light-receiving element
The photodetector design with a metalens and aperture region addresses the limitation of conventional metalenses by focusing light closer to the incident surface, enhancing sensitivity and resolution.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- HAMAMATSU PHOTONICS KK
- Filing Date
- 2022-05-09
- Publication Date
- 2026-06-17
Smart Images
Figure 0007875021000009 
Figure 0007875021000010 
Figure 0007875021000011
Abstract
Description
Technical Field
[0001] This disclosure relates to a light-receiving element.
Background Art
[0002] Conventionally, as disclosed in Non-Patent Document 1, a metalens composed of a plurality of fine unit structures (pillars) arranged in a lattice pattern is known.
Prior Art Documents
Non-Patent Documents
[0003]
Non-Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] The metalens described above may be placed, for example, on the surface (light incident surface) of a photodetector (photosensor) to increase the photodetector's sensitivity. By placing a metalens on the surface of the photodetector, it is expected that the light to be detected can be efficiently focused into the light-receiving region (effective sensitivity region). However, from the viewpoint of effectively increasing the photodetector's sensitivity, it is sometimes desirable to focus the light to be detected as close to the surface as possible on the photodetector's light-detecting substrate (silicon substrate, etc.). However, when using conventional metalens such as those disclosed in Non-Patent Document 1, the focal length could only be shortened to the extent possible by phase design under structural constraints, such as the fact that the metalens is composed of a periodic structure of multiple unit structures.
[0005] Therefore, one aspect of this disclosure aims to provide a light-receiving element that can focus light to a position closer to the light incident surface than the focal position based on the phase design of the metalens. [Means for solving the problem]
[0006] This disclosure includes the following photodetectors [1] to
[10] .
[0007] [1] A photodetector comprising: a photodetector substrate having at least one light-receiving region and a light incident surface into which light to be detected is incident; and a metalens composed of a plurality of unit structures arranged in a grid and positioned on the light incident surface to focus the light to be detected, wherein, when viewed from the thickness direction of the photodetector substrate, an aperture region in which no unit structures are formed is provided in the region including the center of the metalens.
[0008] The above-described photodetector can simultaneously generate diffraction by a metalens (multiple unit structures) and diffraction by an aperture region (aperture diffraction). By generating these two types of diffraction simultaneously, it becomes possible to form a focal point closer to the incident light plane than the focal point when only diffraction by a metalens occurs (i.e., the focal point set by the multiple unit structures constituting the metalens). In other words, by providing an aperture region, the above-described photodetector can focus light to a position closer to the incident light plane than the focal point based on the phase design of the metalens.
[0009] [2] The photodetector according to [1], further comprising a dielectric layer disposed between the photodetector substrate and the metalens, the dielectric layer having a refractive index lower than that of the photodetector substrate. The above photodetector can improve the focusing effect on the focal point formed at a position closer to the incident light surface than the focal point based on the phase design of the metalens.
[0010] [3] The numerical aperture NA of the metalens is set to satisfy the following equation, where λ eff The photodetector described in [1] or [2], wherein is the effective wavelength of the light to be detected after passing through the metalens, and P is the period in which multiple unit structures are arranged. With the above photodetector, it is possible to more reliably form a focal point closer to the incident light plane than the focal point based on the phase design of the metalens.
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[0011] [4] The photodetector described in any of [1] to [3], wherein the aperture region is rectangular when viewed from the thickness direction. The above photodetector makes it possible to improve the focusing effect on the focal point formed closer to the incident light surface than the focal point based on the phase design of the metalens, compared to the case where the aperture region is a shape other than rectangular (e.g., circular).
[0012] [5] A photodetector according to any one of [1] to [4], wherein multiple unit structures are configured such that the phase distribution of the metalens follows a quadratic phase pattern. The above photodetector can improve the focusing effect on the focal point formed closer to the incident light plane than the focal point based on the phase design of the metalens, compared to the case where a pattern other than a quadratic phase pattern (e.g., a Fresnel pattern) is applied to the phase distribution of the metalens.
[0013] [6] The photodetector substrate is a photodetector according to any one of [1] to [5], having an avalanche photodiode. With the above photodetector, the photodetector can effectively improve the light detection sensitivity of the avalanche photodiode by forming a focal point closer to the incident light surface than the focal point based on the phase design of the metalens (i.e., as close as possible to the interface of the PN junction).
[0014] [7] When viewed from the thickness direction, the metalens is formed to overlap with both the adjacent region adjacent to the light-receiving region and the peripheral region which is the region inside the light-receiving region along the boundary between the light-receiving region and the adjacent region, as described in any of [1] to [6]. With the above light-receiving element, even when the light-receiving region is small, the light to be detected can be reliably focused into the light-receiving region by forming a metalens (unit structure) in the part that overlaps with both the adjacent region and the peripheral region.
[0015] [8] The photodetector according to any one of [1] to [7], wherein the metalens is formed such that the first distance is 40% or more and 90% or less of the second distance, the second distance being the distance in the thickness direction from the metalens to the focal point based on the phase design of the metalens, and the first distance being the distance in the thickness direction from the metalens to the focal point formed at a position closer to the light incident surface than the focal point due to the provision of an aperture region.
[0016] [9] The photodetector according to [8], wherein the metalens is formed such that the first distance is 50% or more and 80% or less of the second distance.
