Patent Publication Number: US-2022238573-A1

Title: Photoelectric conversion apparatus and optical detection system

Description:
BACKGROUND 
     Field of the Disclosure 
     The aspect of the embodiments relates to a photoelectric conversion apparatus performing photoelectric conversion and to an optical detection system. 
     Description of the Related Art 
     There is known a photoelectric conversion apparatus including a pixel array in which SPAD (Single Photon Avalanche Diode) pixels are formed in a planar pattern. In the SPAD pixels, a photocarrier attributable to a single photon causes avalanche multiplication in a PN junction region within a semiconductor region. 
     Japanese Patent Laid-Open No. 2018-88488 discloses, in relation to a photoelectric conversion apparatus including SPAD pixels, a first wiring supplying a potential to an N-type diffusion layer (cathode) of an avalanche photodiode (hereinafter abbreviated to an “APD”), and a second wiring supplying a potential to a P-type diffusion layer (anode) of the APD. In Japanese Patent Laid-Open No. 2018-88488, the first wiring and the second wiring are disposed on a surface of a substrate on an opposite side to a light incident surface of the substrate. The first wiring is disposed to cover an avalanche multiplication region. 
     The photoelectric conversion apparatus disclosed in Japanese Patent Laid-Open No. 2018-88488 has room for improvement regarding relationships between the first wiring and the second wiring, including configurations and placement positions of those wirings. For example, when pixels including the avalanche photodiodes are used, a withstand voltage is to be ensured in consideration of the difference between a voltage applied to the cathode and a voltage applied to the anode. In Japanese Patent Laid-Open No. 2018-88488, however, relationships between a first metal wiring supplying a potential to the cathode and a second metal wiring supplying a potential to the anode, including configurations and placement positions of those metal wirings, are not explained in consideration of the withstand voltage. 
     SUMMARY OF THE DISCLOSURE 
     According to one aspect, the present disclosure provides an apparatus including a layer that has a light incident surface and includes conversion elements, and a wiring structure disposed on a surface of the layer on an opposite side to the light incident surface, wherein each of the conversion elements includes a photodiode, the photodiode includes a first region of a first conductivity type in which charges having the same polarity as signal carriers are majority carriers and a second region of a second conductivity type, a voltage is supplied to the second region through a region of the second conductivity type, the wiring structure includes a first wiring positioned closest to the layer among wirings to supply the voltage to the region of the second conductivity type, a plug arranged to connect the first wiring and the region of the second conductivity type, and a second wiring arranged to supply a voltage to the first region, the second wiring is disposed to cover the first region when viewed in plan, and a distance between the second wiring and the layer is shorter than a distance between the first wiring and the layer. 
     According to another aspect, the present disclosure provides a apparatus including a layer that has a light incident surface and includes conversion elements, and a wiring structure disposed on a surface of the layer on an opposite side to the light incident surface, wherein each of the conversion elements includes a photodiode; the photodiode includes a first region of a first conductivity type in which charges having the same polarity as signal carriers are majority carriers and a second region of a second conductivity type; a voltage is supplied to the second region through a region of the second conductivity type; the wiring structure includes a first wiring positioned closest to the layer among wirings to supply the voltage to the region of the second conductivity type, a plug arranged to connect the first wiring and the region of the second conductivity type, and a second wiring disposed to overlap the first region when viewed in plan; the second wiring is disposed to cover the first region when viewed in plan; a distance between the second wiring and the layer is shorter than a distance between the first wiring and the layer; when viewed in plan, a first conversion element among the conversion elements and a second conversion element among the conversion elements are disposed side by side in a first direction, and a third conversion element among the conversion elements and the second conversion element among the conversion elements are disposed side by side in a second direction intersecting the first direction; and when viewed in plan, the second wiring overlapping the first region of the first conversion element when viewed in plan and the second wiring overlapping the first region of the second conversion element are disposed adjacent to each other in the first direction, and the first wiring is not disposed between the second wiring for the first conversion element when viewed in plan and the second wiring for the second conversion element. 
     According to still another aspect, the present disclosure provides an apparatus including a layer that has a light incident surface and includes conversion elements including photodiodes, and a wiring structure disposed on a surface of the layer on an opposite side to the light incident surface, wherein each of the photodiodes includes a first region of a first conductivity type in which charges having the same polarity as signal carriers are majority carriers and a second region of a second conductivity type, a voltage is supplied to the second region through a region of the second conductivity type, the wiring structure includes a first wiring arranged to supply the drive voltage to the region of the second conductivity type and positioned closest to the layer, and a second wiring disposed in the same wiring layer as the first wiring and arranged to supply a drive voltage to the first region, the first wiring has an opening, the second wiring is disposed in the opening of the first wiring when viewed in plan, the second wiring is formed by five or more sides when viewed in plan, and the opening of the first wiring has five or more sides. 
     Further features of the disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a configuration of a photoelectric conversion apparatus. 
         FIG. 2  illustrates a layout example on a sensor substrate. 
         FIG. 3  illustrates a layout example on a circuit substrate. 
         FIG. 4  is a block diagram including an equivalent circuit of a photoelectric conversion element. 
         FIGS. 5A, 5B and 5C  illustrate relationships between operation of an APD and output signals. 
         FIG. 6  is a schematic sectional view of a photoelectric conversion apparatus according to a first embodiment. 
         FIGS. 7A and 7B  are schematic plan views of the photoelectric conversion apparatus according to the first embodiment. 
         FIGS. 8A and 8B  are schematic plan views of a photoelectric conversion apparatus according to a modification of the first embodiment. 
         FIG. 9  is a schematic sectional view of a photoelectric conversion apparatus according to a second embodiment. 
         FIGS. 10A and 10B  are schematic plan views of the photoelectric conversion apparatus according to the second embodiment. 
         FIG. 11  is a schematic sectional view of a photoelectric conversion apparatus according to a third embodiment. 
         FIG. 12  is a schematic sectional view of a photoelectric conversion apparatus according to a fourth embodiment. 
         FIG. 13  is a schematic sectional view of a photoelectric conversion apparatus according to a modification of the fourth embodiment. 
         FIG. 14  is a schematic sectional view of a photoelectric conversion apparatus according to a fifth embodiment. 
         FIG. 15  is a schematic sectional view of a photoelectric conversion apparatus according to a sixth embodiment. 
         FIG. 16  is a schematic sectional view of a photoelectric conversion apparatus according to a seventh embodiment. 
         FIGS. 17A, 17B, 17C, 17D and 17E  are schematic plan views of the photoelectric conversion apparatus according to the seventh embodiment. 
         FIG. 18  is a block diagram of an optical detection system according to an eighth embodiment. 
         FIG. 19  is a block diagram of an optical detection system according to a ninth embodiment. 
         FIG. 20  is a block diagram of an optical detection system according to a tenth embodiment. 
         FIGS. 21A and 21B  are a block diagram and a layout view, respectively, of an optical detection system according to an eleventh embodiment. 
         FIG. 22  is a flowchart for the optical detection system according to the eleventh embodiment. 
         FIGS. 23A and 23B  each illustrate a specific example of an electronic apparatus according to a twelfth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments described below represent examples of implementing the technical concept of the present disclosure and are not purported to limit the present disclosure. Sizes and positional relationships illustrated in the drawings are exaggerated in some cases for clearer understanding of explanation. In the following, the same components are denoted by the same reference numerals and description of those components is omitted in some cases. 
     Configurations common to photoelectric conversion apparatuses according to the embodiments are described with reference to  FIGS. 1 to 4 . 
     Each of the photoelectric conversion apparatuses includes SPAD pixels including avalanche photodiodes. A conductivity type of electric charges of charge pairs generated in the avalanche photodiodes, the electric charges being used as signal carriers, is called a first conductivity type. The first conductivity type indicates the conductivity type of majority carriers having the same polarity as the signal carriers. The conductivity type opposite to the first conductivity type is called a second conductivity type. In an example described below, it is assumed that the signal carriers are electrons, the first conductivity type is N type, and the second conductivity type is P type. However, the signal carriers may be holes, the first conductivity type may be P type, and the second conductivity type may be N type. 
     In this Specification, when the term “impurity concentration” is simply used, it indicates a net impurity concentration obtained by subtracting a concentration compensated by the impurity of the opposite conductivity type. Thus, the term “impurity concentration” indicates a NET doping concentration. The region where a concentration of a P-type dopant impurity is higher than that of an N-type dopant impurity is a P-type semiconductor region. On the contrary, the region where a concentration of the N-type dopant impurity is higher than that of the P-type dopant impurities is an N-type semiconductor region. 
     In this Specification, the wording “when viewed in plan” indicates that a semiconductor substrate (described later) is viewed in a direction perpendicular to a light incident surface. The wording “cross-section” indicates a surface along a cut line when a sensor substrate  11  is cut in a direction perpendicular to a light incident surface of a semiconductor layer  302  of the sensor substrate  11 . When the light incident surface of the semiconductor layer is a rough surface in a microscopic view, the direction “when viewed in plan” is defined on the basis of the light incident surface of the semiconductor layer in a macroscopic view. 
     In this Specification, the wording “depth direction” indicates a direction from the light incident surface (first surface) of the semiconductor layer  302  toward a surface (second surface) on a side where a circuit substrate  21  is disposed. 
     The configurations common to the embodiments are first described. 
       FIG. 1  illustrates a configuration of a multilayer photoelectric conversion apparatus  100  according to an embodiment of the present disclosure. 
     The photoelectric conversion apparatus  100  is constituted by stacking two substrates, namely the sensor substrate  11  and the circuit substrate  21 , and by electrically connecting those two substrates. The sensor substrate  11  includes a first semiconductor layer including a photoelectric conversion element  102  (described later), and a first wiring structure. The circuit substrate  21  includes a second semiconductor layer including circuits such as signal processing sections  103  (described later), and a second wiring structure. The photoelectric conversion apparatus  100  is formed by laminating the second semiconductor layer, the second wiring structure, the first wiring structure, and the first semiconductor layer. The photoelectric conversion apparatus according to the embodiments are each a backside-illuminated photoelectric conversion apparatus in which light is incident on the first surface and the circuit substrate is disposed on the second surface. 
     Although, in the following, the sensor substrate  11  and the circuit substrate  21  are described as being in the form of chips after dicing, those substrates are each not limited to a chip. For example, each substrate may be a wafer. Alternatively, the substrates may be obtained by stacking wafers and then dicing the wafers into chips, or by dicing wafers into chips and then stacking and joining the chips. 
