Patent Publication Number: US-2021185195-A1

Title: Imaging unit and imaging device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of International Application No. PCT/JP2019/032894 filed on Aug. 22, 2019, and claims priority from Japanese Patent Application No. 2018-163210 filed on Aug. 31, 2018, the entire disclosures of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging unit and an imaging device. 
     2. Description of the Related Art 
     Recently, there has been a rapid increase in demand for an information device having an imaging function, such as a digital still camera, a digital video camera, a portable telephone such as a smartphone, a tablet terminal, and an endoscope in accordance with an increase in resolution of an imaging sensor such as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor. An electronic device having an imaging function as described above is referred to as an imaging device. 
     The imaging device comprises an imaging unit including an imaging sensor chip that is a semiconductor chip, a package that houses the imaging sensor chip, and a circuit board on which the package is mounted. 
     JP2017-139258A discloses a configuration in which a central portion of a back surface of an imaging sensor chip and a package substrate are adhered to each other, and a configuration in which four corner portions on a light receiving surface side of the imaging sensor chip and the package substrate are adhered to each other. 
     JP2008-098262A discloses a configuration in which a concave portion is formed on a mounting surface of a package substrate on which an imaging sensor chip is mounted, and the imaging sensor chip and the package substrate are adhered to each other by an adhesive packed in the concave portion. 
     SUMMARY OF THE INVENTION 
     In a case where the package substrate that houses the semiconductor chip is mounted on the circuit board, the unit is placed in a high temperature in a step of electrically connecting the package substrate and the circuit board to each other with a solder. In a case where the temperature of the unit decreases after completion of this step, a warpage due to a bimetal effect occurs due to a difference in linear expansion coefficients of components of the unit. 
     In a case where the semiconductor chip is an imaging sensor chip, a flatness of a light receiving surface of the imaging sensor chip cannot be ensured due to a warpage caused by a bimetal effect. In a case where the light receiving surface warps in this way, a focus shifts around the light receiving surface, which affects an image quality. In a case where a size of the imaging sensor chip is large, for example, a full size, it is particularly important to take measures against a warpage due to a bimetal effect. JP2017-139258A and JP2008-098262A do not recognize such a problem of the warpage of the imaging sensor chip. 
     The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an imaging unit capable of improving an image quality by preventing a warpage of an imaging sensor chip, and an imaging device comprising the imaging unit. 
     An imaging unit according to an aspect of the present invention comprises: an imaging sensor chip; a package substrate on which the imaging sensor chip is mounted; an adhesion member that adheres a back surface of a light receiving surface of the imaging sensor chip and a mounting surface of the package substrate on which the imaging sensor chip is mounted to each other; and a circuit board that is adhered to a back surface of the mounting surface of the package substrate. The adhesion member includes a central adhesion part adhered to a central portion of the imaging sensor chip and a peripheral adhesion part adhered to a peripheral portion of the imaging sensor chip that is separated from the central portion. The peripheral portion is an annular region extending along a peripheral edge of the image element chip. 
     An imaging device of an aspect of the present invention comprises: the above-described imaging unit. 
     According to the present invention, it is possible to provide an imaging unit capable of improving an image quality by preventing a warpage of an imaging sensor, and an imaging device comprising the imaging unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a schematic configuration of a digital camera  100  that is an embodiment of an imaging device of the present invention. 
         FIG. 2  is a schematic cross-sectional view of an imaging unit  50  in the digital camera  100  shown in  FIG. 1 . 
         FIG. 3  is an exploded perspective view schematically showing the imaging unit  50  in the digital camera  100  shown in  FIG. 1 . 
         FIG. 4  is a schematic plan view of an imaging sensor chip  1  of the imaging unit  50  shown in  FIGS. 2 and 3  as viewed from a back surface side of the imaging sensor chip  1  opposite to a light receiving surface  10  in a direction perpendicular to the light receiving surface  10 . 
         FIG. 5  is a diagram showing a first modification example of the shape of a peripheral portion  1 B shown in  FIG. 4 . 
         FIG. 6  is a diagram showing a second modification example of the shape of a peripheral portion  1 B shown in  FIG. 4 . 
         FIG. 7  is a schematic view for describing an image height set on the back surface of the imaging sensor chip  1  in a simulation. 
         FIG. 8  is a diagram showing a central portion  1 A and a peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a verification example A. 
         FIG. 9  is a diagram showing a central portion  1 A and a peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a verification example B. 
         FIG. 10  is a diagram showing a central portion  1 A and a peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a verification example C. 
         FIG. 11  is a diagram showing a central portion  1 A and a peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a verification example D. 
