Patent Publication Number: US-2022238743-A1

Title: Photoelectric conversion device, photoelectric conversion system, and moving body

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a photoelectric conversion device, a photoelectric conversion system, and a moving body. 
     Description of the Related Art 
     There is known a light detection device that can detect weak light of a single photon level using avalanche (electron avalanche) multiplication. Japanese Patent Laid-Open No. 2018-201005 discloses a SPAD (Single Photon Avalanche Diode) in which photo charges generated by a single photon cause avalanche multiplication in the p-n junction region of semiconductor regions forming a photoelectric converter. 
     SUMMARY OF THE INVENTION 
     The invention provides a technique advantageous in suppressing the increase of the dark count rate (to be referred to as the DCR hereinafter) caused by a high electric field in an avalanche photodiode (to be referred to as an APD hereinafter). 
     One of aspects of the invention provides a photoelectric conversion device comprising: a first region of a first conductivity type arranged in a semiconductor layer having a first surface and a second surface; a second region of a second conductivity type arranged between the second surface and the first region and forming an avalanche photodiode together with the first region; a separation region of the second conductivity type arranged between the first surface and the second surface to surround the second region in an orthogonal projection with respect to the first surface; a contact region of the second conductivity type arranged to contact the separation region; a first contact plug connected to the first region; and a second contact plug connected to the contact region, wherein the second region has a shape of a rectangle, and the second contact plug is arranged in a diagonal direction of the rectangle, and in the orthogonal projection with respect to the first surface, a distance between a center of the first contact plug and a center of the second contact plug is larger than a distance between a center of the second region and the center of the second contact plug. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view exemplifying the arrangement of a photoelectric conversion device according to an embodiment; 
         FIG. 2  is a view exemplifying the arrangement of one pixel shown in  FIG. 1 ; 
         FIG. 3A  shows views an APD. 
         FIGS. 3B and 3C  show exemplifying detection of a photon using the APD shown in  FIG. 3A ; 
         FIG. 4  is a plan view of a pixel array according to the first embodiment; 
         FIG. 5  is an enlarged view of a portion shown in  FIG. 4 ; 
         FIG. 6  is an enlarged view of a portion shown in  FIG. 4 ; 
         FIG. 7  is a sectional view taken along a line X-X′ in  FIG. 4 ; 
         FIG. 8  is a plan view of a pixel array according to the second embodiment; 
         FIG. 9  is a plan view of a pixel array according to the third embodiment; 
         FIG. 10  is a plan view of a pixel array according to the fourth embodiment; 
         FIG. 11  is a plan view of a pixel array according to the fifth embodiment; 
         FIG. 12  is a plan view of a pixel array according to the sixth embodiment; 
         FIG. 13  is a plan view of the seventh embodiment of a pixel array; 
         FIG. 14  is a block diagram showing the arrangement of a photoelectric conversion system according to an embodiment; 
         FIGS. 15A and 15B  show views of the arrangement of a vehicle system and a photoelectric conversion system that is incorporated in the vehicle system and performs image capturing according to an embodiment; and 
         FIG. 16  is a flowchart illustrating the operation of the photoelectric conversion system shown in  FIGS. 15A and 15B . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made to an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
       FIG. 1  exemplifies the arrangement of a photoelectric conversion device  100  according to an embodiment. Referring to  FIG. 1 , the photoelectric conversion device  100  can include a pixel array  101 , a controller  115 , a horizontal scanning circuit  111 , a readout circuit  112 , a plurality of signal lines  113 , and a vertical scanning circuit  110 . In the pixel array  101 , a plurality of pixels  104  can be arranged to form a plurality of rows and a plurality of columns. Each pixel  104  can include a photoelectric converter  102  with an APD (avalanche photodiode) and a signal processor  103 . The photoelectric converter  102  converts light into an electrical signal. The signal processor  103  can be configured to output, to the readout circuit  112 , a signal obtained by processing the electrical signal output from the photoelectric converter  102 . The signal processor  103  can include, for example, a counter and a memory. The memory can store a digital value. 
     The vertical scanning circuit  110  can be configured to receive a control signal supplied from the controller  115  and to supply a control pulse to each pixel  104 . The vertical scanning circuit  110  can include, for example, a shift register and an address decoder. The vertical scanning circuit  110  can be configured to select the plurality of pixels  104  of the pixel array  101  on the row basis. The signal processors  103  of the plurality of pixels  104  belonging to the row selected by the vertical scanning circuit  110  output signals to a corresponding one of the plurality of signal lines  113 . 