[0017]
[10] A photodetector according to any of [1] to [9], wherein the width of the aperture region, when viewed from the thickness direction, is 1 / 2 or more of the width of the metalens. With the above photodetector, by making the width of the aperture region sufficiently large relative to the width of the metalens, it is possible to more reliably form a focal point closer to the light incident surface than the focal point based on the phase design of the metalens. [Effects of the Invention]
[0018] According to one aspect of this disclosure, it is possible to provide a light-receiving element that can focus light to a position closer to the incident light surface than the focal position based on the phase design of the metalens. [Brief explanation of the drawing]
[0019] [Figure 1] This figure schematically shows the stacked structure of a photodetector according to one embodiment. [Figure 2] Figure 1 is a schematic plan view of the photodetector substrate shown. [Figure 3] Figure 1 is a circuit diagram of the light-receiving element. [Figure 4] Figure 1 is a plan view of a portion of the photodetector substrate shown. [Figure 5] Figure 1 is a cross-sectional view of a portion of the photodetector substrate. [Figure 6] Figure 1 is a bottom view of the photodetector substrate. [Figure 7] This is a cross-sectional view of a portion of the photodetector shown in Figure 1, corresponding to one of the light detection units. [Figure 8] Figure 7 is a schematic plan view of the metalens shown. [Figure 9] This is an enlarged view of region A1 shown in Figure 8. [Figure 10] This figure shows an example of a unit structure of a metalens. [Figure 11] This figure shows an example of the manufacturing process for metalens. [Figure 12] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the first embodiment. [Figure 13]This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the second embodiment. [Figure 14] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the third embodiment. [Figure 15] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the fourth embodiment. [Figure 16] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the comparative example. [Figure 17] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the first modified example. [Figure 18] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the second modified example. [Figure 19] This is a cross-sectional view of a portion of the photodetector corresponding to one of the light detection parts of the third modified example. [Figure 20] This figure shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the third modified example. [Modes for carrying out the invention]
[0020] Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals will be used for the same or equivalent elements, and redundant explanations will be omitted.
[0021] As shown in Figure 1, the photodetector 1 comprises a photodetector substrate 2, a lens layer 3, and a dielectric layer 4. As shown in Figure 2, the photodetector 1 comprises a plurality of photodetectors 10 arranged in two dimensions and a common electrode E3. The plurality of photodetectors 10 are arranged in a matrix. The lens layer 3 is composed of a plurality of metalens 30, each corresponding to one of the plurality of photodetectors 10. The dielectric layer 4 is positioned between the photodetector substrate 2 and the lens layer 3 (the plurality of metalens 30). For the sake of explanation, some drawings show a three-dimensional Cartesian coordinate system consisting of the X, Y, and Z axes. The X and Y axis directions correspond to the row and column directions in which the plurality of photodetectors 10 are arranged. The Z axis direction is perpendicular to the X and Y axis directions and corresponds to the thickness direction of the photodetector substrate 2.
[0022] The photodetector substrate 2 has multiple light-receiving regions LA (see Figure 7) and a light incident surface 2a into which the light to be detected L (see Figure 7) is incident. One light-receiving region LA is provided for each photodetector 10. The light to be detected L is the light to be detected by the photodetector 1. For example, the wavelength range of the light to be detected L is in the range of 300 nm to 6 μm.
[0023] As shown in Figure 2, the photodetector substrate 2, for example, has a rectangular shape when viewed from the Z-axis direction. The common electrode E3 is located in the center of the photodetector substrate 2 when viewed from the Z-axis direction. Charges generated in each photodetector 10 are collected on the common electrode E3. In other words, the photodetector substrate 2 is a SiPM (Silicon Photomultiplier) having multiple SPADs (Single Photon Avalanche Diodes) (photodetector 10). Note that in Figure 2, only some of the photodetector 10s located in the regions at both ends of the photodetector substrate 2 in the X-axis direction are shown, but multiple photodetector 10s are formed in the entire region of the photodetector substrate 2 excluding the common electrode E3. The photodetector 10, for example, has a rectangular shape when viewed from the Z-axis direction. However, the shape of the photodetector 10 when viewed from the Z-axis direction may be a shape other than rectangular (for example, circular).
[0024] As shown in Figure 3, in this embodiment, as an example, the wiring board 5 that performs signal processing is shared by a plurality of photodetectors 1 (photodetector boards 2). In addition, each photodetector board 2 has an avalanche photodiode APD and a quenching resistor R1. That is, the photodetector board 2 has an avalanche photodiode APD. One end of the quenching resistor R1 is electrically connected to the anode of the avalanche photodiode APD. The other end of the quenching resistor R1 is electrically connected to a common electrode E3 via a readout wiring TL on the photodetector board 2. In other words, the plurality of photodetectors 10 are connected in parallel, and in each photodetector board 10, the avalanche photodiode APD and the quenching resistor R1 are connected in series. In the photodetector board 2, each avalanche photodiode APD is operated in Geiger mode. In Geiger mode, a reverse voltage (reverse bias voltage) greater than the breakdown voltage of the avalanche photodiode (APD) is applied to the avalanche photodiode (APD). That is, a potential V1 is applied to the anode of the avalanche photodiode (APD), and a potential V2, which is positive to potential V1, is applied to the cathode of the avalanche photodiode (APD). The polarity of these potentials is relative; for example, either potential may be the ground potential.
[0025] The wiring board 5 is provided with a signal processing unit SP. The signal processing unit SP processes the signals output from each photodetector board 2, treating each board 2 as a channel. The signal processing unit SP constitutes, for example, an ASIC (Application Specific Integrated Circuit). The signal processing unit SP may also include a CMOS circuit that converts the signals output from each photodetector board 2 into digital pulses.
[0026] As shown in Figure 4, in the photodetector substrate 2, the readout wiring TL includes a plurality of signal lines TL1 and a plurality of signal lines TL2. For example, each signal line TL1 extends in the Y-axis direction between adjacent avalanche photodiodes APD in the X-axis direction. Each signal line TL2 extends in the X-axis direction between adjacent avalanche photodiodes APD in the Y-axis direction. The plurality of signal lines TL1 and the plurality of signal lines TL2 extend in a grid pattern so as to be connected to each other at intersections and are electrically connected to a common electrode E3.
[0027] In each photodetector 10, one end of the quenching resistor R1 is connected to electrode E1, and the other end of the quenching resistor R1 is connected to signal line TL1. In other words, in each photodetector 10, one end of the quenching resistor R1 is electrically connected to the anode of the avalanche photodiode APD via electrode E1, and the other end of the quenching resistor R1 is electrically connected to the common electrode E3 via readout wiring TL.
