Patent Publication Number: US-2022231071-A1

Title: Light detection device

Description:
TECHNICAL FIELD 
     The present invention relates to a photodetecting device. 
     BACKGROUND ART 
     Known photodetecting devices include a semiconductor substrate including a first principal surface and a second principal surface that oppose each other (see, for example, Patent Literature 1). The photodetecting device described in Patent Literature 1 includes a plurality of avalanche photodiodes operating in Geiger mode and through-electrodes electrically connected to the corresponding avalanche photodiodes. The plurality of avalanche photodiodes are two-dimensionally arranged on the semiconductor substrate. Each avalanche photodiode includes a light receiving region disposed at the first principal surface side of the semiconductor substrate. The through-electrode is disposed in a through-hole penetrating through the semiconductor substrate in the thickness direction. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2015-61041 
     SUMMARY OF INVENTION 
     Technical Problem 
     The object of an aspect of the present invention is to provide a photodetecting device in which the aperture ratio is ensured, and an inflow of a surface leakage electric current to the avalanche photodiode is reduced, and structural defects are less likely to occur around the through-hole in the semiconductor substrate. 
     Solution to Problem 
     As a result of researches and studies, the present inventors have newly found the following facts. 
     When a photodetecting device includes a plurality of avalanche photodiodes, a through-electrode is placed in a first area where the plurality of avalanche photodiodes are two-dimensionally arranged, for example, in order to shorten the wiring distance from the avalanche photodiode. When the through-electrode is disposed outside of the first area, the wiring distance between the avalanche photodiode and the through-electrode is large, and the difference in the wiring distances between the avalanche photodiodes is large, as compared with when the through-electrode is disposed in the first area. The wiring distance is related to the wiring resistance, the parasitic capacitance, and the like, and affects the detection accuracy of the photodetecting device. 
     The through-hole where the through-electrode is arranged becomes a dead space for photodetection. Therefore, when the through-electrode is disposed in the first area, an effective area for photodetection is small, as compared with when the through-electrode is disposed outside of the first area. That is, the aperture ratio may decrease. When the aperture ratio decreases, photodetection characteristics of the photodetecting device are deteriorated. 
     In order to suppress the reduction of the aperture ratio, it is desirable that the dead space is as small as possible. For example, the aperture ratio is ensured by reducing the distance between the avalanche photodiode and the through-hole (through-electrode). When the distance between the avalanche photodiode and the through-hole is small, a surface leakage electric current flows easily from the through-hole to the avalanche photodiode, as compared with when the distance between the avalanche photodiode and the through-hole is large. Consequently, this may adversely affect the detection accuracy in the photodetecting device. 
     Therefore, the present inventors keenly studied a configuration in which the aperture ratio is ensured, and the inflow of the surface leakage electric current to the avalanche photodiode is reduced. 
     The present inventors have found a configuration in which a groove surrounding the through-hole at the first principal surface of the semiconductor substrate is formed in an area between the through-hole and a light receiving region of an avalanche photodiode adjacent to the through-hole. In this configuration, the groove surrounding the through-hole is formed in the area between the through-hole and the light receiving region of the avalanche photodiode adjacent to the through-hole, and therefore, even when the distance between the light receiving region and the through-electrode (through-hole) is small, a flow of a surface leakage electric current from the through-hole to the avalanche photodiode is reduced. 
     The present inventors also found that a new problem arises when a groove surrounding the through-hole is formed in the semiconductor substrate. The groove surrounding the through-hole is formed in a narrow area between the through-hole and the light receiving region. For this reason, a structural defect may occur in the area between the groove and the through-hole surrounded by the groove in the semiconductor substrate. The structural defect is, for example, cracking or chipping of the semiconductor substrate. When a first distance from an edge of the groove to an edge of the through-hole surrounded by the groove is equal to or less than a second distance from the edge of the groove to the edge of the light receiving region adjacent to the through-hole surrounded by the groove, a structural defect is likely to occur, as compared with when the first distance is longer than the second distance. 
     The present inventors found a configuration in which the first distance is longer than the second distance. According to this configuration, when the distance between the edge of the light receiving region and the edge of the through-hole adjacent to the light receiving region is small, and a groove surrounding the through-hole is formed between the light receiving region and the through-hole adjacent to the light receiving region in the semiconductor substrate, a structural defect is unlikely to occur in the area between the groove and the through-hole surrounded by the groove in the semiconductor substrate. 
     An aspect of the present invention is a photodetecting device including a semiconductor substrate including a first principal surface and a second principal surface opposing each other, a plurality of avalanche photodiodes operating in Geiger mode, and a through-electrode. Each of the plurality of avalanche photodiodes includes a light receiving region disposed at the first principal surface side of the semiconductor substrate, and the avalanche photodiodes are arranged two-dimensionally on the semiconductor substrate. The through-electrode is electrically connected to a corresponding light receiving region. The through-electrode is provided in a through-hole penetrating through the semiconductor substrate in a thickness direction in an area where the plurality of avalanche photodiodes are arranged two-dimensionally. At the first principal surface side of the semiconductor substrate, a groove surrounding the through-hole is formed between the through-hole and the light receiving region adjacent to the through-hole. A first distance between an edge of the groove and an edge of the through-hole surrounded by the groove is longer than a second distance between the edge of the groove and an edge of the light receiving region adjacent to the through-hole surrounded by the groove. 
     In the photodetecting device according to the aspect, at the first principal surface side of the semiconductor substrate, the groove surrounding the through-hole is formed in the area between the through-hole and the light receiving region adjacent to the through-hole, and therefore, the aperture ratio is ensured, and the flow of surface leakage electric current to the avalanche photodiode is reduced. Since the first distance is longer than the second distance, a structural defect is less likely to occur around the through-hole in the semiconductor substrate. 
     In the photodetecting device according to the aspect, each avalanche photodiode may include a first semiconductor region of a first conductivity type located at the first principal surface side of the semiconductor substrate, a second semiconductor region of a second conductivity type located at the second principal surface side of the semiconductor substrate, a third semiconductor region of the second conductivity type located between the first semiconductor region and the second semiconductor region and having a lower impurity concentration than the second semiconductor region, and a fourth semiconductor region of the first conductivity type formed in the first semiconductor region and having a higher impurity concentration than the first semiconductor region. In which case, the fourth semiconductor region may be the light receiving region, and a bottom surface of the groove may be constituted by the second semiconductor region. In this embodiment, the bottom surface of the groove is deeper than the third semiconductor region. Therefore, even when charges are generated in the area surrounded by the groove in the semiconductor substrate, this suppresses movement of the charges generated in the area to the avalanche photodiode. Since the bottom surface of the groove is formed in the semiconductor substrate, i.e., the groove does not reach the second principal surface of the semiconductor substrate, the semiconductor substrate will not be separated at the position of the groove. Therefore, in the manufacturing process of the photodetecting device, the semiconductor substrate is easily handled. 
     In the photodetecting device according to the aspect, each avalanche photodiode may include a first semiconductor region of a first conductivity type located at the first principal surface side of the semiconductor substrate, a second semiconductor region of the first conductivity type located at the second principal surface side of the semiconductor substrate and having a higher impurity concentration than the first semiconductor region, a third semiconductor region of a second conductivity type formed at the first principal surface side of the first semiconductor region, and a fourth semiconductor region of the first conductivity type formed in the first semiconductor region to be in contact with the third semiconductor region, and having a higher impurity concentration than the first semiconductor region. In which case, the third semiconductor region may be the light receiving region, and a bottom surface of the groove may be constituted by the second semiconductor region. In this embodiment, the bottom surface of the groove is deeper than the third semiconductor region. Therefore, even when charges are generated in the area surrounded by the groove in the semiconductor substrate, this suppresses movement of the charges generated in the area to the avalanche photodiode. Since the bottom surface of the groove is formed in the semiconductor substrate, i.e., the groove does not reach the second principal surface of the semiconductor substrate, the semiconductor substrate will not be separated at the position of the groove. Therefore, in the manufacturing process of the photodetecting device, the semiconductor substrate is easily handled. 
     The photodetecting device according to the aspect may include an electrode pad disposed on the first principal surface and electrically connected to the through-electrode. In which case, when viewed from a direction perpendicular to the first principal surface, the electrode pad may be located in an area surrounded by the groove and spaced apart from the groove. In this embodiment, when a configuration is employed in which the groove is filled with metal, a parasitic capacitance generated between the electrode pad and the metal in the groove is reduced. 