         FIG. 12  is a diagram showing a central portion  1 A and a peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a verification example E. 
         FIG. 13  is a diagram showing a central portion  1 A and a peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a verification example F. 
         FIG. 14  is a diagram showing an adhesion region with an adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a comparative verification example G 1 . 
         FIG. 15  is a diagram showing an adhesion region with an adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a comparative verification example G 2 . 
         FIG. 16  is a diagram showing an adhesion region with an adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a comparative verification example G 3 . 
         FIG. 17  is a diagram showing an adhesion region with an adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of a comparative verification example G 4 . 
         FIG. 18  is a diagram showing simulation results of the amount of warpage of the imaging sensor chip  1  for each image height in the verification examples A to F and the comparative verification examples G 1  to G 4 . 
         FIG. 19  is a diagram summarizing the amount of warpage of each verification example at an image height of 50% and an image height of 80% among the results shown in  FIG. 18 . 
         FIGS. 20A to 20C  are diagrams each showing a simulation result of warpage distribution in the plane of the imaging sensor chip  1  in the verification examples A, C, and E. 
         FIGS. 21A to 21C  are diagrams each showing a simulation result of warpage distribution in the plane of the imaging sensor chip  1  in the verification examples B, D, and F. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a diagram showing a schematic configuration of a digital camera  100  that is an embodiment of an imaging device of the present invention. 
     The digital camera  100  shown in  FIG. 1  comprises a lens device  40  including an imaging lens  41 , a stop  42 , a lens driving unit  43 , a stop driving unit  44 , and a lens control unit  45 . 
     The lens device  40  may be attachable to and detachable from a main body of the digital camera  100 , or may be integrated with the main body of the digital camera  100 . 
     The imaging lens  41  includes a focus lens or a zoom lens that can move in an optical axis direction. 
     The lens control unit  45  of the lens device  40  is configured to be able to communicate with a system control unit  11  of the digital camera  100  by wire or wireless means. 
     According to a command from the system control unit  11 , the lens control unit  45  changes a position of a principal point of the focus lens by driving the focus lens included in the imaging lens  41  via the lens driving unit  43 , or controls the aperture amount of the stop  42  via the stop driving unit  44 . 
     The digital camera  100  further comprises an imaging unit  50  for imaging a subject through an imaging optical system, the system control unit  11 , and an operation unit  14 . 
     The imaging unit  50  comprises an imaging sensor  51  such as a CCD image sensor or a CMOS image sensor, and a circuit board  52  on which the imaging sensor  51  is mounted. 
     The imaging sensor  51  has a light receiving surface (light receiving surface  10  in  FIG. 3  which will be described below) on which a plurality of pixels are two-dimensionally arranged, and converts an image of the subject formed on the light receiving surface  10  by the imaging optical system into an electric signal (pixel signal) by the plurality of pixels and outputs the electric signal. 
     The system control unit  11  drives the imaging sensor  51  to output the image of the subject captured through the imaging optical system of the lens device  40  as a captured image signal. A command signal from a user is input to the system control unit  11  through the operation unit  14 . 
     The system control unit  11  collectively controls the entire digital camera  100 , and has a hardware structure of various processors that execute programs to perform processing. 
     The various processors include a central processing unit (CPU) that is a general-purpose processor executing a program to perform various types of processing, a programmable logic device (PLD) that is a processor of which a circuit configuration can be changed after manufacturing such as a field programmable gate array (FPGA), or a dedicated electric circuit that is a processor having a circuit configuration designed to be dedicated to executing specific processing such as an application specific integrated circuit (ASIC). More specifically, structures of the various processors are electric circuits in which circuit elements such as semiconductor elements are combined. 
     The system control unit  11  may be constituted by one of the various processors, or may be constituted by a combination of two or more processors of the same type or different types (for example, a combination of a plurality of FPGAs or a combination of a CPU and an FPGA). 
     Further, an electric control system of the digital camera  100  comprises a main memory  16  constituted by a random access memory (RAM), a memory control unit  15  that controls data storage in the main memory  16  and data read from the main memory  16 , a digital signal processing unit  17  that performs digital signal processing on the captured image signal output from the imaging unit  50  to generate captured image data according to various formats such as a joint photographic experts group (JPEG) format, an external memory control unit  20  that controls data storage in a storage medium  21  and data read from the storage medium  21 , a display unit  23  that is constituted by an organic electroluminescence (EL) display or a liquid crystal display, and a display control unit  22  that controls a display on the display unit  23 . 
       FIG. 2  is a schematic cross-sectional view of the imaging unit  50  in the digital camera  100  shown in  FIG. 1 .  FIG. 3  is an exploded perspective view schematically showing the imaging unit  50  in the digital camera  100  shown in  FIG. 1 . 