     The horizontal scanning circuit  111  can be configured to scan the plurality of signal lines  113  so as to select, among the plurality of signal lines  113 , the signal line  113  from which the readout circuit  112  is to read out the signals. The readout circuit  112  can supply, to an output circuit  114 , the signals of the signal line  113  selected by the horizontal scanning circuit  111 . The output circuit  114  can output the signals, supplied from the readout circuit  112 , to an external device or a device incorporated in the photoelectric conversion device  100 , for example, a recording unit or a processor. 
       FIG. 1  shows an example in which the plurality of pixels  104  are two-dimensionally arranged but the plurality of pixels  104  may be one-dimensionally arranged. Furthermore, the pixel array  101  may be replaced by one pixel  104 . The function of the signal processor  103  need not always be individually provided in each of all the pixels  104 . For example, at least two pixels  104  may share one signal processor  103 , and signals output from the photoelectric converters  102  of the at least two pixels  104  may sequentially be processed by the shared signal processor  103 . 
       FIG. 2  exemplifies the arrangement of one pixel  104 . The photoelectric converter  102  includes an APD  201 . The APD  201  photoelectrically converts incident light to generate charge pairs. The anode of the APD  201  can be supplied with a voltage VL. The cathode of the APD  201  can be supplied with a voltage VH higher than the voltage VL supplied to the anode. The anode and the cathode are supplied with a reverse bias voltage that causes the APD  201  to perform an avalanche multiplication operation. If light enters the APD  201  in the state in which such voltage is supplied, charges generated by the light cause avalanche multiplication, thereby generating an avalanche current. When a reverse bias voltage is supplied, there are a Geiger mode operating in a state in which the potential difference between the anode and the cathode is larger than the breakdown voltage and a linear mode operating in a state in which the potential difference between the anode and the cathode is around or smaller than the breakdown voltage. An APD operated in the Geiger mode is called a SPAD. For example, the voltage VL is −30 V and the voltage VH is 1 V. 
     A quenching element  202  can be arranged to connect a power supply line for supplying the voltage VH and the cathode of the APD  201 . The quenching element  202  has a function of converting, into a voltage, a change of the avalanche current generated in the APD  201 . The quenching element  202  functions as a load circuit (quenching circuit) at the time of signal multiplication by avalanche multiplication, and performs an operation (quenching operation) of suppressing avalanche multiplication by decreasing the voltage supplied to the APD  201 . 
     The signal processor  103  can include, for example, a waveform shaper  210 , a counter circuit  211 , and a selection circuit  212 . The waveform shaper  210  outputs a pulse signal by shaping the potential change waveform of the cathode of the APD  201  obtained at the time of detection of a photon. The waveform shaper  210  can include, for example, an inverter circuit.  FIG. 2  shows an example in which the waveform shaper  210  is formed by one inverter. However, the waveform shaper  210  may be formed by series-connecting a plurality of inverters or by another circuit having a waveform shaping effect. The counter circuit  211  can count the pulse signal output from the waveform shaper  210 , and hold a thus obtained count value. Furthermore, when the counter circuit  211  is supplied with a control pulse pRES from the vertical scanning circuit  110  via a driving line  213  (not shown in  FIG. 1 ), it resets the held signal. When the selection circuit  212  is supplied with a control pulse pSEL from the vertical scanning circuit  110  via a driving line  214  (not shown in  FIG. 1 ), it electrically connects the counter circuit  211  and the signal line  113 . The selection circuit  212  may include, for example, a buffer circuit for driving the signal line  113 . A switch such as a transistor may be arranged between the quenching element  202  and the photoelectric converter  102  (APD  201 ) and/or between the photoelectric converter  102  and the signal processor  103 . A switch such as a transistor may be arranged in the supply path of the voltage VH or VL to the photoelectric converter  102 . 
     In this embodiment, the arrangement using the counter circuit  211  is described. However, instead of the counter circuit  211 , a time to digital converter (to be referred to as a TDC hereinafter) and a memory may be used. In this case, the photoelectric conversion device  100  can function as a photoelectric conversion device that acquires a pulse detection timing. In this case, the occurrence timing of the pulse signal output from the waveform shaper  210  can be converted into a digital signal by the TDC. The TDC can be supplied with a control pulse pREF (reference signal) from the vertical scanning circuit  110  via the driving line to measure the timing of the pulse signal. The TDC generates a digital signal indicating the input timing of the signal output from the waveform shaper  210  with reference to the control pulse pREF. 
     In the example of the arrangement shown in  FIG. 2 , the anode of the APD  201  is connected to the supply line of the voltage VL, the quenching element  202  is connected between the cathode of the APD  201  and the supply line of the voltage VH, and the cathode is connected to the input terminal of the waveform shaper  210 . In this case, a signal charge is an electron. Instead of this arrangement, the cathode of the APD  201  may be connected to the supply line of the voltage VH, the quenching element  202  may be connected between the anode of the APD  201  and the supply line of the voltage VL, and the anode may be connected to the input terminal of the waveform shaper  210 . In this case, a signal charge is a hole. 