     A pixel region  12  is formed in the sensor substrate  11 , and a circuit region  22  where signals detected in the pixel region  12  are processed is formed in the circuit substrate  21 . 
       FIG. 2  illustrates a layout example on the sensor substrate  11 . Pixels  101  including the photoelectric conversion elements  102  each of which includes an avalanche photodiode (APD)  201  (see  FIG. 4 ) are arranged in a two-dimensional array when viewed in plan, thus forming the pixel region  12 . 
     The pixels  101  are typically pixels for forming an image but are not always required to form an image when the pixels are used in TOF (Time of Flight) systems. In other words, the pixels  101  may be each a pixel for measuring a time at which light arrives and an amount of the light. 
       FIG. 3  illustrates a layout example on the circuit substrate  21 . The circuit substrate  21  includes the signal processing sections  103  for processing electric charges after being photoelectrically converted in the photoelectric conversion elements  102  in  FIG. 2 , a column circuit  112 , a control pulse generation section  115 , a horizontal scanning circuit section  111 , output lines  113 , and a vertical scanning circuit section  110 . 
     The photoelectric conversion elements  102  in  FIG. 2  and the signal processing sections  103  in  FIG. 3  are electrically connected via connection wirings that are disposed per pixel. 
     The vertical scanning circuit section  110  receives control pulses supplied from the control pulse generation section  115  and supplies the control pulses to the individual pixels. Logic circuits, such as a shift register and an address decoder, are used in the vertical scanning circuit section  110 . 
     Signals output from the photoelectric conversion elements  102  in the pixels are processed in the signal processing sections  103 . A counter, a memory, and so on are disposed in each of the signal processing sections  103 , and a digital value is held in the memory. 
     To read out digital signals from the memories of the pixels in which the digital signals are held, the horizontal scanning circuit section  111  inputs, to the signal processing sections  103 , control pulses for sequentially selecting each column. 
     With respect to selected one of the columns, a signal is output from the signal processing section  103  in the pixel selected by the vertical scanning circuit section  110  to the output line  113 . 
     The signal output to the output line  113  is in turn output to a recording unit or a signal processing unit outside the photoelectric conversion apparatus  100  through an output circuit  114 . 
     In  FIG. 2 , the photoelectric conversion elements may be one-dimensionally arrayed in the pixel region. Beneficial effects of the present disclosure can be obtained even in the case of using one pixel, and an advantageous effect resulting from ensuring a withstand voltage in this embodiment is easy to more effectively obtain with a photoelectric conversion apparatus including multiple pixels. The function of the signal processing section is not always required to be disposed one for each of all the photoelectric conversion elements. For example, one signal processing section may be shared by multiple photoelectric conversion elements, and the signal processing may be executed sequentially. 
     As illustrated in  FIGS. 2 and 3 , the signal processing sections  103  are disposed in a region overlapping the pixel region  12  when viewed in plan. Furthermore, the vertical scanning circuit section  110 , the horizontal scanning circuit section  111 , the column circuit  112 , the output circuit  114 , and the control pulse generation section  115  are disposed to overlap regions between edges of the sensor substrate  11  and edges of the pixel region  12  when viewed in plan. In other words, the sensor substrate  11  has the pixel region  12  and a non-pixel region positioned around the pixel region  12 , and the vertical scanning circuit section  110 , the horizontal scanning circuit section  111 , the column circuit  112 , the output circuit  114 , and the control pulse generation section  115  are disposed in regions overlapping the non-pixel region when viewed in plan. 
       FIG. 4  illustrates an example of a block diagram including equivalent circuits of  FIGS. 2 and 3 . 
     In  FIG. 2 , the photoelectric conversion elements  102  each including the APD  201  are disposed in the sensor substrate  11 , and other members are disposed in the circuit substrate  21 . 
     The APD  201  generates charge pairs through photoelectric conversion in accordance with incident light. A voltage VL (first voltage) is supplied to an anode of the APD  201 . A voltage VH (second voltage) higher than the voltage VL supplied to the anode is supplied to a cathode of the APD  201 . A reverse bias voltage is supplied between the anode and the cathode such that the APD  201  is operated to perform avalanche multiplication. By maintaining the state in which the above-mentioned voltages are supplied, electric charges generated due to the incident light cause the avalanche multiplication, and an avalanche current is generated. 
     When the reverse bias voltage is supplied, there are a Geiger mode in which the APD  201  is operated under condition of a potential difference between the anode and the cathode being higher than a breakdown voltage, and a linear mode in which the APD  201  is operated under condition of the potential difference between the anode and the cathode being close to or lower than the breakdown voltage. 
     The APD operated in the Geiger mode is called an SPAD. For example, the voltage VL (first voltage) is −30 V, and the voltage VH (second voltage) is 1 V. The APD  201  may be operated in the linear mode or the Geiger mode. In the case of the SPAD, the potential difference is greater than in the case of the APD operated in the linear mode, and an effect resulting from increasing the withstand voltage is more significant. For that reason, in one embodiment, the APD is the SPAD. 
     A quenching element  202  is connected to a power supply for supplying the voltage VH and to the APD  201 . The quenching element  202  functions as a load circuit (quenching circuit) when signal multiplication is caused due to the avalanche multiplication and has a role of suppressing the voltage supplied to the APD  201 , thereby suppressing the avalanche multiplication (called a quenching operation). In addition, the quenching element  202  has a role of supplying a current in an amount corresponding to a voltage drop caused by the quenching operation and returning the voltage supplied to the APD  201  to the voltage VH (called a recharge operation). 
     The signal processing section  103  includes a waveform shaping section  210 , a counter circuit  211 , and a selection circuit  212 . In this Specification, the signal processing section  103  may include any one or more of the waveform shaping section  210 , the counter circuit  211 , and the selection circuit  212 . 
     The waveform shaping section  210  shapes a potential change obtained at the cathode of the APD  201  in detection of photons and outputs a pulse signal. For example, an inverter circuit is used as the waveform shaping section  210 .  FIG. 4  illustrates an example in which one inverter is used as the waveform shaping section  210 . However, the waveform shaping section  210  may be constituted by a circuit including multiple inverters connected in series, or by another circuit with a waveform shaping effect. 
     The counter circuit  211  counts pulse signals output from the waveform shaping section  210  and holds a count value. Moreover, when a control pulse pRES is supplied via a drive line  213 , the signal held in the counter circuit  211  is reset. 
     A control pulse pSEL is supplied to the selection circuit  212  from the vertical scanning circuit section  110  in  FIG. 3  via a drive line  214  in  FIG. 4  (the drive line  214  being not illustrated in  FIG. 3 ), thereby switching electrical connection and disconnection between the counter circuit  211  and the output line  113 . The selection circuit  212  includes, for example, a buffer circuit for outputting a signal. 
     A switch, such as a transistor, may be disposed between the quenching element  202  and the APD  201  or between the photoelectric conversion element  102  and the signal processing section  103  to switch electrical connection therebetween. Similarly, the supply of the voltage VH or the voltage VL to the photoelectric conversion element  102  may be electrically switched by using a switch such as a transistor. 
     The configuration using the counter circuit  211  is adopted in this embodiment. However, a TDC (Time to Digital Converter) and a memory may be used instead of the counter circuit  211  to constitute the photoelectric conversion apparatus  100  that is to measure timing of pulse detection. In such a case, generation timing of the pulse signal output from the waveform shaping section  210  is converted to a digital signal by the TDC. For measuring the timing of the pulse signal, a control pulse pREF (reference signal) is supplied to the TDC from the vertical scanning circuit section  110  in  FIG. 3  via a drive line. The TDC obtains, as a digital signal, a signal representing a relative time of the input timing of the signal output from each pixel through the waveform shaping section  210  with the control pulse pREF being a reference. 
       FIGS. 5A, 5B and 5C  illustrate relationships between operation of the APD and output signals. 
       FIG. 5A  illustrates the APD  201 , the quenching element  202 , and the waveform shaping section  210  that are extracted from  FIG. 4 . Here, an input side of the waveform shaping section  210  is indicated by a node A, and an output side is indicated by a node B.  FIG. 5B  represents a waveform change at the node A in  FIG. 5A , and  FIG. 5C  represents a waveform change at the node B in  FIG. 5A . 
     During a period from time t 0  to time t 1 , the potential difference of VH−VL is applied to the APD  201  in  FIG. 5A . When a photon is incident on the APD  201  at the time t 1 , the avalanche multiplication generates in the APD  201 , and an avalanche multiplication current flows through the quenching element  202 , whereby a voltage at the node A drops. When an amount of the voltage drop further increases and the potential difference applied to the APD  201  reduces, the avalanche multiplication in the APD  201  is stopped as indicated at the time t 2 , and a voltage level at the node A does not further drop beyond a certain value. Thereafter, a current compensating for the voltage drop flows to the node A from the voltage VL during a period from the time t 2  to time t 3 . At the time t 3 , the potential level at the node A is settled down to an initial level. On that occasion, part of an output waveform at the node A, the part exceeding a certain threshold, is shaped by the waveform shaping section  210  and is output as a signal through the node B. 
     The layout of the output lines  113 , the column circuit  112 , and the output circuit  114  is not limited to that illustrated in  FIG. 3 . For example, the output lines  113  may be disposed to extend in a row direction, and the column circuit  112  may be disposed at extended ends of the output lines  113 . 
     Photoelectric conversion apparatuses according to various embodiments will be described below. 
     First Embodiment 
       FIG. 6  is a schematic sectional view of an SPAD pixel in a first embodiment.  FIGS. 7A and 7B  are schematic plan views taken along VIIA-VIIA and VIIB-VIIB in  FIG. 6 , respectively.  FIGS. 8A and 8B  are schematic plan views taken along VIIIA-VIIIA and VIIIB-VIIIB, in  FIG. 6 , respectively. 