     As shown in  FIGS. 2 and 3 , the imaging unit  50  comprises the imaging sensor  51  and the circuit board  52  adhered to a rear surface of the imaging sensor  51 . 
     The imaging sensor  51  comprises a package substrate  2  that has a bottom portion  2   a  having a plate shape such as a rectangular plate shape or a circular plate shape (rectangular plate shape in the examples of  FIGS. 2 and 3 ) and a wall portion  2   b  erected at an end of the bottom portion  2   a  and having a frame shape such as a rectangular frame shape or a circular frame shape (rectangular frame shape in the examples of  FIGS. 2 and 3 ). The package substrate  2  is configured to have a concave portion  2   c  in a portion surrounded by the wall portion  2   b.    
     The imaging sensor  51  further comprises an imaging sensor chip  1  fixed to a bottom surface  2   d  of the concave portion  2   c  of the package substrate  2 , and a protective cover  3  constituted by a light-transmissive member such as a resin or a glass, the protective cover  3  being fixed to an upper surface of the wall portion  2   b  of the package substrate  2  by an adhesive  4  and sealing the imaging sensor chip  1  by closing the concave portion  2   c  of the package substrate  2 . The bottom surface  2   d  of the package substrate  2  constitutes a mounting surface on which the imaging sensor chip  1  is mounted. 
     The imaging sensor chip  1  is a semiconductor chip including a photoelectric conversion element such as a photodiode, and a light receiving surface  10  on which a readout circuit that converts charges accumulated in the photoelectric conversion element into signals and read out the signals is formed. The imaging sensor chip  1  has a rectangular planar shape and is adhered to the bottom surface  2   d  of the package substrate  2  by an adhesion member  5  such as a resin used as a die bonding material. A semiconductor substrate of the imaging sensor chip  1  is made of, for example, silicon. 
     Although the details will be described below, the adhesion member  5  is composed of a central adhesion part  5   a  adhered to a central portion of the imaging sensor chip  1  and a peripheral adhesion part  5   b  adhered to a peripheral portion in the imaging sensor chip  1  that is separated from the central portion. A linear expansion coefficient of the adhesion member  5  is larger than that of the package substrate  2  and the circuit board  52 . 
     The package substrate  2  is constituted by an insulating material such as alumina ceramic, or has a multilayer structure in which a conductive layer formed of a conductive member such as tungsten and an insulating layer formed of an insulating material such as alumina ceramic are laminated. A linear expansion coefficient of the package substrate  2  is larger than a linear expansion coefficient of the imaging sensor chip  1 . 
     A large number of terminals (not shown) are formed on the bottom surface  2   d  of the concave portion  2   c  of the package substrate  2 , and these terminals are electrically connected to electrode pads formed on the imaging sensor chip  1  by conductive wires (not shown). Further, terminals that are electrically connected to the terminals formed on the bottom surface  2   d  of the concave portion  2   c  of the package substrate  2  are exposed on the rear surface  2   e  of the package substrate  2  opposite to the side to which the protective cover  3  is fixed. 
     The circuit board  52  is adhered and fixed to the rear surface  2   e  of the package substrate  2  by a plurality of conductive members  7 . The conductive member  7  is in contact with each of the plurality of terminals exposed on the rear surface  2   e  of the package substrate  2 . 
     The conductive member  7  may be constituted by a conductive material having an adhesion function, and for example, a solder formed of an alloy of lead and tin or a solder formed of an alloy of tin and copper is used. 
     The circuit board  52  is a plate-shaped member. On the circuit board  52 , a circuit for driving the imaging sensor chip  1 , a circuit for processing a signal output from the imaging sensor chip  1 , and the like are formed. On a surface  52   a  of the circuit board  52  on the side adhered to the package substrate  2 , terminals of these circuits are formed at positions in contact with the conductive member  7 . Therefore, the terminal of the circuit included in the circuit board  52  and the terminal formed on the rear surface of the package substrate  2  are electrically connected to each other by the conductive member  7 . 
     The circuit board  52  is constituted of, for example, a glass epoxy resin and copper, and a linear expansion coefficient thereof is larger than the linear expansion coefficient of the package substrate  2 . A difference in the linear expansion coefficient between the circuit board  52  and the package substrate  2  is a main factor that causes a warpage of the imaging sensor chip  1  due to the above-described bimetal effect. 