     Detection of a photon using the APD  201  will be described with reference to  FIGS. 3A to 3C .  FIG. 3A  shows the APD  201 , the quenching element  202 , and the waveform shaper  210  forming part of the pixel  104 . The input side of the waveform shaper  210  is indicated by node A and the output side of the waveform shaper  210  is indicated by node B.  FIG. 3B  shows the voltage waveform of node A in  FIG. 3A .  FIG. 3C  shows the voltage waveform of node B in  FIG. 3A . 
     During a period from time t 0  to time t 1 , a potential difference of VH−VL is applied to the APD  201  shown in  FIG. 3A . When a photon enters at time t 1 , an avalanche multiplication current flows through the quenching element  202 , thereby dropping the voltage of node A. If the voltage drop amount becomes larger and the potential difference applied to the APD  201  becomes smaller, the avalanche multiplication of the APD  201  stops, and the voltage level of node A does not drop to a value less than a given value. After that, a current that compensates for the voltage drop flows through node A, and node A is set to the original potential level at time t 3 . The voltage waveform of node A is shaped by the waveform shaper  210 . More specifically, the waveform shaper  210  outputs a signal which is set to an active level in a portion where the voltage of node A exceeds a threshold. 
       FIG. 4  shows a plan view or a planar view of the pixel array  101  according to the first embodiment. 
       FIGS. 5 and 6  each show an enlarged view of a portion in  FIG. 4 .  FIG. 7  shows a sectional view taken along a line X-X′ in  FIG. 4 . The photoelectric conversion device  100  includes a semiconductor layer  301  having a first surface S 1  and a second surface S 2 .  FIGS. 4 and 5  may be understood as an orthogonal projection with respect to the first surface S 1  or the second surface S 2 . In the following description, the first conductivity type and the second conductivity type are different from each other. If the first conductivity type is an n type, the second conductivity type is a p type. If the first conductivity type is a p type, the second conductivity type is an n type. Note that referring to  FIGS. 4, 5, 6, and 7 , the signal processor  103  is not illustrated for the sake of descriptive simplicity. The same applies to the remaining drawings to be referred to hereinafter. A region of the first conductivity type and a region of the second conductivity type are both semiconductor regions, in other words, impurity semiconductor regions. In the following description, the impurity concentration of the region of the first conductivity type indicates a net impurity concentration obtained by subtracting the impurity concentration of the second conductivity type from the impurity concentration of the first conductivity type when the region includes an impurity of the second conductivity type in addition to an impurity of the first conductivity type. Similarly, the impurity concentration of the region of the second conductivity type indicates a net impurity concentration obtained by subtracting the impurity concentration of the first conductivity type from the impurity concentration of the second conductivity type when the region includes an impurity of the first conductivity type in addition to an impurity of the second conductivity type. 
     The pixel  104  can include a first region  311  of the first conductivity type, a second region  312  of the second conductivity type, a separation region  314  of the second conductivity type, and a contact region  316  of the second conductivity type. The first region  311  is arranged between the first surface S 1  and the second surface S 2  of the semiconductor layer  301 . The second region  312  can be arranged between the second surface S 2  of the semiconductor layer  301  and the first region  311 . The second region  312  can be arranged apart from the first region  311 . Note that the first region  311  and the second region  312  may contact each other. The second region  312  can form the APD  201  together with the first region  311 . The first region  311  can be the cathode of the APD  201  and the second region  312  can be the anode of the APD  201 . Alternatively, the first region  311  can be the anode of the APD  201  and the second region  312  can be the cathode of the APD  201 . 
     In the plan view or the planar view (the orthogonal projection with respect to the first surface S 1 ), the separation region  314  can be arranged between the first surface S 1  and the second surface S 2  to surround the second region  312 . Furthermore, the separation region  314  can be arranged between the first surface S 1  and the second surface S 2  to surround the first region  311 . The boundary of the separation region  314  may or may not include the first surface S 1 . The boundary of the separation region  314  may or may not include the second surface S 2 . The contact region  316  can be arranged to contact the separation region  314 . 
     The impurity concentration of the second conductivity type of the contact region  316  may be higher than that of the second conductivity type of the separation region  314 . The contact region  316  may be arranged so that its side surface is surrounded by the separation region  314  or contacts the separation region  314 . The contact region  316  may be arranged to cover the entire region of the end face of the separation region  314  on the side of the first surface S 1 . The contact region  316  preferably has at least a portion that contacts the separation region  314 . This can supply a potential to the separation region  314  from the second contact plug  322  (to be described later) via the contact region  316 . Furthermore, if the separation region  314  and the second region  312  contact each other, it is possible to supply a potential to the second region  312  via the separation region  314 . 
