Photoelectric conversion element, imaging element, and imaging system

Abstract

A photoelectric conversion element according to the present invention has a substrate and an optical element. The photoelectric conversion element uses a light source which emits light and receives reflected light that the light is reflected by an object. When the incident energy of the reflected light incident on the photoelectric conversion element is represented by I, an absorption coefficient of the reflected light at the substrate when an average wavelength of the light source is represented by λ is represented by α(λ), the incident energy of the reflected light at a predetermined region on a first surface is represented by A1, the incident energy of the reflected light at the predetermined region on the first surface when the photoelectric conversion element does not include the optical element is represented by A2, and the incident energy of the reflected light at a region moved in parallel from the predetermined region in the thickness direction of the substrate by a prescribed distance z is represented by B(z), A1 ≥ A2 is satisfied, and when assuming a distance z0 = ln(2) / α(λ), in the case of z = z0, a relational expression of 0.95 * exp(-α(λ) * z) ≤ B(z) / A ≤ 1.05 * exp(-α(λ) * z) holds.

Description

Technical Field

[0001] The present invention relates to a photoelectric conversion element, an imaging element in which the photoelectric conversion element is arranged, and an imaging system including the imaging element. This application claims priority based on Japanese Patent Application No. 2019-157643 filed in Japan on August 30, 2019, the content of which is incorporated herein by reference.

Background Art

[0002] Conventionally, as a technique for measuring the distance to an object, an object is irradiated with an optical pulse in the near-infrared region, and the time difference between the time when the optical pulse is irradiated and the time when the reflected light of the irradiated optical pulse is detected by the object is measured, that is, there is a technique for measuring the flight time of the optical pulse. A distance measurement sensor that measures the distance to an object by such a flight time of an optical pulse is called Time of Flight (TOF). And a distance measurement sensor that measures the distance to an object by the time-of-flight technique using a photoelectric conversion element has also been put into practical use.

[0003] In recent years, a configuration for measuring the distance to an object by the time-of-flight technique using a photoelectric conversion element has been developed, and not only the distance to the object but also a two-dimensional image including the object can be obtained, that is, a distance measurement sensor capable of obtaining three-dimensional information about the object has also been put into practical use. Such a distance measurement sensor is also called a distance image sensor. In a distance image sensor, pixels including a photodiode, which is a light receiving unit that receives the reflected light of an optical pulse reflected by an object, are arranged in a two-dimensional matrix on a silicon substrate. And in the distance image sensor, by outputting a photoelectric conversion signal based on the amount of light of the reflected light of the optical pulse received by each of the plurality of pixels for one image, a two-dimensional image including the object and the distance information of each of the plurality of pixels constituting this image can be obtained. Thereby, in the distance image sensor, three-dimensional information combining the distance information of each of the plurality of pixels in a two-dimensional image including the object can be obtained.

[0004] Incidentally, the accuracy of the distance that can be measured by 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. For this reason, 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 improve the sensitivity to light in the near-infrared region for each of the plurality of pixels.

[0005] In an image sensor that acquires an image, for example, various techniques for improving the sensitivity to light have been proposed as in 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 photogate pairs that constitute each sensor unit (pixel) formed on a semiconductor substrate of a sensor system. Thereby, in the 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 the distance image sensor as well, in order to improve the sensitivity to light in the near-infrared region for each of the plurality of pixels, it is conceivable to apply the technique of forming microlenses as disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Incidentally, in a distance image sensor, the sensitivity of each of a plurality of pixels is related to the structure of a photodiode which is a light receiving section, particularly to the distance in the optical axis direction. That is, the sensitivity of each of the plurality of pixels in the distance image sensor is also related to the depth (thickness) of a diffusion layer when forming the photodiode on a silicon substrate. The reason for this is that in the distance image sensor, as described above, the light pulse in the near infrared region receives the reflected light reflected by an object, and since this reflected light is also the light pulse in the near infrared region, more photoelectric conversion is performed at a deeper position in the silicon substrate.

[0008] However, the technique disclosed in Patent Document 1 is a technique for forming microlenses at respective positions of a plurality of pixels in an image sensor. Therefore, by applying the technique disclosed in Patent Document 1, microlenses corresponding to each of the plurality of pixels can be formed also in the distance image sensor, but the formed microlenses are not necessarily suitable for the distance image sensor. The reason for this is that the focal position of the microlens formed in the technique disclosed in Patent Document 1 is near the light receiving surface of the photogate pair, that is, near the incident side surface where light enters the semiconductor substrate on which the photogate is formed. And, as a technique related to the distance image sensor, when forming microlenses corresponding to each of the plurality of pixels, a technique defining the relationship between the position where the microlens condenses light, that is, the focal position of the microlens, and the depth (thickness) of the diffusion layer when forming the photodiode which is the light receiving section in each of the plurality of pixels is not disclosed.

[0009] The present invention has been made based on the above problems, and an object thereof is to provide a photoelectric conversion element having a structure capable of improving the sensitivity to light of each of a plurality of pixels in a distance image sensor in which microlenses corresponding to each of the plurality of pixels are formed, an imaging element in which this photoelectric conversion element is arranged, and an imaging system including this imaging element.

Means for Solving the Problems

[0011] In order to solve the above problems, a photoelectric conversion element according to one aspect of the present invention is Near-infrared a photoelectric conversion element that receives reflected light obtained by reflecting the light by an object using a light source that emits light in a wavelength band. A first surface that is a surface on which the reflected light is incident, a first semiconductor region formed of a first-conductivity-type semiconductor, and a second-conductivity-type semiconductor having a conductivity type different from that of the first-conductivity-type semiconductor and formed so as to spread inward from the first surface in a direction perpendicular to the first surface. A substrate having a second semiconductor region, and an optical element disposed on the first surface side of the substrate to condense the reflected light onto the second semiconductor region. The incident energy of the reflected light incident on the photoelectric conversion element is I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is α(λ), the incident energy of the reflected light in a predetermined region on the first surface is A1, and 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 A2, within the second semiconductor region, parallelly moved by a predetermined distance z in the thickness direction of the substrate from the predetermined region position the incident energy of the reflected light at is B(z), when A1≧A2 and A1≧0.7*I are satisfied, and when the distance z0 = ln(2) / α(λ), for all z satisfying 0≦z≦z0, the thickness of the optical element is set so that the relational expression 0.95*exp(-α(λ)*z)≦B(z) / A1≦1.05*exp(-α(λ)*z) holds.

[0012] In the photoelectric conversion element according to one aspect of the present invention, the predetermined region may be a region obtained by projecting the second semiconductor region perpendicularly to the first surface.

[0014] In the photoelectric conversion element according to one aspect of the present invention, the near-infrared wavelength band may be a wavelength band of 850 nm to 940 nm.