     As illustrated in  FIG. 6 , the circuit substrate  21  and the sensor substrate  11  are stacked. The circuit substrate  21  includes a semiconductor layer  402  including circuit elements that form a signal processing circuit, and a wiring structure  403 . The sensor substrate  11  includes a semiconductor layer  302  in which the APDs are disposed, and a wiring structure  303 . The semiconductor layer  302 , the wiring structure  303 , the wiring structure  403 , and the semiconductor layer  402  are disposed in order from a side closer to a light incident surface. In the photoelectric conversion apparatus, the wiring structure  403  of the circuit substrate  21  and the wiring structure  303  of the sensor substrate  11  are bonded in contact with each other. In a specific example, a wiring  332  included in the wiring structure  303  and a wiring  432  included in the wiring structure  403  are bonded. In other words, a wiring layer in which the wiring  332  is disposed and a wiring layer in which the wiring  432  is disposed are bonded. Thus, the wiring  332  and the wiring  432  form metal bonded portions. As described later, the metal bonded portions include a metal bonded portion that is electrically connected to the semiconductor layer  302  and the semiconductor layer  402 , and a metal bonded portion that is not connected to the semiconductor layer  302  and the semiconductor layer  402 . 
     Here, the wording “wiring layer” indicates one of multiple wiring layers forming the wiring structure  303 . The word “wiring” indicates a specific one of wirings disposed in the layers to which a specific potential is supplied. An interlayer insulating film  329  or  427  is disposed between the wirings. 
     The semiconductor layer  302  of the sensor substrate  11  includes the APDs. Each of the APDs includes a first semiconductor region  311  of the first conductivity type and a second semiconductor region  312  of the second conductivity type. The first semiconductor region  311  and the second semiconductor region  312  form a PN junction. In an end portion of the PN junction forming the APD, a third semiconductor region  313  of the first conductivity type may be formed to relax an electric field. In just aiming to relax the electric field, the third semiconductor region  313  may be formed of a semiconductor region of the second conductivity type. An impurity concentration in the third semiconductor region  313  is lower than in the first semiconductor region  311  when the third semiconductor region  313  is of the first conductivity type, and is lower than in the second semiconductor region  312  when it is of the second conductivity type. 
     A difference in impurity concentration between the third semiconductor region  313  and the first semiconductor region  311  or a difference in impurity concentration between the third semiconductor region  313  and the second semiconductor region  312  is twice or more. 
     A thickness of the semiconductor layer  302  can be set as appropriate depending on a wavelength of light to be detected. A color of the light to be detected by the photoelectric conversion apparatus can be set to, for example, blue, green, red, or a color of infrared light according to the purpose. A peak wavelength of the light to be detected by the photoelectric conversion apparatus can be set in a range of, for example, 350 nm or longer to 1000 nm or shorter. A reflective metal layer  322  can reflect light having passed through the semiconductor layer  302 . Therefore, this embodiment is easy to further improve sensitivity especially for long-wavelength light such as infrared light. 
     The adjacent APDs are separated by a fourth semiconductor region  314  of the second conductivity type. A fifth semiconductor region  315  of the second conductivity type is disposed on a side closer to the light incident surface. A sixth semiconductor region  316  of the second conductivity type is disposed between the second semiconductor region  312  and the fifth semiconductor region  315 . 
     A concentration of an impurity of the second conductivity type in the sixth semiconductor region  316  in a zone overlapping the first semiconductor region  311  when viewed in plan may be higher than that of the impurity of the second conductivity type in the sixth semiconductor region  316  in a zone not overlapping the first semiconductor region  311  when viewed in plan. 
     A pinning film  341  for suppressing a dark current may be disposed at an interface defined by the light incident surface. A known material can be used for the pinning film  341 . A planarization layer  342 , a filter layer  343 , and microlenses  344  are disposed on a surface of the pinning film  341  on a side closer to the light incident surface. Various optical filters, such as a color filter, an infrared cut filter, and a monochromatic filter, can be optionally used as the filter layer  343 . For example, an RGB color filter or an RGBW color filter can be used as the color filter. 
     Drive voltages are applied to the anode and the cathode of the APD. Voltages enabling a reverse bias to be applied to the APD are applied as the drive voltages. Of the drive voltages, the voltage with a higher absolute value is applied to one of the anode and the cathode of the APD through a pad electrode  352  that is disposed in a pad opening  355 . Of the drive voltages, the other voltage with a lower absolute value is applied to the other one of the anode and the cathode of the APD through a pad electrode  354  that is disposed in a pad opening  353 . In  FIG. 6 , the voltage applied through the pad electrode  352  is applied to the anode of the APD, and the voltage applied through the pad electrode  354  is applied to the cathode of the APD. 
     Maximum diameters of the pad openings  353  and  355  are each set to, for example, 50 μm or greater in one embodiment, and in another embodiment, for example, 80 μm or greater. Depths of the pad openings  353  and  355  are each set to, for example, 1 μm or greater and 30 μm or smaller and in one embodiment, for example, 3 μm or greater and 8 μm or smaller. 
     Of the drive voltages, the voltage with the higher absolute value is applied to the anode through a via  324 . In a specific example, that voltage is applied to the fourth semiconductor region  314  through a wiring  326 , the via  324 , and a contact plug  321  that is connected to the anode of the APD. The wiring  326 , the via  324 , and the contact plug  321  are each held at the same potential as the anode of the APD. The wiring  326  is one of wirings for supplying the drive voltage to the fourth semiconductor region  314 , the one wiring being closest to the semiconductor layer  302 . 
     Of the drive voltages for the APD, the voltage with the lower absolute value is supplied to the circuit substrate  21  through the pad electrode  354 , a via  331  connected to the wiring  332 , and the wiring  332 . Thus, that drive voltage is supplied to the cathode through the circuit substrate  21 . 
     The signal carriers avalanche-multiplied in the PN junction are supplied to the circuit substrate  21  through a contact plug  320 , the reflective metal layer  322 , a via  323 , a wiring  325 , a via  327 , a wiring  328 , the via  331  connected to the wiring  332 , and the wiring  332 . When the avalanche-multiplied electric charges are read out, the contact plug  320 , the reflective metal layer  322 , the via  323 , the wiring  325 , the via  327 , the wiring  328 , the via  331  connected to the wiring  332 , and the wiring  332  are held at the same potential as the cathode of the APD. 
     The reflective metal layer  322  reflects the light having passed through the semiconductor layer  302 . The reflective metal layer  322  is disposed to cover an avalanche multiplication region when viewed in plan. In one embodiment, the reflective metal layer  322  is disposed to cover the entirety of the avalanche multiplication region when viewed in plan. Furthermore, the reflective metal layer  322  is disposed to cover the entirety of the first semiconductor region  311  when viewed in plan. The reflective metal layer  322  is formed in one of the wiring layers in the wiring structure, the one wiring layer being positioned closest to the semiconductor layer  302 . For example, long-wavelength light that has not been fully absorbed by the semiconductor layer  302  can be reflected by the reflective metal layer  322  to enter the semiconductor layer  302  again. This improves sensitivity for the long-wavelength light that is not fully absorbable by the semiconductor layer  302 . 
     In another embodiment, a material of the reflective metal layer  322  is selected so as to maximize reflectance at a wavelength of light to be reflected. For example, copper or aluminum can be used as the material of the reflective metal layer  322 . When copper is used, reflectance for infrared light can be increased in comparison with the case of using aluminum. Here, the wording “copper is used” indicates that copper is used as a main component, and the reflective metal layer  322  is not required to be made of only copper. The main component indicates a material that is contained more than 50% of all materials of the reflective metal layer  322 . When aluminum is used, the wording “aluminum is used” similarly indicates that aluminum is a main component. 
     The pad electrode  352  may be made of aluminum, and the other wirings may be all made of copper. 
     The reflective metal layer  322  may be made of a material different from that of the wiring  326 . 
     As an area of the reflective metal layer  322  increases, an amount of reflected light can be increased with respect to that of incident light. Therefore, the reflective metal layer  322  is formed in an area as large as possible. One of the two drive voltages for the APD is applied to the reflective metal layer  322 . When, as in the related art, the wiring to which the other of the two drive voltages for the APD is applied is disposed in the same layer as the reflective metal layer  322  and an area of the reflective metal layer is increased, there is a possibility that the withstand voltage cannot be ensured because a distance between the reflective metal layer and the wiring to which the other drive voltage is applied becomes short. 
     Accordingly, in this embodiment, the wiring layer including the wiring  326  to which the voltage to be applied to one node of the APD is supplied is made different from the wiring layer including the reflective metal layer  322 . Thus, the reflective metal layer  322  is disposed in a layer positioned closer to the semiconductor layer  302  than the wiring layer in which the wiring  326  is disposed. Stated in another way, a distance between the reflective metal layer  322  and the semiconductor layer  302  is shorter than that between the wiring  326  and the semiconductor layer  302 . For example, in  FIG. 6 , no wiring is disposed between the contact plug  321  and the via  324 , and the contact plug  321  and the via  324  are directly connected by stacking them. In other words, the contact plug is formed by stacking a first via and a second via. In the same wiring layer as the reflective metal layer  322 , at least one of the contact plug  321  and the via  324  is disposed, and the wiring for supplying the potential to the anode of the APD is not disposed. Thus, part of the contact plug formed by stacking the first via and the second via is disposed at the same height as the reflective metal layer  322 . 
     A method of fabricating the contact plug  321  and the via  324  by direct stacking is as follows. 
     First, the contact plug  320  and the contact plug  321  are formed. Then, the reflective metal layer  322  is formed. At that time, the wiring for the anode is not formed. Then, the via  323  and the via  324  are formed. Then, the wiring  325  and the wiring  326  are formed. 
     Without connecting the contact plug  321  and the via  324  by stacking them, the distance between the reflective metal layer  322  and the semiconductor layer  302  may be made shorter than that between the wiring  326  and the semiconductor layer  302 . In other words, the contact plug  321  may be formed deeper than the contact plug  320 , and the contact plug  321  and the wiring  326  may be directly connected. 
     A fabrication method in the above-mentioned case is as follows. First, the contact plug  320  is formed. Then, the reflective metal layer  322  is formed. Then, the via  323  is formed. Then, the contact plug  321  is formed to continuously extend from the second surface of the semiconductor layer  302  to the same depth as the wiring  325  on a side facing the circuit substrate  21 . Then, the wiring  325  and the wiring  326  are formed. 
     Since the reflective metal layer  322  is positioned closer to the semiconductor layer  302  as described above, it is possible to increase the area of the reflective metal layer  322  and to improve the sensitivity of the APD for the long-wavelength light while the withstand voltage between the reflective metal layer  322  and the wiring  326  is ensured. 