     In the photodetecting device according to the one aspect, when viewed from a direction perpendicular to the first principal surface, an area surrounded by the groove may have a polygonal shape, and the light receiving region may have a polygonal shape. When the area surrounded by the groove and the light receiving region have polygonal shapes, it is possible to employ a configuration in which the area surrounded by the groove and the light receiving region are arranged in such manner that a side of the area surrounded by the groove is along a side of the light receiving region. In the photodetecting device adopting this configuration, the dead space is small and the aperture ratio is high. 
     In the photodetecting device according to the aspect, when viewed from a direction perpendicular to the first principal surface, an opening of the through-hole may have a circular shape, and an insulating layer may be arranged on the inner peripheral surface of the through-hole. When the insulating layer is arranged on the inner peripheral surface of the through-hole, the through-electrode and the semiconductor substrate are electrically insulated from each other. When there is a corner at the opening of the through-hole, a crack may be formed at the corner of the insulating layer when the insulating layer is formed. Since the through-hole has a circular shape when viewed from a direction perpendicular to the first principal surface, cracks tend not to be generated in the insulating layer when the insulating layer is formed. Therefore, in this embodiment, electrical insulation between the through-electrode and the semiconductor substrate is ensured. 
     In the photodetecting device according to the aspect, the plurality of avalanche photodiodes may be arranged in a matrix. In which case, the through-hole may be formed in each area surrounded by four mutually adjacent avalanche photodiodes of the plurality of avalanche photodiodes. The through-hole may be provided with the through-electrode electrically connected to the light receiving region of one of the four mutually adjacent avalanche photodiodes. The groove may be formed in an area between the through-hole and the light receiving region of each of the four mutually adjacent avalanche photodiodes. In this embodiment, since the wiring distance between the through-electrode and the light receiving region electrically connected to the through-electrode is relatively short, it is unsusceptible to influence by the wiring resistance and the parasitic capacitance. Therefore, this suppresses degradation of the detection accuracy of the photodetecting device. 
     When viewed from a direction perpendicular to the first principal surface, the light receiving region may have a polygonal shape. In which case, when viewed from a direction perpendicular to the first principal surface, the groove may extend along a side adjacent to the through-hole, of a plurality of sides of the light receiving region of each of the four avalanche photodiodes adjacent to the through-hole. In this embodiment, the groove extends along the side of the light receiving region, and therefore, a configuration in which the distance between the through-hole and the light receiving region is short can be employed even when the through-hole is formed in each area surrounded by the four adjacent avalanche photodiodes. The photodetecting device that employs this configuration has a smaller dead space and a higher aperture ratio. 
     Advantageous Effects of Invention 
     An aspect of the present invention provides a photodetecting device in which the aperture ratio is ensured, and an inflow of a surface leakage electric current to the avalanche photodiode is reduced, and a structural defect is less likely to occur around the through-hole in the semiconductor substrate. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view illustrating a photodetecting device according to an embodiment. 
         FIG. 2  is a schematic plan view illustrating a semiconductor photodetecting element. 
         FIG. 3  is a schematic enlarged view illustrating the semiconductor photodetecting element. 
         FIG. 4  is a diagram for describing a cross-sectional configuration along line IV-IV illustrated in  FIG. 2 . 
         FIG. 5  is a schematic plan view illustrating a mounting substrate. 
         FIG. 6  is a circuit diagram of the photodetecting device. 
         FIG. 7  is a diagram for describing a cross-sectional configuration of a photodetecting device according to a modification of the embodiment. 
         FIG. 8  is a schematic plan view illustrating a modification of a semiconductor photodetecting element. 
         FIG. 9  is a schematic plan view illustrating a modification of a semiconductor photodetecting element. 
         FIG. 10  is a schematic plan view illustrating a modification of a semiconductor photodetecting element. 
         FIG. 11  is a schematic plan view illustrating a modification of a semiconductor photodetecting element. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be hereinafter described in detail with reference to the accompanying drawings. In the description, the same reference numerals are used for the same elements or elements having the same functions, and redundant descriptions thereabout are omitted. 
     First, a configuration of a photodetecting device  1  according to the present embodiment will be described with reference to  FIG. 1  to  FIG. 4 .  FIG. 1  is a schematic perspective view illustrating the photodetecting device according to the present embodiment.  FIG. 2  is a schematic plan view illustrating semiconductor photodetecting elements.  FIG. 3  is a schematic enlarged view illustrating a semiconductor photodetecting element.  FIG. 4  is a diagram for describing a cross-sectional configuration along line IV-IV illustrated in  FIG. 2 . 
     As illustrated in  FIG. 1 , the photodetecting device  1  includes a semiconductor photodetecting element  10 A, a mounting substrate  20 , and a glass substrate  30 . The mounting substrate  20  opposes the semiconductor photodetecting element  10 A. The glass substrate  30  opposes the semiconductor photodetecting element  10 A. The semiconductor photodetecting element  10 A is disposed between the mounting substrate  20  and the glass substrate  30 . In the present embodiment, a plane in parallel with each principal surface of the semiconductor photodetecting element  10 A, the mounting substrate  20 , and the glass substrate  30  is the XY-axis plane, and a direction perpendicular to each principal surface is the Z-axis direction. 
     The semiconductor photodetecting element  10 A includes a semiconductor substrate  50 A having a rectangular shape in a plan view. The semiconductor substrate  50 A is made of Si and is an N type (second conductivity type) semiconductor substrate. The semiconductor substrate  50 A includes a principal surface  1 Na and a principal surface  1 Nb that oppose each other. 
     As illustrated in  FIG. 2 , the semiconductor photodetecting element  10 A includes a plurality of avalanche photodiodes APD and a plurality of through-electrodes TE. The plurality of avalanche photodiodes APD are two-dimensionally arranged on the semiconductor substrate  50 A. In the present embodiment, the avalanche photodiodes APD are arranged in a matrix. In the present embodiment, the row direction is X-axis direction and the column direction is Y-axis direction. The avalanche photodiodes APD are arranged with an equal distance on a straight line when the avalanche photodiodes APD are viewed from each of the X-axis direction and the Y-axis direction. 
     Each avalanche photodiode APD includes a light receiving region S 1  and operates in Geiger mode. The light receiving region S 1  is arranged at a principal surface  1 Na side of the semiconductor substrate  50 A. As illustrated in  FIG. 6 , the avalanche photodiodes APD are connected in parallel in such a manner that a quenching resistor R 1  is connected in series with each avalanche photodiode APD. A reverse bias voltage is applied to each avalanche photodiode APD from a power supply. The output electric current from each avalanche photodiode APD is detected by a signal processing unit SP. The light receiving region S 1  is a charge generating region (a photosensitive region) configured to generate charges in response to incident light. That is, the light receiving region S 1  is a photodetecting region. 
     The glass substrate  30  includes a principal surface  30   a  and a principal surface  30   b  that oppose each other. The glass substrate  30  has a rectangular shape in a plan view. The principal surface  30   b  opposes the principal surface  1 Na of the semiconductor substrate  50 A. The principal surface  30   a  and the principal surface  30   b  are flat. The glass substrate  30  and the semiconductor photodetecting element  10 A are optically connected by an optical adhesive OA. The glass substrate  30  may be formed directly on the semiconductor photodetecting element  10 A. 
     A scintillator (not illustrated) may be optically connected to the principal surface  30   a  of the glass substrate  30 . In which case, the scintillator is connected to the principal surface  30   a  by an optical adhesive. The scintillation light from the scintillator passes through the glass substrate  30  and is incident on the semiconductor photodetecting element  10 A. 
     The mounting substrate  20  includes a principal surface  20   a  and a principal surface  20   b  that oppose each other. The mounting substrate  20  has a rectangular shape in a plan view. The principal surface  20   a  opposes the principal surface  1 Nb of the semiconductor substrate  50 A. The mounting substrate  20  includes a plurality of electrodes arranged on the principal surface  20   a . These electrodes are arranged corresponding to the through-electrodes TE. 
     The side surface  1 Nc of the semiconductor substrate  50 A, the side surface  30   c  of the glass substrate  30 , and the side surface  20   c  of the mounting substrate  20  are flush with each other. That is, in the plan view, the outer edge of the semiconductor substrate  50 A, the outer edge of the glass substrate  30 , and the outer edge of the mounting substrate  20  match each other. The outer edge of the semiconductor substrate  50 A, the outer edge of the glass substrate  30 , and the outer edge of the mounting substrate  20  do not have to match each other. For example, in the plan view, the area of the mounting substrate  20  may be larger than the area of each of the semiconductor substrate  50 A and the glass substrate  30 . In which case, the side surface  20   c  of the mounting substrate  20  is located outside, in the XY-axis plane direction, of the side surface  1 Nc of the semiconductor substrate  50 A and the side surface  30   c  of the glass substrate  30 . 