     The pixel  104  can include a first contact plug  321  electrically connected to the first region  311 . The pixel  104  can also include a second contact plug  322  electrically connected to the contact region  316 . The pixel  104  may further include a ring-shaped region  313  of the first conductivity type. The impurity concentration of the first conductivity type of the ring-shaped region  313  may be lower than that of the first conductivity type of the first region  311 . The ring-shaped region  313  can function to relax local concentration of an electric field in a region between the first region  311  and the separation region  314  and/or the contact region  316 . The ring-shaped region  313  can be arranged to cover the side surface of the first region  311 . The ring-shaped region  313  can be arranged not to cover the central portion of a surface facing the second region  312  among the surfaces of the first region  311  and to cover all or part of the peripheral portion outside the central portion. The second region  312  can be arranged apart from the ring-shaped region  313 . The separation region  314  can be arranged apart from the ring-shaped region  313 . Referring to  FIGS. 4 to 6 , the ring-shaped region  313  is a circle but may be a rectangle or polygon. 
     The pixel  104  may include a third region  300 . The third region  300  can be a region of the first conductivity type arranged between the first region  311  of the first conductivity type and the second region  312  of the second conductivity type and between the ring-shaped region  313  of the first conductivity type and the second region  312  of the second conductivity type. In this case, the impurity concentration of the first conductivity type of the third region  300  is lower than that of the first conductivity type of the ring-shaped region  313 . Alternatively, the third region  300  can be a region of the second conductivity type arranged between the first region  311  of the first conductivity type and the second region  312  of the second conductivity type and between the ring-shaped region  313  of the first conductivity type and the second region  312  of the second conductivity type. In this case, the impurity concentration of the second conductivity type of the third region  300  is lower than those of the second conductivity type of the second region  312  and the separation region  314 . 
     The pixel  104  may further include a fourth region  315  of the second conductivity type. The fourth region  315  can be arranged between the second region  312  and the second surface S 2 . The boundary of the fourth region  315  may or may not include the second surface S 2 . A region of the second conductivity type having an impurity concentration lower than the impurity concentrations of the second conductivity type of the second region  312  and the fourth region  315  can be arranged between the second region  312  of the second conductivity type and the fourth region  315  of the second conductivity type. Alternatively, a region of the first conductivity type can be arranged between the second region  312  of the second conductivity type and the fourth region  315  of the second conductivity type. Charges generated in the region between the second region  312  and the fourth region  315  are collected to a strong electric field region formed by the first region  311  and the second region  312 . Then, the generated charges cause avalanche multiplication in the strong electric field region. The fourth region  315  and the separation region  314  preferably contact each other. This surrounds the region between the second region  312  and the fourth region  315  by the second region  312 , the separation region  314 , and the fourth region  315 , thereby making it easy to collect the generated charges to the strong electric field region. 
     The second contact plug  322  and the contact region  316  of the second conductivity type can be shared by at least two pixels  104 , for example, four pixels  104 . For example, the second contact plug  322  can be surrounded by the four pixels  104  and shared by the four pixels. From another viewpoint, the number of second contact plugs  322  is smaller than that of first contact plugs  321 . The four pixels  104  sharing the second contact plug  322  can be arranged to have symmetry (point symmetry) with respect to the second contact plug  322 . From another viewpoint, four adjacent pixels  104  which can arbitrarily be extracted can be arranged to have symmetry (point symmetry) with respect to the center of the four pixels. From another viewpoint, the second region  312  has a shape of a rectangle, and the second contact plug  322  assigned to the second region  312  can be arranged in one of the four diagonal directions of the second region  312 . 
     Alternatively, the third region  300  has a shape of a rectangle, and the second contact plug  322  assigned to the third region  300  can be arranged in one of the four diagonal directions of the third region  300 . 
     The first contact plug  321  electrically connected to the first region  311  forming the cathode can be supplied with the voltage VH via the quenching element  202 . The second contact plug  322  electrically connected, via the separation region  314 , to the second region  312  forming the anode can be supplied with the voltage VL lower than the voltage VH supplied to the cathode. The anode and the cathode can be supplied with a reverse bias voltage that causes the APD  201  to perform an avalanche multiplication operation. With this reverse bias voltage, in an avalanche multiplication region  302  between the first region  311  and the second region  312 , charges generated by photoelectric conversion of incident light cause avalanche multiplication and an avalanche current thus flows. 
