Optical sensor

Abstract

To provide an optical sensor with higher sensitivity.SOLUTION: An image sensor 100 as one example of an optical sensor has such a structure that a plurality of avalanche-type photodiodes 18A, 18B, 18C, 18D, ... in each of which a P-type semiconductor layer 13A, 13B, 13C, 13D, ... is a thin film of crystallized selenium are arranged in an array.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to optical sensors. [Background technology]

[0002] As an example of a conventional image sensor, "Development of an image sensor with a heterojunction crystalline selenium photoelectric conversion film stacked" (hereinafter referred to as Non-Patent Document 1) by Tamemura Shigeyuki et al. discloses an image sensor in which a photoelectric conversion film capable of avalanche multiplication of signal charges is stacked on a substrate on which a CMOS (Complementary Metal Oxide Semiconductor) circuit is provided.

[0003] In such a conventional optical sensor equipped with an avalanche-type photodiode, the photodiode has a structure formed by laminating thin films, and therefore one optical sensor is composed of one photodiode. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Shigeyuki Tamemura and four others, "Development of an Image Sensor Stacked with Heterojunction Crystalline Selenium Photoelectric Conversion Films," NHK Science & Technology Research Institute R&D, NHK Packaging Technology Research Institute, September 2015, No. 153, Report 04, pp. 42-48 (https: / / www.nhk.or.jp / strl / publica / rd / 153 / 6.html) Summary of the Invention

[0005] It is desirable to improve the sensitivity of the image sensor disclosed in Non-Patent Document 1. Therefore, an optical sensor with higher sensitivity is provided.

[0006] According to one embodiment, the optical sensor has a structure in which a plurality of avalanche-type photodiodes are arranged in an array, and the P-type semiconductor layer is a thin film of crystallized selenium.

[0007] Further details will be described in the following embodiments. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic plan view of an image sensor according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view taken along line AA in FIG. [Figure 3] FIG. 3 is a schematic diagram showing another example of the relationship between the first electrode and the module. [Figure 4] FIG. 4 is a diagram schematically illustrating another example of a method for connecting modules. [Figure 5] FIG. 5 shows the results of measurements made by the inventors on the current density of modules of different sizes, which are accompanied by voltage changes from no-bias voltage to near the open-circuit voltage when irradiated with light. [Figure 6] FIG. 6 shows the results of fitting the measurement results of FIG. 5 to the theoretical equation for the diode using the least squares method, and also shows the measurement results of the S / N ratio when a bias voltage (−3 V) is applied. [Figure 7] FIG. 7 is a graph showing the IV characteristics, which are the measurement results of the change in current density with respect to the voltage applied to a micro-sized photodiode (35 μm square). [Figure 8] FIG. 8 is a schematic diagram of an image sensor according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0009] <1. Optical sensor overview>

[0010] (1) The photosensor according to the embodiment has a structure in which a plurality of avalanche-type photodiodes, each having a P-type semiconductor layer made of a thin film of crystallized selenium, are arranged in an array.

[0011] By configuring the optical sensor with multiple photodiodes arranged in an array, the size of each photodiode used in the optical sensor can be reduced compared to a structure consisting of a single photodiode. Experiments by the inventors have confirmed that miniaturizing the photodiode reduces the series resistance, thereby increasing the light detection sensitivity. Therefore, by configuring the optical sensor with this structure, it is possible to achieve higher sensitivity than a structure consisting of a single photodiode.

[0012] (2) Preferably, some of the photodiodes among the plurality of photodiodes are connected to apply a reverse bias to the other photodiodes. This allows some of the photodiodes to function as photovoltaic cells (self-biased photovoltaic cells). As a result, connection to an external power source can be eliminated or reduced. This allows for easier wiring and shorter wiring. As a result, resistance loss can be reduced.

[0013] (3) Preferably, the part of the photodiodes connected to apply a reverse bias includes a plurality of photodiodes connected in series, which makes it easier to ensure a voltage required for the reverse bias voltage.

[0014] (4) Preferably, some photodiodes connected to apply a reverse bias are provided adjacent to other photodiodes. Therefore, the photodiode functioning as a photocell and the other photodiodes form a single light-receiving surface. This prevents interference between the photodiode and the photocell and enhances the signal-to-noise ratio of the image sensor in both dark and bright conditions.