     Next, the structure of the semiconductor photodetecting element  10 A will be described with reference to  FIG. 2  and  FIG. 3 .  FIG. 2  is a view illustrating the semiconductor photodetecting element  10 A that is viewed from the direction perpendicular to the principal surface  1 Na of the semiconductor substrate  50 A (Z-axis direction).  FIG. 3  illustrates an area where the groove is formed. 
     One avalanche photodiode APD constitutes one cell in the semiconductor photodetecting element  10 A. Each avalanche photodiode APD includes one light receiving region S 1 . That is, the semiconductor photodetecting element  10 A includes a plurality of light receiving regions S 1 . The light receiving region S 1  has a polygonal shape when viewed from the Z-axis direction. The light receiving region S 1  of the semiconductor photodetecting element  10 A has a substantially regular octagonal shape when viewed from the Z-axis direction. 
     The plurality of light receiving regions S 1  are two-dimensionally arranged when viewed from the Z-axis direction. In the present embodiment, the plurality of light receiving regions S 1  are arranged in a matrix. The light receiving regions S 1  are arranged with an equal distance on a straight line when viewed from each of the X-axis direction and the Y-axis direction. In the present embodiment, the light receiving regions S 1  are arranged with a pitch of 100 μm. In the semiconductor photodetecting element  10 A, two adjacent light receiving regions S 1  are arranged in such a manner that one side of an octagon shape opposes each other. 
     Each avalanche photodiode APD includes an electrode E 1 . The electrode E 1  is arranged on the principal surface  1 Na side of the semiconductor substrate  50 A. The electrode E 1  is provided along the contour of the light receiving region S 1  and has an octagonal ring shape. 
     The electrode E 1  includes a connected portion C that is electrically connected to the light receiving region S 1 . The connected portions C are provided on the four sides of the light receiving region S 1 . The connected portions C are provided alternately on the sides of the light receiving region S 1 . In which case, the detection accuracy of the signal from the light receiving region S 1  is ensured. As illustrated in  FIG. 3 , the connected portion C includes a first end portion Ela and a second end portion E 1   b  and extends on the XY-axis plane from the outer edge toward the center of the light receiving region S 1 . As also illustrated in  FIG. 4 , the electrode E 1  extends in the Z-axis direction at the second end portion E 1   b . Accordingly, a step is formed at the position of the second end portion Eb in the electrode E 1 . The electrode E 1  extends from the step in the direction opposite to the center of the light receiving region S 1 . The electrode E 1  includes a third end portion E 1   c  that is electrically connected to the wiring F. 
     As illustrated also in  FIG. 4 , the wiring F extends from the third end portion E 1   c  in the direction opposite to the center of the light receiving region S 1 . The wiring F electrically connects the electrode E 1  and an electrode pad  12 . The wiring F is located above the semiconductor substrate  50 A outside of the light receiving region S 1 . The wiring F is formed above the semiconductor substrate  50 A with an insulating layer L 1  interposed therebetween. 
     The electrode E 1  and a through-electrode TE are made of metal. The electrode E 1  and the through-electrode TE are made of, for example, aluminum (Al). When the semiconductor substrate is made of Si, copper (Cu) is used as an electrode material instead of aluminum. The electrode E 1  and the through-electrode TE may be integrally formed. The electrode E 1  and the through-electrode TE are formed, for example, by sputtering. 
     The semiconductor photodetecting element  10 A includes a plurality of the through-electrodes TE and a plurality of the electrode pads  12 . Each through-electrode TE is electrically connected to a corresponding avalanche photodiode APD. Each electrode pad  12  is electrically connected to a corresponding through-electrode TE. The electrode pad  12  is electrically connected to the electrode E 1  through the wiring F. The electrode pad  12  is arranged on the principal surface  1 Na. Each through-electrode TE is electrically connected to the light receiving region S 1  through the electrode pad  12 , the wiring F, and the electrode E 1 . The electrode pad  12  is positioned in an area (the inner area of the groove  13 ) AR 1  surrounded by the groove  13  when viewed from the Z-axis direction, and the electrode pad  12  is away from the groove  13 . 
     The through-electrode TE is arranged for each avalanche photodiode APD. The through-electrode TE penetrates through the semiconductor substrate  50 A from the principal surface  1 Na side to the principal surface  1 Nb side. The through-electrode TE is disposed in a through-hole TH penetrating through the semiconductor substrate  50 A in the thickness direction (Z-axis direction). The through-hole TH is located in the area where multiple avalanche photodiodes APD are arranged two-dimensionally. In the semiconductor substrate  50 A, a plurality of the through-holes TH are formed. 
     The opening of the through-hole TH is located in the XY-axis plane and has a circular shape when viewed from the Z-axis direction. The cross-sectional shape of the through-hole TI in the cross section in parallel with the XY-axis plane is a circular shape. The semiconductor photodetecting element  10 A includes the insulating layer L 2  on the inner peripheral surface of the through-hole TH. The through-electrode TE is arranged in the through-hole TH with the insulating layer L 2  interposed therebetween. 
     The plurality of through-holes TH are arranged in such a manner that the centers of the openings are located in a matrix when viewed from the Z-axis direction. In the present embodiment, the row direction is X-axis direction and the column direction is Y-axis direction. The plurality of through-holes TH are arranged in such a manner that the centers of the openings are arranged with an equal distance on a straight line when viewed from each of the X-axis direction and the Y-axis direction. The through-holes TH are arranged with a pitch of 100 μm. 
     Each of the plurality of through-holes TH is formed in an area surrounded by four mutually adjacent avalanche photodiodes APD, of the plurality of avalanche photodiodes APD. In the through-hole TH, the through-electrode TE is arranged, the through-electrode TE being electrically connected to the light receiving region S 1  of one of the four mutually adjacent avalanche photodiodes APD. That is, the through-electrode TE is electrically connected to the light receiving region S 1  of the avalanche photodiode APD of one of the four avalanche photodiodes APD surrounding the through-hole TH in which the through-electrode TB is arranged. 
     The plurality of through-holes TH and the plurality of light receiving regions S 1  are arranged in such a manner that, when viewed from the Z-axis direction, four through-holes TH surround one light receiving region S 1  and four light receiving regions S 1  surround one through-hole TH. The through-hole TH and the light receiving region S 1  are alternately arranged in directions crossing the X-axis and the Y-axis. 
     Each of four sides of the eight sides of the light receiving region S 1  opposes a side of an adjacent light receiving region S 1 , and the remaining four sides oppose the adjacent through-holes TH. One through-hole TH is surrounded by one side of the four light receiving regions S 1  when viewed from the Z-axis direction. The connected portions C are provided on four sides opposing the through-hole TH, of the eight sides of the light receiving region S 1 . 
     The principal surface  1 Na of the semiconductor substrate  50 A includes the light receiving region S 1 , an intermediate area S 2 , and an opening peripheral area S 3 . The opening peripheral area S 3  is an area located at the periphery of the opening of the through-hole TH of the principal surface  1 Na. The intermediate area S 2  is an area excluding the light receiving region S 1  and the opening peripheral area S 3  in the principal surface  1 Na. 
     A groove  13  is formed in an intermediate area S 2  between the light receiving regions S 1  of the four mutually adjacent avalanche photodiodes APD and the through-hole TH surrounded by these avalanche photodiodes APD. The groove  13  extends along the sides adjacent to the through-hole TH, of the plurality of sides of the light receiving regions S 1  of the four mutually adjacent avalanche photodiodes APD when viewed from the Z-axis direction. In the semiconductor photodetecting element  10 A, the groove  13  surrounds the entire circumference of the through-hole TH when viewed from the Z-axis direction. The area AR 1  surrounded by the groove  13  is a substantially square when viewed from the Z-axis direction. One through-hole TH is formed in any given area AR 1 . 
     A groove  14  is formed in the intermediate area S 2  between two mutually adjacent light receiving regions S 1 . The groove  14  extends along two opposing sides of two adjacent light receiving regions S 1  when viewed from the Z-axis direction. The groove  14  connects the grooves  13  surrounding different through-holes TH. In the semiconductor photodetecting element  10 A, the entire circumference of the light receiving region S 1  is surrounded by the grooves  13  and  14 . In one area AR 2 , one light receiving region S 1  is provided. The area AR 2  has substantially the same regular octagonal shape as the shape of the light receiving region S 1 . The areas AR 1  and AR 2  have a polygonal shape when viewed from the 2-axis direction. 
     The groove  14  extends in a straight line in the area between two adjacent light receiving regions S 1 . The groove  14  surrounding the two adjacent light receiving regions S 1  is shared by two adjacent light receiving regions S 1 . The groove  14  located in the area between two adjacent light receiving regions S 1  is not only a groove surrounding one light receiving region S 1  abut also a groove surrounding the other light receiving region S 1 . 
