Photoelectric conversion element, imaging element, and imaging system

Abstract

The photoelectric conversion element of the present invention has a substrate and an optical element, and receives light from a light source that emits light and reflected light obtained by reflecting the light from an object. When the incident energy of the reflected light incident on the photoelectric conversion element is set as I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is set as α(λ), the incident energy of the reflected light in a predetermined region on the first surface is set as A1, the incident energy of the reflected light in the predetermined region on the first surface when the photoelectric conversion element does not have the optical element is set as A2, and the incident energy of the reflected light in a region that has been translated a prescribed distance z in the thickness direction of the substrate from the predetermined region is set as B(z), when A1≧A2 and the distance z0 = ln(2) / α(λ) is set, when z = z0, the relational expression 0.95*exp(‑α(λ)*z)≦B(z) / A1≦1.05*exp(‑α(λ)*z) holds.

Description

Technical Field

[0001] The present invention relates to a photoelectric conversion element, an imaging element provided with the photoelectric conversion element, and an imaging system including the imaging element.

[0002] This application is based on Japanese Patent Application No. 2019-157643 filed in Japan on August 30, 2019, and claims priority thereto, the content of which is incorporated herein by reference. Background Art

[0003] Conventionally, as a technique for measuring the distance to an object, there has been a technique of irradiating the object with a light pulse in the near-infrared region and measuring the time difference between the time when the light pulse is irradiated and the time when the reflected light of the irradiated light pulse reflected by the object is detected, that is, measuring the flight time of the light pulse. Such a technique for measuring the distance to an object based on the flight time of a light pulse is called Time of Flight (TOF). Then, a distance measuring sensor that uses a photoelectric conversion element to measure the distance to an object based on the flight time technique has also been put into practical use.

[0004] Furthermore, in recent years, a configuration that uses a photoelectric conversion element to measure the distance to an object based on the flight time technique has been developed, and a distance measuring sensor has been put into practical use, which can not only obtain the distance to an object but also obtain a two-dimensional image including the object, that is, can obtain three-dimensional information about the object. Such a distance measuring sensor is also called a distance image sensor. In the distance image sensor, a plurality of pixels are arranged in a two-dimensional matrix on a silicon substrate, and each pixel includes a light receiving portion, i.e., a photodiode, that receives the reflected light of the light pulse reflected by the object. Then, in the distance image sensor, by outputting the amount of a photoelectric conversion signal obtained based on the amount of light of the reflected light of the light pulse received by each of the plurality of pixels as an image, a two-dimensional image including the object and the distance information of each of the plurality of pixels constituting the image can be obtained. Thus, in the distance image sensor, three-dimensional information combining the two-dimensional image including the object and the distance information of each of the plurality of pixels can be obtained.

[0005] However, the accuracy of the distance that can be measured in the distance image sensor varies depending on the amount of light of the reflected light of the light pulse that each of the plurality of pixels can receive at the same time. That is, in the distance image sensor, if each of the plurality of pixels can receive more reflected light at the same time, the distance can be measured with high accuracy. Therefore, in the distance image sensor, it is desired to increase the amount of light of the reflected light of the light pulse that each of the plurality of pixels can receive, that is, to increase the sensitivity of each of the plurality of pixels to light in the near-infrared region.

[0006] In addition, in an image sensor that acquires an image, various techniques for improving sensitivity to light have been proposed, such as the technique disclosed in Patent Document 1. In the technique disclosed in Patent Document 1, a plurality of microlenses are formed on each of a plurality of grating pairs that constitute respective sensor units (pixels) formed on a semiconductor substrate of a sensor system. As a result, in a sensor system to which the technique disclosed in Patent Document 1 is applied, the light-receiving area of the array increases, and the sensitivity of the sensor system increases. Therefore, in a distance image sensor, in order to improve the sensitivity of each of a plurality of pixels to light in the near-infrared region, it is possible to consider applying the technique of forming microlenses as disclosed in Patent Document 1.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent No. 6001236 Gazette Summary of the Invention

[0010] Problems to be Solved by the Invention

[0011] However, in a distance image sensor, the sensitivity of each of a plurality of pixels is related to the structure of a light-receiving portion of light, that is, a photodiode, particularly the distance in the optical axis direction. That is, the sensitivity of each of a plurality of pixels in a distance image sensor is also related to the depth (thickness) of a diffusion layer when a photodiode is formed on a silicon substrate. The reason is that in a distance image sensor, as described above, a light pulse in the near-infrared region is received as reflected light reflected by an object, and since this reflected light is also a light pulse in the near-infrared region, more photoelectric conversion is performed at a deeper position in the silicon substrate.

[0012] However, the technique disclosed in Patent Document 1 is a technique for forming microlenses at the positions of respective pixels in an image sensor. Therefore, by applying the technique disclosed in Patent Document 1, microlenses corresponding to a plurality of pixels can also be formed in a distance image sensor, but the formed microlenses are not necessarily suitable for a distance image sensor. The reason is that the focal position of the microlenses formed in the technique disclosed in Patent Document 1 is near the light-receiving surface of the grating pair, that is, the surface on the incident side where light enters the semiconductor substrate on which the grating is formed. Then, as a technique related to a distance image sensor, a technique for defining the relationship between the position where the microlenses condense light, that is, the focal position of the microlenses, and the depth (thickness) of the diffusion layer when forming the light-receiving portions, that is, photodiodes, of respective pixels is not disclosed.

[0013] The present invention has been made based on the above problems, and an object thereof is to provide a photoelectric conversion element, an imaging element including the photoelectric conversion element, and an imaging system including the imaging element. The structure of the photoelectric conversion element is such that in a distance image sensor having microlenses respectively corresponding to a plurality of pixels, the sensitivity of each of the plurality of pixels to light can be improved.

[0014] Means for Solving the Problems

[0015] To solve the above problems, a photoelectric conversion element according to one aspect of the present invention is a photoelectric conversion element that receives reflected light obtained by reflecting light in a predetermined wavelength band emitted from a light source by an object. The photoelectric conversion element includes: a substrate having a first surface on which the reflected light is incident, a first semiconductor region formed of a first conductive type semiconductor, and a second semiconductor region formed of a second conductive type semiconductor having a conductive type different from that of the first conductive type semiconductor and extending from the first surface toward the inside in a direction perpendicular to the first surface; and an optical element disposed on the first surface side of the substrate that condenses the reflected light on the second semiconductor region. When the incident energy of the reflected light incident on the photoelectric conversion element is set to I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is set to α(λ), the incident energy of the reflected light in a predetermined region on the first surface is set to A1, the incident energy of the reflected light in the predetermined region on the first surface when the photoelectric conversion element does not have the optical element is set to A2, and the incident energy of the reflected light in a region parallelly shifted by a predetermined distance z in the thickness direction of the substrate from the predetermined region is set to B(z), when A1≧A2 and a distance z0 = ln(2) / α(λ) is set, when z = z0, a relational expression of 0.95*exp(-α(λ)*z)≦B(z) / A1≦1.05*exp(-α(λ)*z) holds.

[0016] In order to solve the above problems, a photoelectric conversion element according to one embodiment of the present invention is a photoelectric conversion element that receives light emitted from a light source that emits light in a specified wavelength band and reflected light obtained by reflecting the light from an object. The photoelectric conversion element includes: a substrate having a first surface on which the reflected light is incident, a first semiconductor region formed of a first-conductive-type semiconductor, and a second semiconductor region formed of a second-conductive-type semiconductor having a conductivity type different from that of the first-conductive-type semiconductor and extending from the first surface toward the inside in a direction perpendicular to the first surface; and an optical element disposed on the first surface side of the substrate that condenses the reflected light onto the second semiconductor region. When the incident energy of the reflected light incident on the photoelectric conversion element is set to I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is set to α(λ), the incident energy of the reflected light in a predetermined region on the first surface is set to A1, the incident energy of the reflected light in the predetermined region on the first surface when the photoelectric conversion element does not have the optical element is set to A2, and the incident energy of the reflected light in a region parallelly shifted by a specified distance z in the thickness direction of the substrate from the predetermined region is set to B(z), when A1≧A2 and a distance z0 = ln(2) / α(λ) is set, for all z satisfying 0≦z≦z0, a relational expression of 0.95*exp(-α(λ)*z)≦B(z) / A1≦1.05*exp(-α(λ)*z) holds.