[0015] An imaging element according to one aspect of the present invention is Near-infraredAn imaging device that receives reflected light reflected by an object using a light source that emits light in a wavelength band, and has a plurality of photoelectric conversion elements according to the above-described aspect, wherein a plurality of pixels are provided in a two-dimensional matrix arrangement. A light receiving region. In the light receiving region, a plurality of pixels are arranged along a first direction and a second direction orthogonal to each other. When the optical element is cut along the first direction and the second direction, the height of the valleys of two adjacent optical elements is defined as a first height. When the optical element is cut along the diagonal direction of the pixel, the height of the valleys of two adjacent optical elements is defined as a second height. The first height and the second height are different from each other.

[0016] An imaging system according to an aspect of the present invention Near-infrared A light source unit that emits light in a wavelength band, an imaging device according to the above-described aspect, and a light receiving unit that receives reflected light reflected by an object.

Effect of the Invention

[0017] According to an imaging system according to an aspect of the present invention, in a distance image sensor in which microlenses corresponding to each of a plurality of pixels are formed, a photoelectric conversion element having a structure capable of improving the sensitivity of each of the plurality of pixels to light, An imaging device in which the photoelectric conversion element is arranged, and an imaging system including the imaging device can be provided.

Brief Description of the Drawings

[0018]

Figure 1

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Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The 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). Inside the silicon substrate, a photodiode that functions as a photoelectric conversion unit is provided. 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 the Time of Flight (TOF) technology. Further, the photoelectric conversion element (pixel) is formed in a distance image sensor which is an imaging device according to an embodiment of the present invention. That is, in the following description, the photoelectric conversion element according to an embodiment of the present invention receives the reflected light of the light in the long-wavelength near-infrared wavelength band (for example, the light in the wavelength band of 850 nm to 940 nm) emitted by the imaging system according to an embodiment of the present invention and reflected by the object, and is formed in the photoelectric conversion element (distance image sensor) according to an embodiment of the present invention that outputs a signal for measuring the distance to the object.

[0020] FIG. 1 is a block diagram showing the overall schematic configuration of the imaging device according to an embodiment of the present invention. In FIG. 1, the distance image sensor 10 includes a light-receiving region 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. Note that in the distance image sensor 10 shown in FIG. 1, an example of the light-receiving region 100 in which a plurality of pixels 101 are arranged in a two-dimensional matrix of 6 rows and 8 columns is shown so as to form a plurality of pixel columns. In other words, the plurality of pixels 101 constituting the pixel array constituting the distance image sensor 10 are arranged along the first direction and the second direction orthogonal to each other.

[0021] The control circuit 200 controls the components provided in the distance image sensor 10 such as the vertical drive circuit 300, the horizontal drive circuit 400, and the AD conversion circuit 500. The control circuit 200 controls the operation of the components provided in the distance image sensor 10, for example, in response to control from a control device (not shown) provided in the imaging system.

[0022] The vertical drive circuit 300 is a drive circuit that controls each of the plurality of pixels 101 arranged in the light-receiving region 100 in accordance with the control from the control circuit 200. By driving the vertical drive circuit 300, each of the plurality of pixels 101 photoelectrically converts the light (incident light) incident on the pixel 101 and generates signal charges. The vertical drive circuit 300 causes (reads out) a pixel signal corresponding to the signal charges of each of the plurality of pixels 101 to be output to the corresponding vertical signal line. The vertical drive circuit 300 outputs a drive signal for driving (controlling) the pixel 101 for each row of the pixels 101 arranged in the light-receiving region 100. As a result, the pixel signals output by the pixels 101 are read out row by row to the vertical signal lines and output to the AD conversion circuit 500.

[0023] Each pixel 101 arranged in the light-receiving region 100 outputs a pixel signal obtained by converting the incident light into an electrical signal. The pixel 101 includes components such as a photodiode (photoelectric conversion unit) that converts the incident light into an electrical signal by generating and accumulating signal charges corresponding to the amount of incident light (light amount). Each of the plurality of pixels 101 outputs a pixel signal corresponding to the amount of incident light (light amount) to the corresponding vertical signal line in accordance with the drive signal input from the vertical drive circuit 300. Details of the structure of the pixel 101 will be described later.

[0024] The AD conversion circuit 500 is an analog / digital conversion circuit that converts, in accordance with the control from the control circuit 200, the magnitude of the analog pixel signal output from the pixels 101 in the corresponding column to the corresponding vertical signal line into a digital value representing the magnitude. Note that the AD conversion circuit 500 may be an AD conversion circuit group including a plurality of AD conversion circuits corresponding to each of 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 to the horizontal signal line as an output signal in accordance with the control from the horizontal drive circuit 400.

[0025] 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 lines. That is, due to the drive of the horizontal drive circuit 400, in accordance with the control from the control circuit 200, the pixel signals after analog / digital conversion corresponding to each column (pixel column) of the pixels 101 arranged in the light-receiving region 100 and output from the AD conversion circuit 500 are sequentially output to the horizontal signal lines. The horizontal drive circuit 400 sequentially outputs control signals for outputting the output signals corresponding to the pixels 101 in each of the plurality of columns to the AD conversion circuit 500. Thereby, the output signals output by the AD conversion circuit 500 are sequentially output to the output circuit 600 via the horizontal signal lines.

[0026] The output circuit 600 is a circuit that outputs the output signals from the AD conversion circuit 500 output to the horizontal signal lines 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.

[0027] Next, the structure of the semiconductor substrate constituting the pixel 101 arranged in the light-receiving region 100 provided in the distance image sensor 10 will be described. FIG. 2 is a cross-sectional view schematically showing a schematic configuration of a cross-section of an image pickup device (distance image sensor 10) in which a photoelectric conversion element according to an embodiment of the present invention is formed. 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. FIG. 2 schematically shows an example of a cross-section of the semiconductor substrate of a region corresponding to two adjacent pixels in the lateral direction (horizontal direction H) when the light-receiving region 100 of the distance image sensor 10 is viewed from the incident direction of light. The pixel 101 is configured to include at least a photodiode PD, a floating diffusion FD, and a gate electrode G.

[0028] The photodiode PD is an embedded photodiode that generates and accumulates signal charges according to the amount of incident light (light quantity). The gate electrode G is an electrode for applying from outside the pixel 101 a potential necessary for transferring the signal charges generated and accumulated by the photodiode PD to the floating diffusion FD. The gate electrode G functions as a shutter for the photodiode PD that is receiving the incident light. The floating diffusion FD is a charge storage capacitance that stores the signal charges transferred by the gate electrode G. In the distance image sensor 10, the signal charges accumulated in the floating diffusions FD provided in each of the plurality of pixels 101 are read out as pixel signals to the AD conversion circuit 500.

