Patent Publication Number: US-2023154948-A1

Title: Imaging element and imaging device

Description:
TECHNICAL FIELD 
     The present disclosure relates to an imaging element and an imaging device. Specifically, the present disclosure relates to an imaging element and an imaging device using the imaging element, the imaging element being configured such that pixels each having a vertical transistor are disposed, and the vertical transistor transferring charge generated by photoelectric conversion in a photoelectric conversion unit disposed on a semiconductor substrate in a thickness direction of the semiconductor substrate. 
     BACKGROUND ART 
     In the related art, in an imaging element that images a subject, an imaging element in which pixels generating an image signal based on incident light are disposed in a two-dimensional lattice shape is used. In each of the pixels, a photodiode that generates charge corresponding to incident light by photoelectric conversion and a floating diffusion to which the generated charge is transferred are disposed. An image signal is generated based on the charge transferred to the floating diffusion. Further, in the pixel, a transfer transistor that transfers charge generated by the photodiode to the floating diffusion is further disposed. An imaging element in which a vertical transistor is used as the transfer transistor has been proposed (see, for example, PTL 1). The vertical transistor is a transistor configured with an embedded gate electrode and gate insulating film in the semiconductor substrate. 
     The vertical transistor is configured with the gate insulating film and the gate electrode disposed in a hole formed in the semiconductor substrate by dry etching. The gate insulating film is constituted by a silicon oxide (SiO 2 ) film formed on a bottom surface and a side surface of the hole by radical oxidation or plasma oxidation. The gate electrode is constituted by polycrystalline silicon and disposed adjacent to the gate insulating film of the hole. A channel of the vertical transistor is formed along the outer circumference of the embedded gate insulating film. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     JP 2010-287743 A 
     SUMMARY 
     Technical Problem 
     The related art described above has a problem that a charge transfer path of the transfer transistor becomes long. As described above, a vertical transistor is configured in a form in which a gate insulating film and a gate electrode are embedded in a hole formed in a semiconductor substrate, and thus fine machining of a gate region is difficult, and a distance between a photodiode and a floating diffusion becomes long. For this reason, a charge transfer path becomes longer, and transfer efficiency deteriorates. 
     The present disclosure is contrived in view of the above-described problem, and an object thereof is to reduce a charge transfer path of a transfer transistor constituted by a vertical transistor. 
     Solution to Problem 
     The present disclosure is contrived in order to solve the above-described problem, and a first aspect thereof is an imaging element including a photoelectric conversion unit configured to be disposed on a semiconductor substrate and to generate charge corresponding to incident light by photoelectric conversion, a charge holding unit configured to hold the charge, a charge transfer unit configured to include an opening portion, which is formed in the semiconductor substrate and having a polygonal shape in a plan view, and an embedded gate disposed in the opening portion and to transfer the charge from the photoelectric conversion unit to the charge holding unit, and an image signal generation unit configured to generate an image signal based on the held charge. 
     Further, in the first aspect, the charge transfer unit may include the opening portion having a polygonal shape with six or more sides. 
     Further, in the first aspect, the charge transfer unit may include the opening portion having a polygonal shape in which an interior angle of a vertex is 120 to 150 degrees. 
     Further, in the first aspect, the charge transfer unit may include the opening portion having an octagonal shape. 
     Further, in the first aspect, the charge transfer unit may include the opening portion having a polygonal shape which is formed by performing recrystallization of a member constituting the semiconductor substrate. 
     Further, in the first aspect, the photoelectric conversion unit may include a boundary surface parallel to a side of the polygon of the opening portion of the charge transfer unit. 
     Further, in the first aspect, the charge holding unit may include a boundary surface parallel to a side of the polygon of the opening portion of the charge transfer unit. 
     Further, in the first aspect, the charge transfer unit may further include a gate insulating film disposed between the semiconductor substrate and the embedded gate. 
     Further, in the first aspect, the charge transfer unit may include the gate insulating film formed by oxidizing the semiconductor substrate. 
     Further, in the first aspect, the charge transfer unit may include the gate insulating film formed by oxidizing the semiconductor substrate with oxygen radicals. 
     Further, in the first aspect, the semiconductor substrate may be formed of silicon. 
     Further, in the first aspect, the charge transfer unit may further include a high impurity concentration region which is disposed on the semiconductor substrate adjacent to the opening portion and configured to have a high impurity concentration. 
     Further, in the first aspect, the charge transfer unit may further include a substrate surface gate which is adjacent to the embedded gate and configured to have a shape covering the high impurity concentration region on a front surface side of the semiconductor substrate. 
     Further, in the first aspect, the imaging element may further include a second high impurity concentration region which is adjacent to the photoelectric conversion unit, disposed on a front surface side of the semiconductor substrate, and configured to have a high impurity concentration. 