     If the distance between the first region  311  of the first conductivity type and the contact region  316  of the second conductivity type decreases along with reduction of the size of the pixel  104 , a local high electric field region can be formed between the first region  311  and the contact region  316 . The distance between the first region  311  of the first conductivity type and the contact region  316  of the second conductivity type should be made as large as possible. 
     As shown in  FIG. 5 , in the plan view or the planar view (the orthogonal projection with respect to the first surface S 1 ), D 2 &gt;D 1  is preferably satisfied. D 1  represents a distance between a center C 1  of the second region  312  (third region  300 ) and the center of the second contact plug  322 . D 2  represents a distance between the center of the first contact plug  321  and the center of the second contact plug  322 . 
     From another viewpoint, as shown in  FIG. 6 , in the plan view or the planar view (the orthogonal projection with respect to the first surface S 1 ), L 1 &gt;0.5L 2  is preferably satisfied. L 1  represents a distance between the center of the second contact plug  322  and the center of a first contact plug (to be referred to as “recent first contact plug” hereinafter)  321   a  closest to the second contact plug  322  among the plurality of first contact plugs  321 . L 2  represents a distance between a first region (to be referred to as “recent first region” hereinafter)  311   a  connected to the recent first contact plug  321   a  among the plurality of first regions  311  and a first region (to be referred to as “adjacent first region” hereinafter)  311   b  closest to the recent first region  311   a  among the plurality of first regions  311  on a straight line (straight line SL) passing through the center of the second contact plug  322  and the center of the recent first contact plug  321   a.    
     From another viewpoint, as shown in  FIG. 6 , in the plan view or the planar view (the orthogonal projection with respect to the first surface S 1 ), L 3 &gt;L 4  is preferably satisfied. In this example, L 3  and L 4  are located on a straight line passing through the contact region  316 , a first portion P 1  of the separation region  314  contacting the contact region  316 , the first region  311 , and a second portion P 2  of the separation region  314 . The first region  311  is located between the contact region  316  and the second portion P 2  of the separation region  314 . L 3  represents a distance between the contact region  316  and the first region  311 . L 4  represents a distance between the second portion of the separation region  314  and the first region  311 . The second portion P 2  is the separation region  314  where the contact region  316  is not arranged. 
     With this arrangement, the electric field between the first region  311  of the first conductivity type and the contact region  316  of the second conductivity type is hardly influenced by reduction of the size of the pixel  104 . That is, the increase of the DCR caused by reduction of the size of the pixel  104  can be suppressed. 
       FIG. 8  shows a plan view or a planar view of a pixel array  101  according to the second embodiment. 
     In the second embodiment, similar to the arrangement exemplified in the first embodiment, a plurality of pixels  104  arranged in a semiconductor layer  301  form the rectangular pixel array  101 . Each pixel  104  includes a photoelectric converter  102  with an APD  201 . The plurality of pixels  104  are arranged in the semiconductor layer  301 . A second contact plug  322  is arranged at each of positions in the diagonal directions of the pixel array  101 , and the total number of second contact plugs  322  is four. The second embodiment is advantageous in reducing dark electrons from a contact region  316  to which the second contact plug  322  is electrically connected, and this is effective for reducing the DCR. In the second embodiment as well, the arrangement described with reference to  FIGS. 5 and 6  can be adopted, thereby making it possible to suppress the increase of the DCR. 
       FIG. 9  shows a plan view or a planar view of a pixel array  101  according to the third embodiment. 
     An arrangement of the third embodiment is a modification of the second embodiment and matters not mentioned in the third embodiment can comply with the first and/or second embodiment. In the third embodiment, the arrangement described with reference to  FIGS. 5 and 6  is adopted for pixels  104  at the four corners of the pixel array  101 . With respect to other pixels  104 , a first contact plug  321  can be arranged at the center of a second region  312  and/or a fourth region  315  but may be arranged at a position deviated from the center. 
     From another viewpoint, the pixels  104  other than the pixels  104  at the four corners may be arranged at equal intervals. In this arrangement, it is possible to reduce variations in time until photoelectrically converted charges are detected as signals in the plurality of pixels  104 . 