[0029] Note that the configuration of the distance image sensor 10 shown in FIG. 2 corresponds to the structure of a front side illumination (FSI) type image sensor when compared to a general image sensor. Therefore, the distance image sensor 10 shown in FIG. 2 includes a silicon substrate Si (substrate), which is a semiconductor substrate composed of a first conductivity type semiconductor (P-type semiconductor) that becomes the distance image sensor 10. A plurality of components including the photodiode PD that constitutes the pixel 101 are formed on the incident side where light enters the silicon substrate Si, that is, on the surface side of the silicon substrate Si.

[0030] More specifically, as shown in FIG. 2, a photodiode PD (photoelectric conversion unit) that constitutes the pixel 101 is formed from the surface of the silicon substrate Si (hereinafter referred to as the "first surface"). As shown in FIG. 2, the silicon substrate Si includes a first surface that is a surface on which reflected light is incident, a first semiconductor region composed of a first conductivity type semiconductor (P+ type semiconductor), and a second semiconductor region composed of a second conductivity type semiconductor (N type semiconductor) having a different conductivity type from the first conductivity type semiconductor. Thereby, a photodiode PD having a configuration in which electrons corresponding to the amount of incident light (light amount) are generated and accumulated as signal charges is formed. The photodiode PD having the configuration shown in FIG. 2 can be realized by doping the silicon substrate Si with an impurity that becomes a second conductivity type semiconductor (N type semiconductor) to form an N type semiconductor region (second semiconductor region), and then doping the silicon substrate Si with an impurity that becomes a first conductivity type semiconductor (P+ type semiconductor) to form a P+ type semiconductor region. That is, the second semiconductor region is formed so as 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.

[0031] Note that the floating diffusion FD shown in FIG. 2 is formed when forming the photodiode PD. In FIG. 2, the floating diffusion FD of the N+ type semiconductor is formed on the first surface of the silicon substrate Si.

[0032] Thereafter, as shown in FIG. 2, a wiring layer W including a gate electrode G is formed on the first surface of the silicon substrate Si on which the photodiode PD is formed. FIG. 2 shows a wiring layer W in which wirings are formed in four layers. Each of the plurality of wirings formed in the wiring layer W is formed of polysilicon (poly-Si) or aluminum (Al). Note that the gate electrode G shown in FIG. 2 is, for example, a polysilicon gate electrode formed of polysilicon. Also, the other wirings shown in FIG. 2 are formed of, for example, aluminum, and are wirings connected to other components that constitute the pixel 101 and circuit elements (not shown) formed in the distance image sensor 10.

[0033] Subsequently, as shown in FIG. 2, a microlens ML, which is an optical element that condenses incident light onto a photodiode PD, is formed on the surface side of the wiring layer W, that is, on the microlens layer L formed on the incident surface (the surface, the first surface) where light enters the distance image sensor 10. In the distance image sensor 10, microlenses ML are formed at positions corresponding to the photodiodes PD that constitute each of the plurality of pixels 101.

[0033] In the example shown in FIG. 2, two adjacent pixels 101, that is, the first pixel and the second pixel, are shown. Microlenses ML are formed on each of the first pixel and the second pixel. In this structure, two adjacent microlenses ML, that is, the first microlens and the second microlens, are formed. At this time, in the distance image sensor 10, as shown in FIG. 2, each of the plurality of microlenses ML is formed so that there is no gap between adjacent microlenses ML corresponding to adjacent pixels, that is, so that the so-called lens gap becomes 0. However, there is also a method of forming the microlens ML in which the lens gap cannot be made 0. When the microlens ML is formed by a method in which the lens gap cannot be made 0 in the distance image sensor 10, the height of the valley portion between two adjacent microlenses ML is made different. The height of the valley portion is the distance from the surface between the wiring layer W and the microlens ML to the portion at the lowest position in the valley portion. Specifically, when the microlens ML is cut along the first direction and the second direction (directions orthogonal to each other, the vertical direction V and the horizontal direction H) in which the plurality of pixels 101 in the pixel array shown in FIG. 1 are arranged, the valley height between two adjacent microlenses ML is set to a height H1 (the first height). On one hand, when the microlens ML is cut along the direction inclined 45° with respect to the first direction and the second direction in which a plurality of pixels 101 are arranged, that is, along the diagonal direction of the pixel 101, the height of the valley portion between two adjacent microlenses ML is set as height H2 (the second height). In this case, the heights H1 and H2 are made different from each other, and the height H1 is made higher than the height H2.

[0034] The above-described configuration regarding the pixel 101 can be said to be the same as that of a general surface-irradiation type image sensor. That is, the pixel 101 can be manufactured by the same process as that of a general surface-irradiation type image sensor. However, in the distance image sensor 10, the light incident on the photodiode PD constituting each of the plurality of pixels 101 is light in the near-infrared wavelength band with a long wavelength. For this reason, in the distance image sensor 10, the light in the near-infrared wavelength band incident on each of the plurality of pixels 101 reaches a deep position in the silicon substrate Si, that is, a position where the optical axis direction distance in the light in the near-infrared wavelength band collected by the microlens ML is long (far). Therefore, in the distance image sensor 10, in the photodiode PD constituting each of the plurality of pixels 101, electrons corresponding to the light in the near-infrared wavelength band are generated even in the deep position region.

[0035] FIG. 3 is a cross-sectional view of the pixel 101 showing a state in which electrons corresponding to the light (light in the near-infrared wavelength band) incident on the imaging element (distance image sensor 10) in which the photoelectric conversion element (pixel 101) of the embodiment of the present invention is formed are generated. FIG. 3 schematically shows a state in which electrons e− corresponding to the light in the near-infrared wavelength band are generated inside the silicon substrate Si when parallel light (so-called collimated light) in the near-infrared wavelength band is incident on one pixel 101 arranged in the distance image sensor 10.

[0036] In the distance image sensor 10, in each of the plurality of pixels 101, as shown in FIG. 3, electrons e- are generated in a region at a deep position in the silicon substrate Si in response to light in the near-infrared wavelength band. Therefore, in the distance image sensor 10, the structure of the photodiode PD constituting the pixel 101 is made into a structure suitable for light in the near-infrared wavelength band. More specifically, an N-type semiconductor region that spreads 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 deeper than 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 spread from the surface toward the inside of the silicon substrate Si. FIG. 3 shows a state in which the range R1 in which the N-type semiconductor region spreads is expanded in the thickness direction D to a range R2. In FIG. 3, the range R2 of the N-type semiconductor region is shown as also spreading in the lateral direction (horizontal direction X). As a result, 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 deep position in the silicon substrate Si by the photodiode PD in response to light in the near-infrared wavelength band incident on the pixel to the floating diffusion FD. As a result, in the distance image sensor 10, the sensitivity of each pixel 101 to light in the near-infrared wavelength band can be improved. That is, in the distance image sensor 10, a pixel signal having a larger value can be output.