[0017] In the photoelectric conversion element according to one embodiment of the present invention, the predetermined region may be a region obtained by vertically projecting the second semiconductor region onto the first surface.

[0018] In the photoelectric conversion element according to one embodiment of the present invention, the wavelength band may be a near-infrared wavelength band.

[0019] In the photoelectric conversion element according to one embodiment of the present invention, the near-infrared wavelength band may be a wavelength band of 850 nm to 940 nm.

[0020] An imaging element according to one embodiment of the present invention is an imaging element that receives light from a light source that emits light in a specified wavelength band and reflected light obtained by reflecting the light from an object. The imaging element includes a plurality of photoelectric conversion elements according to the above embodiment, and the photoelectric conversion element includes a light-receiving region in which a plurality of pixels are arranged in a two-dimensional matrix. In the light-receiving region, the plurality of pixels are arranged along a first direction and a second direction that are orthogonal to each other. When the optical element is cut along the first direction and the second direction, the height of the valley portions of two adjacent optical elements is set as a first height. When the optical element is cut along the diagonal direction of the pixel, the height of the valley portions of two adjacent optical elements is set as a second height. The first height and the second height are different from each other.

[0021] An imaging system according to one embodiment of the present invention includes: a light source unit that emits light in a specified wavelength band; the imaging element according to the above embodiment; and a light-receiving unit that receives the reflected light obtained by reflecting the light from an object.

[0022] Effect of the Invention

[0023] According to the imaging system according to one embodiment of the present invention, the following effect can be obtained: It is possible to provide a photoelectric conversion element, an imaging element in which the photoelectric conversion element is arranged, and an imaging system including the imaging element, and the structure of the photoelectric conversion element can improve the sensitivity of each of the plurality of pixels to light in a distance image sensor in which microlenses corresponding to the plurality of pixels are formed. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a block diagram schematically showing the overall schematic configuration of the imaging element according to the embodiment of the present invention.

[0025] Figure 2 It is a cross-sectional view schematically showing the schematic configuration of a cross-section of an imaging element in which a photoelectric conversion element according to the embodiment of the present invention is formed.

[0026] Figure 3 It is a cross-sectional view of a pixel showing a state in which electrons corresponding to light incident on an imaging element in which a photoelectric conversion element according to the embodiment of the present invention is formed are generated.

[0027] Figure 4 It is a cross-sectional view of a pixel schematically showing a concept when forming a photoelectric conversion element according to the embodiment of the present invention.

[0028] Figure 5 It is a graph showing the attenuation of the incident energy of light incident on an imaging element in which a photoelectric conversion element according to the embodiment of the present invention is formed.

[0029] Figure 6It is a diagram showing parameters of simulation performed when forming a microlens in an imaging element having a photoelectric conversion element according to an embodiment of the present invention.

[0030] Figure 7A It is a diagram showing parameters of simulation performed when forming a microlens in an imaging element having a photoelectric conversion element according to an embodiment of the present invention.

[0031] Figure 7B It is a diagram showing parameters of simulation performed when forming a microlens in an imaging element having a photoelectric conversion element according to an embodiment of the present invention.

[0032] Figure 8 It is a graph showing an example of a result of simulating attenuation of incident energy of light incident on an imaging element having a photoelectric conversion element according to an embodiment of the present invention.

[0033] Figure 9A It is a graph showing an example of a result of simulation comparing attenuation of incident energy of light incident on an imaging element.

[0034] Figure 9B It is a graph showing an example of a result of simulation comparing attenuation of incident energy of light incident on an imaging element.

[0035] Figure 10 It is a block diagram showing a schematic configuration of an imaging system according to an embodiment of the present invention in which an imaging element according to an embodiment of the present invention is mounted. Detailed Embodiment

[0036] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0037] A photoelectric conversion element according to an embodiment of the present invention is a pixel including a silicon substrate (substrate), a wiring layer W, and a microlens (optical element). A photodiode that functions as a photoelectric conversion unit is provided inside the silicon substrate.

[0038] The photoelectric conversion element (pixel) is mounted on an imaging system according to an embodiment of the present invention that measures the distance to an object by time-of-flight (TOF) technology. Further, the photoelectric conversion element (pixel) is formed in an imaging element according to an embodiment of the present invention, that is, a distance image sensor. That is, in the following description, the photoelectric conversion element according to an embodiment of the present invention is formed as an imaging element (distance image sensor) according to an embodiment of the present invention that receives reflected light obtained by reflecting light in a near-infrared band having a relatively long wavelength (for example, light in a band of 850 nm to 940 nm) emitted from the imaging system according to an embodiment of the present invention and outputs a signal for measuring the distance to the object.

[0039] Figure 1 is a block diagram showing a schematic configuration of an entire imaging element according to an embodiment of the present invention. In Figure 1 , the distance image sensor 10 includes a light-receiving area 100 in which a plurality of pixels 101 (pixel array) are arranged, a control circuit 200, a vertical drive circuit 300, a horizontal drive circuit 400, an AD conversion circuit 500, and an output circuit 600. Further, in Figure 1 the distance image sensor 10 shown, an example of the light-receiving area 100 is shown in which a plurality of pixels 101 are arranged in a two-dimensional matrix of 6 rows and 8 columns to form a plurality of pixel columns. In other words, the plurality of pixels 101 constituting the pixel array for constituting the distance image sensor 10 are arranged along a first direction and a second direction orthogonal to each other.

[0040] The control circuit 200 controls components included in the distance image sensor 10, such as the vertical drive circuit 300, the horizontal drive circuit 400, and the AD conversion circuit 500. For example, the control circuit 200 controls the operations of the components included in the distance image sensor 10 according to control from a control device (not shown) included in the imaging system.

[0041] The vertical drive circuit 300 is a drive circuit that controls each of the plurality of pixels 101 arranged in the light-receiving area 100 according to control from the control circuit 200. By driving of the vertical drive circuit 300, each of the plurality of pixels 101 photoelectrically converts light (incident light) incident on the pixel 101 to generate signal charges. The vertical drive circuit 300 outputs (reads out) pixel signals corresponding to the signal charges of the respective plurality of pixels 101 to corresponding vertical signal lines. The vertical drive circuit 300 outputs drive signals for driving (controlling) the pixels 101 for each row of the pixels 101 arranged in the light-receiving area 100. Thereby, the pixel signals output from the pixels 101 are read out to the vertical signal lines for each row and output to the AD conversion circuit 500.

[0042] Each pixel 101 arranged in the light-receiving area 100 outputs a pixel signal obtained by converting incident light into an electric signal. The pixel 101 is configured to include components such as a photodiode (photoelectric conversion unit) that generates and accumulates signal charges corresponding to the amount of incident light (light amount) to convert the incident light into an electric signal. Each of the plurality of pixels 101 outputs a pixel signal corresponding to the amount of incident light (light amount) to a corresponding vertical signal line according to a drive signal input from the vertical drive circuit 300. Further, a detailed description of the structure of the pixel 101 will be given later.

[0043] The AD conversion circuit 500 is an analog / digital conversion circuit that converts the analog pixel signal output from the pixels 101 in the corresponding column to the vertical signal line into a digital value representing the magnitude of the pixel signal according to the control from the control circuit 200. In addition, the AD conversion circuit 500 may also be an AD conversion circuit group composed of a plurality of AD conversion circuits corresponding to the plurality of columns in which the pixels 101 are arranged in the light-receiving region 100. The AD conversion circuit 500 outputs the pixel signal after analog / digital conversion according to the control from the horizontal drive circuit 400 as an output signal to the horizontal signal line.