       FIG. 10  shows a plan view or a planar view of a pixel array  101  according to the fourth embodiment. An arrangement of the fourth embodiment is a modification of the first embodiment and matters not mentioned in the fourth embodiment can comply with the first embodiment. In the fourth embodiment, similar to the arrangement exemplified in the first embodiment, a plurality of pixels  104  arranged in a semiconductor layer  301  form the rectangular pixel array  101 . Each pixel  104  includes a photoelectric converter  102  with an APD  201 . A photoelectric conversion device  100  according to the fourth embodiment includes a plurality of second contact plugs  322 , and the plurality of second contact plugs  322  can be arranged so that two second contact plugs  322  sandwich at least two pixels  104 . In one example, the plurality of second contact plugs  322  can be arranged so that two second contact plugs  322  sandwich at least two pixels  104  arrayed in a row direction (a direction orthogonal to a signal line  113 ). In another example, the plurality of second contact plugs  322  can be arranged so that the second contact plugs  322  sandwich at least two pixels  104 . In one example, the plurality of second contact plugs  322  can be arranged so that two second contact plugs  322  sandwich at least two pixels  104  arrayed in a column direction (a direction parallel to the signal line  113 ). In the fourth embodiment, the arrangement described with reference to  FIGS. 5 and 6  can be adopted, thereby suppressing the increase of the DCR. 
     In one example, the fourth embodiment can be implemented so that two second contact plugs  322  sandwich two pixels  104 . In this arrangement, the number of second contact plugs  322  is larger than that in the arrangement exemplified in the first embodiment in which four pixels  104  share one second contact plug  322 . This is advantageous in suppressing the voltage drop amount at the time of the operation of the APD. 
       FIG. 11  shows a plan view or a planar view of a pixel array  101  according to the fifth embodiment. An arrangement of the fifth embodiment is a modification of the first embodiment and matters not mentioned in the fifth embodiment can comply with the first embodiment. In the fifth embodiment, similar to the arrangement exemplified in the first embodiment, a plurality of pixels  104  arranged in a semiconductor layer  301  form the rectangular pixel array  101 . Each pixel  104  includes a photoelectric converter  102  with an APD  201 . 
     In the fifth embodiment, in a group of four pixels  104  surrounded by four second contact plugs  322  arranged at the vertices of a virtual rectangle, four ring-shaped regions  313  are coupled and arranged. This arrangement is advantageous since the condition described with reference to  FIG. 6  is satisfied even if the size of the pixel  104  is reduced. 
       FIG. 12  shows a plan view or a planar view of a pixel array  101  according to the sixth embodiment. An arrangement of the sixth embodiment is a modification or application of each of the first to fifth embodiments and matters not mentioned in the sixth embodiment can comply with the first embodiment. In the sixth embodiment, similar to the arrangement exemplified in the first embodiment, a plurality of pixels  104  arranged in a semiconductor layer  301  form the rectangular pixel array  101 . Each pixel  104  includes a photoelectric converter  102  with an APD  201 . A photoelectric conversion device  100  according to the sixth embodiment includes a microlens  331  in each pixel  104 . The microlens  331  can be provided on, for example, the side of a first surface S 1  but may be provided on the side of a second surface S 2 . If the microlens  331  is provided on the side of the first surface S 1 , the first surface S 1  is located between the microlens  331  and the second surface S 2 . 
     If the microlens  331  is provided on the side of the second surface S 2 , the second surface S 2  is located between the microlens  331  and the first surface S 1 . 
     In an orthogonal projection with respect to the first surface S 1 , the microlens  331  can be arranged so that the center of the microlens  331  matches the center of a second region  312 . Alternatively, in the orthogonal projection with respect to the first surface S 1 , the microlens  331  can be arranged so that the center of the microlens  331  matches the center of a third region  300 . 
     In one example, the first contact plug  321  can be arranged so that the center of the first contact plug  321  deviates from the center of the second region  312 , and the microlens  331  can be arranged so that the center of the microlens  331  matches the center of the second region  312 . In one example, the first contact plug  321  can be arranged so that the center of the first contact plug  321  deviates from the center of the third region  300 , and the microlens  331  can be arranged so that the center of the microlens  331  matches the center of the third region  300 . This arrangement is advantageous since the APD  201  efficiently receives light or photons when the microlens  331  is provided on the side of the first surface S 1 . 
       FIG. 13  shows a plan view or a planar view of a pixel array  101  according to the seventh embodiment. An arrangement of the seventh embodiment is a modification or application of each of the first to sixth embodiments and matters not mentioned in the seventh embodiment can comply with the first embodiment. In the seventh embodiment, similar to the arrangement exemplified in the first embodiment, a plurality of pixels  104  arranged in a semiconductor layer  301  form the rectangular pixel array  101 . The seventh embodiment provides an example of a photoelectric conversion device  100  formed as a back-side illumination type. A contact region  316  is arranged on the side of a second surface S 2 , and a second contact plug  322  is also arranged on the side of the second surface S 2 . For example, a voltage line  332  for supplying a voltage VL is electrically connected to the second contact plug  322 . In this arrangement, an electric field between a first region  311  of the first conductivity type and the contact region  316  of the second conductivity type is irrelevant to reduction of the size of the pixel  104 . Therefore, it is possible to suppress the increase of the DCR caused by reduction of the size of the pixel  104 . 