[0037] Therefore, in the distance image sensor 10, the control of doping impurities that become the N-type semiconductor when forming the photodiode PD constituting each of the plurality of pixels 101 is made different from the control of a general surface-irradiation type image sensor.

[0038] <Concept> Here, the concept of forming the photodiode PD that constitutes each of the plurality of pixels 101 in the distance image sensor 10 will be described. More specifically, the concept of controlling the doping of impurities that become N-type semiconductors when forming the photodiode PD that constitutes each of the plurality of pixels 101 in the distance image sensor 10 will be described. FIG. 4 is a cross-sectional view of the pixel 101 schematically showing the concept of forming the photoelectric conversion element (pixel 101) according to the embodiment of the present invention.

[0039] In the concept of forming the photodiode PD in the distance image sensor 10, based on the ratio B / A between the light intensity A in a predetermined region (first region) on the first surface where light is incident on the silicon substrate Si and the light intensity B in a predetermined region (second region) at a predetermined distance in the thickness direction D (depth direction of the silicon substrate Si) from the first surface, the doping of impurities that become N-type semiconductors is controlled. Specifically, the incident energy (intensity) of the reflected light in a region (second region) translated parallel by a predetermined distance z in the thickness direction of the silicon substrate Si from the predetermined region (first region) is defined as B(z). Then, based on the ratio B(z) / A between the light intensity A and the light intensity B(z), the doping of impurities that become N-type semiconductors is controlled. That is, in the concept of forming the photodiode PD in the distance image sensor 10, based on the attenuation rate of the light incident on the photodiode PD, the depth of the N-type semiconductor region in the photodiode PD is controlled. Furthermore, the "predetermined region" is a region obtained by vertically projecting the second semiconductor region onto the first surface.

[0040] More specifically, as shown in FIG. 4, when the incident energy of the reflected light incident on the photodiode PD is 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 α(λ), the incident energy of the reflected light in a predetermined region on the first surface is A, and the incident energy of the reflected light in a predetermined region that is separated from the first surface by a predetermined distance z in the thickness direction D of the silicon substrate Si is B(z). Then, when a predetermined distance z0 is defined in the thickness direction D of the silicon substrate Si as shown in the following formula (1), the doping of the impurity that becomes an N-type semiconductor is controlled so that the relational expressions of the following formulas (2) and (3) are satisfied.

[0041] z0 = ln(2) / α(λ) ···(1)

[0042] A ≧ 0.5*I ···(2)

[0043] ∀z[0 ≦ z ≦ z0 ⇒ 0.95*exp(-α(λ)*z) ≦ B(z) / A ≦ 1.05*exp(-α(λ)*z)] ···(3)

[0044] Note that the average wavelength of the light source is defined by the following formula (4).

Number

[0045] Next, the above-described conditions will be explained. In FIG. 4, the light of the incident energy I incident on the pixel 101 is condensed by the microlens ML, passes through the wiring layer W, and is incident on the silicon substrate Si. Here, let the incident energy of the light incident on a predetermined region 110a on the first surface FS of the silicon substrate Si be A. In FIG. 4, the predetermined region 110a is defined as a region obtained by vertically projecting an opening (light transmission region) of the wiring Wi formed in the wiring layer W onto the silicon substrate Si.

[0046] Let the incident energy in a predetermined region 110b inside the silicon substrate Si at a distance z from the first surface FS in the thickness direction D (depth direction) be B(z). At this time, the thicknesses of the microlens ML and the wiring layer W and the width of the wiring Wi are determined so as to satisfy the above equation (2). 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 equation (2). Therefore, the microlens ML needs to be set to an appropriate thickness (aspect ratio) that satisfies the above equation (2).

[0047] Here, let the absorption coefficient of the silicon substrate Si when the average wavelength of a light source (not shown) is λ be α(λ). At this time, when the distance z0 is defined as in the above equation (1), the above equation (3) is satisfied. Note that the distance z0 is the solution of exp(-α(λ)*z)=0.5 and represents the depth at which the light incident perpendicularly to the surface of the silicon substrate Si is attenuated by half. Therefore, the incident energy in a 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).

[0048] FIG. 5 is a graph showing the attenuation of the incident energy of light (light in the near-infrared wavelength band) incident on the imaging device (distance image sensor 10) on which the photoelectric conversion element (pixel 101) of the embodiment of the present invention is formed. When considering the absorption of the incident light 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 a predetermined region. In this case, the incident energy B(z) / A draws a graph of an exponential function as shown by the solid line (a) in FIG. 5.

[0049] A semiconductor region formed of a second-conductivity-type semiconductor (N-type semiconductor) formed on a silicon substrate Si, which is a semiconductor substrate made of a first-conductivity-type semiconductor (P-type semiconductor), is generally formed in a region including predetermined regions 110a to 110c. That is, a high built-in electric field is formed in the predetermined regions 110a to 110c by 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 transfer electrons at high speed after photoelectric conversion, it is desirable to design the photoelectric conversion element (pixel 101) so that the incident energy B(z) / A, at which the amount of light incident on the predetermined regions 110a to 110c is maximized, lies on the solid line (a) shown in FIG. 5. However, the solid line (a) shown in FIG. 5 represents an ideal case. Due to variations in dimensions such as the thickness of the microlens ML and wiring layer W and the width of the wiring Wi that make up the pixel 101, and variations in optical constants in the microlens ML, the characteristics of the actual photodiode PD rarely match exactly the characteristics of the solid line (a), which are the characteristics obtained by simulation. For this reason, for example, as shown in FIG. 5, it is assumed that an error of 5% occurs between the characteristics of the actual photodiode PD and the characteristics of the photodiode PD obtained by simulation. Then, the incident energy B(z) / A = 0.95*exp(-α(λ)*z), shown by the solid line (a') in FIG. 5, is taken as the allowable lower limit, and the incident energy B(z) / A = 1.05*exp(-α(λ)*z), shown by the solid line (a'') in FIG. 5, is taken as the allowable upper limit. That is, the above equation (3) is an equation assuming that an error of 5% occurs between the characteristics of the actual photodiode PD and the characteristics of the photodiode PD obtained by simulation.

[0050] 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 precisely, this theoretical value is the numerical value in the case of parallel light rays in which the incident light in the predetermined regions 110a to 110c is incident perpendicularly to the pixel 101. Therefore, when the light condensed by the microlens ML such as the pixel 101 is used as the incident light, the incident energy B(z) / A becomes smaller than exp(-α(λ)*z). However, the difference in the theoretical value due to the above reasons is slight. For this reason, when obtaining the characteristics of the photodiode PD by simulation, it is considered that even if the difference in the incident energy B(z) / A due to the difference in the incident light rays is ignored, there is no influence on the characteristics of the photodiode PD to be obtained.