[0044] The horizontal drive circuit 400 is a drive circuit that sequentially outputs (reads out) the pixel signals (output signals) after analog / digital conversion to the horizontal signal line. That is, through the drive of the horizontal drive circuit 400, according to the control from the control circuit 200, the pixel signals after analog / digital conversion corresponding to the respective columns (pixel columns) of the pixels 101 arranged in the light-receiving region 100 output from the AD conversion circuit 500 are sequentially output to the horizontal signal line. The horizontal drive circuit 400 sequentially outputs control signals for outputting output signals corresponding to the pixels 101 in each of the plurality of columns to the AD conversion circuit 500. Thus, the output signals output from the AD conversion circuit 500 are sequentially output to the output circuit 600 via the horizontal signal line.

[0045] The output circuit 600 is a circuit that outputs the output signal from the AD conversion circuit 500 output to the horizontal signal line by the horizontal drive circuit 400 to the outside of the distance image sensor 10. The output circuit 600 is, for example, an output amplifier or the like.

[0046] Next, the structure of the semiconductor substrate constituting the pixels 101 arranged in the light-receiving region 100 provided in the distance image sensor 10 will be described. Figure 2 FIG. is a cross-sectional view schematically showing a schematic configuration of a cross-section of an imaging element (distance image sensor 10) in which a photoelectric conversion element according to an embodiment of the present invention is formed. In addition, in the distance image sensor 10, a plurality of pixels 101 including the photoelectric conversion element of the present invention are arranged in a two-dimensional matrix in the light-receiving region 100 formed on the semiconductor substrate of the distance image sensor 10. Figure 2 FIG. schematically shows an example of a cross-section of the semiconductor substrate in a region of two adjacent pixel amounts in the horizontal width direction (horizontal direction H) when observing the light-receiving region 100 of the distance image sensor 10 from the incident direction of light. The pixel 101 includes at least a photodiode PD, a floating diffusion region FD, and a gate electrode G.

[0047] The photodiode PD is an embedded photodiode that generates and accumulates signal charges corresponding to the amount of incident light (light quantity). The gate electrode G is an electrode for applying, from the outside of the pixel 101, the potential required to transfer the signal charges generated and accumulated by the photodiode PD to the floating diffusion region FD. The gate electrode G functions as a shutter with respect to the photodiode PD that receives the incident light. The floating diffusion region FD is a charge storage capacitor that stores the signal charges transferred by the gate electrode G. In the distance image sensor 10, the signal charges stored in the floating diffusion regions FD respectively provided in the plurality of pixels 101 are read out as pixel signals to the AD conversion circuit 500.

[0048] In addition, when compared to a general image sensor, Figure 2 the configuration of the shown distance image sensor 10 becomes equivalent to the structure of a Front Side Illumination (FSI) type image sensor. Therefore, Figure 2 the shown distance image sensor 10 includes a semiconductor substrate, i.e., a silicon substrate Si (substrate), made of a first conductive type semiconductor (P-type semiconductor) that forms the distance image sensor 10. On the incident side where light enters the silicon substrate Si, i.e., the surface side of the silicon substrate Si, a plurality of components including the photodiode PD that forms the pixel 101 are formed.

[0049] More specifically, as Figure 2 shown, the photodiode PD (photoelectric conversion section) that forms the pixel 101 is formed starting from the surface of the silicon substrate Si (hereinafter referred to as the "first surface"). As Figure 2 shown, the silicon substrate Si has a first surface that is the surface where the reflected light enters, a first semiconductor region made of a first conductive type semiconductor (P+-type semiconductor), and a second semiconductor region made of a second conductive type semiconductor (N-type semiconductor) whose conductivity type is different from that of the first conductive type semiconductor. Thereby, a photodiode PD having a configuration that generates and accumulates electrons corresponding to the amount of incident light (light quantity) as signal charges is formed. The photodiode PD having the Figure 2 shown configuration can be realized by doping an impurity that becomes a second conductive type semiconductor (N-type semiconductor) into the silicon substrate Si to form an N-type semiconductor region (second semiconductor region), and then doping an impurity that becomes a first conductive type semiconductor (P+-type semiconductor) into the silicon substrate Si to form a P+-type semiconductor region. That is, the second semiconductor region is formed to extend toward the inside of the silicon substrate Si in a direction perpendicular to the first surface. Inside the silicon substrate Si, the second semiconductor region is surrounded by the first semiconductor region.

[0050] In addition, when forming the photodiode PD, the Figure 2 shown floating diffusion region FD is formed. InFigure 2 In this case, a floating diffusion region FD of an N+-type semiconductor is formed on a first surface of a silicon substrate Si.

[0051] Then, as Figure 2 shown, on the first surface of the silicon substrate Si on which a photodiode PD is formed, a wiring layer W including a gate electrode G is formed. Figure 2 In [the figure], a wiring layer W having four layers of wirings is shown. The multiple wirings formed by the wiring layer W are formed of polysilicon (poly-Si) and aluminum (Al), respectively. In addition, Figure 2 the gate electrode G shown, for example, is a polysilicon gate electrode formed of polysilicon. In addition, Figure 2 the other wirings shown, for example, are formed of aluminum and are wirings connected to other components constituting the pixel 101 and circuit components (not shown) formed at a distance from the image sensor 10.

[0052] Then, as Figure 2 shown, in a microlens layer L formed on the surface side of the wiring layer W, that is, on an incident surface (surface, first surface) where light is incident on the image sensor 10 at a distance, a microlens ML, which is an optical element that condenses the incident light onto the photodiode PD, is formed. In addition, in the image sensor 10 at a distance, microlenses ML are formed at positions corresponding to the photodiodes PD of the respective multiple pixels 101.

[0053] In Figure 2 the example shown, two adjacent pixels 101, that is, a first pixel and a second pixel, are shown. Microlenses ML are respectively formed in the first pixel and the second pixel. In this configuration, two adjacent microlenses ML, that is, a first microlens and a second microlens, are formed.

[0054] At this time, in the image sensor 10 at a distance, as Figure 2 shown, each of the multiple microlenses ML is formed such that there is no gap between the microlenses ML corresponding to adjacent pixels, that is, in a so-called state where the lens gap is 0. However, in the method of forming the microlens ML, there is also a method in which the lens gap cannot be made 0. When the microlens ML is formed in the image sensor 10 at a distance by a method in which the lens gap cannot be made 0, the height of the valley portion between two adjacent microlenses ML is made different.

[0055] The height of the valley portion refers to the distance from the surface between the wiring layer W and the microlens ML to the portion that is at the lowest position in the valley portion.

[0056] Specifically, along Figure 1When the first direction and the second direction (orthogonal directions, vertical direction V and horizontal direction H) in which the plurality of pixels 101 in the pixel array shown are arranged cut the microlens ML, the valley height between two adjacent microlenses ML is set as height H1 (first height).

[0057] On the other hand, when the microlens ML is cut along a direction inclined 45° with respect to the first direction and the second direction in which the plurality of pixels 101 are arranged, that is, along the diagonal direction of the pixel 101, the valley height between two adjacent microlenses ML is set as height H2 (second height). In this case, height H1 and height H2 are made different from each other, and height H1 is made higher than height H2.

[0058] The above configuration related to the pixel 101 is the same as that of a general surface illumination type image sensor. That is, the pixel 101 can be manufactured by the same process as a general surface illumination type image sensor. However, in the distance image sensor 10, the light incident on the photodiodes PD constituting the respective pixels 101 is light in the near-infrared band with a longer wavelength. Therefore, in the distance image sensor 10, the near-infrared band light incident on each of the plurality of pixels 101 reaches a deeper position in the silicon substrate Si, that is, a position where the distance in the optical axis direction of the near-infrared band light condensed by the microlens ML is longer (farther). Therefore, in the distance image sensor 10, in the photodiodes PD constituting the respective pixels 101, electrons corresponding to the near-infrared band light are generated even in the region at a deeper position.

[0059] Figure 3 It is a cross-sectional view of the pixel 101 showing the state in which electrons corresponding to the light (near-infrared band light) incident on the imaging element (distance image sensor 10) forming the photoelectric conversion element (pixel 101) of the embodiment of the present invention are generated. Figure 3 It schematically shows the following state: In one pixel 101 arranged in the distance image sensor 10, when parallel light (so-called collimated light) in the near-infrared band is incident on the microlens ML, electrons e- corresponding to the near-infrared band light are generated inside the silicon substrate Si.