[0015] (5) Preferably, the photodiode is provided between the pixel electrode and the counter electrode corresponding to the pixel electrode, and the pixel electrode is connected to one photodiode at a time. This allows the size of the photodiode to be reduced to approximately the size of the pixel electrode, resulting in a high sensitivity of the photosensor.

[0016] (6) Preferably, the photodiode is provided between the pixel electrode and the counter electrode corresponding to the pixel electrode, and a plurality of pixel electrodes are connected to one photodiode. Even in this case, the size of the photodiode can be reduced compared to a structure in which the optical sensor is composed of a single photodiode. As a result, the optical sensor has high sensitivity.

[0017] (7) Preferably, the equivalent diameter of the photodiode is 100 μm or less. Experiments by the inventors have confirmed that the smaller the photodiode, the more effective the increase in photocurrent due to the combination of the scale effect and the avalanche effect. Therefore, by providing a micro-sized photodiode, the sensitivity of the optical sensor can be increased.

[0018] <2. Optical sensor example>

[0019] Fig. 1 is a schematic plan view of an image sensor 100 according to the present embodiment, which serves as an example of an optical sensor. The image sensor 100 has a structure in which a plurality of avalanche-type photodiodes, each having a P-type semiconductor layer made of a thin film of crystalline selenium, are arranged in an array. Fig. 1 shows a schematic diagram of the array arrangement of the photodiodes.

[0020] 2 is a schematic diagram showing the AA cross section of FIG. 1, in which image sensor 100 is cut along center line AA in the row direction of modules 10A, 10B, 10C, and 10D arranged in the row direction, and the cross section is viewed from the front. FIG. 2 shows the cross sections of modules 10A, 10B, 10C, and 10D and substrate 19.

[0021] 1 and 2, the image sensor 100 has a plurality of extremely small modules 10A, 10B, 10C, ... arranged in an array on the upper surface of a substrate 19. The plurality of modules 10A, 10B, 10C, ... are arranged in an array on the upper surface of the substrate 19. The arrayed state refers to a state in which a plurality of modules are arranged with gaps in at least one direction, and as an example, refers to a state in which a plurality of modules are arranged with gaps in one direction and a plurality of modules are arranged with gaps in the other direction.

[0022] In the example of FIG. 1, for ease of explanation, 16 modules 10A, 10B, 10C, ... 10P are arranged on the upper surface of a substrate 19, with four modules in the row direction and four modules in the column direction. The multiple modules 10A, 10B, 10C, ... are collectively referred to as module 10. An optical sensor can detect color by having at least three modules 10. An image sensor, which is an example of an optical sensor, may have approximately ten modules 10 arranged vertically and ten modules 10 arranged horizontally.

[0023] 2, the modules 10A, 10B, 10C, and 10D each include first electrodes 11A, 11B, 11C, and 11D that function as pixel electrodes and are disposed on a substrate 19, and arrayed photodiodes 18A, 18B, 18C, and 18D that are provided between second electrodes 15A, 15B, 15C, and 15D that are opposite electrodes to the first electrodes 11A, 11B, 11C, and 11D. The photodiodes 18A, 18B, 18C, and 18D each include P-type semiconductors 13A, 13B, 13C, and 13D, and N-type semiconductors 14A, 14B, 14C, and 14D.

[0024] The first electrodes 11A, 11B, 11C, 11D... will also be referred to as first electrode 11, the second electrodes 15A, 15B, 15C, 15D... will also be referred to as second electrode 15, and the photodiodes 18A, 18B, 18C, 18D... will also be referred to as photodiode 18. The P-type semiconductors 13A, 13B, 13C, 13D... will also be referred to as P-type semiconductor 13, and the N-type semiconductors 14A, 14B, 14C, 14D... will also be referred to as N-type semiconductor 14.

[0025] The module 10 has a multilayer structure including, in order from closest to the substrate 19, a first electrode 11, a P-type semiconductor 13, an N-type semiconductor 14, and a second electrode 15. When a reverse bias is applied to the first electrode 11 and the second electrode, a current generated by irradiating the photodiode 18 with light is amplified by the avalanche effect. The current is extracted as a signal current from a power line connected to the first electrode 11 and the second electrode.