     In one embodiment, the distance between the reflective metal layer  322  and the semiconductor layer  302  is set to, for example, 0.05 μm or longer and 2 μm or shorter and in another embodiment, for example, 0.1 μm or longer and 0.8 μm or shorter. By setting the above-mentioned distance to a predetermined value or more, the withstand voltage between the semiconductor layer  302  and the reflective metal layer  322  can be ensured. On the other hand, by setting the above-mentioned distance to a predetermined value or less, it is easy to further reflect the light having passed through the semiconductor layer  302  toward the semiconductor layer  302  and improve the sensitivity of the APD. 
     In one embodiment, the wiring  326  held at the same potential as that applied to the anode of the APD is disposed to cover a gap between the via  324  and the reflective metal layer  322  when viewed in plan. This arrangement enables light having passed through the gap between the via  324  and the reflective metal layer  322  to be reflected by the wiring  326  and to be absorbed by the semiconductor layer  302 . The wiring  326  is disposed to continuously overlap the reflective metal layer  322  and the via  324  when viewed in plan. 
     An active region  411  and an element isolation region  412  are formed in the semiconductor layer  402  of the circuit substrate  21 . For example, PN junction isolation or isolation separation, such as Shallow Trench Isolation (STI) or Deep Trench Isolation (DTI), can be utilized for the element isolation region  412 . 
     A signal output from the APD in the sensor substrate  11  is supplied to the processing circuit in the circuit substrate  21  through the wiring  432 , a via  431 , a wiring  426 , a via  425 , a wiring  424 , a via  423 , a wiring  422 , and a contact plug  421 , those wirings, vias and contact plug providing metal bonding. 
     As illustrated in  FIG. 6 , the pad electrode  354  and the pad electrode  352  are disposed in the sensor substrate  11 . The voltage supplied from the pad electrode  352  is supplied to only the sensor substrate  11  and is not supplied to the circuit substrate  21 . The voltage supplied from the pad electrode  354  is supplied to the semiconductor layer  402  through some of the metal bonded portions. Thus, due to the configuration that, of the drive voltages applied to the APD, the voltage with the higher absolute value is not applied to the circuit substrate  21 , a reduction in reliability of the photoelectric conversion apparatus is suppressed. 
     As illustrated in  FIG. 6 , some of the metal bonded portions are disposed between the pad electrode  352  and the semiconductor layer  402 . Those metal bonded portions are not connected to the wirings in the other wiring layers. 
     With the presence of those metal bonded portions, bonding strength between the sensor substrate  11  and the circuit substrate  21  near the pad electrode  352  is ensured. 
       FIG. 6  discloses that some of the metal bonded portions not connected to the semiconductor layers  302  and  402  are disposed in the pixel region. With the presence of the metal bonded portions, a reduction in the bonding strength between the sensor substrate  11  and the circuit substrate  21  can be suppressed in the pixel region as well. Those metal bonded portions are not essential, and one metal bonded portion for reading out the signal from the APD is to be disposed in the pixel region corresponding to each APD. In other words, it is at least required that only the metal bonded portion for reading out the signal from the APD is disposed corresponding to each APD. Although, in  FIG. 6 , the pad electrode is drawn in a smaller size than the APD for easier understanding of explanation, the APD may have a smaller size than the pad electrode. The number of the metal bonded portions connected to the pad electrode  354  may be three or more. 
     Although, in  FIG. 6 , the pad electrode  354  and the pad electrode  352  are disposed in the sensor substrate  11 , those pad electrodes  354  and  352  may be disposed in the circuit substrate  21 . In such a case, from the viewpoint of the withstand voltage, the pad electrodes are formed such that the potential applied from the pad electrode  352  is not applied to the semiconductor layer  402 . 
       FIG. 7A  is a schematic plan view taken along VIIA-VIIA in  FIG. 6  at the height at which the reflective metal layer  322  is disposed, and  FIG. 7B  is a schematic plan view taken along VIIB-VIIB in  FIG. 6  at the height at which the wirings  325  and  326  are disposed. 
     As illustrated in  FIG. 7A , the multiple vias  324  are disposed to surround the reflective metal layer  322  when viewed in plan. In  FIG. 6 , the fourth semiconductor region  314  is disposed to surround the first semiconductor region  311  when viewed in plan. The potential is supplied to the fourth semiconductor region  314  through the vias  324 . 
     One of the two voltages for driving the APD is applied to the reflective metal layer  322 , and the other of the two voltages for driving the APD is applied to each of the vias  324 . Accordingly, as described above, the reflective metal layer  322  and the via  324  are to be disposed to ensure a certain distance at which dielectric breakdown is not caused. In one embodiment, the distance between the reflective metal layer  322  and the via  324  is, for example, 0.4 μm or longer and 1.5 μm or shorter and in another embodiment, for example, 0.5 μm or longer and 1.0 μm or shorter. The withstand voltage between the reflective metal layer  322  and the via  324  can be ensured by ensuring a predetermined distance or more therebetween. On the other hand, the sensitivity of the APD for the long-wavelength light can be improved by setting the above-mentioned distance to a predetermined value or less. 
     As seen from  FIG. 7B , the wiring  325  at the same potential as the cathode of the APD is electrically connected to the reflective metal layer  322  in  FIG. 7A , and the anode of the APD and the wiring  326  are electrically connected to the vias  324  in  FIG. 7A . 
     As illustrated in  FIG. 7B , the wiring  326  is a continuous planar member when viewed in plan. The potential supplied from the pad electrode  352  illustrated in  FIG. 6  is supplied to the wiring  326 , whereby the voltage is applied to the anodes of the multiple APDs. Stated in another way, the common potential is supplied to the anodes of the multiple APDs through the common wiring  326 . On the other hand, the reflective metal layer  322  and the wiring  325  are disposed individually for each APD. Thus, one reflective metal layer  322  and one wiring  325  are disposed for one APD. Accordingly, the signal for each of the APDs can be read out individually. 
     Another example of the reflective metal layer  322 , the wiring  325 , and the wiring  326  is described with reference to  FIGS. 8A and 8B .  FIG. 8A  represents another example of the reflective metal layer  322  illustrated in  FIG. 7A , and  FIG. 8B  represents another example of the wirings  325  and  326  illustrated in  FIG. 7B . 
       FIGS. 8A and 8B  are different from  FIGS. 7A and 7B  in that, when viewed in plan, corners of the reflective metal layer  322  and the wiring  325  are cut and the wiring  326  has a shape in match with the shape of the wiring  325 . Stated in another way, in the other examples, each of the reflective metal layer  322  and the wiring  325  has an octagonal shape when viewed in plan, and an opening in the wiring  326  corresponding to the wiring  325  also has an octagonal shape. Thus, the reflective metal layer  322  has eight sides, and the opening in the wiring  326  has eight sides. As described above, the cathode voltage for driving the APD is supplied to the reflective metal layer  322  and the wiring  325 . The anode voltage for driving the APD is supplied to the vias  324  and the wiring  326 . An electric field has properties of tending to concentrate at corners. Therefore, when the reflective metal layer  322 , the via  324 , the wiring  325 , and the wiring  326  have corners, there is a possibility that the electric field concentrates at the corners. By contrast, in  FIG. 8 , since the reflective metal layer  322  has no corners with an angle of 90° or less, the electric field can be suppressed from excessively concentrating at the corners. 
     Here, the wording “quadrangular or octagonal shape when viewed in plan” includes the case in which corners are chamfered. Thus, in this Specification, a shape being quadrangular or octagonal in a macroscopic view without having corners is also called the quadrangular or octagonal shape. Although the drawing illustrates an example in which the reflective metal layer  322  has eight sides, an effect of suppressing concentration of the electric field can be obtained when the reflective metal layer  322  has at least five sides. 
     Second Embodiment 
     A photoelectric conversion apparatus according to a second embodiment will be described below with reference to  FIGS. 9, 10A, and 10B .  FIG. 9  is a schematic sectional view of the photoelectric conversion apparatus according to this embodiment.  FIG. 10A  is a schematic sectional view taken along XA-XA in  FIG. 9 , and  FIG. 10B  is a schematic sectional view taken along XB-XB in  FIG. 9 . 
     The photoelectric conversion apparatus according to this embodiment is different from the photoelectric conversion apparatus according to the first embodiment in positions at which the vias  324  are disposed and in that the wiring  326  does not have a continuous shape. Features except for the above points and matters described below are substantially similar to those in the first embodiment. Hence similar components to those in the first embodiment are denoted by the same reference signs, and description of those components is omitted in some cases. 
       FIG. 9  is a pixel sectional view taken in a first direction when a pixel array in the sensor substrate  11  is viewed in plan. 
     The first direction is, for example, an opposite side direction of the APD. Thus,  FIG. 9  is a schematic sectional view of the APDs arrayed in a horizontal direction in  FIG. 10A . Because the vias  324  connected to the fourth semiconductor region  314  forming the anode of the APD are arranged in a second direction intersecting the first direction, the vias  324  do not appear in  FIG. 9 . The second direction is, for example, a diagonal direction of the APD. The vias  324  are arranged on the diagonal line of the APD and hence do not appear on  FIG. 9 . As illustrated in  FIG. 10A , the vias  324  disposed around the reflective metal layer  322  are arranged only in the diagonal direction between the pixels and are not arranged in the opposite side direction. In other words, the vias  324  are disposed between diagonally adjacent ones among six APDs that are arrayed in two rows and three columns, while the vias  324  are not disposed between the APDs that are arrayed in a left-right direction and an up-down direction. 
     As in the first embodiment, the voltage with the lower absolute value for driving the APD is applied to one of the reflective metal layer  322  and the via  324 , and the voltage with the higher absolute value for driving the APD is applied to the other. Accordingly, a certain distance at which dielectric breakdown is not caused is to be ensured between the reflective metal layer  322  and the via  324 . For example, the voltage with the lower absolute value for driving the APD is applied to the reflective metal layer  322 , and the voltage with the higher absolute value for driving the APD is applied to the via  324 . 
     In this embodiment, gaps between adjacent twos of the pixels in the opposite side direction face the reflective metal layers  322  that are held at the same potential. In other words, the via  324  is not disposed between the adjacent pixels in the opposite side direction. Accordingly, a space between the reflective metal layers  322  arrayed in the left-right direction and the up-down direction can be reduced in comparison with that in the first embodiment. With the structure according to this embodiment, an area of the reflective metal layer  322  can be increased in comparison with that in the first embodiment, and sensitivity of the pixel, especially sensitivity for the long-wavelength light, can be easily increased. 