[0060] In the distance image sensor 10, in each of the plurality of pixels 101, as Figure 3As shown, even in the region at a deeper position in the silicon substrate Si, electrons e− corresponding to light in the near-infrared band are generated. Therefore, in the distance image sensor 10, the structure of the photodiode PD constituting the pixel 101 is made suitable for light in the near-infrared band. More specifically, the N-type semiconductor region extending in the depth direction of the N-type semiconductor region in the silicon substrate Si, that is, in the thickness direction D from the surface (first surface) of the silicon substrate Si toward the back surface, is formed to extend to a deeper position compared to the N-type semiconductor region of the photodiode PD formed in the pixel of a general surface-irradiation type image sensor. In other words, in the direction perpendicular to the surface of the silicon substrate Si, the N-type semiconductor region is formed in a part of the silicon substrate Si so as to extend from its surface toward the inside of the silicon substrate Si.

[0061] Figure 3 represents a state in which the range R1 in which the N-type semiconductor region extends is extended to range R2 in the thickness direction D. In addition, in Figure 3 represents a case where the range R2 of the N-type semiconductor region also extends in the horizontal width direction (horizontal direction H). Thus, in the distance image sensor 10, each of the plurality of pixels 101 can transfer and accumulate, as signal charges generated by the photodiode PD, the electrons e− generated at a deeper position in the silicon substrate Si by the photodiode PD corresponding to the light in the near-infrared band incident on the pixel. Thus, in the distance image sensor 10, the sensitivity of each pixel 101 to light in the near-infrared band can be improved. That is, in the distance image sensor 10, a pixel signal with a larger value can be output.

[0062] Therefore, in the distance image sensor 10, the control of the doping of the impurity that becomes an N-type semiconductor when forming the photodiodes PD constituting each of the plurality of pixels 101 is made different from that of a general surface-irradiation type image sensor.

[0063] <Thought process>

[0064] Here, the thought process when forming the photodiodes PD constituting each of the plurality of pixels 101 in the distance image sensor 10 is described. More specifically, the thought process when controlling the doping of the impurity that becomes an N-type semiconductor when forming the photodiodes PD constituting each of the plurality of pixels 101 in the distance image sensor 10 is described. Figure 4 is a cross-sectional view of the pixel 101 schematically showing the thought process when forming the photoelectric conversion element (pixel 101) of the embodiment of the present invention.

[0065] In the way of thinking when forming the photodiode PD in the distance image sensor 10, the doping of the impurity that becomes an N-type semiconductor is controlled based on the ratio B / A of the intensity A of light in a predetermined region (first region) on the first surface where light is incident on the silicon substrate Si and the intensity B of light in a predetermined region (second region) at a position that is a predetermined distance away from the first surface in the thickness direction D (depth direction of the silicon substrate Si).

[0066] Specifically, the incident energy (intensity) of the reflected light in the region (second region) that has been translated a specified distance z in the thickness direction of the silicon substrate Si from the above-mentioned predetermined region (first region) is defined as B(z). Then, the doping of the impurity that becomes an N-type semiconductor is controlled based on the ratio B(z) / A of the intensity A of light and the intensity B(z) of light.

[0067] That is, in the way of thinking when forming the photodiode PD in the distance image sensor 10, the depth of the N-type semiconductor region in the photodiode PD is controlled based on the attenuation rate of the light incident on the photodiode PD.

[0068] And, the "predetermined region" is the region obtained by vertically projecting the second semiconductor region onto the first surface.

[0069] More specifically, as Figure 4 shown, the incident energy of the reflected light incident on the photodiode PD is set as I, the absorption coefficient of the reflected light in the silicon substrate Si when the average wavelength of the light source (not shown) is λ is set as α(λ), the incident energy of the reflected light in the predetermined region on the first surface is set as A, and the incident energy of the reflected light in the predetermined region at a position that is a specified distance z away from the first surface in the thickness direction D of the silicon substrate Si is set as B(z). Then, when defining a specified distance z0 away from the first surface in the thickness direction D of the silicon substrate Si as in the following formula (1), the doping of the impurity that becomes an N-type semiconductor is controlled in such a way that the relational expressions of the following formula (2) and formula (3) hold.

[0070] z0 = ln(2) / α(λ)……(1)

[0071] A ≥ 0.5 * I……(2)

[0072]

[0073] In addition, the average wavelength of the light source is defined by the following formula (4).

[0074] [Equation 1]

[0075]

[0076] Here,

[0077] I(λ) represents the intensity distribution of the light source.

[0078] λ0 represents the peak wavelength of the light source intensity.

[0079] Λ represents the wavelength range used for calculating the average value, and is set to 10 nm, for example.

[0080] Next, the above conditions will be described. In Figure 4 , the light of the incident energy I incident on the pixel 101 is condensed by the microlens ML, and after passing through the transmission wiring layer W, it is incident on the silicon substrate Si. Here, the incident energy of the light incident on the predetermined region 110a on the first surface FS of the silicon substrate Si is set to A. In Figure 4 , the predetermined region 110a is a region obtained by vertically projecting the opening (light transmission region) of the wiring Wi formed in the wiring layer W onto the silicon substrate Si.

[0081] The incident energy in the predetermined region 110b inside the silicon substrate Si at a distance z from the first surface FS in the thickness direction D (depth direction) is set to B(z). At this time, in such a manner that the above formula (2) is satisfied, the thicknesses of the microlens ML and the wiring layer W and the width of the wiring Wi are determined. For example, when the thickness of the microlens ML, that is, the aspect ratio of the microlens ML is small, the incident light is reflected by the wiring Wi, and the incident energy A of the incident light does not satisfy the above formula (2). Therefore, the microlens ML needs to be set to an appropriate thickness (aspect ratio) that satisfies the above formula (2).

[0082] Here, the absorption coefficient of the silicon substrate Si when the average wavelength of the light source (not shown) is λ is set to α(λ). At this time, when the distance z0 is defined as in the above formula (1), the above formula (3) is satisfied. In addition, the distance z0 is the solution of exp(-α(λ)*z)=0.5, and represents the depth at which the light vertically incident on the surface of the silicon substrate Si is attenuated to half. Therefore, the incident energy in the predetermined region 110c inside the silicon substrate Si at a distance z0 from the first surface FS in the thickness direction D (depth direction) is represented by B(z0).

[0083] Figure 5 is a graph showing the attenuation of the incident energy of the light (light in the near-infrared band) incident on the imaging element (distance image sensor 10) on which the photoelectric conversion element (pixel 101) of the embodiment of the present invention is formed. When considering that the incident light is absorbed by the silicon substrate Si, theoretically the incident energy B(z) / A does not exceed exp(-α(λ)*z). The incident energy B(z) / A becomes exp(-α(λ)*z) when the incident light does not leak at all from the side surfaces 120a and 120b of the predetermined region. In this case, the incident energy B(z) / A depictsFigure 5 The curve of an exponential function as shown by the solid line (a).

[0084] A semiconductor region formed of a second conductivity type semiconductor (N-type semiconductor) on a semiconductor substrate, i.e., a silicon substrate Si, made of a first conductivity type semiconductor (P-type semiconductor) is generally formed to include regions such as a predetermined region 110a to a region 110c. That is, in the predetermined regions 110a to 110c, a relatively high built-in electric field is generated due to the junction of the first semiconductor region of the first conductivity type semiconductor (P-type semiconductor) and the second semiconductor region of the second conductivity type semiconductor (N-type semiconductor). Therefore, in order to transmit the electrons after photoelectric conversion at high speed, it is preferable to design the photoelectric conversion element (pixel 101) such that the incident light amount incident on the predetermined regions 110a to 110c has the maximum incident energy B(z) / A on Figure 5 the solid line (a) shown. However, Figure 5 the solid line (a) shown represents an ideal case. Due to differences in dimensions such as the thickness of the microlens ML and the wiring layer W and the width of the wiring Wi that make up the pixel 101, and optical constants in the microlens ML, etc., the characteristics of the actual photodiode PD rarely match exactly the characteristics obtained by simulation, i.e., the solid line (a). Therefore, for example Figure 5 as shown, it is assumed that a 5% error occurs between the characteristics of the actual photodiode PD and the characteristics of the photodiode PD obtained by simulation. Then, Figure 5 the incident energy B(z) / A = 0.95 * exp(-α(λ) * z) shown by the solid line (a') in Figure 5 is set as the allowable lower limit value, and the incident energy B(z) / A = 1.05 * exp(-α(λ) * z) shown by the solid line (a'') in

[0085] In addition, in the above description, when the incident light does not leak at all from the side surfaces 120a and 120b of the predetermined region, the theoretical value of the incident energy B(z) / A is set to exp(-α(λ) * z). However, more strictly speaking, this theoretical value is the value in the case of parallel light rays incident perpendicularly to the pixel 101 for the incident light in the predetermined regions 110a to 110c. Therefore, in the case where the light condensed by the microlens ML is used as the incident light as in the pixel 101, the incident energy B(z) / A becomes smaller than exp(-α(λ) * z). However, the difference in the theoretical value caused by the above reason is relatively small. Therefore, it can be considered that when obtaining the characteristics of the photodiode PD through simulation, even if the difference in the incident energy B(z) / A caused by the difference in the incident light rays is ignored, it will not affect the characteristics of the obtained photodiode PD.