[0028] As shown in Figure 5, the photodetector substrate 2 has a semiconductor layer 11. The semiconductor layer 11 includes an N-type (first conductivity type) semiconductor region 12, a P-type (second conductivity type) semiconductor region 13, and a plurality of P-type semiconductor regions 14. The semiconductor region 13 is formed on the light-incident side surface of the semiconductor region 12. The plurality of semiconductor regions 14 are formed within the semiconductor region 13 along the light-incident side surface of the semiconductor region 13. The impurity concentration of each semiconductor region 14 is higher than the impurity concentration of the semiconductor region 13.
[0029] In the photodetector substrate 2, one avalanche photodiode APD is composed of one semiconductor region 14 and one of the semiconductor regions 12 and 13 that overlap with the semiconductor region 14 in the Z-axis direction (i.e., the light-receiving region LA (see Figure 7)). In other words, each avalanche photodiode APD includes an N-type semiconductor region 12 and P-type semiconductor regions 13 and 14 that form a PN junction with the N-type semiconductor region 12. The P-type semiconductor regions 13 and 14 are located on the light incident surface 2a side of the photodetector substrate 2 relative to the N-type semiconductor region 12.
[0030] An insulating layer 16 is formed on the light-incident surface of the semiconductor region 13. A common electrode E3 and a readout wiring TL are arranged on the insulating layer 16. The common electrode E3 and the readout wiring TL are covered by an insulating layer 17. In the photodetector substrate 2, the light-incident surface of the insulating layer 17 corresponds to the light-incident surface 2a. In each photodetector 10, one end of the quenching resistor R1 (see Figure 4) is electrically connected to the semiconductor regions 13 and 14 of the avalanche photodiode APD, and the other end of the quenching resistor R1 is electrically connected to the readout wiring TL.
[0031] A through-hole TH is formed in the semiconductor layer 11. An insulating layer 18 is formed on the inner surface of the through-hole TH and on the surface of the semiconductor region 12 opposite to the light incident side. A through-electrode TE is placed on the inner surface of the through-hole TH via the insulating layer 18. The through-electrode TE is connected to a common electrode E3 at the light incident side opening of the through-hole TH. A bump electrode B1 is placed on the through-electrode TE via an under-bump metal BM. The through-electrode TE and the insulating layer 18 are covered by a passivation film PF. An N-type semiconductor region 1PC is formed in the region surrounding the through-hole TH on the light incident side surface of the semiconductor region 12. The semiconductor region 1PC prevents the PN junction, which is composed of the N-type semiconductor region 12 and the P-type semiconductor regions 13 and 14, from reaching the through-hole TH.
[0032] As shown in Figure 6, the passivation film PF has grooves formed in such a way that they surround the through-holes TH when viewed from the Z-axis direction, and the semiconductor region 12 is exposed within these grooves. Multiple bump electrodes B2 are arranged on the semiconductor region 12 exposed within these grooves. Bump electrode B1 and the multiple bump electrodes B2 are electrically and physically connected to the wiring substrate 5, which is located on the side of the photodetector substrate 2 opposite to the side where the lens layer 3 is located. In other words, the photodetector substrate 2 is electrically and physically connected to the wiring substrate 5.
[0033] In the photodetector substrate 2 configured as described above, each photodetector 10 operates an avalanche photodiode APD in Geiger mode. In this state, when light is incident on the semiconductor region 12 from the light incident surface 2a, photoelectric conversion occurs in the semiconductor region 12, and photoelectrons (charges) are generated in the semiconductor region 12. In the avalanche photodiode APD where photoelectrons have been generated, avalanche multiplication occurs in the semiconductor region 13, and the amplified group of electrons (charges) are collected on the common electrode E3 via the semiconductor region 14 and the quenching resistor R1. The charges collected on the common electrode E3 from each photodetector 10 are input as signals to the signal processing unit SP (see Figure 3) of the wiring board 5.
[0034] The semiconductor layer 11 is formed of, for example, Si (silicon). In the semiconductor layer 11, P-type impurities are, for example, Group 3 elements such as B (boron), and N-type impurities are, for example, Group 5 elements such as N (nitrogen), P (phosphorus), and As (arsenic). Methods for adding these impurities include, for example, diffusion and ion implantation. Each insulating layer 16, 17, and 18 is formed of, for example, SiO2 and SiN. Methods for forming each insulating layer 16, 17, and 18 include, for example, thermal oxidation and sputtering. Electrode E1, common electrode E3, and through electrode TE are formed of, for example, a metal such as aluminum. Methods for forming electrode E1, common electrode E3, and through electrode TE include, for example, sputtering. The resistivity of the quenching resistor R1 is higher than the resistivity of electrode E1 and common electrode E3. The quenching resistor R1 is formed of, for example, polysilicon. The method for forming the quenching resistance R1 is, for example, the CVD (Chemical Vapor Deposition) method. The material of the quenching resistance R1 may be, for example, SiCr, NiCr, TaNi, FeCr, etc.
[0035] As shown in Figure 7, the photodetector substrate 2 has trenches 19. The trenches 19 are provided on the light incident side surface of the semiconductor layer 11 so as to separate adjacent photodetectors 10 from each other. The trenches 19 function as isolation regions that separate each avalanche photodiode APD that forms each light-receiving region LA. Insulating materials such as silicon oxide, metallic materials such as tungsten, and polysilicon may be placed inside the trenches 19.
[0036] As shown in Figures 7, 8, and 9, in this embodiment, each metalens 30 included in the lens layer 3 is provided on the upper surface 4a of the dielectric layer 4 opposite to the photodetector substrate 2. That is, each metalens 30 is arranged on the light incident surface 2a of the photodetector substrate 2 via the dielectric layer 4. The metalens 30 is a metasurface structure that functions as a lens that focuses the light to be detected incident on the light incident surface 2a.
[0037] The dielectric layer 4 is an optical transmission substrate that transmits the detected light L. The dielectric layer 4 is bonded to the light incident surface 2a of the light detection substrate 2 by an adhesive having optical transparency. The dielectric layer 4 is formed of a material having a refractive index n2 (n2 < n1) lower than the refractive index n1 of the light detection substrate 2 (semiconductor layer 11). The dielectric layer 4 can be formed of, for example, SiO2, GaAs, GaP, Si, SiC, or the like. The thickness t (length in the Z-axis direction) of the dielectric layer 4 is, for example, 1 μm to 10 μm.