       FIG. 4  is a schematic plan view of the imaging sensor chip  1  of the imaging unit  50  shown in  FIGS. 2 and 3  as viewed from a back surface side of the imaging sensor chip  1  opposite to a light receiving surface  10  in a direction perpendicular to the light receiving surface  10 .  FIG. 4  shows a central portion  1 A of the imaging sensor chip  1  adhered to the central adhesion part  5   a  of the adhesion member  5 , a peripheral portion  1 B of the imaging sensor chip  1  adhered to the peripheral adhesion part  5   b  of the adhesion member  5 , a peripheral edge  1 C of the imaging sensor chip  1 , and the center  1 P of the imaging sensor chip  1 . The center  1 P coincides with the center of the light receiving surface  10 . 
     The central portion  1 A is a region such as a rectangular shape, a polygonal shape, or a circular shape centered on the center  1 P on the back surface of the imaging sensor chip  1 . In the example of  FIG. 4 , the central portion  1 A has a rectangular shape. It is desired that the central portion  1 A is a region of line symmetry with respect to a straight line passing through the center  1 P and extending in the direction along the long side of the imaging sensor chip  1 , and with respect to a straight line passing through the center  1 P and extending in the direction along the short side of the imaging sensor chip  1 . 
     The peripheral portion  1 B is a region on the back surface of the imaging sensor chip  1  that is separated from the central portion  1 A and is adjacent to the peripheral edge  1 C. In the example of  FIG. 4 , the peripheral portion  1 B is an annular (square frame shape in the example of  FIG. 4 ) region extending along the peripheral edge  1 C, and an outer peripheral edge of the peripheral portion  1 B coincides with the peripheral edge  1 C. 
     As a result of verification, the inventor found that a warpage of the imaging sensor chip  1  caused by the bimetal effect can be reduced by constituting the adhesion member  5  in the imaging unit  50  by the central adhesion part  5   a  adhered to the central portion  1 A having the configuration described above and the peripheral adhesion part  5   b  adhered to the peripheral portion  1 B having the configuration described above. 
     In a case where the central portion  1 A and the peripheral portion  1 B of the imaging sensor chip  1  are adhered to the package substrate  2  by the adhesion member  5 , an adhesion region adhered to the package substrate  2 , a non-adhesion region not adhered to the package substrate  2 , and an adhesion region adhered to the package substrate  2  are arranged on the back surface of the imaging sensor chip  1  from the center  1 P toward the peripheral edge  1 C. 
     A warpage of the imaging sensor chip  1  can be caused by being adhered to the package substrate  2 . That is, the adhesion member  5  itself causes stress that can cause a warpage. 
     In fact, it is clarified from simulation results of  FIG. 18  which will be described below that the amount of warpage of the imaging sensor chip  1  at a position apart from the center  1 P by a predetermined distance r is proportional to the square of the predetermined distance r in the adhesion region (central portion  1 A and peripheral portion  1 B) adhered to the package substrate  2 , and is proportional to the predetermined distance r in the non-adhesion region (portion other than central portion  1 A and peripheral portion  1 B) not adhered to the package substrate  2 . 
     In the imaging sensor chip  1 , in a case where an image height at a position of a peripheral edge of the light receiving surface  10  from the center  1 P of the imaging sensor chip  1  is defined as a reference image height, it is important to reduce the amount of warpage at a position where an image height is about 50% of the reference image height and the amount of warpage at a position where an image height is about 80% of the reference image height, in order to prevent deterioration in quality of a captured image. 
     Therefore, the deterioration in quality of the captured image can be most effectively prevented by providing no adhesion region between a position where an image height is about 50% of the reference image height and a position where an image height is about 80% of the reference image height. Specifically, the central portion  1 A shown in  FIG. 4  is preferably a region inside (center  1 P side) a position where an image height is 45% or less of the reference image height, and the peripheral portion  1 B shown in  FIG. 4  is preferably a region from a position where an image height is 85% or more of the reference image height to the peripheral edge  1 C of the imaging sensor chip  1 . 
     In addition, as a result of verification, the inventor found that a warpage of the imaging sensor chip  1  caused by the bimetal effect can be reduced by using the peripheral portion  1 B shown in  FIG. 4  as four corner portions of the imaging sensor chip  1  instead of the annular region surrounding the central portion  1 A. The corner portion of the imaging sensor chip  1  refers to a region such as a rectangular shape, a polygonal shape, or a circular shape that is adjacent to the corner of the peripheral edge  1 C and is in contact with two sides of the peripheral edge  1 C extending from this corner. 
       FIG. 5  is a diagram showing a first modification example of the shape of the peripheral portion  1 B shown in  FIG. 4 . In the example shown in  FIG. 5 , each of the four corner portions of the imaging sensor chip  1  is the peripheral portion  1 B.  FIG. 5  shows an example in which the corner portion of the imaging sensor chip  1  has a rectangular shape including the corner of the peripheral edge  1 C. 
