteristics before and after application of a gate voltage are shown. The horizontal axis represents the gate voltage, and the vertical axis represents the drain current. The solid line represents changes in the drain current caused by a first scan of the gate voltage, and the broken line represents changes in the drain current caused by a second scan of the gate voltage. As Figure 13A indicated, there is no significant change in characteristics between the first measurement results and the second measurement results.

[0102] Figure 13B Measurement results of changes in IV characteristics before and after X-ray irradiation are shown. The horizontal axis represents the gate voltage, and the vertical axis represents the drain current. The solid line represents measurement results before the X-ray irradiation, and the broken line represents measurement results after the X-ray irradiation. From Figure 13B From the measurement results in FIG, it can be understood that there is no significant change in the characteristics before and after X-ray irradiation.

[0103] The above description explains that the silicon oxynitride film having the second composition ratio effectively functions as an electron blocking layer and suppresses the threshold voltage drift of the oxide semiconductor transistor caused by X-ray irradiation.

[0104] Figure 14 Composition analysis results of an oxide semiconductor transistor including an electron blocking layer having a first composition ratio and an oxide semiconductor transistor including an electron blocking layer having a second composition ratio are provided. The measurements were performed by electron energy loss spectroscopy (EELS). Figure 14 A graph 71 of nitrogen concentration ratio, a graph 72 of oxygen concentration ratio, and a graph 73 of silicon concentration ratio are provided.

[0105] In each graph, the horizontal axis represents the distance from the top surface of the oxide semiconductor (IGZO) layer, and the vertical axis represents the concentration ratio (atomic ratio) of a specific element. In each graph, the solid line represents the measurement results of an oxide semiconductor transistor including a silicon oxynitride film having a first composition ratio, and the dotted line represents the measurement results of an oxide semiconductor transistor including a silicon oxynitride film having a second composition ratio.

[0106] like Figure 14 As shown, the thickness of the silicon oxynitride (SiOxNy) film is 3 nm. In the silicon oxynitride film having a first composition ratio, the nitrogen concentration ratio (29%) is higher than the silicon concentration ratio (28%). However, in the silicon oxynitride film having a second composition ratio, the nitrogen concentration ratio (24%) is lower than the silicon concentration ratio (28%). As described above, configuring an electron blocking layer with a silicon oxynitride film having a lower nitrogen atomic ratio than a silicon atomic ratio can effectively suppress characteristic changes of the oxide semiconductor transistor caused by gate voltage application and X-ray irradiation.

[0107] A silicon oxynitride film can be formed by annealing the silicon nitride film. For example, after depositing an oxide semiconductor layer (IGZO layer) on the silicon nitride film and patterning the IGZO layer, annealing is performed at 400°C for one hour. Annealing can be performed in air. Oxygen from the oxide semiconductor layer can be used to form a thin silicon oxynitride film within the silicon nitride film.

[0108] Third embodiment

[0109] Hereinafter, other configuration examples of oxide semiconductor transistors related to the embodiments of this specification are described.

[0110] Figure 15is a cross-sectional view schematically showing a structure example of a switching thin-film transistor in an X-ray sensor of one embodiment of the present specification. The switching thin-film transistor 520 includes a gate electrode 522, a gate insulating film 523 over the gate electrode 522, and an oxide semiconductor layer 524 over the gate insulating film 523. An interlayer insulating film 527 covers the entire switching thin-film transistor 520. Although the thin-film transistor 520 in this embodiment is of a channel-etch type, it can be of a channel-protect type so that a channel region of the oxide semiconductor layer 524 is protected by an insulating film.

[0111] The gate insulating film 523 has a four-layer structure of a lower silicon nitride (SiNx) film 531, a silicon oxide (SiOx) film 532 over the lower silicon nitride film 531, an upper silicon nitride film 533 over the silicon oxide film 532, and another silicon oxide film 534 over the upper silicon nitride film 533.

[0112] The silicon oxide film 534 is in direct contact with and has an interface with the upper silicon nitride film 533 and the oxide semiconductor layer 524. The silicon oxide film 534 has a thickness greater than or equal to 1 nm and less than or equal to 4 nm, and functions as an electron barrier layer that blocks flow of electrons from the oxide semiconductor layer 524 to the upper silicon nitride film 533. The silicon oxide film 534 can be thinner than any of the lower silicon nitride film 531, the silicon oxide film 532, and the upper silicon nitride film 533. The silicon oxide film 534 as the electron barrier layer can be replaced with the silicon oxynitride film in the first embodiment or the second embodiment. The description of the electron barrier layer in the first embodiment or the second embodiment is applicable to this configuration example.