     The contact region  316  can be arranged to contact a separation region  314 . Furthermore, the contact region  316  can be arranged to contact a fourth region  315 . In one example, the side surface of the contact region  316  contacts the fourth region  315  and is surrounded by the fourth region  315 . In another example, the side surface of the contact region  316  contacts the separation region  314  and is surrounded by the separation region  314 . 
     The end face of the separation region  314  on the side of a first surface S 1  can be arranged between the first surface S 1  and the second surface S 2 . From another viewpoint, the end face of the separation region  314  on the side of the first surface S 1  can be arranged apart from the first surface S 1 . In this arrangement, an electric field between the first region  311  of the first conductivity type and the separation region  314  of the second conductivity type is hardly influenced by reduction of the size of the pixel  104 . That is, it is possible to suppress the increase of the DCR caused by reduction of the size of the pixel  104 . 
     In another example, the end face of the separation region  314  on the side of the first surface S 1  may match the first surface S 1 . This arrangement is advantageous in improving the separation characteristic between the pixels  104 . 
     An example of a photoelectric conversion system using a photoelectric conversion device of each of the above-described embodiments will be described below. 
       FIG. 14  is a block diagram showing the arrangement of a photoelectric conversion system  1200  according to this embodiment. The photoelectric conversion system  1200  according to this embodiment includes a photoelectric conversion device  1215 . Any one of the photoelectric conversion devices described in the above embodiments can be applied as the photoelectric conversion device  1215 . The photoelectric conversion system  1200  can be used as, for example, an image capturing system. Practical examples of the image capturing system are a digital still camera, a digital camcorder, and a monitoring camera.  FIG. 14  shows an example of a digital still camera as the photoelectric conversion system  1200 . 
     The photoelectric conversion system  1200  shown in  FIG. 14  includes the photoelectric conversion device  1215 , a lens  1213  for forming an optical image of an object on the photoelectric conversion device  1215 , an aperture  1214  for changing the amount of light passing through the lens  1213 , and a barrier  1212  for protecting the lens  1213 . The lens  1213  and aperture  1214  form an optical system for concentrating light to the photoelectric conversion device  1215 . 
     The photoelectric conversion system  1200  includes a signal processor  1216  for processing an output signal output from the photoelectric conversion device  1215 . The signal processor  1216  performs an operation of signal processing of performing various kinds of correction and compression for an input signal, as needed, thereby outputting the resultant signal. The photoelectric conversion 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 an external computer or the like. 
     Furthermore, the photoelectric conversion system  1200  includes a recording medium  1211  such as a semiconductor memory for recording or reading out image capturing data, and a recording medium control interface unit (recording medium control I/F unit)  1210  for performing a recording or readout operation in or from the recording medium  1211 . The recording medium  1211  may be incorporated in the photoelectric conversion system  1200  or may be detachable. 
     In addition, communication with the recording medium  1211  from the recording medium control I/F unit  1210  or communication from the external I/F unit  1209  may be performed wirelessly. 
     Furthermore, the photoelectric conversion system  1200  includes a general control/arithmetic unit  1208  that controls various kinds of operations and the entire digital still camera, and a timing generation unit  1217  that outputs various kinds of timing signals to the photoelectric conversion device  1215  and the signal processor  1216 . In this example, the timing signal and the like may be input from the outside, and the photoelectric conversion system  1200  need only include at least the photoelectric conversion device  1215  and the signal processor  1216  that processes an output signal output from the photoelectric conversion device  1215 . As described in the fourth embodiment, the timing generation unit  1217  may be incorporated in the photoelectric conversion device. The general control/arithmetic unit  1208  and the timing generation unit  1217  may be configured to perform some or all of the control functions of the photoelectric conversion device  1215 . 
     The photoelectric conversion device  1215  outputs an image signal to the signal processor  1216 . The signal processor  1216  performs predetermined signal processing for the image signal output from the photoelectric conversion device  1215  and outputs image data. The signal processor  1216  also generates an image using the image signal. Furthermore, the signal processor  1216  may perform distance measurement calculation for the signal output from the photoelectric conversion device  1215 . Note that the signal processor  1216  and the timing generation unit  1217  may be incorporated in the photoelectric conversion device. That is, each of the signal processor  1216  and the timing generation unit  1217  may be provided on a substrate on which pixels are arranged or may be provided on another substrate. An image capturing system capable of acquiring a higher-quality image can be implemented by forming an image capturing system using the photoelectric conversion device of each of the above-described embodiments. 