[0051] Note that in each photodiode PD, the incident energy B(z) in the region 110b, which is at a distance z from the first surface FS in the thickness direction D (depth direction), is made equal to the incident energy B(z0). In other words, in the photodiode PD, the depth of the region 110b is set to the depth at which the light incident perpendicularly to the surface of the silicon substrate Si is attenuated by half. That is, the distance z is made equal to the distance z0 (the distance z = the distance z0). In this case, the above equation (3) can be expressed as the following equation (5) by substituting the above equation (1), that is, exp(-α(λ)*z0)=0.5.

[0052] ∀z[z = z0 ⇒ 0.475 ≦ B(ln(2) / α(λ)) / A ≦ 0.525] ···(5)

[0053] Therefore, when forming the photodiode PD that constitutes each pixel 101, the doping of the impurity that becomes the N-type semiconductor is controlled so that the relational expressions of the above equations (2) and (5) are satisfied.

[0054] With such a concept, in the distance image sensor 10, an N-type semiconductor region is formed in the photodiode PD that constitutes each of the plurality of pixels 101. As a result, in the distance image sensor 10, in particular, the depth of the N-type semiconductor region that constitutes the photodiode PD in each of the plurality of pixels 101 is deeper than the depth of the N-type semiconductor region in the photodiode formed in a general surface-irradiation type image sensor. Thereby, in the distance image sensor 10, it is possible to improve the sensitivity of the photodiode PD that constitutes each of the plurality of pixels 101 to light in the near-infrared wavelength band.

[0055] <Aspect ratio of the microlens ML> Also, as described above, in the distance image sensor 10, the light in the near-infrared wavelength band incident on each of the plurality of pixels 101 is made to reach the N-type semiconductor region formed to a deep position in the silicon substrate Si. That is, in order to generate electrons corresponding to the light in the near-infrared wavelength band in the N-type semiconductor region, the thickness (aspect ratio) of the microlens ML is set so as to satisfy the above formula (2). The distance (depth) in the optical axis direction where the light in the near-infrared wavelength band collected by the microlens ML reaches can be confirmed by simulation. Then, the aspect ratio of the microlens ML formed in each pixel 101 in the distance image sensor 10 can be determined to be an appropriate thickness (aspect ratio) that satisfies the above formula (2) by performing a simulation of a general optical lens.

[0056] In the simulation of the optical lens, various parameters are set, such as the structure of pixel 101, the shape of microlens ML, and the optical properties of the materials forming pixel 101 and microlens ML respectively. Examples of the parameters related to the structure of pixel 101 and the shape of microlens ML set in the simulation of the optical lens include, for example, the pixel size of pixel 101, the height of microlens ML, and the thickness of wiring layer W in pixel 101. Further, examples of the parameters related to the properties of the materials set in the simulation of the optical lens include, for example, the refractive index and attenuation coefficient of light in each of the materials such as microlens ML, wiring layer W, and wiring Wi with respect to light. By setting these parameters and performing the simulation of the optical lens, the change in the intensity of incident light (light in the near-infrared wavelength band) in the thickness direction (depth direction) of silicon substrate Si can be confirmed, and the aspect ratio of microlens ML can be determined.

[0057] Here, an example of the simulation when the thickness (aspect ratio) of microlens ML is changed will be described. First, each parameter for performing the simulation will be described. FIGS. 6, 7A, and 7B are diagrams for explaining the parameters of the simulation performed when forming microlens ML on the imaging device (distance image sensor 10) forming the photoelectric conversion element (pixel 101) of the embodiment of the present invention. FIGS. 6, 7A, and 7B show one pixel 101 arranged in distance image sensor 10. And FIG. 6 shows a top view of pixel 101 as seen from the incident direction of light. Further, FIGS. 7A and 7B show cross-sectional views of pixel 101 shown in FIG. 6 as seen from the side. More specifically, FIG. 7A shows a cross-sectional view of the A-A' cross-section in the top view of pixel 101 shown in FIG. 6. FIG. 7B shows a cross-sectional view of the B-B' cross-section in the top view of pixel 101 shown in FIG. 6.

[0058] FIG. 6, FIG. 7A, and FIG. 7B are examples of pixel 101 with a pixel size of 16 μm square and an opening of 8.5 μm square. In FIGS. 6, 7A, and 7B, for ease of simulation, the region other than the opening shows a case where aluminum (Al) exists as a mass in the entire depth direction and wiring Wi is present. Consider a case where a microlens ML with a diameter of 20 μm (φ20 μm) is formed in pixel 101 having such a configuration. In pixel 101, as shown in FIGS. 7A and 7B, a planarization layer FL is formed on the surface side of wiring layer W where wiring Wi is formed, that is, on the surface of the wiring layer W on the incident side where light enters pixel 101, and a microlens ML is formed on the planarization layer FL, that is, on the incident side where light enters pixel 101. The planarization layer FL is a layer (underlayer) that serves as the base of microlens ML in microlens layer L and is a part of microlens ML. For this reason, although the thickness of the planarization layer FL is a constant thickness, the distance (depth) in the optical axis direction where light in the near-infrared wavelength band collected by microlens ML reaches depends on the height (thickness) in the optical axis direction obtained by combining microlens ML itself and the planarization layer FL from the surface side of the wiring layer W.

[0059] As shown in FIGS. 7A and 7B, the shape of microlens ML can be considered as a part of an ellipse. FIGS. 7A and 7B show a case where a part of an ellipse with φ20 μm, that is, a short axis of 20 μm and a long axis of 26 μm, is the shape of microlens ML. In FIGS. 7A and 7B, the value of half the length of the short axis (short radius) of 10 μm and half the length of the long axis (long radius) of 13 μm in the ellipse that can be considered as microlens ML is shown. The height of microlens ML is the height from the surface side surface where light enters the planarization layer FL. And in FIGS. 7A and 7B, the short axis of the ellipse is along the surface side surface of the planarization layer FL. In the case of pixel 101 shown in FIGS. 7A and 7B, the height of microlens ML is the length of the long radius, that is, 13 μm. And the aspect ratio of microlens ML can be calculated by the following formula (6) from the respective values representing the ellipse.