[0086] In addition, in each photodiode PD, the incident energy B(z) in the region 110b at a distance z from the first surface FS in the thickness direction D (depth direction) is made the incident energy B(z0). In other words, in the photodiode PD, the depth of the region 110b is the depth at which the light incident perpendicularly to the surface of the silicon substrate Si is attenuated to half. That is, the distance z is made the distance z0 (distance z = distance z0). In this case, the above formula (3) can be expressed as follows by substituting the above formula (1), that is, exp(-α(λ) * z0) = 0.5, as shown in the following formula (5).

[0087]

[0088] Therefore, when forming the photodiodes PD constituting each pixel 101, the doping of the impurity that becomes the N-type semiconductor is controlled so that the relational expressions of the above formula (2) and the above formula (5) hold.

[0089] By such a way of thinking, in the distance image sensor 10, the N-type semiconductor regions constituting the photodiodes PD of the respective plurality of pixels 101 are formed. As a result, in the distance image sensor 10, particularly the depth of the N-type semiconductor regions of the photodiodes PD in each of the plurality of pixels 101 becomes deeper than the depth of the N-type semiconductor regions in the photodiodes formed in a general surface-irradiation type image sensor. As a result, in the distance image sensor 10, in the photodiodes PD constituting each of the plurality of pixels 101, the sensitivity to light in the near-infrared band can be improved.

[0090] <Aspect ratio of microlens ML>

[0091] In addition, as described above, in the distance image sensor 10, in order for the light in the near-infrared band incident on each of the plurality of pixels 101 to reach the N-type semiconductor region formed at a deeper position in the silicon substrate Si, that is, in order to generate electrons corresponding to the light in the near-infrared band in the N-type semiconductor region, the thickness (aspect ratio) of the microlens ML is set to satisfy the above formula (2). The distance (depth) in the optical axis direction that the light in the near-infrared band condensed by the microlens ML reaches can be confirmed by simulation. Then, in the distance image sensor 10, the aspect ratio of the microlens ML formed in each pixel 101 is an appropriate thickness (aspect ratio) that can be determined by performing a simulation of a general optical lens to satisfy the above formula (2).

[0092] In the simulation of the optical lens, various parameters such as the structure of the pixel 101, the shape of the microlens ML, and the optical characteristics of the materials forming the pixel 101 and the microlens ML are set. As the parameters related to the structure of the pixel 101 and the shape of the microlens ML set in the simulation of the optical lens, for example, there are the pixel size of the pixel 101, the height of the microlens ML, the thickness of the wiring layer W in the pixel 101, etc. In addition, as the parameters related to the characteristics of the materials set in the simulation of the optical lens, for example, there are the refractive index or extinction coefficient of the materials of the microlens ML, the wiring layer W, the wiring Wi, etc. with respect to light. By setting these parameters and performing the simulation of the optical lens, the intensity change of the incident light (light in the near-infrared band) in the thickness direction (depth direction) of the silicon substrate Si can be confirmed, and the aspect ratio of the microlens ML can be determined.

[0093] Here, an example of the simulation in the case where the thickness (aspect ratio) of the microlens ML is changed is described. First, each parameter when performing the simulation is described. Figure 6 , Figure 7A and Figure 7B are diagrams for explaining the parameters of the simulation performed when forming the microlens ML in the imaging element (distance image sensor 10) in which the photoelectric conversion element (pixel 101) of the embodiment of the present invention is formed. Figure 6 , Figure 7A and Figure 7B show one pixel 101 arranged in the distance image sensor 10. Then, Figure 6 shows a top view of the pixel 101 observed from the incident direction of light. In addition, Figure 7A and Figure 7B show a cross-sectional view of the pixel 101 shown in Figure 6 observed from the side. More specifically, Figure 7A shows Figure 6 a cross-sectional view of the A-A' cross-section in the top view of the pixel 101 shown. Figure 7B shows Figure 6Cross-sectional view of the B-B' cross-section in the top view of the pixel 101 shown.

[0094] Figure 6 and Figure 7A and Figure 7B is an example of the pixel 101 with a pixel size of 16 μm square and an opening of 8.5 μm square. In addition, in Figure 6 and Figure 7A and Figure 7B it is shown that for easy simulation, in the area outside the opening, there are wirings Wi formed in a block manner by aluminum (Al) over the entire depth direction. Consider the case of forming a microlens ML with a diameter of in such a configured pixel 101. In addition, in the pixel 101, as Figure 7A and Figure 7B shown, a planarization layer FL is formed on the surface side of the wiring layer W where the wiring Wi is formed, that is, on the surface of the wiring layer W on the incident side where light enters the pixel 101, and the microlens ML is formed above the planarization layer FL, that is, on the incident side where light enters the pixel 101. The planarization layer FL is a layer (base layer) that becomes the base of the microlens ML in the microlens layer L and is a part of the microlens ML. Therefore, the thickness of the planarization layer FL is a certain thickness, but the distance (depth) in the optical axis direction that the light in the near-infrared band focused by the microlens ML reaches depends on the height (thickness) in the optical axis direction that combines the microlens ML itself and the planarization layer FL starting from the surface side of the wiring layer W.

[0095] As Figure 7A and Figure 7B shown, the shape of the microlens ML can be considered as a part of an ellipse. In Figure 7A and Figure 7B it is shown that the shape of the microlens ML is i.e., a part of an ellipse with a minor axis of 20 μm and a major axis of 26 μm. In addition, Figure 7A and Figure 7B show the values of 10 μm, which can be considered as half the length of the minor axis (minor radius) of the ellipse that is the microlens ML, and 13 μm, which is half the length of the major axis (major radius). The height of the microlens ML is the height from the surface side where light enters the planarization layer FL. Then, in Figure 7A and Figure 7B the minor axis of the ellipse is along the surface side of the planarization layer FL. In the case of the pixel 101 shown in Figure 7A and Figure 7B the height of the microlens ML is the length of the major radius, i.e., 13 μm. Then, the aspect ratio of the microlens ML can be calculated by the following formula (6) based on the respective values representing the ellipse.

[0096] Aspect ratio of the microlens ML

[0097] = height of the microlens ML / diameter of the microlens ML

[0098] = major radius / minor axis

[0099] = 13 μm / 20 μm

[0100] = 0.65……(6)

[0101] In addition, in the case where the microlens ML is formed in the 16-μm square pixel 101, as shown, on the sides in the vertical direction ( Figure 6 the up-and-down direction) and the horizontal direction ( Figure 6 the left-and-right direction) of the pixel 101, a part of the area of the microlens ML protrudes. A part of the area of the microlens ML that protrudes from the area of the pixel 101 overlaps with a part of the area of the microlens ML that protrudes from the area of the pixel 101 formed in the adjacent pixel 101. In addition, as Figure 6 shown, in the diagonal direction ( Figure 6 the 45° oblique direction) of the pixel 101, there is an area where the microlens ML is not formed in a part of the pixel 101. The area where the microlens ML is not formed only becomes the planarization layer FL. In Figure 6 and Figure 7A and Figure 7B , the height (thickness) in the optical axis direction of the planarization layer FL that serves as the base of the microlens ML is set to 2 μm. In addition, the thickness of the planarization layer FL is a constant thickness (2 μm here) regardless of the height of the microlens ML. In addition, in Figure 7A and Figure 7B , the thickness of the wiring layer W constituting the pixel 101, that is, the height of the area other than the opening, is set to 3 μm. In addition, the microlens ML can also be formed such that the area overlapping with a part of the microlens ML formed in the adjacent pixel 101 is increased and the area where the microlens ML is not formed disappears. That is, it can also be formed such that the diameter of the microlens ML is increased and the area where only the planarization layer FL is formed disappears.