       FIG. 6  is a diagram showing a second modification example of the shape of the peripheral portion  1 B shown in  FIG. 4 . In the example shown in  FIG. 6 , each of the four corner portions of the imaging sensor chip  1  is the peripheral portion  1 B.  FIG. 6  shows an example in which the corner portion of the imaging sensor chip  1  has an L shape including the corner of the peripheral edge  1 C. 
     In the modification examples shown in  FIGS. 5 and 6 , the peripheral portions  1 B are separated from each other, and a non-adhesion region is present between the adjacent peripheral portions  1 B. Therefore, at a position where an image height is 80% of the reference image height, a range in which the adhesion region is present can be made smaller than that in the example shown in  FIG. 4 . For this reason, the deterioration in quality of the captured image can be most effectively prevented by making an adhesion region not present at least at a position where an image height is about 50% of the reference image height. 
     Specifically, the central portion  1 A shown in  FIGS. 5 and 6  is preferably a region inside a position where an image height is 45% or less of the reference image height, and the peripheral portion  1 B shown in  FIGS. 5 and 6  is preferably provided in a region between a position where an image height is 55% of the reference image height and the peripheral edge  1 C of the imaging sensor chip  1 . 
     In addition, it is preferable that a total value of areas of the four peripheral portions  1 B shown in  FIGS. 5 and 6  is the same as an area of the peripheral portion  1 B shown in  FIG. 4 . As described above, since the peripheral portion  1 B shown in  FIG. 4  is a region from a position where an image height is 85% or more of the reference image height to the peripheral edge  1 C of the imaging sensor chip  1 , the total value of the areas of the four peripheral portions  1 B shown in  FIGS. 5 and 6  is preferably the same as an area of a region from the peripheral edge  1 C to a position where an image height is 85% or more of the reference image height. 
     Hereinafter, results of verifying the amount of warpage of the imaging sensor chip  1  in the imaging unit  50  by simulation using a finite element method will be described. 
       FIG. 7  is a schematic view for describing an image height set on the back surface of the imaging sensor chip  1  in a simulation. In this simulation, an image height at a position of a peripheral edge of the light receiving surface  10  from the center  1 P of the imaging sensor chip  1  is defined as a reference image height. 
     An image height at a position of the peripheral edge  1 C of the imaging sensor chip  1  is 103% of the reference image height. An image height at a position of a rectangular frame Z 1  shown in  FIG. 7  is 92% of the reference image height. An image height at a position of a rectangular frame Z 2  shown in  FIG. 7  is 80% of the reference image height. An image height at a position of a rectangular frame Z 3  shown in  FIG. 7  is 65% of the reference image height. An image height at a position of a rectangular frame Z 4  shown in  FIG. 7  is 45% of the reference image height. 
     Outer shapes of the peripheral edge  1 C, the light receiving surface  10 , the rectangular frame Z 1 , the rectangular frame Z 2 , the rectangular frame Z 3 , and the rectangular frame Z 4  have similar shapes centered on the center  1 P. An area of a region inside the rectangular frame Z 4  on the back surface of the imaging sensor chip  1 , an area of a region between the rectangular frame Z 4  and the rectangular frame Z 3  on the back surface of the imaging sensor chip  1 , an area of a region between the rectangular frame Z 3  and the rectangular frame Z 2  on the back surface of the imaging sensor chip  1 , an area of a region between the rectangular frame Z 2  and the rectangular frame Z 1  on the back surface of the imaging sensor chip  1 , and an area of a region between the rectangular frame Z 1  and the peripheral edge  1 C on the back surface of the imaging sensor chip  1  are the same as one another. 
     The amount of warpage of the imaging sensor chip  1  was simulated for verification examples A and B corresponding to the configuration shown in  FIG. 4 , verification examples C and D corresponding to the configuration shown in  FIG. 5 , verification examples E and F corresponding to the configuration shown in  FIG. 6 , and comparative verification examples G 1  to G 4  as the configuration of the region adhered to the adhesion member  5  on the back surface of the imaging sensor chip  1  of the imaging unit  50 . In all verification examples including the verification examples A to F and the comparative verification examples G 1  to G 4 , material physical property values of the adhesion member  5  were the same as one another. 
       FIG. 8  is a diagram showing the central portion  1 A and the peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the verification example A. In the verification example A, a region inside the rectangular frame Z 4  is the central portion  1 A, and a region between the rectangular frame Z 1  and the peripheral edge  1 C is the peripheral portion  1 B. 