[0060] Aspect ratio of microlens ML = Height of the microlens ML / Diameter of the microlens ML = Major axis radius / Minor axis = 13 μm / 20 μm = 0.65 ···(6)

[0061] In addition, when forming a φ20 μm microlens ML on a 16 μm square pixel 101, as shown in FIG. 6, a partial region of the microlens ML protrudes on the sides in the vertical direction (the up and down direction in FIG. 6) and the horizontal direction (the left and right direction in FIG. 6) of the pixel 101. A partial region that protrudes from the region of the pixel 101 in the microlens ML will overlap with a partial region that protrudes from the region of the pixel 101 in the microlens ML formed on an adjacent pixel 101. Also, as shown in FIG. 6, in the diagonal direction of the pixel 101 (the 45° diagonal direction in FIG. 6), there is a region where the microlens ML is not formed on a part of the pixel 101. This region where the microlens ML is not formed consists only of the planarization layer FL. In FIGS. 7A and 7B, the height (thickness) of the planarization layer FL serving as the base of the microlens ML in the optical axis direction is set to 2 μm. Note that the thickness of the planarization layer FL is a constant thickness (here, 2 μm) regardless of the height of the microlens ML. Also, in FIGS. 7A and 7B, the thickness of the wiring layer W constituting the pixel 101, that is, the height of the region other than the opening, is set to 3 μm. Note that the microlens ML may be formed such that the region overlapping with a partial region of the microlens ML formed on an adjacent pixel 101 is increased so that there is no region where the microlens ML is not formed. That is, the diameter of the microlens ML may be increased so that there is no region where only the planarization layer FL is formed.

[0062] In the pixel 101 formed with the microlens ML having such a configuration, an example of the result of simulating the change in the intensity of incident near-infrared light in the thickness direction (depth direction) of the silicon substrate Si will be described. FIG. 8 is a graph showing an example of the result of simulating the attenuation of the incident energy of light (light in the near-infrared wavelength band) incident on the imaging device (distance image sensor 10) formed with the photoelectric conversion element (pixel 101) according to the embodiment of the present invention. The graph of the simulation result shown in FIG. 8 is a graph showing the state in which the intensity of near-infrared light with a wavelength of 940 nm is attenuated according to the depth of the silicon substrate Si in the structure of the pixel 101 and the shape of the microlens ML shown in FIGS. 6, 7A, and 7B, and representing the change in the intensity of near-infrared light as a relative value.

[0063] In performing the simulation, the refractive index of light in the silicon substrate Si, that is, silicon (Si), is set to 3.59. The refractive index of light in the material of the microlens ML (including the planarization layer FL) is set to 1.6. The refractive index of light in aluminum (Al) formed as the wiring Wi is set to 1.66. In the pixel 101, the refractive index of light in the silicon dioxide (SiO 2 ) formed in the wiring layer W including openings and the like as an insulating material is set to 1.46. Further, in performing the simulation, the attenuation coefficient of light in silicon (Si) is set to 0.01, and the attenuation coefficient of light in aluminum is set to 8.71.

[0064] Also, FIG. 8 shows, for comparison, the cases where the height of the microlens ML is 15 μm (aspect ratio = 0.75) and 19 μm (aspect ratio = 0.95). When the height of the microlens ML itself changes with the change in 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 on the adjacent pixel 101, that is, the height (thickness) in the optical axis direction other than the planarization layer FL, changes. Also, when the height (thickness) in the optical axis direction of the microlens ML that overlaps with the region of the microlens ML formed on 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 to be simulated (the pixels adjacent in the vertical direction V and the horizontal direction H in the pixel array shown in FIG. 1) also changes. The simulation results shown in FIG. 8 are the results considering the height (thickness) in the optical axis direction of the microlens ML that overlaps with the region of the microlens ML formed on the adjacent pixel 101 and the amount of light leakage accompanying the change in the aspect ratio of the microlens ML. That is, the simulation results shown in FIG. 8 are the results reflecting each parameter that changes with the change in the height of the microlens ML itself (the aspect ratio of the microlens ML).

[0065] When the height of the microlens ML is 13 μm (aspect ratio = 0.65), as shown in Fig. 8, the intensity of near-infrared light with a wavelength of 940 nm decreases exponentially with respect to the depth of the silicon substrate Si. On the other hand, 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 in the same manner as when the height of the microlens ML is 13 μm until the depth of the silicon substrate Si reaches about 20 μm, which corresponds to the distance z0 shown in Fig. 4. Further, in this case, the intensity of the near-infrared light decreases rapidly from around when the depth of the silicon substrate Si exceeds about 20 μm. Also, when the height of the microlens ML is 19 μm (aspect ratio = 0.95), the intensity of the near-infrared light decreases rapidly from around 6 - 7 μm in the depth of the silicon substrate Si.

[0066] Here, the reason for the rapid decrease in the intensity of the near-infrared light when the height of the microlens ML is 15 μm and 19 μm is that when the aspect ratio of the microlens ML is too high, there is more near-infrared light that diffuses beyond the focal point of the microlens ML, that is, the focus position. The diffusion of the near-infrared light after passing through this focal point is a factor that reduces the efficiency of electron generation in the N-type semiconductor region formed to a deep position in the silicon substrate Si, that is, reduces the sensitivity of the photodiode PD to light in the near-infrared wavelength band. For this reason, in an example of the simulation results shown in Fig. 8, it can be said that the appropriate thickness (aspect ratio) of the microlens ML in the structure of the pixel 101 shown in Figs. 6, 7A, and 7B is a height of 13 μm (aspect ratio = 0.65).

[0067] Note that when actually determining the thickness (aspect ratio) of the microlens ML that satisfies the above formula (2), it is conceivable to increase the accuracy of each parameter set in the simulation. However, as shown in Fig. 8, even when the simulation parameters are set simply, it is possible to confirm the state in which the intensity of the near-infrared light decreases with respect to the depth of the silicon substrate Si, that is, the change in intensity.

[0068] Next, for comparison, an example of the difference in the change in the intensity of light in the thickness direction (depth direction) of the silicon substrate Si due to the difference in the height between the microlens formed on the pixel of a general surface-irradiation type image sensor and the microlens ML formed on the pixel 101 of the distance image sensor 10 of the present embodiment will be described. FIGS. 9A and 8B are graphs showing an example of the result of a simulation for comparing the attenuation of the incident energy of light incident on the imaging device. The graphs of the simulation results shown in FIGS. 9A and 8B show the simulation result shown in FIG. 8 and the simulation result when the height of the microlens ML is set to the height of the microlens formed on the pixel of a general image sensor. That is, the simulation results shown in FIGS. 9A and 8B 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. FIG. 9A shows a graph representing the change in the intensity of near-infrared light with respect to the depth of the silicon substrate Si as a relative value, and FIG. 9B shows a graph representing the change in the intensity of near-infrared light with respect to the depth of the silicon substrate Si as an absolute value.

[0069] In the simulation results shown in FIGS. 9A and 9B, the height of the microlens ML formed on the pixel 101 is 13 μm (aspect ratio = 0.65), and the height of the microlens formed on the pixel of a general image sensor is 3 μm (aspect ratio = 0.15). Note that the parameters other than the height of the microlens ML in the simulation results shown in FIGS. 9A and 9B are the same as those in the simulation result shown in FIG. 8.