     In addition, a second aspect of the present disclosure is an imaging device including a photoelectric conversion unit configured to be disposed on a semiconductor substrate and to generate charge corresponding to incident light by photoelectric conversion, a charge holding unit configured to hold the charge, a charge transfer unit configured to include an opening portion, which is formed in the semiconductor substrate and having a polygonal shape in a plan view, and an embedded gate disposed in the opening portion and to transfer the charge from the photoelectric conversion unit to the charge holding unit, an image signal generation unit configured to generate an image signal based on the held charge, and a processing circuit configured to process the generated image signal. 
     According to the aspects of the present disclosure, an effect of forming a channel along an opening portion having a polygonal shape in a charge transfer unit section is obtained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a configuration example of an imaging element according to an embodiment of the present disclosure. 
         FIG.  2    is a diagram illustrating a configuration example of a pixel according to the embodiment of the present disclosure. 
         FIG.  3    is a cross-sectional view illustrating a configuration example of the pixel according to a first embodiment of the present disclosure. 
         FIG.  4    is a plan view illustrating a configuration example of the pixel according to the first embodiment of the present disclosure. 
         FIG.  5    is a plan view illustrating a configuration example of an embedded gate according to the first embodiment of the present disclosure. 
         FIG.  6    is a diagram illustrating an example of a method of manufacturing the pixel according to the first embodiment of the present disclosure. 
         FIG.  7    is a diagram illustrating an example of a method of manufacturing the pixel according to the first embodiment of the present disclosure. 
         FIG.  8    is a diagram illustrating an example of a method of manufacturing the pixel according to the first embodiment of the present disclosure. 
         FIG.  9    is a diagram illustrating an example of a method of manufacturing the pixel according to the first embodiment of the present disclosure. 
         FIG.  10    is a plan view illustrating another configuration example of the pixel according to the first embodiment of the present disclosure. 
         FIG.  11    is a cross-sectional view illustrating a configuration example of a pixel according to a second embodiment of the present disclosure. 
         FIG.  12    is a cross-sectional view illustrating a configuration example of a pixel according to a third embodiment of the present disclosure. 
         FIG.  13    is a block diagram illustrating a schematic configuration example of a camera which is an example of an imaging device to which the present technology can be applied. 
         FIG.  14    is a diagram illustrating an example of a schematic configuration of an endoscopic operation system. 
         FIG.  15    is a block diagram illustrating an example of a functional configuration of a camera head and a CCU. 
         FIG.  16    is a block diagram illustrating an example of a schematic configuration of a vehicle control system. 
         FIG.  17    is a diagram illustrating an example of installation positions of a vehicle external information detection unit and an imaging unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, embodiments for implementing the present disclosure (hereinafter referred to as embodiments) will be described with reference to the drawings. In the following drawings, the same or similar portions are denoted by the same or similar reference numerals and signs. In addition, the embodiments will be described in the following order. 
     1. First embodiment 
     2. Second embodiment 
     3. Third embodiment 
     4. Example of application to camera 
     5. Application example to endoscopic operation system 
     6. Example of application to moving body 
     1. First Embodiment 
     Configuration of Imaging Element 
       FIG.  1    is a diagram illustrating a configuration example of an imaging element according to an embodiment of the present disclosure. In the drawing, an imaging element  1  includes a pixel array portion  10 , a vertical driving unit  20 , a column signal processing unit  30 , and a control unit  40 . 
     The pixel array portion  10  is configured with pixels  100  disposed in a two-dimensional lattice shape. Here, the pixels  100  generate image signals in response to emitted light. Each of the pixels  100  includes a photoelectric conversion unit that generates charge in response to the emitted light. In addition, each of the pixels  100  further includes a pixel circuit. The pixel circuit generates an image signal based on charge generated by the photoelectric conversion unit. The generation of the image signal is controlled by a control signal generated by the vertical driving unit  20 , which will be described later. Signal lines  11  and  12  are disposed in an XY matrix form in the pixel array portion  10 . The signal line  11  is a signal line through which a control signal of the pixel circuit in the pixels  100  is transmitted, is disposed for each row of the pixel array portion  10 , and is wired in common for pixels  100  disposed in each row. The signal line  12  is a signal line through which an image signal generated by the pixel circuit of the pixel  100  is transmitted, is disposed for each column of the pixel array portion  10 , and is wired in common for pixels  100  disposed in each column. The photoelectric conversion unit and the pixel circuit are formed on a semiconductor substrate. 