[0102] An example of the result of simulating the intensity change of the light in the near-infrared band incident in the thickness direction (depth direction) of the silicon substrate Si in the pixel 101 in which the microlens ML having such a configuration is formed is described. Figure 8 is a graph showing an example of the result of simulating the attenuation of the incident energy of the light (light in the near-infrared band) incident on the imaging element (distance image sensor 10) having the photoelectric conversion element (pixel 101) of the embodiment of the present invention. Figure 8 The graph of the simulation result shown is in Figure 6, Figure 7A and Figure 7B Under the structure of the pixel 101 and the shape of the microlens ML shown, the state of the intensity attenuation of the near-infrared light with a wavelength of 940 nm according to the depth of the silicon substrate Si was simulated, and a graph showing the intensity change of the near-infrared light is represented by a relative value.

[0103] In addition, when performing the simulation, the refractive index of light in the silicon substrate Si, that is, silicon (Si), was set to 3.59. The refractive index of light in the material of the microlens ML (including the planarization layer FL) was set to 1.6. The refractive index of light in the aluminum (Al) formed as the wiring Wi was set to 1.66. The refractive index of light in the silicon dioxide (SiO2) formed as an insulating material in the pixel 101 and forming the wiring layer W including openings and the like was set to 1.46. In addition, when performing the simulation, the extinction coefficient of light in silicon (Si) was set to 0.01, and the extinction coefficient of light in aluminum was set to 8.71.

[0104] In addition, in Figure 8 , for comparison, the case where the height of the microlens ML is 15 μm (aspect ratio = 0.75) and the case where it is 19 μm (aspect ratio = 0.95) are shown simultaneously. In addition, if the height of the microlens ML itself changes with the change of the aspect ratio of the microlens ML, the height (thickness) in the optical axis direction of the microlens ML that overlaps with the region of the microlens ML formed in the adjacent pixel 101, that is, the height (thickness) in the optical axis direction other than the planarization layer FL, changes. In addition, if the height (thickness) in the optical axis direction of the microlens ML that overlaps with the region of the microlens ML formed in the adjacent pixel 101 changes, the amount of light leakage from the adjacent pixel 101, that is, the amount of light leakage from the pixel 101 located around the pixel 101 being simulated (in Figure 1 the pixels located at adjacent positions in the vertical direction V and the horizontal direction H in the pixel array shown) also changes. Figure 8 The simulation results shown are the results obtained by also considering the height (thickness) in the optical axis direction of the microlens ML that overlaps with the region of the microlens ML formed in the adjacent pixel 101 and the amount of light leakage accompanying the change of the aspect ratio of the microlens ML. That is, Figure 8 the simulation results shown are the results reflecting each parameter that changes with the change of the height (aspect ratio of the microlens ML) of the microlens ML itself.

[0105] When the height of the microlens ML is 13 μm (aspect ratio = 0.65), as Figure 8As shown, the intensity of near-infrared light with a wavelength of 940 nm decreases exponentially with respect to the depth of the silicon substrate Si. In contrast, when the height of the microlens ML is 15 μm (aspect ratio = 0.75), the intensity of the near-infrared light decreases exponentially with respect to the depth of the silicon substrate Si until the depth of the silicon substrate Si reaches around 20 μm, which corresponds to the distance z0 shown in Figure 4 and is the same as the case where the height of the microlens ML is 13 μm. And in this case, the intensity of the near-infrared light decreases sharply from around when the depth of the silicon substrate Si exceeds 20 μm. In addition, when the height of the microlens ML is 19 μm (aspect ratio = 0.95), the intensity of the near-infrared light decreases sharply from around when the depth of the silicon substrate Si is 6 - 7 μm.

[0106] Here, in the cases where the height of the microlens ML is 15 μm and 19 μm, the reason for the sharp decrease in the intensity of the near-infrared light is that if the aspect ratio of the microlens ML is too high, more near-infrared light diffuses beyond the focal point of the microlens ML, that is, the focal position. The diffusion of the near-infrared light after exceeding this focal point reduces the generation efficiency of electrons in the N-type semiconductor region formed at a deeper position in the silicon substrate Si, that is, it becomes an important reason for the decrease in the sensitivity of the photodiode PD to light in the near-infrared band. Therefore, in Figure 8 an example of the simulation results shown, Figure 6 , Figure 7A and Figure 7B the appropriate thickness (aspect ratio) of the microlens ML in the structure of the pixel 101 shown can be 13 μm in height (aspect ratio = 0.65).

[0107] In addition, when actually determining the thickness (aspect ratio) of the microlens ML that satisfies the above formula (2), the accuracy of each parameter set in the simulation can be considered to be increased. However, as shown in Figure 8 , even when the parameters of the simulation are simply set, the state of the attenuation of the intensity of the near-infrared light with respect to the depth of the silicon substrate Si, that is, the change in intensity, can be confirmed.

[0108] Next, for comparison, an example of the difference in the intensity change of light in the thickness direction (depth direction) of the silicon substrate Si caused by the difference in height between the microlens formed in the pixel of a general surface-irradiation type image sensor and the microlens ML formed in the pixel 101 of the distance image sensor 10 of the present embodiment is described. Figure 9A And Figure 9B are graphs showing an example of the simulation results comparing the attenuation of the incident energy of light incident on the imaging element. In Figure 9A and Figure 9BIn the graph of the simulation results shown, both Figure 8 the simulation results shown, and the simulation results when the height of the microlens ML is set to the height of the microlens formed in the pixel of a general image sensor are shown. That is, Figure 9A and Figure 9B the simulation results shown are the results of simulating the state in which the intensity of near-infrared light with a wavelength of 940 nm attenuates according to the depth of the silicon substrate Si. Figure 9A A graph showing the change in the intensity of near-infrared light relative to the depth of the silicon substrate Si in relative values is shown, Figure 9B and a graph showing the change in the intensity of near-infrared light relative to the depth of the silicon substrate Si in absolute values is shown.

[0109] In Figure 9A and Figure 9B the simulation results shown, the height of the microlens ML formed in the pixel 101 is set to 13 μm (aspect ratio = 0.65), and the height of the microlens formed in the pixel of a general image sensor is set to 3 μm (aspect ratio = 0.15). In addition, Figure 9A and Figure 9B the simulation results shown are also such that the parameters other than the height of the microlens ML are the same as those in Figure 8 the simulation results shown.

[0110] As Figure 9A shown, when the change in the intensity of near-infrared light is represented in relative values, in the case where the height of the microlens ML is 13 μm (aspect ratio = 0.65) and in the case where the height of the microlens is 3 μm (aspect ratio = 0.15), the change in the intensity of near-infrared light relative to the depth of the silicon substrate Si is similar. That is, regardless of whether the height of the microlens ML is 13 μm or 3 μm, the intensity of near-infrared light with a wavelength of 940 nm decreases in an exponential function manner with respect to the depth of the silicon substrate Si.

[0111] However, as Figure 9BAs shown, when the intensity change of near-infrared light is represented by an absolute value, the intensity of near-infrared light when the height of the microlens is 3 μm is overall lower compared to the intensity of near-infrared light when the height of the microlens ML is 13 μm. More specifically, when the intensity of the near-infrared light incident on pixel 101 is set to 100%, the intensity of the near-infrared light on the surface (the first surface) where the light is incident on the silicon substrate Si is 70% or more when the height of the microlens ML is 13 μm, and 40% when the height of the microlens is 3 μm. The reason is that when the height of the microlens is 3 μm, the aspect ratio of the microlens ML is relatively low, so the light condensing ability of the microlens ML is weak (the light condensing characteristic is low), and in the region of the wiring layer W that the near-infrared light transmits before reaching the first surface of the silicon substrate Si, the near-infrared light attenuates more. Therefore, the amount of near-infrared light reaching the first surface of the silicon substrate Si when the height of the microlens is 3 μm is reduced compared to when the height of the microlens is 13 μm, and its intensity decreases accordingly.