     As illustrated in  FIG. 3 , a distance β from an edge  13   e  of the groove  13  to an edge D 2  of the through-hole TH surrounded by the groove  13  is longer than a distance α from an edge  13   f  of the groove  13  to an edge D 1  of the light receiving region S 1  adjacent to the through-hole TH. In the present embodiment, the distance α is 5.5 μm and the distance β is 7.5 μm. The distance α is the shortest distance from the edge  13   f  of the groove  13  to the edge DI of the light receiving region S 1  adjacent to the through-hole TH when viewed from the Z-axis direction. The distance β is the shortest distance from the edge  13   e  of the groove  13  to the edge D 2  of the through-hole TH surrounded by the groove  13  when viewed from the Z-axis direction. 
     Next, the cross-sectional configuration of the semiconductor photodetecting element according the present embodiment will be described with reference to  FIG. 4 . In  FIG. 4 , the glass substrate  30  and the optical adhesive OA are not illustrated. 
     Each avalanche photodiode APD includes the light receiving region S 1 . Each avalanche photodiode APD includes a first semiconductor region  1 PA of a P-type (first conductivity type), a second semiconductor region  1 NA of an N-type (second conductivity type), a third semiconductor region  1 NB of an N-type, and a fourth semiconductor region  1 PB of P-type. 
     The first semiconductor region  1 PA is located at the principal surface  1 Na side of the semiconductor substrate  50 A. The second semiconductor region  1 NA is located at the principal surface  1 Nb side of the semiconductor substrate  50 A. The third semiconductor region  1 NB is located between the first semiconductor region  1 PA and the second semiconductor region  1 NA and has a lower impurity concentration than the second semiconductor region  1 NA. The fourth semiconductor region  1 PB is formed inside of the first semiconductor region  1 PA and has a higher impurity concentration than the first semiconductor region  1 PA. The fourth semiconductor region  1 PB is the light receiving region S 1 . Each avalanche photodiode APD is configured to include: a P +  layer serving as the fourth semiconductor region  1 PB; a P layer serving as the first semiconductor region  1 PA; an N layer serving as the third semiconductor region  1 NB; and an N +  layer serving as the second semiconductor region  1 NA, which are arranged in this order from the principal surface  1 Na. 
     The first semiconductor region  1 PA is located in the intermediate area S 2  when viewed from the Z-axis direction and is positioned to surround the fourth semiconductor region  1 PB (light receiving region S 1 ). Although not illustrated in the drawing, the first semiconductor region  1 PA is also located in the intermediate area S 2  between two mutually adjacent light receiving regions S 1  when viewed from the Z-axis direction. The intermediate area S 2  of the semiconductor substrate  50 A is configured to include: a P layer serving as the first semiconductor region  1 PA; an N layer serving as the third semiconductor region  1 NB; and an N +  layer serving as the second semiconductor region  1 NA, which are arranged in this order from the principal surface  1 Na except the portion where the grooves  13 ,  14  are formed. 
     The inner surface  13   b  of the groove  13  is formed by the same N +  layer as the second semiconductor region  1 NA. On the inner surface  13   b , an insulating layer  13   c  is provided. A filling material  13   a  is provided in the area surrounded by the insulating layer  13   c  in the groove  13 . The filling material  13   a  is made of, for example, a material that is easy to fill and has a high light shielding property. In the present embodiment, the filling material  13   a  is made of tungsten (W). Like the inner surface  13   b , the inner surface of the groove  14  is formed by the same N layer as the second semiconductor region  1 NA. An insulating layer  13   c  and a filling material  13   a  are provided in the groove  14  like the groove  13 .  FIG. 4  does not illustrate the groove  14 , and the insulating layer  13   c  and the filling material  13   a  provided in the groove  14 . The filling material  13   a  may be made of copper or aluminum instead of tungsten. 
     The depth of the grooves  13  and  14 , i.e., a distance from the principal surface  1 Na to the bottom surfaces of the grooves  13  and  14  in the Z-axis direction (the thickness direction of the semiconductor substrate  50 A), is longer than a distance in the Z-axis direction from the principal surface  1 Na to the interface between the second semiconductor region  1 NA and the third semiconductor region  1 NB, and shorter than the thickness of the semiconductor substrate  50 A. The bottom surface  13   d  of the groove  13  is constituted by the second semiconductor region  1 NA and is located closer to the principal surface  1 Nb than the third semiconductor region  1 NB. 
     The semiconductor substrate  50 A includes an N-type fifth semiconductor region INC. The fifth semiconductor region INC is formed between the edge D 2  of the through-hole TH and the first semiconductor region  1 PA when viewed from the Z-axis direction. Like the second semiconductor region  1 NA, the fifth semiconductor region INC is an N +  layer with a higher impurity concentration than the third semiconductor region  1 NB. On the principal surface  1 Na, an area where the fifth semiconductor region INC is formed is the opening peripheral area S 3 . The opening peripheral area S 3  of the semiconductor substrate  50 A is configured to include: an N +  layer serving as the fifth semiconductor region INC; and an N +  layer serving as the second semiconductor region  1 NA, which are arranged in this order from the principal surface  1 Na. 
     The inner peripheral surface (edge D 2 ) of the through-hole TH is configured to include the fifth semiconductor region INC and the second semiconductor region  1 NA, which are arranged in this order from the principal surface  1 Na. Therefore, a PN junction formed by the first semiconductor region  1 PA and the third semiconductor region  1 NB is not exposed to the through-hole TH. 
     The avalanche photodiode APD includes an electrode E 1 . The connected portion C of the electrode E 1  is connected to the fourth semiconductor region  1 PB (light receiving region S 1 ). As described above, the connected portion C includes the first end portion Ela and the second end portion E 1   b . The electrode E 1  includes the third end portion E 1   c.    
     The first semiconductor region  1 PA is electrically connected to the electrode E 1  through the fourth semiconductor region  1 PB. 
     The electrode pad  12  is electrically connected to the through-electrode TE. The through-electrode TE extends to the back side (adjacent to the principal surface  1 Nb) of the semiconductor substrate  50 A. The through-electrode TE is provided with an insulating layer L 3  adjacent to the mounting substrate  20 . The through-electrode TE is electrically connected to the mounting substrate  20  through a bump electrode BE on the back side of the semiconductor substrate  50 A. The electrode E 1  and the mounting substrate  20  are electrically connected to each other through the wiring F, the electrode pad  12 , the through-electrode TE, and the bump electrode BE. The fourth semiconductor region  1 PB is electrically connected to the mounting substrate  20  through the electrode E 1 , the wiring F, the electrode pad  12 , the through-electrode TE, and the bump electrode BE. The bump electrode BE is made of, for example, solder. 
     The bump electrode BE is formed on the through-electrode TE extending on the principal surface  1 Nb with an under bump metal (UBM), not illustrated, interposed therebetween. The UBM is made of a material with excellent electrical and physical connection with the bump electrode BE. The UBM is formed by, for example, an electroless plating method. The bump electrode BE is formed by, for example, a method of mounting a solder ball or a printing method. 
     Next, the mounting substrate according to the present embodiment will be described with reference to  FIG. 5 .  FIG. 5  is a schematic plan view of the mounting substrate. As illustrated in  FIG. 5 , the mounting substrate  20  includes a plurality of electrodes E 9 , a plurality of quenching resistors R 1 , and a plurality of signal processing units SP. The mounting substrate  20  constitutes an application specific integrated circuit (ASIC). The quenching resistor R 1  may be located at the semiconductor photodetecting element  10 A instead of the mounting substrate  20 . 
     Each electrode E 9  is electrically connected to the bump electrode BE. The electrode E 9  is made of a metal just like the electrode E 1  and the through-electrode TE. The electrode E 9  is made of, for example, aluminum. The material constituting the electrode E 9  may be copper instead of aluminum. 
     Each quenching resistor R 1  is disposed on the principal surface  20   a  side. One end of the quenching resistor R 1  is electrically connected to the electrode E 9 , and the other end of the quenching resistor R 1  is connected to a common electrode CE. The quenching resistor R 1  constitutes a passive quenching circuit. A plurality of quenching resistors R 1  are connected in parallel to the common electrode CE. 
     Each signal processing unit SP is located on the principal surface  20   a  side. An input terminal of the signal processing unit SP is electrically connected to the electrode E 9  and an output terminal of the signal processing unit SP is connected to the signal line TL. Each signal processing unit SP receives an output signal from the corresponding avalanche photodiode APD (semiconductor photodetecting element  10 A) through the electrode E 1 , the through-electrode TE, the bump electrode BE, and the electrode E 9 . Each signal processing unit SP processes the output signal from the corresponding avalanche photodiode APD. Each signal processing unit SP includes a CMOS circuit that converts the output signal from the corresponding avalanche photodiode APD into a digital pulse. 