       FIG. 10B  is a schematic plan view taken along XB-XB in  FIG. 9  at the height of the wiring layer in which the wirings  325  and  326  are disposed. The wiring  325  at the same potential as the cathode of the APD is electrically connected to the reflective metal layer  322  in  FIG. 10A , and the wiring  326  at the same potential as the anode of the APD is electrically connected to the via  324  in  FIG. 10A . In  FIG. 10B , the wiring  326  is disposed at a level position corresponding to the wiring  325 . Stated in another way, the individual wirings  326  are disposed instead of the continuous wiring  326 . Without being limited to the above-described example, the wiring  326  may be continuously disposed as illustrated in  FIG. 7B . 
     According to this embodiment, as in the first embodiment, the sensitivity of the APD can be improved while the withstand voltage between the via  324  and the reflective metal layer  322  is ensured. Furthermore, this embodiment is easy to further improve the sensitivity of the APD than in the first embodiment. 
     Comparing the area of the reflective metal layer between the case of adopting this embodiment and the case of not adopting this embodiment, by way of example, it can be expected that the area of the reflective metal layer increases about 10% or more with respect to a pixel size of 5 μm or less, although depending on a wavelength, by adopting this embodiment. 
     Third Embodiment 
       FIG. 11  is a schematic sectional view of a photoelectric conversion apparatus according to a third embodiment. The photoelectric conversion apparatus according to this embodiment is different from the photoelectric conversion apparatus according to the first embodiment in that a DTI  351  is disposed as an isolation portion between the adjacent pixels. Features except for the above point and matters described below are substantially similar to those in the first embodiment. Hence similar components to those in the first embodiment are denoted by the same reference signs, and description of those components is omitted in some cases. 
     As illustrated in  FIG. 11 , the DTI  351  is disposed between the adjacent pixels. This provides an effect of suppressing light reflected by the reflective metal layer  322  from cross-talking with the adjacent pixels. 
     In  FIG. 11 , the DTI  351  is disposed to penetrate through the semiconductor layer  302  from the first surface to the second surface. Since the DTI  351  is disposed in a penetration structure as mentioned above, the effect of suppressing the crosstalk to the adjacent pixels can be increased in comparison with the case of disposing the DTI  351  in a non-penetration structure. 
     The DTI  351  is not always required to penetrate through the semiconductor layer  302  and may be disposed to partially extend in the semiconductor layer  302 . For example, trench isolation may be partially provided to extend from the surface (second surface) of the semiconductor layer  302  on the side where the circuit substrate  21  is disposed toward the first surface. In one embodiment, a depth of the DTI  351  not penetrating through the semiconductor layer  302  is not limited to a specific value, but the depth is ½ or more of a thickness of the semiconductor layer  302  from the viewpoint of suppressing the crosstalk. 
     The DTI  351  may be filled with an oxide film or metal. In one embodiment, the metal is filled from the viewpoint of suppressing the crosstalk. The filled metal enables the light reflected by the reflective metal layer  322  to be further suppressed from cross-talking with the adjacent pixels. For example, when the light reflected by the reflective metal layer  322  hits the DTI  351  at an angle θ, a material and a film thickness of the DTI are selected and designed to increase reflectance at the wavelength of the light and the angle θ. 
     In one embodiment, the fourth semiconductor region  314  of the second conductivity type is disposed at a sidewall of the DTI  351 . This is effective in suppressing the influence of a dark current generating from the sidewall of the DTI  351 . 
     In one embodiment, the contact plug  321  connected to the anode of the APD is disposed to cover the DTI  351  when viewed in plan. For example, the contact plug  321  is disposed to be in contact with the fourth semiconductor region  314  of one pixel, the DTI  351 , and the fourth semiconductor region  314  of another pixel adjacent to the one pixel. This enables the APD drive voltage to be supplied to both the fourth semiconductor regions  314  of the adjacent pixels through one contact plug  321 . 
     According to this embodiment, as in the first embodiment, the sensitivity of the APD can be improved while the withstand voltage between the via  324  and the reflective metal layer  322  is ensured. Furthermore, this embodiment is easy to further improve the sensitivity of the APD than in the first embodiment. Moreover, according to this embodiment, in comparison with the first embodiment, an effective length over which the light reflected by the reflective metal layer  322  is absorbed in the semiconductor layer  302  can be increased, and hence the sensitivity for the long-wavelength light can be improved. In addition, according to this embodiment, since the DTI  351  is disposed, electric charges generated in one pixel can be suppressed from mixing into the adjacent pixels. The provision of the DTI  351  makes it easy to further reduce incidence of light emitted from the avalanche multiplication region of the APD into the adjacent pixels. 
     Fourth Embodiment 
       FIG. 12  is a schematic sectional view of a photoelectric conversion apparatus according to a fourth embodiment. The photoelectric conversion apparatus according to this embodiment is different from the photoelectric conversion apparatus according to the third embodiment in that a scattering structure  356  is disposed in the first surface of the semiconductor layer  302 . Features except for the above point and matters described below are substantially similar to those in the third embodiment. Hence similar components to those in the third embodiment are denoted by the same reference signs, and description of those components is omitted in some cases. 
     As illustrated in  FIG. 12 , the structure  356  for scattering or diffracting light is disposed in the light incident surface of the semiconductor layer  302 . The scattered or diffracted light is reflected by not only the reflective metal layer  322 , but also the DTI  351  between the adjacent pixels. Accordingly, an effective length through which the incident light advances in the semiconductor layer  302  is increased. Thus, even when the semiconductor layer  302  has the same thickness as in the third embodiment, a rate of absorption of the light by the semiconductor layer  302  is increased, and the sensitivity can be improved in comparison with that in the third embodiment. 
     Multiple recesses are formed as the scattering structure  356 . The scattering structure  356  enables the light incident on the first surface of the semiconductor layer  302  to be scattered, whereby an optical path until reaching the second surface can be prolonged. In one embodiment, when the incident light is infrared light, it is necessary to prolong a distance through which the infrared light advances before the photoelectric conversion, and to reduce an amount of the light that disappears without being subjected to the photoelectric conversion. Hence an effect obtained with the scattering structure  356  is more significant in the case of using the infrared light. 
     The number and shape of the recesses formed as the scattering structure  356  can be designed as appropriate according to the incident light. As illustrated in  FIG. 12 , the recesses may be formed in a triangular shape over the entirety of the first surface within the pixel in the sectional view. Alternatively, the recesses may have a pyramidal shape. 
     As illustrated in  FIG. 13 , the recesses may be partially formed in the first surface within the pixel. With the configuration of  FIG. 13 , a scattering or diffraction angle of the incident light can be made smaller with respect to a light incident angle. Accordingly, the sensitivity for the infrared light can be improved while leak of the light to the adjacent pixels is suppressed. 
     As illustrated in  FIG. 12 , a width of one recess is set to, for example, 1/30 or more and ⅓ or less with respect to the width of the pixel. According to another aspect, the width of one recess may be set to, for example, 10 nm or more and 1 μm or less. A depth of one recess may be set to, for example, 1/50 or more and ⅓ or less with respect to the distance from the first surface to the second surface. 
     The shape of the recesses may be, for example, triangular or trapezoidal. Apexes of the triangular or trapezoidal shape may be rounded. The recesses may not need to be continuously formed in the first surface. For example, a region where no recesses are formed may exist between the adjacent recesses and between the recess and the DTI  351 . The recesses can be formed by a known method such as dry etching. 
     An insulator is disposed in the recesses of the scattering structure  356 . For example, silicon oxide or silicon nitride can be used as a material of the insulator. 
     According to this embodiment, as in the third embodiment, the sensitivity of the APD can be improved while the withstand voltage between the via  324  and the reflective metal layer  322  is ensured. Furthermore, since the light is scattered by the scattering structure  356 , an optical path up to the avalanche multiplication region can be prolonged in comparison with that in the third embodiment. As a result, the infrared light can be photoelectrically converted with higher efficiency. 
     Fifth Embodiment 
       FIG. 14  is a schematic sectional view of a photoelectric conversion apparatus according to a fifth embodiment. The photoelectric conversion apparatus according to this embodiment is different from the photoelectric conversion apparatus according to the fourth embodiment in that the DTI  351  has a greater width. Features except for the above point and matters described below are substantially similar to those in the fourth embodiment. Hence similar components to those in the fourth embodiment are denoted by the same reference signs, and description of those components is omitted in some cases. 
       FIG. 14  is a schematic sectional view of the pixel in the opposite side direction when the pixel region is viewed in plan. A width of the DTI  351  between the adjacent pixels is set, for example, such that the DTI  351  and the reflective metal layer  322  overlap when viewed in plan. In a cross-section passing the APD, the reflective metal layer  322  is disposed under the APD, and a length of the reflective metal layer  322  from one end to the other end is longer than a length from one DTI  351  to another adjacent DTI  351 . The DTI  351  and the reflective metal layer  322  are disposed to overlap by a distance L 1 , for example. 
     According to this embodiment, since the light scattered or diffracted by the DTI  351  disposed in the light incident surface of the semiconductor layer  302  is reflected to the semiconductor layer  302  without leaking, the sensitivity of the pixel can be improved in comparison with that in the fourth embodiment. 
     Sixth Embodiment 
       FIG. 15  is a schematic sectional view of a photoelectric conversion apparatus according to a sixth embodiment. The photoelectric conversion apparatus according to this embodiment is different from the photoelectric conversion apparatus according to the first embodiment in that the avalanche multiplication region is smaller than in the first embodiment, and that the signal carriers are collected into the avalanche multiplication region. Features except for the above points and matters described below are substantially similar to those in the first embodiment. Hence similar components to those in the first embodiment are denoted by the same reference signs, and description of those components is omitted in some cases. 
     The PN junction is formed between the first semiconductor region  311  of the first conductivity type and the second semiconductor region  312  of the second conductivity type. A seventh semiconductor region  317  of the second conductivity type is disposed at a position overlapping the first semiconductor region  311  when viewed in plan. The seventh semiconductor region  317  is given with a lower potential for the signal carriers than that given to the second semiconductor region  312 . The electric charges photoelectrically converted in the sixth semiconductor region  316  are multiplied in the avalanche multiplication region between the first semiconductor region  311  and the second semiconductor region  312  while passing through the seventh semiconductor region  317  and are then read out from the contact plug  320  serving as the cathode. 