[0113] The silicon oxide film 532 is in direct contact with and has an interface with the lower silicon nitride film 531 and the upper silicon nitride film 533. The lower silicon nitride film 531 has an interface with the gate electrode 522. The upper silicon nitride film 533 and the lower silicon nitride film 531 sandwich the silicon oxide film 532, which reduces the influence of the silicon oxide film 532 on the threshold voltage caused by X-ray irradiation.

[0114] The switching thin-film transistor 520 has a bottom gate structure; the gate electrode 522 is positioned below the oxide semiconductor layer 524. The switching thin-film transistor 520 further includes one of source / drain electrodes 525 and another of source / drain electrodes 526 over the gate insulating film 523. The source / drain electrodes 525 and 526 are connected to the oxide semiconductor layer 524, respectively. Reference is made to Figure 3 or Figure 4 The description given on the components is applicable to the gate electrode 522, the source / drain electrodes 525 and 526, and the interlayer insulating film 527.

[0115] Figure 16is a cross-sectional view showing a structure example of a switching thin-film transistor having a top-gate structure. The switching thin-film transistor 700 includes an oxide semiconductor layer 704, a gate insulating film 703 over the oxide semiconductor layer 704, and a gate electrode 702 over the gate insulating film 703. The gate insulating film 703 has a two-layer structure composed of a silicon oxide (SiOx) film 711 and a silicon nitride (SiNx) film 712 over the silicon oxide film 711.

[0116] The silicon nitride film 712 is in direct contact with and has an interface with the gate electrode 702 and the silicon oxide film 711. The silicon oxide film 711 is in direct contact with and has an interface with the oxide semiconductor layer 704 and the silicon nitride film 712.

[0117] The silicon oxide film 711 has a thickness greater than or equal to 1 nm and less than or equal to 4 nm and functions as an electron barrier layer that blocks flow of electrons from the oxide semiconductor layer 704 to the silicon nitride film 712. The silicon oxide film 711 is thinner than the silicon nitride film 712. The silicon oxide film 711 as the electron barrier layer can be replaced with the silicon oxynitride film in the first embodiment or the second embodiment. The description of the electron barrier layer in the first embodiment or the second embodiment is applicable to this configuration example.

[0118] The silicon nitride film 712 can be replaced with an insulating film having a three-layer structure composed of an upper silicon nitride film and a lower silicon nitride film and a silicon oxide film therebetween, as described with reference to Figure 15 The silicon oxide film 711 can be thinner than any of the layers constituting the three-layer insulating film.

[0119] The switching thin-film transistor 700 has a top-gate structure; the gate electrode 702 is positioned over the oxide semiconductor layer 704. The switching thin-film transistor 700 further includes one of source / drain electrodes 705 and the other of source / drain electrodes 706. Each of the source / drain electrodes 705 and 706 extends through an interlayer insulating film 707 and the gate insulating film 703 to be connected to the oxide semiconductor layer 704. The above description on the thin-film transistor having a bottom-gate structure is applicable to the gate insulating film 703, and the description provided with reference to Figure 3 or Figure 4 The description provided with reference to

[0120] Figure 17is a cross-sectional view illustrating a structure example of a switching thin film transistor having a dual-gate structure. The switching thin film transistor 720 includes a gate electrode 722, a gate insulating film 723 over the gate electrode 722, and an oxide semiconductor layer 724. The switching thin film transistor 720 further includes another gate insulating film 743 over the oxide semiconductor layer 724 and another gate electrode 762 over the gate insulating film 743.

[0121] The gate insulating film 723 has a two-layer structure of a silicon nitride film 731 and a silicon oxide film 732 over 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, and has interfaces with them. The silicon nitride film 731 is in direct contact with the gate electrode 722 and the silicon oxide film 732, and has interfaces with them.

[0122] The gate insulating film 743 has a two-layer structure of a silicon oxide film 751 and a silicon nitride film 752 over 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, and has interfaces with them. The silicon nitride film 752 is in direct contact with the silicon oxide film 751 and the gate electrode 762, and has interfaces with them.