     The photoelectric conversion system and a moving body according to this embodiment will be described with reference to  FIGS. 15A, 15B and 16 .  FIGS. 15A and 15B  are schematic views showing an example of the arrangement of the photoelectric conversion system and the moving body according to this embodiment.  FIG. 16  is a flowchart illustrating the operation of the photoelectric conversion system according to this embodiment. This embodiment will describe an example of an in-vehicle camera as the photoelectric conversion system. 
       FIGS. 15A and 15B  show examples of a vehicle system and a photoelectric conversion system that is incorporated in the vehicle system and performs image capturing. A photoelectric conversion system  1301  includes a photoelectric conversion device  1302 , an image preprocessing unit  1315 , an integrated circuit  1303 , and an optical system  1314 . The optical system  1314  forms an optical image of an object on the photoelectric conversion device  1302 . The photoelectric conversion device  1302  converts, into an electrical signal, the optical image of the object formed by the optical system  1314 . The photoelectric conversion device  1302  is one of the photoelectric conversion devices according to the above-described embodiments. The image preprocessing unit  1315  performs predetermined signal processing for the signal output from the photoelectric conversion device  1302 . The function of the image preprocessing unit  1315  may be incorporated in the photoelectric conversion device  1302 . In the photoelectric conversion system  1301 , at least two sets of the optical systems  1314 , the photoelectric conversion devices  1302 , and the image preprocessing units  1315  are arranged, and an output from the image preprocessing unit  1315  of each set is input to the integrated circuit  1303 . 
     The integrated circuit  1303  is an image capturing system application specific integrated circuit, and includes an image processing unit  1304  with a memory  1305 , an optical distance measurement unit  1306 , a distance measurement calculation unit  1307 , an object recognition unit  1308 , and an abnormality detection unit  1309 . The image processing unit  1304  performs image processing such as development processing and defect correction for the output signal from each image preprocessing unit  1315 . The memory  1305  temporarily stores a captured image, and stores the position of a defect in the captured image. The optical distance measurement unit  1306  performs focusing or distance measurement of an object. The distance measurement calculation unit  1307  calculates distance measurement information from a plurality of image data acquired by the plurality of photoelectric conversion devices  1302 . The object recognition unit  1308  recognizes objects such as a vehicle, a road, a road sign, and a person. Upon detecting an abnormality of the photoelectric conversion device  1302 , the abnormality detection unit  1309  notifies a main controller  1313  of the abnormality. 
     The integrated circuit  1303  may be implemented by dedicated hardware, a software module, or a combination thereof. Alternatively, the integrated circuit may be implemented by an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), or a combination thereof. 
     The main controller  1313  comprehensively controls the operations of the photoelectric conversion system  1301 , vehicle sensors  1310 , a controller  1320 , and the like. A method in which the photoelectric conversion system  1301 , the vehicle sensors  1310 , and the controller  1320  each individually include a communication interface and transmit/receive control signals via a communication network (for example, CAN standards) may be adopted without providing the main controller  1313 . 
     The integrated circuit  1303  has a function of transmitting a control signal or a setting value to each photoelectric conversion device  1302  by receiving the control signal from the main controller  1313  or by its own controller. 
     The photoelectric conversion system  1301  is connected to the vehicle sensors  1310  and can detect the traveling state of the self-vehicle such as the vehicle speed, the yaw rate, and the steering angle, the external environment of the self-vehicle, and the states of other vehicles and obstacles. The vehicle sensors  1310  also serve as a distance information acquisition unit that acquires distance information to a target object. Furthermore, the photoelectric conversion system  1301  is connected to a driving support controller  1311  that performs various driving support operations such as automatic steering, adaptive cruise control, and anti-collision function. More specifically, with respect to a collision determination function, based on the detection results from the photoelectric conversion system  1301  and the vehicle sensors  1310 , a collision with another vehicle or an obstacle is estimated or the presence/absence of a collision is determined. This performs control to avoid a collision when the collision is estimated or activates a safety apparatus at the time of a collision. 
     Furthermore, the photoelectric conversion system  1301  is also connected to an alarm device  1312  that generates an alarm to the driver based on the determination result of a collision determination unit. For example, if the determination result of the collision determination unit indicates that the possibility of a collision is high, the main controller  1313  performs vehicle control to avoid a collision or reduce damage by braking, releasing the accelerator pedal, or suppressing the engine output. The alarm device  1312  sounds an alarm such as a sound, displays alarm information on the screen of a display unit such as a car navigation system or a meter panel, applies a vibration to the seat belt or a steering wheel, thereby giving an alarm to the user. 
     The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made. 
     According to the present invention, there is provided a technique advantageous in suppressing the increase of the DCR caused by a high electric field in the APD. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention 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. 2021-008942 filed Jan. 22, 2021, which is hereby incorporated by reference herein in its entirety.