     Although the above description is made in connection with the case in which the seventh semiconductor region is of the second conductivity type, the seventh semiconductor region  317  may be a semiconductor region of the first conductivity type insofar as the above-mentioned potential relationship can be realized. The seventh semiconductor region  317  may be formed by setting the impurity concentration to be lower than in the second semiconductor region  312  at the time of ion injection. As an alternative, the impurity concentration may be the same as in the second semiconductor region  312  at the time of the ion injection and may be eventually reduced to be lower than that in the second semiconductor region  312  due to, for example, an influence of ion injection into the first semiconductor region  311 . 
     In this embodiment, the sixth semiconductor region  316  is formed by a semiconductor region of the first conductivity type. The impurity concentration in the sixth semiconductor region  316  may be uniform or may be set such that the impurity concentration in a region overlapping the seventh semiconductor region  317  when viewed in plan is higher than in a region overlapping the second semiconductor region  312  when viewed in plan. This can form a potential structure capable of causing the electric charges generated in end portions of the sixth semiconductor region  316  to be moved to the seventh semiconductor region  317 . The end portions of the sixth semiconductor region  316  are, for example, a portion near a region where the fourth semiconductor region  314  and the fifth semiconductor region  315  intersect, and a portion near a region where the fourth semiconductor region  314  and the second semiconductor region  312  intersect. 
     As illustrated in  FIG. 15 , the fifth semiconductor region  315  may be disposed to extend up to a position between the pixel region and the pad region. In other words, the fifth semiconductor region  315  may be disposed such that a region where the fifth semiconductor region  315  is disposed is larger than the pixel region when viewed in plan. 
     According to this embodiment, as in the first embodiment, the sensitivity of the APD can be improved while the withstand voltage between the via  324  and the reflective metal layer  322  is ensured. Furthermore, since the electric charges generated in the sixth semiconductor region  316  can be collected and subjected to the avalanche multiplication, it is easy to further improve the sensitivity of the APD. In addition, the avalanche multiplication region can be reduced, and hence the dark current can be reduced. 
     Seventh Embodiment 
       FIGS. 16, 17A, 17B, 17C, 17D and 17E  are schematic sectional views of a photoelectric conversion apparatus according to a seventh embodiment. The photoelectric conversion apparatus according to this embodiment is different from the photoelectric conversion apparatus according to the sixth embodiment in shapes of the wiring layers. Another different point from the sixth embodiment is that the reflective metal layer  322  is not disposed. Features except for the above points and matters described below are substantially similar to those in the sixth embodiment. Hence similar components to those in the sixth embodiment are denoted by the same reference signs, and description of those components is omitted in some cases. 
       FIG. 16  is a schematic sectional view of the pixels arrayed in the opposite side direction.  FIG. 17A  is a schematic plan view taken along XVIIA-XVIIA in  FIG. 16 , and  FIG. 17B  is a schematic plan view taken along XVIIB-XVIIB in  FIG. 16 .  FIG. 17C  is a schematic plan view taken along XVIIC-XVIIC in  FIG. 16 , and  FIG. 17D  is a schematic plan view taken along XVIID-XVIID in  FIG. 16 .  FIG. 17A  further illustrates part of the semiconductor region of the semiconductor layer  302  in dotted lines for easier understanding regarding to which semiconductor region the contact plug is connected. 
     The contact plug  321  is connected to the fourth semiconductor region  314  through an eighth semiconductor region  318  of the second conductivity type. The eighth semiconductor region  318  is a semiconductor region with a higher impurity concentration than the fourth semiconductor region  314 . Positions at which the fourth semiconductor region  314  and the eighth semiconductor region  318  are disposed may not need to be in match with each other as illustrated in  FIG. 17A . Alternatively, the fourth semiconductor region  314  and the eighth semiconductor region  318  may be positioned in match with each other. The eighth semiconductor region  318  may be disposed to overlap part of the fourth semiconductor region  314  when viewed in plan. In such a case, the contact plug  321  and the fourth semiconductor region  314  are disposed to overlap when viewed in plan. 
     As illustrated in  FIGS. 16 and 17B , a wiring  330  to which the drive voltage for the APD is supplied is disposed in a wiring layer in which a wiring  335  is disposed. Thus, in this embodiment, the wiring for supplying one of the drive voltages for the APD and the wiring for supplying the other drive voltage are disposed in the same wiring layer. Even in this case, the withstand voltage between the wiring  335  and the wiring  330  can be ensured by holding a certain distance therebetween. Furthermore, at corners of the wirings illustrated in  FIG. 17B , there is a possibility that an electric field concentrates at the corners and dielectric breakdown is more likely to occur. Accordingly, reliability is ensured by forming the wiring  330  not to have corners with an angle of 90 degree or less, as illustrated in  FIG. 17B . In addition, the shape of an opening in the wiring  335  is made in match with the shape of the wiring  330 . 
     As illustrated in  FIG. 17C , the via  324  may be disposed as multiple vias. Of the drive voltages for the APD, the voltage with the higher absolute value is applied to the vias  324  that are connected to the wiring  326 . By connecting the above voltage to the wiring  326  via the multiple vias  324 , resistance can be reduced, and hence a voltage drop caused during the driving of the APD can be reduced. In one embodiment, intervals between the vias  324  are the same, but the vias  324  may be disposed at different intervals. 
     The wiring  326  and the wiring  325  are disposed as illustrated in  FIG. 17D . The wiring layer in which the wiring  325  is disposed and the wiring layer in which the wiring  330  is disposed have substantially the same shape when viewed in plan. The wiring layer in which the wiring  326  is disposed may not need to be disposed, but in one embodiment, it is disposed. The wiring layer in which the wiring  328  illustrated in  FIG. 17E  is disposed is the same wiring layer as the pad electrode  352 . 
     The drive voltage supplied from the pad electrode  352  is in turn supplied to the anode of the APD, as illustrated in the sectional view of  FIG. 15 , through a wiring disposed in the same wiring layer as the wiring  325 . 
     On that occasion, as illustrated in  FIG. 15 , a via connecting the pad electrode and the wiring may be disposed at a position deviated from a via in a layer just above the former via toward the pad electrode. With such a configuration, since the number of the vias disposed in the end portion of the pixel array can be increased in comparison with that disposed in an inner portion of the pixel array, a voltage drop from a PAD level during the driving of the APD can be reduced. 
     In  FIG. 16 , wiring layers other than the wirings forming the metal bonded portions in the wiring structure  303  are made of aluminum, and the wirings forming the metal bonded portions and the wirings in the wiring structure  403  are made of copper. However, the wiring layers other than the wirings forming the metal bonded portions in the wiring structure  303  may also be made of copper. 
     According to this embodiment, since the materials and the shapes of the wiring through which the drive voltages for the APD are supplied are selected as described above, the photoelectric conversion apparatus with higher reliability can be provided. 
     Eighth Embodiment 
       FIG. 18  is a block diagram illustrating a configuration of an optical detection system  1200  according to an eighth embodiment. 
     The optical detection system  1200  according to this embodiment includes a photoelectric conversion apparatus  1204 . Here, one of the photoelectric conversion apparatuses according to the above-described embodiments can be applied to the photoelectric conversion apparatus  1204 . The optical detection system  1200  can be used as, for example, an imaging system. Practical examples of the imaging system are a digital still camera, a digital camcorder, and a monitoring camera.  FIG. 18  illustrates an example in which the optical detection system  1200  is a digital still camera. 
     The optical detection system  1200  illustrated in  FIG. 18  includes the photoelectric conversion apparatus  1204 , a lens  1202  for focusing an optical image of an object onto the photoelectric conversion apparatus  1204 , a diaphragm  1203  for varying a quantity of light passing through the lens  1202 , and a barrier  1201  for protecting the lens  1202 . The lens  1202  and the diaphragm  1203  form an optical system for condensing light to the photoelectric conversion apparatus  1204 . 
     The optical detection system  1200  includes a signal processing unit  1205  for processing an output signal output from the photoelectric conversion apparatus  1204 . The signal processing unit  1205  executes a signal processing operation of executing various corrections and compressions on an input signal as required and then outputting a signal after the processing. The optical detection system  1200  further includes a buffer memory unit  1206  for temporarily storing image data, and an external interface unit (external I/F unit)  1209  for communicating with, for example, an external computer. In addition, the optical detection system  1200  includes a recording medium  1211 , such as a semiconductor memory, on which or from which image data is recorded or read out, and a recording-medium control interface unit (recording-medium control I/F unit)  1210  for recording or reading out image data on or from the recording medium  1211 . The recording medium  1211  may be incorporated in the optical detection system  1200  or may be detachably mounted on the same. Communication between the recording-medium control I/F unit  1210  and the recording unit  1211  and communication from the external I/F unit  1209  may be performed wirelessly. 
     Moreover, the optical detection system  1200  includes a general control/calculation unit  1208  for executing various calculations and controlling the entirety of the digital still camera, and a timing generation unit  1207  for outputting various timing signals to the photoelectric conversion apparatus  1204  and the signal processing unit  1205 . Here, the timing signals and so on may be input from the outside, and the optical detection system  1200  is to be included at least the photoelectric conversion apparatus  1204  and the signal processing unit  1205  for processing the output signal from the photoelectric conversion apparatus  1204 . The timing generation unit  1207  may be mounted on the photoelectric conversion apparatus. The general control/calculation unit  1208  and the timing generation unit  1207  may be constituted to execute part or all of control functions of the photoelectric conversion apparatus  1204 . 
     The photoelectric conversion apparatus  1204  outputs an image signal to the signal processing unit  1205 . The signal processing unit  1205  executes predetermined signal processing on the image signal output from the photoelectric conversion apparatus  1204  and outputs image data. Furthermore, the signal processing unit  1205  forms an image from the image signal. In addition, the signal processing unit  1205  may execute distance measurement calculation on the signal output from the photoelectric conversion apparatus  1204 . The signal processing unit  1205  and the timing generation unit  1207  may be mounted on the photoelectric conversion apparatus. In other words, the signal processing unit  1205  and the timing generation unit  1207  may be mounted on a substrate on which pixels are disposed or may be disposed on a different substrate. When the imaging system is constituted by using the photoelectric conversion apparatus according to any of the above-described embodiments, the imaging system capable of providing an image with higher quality can be realized. 