     Next, the circuit configuration of the photodetecting device  1  will be described with reference to  FIG. 6 .  FIG. 6  is a circuit diagram of the photodetecting device. In the photodetecting device  1  (semiconductor photodetecting element  10 A), an avalanche photodiode APD is formed by a PN junction formed between the N-type third semiconductor region  1 NB and the P-type first semiconductor region  1 PA. The semiconductor substrate  50 A is electrically connected to an electrode (not illustrated) arranged on the back side, and the first semiconductor region  1 PA is connected to the electrode E 1  through the fourth semiconductor region  1 PB. Each quenching resistor R 1  is connected in series with the corresponding avalanche photodiode APD. 
     In the semiconductor photodetecting element  10 A, each avalanche photodiode APD operates 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 between the anode and the cathode of the avalanche photodiode APD. For example, a (−) potential V 1  is applied to the anode and a (+) potential V 2  is applied to the cathode. The polarities of these potentials are relative to each other, and one potential may be the ground potential. 
     The anode is the first semiconductor region  1 PA and the cathode is the third semiconductor region  1 NB. When light (photon) is incident on the avalanche photodiode APD, photoelectric conversion is performed inside of the substrate to generate photoelectrons. At an area near the PN junction interface of the first semiconductor region  1 PA, avalanche multiplication is performed and the amplified electron group moves toward the electrode arranged on the back side of the semiconductor substrate  50 A. When light (photon) is incident on any cell (avalanche photodiode APD) of the semiconductor photodetecting element  10 A, the light is multiplied and obtained from the electrode E 9  as a signal. The signal retrieved from the electrode E 9  is input to the corresponding signal processing unit SP. 
     As described above, the photodetecting device  1  is configured in such a manner that, on the principal surface  1 Na side of the semiconductor substrate  50 A, the groove  13  surrounding the through-hole TH is formed in the intermediate area S 2  between the through-hole TH and the light receiving region S 1  adjacent to the through-hole TH. Therefore, in the intermediate area S 2  between the through-electrode TE and the light receiving region S 1 , the principal surface  1 Na of the semiconductor substrate  50 A is divided. As a result, even if the light receiving region S 1  and the through-electrode TE are close to each other in order to ensure the aperture ratio of the avalanche photodiode APD, the flow of the surface leakage electric current from the through-electrode TE to the avalanche photodiode APD is reduced. 
     The distance β is longer than the distance α. Therefore, structural defects tend not to be generated around the through-holes TH in the semiconductor substrate  50 A. 
     The bottom surface  13   d  of the groove  13  is constituted by the second semiconductor region  1 NA. The bottom surface  13   d  of the groove  13  is located deeper than the third semiconductor region  1 NB. Therefore, even when charges are generated in the area surrounded by the groove  13  in the semiconductor substrate  50 A, this suppresses movement of the charges generated in the area to the avalanche photodiode APD. Since the bottom surface  13   d  of the groove  13  is formed in the semiconductor substrate  50 A, i.e., the groove  13  does not reach the principal surface  1 Nb of the semiconductor substrate  50 A, the semiconductor substrate  50 A will not be separated at the position of the groove  13 . Therefore, in the manufacturing process of the photodetecting device  1 , the semiconductor substrate  50 A is easily handled. 
     In the groove  13 , a filling material  13   a  made of tungsten is provided. Since the electrode pad  12  is spaced apart from the groove  13 , the parasitic capacitance generated between the electrode pad  12  and the filling material  13   a  is reduced. 
     When viewed from the Z-axis direction, the area AR 1  and the area AR 2  have a polygonal shape and the light receiving region S 1  has a polygonal shape. When the light receiving region S 1  has a circular shape, there is no corner where the electric field concentrates. In the case where the light receiving region S 1  has a circular shape, the dead space generated between the light receiving region S 1  and the through-hole TH is large, as compared with when the light receiving region S 1  has a polygonal shape. Therefore, it is difficult to ensure the aperture ratio. The areas AR 1  and AR 2 , and the light receiving region S 1  have a polygonal shape. The areas AR 1  and AR 2  and the light receiving region S 1  are arranged in such a manner that the sides of the areas AR 1  and AR 2  are along the side of the light receiving region S 1 . For this reason, the photodetecting device  1  has a small dead space, and a high aperture ratio. 
     When viewed from the Z-axis direction, the opening of the through-hole TH has a circular shape, and the insulating layer L 2  is arranged in the inner peripheral surface of the through-hole TR Since the insulating layer L 2  is disposed on the inner peripheral surface of the through-hole TH, the through-electrode TE and the semiconductor substrate  50 A are electrically insulate from each other. When there is a corner at the opening of the through-hole TH, a crack may be formed at the corner of the insulating layer L 2  when the insulating layer L 2  is formed. In the present embodiment, since the through-hole TH has a circular shape when viewed from a direction perpendicular to the principal surface  1 Na, cracks tend not to be generated in the insulating layer L 2  when the insulating layer L 2  is formed. Therefore, in the photodetecting device  1 , electrical insulation between the through-electrode TE and the semiconductor substrate  50 A is ensured. 
     The through-electrode TE is electrically connected to the light receiving region S 1  of avalanche photodiode APD of one of the four mutually adjacent avalanche photodiodes APD. In which case, since the wiring distance between the through-electrode TE and the light receiving region S 1  electrically connected to the through-electrode TE is relatively short, it is unsusceptible to influence by the wiring resistance and the parasitic capacitance. Therefore, this suppresses degradation of the detection accuracy of the photodetecting device  1 . 
     The light receiving region S 1  has a polygonal shape when viewed from the Z-axis direction. When viewed from the Z-axis direction, the groove  13  extends along the sides adjacent to the through-hole TH 1 , of the plurality of the sides of the light receiving regions S 1  of four avalanche photodiodes APD adjacent to the through-hole TH. Since the groove  13  extends along the sides of the light receiving regions S 1 , the distance between the through-hole TH and light receiving region S 1  can be configured to be narrow even when the through-hole TH is formed in each area surrounded by four mutually adjacent avalanche photodiodes APD. For this reason, the photodetecting device  1  has a small dead space, and a high aperture ratio. 
     When the light receiving region S 1  has an octagonal shape when viewed from the Z-axis direction, the area other than the through-electrode TE in the principal surface  1 Na can be efficiently made use of. Therefore, the photodetecting device  1  achieves a configuration having a short wiring distance between the through-electrode TE and the light receiving region S 1 , and the aperture ratio is improved, as compared with in the case where the light receiving region S 1  has other shapes. 
     When the filling material  13   a  disposed in the grooves  13 ,  14  is made of a metal, a parasitic capacitance may be generated between the filling material  13   a  and the light receiving region S 1 . When the value of parasitic capacitance differs according to the position between the filling material  13   a  and the light receiving region S 1 , i.e., when the value of parasitic capacitance is deviated, the photodetecting accuracy of the avalanche photodiode APD may be reduced. In the photodetecting device  1 , the grooves  13  and  14  are formed in such a manner that the edges of the grooves  13  and  14  are along the edge D 1  of the light receiving region S 1  when viewed from the Z-axis direction. Therefore, even when a parasitic capacitance is generated between the filling material  13   a  and the light receiving region S 1 , the value of the parasitic capacitance is less likely to be biased. As a result, the avalanche photodiode APD is less affected by the parasitic capacitance. 
     The groove  14  surrounding the two adjacent light receiving regions S 1  is formed in such a manner that the edge of the groove  14  is along the edge DI of the light receiving region S 1 . The groove  14  is shared by two adjacent light receiving regions S 1 . Therefore, the avalanche photodiode APD is less affected by the parasitic capacitance. Furthermore, the area of the principal surface  1 Na is effectively utilized, so that the light receiving regions S 1  of the avalanche photodiodes APD are densely arranged. As a result, not only a reduction of the influence of the parasitic capacitance on the avalanche photodiode APD but also an improvement of the aperture ratio is realized. 
     Next, a configuration of a photodetecting device according to a modification of the present embodiment will be described with reference to  FIG. 7 .  FIG. 7  is a diagram for describing a cross-sectional configuration of a photodetecting device according to the modification of the present embodiment.  FIG. 7  illustrates a cross-sectional configuration obtained when the photodetecting device according to this modification is cut along the plane corresponding to line IV-IV illustrated in  FIG. 2 .  FIG. 7  also does not illustrate the glass substrate  30  and the optical adhesive OA. The modification is generally similar or the same as the above-described embodiment, but the modification differs from the above-described embodiment in the configuration of the avalanche photodiodes APD, as described below. 