[0026] The first electrode 11 is formed as a thin film, and is, for example, a gold (Au) electrode. The thickness of the first electrode 11 is, for example, about 150 nm. The gold electrode does not transmit light but reflects it. Therefore, the module 10 does not detect light on the first electrode 11 side. In other words, the image sensor 100 does not detect light on the top side.

[0027] The second electrode 15 is formed as a thin film, and is, for example, an ITO (Indium-Tin Oxide) transparent electrode. The thickness of the second electrode 15 is, for example, 200 nm or less. Because the second electrode 15 is transparent, light that has passed through the second electrode 15 can be transmitted to the N-type semiconductor 14 side. Therefore, the module 10 can detect light on the second electrode 15 side. In other words, the image sensor 100 can detect light on the bottom side.

[0028] The P-type semiconductor 13 is formed as a thin film, such as a crystalline selenium (Se) thin film. Selenium has a high optical absorption coefficient for visible light. When a crystalline selenium thin film photodiode is irradiated with light while a negative external bias voltage is applied, the photo-generated current is amplified by the avalanche effect. This is advantageous for increasing the sensitivity of the image sensor 100.

[0029] The N-type semiconductor 14 is formed as a thin film and is made of, for example, titanium oxide (TiO2). Alternatively, the N-type semiconductor 14 may be made of magnesium zinc oxide (ZnMgO) or gallium oxide (Ga2O3). The thickness of the N-type semiconductor 14 is, for example, 40 nm or less.

[0030] 2, the first electrodes 11 are provided in a one-to-one correspondence with the modules 10. The first electrodes 11 are pixel electrodes, so in other words, the photodiodes 18 are as minute in size as the pixel electrodes.

[0031] Note that, as shown in Fig. 3, a plurality of first electrodes 11 may be connected to the module 10. Fig. 3 is a schematic diagram showing another example of the relationship between the first electrodes and the module 10. In the example of Fig. 3, two first electrodes 11Q1 and 11Q2 are connected to the module 10Q, and two first electrodes 11R1 and 11R2 are connected to the module 10R. Even in this case, the image sensor 100 has a plurality of photodiodes 18, and therefore can be made smaller than when the image sensor 100 is configured with a single photodiode 18, as will be described later.

[0032] 1, module 10 is manufactured by laminating, in that order, an ITO thin film 31, a TiO thin film 32, and a gold (Au)-deposited thin film 33 on a surface of a light-transmitting substrate 30 such as glass. The enlarged view of portion B in FIG. 1 shows module 10 as viewed from the back side of substrate 30.

[0033] In the image sensor 100, multiple photodiodes 18 are disposed on the substrate 19. Therefore, the photodiodes 18 are smaller than when a single photodiode 18 is disposed on the substrate 19. Preferably, the photodiodes 18 are minute in size, with an equivalent diameter of 100 μm or less. The equivalent diameter is obtained by dividing four times the cross-sectional area by the edge length (perimeter of the cross section). When the cross section of the photodiode 18 is square, each side is preferably 100 μm or less. More preferably, the photodiode 18 is minute in size, with an equivalent diameter of 35 μm or less, for example, with each side being 35 μm.

[0034] 1, the manufacturing method of module 10 includes forming a thin film 31 of ITO on the entire surface of substrate 30 by sputtering, resistance heating evaporation, or the like. Substrate 30 is, for example, 10 mm x 10 mm in size.

[0035] The manufacturing method of the module 10 includes a first etching process for patterning the thin film 31 into a first shape. The first etching process enables micro-sized patterning. The first shape is a bar shape in a first direction with a predetermined spacing on the substrate 30. The first etching process is, for example, wet etching using a 36% hydrochloric acid solution.

[0036] The method for manufacturing module 10 then includes forming a titanium oxide (TiO2) film by sputtering on thin film 31 patterned into the first shape to form TiO2 thin film 32. The thickness of thin film 32 is, for example, about 50 nm.

[0037] The method for manufacturing module 10 includes a second etching process for patterning thin film 32 into a second shape. The second etching process enables micropatterning. The second shape may be any shape that can cover intersections 35, which will be described later, such as a square with one side longer than the intersection.