[0112] In addition, as described above, in pixel 101, it is necessary to form the N-type semiconductor region of the photodiode PD to a deeper position in the silicon substrate Si, and determine the microlens ML to have a thickness (aspect ratio) that satisfies the above formula (2). That is, in pixel 101, it is necessary to make the incident energy A of the light incident on a predetermined region (here, the region of the opening) on the first surface of the silicon substrate Si be 50% or more of the incident energy I of the light incident on pixel 101. Therefore, according to Figure 9A and Figure 9B the simulation results shown, it can be confirmed that the microlens ML with a relatively small aspect ratio such as a height of 3 μm formed in the pixels of a general image sensor does not satisfy the above formula (2) and is not a microlens ML with a suitable thickness (aspect ratio) in the structure of pixel 101.

[0113] In this way, in the distance image sensor 10, it is possible to confirm by simulation whether the thickness of the microlens ML is a thickness (aspect ratio) suitable for pixel 101. That is, in the distance image sensor 10, in the photodiode PD in which the depth of the N-type semiconductor region formed in each of the plurality of pixels 101 makes the depth of the N-type semiconductor region in the photodiode formed in a general surface-irradiation type image sensor deeper, it is possible to confirm whether it is a thickness (aspect ratio) suitable for improving the sensitivity to light in the near-infrared band.

[0114] In this way, in the distance image sensor 10, by the depth of the N-type semiconductor region constituting the photodiode PD and the suitable thickness (aspect ratio) of the microlens ML, it is possible to improve the sensitivity of each of the plurality of pixels 101 to light in the near-infrared band.

[0115] Accordingly, in the imaging system of the embodiment of the present invention equipped with the distance image sensor 10, it is possible to measure the distance to an object with higher accuracy using the time-of-flight (TOF) technology. Here, the imaging system of the embodiment of the present invention will be described.

[0116] Figure 10 It is a block diagram showing a schematic configuration of the imaging system of the embodiment of the present invention equipped with the imaging element (distance image sensor 10) of the embodiment of the present invention. Figure 10 The imaging system of the embodiment of the present invention shown, i.e., the TOF sensor module 1, includes a light source unit 2 and a light receiving unit 3. In addition, the light source unit 2 includes a light source device 21 and a diffuser plate 22. In addition, the light receiving unit 3 includes a distance image sensor 10 and a lens 31. In addition, Figure 10 also shown therein is the object O whose distance is measured in the TOF sensor module 1 which is the imaging system of the embodiment of the present invention.

[0117] In the Figure 10 TOF sensor module 1 having the configuration shown, a light pulse PL in the near-infrared band is irradiated from the light source unit 2 to the object O. Then, in the TOF sensor module 1, the light receiving unit 3 receives the reflected light RL of the light pulse PL reflected by the object O and outputs a signal for measuring the distance to the object O (hereinafter, referred to as a "measurement signal").

[0118] The light source unit 2 irradiates the light pulse PL to the object O which is the object whose distance is measured in the TOF sensor module 1. The light source unit 2 is, for example, a surface-emitting type semiconductor laser module such as a vertical cavity surface emitting laser (VCSEL). The light source device 21 is a light source that emits laser light in the near-infrared band (for example, a band with a wavelength of 850 nm to 940 nm) as the light pulse PL irradiated to the object O. The light source device 21 is, for example, a semiconductor laser light-emitting element. The light source device 21 emits pulsed laser light according to the control from a light source control unit (not shown). The diffuser plate 22 is an optical lens that diffuses the laser light in the near-infrared band emitted by the light source device 21 into an area of a surface irradiated to the object O. The pulsed laser light diffused by the diffuser plate 22 is emitted from the light source unit 2 as the light pulse PL and irradiated to the object O.

[0119] The light-receiving unit 3 receives the reflected light RL of the light pulse PL reflected by the object O whose distance is measured in the TOF sensor module 1, and outputs a measurement signal corresponding to the received reflected light RL. The lens 31 is an optical lens that guides the incident reflected light RL to the distance image sensor 10. The lens 31 emits the incident reflected light RL toward the distance image sensor 10 side, and causes the entire surface of the light-receiving area 100 provided in the distance image sensor 10, that is, each of the plurality of pixels 101 arranged in the light-receiving area 100 to receive light (incidence).

[0120] According to such a configuration, in the TOF sensor module 1, the reflected light RL obtained by reflecting the light pulse PL in the near-infrared band irradiated by the light source unit 2 to the object O is received by the light-receiving unit 3, and the distance image sensor 10 included in the light-receiving unit 3 outputs a measurement signal for measuring the distance to the object O.

[0121] In addition, in the TOF sensor module 1, the irradiation of the light pulse PL by the light source unit 2 and the reception of the reflected light RL by the light-receiving unit 3 are performed, for example, by a module control unit (not shown) provided outside or inside the TOF sensor module 1. More specifically, the period of the pulse of the light pulse PL irradiated by the light source unit 2 to the object O and the timing of the distance image sensor 10 included in the light-receiving unit 3 receiving the reflected light RL are performed by the module control unit (not shown). In addition, the measurement signal output by the TOF sensor module 1 (more specifically, the distance image sensor 10) is processed, for example, by a distance image processing unit (not shown) provided outside or inside the TOF sensor module 1 to generate a two-dimensional image including the object O and information on the distance to the object O. In addition, the distance image processing unit (not shown) may also generate, for example, a two-dimensional image (distance image) including the object O that represents the information on the distance to the object O by differentiating colors.

[0122] As described above, according to the embodiment of the present invention, in the silicon substrate that is the imaging element (distance image sensor) of the present invention, the structure of the photoelectric conversion element (pixel) of the present invention that constitutes the pixel arranged in the light-receiving area is made suitable for light in the near-infrared band. More specifically, when forming the photoelectric conversion element, it is formed such that the depth (thickness) of the N-type semiconductor region that constitutes the photoelectric conversion element extends deeper in the silicon substrate than the N-type semiconductor region in the photoelectric conversion element formed in the pixel of a general surface-irradiation type image sensor. Thus, in the imaging element of the embodiment of the present invention, in the photoelectric conversion elements that constitute the respective plurality of pixels, the sensitivity to light in the near-infrared band can be improved. That is, in the imaging element of the embodiment of the present invention, a signal that more correctly represents the amount (light amount) of the incident light in the near-infrared band can be output.

[0123] In addition, in an embodiment of the present invention, the imaging system of the present invention (TOF sensor module 1) equipped with the imaging element of the present invention outputs a signal that more accurately represents the amount of light (light quantity) in the near-infrared band of the output of the imaging element. Thus, in the imaging system of the present invention equipped with the imaging element of the present invention, using the time-of-flight (TOF) technology, it is possible to output a measurement signal that enables more accurate distance measurement between the object and the system.

[0124] In addition, in an embodiment of the present invention, the case where the imaging element of the present invention has a structure equivalent to that of a surface illumination type image sensor has been described. However, the structure of the imaging element of the present invention is not limited to the structure equivalent to that of the surface illumination type image sensor shown in the embodiment of the present invention. That is, in general image sensors, in addition to the surface illumination type image sensor, there is also a backside illumination (BSI) type image sensor. Therefore, the structure of the imaging element of the present invention can also be a structure equivalent to that of a backside illumination type image sensor. Further, even when the structure of the imaging element of the present invention is a structure equivalent to that of a backside illumination type image sensor, the concept when forming the photoelectric conversion element of the present invention is the same as the concept shown in the embodiment of the present invention. And, the structure of the imaging element of the present invention in this case can be easily conceived based on the structure of a general backside illumination type image sensor. Therefore, the detailed description of the case where the imaging element of the present invention has a structure equivalent to that of a backside illumination type image sensor is omitted.