     The photodetecting device according to the present modification includes a semiconductor photodetecting element  10 B. The semiconductor photodetecting element  10 B is disposed between the mounting substrate  20  and the glass substrate  30 . The semiconductor photodetecting element  10 B includes a semiconductor substrate  50 B having a rectangular shape in a plan view. The semiconductor substrate  50 B is made of Si and is an N type (second conductivity type) semiconductor substrate. The semiconductor substrate  50 B includes a principal surface  1 Na and a principal surface  1 Nb that oppose each other. The semiconductor photodetecting element  10 B includes a plurality of avalanche photodiodes APD and a plurality of through-electrodes TE. The plurality of avalanche photodiodes APD are two-dimensionally arranged on the semiconductor substrate  50 B. In the present modification, the avalanche photodiodes APD are arranged in a matrix. 
     A groove  23  formed in the semiconductor photodetecting element  10 B has the same configuration as the groove  13  formed in the semiconductor photodetecting element  10 A. The groove  23  is formed in an intermediate area S 2  between the light receiving regions S 1  of the four mutually adjacent avalanche photodiodes APD and the through-hole TH surrounded by these avalanche photodiodes APD. The groove  23  extends along the sides adjacent to the through-hole TH, of the plurality of sides of the light receiving regions S 1  of the four avalanche photodiodes APD adjacent to the through-hole TH when viewed from the Z-axis direction. In the semiconductor photodetecting element  10 B, the groove  23  surrounds the entire circumference of the through-hole TH. The area AR 1  surrounded by the groove  23  is substantially square when viewed from the Z-axis direction. In the present modification, one through-hole TH is also formed in any given area AR 1 . 
     A groove  14  is formed in the intermediate area S 2  between two mutually adjacent light receiving regions S 1 .  FIG. 7  does not illustrate the groove  14 . The groove  14  extends along two opposing sides of two adjacent light receiving regions S 1  when viewed from the Z-axis direction. The groove  14  connects the grooves  23  surrounding different through-holes TH. In the semiconductor photodetecting element  10 B, the entire circumference of the light receiving region S 1  is surrounded by the grooves  23  and  14 . The area AR 2  surrounded by the grooves  23  and  14  has substantially the same regular octagonal shape as the shape of the light receiving region S 1 . In the present modification, the areas AR 1  and AR 2  have a polygonal shape when viewed from the Z-axis direction. One light receiving region S 1  is disposed in any given area AR 2 . 
     In the present modification, the groove  14  extends in a straight line in the area between two adjacent light receiving regions S 1 . The groove  14  surrounding the two adjacent light receiving regions S 1  is shared by two adjacent light receiving regions S 1 . 
     As illustrated in  FIG. 7 , a distance β from an edge  23   e  of the groove  23  to an edge D 2  of the through-hole TH surrounded by the groove  23  is longer than a distance α from an edge  23   f  of the groove  23  to an edge D 1  of the light receiving region S 1  adjacent to the through-hole TH. In the present modification, the distance α is 5.5 μm and the distance β is 7.5 μm. The distance α is the shortest distance from the edge  23   f  of the groove  23  to the edge D 1  of the light receiving region S 1  adjacent to the through-hole TH when viewed from the Z-axis direction. The distance β is the shortest distance from the edge  23   e  of the groove  23  to the edge D 2  of the through-hole TH surrounded by the groove  23  when viewed from the Z-axis direction. 
     In the semiconductor photodetecting element  10 B, each avalanche photodiode APD also includes the light receiving region S 1 . Each avalanche photodiode APD includes a first semiconductor region  2 PA of P-type (first conductivity type), a second semiconductor region  2 PB of P-type, a third semiconductor region  2 NA of N-type, and a fourth semiconductor region  2 PC of P-type. 
     The first semiconductor region  2 PA is located at the principal surface  1 Na side of the semiconductor substrate  50 B. The second semiconductor region  2 PB is located at the principal surface  1 Nb side of the semiconductor substrate  50 B, and has a higher impurity concentration than the first semiconductor region  2 PA. The third semiconductor region  2 NA is formed at the principal surface  1 Na side of the first semiconductor region  2 PA. The fourth semiconductor region  2 PC is formed in the first semiconductor region  2 PA to be in contact with the third semiconductor region  2 NA and has a higher impurity concentration than the first semiconductor region  2 PA. The third semiconductor region  2 NA is the light receiving region S 1 . Each avalanche photodiode APD is configured to include: an N +  layer serving as the third semiconductor region  2 NA; a P layer serving as the fourth semiconductor region  2 PC; a P −  layer serving as the first semiconductor region  2 PA; and a P +  layer serving as the second semiconductor region  2 PB, which are arranged in this order from the principal surface  1 Na. 
     The first semiconductor region  2 PA is located in the intermediate area S 2  when viewed from the Z-axis direction and is positioned to surround the third semiconductor region  2 NA that is the light receiving region S 1 . Although not illustrated in the drawing, the first semiconductor region  2 PA is also located in the intermediate area S 2  between two mutually adjacent light receiving regions S 1  when viewed from the Z-axis direction. The intermediate area S 2  of the semiconductor substrate  50 B is configured to include: a P −  layer serving as the first semiconductor region  2 PA; and a P +  layer serving as the second semiconductor region  2 PB, which are arranged in this order from the principal surface  1 Na except the portion where the grooves  23 ,  14  are formed. 
     The inner surface  23   b  of the groove  23  is formed by the same P +  layer as the second semiconductor region  2 PB. On the inner surface  23   b , an insulating layer  23   c  is provided. A filling material  23   a  is provided in the area surrounded by the insulating layer  23   c  in the groove  23 . The filling material  23   a  is made of, for example, a material that is easy to fill and has a high light shielding property. In the present modification, the filling material  23   a  is made of tungsten (W), which is the same as the filling material  13   a . Like the inner surface  23   b , the inner surface of the groove  14  is formed by the P +  layer having a higher impurity concentration than the first semiconductor region  2 PA. An insulating layer  23   c  and a filling material  23   a  are provided in the groove  14  like the groove  23 . As described above,  FIG. 7  does not illustrate the groove  14 , and the insulating layer  23   c  and the filling material  23   a  provided in the groove  14 . The filling material  13   a  may be made of copper or aluminum instead of tungsten. 
     The depth of the grooves  23  and  14 , i.e., a distance from the principal surface  1 Na to the bottom surfaces of the grooves  23  and  14  in the Z-axis direction (the thickness direction of the semiconductor substrate SOB), is longer than a distance in the Z-axis direction from the principal surface  1 Na to the interface between the first semiconductor region  2 PA and the second semiconductor region  2 PB, and shorter than the thickness of the semiconductor substrate  50 B. The bottom surface  23   d  of the groove  23  is constituted by the second semiconductor region  2 PB and is located closer to the principal surface  1 Nb than the first semiconductor region  2 PA. 
     The semiconductor substrate  50 B includes a P-type fifth semiconductor region  2 PD. The fifth semiconductor region  2 PD is formed between the edge D 2  of the through-hole TH and the first semiconductor region  2 PA when viewed from the Z-axis direction. Like the second semiconductor region  2 PB, the fifth semiconductor region  2 PD is a P +  layer with a higher impurity concentration than the first semiconductor region  2 PA. On the principal surface  1 Na, an area where the fifth semiconductor region  2 PD is formed is the opening peripheral area S 3 . The opening peripheral area S 3  of the semiconductor substrate SOB is configured to include: a P +  layer serving as the fifth semiconductor region  2 PD; and a P +  layer serving as the second semiconductor region  2 PB, which are arranged in this order from the principal surface  1 Na. 
     The inner peripheral surface (edge D 2 ) of the through-hole TH is configured to include the fifth semiconductor region  2 PD and the second semiconductor region  2 PB, which are arranged in this order from the principal surface  1 Na. Therefore, a PN junction formed by the third semiconductor region  2 NA and the fourth semiconductor region  2 PC is not exposed to the through-hole TH. 
     The avalanche photodiode APD includes an electrode E 1 . The electrode E 1  is arranged at the principal surface  1 Na side of the semiconductor substrate  50 B. In the present modification, the electrode E 1  is provided along the contour of the light receiving region S 1  and has an octagonal ring shape. 
     The electrode E 1  includes a connected portion C that is electrically connected to the light receiving region S 1 . In the present modification, as illustrated in  FIG. 7 , the connected portion C includes a first end portion Ela and a second end portion E 1   b . The electrode E 1  includes a third end portion E 1   c  that is electrically connected to the wiring F. 