       FIG. 9  is a diagram showing the central portion  1 A and the peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the verification example B. The central portion  1 A in the verification example B is a reduced configuration of the central portion  1 A in the verification example A of  FIG. 8 . The peripheral portion  1 B in the verification example B is an enlarged configuration of the peripheral portion  1 B in the verification example A of  FIG. 8 . A total value of areas of the central portion  1 A and the peripheral portion  1 B shown in  FIG. 9  is set to be the same as a total value of areas of the central portion  1 A and the peripheral portion  1 B shown in  FIG. 8 . 
     An image height at a position of a peripheral edge of the central portion  1 A in the verification example B is 35% of the reference image height. An image height at a position of an inner peripheral edge of the peripheral portion  1 B in the verification example B is 87% of the reference image height. 
       FIG. 10  is a diagram showing the central portion  1 A and the peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the verification example C. In the verification example C, a region inside the rectangular frame Z 4  is the central portion  1 A, and four corner portions (rectangular regions including corners of peripheral edge  1 C) of the imaging sensor chip  1  are the peripheral portions  1 B. Each peripheral portion  1 B in the verification example C is located on the peripheral edge  1 C side from a position where an image height is 63% of the reference image height. A total value of areas of the four peripheral portions  1 B shown in  FIG. 10  is the same as the area of the peripheral portion  1 B of  FIG. 8 . 
       FIG. 11  is a diagram showing the central portion  1 A and the peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the verification example D. The central portion  1 A in the verification example D is a reduced configuration of the central portion  1 A in the verification example C. The peripheral portion  1 B in the verification example D is an enlarged configuration of the peripheral portion  1 B in the verification example C. Each peripheral portion  1 B in the verification example D is located on the peripheral edge  1 C side from a position where an image height is 55% of the reference image height. An image height at a position of a peripheral edge of the central portion  1 A in the verification example D is 35% of the reference image height. A total value of areas of the four peripheral portions  1 B shown in  FIG. 11  is the same as the area of the peripheral portion  1 B of  FIG. 9 . 
       FIG. 12  is a diagram showing the central portion  1 A and the peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the verification example E. In the verification example E, a region inside the rectangular frame Z 4  is the central portion  1 A, and four corner portions (L-shaped regions including corners of peripheral edge  1 C) of the imaging sensor chip  1  are the peripheral portions  1 B. Each peripheral portion  1 B in the verification example E is located on the peripheral edge  1 C side from the rectangular frame Z 3 . A total value of areas of the four peripheral portions  1 B shown in  FIG. 12  is the same as the area of the peripheral portion  1 B of  FIG. 8 . 
       FIG. 13  is a diagram showing the central portion  1 A and the peripheral portion  1 B set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the verification example F. The central portion  1 A in the verification example F is a reduced configuration of the central portion  1 A in the verification example E. The peripheral portion  1 B in the verification example F is an enlarged configuration of the peripheral portion  1 B in the verification example E. Each peripheral portion  1 B in the verification example F is located on the peripheral edge  1 C side from the rectangular frame Z 3 . A total value of areas of the four peripheral portions  1 B shown in  FIG. 13  is the same as the area of the peripheral portion  1 B of  FIG. 9 . 
       FIG. 14  is a diagram showing an adhesion region with the adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the comparative verification example G 1 . In the comparative verification example G 1 , only an inner region of the rectangular frame Z 3  is an adhesion region  1 D adhered to the adhesion member  5 . An area of the adhesion region  1 D shown in  FIG. 14  is set to be the same as a total value of areas of the central portion  1 A and the peripheral portion  1 B shown in  FIG. 8 . 
       FIG. 15  is a diagram showing an adhesion region with the adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the comparative verification example G 2 . In the comparative verification example G 2 , only a region between the rectangular frame Z 4  and the rectangular frame Z 2  is an adhesion region  1 E adhered to the adhesion member  5 . An area of the adhesion region  1 E shown in  FIG. 15  is set to be the same as a total value of areas of the central portion  1 A and the peripheral portion  1 B shown in  FIG. 8 . 
       FIG. 16  is a diagram showing an adhesion region with the adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the comparative verification example G 3 . In the comparative verification example G 3 , only a region between the rectangular frame Z 3  and the rectangular frame Z 1  is an adhesion region  1 F adhered to the adhesion member  5 . An area of the adhesion region  1 F shown in  FIG. 16  is set to be the same as a total value of areas of the central portion  1 A and the peripheral portion  1 B shown in  FIG. 8 . 