[0038] The method for manufacturing the module 10 then includes forming a selenium (Se) thin film 33 on the second-shape patterned thin film 32, followed by crystallization. Crystallization of the selenium can be achieved, for example, by depositing tellurium (Te) and selenium (Se) by, for example, resistive evaporation, followed by high-temperature annealing.

[0039] The manufacturing method of the module 10 then includes evaporating gold (Au) over the entire crystallized selenium thin film 33 to form a gold thin film 34. The thickness of the evaporated gold is, for example, 120 nm. The evaporation may be, for example, by resistive heating.

[0040] The manufacturing method for module 10 includes a third etching process for patterning thin film 34 into a third shape. The third etching process enables micropatterning. The third shape is a rod shape similar to the first shape, but is a shape in which multiple rods are arranged at intervals in a direction perpendicular to the first shape. This results in an intersection 35 between the first shape and the third shape. The third etching process is, for example, wet etching using an etching solution made of a mixture of 1 g of iodine, 2 g of potassium iodide, and 40 mL of pure water.

[0041] The method for manufacturing the module 10 then includes a fourth etching process in which the crystallized selenium thin film 33 formed below the gold thin film 34 is patterned into the same shape as the thin film 34 using the gold thin film 34 patterned into the third shape as an etching mask. The fourth etching process enables micro-sized patterning. The fourth etching process is, for example, a process in which the crystallized selenium is etched at 400 W in a CF4 gas atmosphere with a flow rate of 30 sccm.

[0042] The intersection 35 between the first-shape thin film 31 and the second-shape thin films 33 and 34 obtained by the manufacturing method of the module 10 including the above steps functions as the micro-sized module 10. At the intersection 35, the ITO thin film 31, the TiO thin film 32, the crystallized selenium thin film 33, and the gold (Au) thin film 34 are stacked in this order on the substrate 30. As shown in FIG. 1 , by etching the first and second shapes on the substrate 19 so that the intersections 35 are arranged in an array, an image sensor 100 is obtained in which the micro-sized modules 10 are arranged in an array. That is, the ITO thin film 31 becomes the second electrode 15, the TiO thin film 32 becomes the N-type semiconductor 14, the crystallized selenium thin film 33 becomes the P-type semiconductor 13, and the gold (Au) thin film 34 becomes the first electrode 11.

[0043] In the manufacturing method of module 10 described above, the thin film 31 is formed on the substrate 30 in that order. In module 10, ITO is used as the second electrode 15, and titanium oxide (TiO2) is used as the N-type semiconductor 14, and these are etched by a first etching process and a second etching process, respectively. In this way, by first etching the ITO, which is less susceptible to corrosion, before the TiO2, which is more susceptible to corrosion, by the first etching process, the TiO2 is more easily etched.

[0044] The photodiodes 18 included in each of the multiple modules 10 arranged on the substrate 19 are avalanche photodiodes. Some of the multiple modules 10 are connected so that the photodiodes 18 included therein provide a reverse bias to the photodiodes 18 of the other modules 10. This allows some of the modules 10 to function as photovoltaic cells (self-biased photovoltaic cells).

[0045] Preferably, the module 10 functioning as a photovoltaic cell is the module adjacent to the module 10 that applies the reverse bias, as shown in Fig. 2. This makes wiring easier and shortens the wiring, thereby reducing resistance loss.

[0046] In the example of Fig. 2, modules 10A and 10C are connected to apply a reverse bias to photodiodes 18B and 18D provided in modules 10B and 10D, respectively. Specifically, referring to Fig. 2, first electrode 11B of module 10B is connected to second electrode 15A of module 10A via conductive line 16A, and second electrode 15B of module 10B is connected to first electrode 12A of module 10A via conductive line 16B. Furthermore, first electrode 11D of module 10D is connected to second electrode 15C of module 10C via conductive line 17A, and second electrode 15D of module 10D is connected to first electrode 12C of module 10C via conductive line 17B.

[0047] As a result, the modules 10A and 10C are connected to apply a reverse bias to the photodiodes 18B and 18D provided in the modules 10B and 10D, respectively, and the modules 10A and 10C therefore function as photovoltaic cells (self-biased photovoltaic cells).