     As illustrated in  FIG. 7 , the wiring F extends from the third end portion E 1   c  in the direction opposite to the center of the light receiving region S 1 . The wiring F electrically connects the electrode E 1  and an electrode pad  12 . The wiring F is located above the semiconductor substrate  50 B outside of the light receiving region S 1 . The wiring F is formed above the semiconductor substrate  50 B with an insulating layer L 1  interposed therebetween. 
     In the present modification, the electrode pad  12  is also electrically connected to the through-electrode TE. The through-electrode TE extends to the back side (adjacent to the principal surface  1 Nb side) of the semiconductor substrate  50 B. The through-electrode TE is provided with an insulating layer L 3 . The through-electrode TE is electrically connected to the mounting substrate  20  via the bump electrode BE. The electrode E 1  and the mounting substrate  20  are electrically connected to each other through the wiring F, the electrode pad  12 , the through-electrode TE, and the bump electrode BE. The third semiconductor region  2 NA is electrically connected to the mounting substrate  20  through the electrode E 1 , the wiring F, the electrode pad  12 , the through-electrode TE, and the bump electrode BE. 
     As described above, according to the present modification, on the principal surface  1 Na side of the semiconductor substrate SOB, the groove  23  surrounding the through-hole TH is formed in the intermediate area S 2  between the through-hole TH and the light receiving region S 1  adjacent to the through-hole TH. Therefore, in the intermediate area S 2  between the through-electrode TE and the light receiving region S 1 , the principal surface  1 Na of the semiconductor substrate  50 B is divided. As a result, even if the light receiving region S 1  and the through-electrode TE are close to each other in order to ensure the aperture ratio of the avalanche photodiode APD, the flow of the surface leakage electric current from the through-electrode TE to the avalanche photodiode APD is reduced. 
     The distance β is longer than the distance α. Therefore, structural defects tend not to be generated around the through-holes TH in the semiconductor substrate  50 B. 
     The bottom surface  23   d  of the groove  23  is constituted by the second semiconductor region  2 PB. The bottom surface  23   d  of the groove  23  is located deeper than the first semiconductor region  2 PA. Therefore, even when charges are generated in the area surrounded by the groove  23  in the semiconductor substrate SOB, this suppresses movement of the charges generated in the area to the avalanche photodiode APD. Since the bottom surface  23   d  of the groove  23  is formed in the semiconductor substrate SOB, i.e., the groove  23  does not reach the principal surface  1 Nb of the semiconductor substrate  50 B, the semiconductor substrate  50 B will not be separated at the position of the groove  23 . Therefore, in the manufacturing process of the photodetecting device according to the present modification, the semiconductor substrate  50 B is easily handled. 
     Next, the configurations of modifications of the semiconductor photodetecting element will be described with reference to  FIG. 8  to  FIG. 11 .  FIG. 8  to  FIG. 11  are schematic plan views illustrating the modifications of the semiconductor photodetecting element. 
     Semiconductor photodetecting elements  10 C,  10 D,  10 E, and  10 F are disposed between a mounting substrate  20  and a glass substrate  30 . Like the semiconductor photodetecting element  10 A, the semiconductor photodetecting elements  10 C,  10 D,  10 E, and  10 F include a semiconductor substrate  50 A having a rectangular shape in a plan view. The semiconductor photodetecting elements  10 C,  10 D,  10 E, and  10 F include a plurality of avalanche photodiodes APD and a plurality of through-electrodes TE. 
     In the semiconductor photodetecting element  10 C as illustrated in  FIG. 8 , a groove  13  is formed in an intermediate area S 2  between the through-hole TH and the light receiving region S 1  adjacent to the through-hole TH. The groove  13  surrounds the through-hole TH. The groove  13  is not formed in an area arranged with the wiring F that electrically connects the through-electrode TE and the light receiving region S 1  when viewed from the Z-axis direction. The groove  13  surrounds the through-hole TH in such a state that the groove  13  is divided by the area where the wiring F is arranged when viewed from the Z-axis direction. 
     In the semiconductor photodetecting element  10 D as illustrated in  FIG. 9 , a groove  13  is formed in the intermediate area S 2  between a through-hole TH and the light receiving region S 1  adjacent to the through-hole TH. The groove  13  surrounds the through-hole TH. 
       FIG. 2  and  FIG. 9  are scaled differently. The size of the electrode pad  12  of the semiconductor photodetecting element  10 D is the same as the size of the electrode pad  12  of the semiconductor photodetecting element  10 A. 
     The through-holes TH and the light receiving regions S 1  are two-dimensionally arranged. Each pitch of the through-hole TH and the light receiving region S 1  is less than those of the semiconductor photodetecting element  10 A. In the semiconductor photodetecting element  10 D, the through-hole TH and the light receiving region S 1  are arranged in a one-to-one relationship to achieve a higher resolution than the semiconductor photodetecting element  10 A. Each pitch of the light receiving region S 1  and the through-hole TH is, for example, 70 μM. 
     In the semiconductor photodetecting element  10 D, the groove  13  surrounds the through-hole TH, like the semiconductor photodetecting element  10 A. Like the semiconductor photodetecting element  10 A, the groove  14  also extends along two opposing sides of two adjacent light receiving regions S 1  when viewed from the Z-axis direction. The groove  14  connects the grooves  13  surrounding different through-holes TH. In the semiconductor photodetecting element  10 D, the entire circumference of the light receiving region S 1  is also surrounded by the grooves  13  and  14 . 
     In the semiconductor photodetecting element  10 D, each pitch of the through-electrode TE and the light receiving region S 1  is smaller than those of the semiconductor photodetecting element  10 A. In the semiconductor photodetecting element  10 D, the groove  14  surrounding the two adjacent light receiving regions S 1  is formed in such a manner that the edge of the groove  14  is along the edge D 1  of the light receiving region S 1 , like the semiconductor photodetecting element  10 A. The groove  14  is shared by two adjacent light receiving regions S 1 . 
     Therefore, the avalanche photodiode APD is less affected by the parasitic capacitance. In addition, the area of the principal surface  1 Na is effectively utilized, and the light receiving regions S 1  of the avalanche photodiodes APD are densely arranged. 
     It is difficult to reduce the size of the through-electrode TE because of problems in machining accuracy or ensuring electrical connection. In order to reduce the parasitic capacitance generated between the electrode pad  12  and the filling material  13   a  in the grooves  13  and  14 , the grooves  13  and  14  are separated from the electrode pad  12 . In order to improve the aperture ratio, the light receiving region S 1  has a polygonal shape. 
     Under these conditions, the light receiving region S 1  of the semiconductor photodetecting element  10 D has a polygonal shape different from the light receiving region S 1  of the semiconductor photodetecting element  10 A. More specifically, the light receiving region S 1  of the semiconductor photodetecting element  10 D has a polygonal shape in which the length of the side opposing the adjacent light receiving region S 1  is shorter than the length of the side opposing the adjacent through-hole TH. 
     With this configuration, in the semiconductor photodetecting element  10 D, the resolution is higher than that of the semiconductor photodetecting element  10 A, and the semiconductor photodetecting element  10 D achieves a higher aperture ratio. The parasitic capacitance generated among the avalanche photodiode APD, the filling material  13   a , and the electrode pad  12  is reduced. 
     In the semiconductor photodetecting element  10 E illustrated in  FIG. 10 , the groove  13  is formed in the intermediate area S 2  between the through-hole TH and the light receiving region S 1  adjacent to the through-hole TH. The groove  13  surrounds the through-hole TH.  FIG. 2  and  FIG. 10  are scaled differently. The size of the electrode pad  12  of the semiconductor photodetecting element  10 E is the same as the size of the electrode pad  12  of the semiconductor photodetecting element  10 A. 
     In the semiconductor photodetecting element  10 E, the pitch of the through-hole TH is the same as the pitch of the through-hole TH of the semiconductor photodetecting element  10 A, the pitch of the light receiving region S 1  is the same as the pitch of the light receiving region S 1  of the semiconductor photodetecting element  10 A. The through-hole TH and the light receiving region S 1  are arranged in such a manner that the through-hole TH and the light receiving region S 1  are arranged in a one-to-one relationship. Like the light receiving region S 1  of the semiconductor photodetecting element  10 A, the light receiving region S 1  of the semiconductor photodetecting element  10 E has a substantially octagonal shape. The area of the light receiving region S 1  of the semiconductor photodetecting element  10 E is smaller than the area of the light receiving region S 1  of the semiconductor photodetecting element  10 A. In the semiconductor photodetecting element  10 E, two grooves  14  extend in the area between two mutually adjacent light receiving regions S 1 . One groove  14  surrounds one light receiving region S 1  and the other groove  14  surrounds the other light receiving region S 1 . That is, the groove  14  is not shared by two adjacent light receiving regions S 1 . 