[0070] As shown in FIG. 9A, when the change in the intensity of near-infrared light is represented as a relative value, the change in the intensity of near-infrared light with respect to the depth of the silicon substrate Si is similar between the case where the height of the microlens ML is 13 μm (aspect ratio = 0.65) and the case where the height of the microlens is 3 μm (aspect ratio = 0.15). That is, 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 exponentially in the same manner with respect to the depth of the silicon substrate Si.

[0071] However, as shown in FIG. 9B, when the change in the intensity of the near-infrared light is represented by the absolute value, the intensity of the near-infrared light when the height of the microlens is 3 μm is overall lower than the intensity of the 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 the pixel 101 is set to 100%, the intensity of the near-infrared light on the surface (the first surface) of the silicon substrate Si where the light is incident is 70% or more when the height of the microlens ML is 13 μm, whereas it is 40% when the height of the microlens is 3 μm. The reason for this is that when the height of the microlens is 3 μm, due to the low aspect ratio of the microlens ML, the light condensing by the microlens ML is weak (the condensing characteristics are low), and a large amount of near-infrared light is attenuated in the region of the wiring layer W that transmits until it reaches the first surface of the silicon substrate Si. For this reason, 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 less than that when the height of the microlens is 13 μm, and the intensity is correspondingly lower.

[0072] As described above, in the pixel 101, it is necessary to form the N-type semiconductor region of the photodiode PD to a deep position in the silicon substrate Si and determine the thickness (aspect ratio) of the microlens ML so as to satisfy the above formula (2). That is, in the 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 50% or more of the incident energy I of the light incident on the pixel 101. For this reason, from the simulation results shown in FIGS. 9A and 9B, it can be confirmed that the microlens ML with a small aspect ratio having a height of 3 μm formed in the pixel of a general image sensor does not satisfy the above formula (2) and is not a microlens ML with an appropriate thickness (aspect ratio) in the structure of the pixel 101.

[0073] In this way, in the distance image sensor 10, it is possible to confirm by simulation whether the thickness of the microlens ML is appropriate (aspect ratio) for the pixel 101. That is, in the distance image sensor 10, for each of the plurality of pixels 101, it can be confirmed whether the microlens ML formed on each of them has an appropriate thickness (aspect ratio) to improve the sensitivity to light in the near-infrared wavelength band in the photodiode PD in which the depth of the N-type semiconductor region is deeper than the depth of the N-type semiconductor region in the photodiode formed in a general surface-irradiation type image sensor.

[0074] In this way, in the distance image sensor 10, the sensitivity to light in the near-infrared wavelength band in each of the plurality of pixels 101 can be improved by the depth of the N-type semiconductor region constituting the photodiode PD and the appropriate thickness (aspect ratio) of the microlens ML.

[0075] Thereby, in the imaging system according to the embodiment of the present invention equipped with the distance image sensor 10, the measurement of the distance to an object using the time-of-flight (TOF) technology can be performed with higher accuracy. Here, the imaging system according to the embodiment of the present invention will be described.

[0076] FIG. 10 is a block diagram showing a schematic configuration of an imaging system according to an embodiment of the present invention equipped with an imaging device (distance image sensor 10) according to the embodiment of the present invention. The TOF sensor module 1, which is the imaging system according to the embodiment of the present invention shown in FIG. 10, includes a light source unit 2 and a light receiving unit 3. The light source unit 2 includes a light source device 21 and a diffusion plate 22. The light receiving unit 3 includes a distance image sensor 10 and a lens 31. Note that FIG. 10 also shows an object O whose distance is measured in the TOF sensor module 1, which is the imaging system according to the embodiment of the present invention.

[0077] In the TOF sensor module 1 having the configuration shown in FIG. 10, the light source unit 2 irradiates the object O with an optical pulse PL in the near-infrared wavelength band from the light source unit 2. Then, in the TOF sensor module 1, the light receiving unit 3 receives the reflected light RL of the optical pulse PL reflected by the object O and outputs a signal (hereinafter referred to as a "measurement signal") for measuring the distance to the object O.

[0078] The light source unit 2 irradiates the object O, which is the object to be measured for distance in the TOF sensor module 1, with the optical pulse PL. The light source unit 2 is, for example, a surface-emitting 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 wavelength band (for example, a wavelength band with a wavelength of 850 nm to 940 nm) that becomes the optical pulse PL for irradiating the object O. The light source device 21 is, for example, a semiconductor laser emitting element. The light source device 21 emits pulsed laser light in response to 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 wavelength band emitted by the light source device 21 to the area of the surface for irradiating the object O. The pulsed laser light diffused by the diffuser plate 22 is emitted from the light source unit 2 as the optical pulse PL and irradiates the object O.

[0079] The light receiving unit 3 receives the reflected light RL of the optical pulse PL reflected by the object O, which is the object to be measured for distance 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 it to be received (incident) on the entire light receiving area 100 of the distance image sensor 10, that is, on each of the plurality of pixels 101 arranged in the light receiving area 100.

[0080] With such a configuration, in the TOF sensor module 1, the light receiving unit 3 receives the reflected light RL of the light pulse PL in the near-infrared wavelength band irradiated by the light source unit 2 onto the object O, and the distance image sensor 10 provided in the light receiving unit 3 outputs a measurement signal for measuring the distance to the object O.

[0081] 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 by, for example, 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 onto the object O and the timing at which the distance image sensor 10 provided in the light receiving unit 3 receives the reflected light RL are controlled by the module control unit (not shown). Further, the measurement signal output by the TOF sensor module 1 (more specifically, the distance image sensor 10) is processed by, for example, a distance image processing unit (not shown) provided outside or inside the TOF sensor module 1, and a two-dimensional image including the object O and information on the distance to the object O are generated. Note that the distance image processing unit (not shown) may generate, for example, a two-dimensional image (distance image) including the object O in which the information on the distance to the object O is color-coded and shown.

[0082] As described above, according to the embodiment of the present invention, in the silicon substrate serving as the image sensor (distance image sensor) of the present invention, the structure of the photoelectric conversion element (pixel) of the present invention that forms the pixel disposed in the light receiving region is made suitable for light in the near-infrared wavelength band. More specifically, when forming the photoelectric conversion element, the depth (thickness) to which the N-type semiconductor region constituting the photoelectric conversion element spreads in the silicon substrate is formed to be deeper than the N-type semiconductor region in the photoelectric conversion element formed in the pixel of a general surface-irradiation type image sensor. Thereby, in the image sensor of the embodiment of the present invention, the sensitivity to light in the near-infrared wavelength band can be improved in the photoelectric conversion elements each constituting a plurality of pixels. That is, the image sensor of the embodiment of the present invention can output a signal that more accurately represents the amount (light amount) of the incident light in the near-infrared wavelength band.