     Ninth Embodiment 
       FIG. 19  is a block diagram illustrating an exemplary configuration of a distance image sensor  401  that is an electronic device utilizing the photoelectric conversion apparatus according to any of the above-described embodiments. 
     As illustrated in  FIG. 19 , the distance image sensor  401  includes an optical system  407 , a photoelectric conversion apparatus  408 , an image processing circuit  404 , a monitor  405 , and a memory  406 . The distance image sensor  401  can obtain a distance image corresponding to a distance up to an object by receiving light (modulated light or pulse light) that has been emitted to the object from a light source device  409  and that has been reflected by a surface of the object. 
     The optical system  407  includes one or more lenses, guides image light (incident light) from the object to the photoelectric conversion apparatus  408 , and focuses the incident light on a light receiving surface (sensor unit) of the photoelectric conversion apparatus  408 . 
     The photoelectric conversion apparatus according to any of the above-described embodiments is used as the photoelectric conversion apparatus  408 , and a distance signal representing a distance to be obtained from a received optical signal output from the photoelectric conversion apparatus  408  is supplied to the image processing circuit  404 . 
     The image processing circuit  404  executes image processing to form a distance image based on the distance signal supplied from the photoelectric conversion apparatus  408 . A distance image (image data) obtained with the image processing is supplied to the monitor  405  to be displayed or is supplied to the memory  406  to be stored (recorded). 
     According to distance image sensor  401  configured as described above, since the above-described photoelectric conversion apparatus is applied to the distance image sensor  401 , pixel characteristics are improved, and the distance image can be obtained at higher accuracy, for example. 
     Tenth Embodiment 
     The technique according to this disclosure (present technique) can be applied to a variety of products. For example, the technique according to this disclosure may be applied to endoscopic surgery systems. 
       FIG. 20  is a schematic view of an exemplary configuration of an endoscopic surgery system  1003  to which the technique according to this disclosure (present technique) can be applied. 
       FIG. 20  illustrates a situation that a surgeon (doctor)  1131  is performing surgery on a patient  1132  on a patient bed  1133  with the endoscopic surgery system  1003 . As illustrated in  FIG. 20 , the endoscopic surgery system  1003  is constituted by an endoscope  1100 , a surgical tool  1110 , and a cart  1134  on which various devices for use in the endoscopic surgery are placed. 
     The endoscope  1100  is constituted by a lens barrel  1101  part of which is inserted into a body cavity of the patient  1132 , the part ranging over a predetermined length from a distal end, and a camera head  1102  connected to a base end of the lens barrel  1101 . Although the endoscope  1100  including the rigid lens barrel  1101 , namely the so-called rigid endoscope, is used in the illustrated example, the endoscope  1100  may be constituted as the so-called soft endoscope including a soft lens barrel. 
     An opening is formed at the distal end of the lens barrel  1101 , and an objective lens is fitted to the opening. A light source device  1203  is connected to the endoscope  1100 . Light generated from the light source device  1203  is guided to the distal end of the lens barrel  1101  by a light guide extending through the lens barrel  1101  and is applied to an observation target in the body cavity of the patient  1132  through the objective lens. The endoscope  1100  may be a direct-viewing endoscope, an oblique-viewing endoscope, or a side-viewing endoscope. 
     The optical system and the photoelectric conversion apparatus are disposed inside the camera head  1102 , and light (observation light) reflected from the observation target is condensed onto the photoelectric conversion apparatus through the optical system. The observation light is photoelectrically converted by the photoelectric conversion apparatus, and an electrical signal corresponding to the observation light, namely an image signal corresponding to an image to be observed, is produced. The photoelectric conversion apparatus may be one of the photoelectric conversion apparatuses according to the above-described embodiments. The image signal is sent as RAW data to a camera control unit (CCU)  1135 . 
     The CCU  1135  is constituted by a CPU (Central Processing Unit) a GPU (Graphics Processing Unit), and so on and controls operations of the endoscope  1100  and a display unit  1136  in a supervising manner Furthermore, the CCU  1135  receives the image signal from the camera head  1102  and executes various types of image processing, such as RAW processing (demosaic processing), on the image signal to display an image based on the image signal. 
     The display unit  1136  displays, in accordance with control of the CCU  1135 , the image based on the image signal that has been subjected to the image processing by the CCU  1135 . 
     The light source device  1203  is constituted by a light source such as an LED (Light Emitting Diode), for example, and applies, to the endoscope  1100 , illumination light to take an image of, for example, part under surgery. 
     An input device  1137  is an input interface for the endoscopic surgery system  1003 . A user can input various items of information and various instructions to the endoscopic surgery system  1003  through the input device  1137 . 
     A surgical tool controller  1138  controls driving of an energy tool  1112  to perform, for example, cauterization or incision of tissues and sealing of blood vessels. 
     The light source device  1203  for applying, to the endoscope  1100 , the illumination light to take the image of the part under surgery may be a white light source that is constituted by, for example, an LED, a laser beam source, or a combination of them. When the white light source is constituted by a combination of RGB laser beam sources, white balance of a taken image can be adjusted in the light source device  1203  because output intensity and output timing for each color (each wavelength) can be controlled with high accuracy. Furthermore, in the above case, individual images corresponding to RGB can be taken in time division by applying individual laser beams from the RGB laser beam sources to the observation target and by controlling driving of imaging elements of the camera head  1102  in synchronism with timings of applying the laser beams. Such a method enables a color image to be obtained without disposing a color filter for each of the imaging elements. 
     Driving of the light source device  1203  may be controlled such that intensity of the light output from the light source device  1203  is changed per predetermined time. A high dynamic range image free from the so-called crushed shadows and blown highlights can be produced by controlling the driving of the imaging elements in the camera head  1102  in synchronism with timing at which the intensity of the above-mentioned output light is changed, thus obtaining images in time division, and then synthesizing those images. 
     The light source device  1203  may be constituted to be able to supply light in a predetermined wavelength range corresponding to special light observation. The special light observation is performed, for example, by utilizing wavelength dependency of absorption of light in body tissues. In a specific example, an image of a predetermined tissue, such as blood vessels in a surface layer of the mucous membrane, is taken with high contrast by applying light in a narrower range than illumination light (namely, white light) that is used in usual observation. 
     Alternatively, fluorescence observation for obtaining an image with fluorescence generated upon illumination with excitation light may be performed in the special light observation. In the fluorescence observation, it is possible, for example, to perform an operation of applying excitation light to a body tissue and observing fluorescence from the body tissue, or an operation of locally injecting a reagent, such as indocyanine green (ICG), to a body tissue, applying excitation light adapted for a fluorescence wavelength of the reagent to the body tissue, and obtaining a fluorescence image. The light source device  1203  can be constituted to be able to supply the narrow-range light and/or the excitation light adapted for the above-described special light observation. 
     Eleventh Embodiment 
     An optical detection system and a moving body according to an eleventh embodiment will be described below with reference to  FIGS. 21A, 21B, and 22 .  FIG. 21A  is a block diagram illustrating an exemplary configuration of the optical detection system and the moving body according to this embodiment.  FIG. 22  is a flowchart representing operation of the optical detection system according to this embodiment. In this embodiment, the optical detection system is described in connection with an example of an on-vehicle camera. 
       FIGS. 21A and 21B  illustrate an example of a vehicle system and an optical detection system  1301  that is mounted on the vehicle system to take an image. The optical detection system  1301  includes a photoelectric conversion apparatus  1302 , an image pre-processing unit  1315 , an integrated circuit  1303 , and an optical system  1314 . The optical system  1314  focuses an optical image of an object on the photoelectric conversion apparatus  1302 . The photoelectric conversion apparatus  1302  converts the optical image of the object, focused by the optical system  1314 , to an electrical signal. The photoelectric conversion apparatus  1302  is one of the photoelectric conversion apparatuses according to the above-described embodiments. The image pre-processing unit  1315  executes predetermined signal processing on a signal output from the photoelectric conversion apparatus  1302 . The function of the image pre-processing unit  1315  may be incorporated in the photoelectric conversion apparatus  1302 . The optical detection system  1301  includes at least two sets of the optical systems  1314 , the photoelectric conversion apparatuses  1302 , and the image pre-processing units  1315 . An output from the image pre-processing unit  1315  in each set is input to the integrated circuit  1303 . 
     The integrated circuit  1303  is an integrated circuit designed for an imaging system and includes an image processing unit  1304  equipped with a memory  1305 , an optical distance measuring unit  1306 , a distance measurement calculation unit  1307 , an object recognition unit  1308 , and an abnormality detection unit  1309 . The image processing unit  1304  executes image processing, such as RAW processing and defect correction, on an output signal from the image pre-processing unit  1315 . The memory  1305  serves as a primary storage for the taken image and further stores defect positions of pixels in the taken image. The optical distance measuring unit  1306  performs focusing on the object and distance measurement. The distance measurement calculation unit  1307  calculates measured distance information from multiple sets of image data obtained from the multiple photoelectric conversion apparatuses  1302 . The object recognition unit  1308  recognizes objects such as vehicles, roads, traffic signs, and persons. The abnormality detection unit  1309  issues, upon detection of an abnormality in any of the photoelectric conversion apparatuses  1302 , a notice indicating the occurrence of the abnormality to a main control unit  1313 . 
     The integrated circuit  1303  may be realized with hardware designed for dedicated use, software modules, or a combination of them. Alternatively, the integrated circuit  1303  may be realized with, for example, FPGA (Field Programmable Gate Array), ASIC (Application Specific Integrated Circuit), or a combination of them. 
     The main control unit  1313  supervises and controls operations of the optical detection system  1301 , vehicle sensors  1310 , control units  1320 , and so on. As another method, the optical detection system  1301 , the vehicle sensors  1310 , and the control units  1320  may have individual communication interfaces without using the main control unit  1313 , and control signals may be sent and received via a communication network (in accordance with, for example, the CAN (Controller Area Network) standards). 
     The integrated circuit  1303  has a function of sending control signals and setting values to the photoelectric conversion apparatuses  1302  upon receiving control signals from the main control unit  1313  or under control of a control unit incorporated therein. 