[0038] Each meta-lens 30 is arranged so as to correspond to each light detection unit 10. That is, one meta-lens 30 is provided for one light detection unit 10. The meta-lens 30 corresponding to a certain light detection unit 10 is arranged on the light incident surface 2a so as to condense the detected light L onto the light receiving region LA of the light detection unit 10. Each meta-lens 30 is formed of, for example, a meta-lens material such as a-Si (amorphous silicon), HfO2, Nb2O5, TiO2, or the like.
[0039] As shown in FIG. 8, one meta-lens 30 corresponding to one light detection unit 10 is, as an example, formed in a rectangular shape (square shape in this embodiment) when viewed from the Z-axis direction (thickness direction of the light detection substrate 2). The meta-lens 30 is composed of a plurality of unit structures 31. In this embodiment, the unit structure 31 is a cylindrical pillar.
[0040] When viewed from the Z-axis direction, an opening region 30a in which no unit structure 3 is formed is provided in a region including the center C of the meta-lens 30. That is, the plurality of unit structures 31 are formed in an annular region A (square annular region in this embodiment) excluding the opening region 30a in the region (square region in this embodiment) where the meta-lens 30 is arranged, and are not formed in the opening region 30a. In FIG. 8, only some of the unit structures 31 formed in the upper left corner portion of the region where the meta-lens 30 is arranged are shown, but the plurality of unit structures 31 are formed over the entire annular region A excluding the opening region 30a.
[0041] When viewed from the Z-axis direction, the aperture region 30a is rectangular. In this embodiment, as an example, the aperture region 30a is square. The width R of the aperture region 30a is 1 / 2 or more of the width D of the metal lens 30. When the aperture region 30a is formed in a square shape as in this embodiment, the width R of the aperture region 30a is the length of one side of the aperture region 30a. Also, when the aperture region 30a is formed in a circular shape as in the second modified example described later, the width R of the aperture region 30a is the diameter of the aperture region 30a. The width D of the metal lens 30 is the length of one side of the metal lens 30 when the metal lens 30 is square, and the diameter of the metal lens 30 when the metal lens 30 is circular, similar to the width R of the aperture region 30a.
[0042] Furthermore, as shown in Figure 7, when viewed from the Z-axis direction, the metalens 30 (i.e., region A on which the unit structure 31 is formed) is formed to overlap with both the adjacent region (the outer part of the light-receiving region LA) adjacent to the light-receiving region LA, and the peripheral region, which is the inner region of the light-receiving region LA along the boundary between the light-receiving region LA and the adjacent region. In other words, the metalens 30 (region A) has a portion 30A that overlaps with the adjacent region and a portion 30B that overlaps with the peripheral region. With the above configuration, even when the light-receiving region LA is small, by forming the metalens 30 (unit structure 31) in the portion that overlaps with both the adjacent region and the peripheral region, the light to be detected L can be reliably focused onto the light-receiving region LA.
[0043] Figure 9 is an enlarged view of region A1 shown in Figure 8. As shown in Figure 9, the multiple unit structures 31 are arranged in a grid pattern (a square grid pattern in this embodiment). More specifically, the unit regions U separated by dashed lines in Figure 9 are arranged in a grid pattern. One unit structure 31 is provided in one unit region U1 included in region A. The width d (see Figure 10) of the unit structure 31 included in each unit region U1 is set based on the phase design of the metalens 30. On the other hand, no unit structures 31 constituting the metalens 30 are provided in each unit region U2 included in region A2, which is included in the aperture region 30a of region A1. Note that the unit structures 31 do not necessarily have to be provided in all unit regions U1 included in region A, and it is sufficient if they are provided in multiple unit regions U1 included in region A to the extent that the effects of the present invention are fully realized. That is, unit structures 31 do not need to be formed in some of the unit regions U1 included in region A outside the aperture region 30a. In relation to the above, the opening region 30a does not necessarily have to be a region whose entire circumference is surrounded by multiple unit structures 31. For example, as shown in Figure 9, in a part of region A3 included in region A, unit structures 31 may not be formed.
[0044] As shown in Figure 10, the metalens 30 is a nanostructure (micro-uneven structure) in which the basic components (unit cell), the unit region U1, are periodically arranged in a grid pattern along the X-axis and Y-axis directions in region A where the unit structure 31 is formed. For example, the unit region U1 is a square-shaped region when viewed from the Z-axis direction. The unit structure 31 is erected on the upper surface 4a of the dielectric layer 4 in the central part of the unit region U1.
[0045] The period P of the unit structure 31 (i.e., the distance between the centers of adjacent unit structures 31, which is the length of one side of the unit region U1) is set to be shorter than the wavelength of the light to be analyzed, L. In other words, the metalens 30 has a subwavelength structure for the light to be analyzed, L.
[0046] For example, if the wavelength λ of the light to be detected L is 905 nm and the unit structure 31 is formed of a-Si, then the period P of the unit structure 31 is set to, for example, 375 nm, the height h of the unit structure 31 is set to, for example, 490 nm, and the width d (diameter) of the unit structure 31 is selected from, for example, a range of 140 nm to 270 nm, depending on the phase design and the position of the unit region U1.
[0047] As another example, if the wavelength λ of the light to be detected L is 905 nm and the unit structure 31 is formed of TiO2, the period P of the unit structure 31 is set to, for example, 441 nm, the height h of the unit structure 31 is set to, for example, 1000 nm, and the width d (diameter) of the unit structure 31 can be selected from, for example, a range of 80 nm to 380 nm, depending on the phase design and the position of the unit region U1.