[0083] In addition, in the embodiments of the present invention, the imaging system of the present invention (TOF sensor module 1) equipped with the imaging device of the present invention outputs a signal that more accurately represents the amount of light (light quantity) in the near-infrared wavelength band output by the imaging device. As a result, in the imaging system of the present invention equipped with the imaging device of the present invention, it is possible to output a measurement signal capable of measuring the distance to an object with higher accuracy using time-of-flight (TOF) technology.

[0084] In the embodiments of the present invention, the case where the imaging device of the present invention has a structure corresponding to a surface-irradiation type image sensor has been described. However, the structure of the imaging device of the present invention is not limited to the structure corresponding to the surface-irradiation type image sensor shown in the embodiments of the present invention. That is, in general image sensors, in addition to the surface-irradiation type image sensor, there is also a backside illumination (BSI) type image sensor. Therefore, the structure of the imaging device of the present invention can also be a structure corresponding to a backside illumination type image sensor. Even when the structure of the imaging device of the present invention is a structure corresponding to 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 embodiments of the present invention. And the structure of the imaging device of the present invention in this case can be easily considered from the structure of a general backside illumination type image sensor. For this reason, a detailed description in the case where the imaging device of the present invention has a structure corresponding to a backside illumination type image sensor is omitted.

[0085] In addition, in the embodiment of the present invention, the configuration of the pixel disposed in the light-receiving region of the imaging device of the present invention has been described in the case where the signal charge generated and accumulated by the photoelectric conversion element of the present invention is transferred and accumulated by a set of one gate electrode G and one floating diffusion FD. However, the set of the gate electrode G and the floating diffusion FD provided in the pixel disposed in the light-receiving region of the imaging device of the present invention is not limited to the one set shown in the embodiment of the present invention. That is, the pixel disposed in the light-receiving region of the imaging device of the present invention can also be configured to include two or more sets of the gate electrode G and the floating diffusion FD. As a result, in the imaging device of the present invention in which pixels including two or more sets of the gate electrode G and the floating diffusion FD are disposed, the signal charge generated and accumulated by the photoelectric conversion element of the present invention can be distributed and transferred to and accumulated in the respective floating diffusions FD. That is, in the imaging device of the present invention in which pixels including two or more sets of the gate electrode G and the floating diffusion FD are disposed, the highly sensitive signal charge generated and accumulated by the photoelectric conversion element of the present invention can be more effectively utilized. As a result, in the imaging system of the present invention equipped with the imaging device of the present invention in which pixels including two or more sets of the gate electrode G and the floating diffusion FD are disposed, the accuracy of distance measurement using the time-of-flight (TOF) technology can be further improved.

[0086] In addition, in the embodiment of the present invention, the case where the photoelectric conversion element of the present invention that constitutes the pixel disposed in the light-receiving region in the imaging device of the present invention is a photoelectric conversion element that generates and accumulates electrons corresponding to the amount of incident light (light amount) as signal charges has been described. 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 embodiment of the present invention. That is, the photoelectric conversion element that constitutes the pixel disposed in a general image sensor is not only in the form of using electrons as signal charges, but also includes a photoelectric conversion element that generates and accumulates holes (so-called holes) corresponding to the amount of incident light (light amount) as signal charges. Therefore, the photoelectric conversion element of the present invention can also be in the form of generating and accumulating holes as signal charges. Note that even when the photoelectric conversion element of the present invention is in the form of generating and accumulating holes as signal charges, 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 photoelectric conversion element of the present invention in this case can be easily considered by replacing electrons with holes in the description of the embodiment of the present invention, including the conductivity type of the semiconductor in the silicon substrate Si and the photodiode PD. For this reason, a detailed description in the case where the photoelectric conversion element of the present invention is in the form of generating and accumulating holes as signal charges is omitted.

[0087] As described above, the embodiments of the present invention have been described with reference to the 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.

Description of Reference Numerals

[0088] 1 ··· TOF sensor module 2 ··· Light source unit 21 ··· Light source device 22 ··· Diffusion plate 3 ··· Light-receiving unit 31 ··· Lens 10 ··· Distance image sensor 101 ··· Pixel (photoelectric conversion element) Si ··· Silicon substrate PD... Photodiode (photoelectric conversion section) FD... Floating diffusion G... Gate electrode W... Wiring layer Wi... Wiring FS... First surface L... Microlens layer ML... Microlens FL... Flattening layer e-... Electron PL... Light pulse RL... Reflected light O... Object 110a, 110b, 110c... Region 120a, 120b... Side surface I, A, B(z), B(z0)... Incident energy

Claims

A photoelectric conversion element that receives reflected light reflected by an object using a light source that emits light in the near-infrared wavelength band, a first surface that is the surface on which the reflected light is incident, a first semiconductor region formed of a first-conductivity-type semiconductor, and a second-conductivity-type semiconductor having a conductivity type different from that of the first-conductivity-type semiconductor and formed so as to spread inward from the first surface in a direction perpendicular to the first surface, and a substrate having a second semiconductor region, an optical element disposed on the first surface side of the substrate for condensing the reflected light onto the second semiconductor region, having, when the incident energy of the reflected light incident on the photoelectric conversion element is I, the absorption coefficient of the reflected light in the substrate when the average wavelength of the light source is λ is α(λ), the incident energy of the reflected light in a predetermined region on the first surface is 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 A2, and the incident energy of the reflected light at a position parallel to the thickness direction of the substrate by a predetermined distance z from the predetermined region within the second semiconductor region is B(z), A1≥A2, and A1≥0.7*I are satisfied, and when the distance z0 = ln(2) / α(λ), for all z satisfying 0≤z≤z0, the thickness of the optical element is set so that the relational expression 0.95*exp(−α(λ)*z)≤B(z) / A1≤1.05*exp(−α(λ)*z) holds, a photoelectric conversion element.

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

3. The near-infrared wavelength band is a wavelength band of 850 nm to 940 nm, The photoelectric conversion element according to claim 1. An imaging element that receives reflected light reflected by an object using a light source that emits light in the near-infrared wavelength band, comprising the photoelectric conversion element according to any one of claims 1 to 3, having a light-receiving region in which a plurality of pixels are arranged in a two-dimensional matrix, in the light-receiving region, a plurality of pixels are arranged along a first direction and a second direction orthogonal to each other, when the optical element is cut along the first direction and the second direction, the height of the valley portion of two adjacent optical elements is defined as the first height, When the optical element is cut along the diagonal direction of the pixel, the height of the valleys of two adjacent optical elements is defined as the second height, and the first height and the second height are different from each other, an imaging device.

5. A light source unit that emits light in the near-infrared wavelength band, The imaging device according to claim 4, A light receiving unit that receives the reflected light reflected by the object from the light, having an imaging system.

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