     The optical detection system  1301  is connected to the vehicle sensors  1310  to be able to detect driving conditions of the vehicle, such as a vehicle speed, a yaw rate, and a steering angle, environments outside the vehicle, and conditions of other vehicles and obstacles. The vehicle sensors  1310  also serve as a distance information acquisition unit for obtaining distance information up to the object. Furthermore, the optical detection system  1301  is connected to a driving support control unit  1311  for providing various driving supports such as automatic steering, automatic cruising, and a collision avoidance function. Regarding the collision avoidance function, estimation of collision and the occurrence of collision with another vehicle or any obstacle are determined based on the detection results of the optical detection system  1301  and the vehicle sensors  1310 . In accordance with the determination, the optical detection system  1301  performs avoidance control when the collision is estimated and activation of a safety device in case of the collision. 
     The optical detection system  1301  is further connected to a warning device  1312  that issues warnings to a driver based on the determination result of a collision determination unit. For example, when the determination result of the collision determination unit indicates high possibility of the collision, the main control unit  1313  performs vehicle control for avoiding the collision or reducing damage by, for example, braking the vehicle, retracting an accelerator, and/or reducing an engine output. The warning device  1312  issues warnings to the user by, for example, giving an alarm such as a sound, displaying alarm information on a display screen of a car navigation system or a meter panel, and/or vibrating a sheet belt or a steering wheel. 
     In this embodiment, the optical detection system  1301  takes an image of the surrounding of the vehicle, for example, the front or the back of the vehicle.  FIG. 21B  illustrates a layout example of the optical detection system  1301  when the image of the front of the vehicle is taken by the optical detection system  1301 . 
     Two photoelectric conversion apparatuses  1302  are disposed in front of a vehicle  1300 . In a specific example, when a vehicular center line when viewed in a traveling direction of the vehicle  1300  or with respect to an outline (for example, a vehicle width) of the vehicle  1300  is regarded as a symmetric axis, the two photoelectric conversion apparatuses  1302  are disposed in line symmetry relative to the symmetry axis. That layout is desired in obtaining the information of the distance between the vehicle  1300  and the object of which image is to be taken, and in determining a possibility of the collision. Furthermore, in one embodiment, the photoelectric conversion apparatuses  1302  are disposed at positions not interfering with the visual field of a driver when the driver visually recognizes situations outside the vehicle  1300  from a driver&#39;s seat. The warning device  1312  is disposed at a position reliably falling within the visual field of the driver. 
     A failure detection operation of the photoelectric conversion apparatus  1302  in the optical detection system  1301  will be described below with reference to  FIG. 22 . The failure detection operation of the photoelectric conversion apparatus  1302  is performed in accordance with steps S 1410  to S 1480  illustrated in  FIG. 22 . 
     In step S 1410 , the setting at startup of the photoelectric conversion apparatus  1302  is performed. In more detail, the setting for the operation of the photoelectric conversion apparatus  1302  is sent from the outside (for example, the main control unit  1313 ) or the inside of the optical detection system  1301  to start the imaging operation and the failure detection operation of the photoelectric conversion apparatus  1302 . 
     Next, in step S 1420 , pixel signals are obtained from the effective pixels. Furthermore, in step S 1430 , an output value from a failure detection pixel disposed for detecting a failure is obtained. The failure detection pixel includes a photoelectric conversion element as in the effective pixel. A predetermined voltage is written in the photoelectric conversion element of the failure detection pixel. The failure detection pixel outputs a signal corresponding to the voltage written into the photoelectric conversion element. The order of step S 1420  and step S 1430  may be reversed. 
     Next, in step S 1440 , match/mismatch determination between an expected output value from the failure detection pixel and an actual output value from the failure detection pixel is executed. If a result of the match/mismatch determination in step S 1440  indicates that the expected output value and the actual output value match, the processing step shifts to step S 1450  where it is determined that the imaging operation is normally performed. Then, the processing step shifts to step S 1460 . In step S 1460 , the pixel signals from a row under scanning are sent to the memory  1305  for primary storage. Thereafter, the processing step returns to step S 1420  to continue the failure detection operation. On the other hand, if a result of the match/mismatch determination in step S 1440  indicates that the expected output value and the actual output value mismatch, the processing step shifts to step S 1470 . In step S 1470 , the imaging operation is determined to be abnormal, and a warning notification is informed to the main control unit  1313  or the warning device  1312 . The warning device  1312  causes the display unit to display information indicating the detection of abnormality. Then, the photoelectric conversion apparatus  1302  is stopped in step S 1480 , and the operation of the optical detection system  1301  is ended. 
     Although the above embodiment has been described in connection with an example of looping the flowchart per row, the flowchart may be looped per multiple rows, or the failure detection operation may be executed per frame. The warning issued in step S 1470  may be notified to the outside of the vehicle via a wireless network. 
     Although the above embodiment has been described in connection with the control of avoiding the collision with another vehicle, the optical detection system  1301  can also be applied to control for automatic operation following another vehicle or control for automatic operation keeping the vehicle from not going out of the lane. Without being limited to vehicles such as cars, the optical detection system  1301  can be further applied to other moving bodies (moving apparatuses) such as ships, aircrafts, and industrial robots, for example. In addition, the optical detection system  1301  can be applied to a variety of equipment utilizing recognition of objects, such as an intelligent traffic system (ITS), without being limited to the moving bodies. 
     Moreover, the photoelectric conversion apparatus according to the present disclosure may be constituted to be able to obtain various items of information including distance information, for example. 
     Twelfth Embodiment 
       FIG. 23A  illustrates a pair of eyeglasses  1600  (smart glasses) according to one application example of a twelfth embodiment. The pair of eyeglasses  1600  includes a photoelectric conversion apparatus  1602 . The photoelectric conversion apparatus  1602  is one of the photoelectric conversion apparatuses according to the above-described embodiments. A display device including a light emitting device, such as an OLED or an LED, may be attached to a rear surface side of each lens  1601 . One or more photoelectric conversion apparatuses  1602  may be disposed. Multiple types if photoelectric conversion apparatuses may be combined with each other. A layout position of the photoelectric conversion apparatus  1602  is not limited to the position illustrated in  FIG. 23A . 
     The pair of eyeglasses  1600  further includes a control device  1603 . The control device  1603  functions as a power supply for supplying electric power to the photoelectric conversion apparatus  1602  and the above-mentioned display device. Furthermore, the control device  1603  controls operations of the photoelectric conversion apparatus  1602  and the display device. An optical system for condensing light to the photoelectric conversion apparatus  1602  is formed on the lens  1601 . 
       FIG. 23B  illustrates a pair of eyeglasses  1610  (smart glasses) according to another application example of the twelfth embodiment. The pair of eyeglasses  1610  includes a control device  1612  on which a photoelectric conversion apparatus corresponding to the photoelectric conversion apparatus  1602  and a display device are mounted. An optical system for projecting lights emitted from both the photoelectric conversion apparatus and the display device disposed in the control device  1612  is formed on each lens  1611 , and an image is projected on the lens  1611 . The control device  1612  not only functions as a power supply for supplying electric power to the photoelectric conversion apparatus and the display device, but also controls operations of the photoelectric conversion apparatus and the display device. The control device may include a line-of-sight detection unit for detecting the line of sight of a wearer. Infrared light may be used to detect the line of the sight. An infrared light emitting unit emits infrared light to an eyeball of a user looking at a display image. An image of the eyeball is obtained by an imaging unit detecting reflected light of the emitted infrared light from the eyeball, the imaging unit including a light receiving element. A reduction in quality of the image is reduced with the provision of a unit for reducing light that enters the display device from the infrared light emitting unit when viewed in plan. 
     The line of sight of the user for the display image is detected from the eyeball image obtained with the above-described infrared imaging. A suitable one of known methods can be used to detect the line of sight from the eyeball image. For example, a line-of-sight detection method based on a Purkinje image formed by reflection of illuminated light at the cornea can be used. 
     In more detail, a line-of-sight detection process based on a pupil and corneal reflection method is performed. The line of sight of the user is detected by detecting a line-of-sight vector representing a direction (rotation angle) of the eyeball in accordance with the pupil and corneal reflection method based on an image of the pupil and the Purkinje image that are included in the obtained eyeball image. 
     The display device in this embodiment may include a photoelectric conversion apparatus including a light receiving element and may control an image displayed on the display device in accordance with line-of-sight information of the user given from the photoelectric conversion apparatus. 
     In more detail, for the display device, a first visual field region at which the user is looking and a second visual field region other than the first visual field region are determined based on the line-of-sight information. The first visual field region and the second visual field region may be determined by a control device in the display device, or those regions determined by an external control device may be received from the external control device. Of a display region of the display device, a display resolution in the first visual field region may be controlled to be higher than that in the second visual field region. In other words, a display resolution in the second visual field region may be controlled to be lower than that in the first visual field region. 
     Alternatively, the display region may include a first display region and a second display region different from the first display region, and which one of the first display region and the second display region is to be given with higher priority may be determined based on the line-of-sight information. The first display region and the second display region may be determined by the control device in the display device, or those regions determined by an external control device may be received from the external control device. A resolution in a region with higher priority may be controlled to be higher than that in a region other than the region with the higher priority. In other words, a resolution in the region with relatively low priority may be set to be relatively low. 
     An AI program may be used to determine the first visual field region and the region with higher priority. The AI program may be a model that is designed to use, as teacher data, an eyeball image and an actually viewing direction of an eyeball in the image, and to estimate an angle of the line of sight and a distance up to an object ahead of the line of sight from the eyeball image. The AI program may be installed in the display device, the photoelectric conversion apparatus, or an external device. When the AI program is installed in the external device, it is transmitted to the display device via communication. 
     When display control is to be performed based on visual recognition and detection, the aspect of the embodiment is applied to smart glasses that further include a photoelectric conversion apparatus for imaging the outside. The smart glasses can display external information obtained by imaging the outside in real time. 
     Other Embodiments 
     Although the embodiments have been described above, the present disclosure is not limited to those embodiments, and various alterations and modifications can be made on the embodiments. In addition, the features of the embodiments can be optionally applied to each other. 
     According to the present disclosure, the photoelectric conversion apparatus can be obtained in which, regarding a first wiring for supplying a drive voltage to one of two nodes of the APD and a second wiring for supplying a drive voltage to the other node, concrete configurations and layout positions are designed in consideration of the withstand voltage between the first wiring and the second wiring. 
     While the disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-008442 filed Jan. 22, 2021, which is hereby incorporated by reference herein in its entirety.