[0048] As in the example above, the height h of the unit structure 31 is set to a constant value throughout the entire metalens 30 (i.e., in all unit regions U1). On the other hand, the width d of the unit structure 31 in each unit region U1 is selected from the above range according to the position of each unit region U1. In this way, by setting the width d of the unit structure 31 in each unit region U1 according to the position of each unit region U1, the amount of phase modulation is controlled for each position of each unit region U1, and a metalens 30 that functions as a focusing lens is obtained. For example, the metalens 30 has a structure in which multiple repeating regions (regions containing multiple unit regions U1) are arranged along the above direction, such that the phase changes continuously by 2π along the radial direction from the outside of the metalens 30 toward the center C when viewed from the Z-axis direction.
[0049] The structure of the metalens 30 described above will be supplemented. Two types of metalens structures (metasurface structures) are known: the refractive index modulation type and the resonance type. The metalens 30 may have any of the above metasurface structures. The refractive index modulation type metasurface structure controls the effective refractive index determined by the filling rate (occupancy rate) of the metalens material in each unit region U1. The resonance type metasurface structure controls the phase and transmittance by adjusting the electrical and magnetic resonances of the structure of each unit region U1 (i.e., the shape and size of the nanostructure consisting of a plurality of regularly arranged uneven structures). More specifically, the resonance type metasurface structure realizes the lens function described above by adjusting the transmittance coefficient t shown by the following equation (1). Note that in the following equation (1), ω e,k ω represents the resonance frequency for the k-th mode of electrical resonance, and m,k ω represents the resonance frequency for the k-th mode magnetic resonance. ω represents the resonance angular frequency in the Lorentz oscillator model describing electronic polarization. e,k γ represents the damping coefficient for the k-th mode electrical resonance in the Lorentz oscillator model described above, m,k This represents the damping coefficient for the k-th mode magnetic resonance in the Lorentz oscillator model described above. k b is a parameter that represents the contribution of the k-th mode's electrical resonance in the Lorentz oscillator model described above, and b k This parameter represents the contribution of the k-th mode magnetic resonance in the Lorentz oscillator model described above. The resonant metasurface structure includes the Huygens type (Nanodisk type) corresponding to the case where "m=n=1" in equation (1) below (i.e., when using the resonance of a single-mode electric dipole and magnetic dipole), and the HCG type (Micropost type) corresponding to the case where "m=n=1" in equation (1) below (i.e., when using the resonance of higher-order modes). When the metalens 30 is constructed using a resonant metasurface structure, either the Huygens type or the HCG type described above may be used.
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[0050] When a refractive index-modulated type is adopted as the structure of the metalens 30, robustness against changes in the wavelength of the detected light L can be ensured compared to when a resonance type is adopted. On the other hand, when a resonance type is adopted, the phase change can be made sharper and high transmittance can be ensured compared to the refractive index-modulated type. Furthermore, when a Huygens type is adopted, the aspect ratio of the unit structure 31 can be made lower (i.e., the ratio of the height to the width of the unit structure 31 can be made smaller) compared to the refractive index-modulated type and the HCG type, making the structure of the metalens 30 even more robust. On the other hand, when an HCG type is adopted, the resonance of multiple higher-order modes can be utilized, thus increasing the degree of freedom in the structural design of the metalens 30.
[0051] Referring to Figure 11, an example of the manufacturing process for the metalens 30 will be described. First, a silicon layer 130 (amorphous silicon) containing the portion that will become the metalens 30 (i.e., multiple unit structures 31) is deposited on the upper surface 4a of the dielectric layer 4 (for example, a quartz substrate) by sputtering (step S1). The thickness of the silicon layer 130 is set to the design value of the height h of the unit structure 31 (490 nm in the above example). Next, an EB (electron beam) resist 100 with a thickness of about 300 nm is applied to the surface of the silicon layer 130 (the side opposite to the dielectric layer 4) (step S2). Subsequently, a pre-designed pattern is drawn on the EB resist 100 by EB lithography (step S3). Specifically, an aperture 100a corresponding to the portion of each unit region U1 included in the region A described above where the unit structure 31 is not formed, and the aperture region 30a, is formed on the EB resist 100. Next, etching (for example, dry etching such as inductively coupled (ICP-RIE) etching) is performed using the EB resist 100 as a mask, thereby removing the portion of the silicon layer 130 corresponding to the opening 100a of the EB resist 100 (i.e., the exposed portion). After that, the EB resist 100 is peeled off (step S4). As a result, a metalens 30 (i.e., a structure in which multiple unit structures 31 are periodically arranged) is formed on the upper surface 4a of the dielectric layer 4.
[0052] As shown in Figure 7, if θ is the maximum angle between the detected light L that passes through the metalens 30 and the dielectric layer 4 and the optical axis AX of the metalens 30, and n is the refractive index of the medium through which the detected light L passes after passing through the metalens 30 (in this example, the dielectric layer 4), then the numerical aperture NA of the metalens 30 is determined by the following equation (2). The optical axis AX is an axis that passes through the center C of the metalens 30 and is parallel to the Z-axis direction. NA = n × sinθ …(2)
[0053] Furthermore, the effective wavelength λ of the detected light L passing through the dielectric layer 4 effAssuming that the wavelength of the detected light L in air is λ, it is determined by the following formula (3). The period P of the metasurface 30 is set within the range that satisfies the following formula (4). λ eff =λ / n …(3) λ eff / 2 < P < λ eff …(4)
[0054] At this time, the numerical aperture NA of the metasurface 30 is preferably selected so as to satisfy the following formula (5).
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[0055] By setting the numerical aperture NA within the range that satisfies the above formula (5), the maximum angle θ (see FIG. 7) is set so as to satisfy the following formula (6).