       FIG. 17  is a diagram showing an adhesion region with the adhesion member  5  set on the back surface of the imaging sensor chip  1  in the imaging unit  50  of the comparative verification example G 4 . In the comparative verification example G 4 , only a region between the rectangular frame Z 2  and the peripheral edge  1 C is an adhesion region  1 G adhered to the adhesion member  5 . An area of the adhesion region  1 G shown in  FIG. 17  is set to be the same as a total value of areas of the central portion  1 A and the peripheral portion  1 B shown in  FIG. 8 . 
       FIG. 18  is a diagram showing simulation results of the amount of warpage of the imaging sensor chip  1  for each image height in the verification examples A to F and the comparative verification examples G 1  to G 4 . A horizontal axis of  FIG. 18  indicates the image height percent in a case where the reference image height is 100%. A vertical axis of  FIG. 18  indicates the amount of warpage of the imaging sensor chip  1  from the position of the center  1 P in a direction perpendicular to the light receiving surface  10 . 
     As shown in  FIG. 18 , it can be seen that the imaging unit  50  of the verification examples A to F having the central portion  1 A and the peripheral portion  1 B is, as compared with the imaging unit  50  of the comparative verification example G 1  having only the central portion  1 A and the imaging unit  50  of the comparative verification examples G 2  to G 4  having only the frame-shaped adhesion region, capable of suppressing the amount of warpage of the imaging sensor chip  1  as a whole. 
       FIG. 19  is a diagram summarizing the amount of warpage of each verification example at an image height of 50% and an image height of 80% among the results shown in  FIG. 18 . As shown in  FIG. 19 , in the imaging unit  50  of the verification examples A to F having the central portion  1 A and the peripheral portion  1 B, the amount of warpage at an image height of 50% is lower than an allowable upper limit value of 25 μm, and the amount of warpage at an image height of 80% is lower than an allowable upper limit value of 50 μm. As described above, according to the imaging unit  50  of the verification examples A to F, both the amount of warpage at the image height of 50% and the amount of warpage at the image height of 80%, which affect the quality of the captured image, can be set to be equal to or less than the allowable upper limit value, and the deterioration in quality of the captured image can be prevented. 
     In the verification example A shown in  FIG. 8 , the central portion  1 A is a region inside a position where an image height is 45% of the reference image height, and the peripheral portion  1 B is a region from a position where an image height is 92% of the reference image height to the peripheral edge  1 C. In addition, in the verification example B shown in  FIG. 9 , the central portion  1 A is a region inside a position where an image height is 35% of the reference image height, and the peripheral portion  1 B is a region from a position where an image height is 87% of the reference image height to the peripheral edge  1 C. Therefore, from the results of the verification examples A and B shown in  FIG. 19 , in the configuration shown in  FIG. 4 , it is proved that the deterioration in quality of the captured image can be effectively prevented in a case where the central portion  1 A is located inside a position where an image height is 45% or less of the reference image height, and the peripheral portion  1 B is located in a region from a position where an image height is 85% or more of the reference image height to the peripheral edge  1 C. 
     In addition, in the verification example C shown in  FIG. 10 , the central portion  1 A is a region inside a position where an image height is 45% of the reference image height, and the peripheral portion  1 B is located in a region from a position where an image height is 63% of the reference image height to the peripheral edge  1 C. In addition, in the verification example D shown in  FIG. 11 , the central portion  1 A is a region inside a position where an image height is 35% of the reference image height, and the peripheral portion  1 B is located in a region from a position where an image height is 55% of the reference image height to the peripheral edge  1 C. In addition, in the verification example E shown in  FIG. 12 , the central portion  1 A is a region inside a position where an image height is 45% of the reference image height, and the peripheral portion  1 B is on the peripheral edge  1 C side from a position where an image height is 65% of the reference image height. In addition, in the verification example F shown in  FIG. 13 , the central portion  1 A is a region inside a position where an image height is 35% of the reference image height, and the peripheral portion  1 B is on the peripheral edge  1 C side from a position where an image height is 65% of the reference image height. 
     Therefore, from the results of the verification examples C to F shown in  FIG. 19 , in the configurations shown in  FIGS. 5 and 6 , it is proved that the deterioration in quality of the captured image can be effectively prevented in a case where the central portion  1 A is located inside a position where an image height is 45% or less of the reference image height, and the peripheral portion  1 B is located in a region from a position where an image height is 55% of the reference image height to the peripheral edge  1 C. 
       FIGS. 20A to 20C  are diagrams each showing a simulation result of warpage distribution in the plane of the imaging sensor chip  1  in the verification examples A, C, and E.  FIGS. 21A to 21C  are diagrams each showing a simulation result of warpage distribution in the plane of the imaging sensor chip  1  in the verification examples B, D, and F. In  FIGS. 20 and 21 , the hatched region has a larger amount of warpage as the hatched region has a thinner hatching. In  FIGS. 20 and 21 , the vertical direction is a direction along one of the two orthogonal sides of the imaging sensor chip  1 , and the horizontal direction is a direction along the other side of the two sides of the imaging sensor chip  1 . 