     The groove  13  of the semiconductor photodetecting element  10 E surrounds the through-hole TH in such a state that the grooves  13  are separated in the row direction and the column direction in which the through-holes TH are arranged. Like the groove  14  of the semiconductor photodetecting element  10 A, the groove  14  of the semiconductor photodetecting element  10 E also extends along two opposing sides of two adjacent light receiving regions S 1  when viewed from the Z-axis direction. The groove  14  connects the grooves  13  surrounding different through-holes TI. In the semiconductor photodetecting element  10 E, the entire circumference of the light receiving region S 1  is also surrounded by the grooves  13  and  14 . 
     In order to reduce the influence of the parasitic capacitance on the avalanche photodiode APD, the grooves  13  and  14  are formed in such a manner that the edges of the grooves  13  and  14  are along the edge D 1  of the light receiving region S 1 . It is difficult to reduce the size of the through-electrode TE because of problems in machining accuracy or ensuring electrical connection. In order to reduce the parasitic capacitance generated between the electrode pad  12  and the filling material  13   a  in the grooves  13  and  14 , the grooves  13  and  14  are separated from the electrode pad  12 . 
     In the semiconductor photodetecting element  10 E, since two grooves  14  extend in the area between two mutually adjacent light receiving regions S 1 , the crosstalk between the light receiving regions S 1  is reduced to a level lower than the semiconductor photodetecting element  10 A. Therefore, in the semiconductor photodetecting element  10 E, the crosstalk between the light receiving regions S 1  is reduced to a level lower than the semiconductor photodetecting element  10 A, and the parasitic capacitance generated between the avalanche photodiode APD, the filling material  13   a , and the electrode pad  12  is reduced. 
     In the semiconductor photodetecting element  10 F illustrated in  FIG. 11 , the groove  13  is formed in the intermediate area S 2  between the through-hole TH and the light receiving region S 1  adjacent to the through-hole TH. The groove  13  surrounds the through-hole  111 .  FIG. 10  and  FIG. 11  are scaled differently. The size of the electrode pad  12  of the semiconductor photodetecting element  10 F is the same as the size of the electrode pad  12  of the semiconductor photodetecting element  10 E. 
     The through-holes TH and the light receiving regions S 1  are two-dimensionally arranged. Each pitch of the through-hole TH and the light receiving region S 1  is less than those of the semiconductor photodetecting element  10 E. In the semiconductor photodetecting element  10 F, the through-hole TH and the light receiving region S 1  are arranged in a one-to-one relationship to achieve a higher resolution than the semiconductor photodetecting element  10 E. Each pitch of the light receiving region S 1  and the through-hole TH is, for example, 50 μm. 
     The groove  13  of the semiconductor photodetecting element  10 F surrounds the through-hole TH in such a state that the grooves  13  are separated in the row direction and the column direction in which the through-holes TH are arranged. Like the groove  14  of the semiconductor photodetecting element  10 E, the groove  14  of the semiconductor photodetecting element  10 F also extends along two opposing sides of two adjacent light receiving regions S 1  when viewed from the Z-axis direction. The groove  14  connects the grooves  13  surrounding different through-holes TH. In the semiconductor photodetecting element  10 F, the entire circumference of the light receiving region S 1  is also surrounded by the grooves  13  and  14 . 
     In order to reduce the influence of the parasitic capacitance on the avalanche photodiode APD, the grooves  13  and  14  are formed in such a manner that the edges of the grooves  13  and  14  are along the edge D 1  of the light receiving region S 1 . It is difficult to reduce the size of the through-electrode TE because of problems in machining accuracy or ensuring electrical connection. In order to reduce the parasitic capacitance generated between the electrode pad  12  and the filling material  13   a  in the grooves  13  and  14 , the grooves  13  and  14  are separated from the electrode pad  12 . 
     Under these conditions, the groove  14  is not shared by two adjacent light receiving regions S 1 , and the light receiving region S 1  of the semiconductor photodetecting element  10 F has a polygonal shape different from the light receiving region S 1  of the semiconductor photodetecting element  10 E. More specifically, the light receiving region S 1  of the semiconductor photodetecting element  10 F has a polygonal shape in which the length of the side opposing the adjacent light receiving region S 1  is shorter than the length of the side opposing the adjacent through-hole TH. 
     With this configuration, in the semiconductor photodetecting element  10 F, the resolution is higher than that of the semiconductor photodetecting element  10 E, and the semiconductor photodetecting element  10 F has a higher aperture ratio. The parasitic capacitance generated among the avalanche photodiode APD, the filling material  13   a , and the electrode pad  12  is reduced. 
     Although the preferred embodiments and modifications of the present invention have been described above, the present invention is not necessarily limited to the above-described embodiments and modifications, and various modifications can be made without departing from the gist thereof. 
     In the above-described embodiment and modifications, a single avalanche photodiode APD is electrically connected to a single through-electrode TE (a single electrode pad  12 ), but the present embodiment and modification are not limited thereto. A plurality of avalanche photodiodes APD may be electrically connected to a single through-electrode TE (a single electrode pad  12 ). 
     In the above-described embodiment and modifications, two types of layer structures, i.e., the semiconductor substrate  50 A and the semiconductor substrate  50 B, are illustrated as the avalanche photodiode APD, but the layer structure of the semiconductor substrate is not limited thereto. In the avalanche photodiode APD provided in the semiconductor substrate  50 A, for example, the second semiconductor region  1 NA and the third semiconductor region  1 NB may be made of a single semiconductor region. In which case, the avalanche photodiode APD includes a semiconductor region of a first conductivity type (for example, N-type), a semiconductor region of a second conductivity type (for example, P-type) forming a pn junction with the semiconductor region of the first conductivity type, and another semiconductor region of the second conductivity type that is located in the semiconductor region of the second conductivity type and that has a higher impurity concentration than the semiconductor region of the second conductivity type. In this configuration, the semiconductor region of the second conductivity type having the higher impurity concentration is the light receiving region. In the avalanche photodiode APD provided in the semiconductor substrate  50 B, for example, the first semiconductor region  2 PA, the second semiconductor region  2 PB, and the fourth semiconductor region  2 PC may be made of a single semiconductor region. In which case, the avalanche photodiode APD includes a semiconductor region of a first conductivity type (for example, P-type), and a semiconductor region of a second conductivity type (for example N-type) that is located in the semiconductor region of the first conductivity type and that forms a pn junction with the semiconductor region of the first conductivity type. In this configuration, the semiconductor region of the second conductivity type is the light receiving region. 
     In the semiconductor substrate  50 A and the semiconductor substrate  50 B, each conductivity type of P-type and N-type may be exchanged to be opposite to the above conductivity type. The light receiving region S 1  of the semiconductor substrate  50 A may be configured to include N +  layer, N layer, P layer, and P +  layer, which are arranged in this order from the principal surface  1 Na. The light receiving region S 1  of the semiconductor substrate  50 B is configured to include P +  layer, N layer, N −  layer, N +  layer, which are arranged in this order from the principal surface  1 Na. 
     In the above-described embodiment and modifications, the distance α is 5.5 μm and the distance β is 7.5 μm. The distance α and the distance β may be other than the above values as long as the distance β is longer than the distance α. 
     Although in the above-described embodiment and modifications, the light receiving regions S 1  are described as having the polygonal shape when viewed from the Z-axis direction, other shapes may be used. For example, the light receiving regions S 1  may have a circular shape, or any other suitable shape. Although in the above-described embodiment and modifications, the through-holes TH are described as having the circular shape when viewed from the Z-axis direction, other shapes may be used. For example, the through-holes TH may have a polygonal shape, or any other suitable shape. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for a photodetecting device to detect weak light. 
     REFERENCE SIGNS LIST 
     
         
           1  photodetecting device 
           12  electrode pad 
           13 ,  23  groove 
           13   d ,  23   d  bottom surface 
           50 A,  50 B semiconductor substrate 
           1 Na,  1 Nb principal surface 
         S 1  light receiving region 
         S 2  intermediate area 
         APD avalanche photodiode 
         TH through-hole 
         TE through-electrode 
         a, p distance 
           1 PA first semiconductor region 
           1 PB fourth semiconductor region 
           1 NA second semiconductor region 
           1 NB third semiconductor region 
           2 PA first semiconductor region 
           2 PB second semiconductor region 
           2 PC fourth semiconductor region 
           2 NA third semiconductor region 
         AR 1 , AR 2  area 
         D 1 , D 2 ,  13   e ,  13   f  edge.