Number
[0056] Also, the focal length f to the virtual focus F (the focus when it is assumed that the medium through which the detected light L passes from passing through the metasurface 30 to reaching the focus is only the dielectric layer 4) corresponding to the selected maximum angle θ is determined by the following formula (7). The focal length f is the distance from the origin OP to the focus F when the intersection of the optical axis AX and the upper surface 4a of the dielectric layer 4 is the origin OP. f = D / (2×tanθ) …(7)
[0057] In this embodiment, the detected light L that has passed through the dielectric layer 4 passes through the semiconductor layer 11. Note that the detected light L also passes through the insulating layers 16 and 17, but since the insulating layers 16 and 17 are very thin compared to the dielectric layer 4 and the semiconductor layer 11, they are ignored here as being within the error range. In this case, the effective focal length f eff to the focus F eff (the second distance), representing the thickness of the dielectric layer 4 as t and the refractive index of the semiconductor layer 11 (Si in this embodiment) as n SiExpressed as such, it is determined by the following equation (8). Here, the focus F eff This is a focal point determined based on the phase design of the metalens 30 (and the medium through which the detected light L passes after passing through the metalens 30), and is determined independently of the aperture region 30a. f eff = t + (ft) × n Si / n …(8)
[0058] In this embodiment, as an example, the width d of each unit structure 31 constituting the metalens 30 is set such that the phase φ(x,y) at each coordinate (x,y) satisfies the following equation (9). That is, the multiple unit structures 31 are configured such that the phase distribution of the metalens 30 follows a quadratic phase pattern. Here, the coordinate (x,y) is a two-dimensional coordinate with the center C as the origin (0,0), as shown in Figure 8.
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[0059] Through the inventors' diligent research, the following findings were obtained. That is, by providing the metalens 30 with the aperture region 30a described above, the effective focal point F eff A focal point F, caused by diffraction (aperture diffraction) by the aperture region 30a, is located closer to the light incident surface 2a. ap (See Figure 7) is formed. In the example in Figure 7, the focal point F ap This is the focal point F and the focal point F eff It is located between, but the focal point F ap The aperture can be formed closer to the light incident surface 2a than the focal point F. Furthermore, by adjusting the width R of the aperture region 30a relative to the width D of the metalens 30, the phase distribution pattern of the metalens 30 (in this embodiment, a second-order phase pattern), the numerical aperture NA of the metalens 30, etc., the focusing point F can be adjusted. ap The position can be adjusted. Based on the above findings, preferably the metalens 30 is positioned from the origin OP to the focusing point F ap The focusing distance f ap (First distance) is the focal length f effIt is formed to be 40% or more and 90% or less of the total. More preferably, the metalens 30 has a focusing distance f ap is the focal length f eff It is formed to be more than 50% and less than 80%.
[0060] As described above, the photodetector 1 can simultaneously generate diffraction by the metalens 30 (multiple unit structures 31) and diffraction by the aperture region 30a (aperture diffraction). By generating these two types of diffraction simultaneously, the focal point (i.e., the focal point F set by the multiple unit structures 31 constituting the metalens 30) that would occur if only diffraction by the metalens 30 occurred can be controlled. eff The focal point F is located closer to the light incident surface 2a than the light incident surface 2a. ap It becomes possible to form a focal point F based on the phase design of the metalens 30, according to the light-receiving element 1, by providing an aperture region 30a. eff This allows the light to be focused to a position closer to the light incident surface 2a.
[0061] Furthermore, the photodetector substrate 2 has an avalanche photodiode APD. In this embodiment, the interface between the semiconductor region 12 and the semiconductor region 13 is the interface of the PN junction. By focusing the light as close as possible to this interface (i.e., the portion of the semiconductor region 12 as close as possible to the semiconductor region 13), the light detection sensitivity of the photodetector 10 (avalanche photodiode APD) can be effectively improved. Also, if the photodetector substrate 2 is SiPM as in this embodiment, the time resolution can be improved by focusing the light to be detected L as close as possible to the front side of the photodetector substrate 2 (closer to the light incident surface 2a). As described above, according to the photodetector element 1, the focus F based on the phase design of the metalens 30 eff The focal point F is located closer to the interface between semiconductor region 12 and semiconductor region 13. apThis is formed. As a result, the light-receiving sensitivity of the photodetector 10 can be effectively improved. Furthermore, as in this embodiment, by combining a metalens with an aperture (i.e., a metalens 30 provided with an aperture region 30a) with the photodetector substrate 2, which is a surface-incident photodiode, it becomes possible to focus the light to be detected L at the shallowest possible position on the semiconductor layer 11, and as a result, the light-receiving sensitivity can be effectively improved.
[0062] Furthermore, the numerical aperture NA of the metalens 30 is set to satisfy the above equation (5). According to the inventors' findings, the above configuration more reliably determines the focal point F based on the phase design of the metalens 30. eff The focal point F is located closer to the light incident surface 2a than the light incident surface 2a. ap It is possible to form this.
[0063] Furthermore, when viewed from the Z-axis direction, the width R of the aperture region 30a is 1 / 2 or more of the width D of the metalens 30. According to the inventor's findings, by making the width R of the aperture region 30a sufficiently large relative to the width D of the metalens 30 as described above, the focus F based on the phase design of the metalens 30 can be more reliably achieved. eff The focal point F is located closer to the light incident surface 2a than the light incident surface 2a. ap It is possible to form this.
[0064] The effects of the photodetector 1 of this embodiment (Embodiment 1 to Embodiment 4) will be explained below based on Embodiments 1 to 4 (Figures 12 to 15) and Comparative Example (Figure 16). The parameters of the metalens for each embodiment and comparative example are shown in the table below (Table 1). In all cases, the phase distribution pattern of the metalens is the second-order phase pattern shown in equation (9) above. [Table 1]
[0065] (First embodiment) Figure 12 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector 1 of the first embodiment. More specifically, (A) shows the phase pattern with the center C of the metalens 30 (see Figure 8) as the origin (0,0). (B) shows the intensity distribution of the detected light L at each coordinate when the origin OP is the origin (0,0) and the direction from the origin OP toward the semiconductor layer 11 is the positive direction of the Z axis. (C) shows the intensity distribution of the detected light L at each position (Z axis coordinate) along the optical axis AX. Note that (A) to (C) in Figures 13 to 18 and Figure 20, which will be described later, are the same as (A) to (C) in Figure 12.