     As shown in  FIGS. 20 and 21 , according to the configurations of the verification examples A and B, the amount of warpage in the direction along each of the above two sides of the imaging sensor chip  1  can be made substantially equal. Therefore, even though a warpage of the light receiving surface  10  occurs, a high-quality captured image with inconspicuous distortion can be obtained. 
     As described above, the following items are described in the present specification. 
     (1) 
     An imaging unit comprising: an imaging sensor chip; a package substrate on which the imaging sensor chip is mounted; an adhesion member that adheres a back surface of the imaging sensor chip opposite to a light receiving surface and a mounting surface of the package substrate on which the imaging sensor chip is mounted to each other; and a circuit board that is adhered to a back surface of the package substrate opposite to the mounting surface, in which the adhesion member is composed of a central adhesion part adhered to a central portion of the imaging sensor chip and a peripheral adhesion part adhered to a peripheral portion of the imaging sensor chip that is separated from the central portion. 
     (2) 
     The imaging unit according to (1), 
     in which the peripheral portion is an annular region extending along a peripheral edge of the imaging sensor chip. 
     (3) 
     The imaging unit according to (2), 
     in which, in a case where an image height at a position of a peripheral edge of the light receiving surface from a center of the imaging sensor chip is defined as a reference image height, the central portion is a region inside a position where an image height is 45% or less of the reference image height, and the peripheral portion is a region from a position where an image height is 85% or more of the reference image height to the peripheral edge of the imaging sensor chip. 
     (4) 
     The imaging unit according to (1), in which the peripheral portion is four corner portions of the imaging sensor chip. 
     (5) 
     The imaging unit according to (4), in which, in a case where an image height at a position of a peripheral edge of the light receiving surface from a center of the imaging sensor chip is defined as a reference image height, the central portion is a region inside a position where an image height is 45% or less of the reference image height, and the four corner portions are located in a region from the peripheral edge of the imaging sensor chip to a position where an image height is 55% of the reference image height. 
     (6) 
     The imaging unit according to (5), 
     in which a total value of areas of the four corner portions in a state of being viewed from a direction perpendicular to the light receiving surface is the same as an area of a region from the peripheral edge of the imaging sensor chip to a position where an image height is 85% or more of the reference image height. 
     (7) 
     An imaging device comprising: 
     the imaging unit according to any one of (1) to (6). 
     Although various embodiments have been described with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is obvious that a person skilled in the art is able to find various modification examples and adjustment examples within the scope of the appended claims, and it should be understood that these modification examples and adjustment examples naturally belong to the technical scope of the present invention. Further, the components according to the above-described embodiment may be randomly combined with each other, without departing from the spirit of the invention. 
     This application is based on Japanese Patent Application filed on Aug. 31, 2018 (JP2018-163210), the content of which is incorporated herein by reference. 
     The present invention is highly convenient and effective to be applied to an electronic device having an imaging function, such as a digital camera, a smartphone, a tablet terminal, a personal computer, a robot, or an endoscope. 
     EXPLANATION OF REFERENCES 
       100 : digital camera 
       11 : system control unit 
       14 : operation unit 
       41 : imaging lens 
       42 : stop 
       43 : lens driving unit 
       44 : stop driving unit 
       45 : lens control unit 
       50 : imaging unit 
       51 : imaging sensor 
       52 : circuit board 
       52   a:  surface 
       15 : memory control unit 
       16 : main memory 
       17 : digital signal processing unit 
       20 : external memory control unit 
       21 : storage medium 
       22 : display control unit 
       23 : display unit 
       1 : imaging sensor chip 
       10 : light receiving surface 
       1 A: central portion 
       1 B: peripheral portion 
       1 C: peripheral edge 
       1 P: center 
       1 D,  1 E,  1 F,  1 G: adhesion region 
     Z 1 : rectangular frame (image height of 92%) 
     Z 2 : rectangular frame (image height of 80%) 
     Z 3 : rectangular frame (image height of 65%) 
     Z 4 : rectangular frame (image height of 45%) 
       2 : package substrate 
       2   a:  bottom portion 
       2   b:  wall portion 
       2   c:  concave portion 
       2   d:  bottom surface 
       2   e:  rear surface 
       3 : protective cover 
       4 : adhesive 
       5 : adhesion member 
       5   a:  central adhesion part 
       5   b:  peripheral adhesion part 
       7 : conductive member