[0123] The switching thin film transistor 720 has a dual-gate structure; the oxide semiconductor layer 724 is provided between a top gate electrode 762 and a bottom gate electrode 722. The switching thin film transistor 720 further includes one of source / drain electrodes 725 and another of source / drain electrodes 726. Each of the source / drain electrodes 725 and 726 extends through an interlayer insulating film 727 and the gate insulating film 743 to be connected to the oxide semiconductor layer 724.

[0124] Reference Figure 5 or Figure 15 The description given on the gate insulating film in the bottom-gate structure is applicable to the gate insulating film 723. Specifically, the silicon oxide film 732 has a thickness greater than or equal to 1 nm and less than or equal to 4 nm and functions as an electron barrier layer that blocks flow of electrons from the oxide semiconductor layer 724 to the silicon nitride film 731. The silicon oxide film 732 is thinner than the silicon nitride film 731. The silicon oxide film 732 as the electron barrier layer can be replaced with the silicon oxynitride film in the first embodiment or the second embodiment. The description on the electron barrier layer in the first embodiment or the second embodiment is applicable to this configuration example.

[0125] The silicon nitride film 731 can be replaced with an insulating film having a three-layer structure of an upper silicon nitride film and a lower silicon nitride film and a silicon oxide film therebetween, as described in Reference Figure 15 The silicon oxide film 732 can be thinner than any of the layers that constitute the three-layer insulating film.

[0126] Reference is made to Figure 16 The description provided regarding the gate insulating film in the top gate structure is applicable to the gate insulating film 743. Specifically, the silicon oxide film 751 has a thickness greater than or equal to 1 nm and less than or equal to 4 nm, and functions as an electron blocking layer that blocks the flow of electrons from the oxide semiconductor layer 724 to the silicon nitride film 752. The silicon oxide film 751 is thinner than the silicon nitride film 752. The silicon oxide film 751 as the electron blocking layer can be replaced with the silicon oxynitride film in the first embodiment or the second embodiment. The description regarding the electron blocking layer in the first embodiment or the second embodiment is applicable to this configuration example.

[0127] The silicon nitride film 752 can be replaced with an insulating film having a three-layer structure composed of an upper silicon nitride film and a lower silicon nitride film and a silicon oxide film therebetween, as described with reference to Figure 15 The silicon oxide film 751 can be thinner than any of the layers constituting the three-layer insulating film.

[0128] The gate insulating films 723 and 743 can have the same configuration or different configurations. Reference is made to Figure 3 or Figure 4 The description provided regarding the components is applicable to the gate electrodes 722 and 762, the source / drain electrodes 725 and 726, and the interlayer insulating film 727.

[0129] As described above, the embodiments of the present application have been described; however, the present application is not limited to the above-described embodiments. Each element in the above-described embodiments can be easily modified, added, or converted within the scope of the present application by those skilled in the art. A part of the configuration of one embodiment can be replaced with the configuration of another embodiment, or the configuration of one embodiment can be incorporated into the configuration of another embodiment.

Claims

1. A thin film transistor for a radiation sensor, the thin film transistor 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 and having interfaces with the silicon nitride layer and the oxide semiconductor layer, and wherein a thickness of the silicon oxide layer is greater than or equal to 1 nm and less than or equal to 4 nm.

2. A thin film transistor for a radiation sensor, the thin film transistor 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 and having interfaces with the silicon nitride layer and the oxide semiconductor layer, and wherein a thickness of the silicon oxynitride layer is greater than or equal to 1 nm and less than or equal to 3 nm.

3. A thin film transistor for a radiation sensor, the thin film transistor 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 and having interfaces with the silicon nitride layer and the oxide semiconductor layer, and wherein a proportion of nitrogen atoms in the silicon oxynitride layer is lower than a proportion of silicon atoms.

4. The thin film transistor according to claim 1, 2 or 3, wherein the silicon nitride layer is a first silicon nitride layer, wherein the gate insulating film further comprises: a second silicon nitride layer between the gate electrode and the first silicon nitride layer; and a second silicon oxide layer between the second silicon nitride layer and the first silicon nitride layer, wherein the second silicon oxide layer has an interface with the second silicon nitride layer and the first silicon nitride layer, and wherein the second silicon nitride layer has an interface with the gate electrode.

5. The thin film transistor according to claim 1, 2 or 3, wherein, the gate electrode is between a substrate and the oxide semiconductor layer.

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 between the substrate and the photoelectric conversion element; and a signal line, wherein the thin film transistor is configured to switch between connection and disconnection of the signal line and the photoelectric conversion element.

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