[0066] The central rectangular black area shown in Figure 12(A) corresponds to the aperture region 30a where the unit structure 31 is not formed. As shown in Figures 12(B) and (C), according to the photodetector 1 of the first embodiment, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0067] (Second example) Figure 13 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector 1 of the second embodiment. The rectangular black area in the center shown in Figure 13(A) corresponds to the aperture region 30a where the unit structure 31 is not formed. As shown in Figures 13(B) and (C), in the photodetector 1 of the second embodiment as well, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0068] (Third embodiment) Figure 14 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector 1 of the third embodiment. The rectangular black area in the center shown in Figure 14(A) corresponds to the aperture region 30a where the unit structure 31 is not formed. As shown in Figures 14(B) and (C), in the photodetector 1 of the third embodiment as well, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0069] (Fourth embodiment) Figure 15 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector 1 of the fourth embodiment. The rectangular black area in the center shown in Figure 15(A) corresponds to the aperture region 30a where the unit structure 31 is not formed. As shown in Figures 15(B) and (C), in the photodetector 1 of the fourth embodiment as well, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0070] (Comparative example) Figure 16 shows the simulation results of the metalens phase pattern and light intensity distribution of the comparative example's photodetector. The comparative example differs from the first embodiment in that no aperture region is formed. That is, in the metalens of the comparative example's photodetector, multiple unit structures 31 are formed in the region corresponding to the aperture region 30a of the metalens 30 of the first embodiment, according to the secondary phase pattern shown in equation (9) above. Therefore, as shown in Figure 16(A), a phase pattern is formed throughout the entire region where the metalens 30 is placed. Also, as shown in Figures 16(B) and (C), in the comparative example, the focal point F is determined based on the phase design of the metalens. eff Only is formed, and the focusing point F in the first to fourth embodiments ap A corresponding focal point is not formed.
[0071] As described above, the focal point F depends on the parameters of the metalens (metalens width D, aperture area width R, numerical aperture NA). eff and the focusing point F ap Although the position and light intensity distribution of change, in all embodiments, the focal point F eff The focal point F is closer to the viewer. ap It was confirmed that the effect of forming an aperture region was obtained. In contrast, in a comparative example using a metalens in which no aperture region is formed, the focusing point F ap It was confirmed that no corresponding focal point is formed, and the above effect cannot be obtained. In other words, from the results of the above simulation, by forming an aperture region 30a in the region including the center C of the metalens 30, the focal point F by the metalens 30 alone is achieved. eff The focusing point F is located closer to the metal lens 30. ap It was confirmed that it is possible to form it.
[0072] (First variation) In the above embodiment, the multiple unit structures 31 are configured such that the phase distribution of the metalens 30 follows the quadratic phase pattern shown in equation (9) above. However, a pattern other than the quadratic phase pattern may be used as the phase distribution pattern of the metalens 30. In the first modified light-receiving element 1, the width d of each unit structure 31 constituting the metalens 30 is set such that the phase φ(x,y) at each coordinate (x,y) satisfies the following equation (10). That is, in the first modified light-receiving element 1, the multiple unit structures 31 are configured such that the phase distribution of the metalens 30 follows a Fresnel pattern, not a quadratic phase pattern. Here, the coordinates (x,y) are two-dimensional coordinates with the center C as the origin (0,0), as shown in Figure 8.
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[0073] The first modified example differs from the first embodiment in that the phase pattern of the metalens 30 follows the Fresnel pattern shown in equation (10) above, while the other parameters of the metalens (width D of the metalens, width R of the aperture region, numerical aperture NA) are the same as those of the first embodiment.
[0074] Figure 17 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector of the first modified example. As shown in Figures 17(B) and (C), in the photodetector 1 of the first modified example, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0075] However, the focusing point F in the first embodiment (Figure 12) ap The light intensity (contrast) at point F in the first modified example (Figure 17) is higher. ap The light intensity is greater than that at the focal point F. In other words, by configuring multiple unit structures 31 such that the phase distribution of the metalens 30 follows a second-order phase pattern, the focal point F is greater than when a pattern other than a second-order phase pattern (e.g., a Fresnel pattern) is applied to the phase distribution of the metalens 30. eff The focal point F is formed closer to the incident light surface 2a than the focal point (based on the phase design of the metalens). app It was confirmed that the light-gathering effect could be improved.
[0076] (Second variation) In the above embodiment, the aperture region 30a is formed in a rectangular (square) shape, but the aperture region 30a may be formed in a shape other than a rectangle. In the second modified example of the photodetector 1, the aperture region 30a of the metalens 30 is formed in a circular shape. The second modified example differs from the first embodiment in that the aperture region 30a is formed in a circular shape centered on the center C when viewed from the Z-axis direction, but the other parameters of the metalens (width D of the metalens, width R of the aperture region, numerical aperture NA) are the same as in the first embodiment. That is, in the second embodiment, the aperture region 30a is a circular region with a diameter (width R) of 10 μm centered on the center C.
[0077] Figure 18 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the second modified example. As shown in Figure 18(A), in the second modified example, the aperture region 30a (black region) is provided in a circular shape. Also, as shown in Figures 18(B) and (C), in the photodetector 1 of the second modified example, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0078] However, the focusing point F in the first embodiment (Figure 12) ap The light intensity (contrast) at point F in the second modified example (Figure 18) is better. ap The light intensity is greater than that at [location]. In other words, by forming the aperture region 30a of the metalens 30 in a rectangular (square) shape, the focal point F is greater compared to the case where the aperture region 30a is in a shape other than a rectangle (for example, a circular shape). eff The focal point F is formed closer to the incident light surface 2a than the focal point (based on the phase design of the metalens). app It was confirmed that the light-gathering effect could be improved.
[0079] (Third variation) In the above embodiment, the light-receiving element 1 had a dielectric layer 4 provided between the photodetection substrate 2 and the metalens 30 (lens layer 3), but the dielectric layer 4 may be omitted.
[0080] As shown in Figure 19, the photodetector 1A of the third modified example differs from the photodetector 1 of the above embodiment (Figure 7) in that it does not have a dielectric layer 4, and the metalens 30 is directly arranged on the light incident surface 2a of the photodetector substrate 2. In this case, the refractive index n in equations (2) and (3) in the above embodiment becomes the refractive index of the semiconductor layer 11. Also, the focal point F in the above embodiment and the effective focal point F eff This matches.