[0048] The module 10 functioning as a photovoltaic cell may be a plurality of modules adjacent to the module 10 that applies the reverse bias, including one module adjacent to the module 10 that applies the reverse bias, as shown in FIG. 4. FIG. 4 is a diagram schematically illustrating another example of a method for connecting modules 10. In FIG. 4, modules 10A, 10B, and 10C are connected in series by conductive wires 16C, 16D, and 16E, respectively, and the plurality of modules 10 are connected in series to module 10D that applies the reverse bias by conductive wires 16E and 16F. This makes it easier to ensure the voltage required for the reverse bias voltage.

[0049] By configuring some modules to function as photovoltaic cells and provide the necessary reverse bias to other photodiodes, the image sensor 100 can eliminate or reduce the need for connections to an external power source to provide the necessary reverse bias to the photodiodes. This allows for easier wiring and shorter wiring. As a result, resistance loss can be reduced. Installation is also easier.

[0050] Modules 10A and 10C, which apply a reverse bias to photodiodes 18B and 18D included in modules 10B and 10D that apply the reverse bias, are adjacent to modules 10B and 10D, respectively, so that photocells are disposed in contact with photodiodes 18B and 18D. Therefore, photodiodes 18B and 18D and the photocells form a single light-receiving surface. This prevents interference between photodiodes 18B and 18D and the photocells, and enhances the signal-to-noise ratio (SN ratio) of image sensor 100 in both dark and bright conditions.

[0051] The inventors conducted an experiment using modules 10 of different sizes to confirm the effect of miniaturizing the module 10. In the experiment, as comparative examples, an image sensor 200 in which one 3 mm x 3 mm square module (hereinafter referred to as 3 mm square PD) 210 is arranged, as shown in Fig. 8, and an image sensor 100 according to the embodiment in which a plurality of 35 μm x 35 μm square modules (hereinafter referred to as 35 μm square PD) 10 are arranged in an array were used.

[0052] 8 has a multilayer structure including, in order from closest to substrate 219, first electrodes 211A, 211B, 211C, and 211D which are pixel electrodes, a P-type semiconductor 213, an N-type semiconductor 214, and a second electrode 215. Module 210 is manufactured by a normal manufacturing method that does not include the first to fourth etching processes.

[0053] The inventors measured the current density of modules 10 and 210 as the voltage changed from the no-bias voltage to near the open-circuit voltage under light irradiation, and obtained the results shown in Figure 5. The inventors then measured the DC resistance by fitting the measurement results shown in Figure 5 to a theoretical equation for diodes using the least squares method. The inventors also measured the S / N ratio of modules 10 and 210 when a bias voltage (-3 V) was applied.

[0054] In the experiment, the irradiance (radiant intensity) of the light irradiated on each module was 1000 W / m 2 The artificial sunlight with an air mass (AM) of 1.5 was used. The following equation (1) was used for fitting. J=J0(EXP((q·VJ·R s ) / nkT)-1)+(V+J·R s ) / R sh -J L ) ...Formula (1) (where J: output current density, J0: reverse saturation current density, J L: photocurrent density, q: elementary charge, V: applied voltage, n: diode coefficient, k: Boltzmann coefficient, T: temperature, R s : Series resistance, R sh :parallel resistance)

[0055] 6 shows the measurement results of DC resistance by fitting for module 210 (3 mm square PD) and module 10 (35 μm square PD), and the measurement results of S / N ratio when a bias voltage (−3 V) is applied. From the measurement results of DC resistance in FIG. 6, the series resistance R of module 210 and module 10 can be calculated by fitting using equation (1). s 10 Ω cm and 0.01 Ω cm, respectively. 2 The DC resistance shows the slope at the timing when the generated current in Figure 5 is large, and the smaller the DC resistance, the smaller the resistance loss is. From the results in Figure 6, the series resistance R s is the series resistance R of module 210 (3mm square PD) s It was confirmed that the resistance loss was significantly smaller than that of the series resistance R due to the scale effect. s Therefore, in the image sensor 100 using the module 10, the series resistance R s The influence of