[0125] In addition, in the embodiments of the present invention, the following is described: the configuration of the pixels disposed in the light-receiving region of the imaging element of the present invention is a configuration in which the signal charges generated and accumulated by the photoelectric conversion element of the present invention are transferred and accumulated by a group of one gate electrode G and one floating diffusion region FD. However, the group of the gate electrode G and the floating diffusion region FD included in the pixels disposed in the light-receiving region of the imaging element of the present invention is not limited to the one group shown in the embodiments of the present invention. That is, the pixels disposed in the light-receiving region of the imaging element of the present invention can also be configured to include two or more groups of the gate electrode G and the floating diffusion region FD. Thereby, in the imaging element of the present invention in which pixels including two or more groups of the gate electrode G and the floating diffusion region FD are disposed, the signal charges generated and accumulated by the photoelectric conversion element of the present invention can be distributively transferred and accumulated to each floating diffusion region FD. That is, in the imaging element of the present invention in which pixels including two or more groups of the gate electrode G and the floating diffusion region FD are disposed, the highly sensitive signal charges generated and accumulated by the photoelectric conversion element of the present invention can be more effectively utilized. Thereby, in the imaging system of the present invention equipped with the imaging element of the present invention in which pixels including two or more groups of the gate electrode G and the floating diffusion region FD are disposed, the accuracy of distance measurement using the time-of-flight (TOF) technique can be further improved.

[0126] In addition, in the embodiments of the present invention, the following is described: the photoelectric conversion element of the present invention that constitutes the pixels disposed in the light-receiving region of the imaging element of the present invention is a photoelectric conversion element that generates and accumulates electrons corresponding to the amount of incident light (light quantity) as signal charges. However, the photoelectric conversion element of the present invention is not limited to the form of generating and accumulating electrons as signal charges shown in the embodiments of the present invention. That is, among the photoelectric conversion elements that constitute the pixels disposed in a general image sensor, there are not only photoelectric conversion elements that use electrons as signal charges, but also photoelectric conversion elements that generate and accumulate holes (so-called vacancies) corresponding to the amount of incident light (light quantity) as signal charges. Therefore, the photoelectric conversion element of the present invention can also be configured to generate and accumulate holes (vacancies) as signal charges. In addition, even when the photoelectric conversion element of the present invention is configured to generate and accumulate holes (vacancies) as signal charges, the concept when forming the photoelectric conversion element of the present invention is the same as the concept shown in the embodiments of the present invention. And, in this case, the structure of the photoelectric conversion element of the present invention can be easily conceived by replacing electrons with holes (vacancies) in the description of the embodiments of the present invention, including the conductivity type of the semiconductor in the silicon substrate Si and the photodiode PD. Therefore, the detailed description of the case where the photoelectric conversion element of the present invention generates and accumulates holes (vacancies) as signal charges is omitted.

[0127] As described above, the embodiments of the present invention have been described with reference to the accompanying drawings. However, the specific configuration is not limited to this embodiment, and various modifications within the scope not departing from the gist of the present invention are also included.

[0128] Explanation of Reference Signs

[0129] 1: TOF sensor module

[0130] 2: Light source unit

[0131] 21: Light source device

[0132] 22: Diffusion plate

[0133] 3: Light receiving unit

[0134] 31: Lens

[0135] 10: Distance image sensor

[0136] 101: Pixel (photoelectric conversion element)

[0137] Si: Silicon substrate

[0138] PD: Photodiode (photoelectric conversion unit)

[0139] FD: Floating diffusion region

[0140] G: Gate electrode

[0141] W: Wiring layer

[0142] Wi: Wiring

[0143] FS: First surface

[0144] L: Microlens layer

[0145] ML: Microlens

[0146] FL: Planarization layer

[0147] e-: Electron

[0148] PL: Light pulse

[0149] RL: Reflected light

[0150] O: Object

[0151] 110a, 110b, 110c: Regions

[0152] 120a, 120b: Sides

[0153] I, A, B(z), B(z0): Incident energy

Claims

1. A photoelectric conversion element that receives light from a light source that emits light in a specified wavelength band and reflected light obtained by reflecting the above-mentioned light from an object, characterized in that comprising: a substrate having a first surface on which the reflected light is incident, a first semiconductor region formed of a first-conductivity-type semiconductor, and a second semiconductor region formed of a second-conductivity-type semiconductor having a conductivity type different from that of the first-conductivity-type semiconductor and extending inward from the first surface in a direction perpendicular to the first surface; and an optical element disposed on the first surface side of the substrate to condense the reflected light onto the second semiconductor region, when the incident energy of the reflected light incident on the photoelectric conversion element is set as I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is set as α(λ), the incident energy of the reflected light in a predetermined region on the first surface is set as A1, the incident energy of the reflected light in the predetermined region on the first surface when the photoelectric conversion element does not have the optical element is set as A2, when the incident energy of the reflected light in a region parallelly displaced by a predetermined distance z in the thickness direction of the substrate from the predetermined region is set as B(z), when A1 ≧ A2 and the distance z0 = ln(2) / α(λ) is set, in the case of z = z0, the relational expression 0.95*exp(-α(λ)*z) ≦ B(z) / A1 ≦ 1.05*exp(-α(λ)*z) holds.

2. The photoelectric conversion element according to claim 1, wherein the predetermined region is a region obtained by vertically projecting the second semiconductor region onto the first surface.

3. The photoelectric conversion element according to claim 1 or 2, wherein the wavelength band is a near-infrared wavelength band.

4. The photoelectric conversion element according to claim 3, wherein the near-infrared wavelength band is a wavelength band of 850 nm to 940 nm.

5. A photoelectric conversion element that receives light from a light source that emits light in a specified wavelength band and reflected light obtained by reflecting the above light from an object, characterized in that comprising: a substrate having a first surface on which the reflected light is incident, a first semiconductor region formed of a first-conductivity-type semiconductor, and a second semiconductor region formed of a second-conductivity-type semiconductor having a conductivity type different from that of the first-conductivity-type semiconductor and extending inward from the first surface in a direction perpendicular to the first surface; and an optical element disposed on the first surface side of the substrate to condense the reflected light onto the second semiconductor region, when the incident energy of the reflected light incident on the photoelectric conversion element is set as I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is set as α(λ), the incident energy of the reflected light in a predetermined region on the first surface is set as A1, the incident energy of the reflected light in the predetermined region on the first surface when the photoelectric conversion element does not have the optical element is set as A2, when the incident energy of the reflected light in a region parallelly displaced by a predetermined distance z in the thickness direction of the substrate from the predetermined region is set as B(z), when A1 ≧ A2 and the distance z0 = ln(2) / α(λ) is set, for all z satisfying 0 ≦ z ≦ z0, The relation 0.95*exp(-α(λ)*z)≦B(z) / A1≦1.05*exp(-α(λ)*z) holds.

6. The photoelectric conversion element according to claim 5, wherein the predetermined region is a region obtained by vertically projecting the second semiconductor region onto the first surface.

7. The photoelectric conversion element according to claim 5 or 6, wherein the wavelength band is a near-infrared wavelength band.

8. The photoelectric conversion element according to claim 7, wherein the near-infrared wavelength band is a wavelength band of 850 nm to 940 nm.

9. An imaging element that receives reflected light obtained by reflecting light of a predetermined wavelength band emitted from a light source by an object, wherein it has the photoelectric conversion element according to any one of claims 1 to 8, and the photoelectric conversion element includes a light-receiving region in which a plurality of pixels are arranged in a two-dimensional matrix, in the light-receiving region, the plurality of pixels are arranged along a first direction and a second direction that are orthogonal to each other, when the optical element is cut along the first direction and the second direction, the height of the valley portions of two adjacent optical elements is set as a first height, when the optical element is cut along the diagonal direction of the pixel, the height of the valley portions of two adjacent optical elements is set as a second height, the first height and the second height are different from each other.

10. An imaging system having: a light source unit that emits light of a predetermined wavelength band; and a light-receiving unit that receives reflected light obtained by reflecting the light by an object, wherein it further has the imaging element according to claim 9.

Images