[0081] Figure 20 shows the simulation results of the metalens phase pattern and light intensity distribution of the photodetector in the third modified example. In the third modified example, the width D of the metalens 30 is 15 μm, the width R of the aperture region 30a is 10 μm, and the numerical aperture NA is set to 1.26. As shown in Figures 20(B) and (C), in the photodetector 1 of the third modified example, the focal point F is determined based on the phase design of the metalens 30. eff A focal point F, caused by aperture diffraction due to the aperture region 30a, is located closer to the light incident surface 2a than the light incident surface 2a. ap It was confirmed that it was formed.
[0082] However, the focusing point F in the above embodiment (for example, the first embodiment) in which the dielectric layer 4 is provided. ap The light intensity (contrast) at point F in the third modified example (Figure 19) is higher. ap The light intensity is greater than that at [location]. In other words, by forming the aperture region 30a of the metalens 30 in a rectangular (square) shape, the focal point F is greater compared to the case where the aperture region 30a is in a shape other than a rectangle (for example, a circular shape). eff The focal point F is formed closer to the incident light surface 2a than the focal point (based on the phase design of the metalens). app It was confirmed that the light-gathering effect could be improved.
[0083] Although several embodiments of this disclosure have been described above, this disclosure is not limited to the embodiments described above. The materials and shapes of each component are not limited to those described above; a variety of materials and shapes can be used. Furthermore, some components of one embodiment or modification described above can be arbitrarily applied to components of other embodiments or modifications.
[0084] For example, multiple unit structures 31 (unit regions U) may be arranged in a grid other than a square grid (e.g., a honeycomb grid). Also, the unit structures 31 are not limited to cylindrical pillars. The unit structures 31 may be configured as pillars (protrusions) of shapes other than cylindrical (e.g., prismatic). Furthermore, instead of a pillar structure in which multiple pillars (unit structures 31) are periodically formed as in the above embodiment, the metalens 30 may employ a hole structure in which multiple holes (recesses, unit structures) are periodically formed. The hole structure is a structure that is an inversion of the pillar structure. For example, in the hole structure, the metalens material is formed in the part of the unit region U1 in the above embodiment where no pillars are formed, and holes (recesses) are formed in the part corresponding to the pillars. When using a metalens with a hole structure, the phase design of the metalens can be performed by adjusting the width (hole diameter) of the holes in each unit region U1.
[0085] Furthermore, in the above embodiment, when viewed from the Z-axis direction, the entire aperture region 30a overlaps with the light-receiving region LA, and the aperture region 30a is not provided outside the light-receiving region LA. However, the aperture region 30a may be formed to be larger than the light-receiving region LA, and the aperture region 30a may exist outside the light-receiving region LA.
[0086] Furthermore, when the photodetection substrate 2 is configured as a SiPM having multiple SPADs (photodetection units 10) as in the above embodiment, it may have other configurations, such as a configuration where the N-type and P-type are reversed. Also, the photodetection substrate 2 may have a configuration other than SiPM. [Explanation of symbols]
[0087] 1,1A...Photodetector, 2...Photodetector substrate, 2a...Incident light surface, 4...Dielectric layer, 30...Metalens, 30a...Aperture region, 31...Unit structure, APD...Avalanche photodiode, C...Center, L...Detected light, LA...Photodetector region.
Claims
1. A photodetector substrate having at least one light-receiving region and a light-incident surface into which the light to be detected is incident, A metalens is composed of multiple unit structures arranged in a grid, and is positioned on the light incident surface to focus the light to be detected, Equipped with, When viewed from the thickness direction of the photodetector substrate, an aperture region is provided in the area including the center of the metalens where the unit structure is not formed. The plurality of unit structures are configured such that the phase distribution of the metalens follows a quadratic phase pattern. Light-receiving element.
2. A photodetector substrate having at least one light-receiving region and a light-incident surface into which the light to be detected is incident, A metalens is composed of multiple unit structures arranged in a grid, and is positioned on the light incident surface to focus the light to be detected, Equipped with, When viewed from the thickness direction of the photodetector substrate, an aperture region is provided in the area including the center of the metalens where the unit structure is not formed. The metalens is formed such that the first distance is 40% or more and 90% or less of the second distance. The second distance is the distance in the thickness direction from the metalens to the focal point set by the metalens, The first distance is the distance in the thickness direction from the metalens to the point of convergence, which is formed at a position closer to the light incident surface than the focal point due to the provision of the aperture region. Light-receiving element.
3. The metalens is formed such that the first distance is 50% or more and 80% or less of the second distance. The light-receiving element according to claim 2.
4. The photodetector further comprises a dielectric layer disposed between the photodetector substrate and the metalens, having a refractive index lower than that of the photodetector substrate. A light-receiving element according to any one of claims 1 to 3.
5. The numerical aperture NA of the aforementioned metalens is set to satisfy the following equation (1): In the following equation (1), λ ｅｆｆ P is the effective wavelength of the detected light that has passed through the metalens, and P is the period in which the plurality of unit structures are arranged. A light-receiving element according to any one of claims 1 to 3. [Math 1]
6. When viewed from the thickness direction, the opening region is rectangular in shape. A light-receiving element according to any one of claims 1 to 3.
7. The aforementioned photodetector substrate has an avalanche photodiode. A light-receiving element according to any one of claims 1 to 3.
8. When viewed from the thickness direction, the metalens is formed to overlap with both the adjacent region adjacent to the light-receiving region and the peripheral region which is the region inside the light-receiving region along the boundary between the light-receiving region and the adjacent region. A light-receiving element according to any one of claims 1 to 3.
9. When viewed from the thickness direction, the width of the aperture region is 1 / 2 or more of the width of the metal lens. A light-receiving element according to any one of claims 1 to 3.
10. The metalens is formed such that the first distance is 40% or more and 90% or less of the second distance. The second distance is the distance in the thickness direction from the metalens to the focal point set by the metalens, The first distance is the distance in the thickness direction from the metalens to the point of convergence, which is formed at a position closer to the light incident surface than the focal point due to the provision of the aperture region. The light-receiving element according to claim 1.