[0056] From the measurement results of the S / N ratio when a bias voltage (-3V) is applied in Figure 6, the S / N ratio when a bias voltage is applied is 4.73 for module 210 (3mm square PD), while the S / N ratio for module 10 (35μm square PD) is 202. The S / N ratio is the ratio of the current generated when light is irradiated to the current generated when no light is irradiated, so the higher the S / N ratio, the higher the sensitivity of the image sensor. The results in Figure 6 confirm that the S / N ratio of module 10 (35μm square PD) is extremely high compared to the S / N ratio of module 210 (3mm square PD). This indicates that the series resistance R sIt is believed that the S / N ratio was improved due to the reduction in the signal strength. Therefore, it was confirmed that the image sensor 100 using the module 10 has significantly higher sensitivity than the image sensor 200 according to the comparative example.

[0057] Figure 7 shows the IV characteristics, which are the measurement results of the change in current density versus applied voltage to the micro-sized module 10 (35 μm square PD). Curve L1 shows the characteristics when no light is irradiated, and curve L2 shows the characteristics when light is irradiated. Curve L2 in Figure 7 shows that the current density drops from around 0.7 V to -3 V when light is irradiated, which should be constant in the reverse bias region. This is thought to be due to a combination of the avalanche effect and photoconductive effect of crystalline selenium.

[0058] The above experiments confirmed that the smaller the photodiode 18, the more effective the increase in photocurrent due to the combination of the scale effect and the avalanche effect. Therefore, the image sensor 100, in which multiple modules 10 each having a micro-sized photodiode 18 are arranged, can detect light with higher sensitivity than the image sensor 200 according to the comparative example.

[0059] <3. Notes> The present invention is not limited to the above-described embodiment, and various modifications are possible. [Explanation of symbols]

[0060] 10: Module 10A: Module 10B: Module 10C: Module 10D: Module 10Q: Module 10R: Module 11: 1st electrode 11A: 1st electrode 11B: 1st electrode 11C: 1st electrode 11D: 1st electrode 11Q1: 1st electrode 11Q2: 1st electrode 11R1: 1st electrode 11R2: 1st electrode 12A: 1st electrode 12C: 1st electrode 13: P-type semiconductor 13A: Type semiconductor 13B: type semiconductor 13C: type semiconductor 13D: type semiconductor 14: N-type semiconductor 14A: Type semiconductor 14B: type semiconductor 14C: type semiconductor 14D: type semiconductor 15: 2nd electrode 15A: 2nd electrode 15B: 2nd electrode 15C: 2nd electrode 15D: 2nd electrode 16A: Conductive wire 16B: Conductive wire 16C: Conductive wire 16D: Conductive wire 16E: Conductive wire 16F: Conductive wire 17A: Conductive wire 17B: Conductive wire 18: Photodiode 18A: Photodiode 18B: Photodiode 18C: Photodiode 18D: Photodiode 19: Substrate 30: Substrate 31: Thin film 32: Thin film 33: Thin film 34: Thin film 35: Intersection 100: Image sensor 200: Image sensor 210: Module 211A: 1st electrode 211B: 1st electrode 211C: 1st electrode 211D: 1st electrode 213: P-type semiconductor 214: N-type semiconductor 215: 2nd electrode 219: Substrate AA: Center line

Claims

1. a P-type semiconductor layer is a thin film of crystalline selenium, and a plurality of avalanche-type photodiodes are arranged in an array; Some of the photodiodes are connected to apply a reverse bias to the other photodiodes. Light sensor.

2. The part of photodiodes includes a plurality of the photodiodes connected in series. The optical sensor of claim 1 .

3. The part of the photodiodes is provided adjacent to the other photodiodes.

3. The optical sensor according to claim 1 or 2.

4. the photodiode is provided between a pixel electrode and a counter electrode corresponding to the pixel electrode, The pixel electrodes are connected to one of the photodiodes. The optical sensor according to any one of claims 1 to 3.

5. the photodiode is provided between a pixel electrode and a counter electrode corresponding to the pixel electrode, A plurality of the pixel electrodes are connected to one of the photodiodes. The optical sensor according to any one of claims 1 to 3.

6. The equivalent diameter of the photodiode is 100 μm or less The optical sensor according to any one of claims 1 to 5.

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