     The vertical driving unit  20  generates a control signal of the pixel circuit of the pixel  100 . The vertical driving unit  20  transmits the generated control signal to the pixels  100  through the signal lines  11  in the drawing. The column signal processing unit  30  processes an image signal generated by the pixels  100 . The column signal processing unit  30  processes an image signal transmitted from the pixels  100  through the signal lines  12  in the drawing. Processing in the column signal processing unit  30  corresponds to, for example, analog-to-digital conversion of converting an analog image signal generated in the pixels  100  into a digital image signal. The image signal processed by the column signal processing unit  30  is output as an image signal of the imaging element  1 . The control unit  40  controls the overall imaging element  1 . The control unit  40  generates and outputs control signals for controlling the vertical driving unit  20  and the column signal processing unit  30  to control the imaging element  1 . The control signals generated by the control unit  40  are transmitted to the vertical driving unit  20  and the column signal processing unit  30  through signal lines  41  and  42 . Meanwhile, the column signal processing unit  30  is an example of a processing circuit described in the claims. 
     Configuration of Pixel 
       FIG.  2    is a diagram illustrating a configuration example of the pixel according to the embodiment of the present disclosure. The drawing is a circuit diagram illustrating a configuration example of the pixel  100 . The pixel  100  in the drawing includes a photoelectric conversion unit  101 , a charge holding unit  102 , a charge transfer unit  103 , and MOS transistors  104  to  106 . Note that the charge transfer unit  103  can be constituted by a MOS transistor. An n-channel MOS transistor can be used for the charge transfer unit  103  and the MOS transistors  104  to  106 . 
     An anode of the photoelectric conversion unit  101  is grounded, and a cathode is connected to a source of the charge transfer unit  103 . A drain of the charge transfer unit  103  is connected to a source of the MOS transistor  104 , a gate of the MOS transistor  105 , and an end of the charge holding unit  102 . The other end of the charge holding unit  102  is grounded. Both the drains of the MOS transistors  104  and  105  are connected to a power supply line Vdd, and a source of the MOS transistor  105  is connected to a drain of the MOS transistor  106 . A source of the MOS transistor  106  is connected to a signal line  12 . Gates of the charge transfer units  103 ,  104 , and  106  are respectively connected to a transfer signal line TR, a reset signal line RST, and a selection signal line SEL. Note that the transfer signal line TR, the reset signal line RST, and the selection signal line SEL constitute the signal line  11 . 
     The photoelectric conversion unit  101  generates charge corresponding to emitted light as described above. A photodiode can be used for the photoelectric conversion unit  101 . 
     In addition, the charge holding unit  102  and the MOS transistors  103  to  106  constitute a pixel circuit. 
     The charge transfer unit  103  is a transistor that transfers charge generated by photoelectric conversion of the photoelectric conversion unit  101  to the charge holding unit  102 . The transfer of charge in the charge transfer unit  103  is controlled by a signal transmitted through the transfer signal line TR. The charge holding unit  102  is a capacitor that holds charge transferred by the charge transfer unit  103 . 
     The MOS transistor  105  is a transistor that generates a signal based on charge held in the charge holding unit  102 . The MOS transistor  106  is a transistor that outputs the signal generated by the MOS transistor  105  to the signal line  12  as an image signal. The MOS transistor  106  is controlled by a signal transmitted through the selection signal line SEL. In this manner, the MOS transistors  105  and  106  generate an image signal based on charge held in the charge holding unit  102 . The circuit of the MOS transistors  105  and  106  constitutes an image signal generation unit  110 . 
     The MOS transistor  104  is a transistor that resets the charge holding unit  102  by discharging the charge held in the charge holding unit  102  to a power supply line Vdd. The reset performed by the MOS transistor  104  is controlled by a signal transmitted through the reset signal line RST, and is executed before the charge is transferred by the charge transfer unit  103 . Note that, at the time of the reset, it is also possible to reset the photoelectric conversion unit  101  by setting the charge transfer unit  103  in an electrically conducting state. In this manner, the pixel circuit converts charge generated by the photoelectric conversion unit  101  into an image signal. 
     Configuration of Cross Section of Pixel 
       FIG.  3    is a cross-sectional view illustrating a configuration example of the pixel according to the first embodiment of the present disclosure. The drawing is a schematic cross-sectional view illustrating a configuration example of the pixel  100 . In the drawing, the pixel  100  includes a semiconductor substrate  120 , a wiring region  140 , an insulating film  150 , a color filter  160 , a protection film  170 , and an on-chip lens  180 . 
     The semiconductor substrate  120  is a semiconductor substrate on which diffusion regions of the photoelectric conversion unit  101 , the MOS transistor, and the like are formed. As the semiconductor substrate  120 , a substrate formed of, for example, silicon (Si) can be used. The photoelectric conversion unit  101  and the like are disposed in a well region formed in the semiconductor substrate  120 . For convenience, it is assumed that the semiconductor substrate  120  in the drawing is configured in a p-type well region. By forming an n-type semiconductor region in the p-type well region, it is possible to form the photoelectric conversion unit  101  and the like. A white region of the semiconductor substrate  120  in the drawing represents an n-type semiconductor region. 
     In the semiconductor substrate  120  in the drawing, the photoelectric conversion unit  101 , the charge holding unit  102 , and the charge transfer unit  103  are illustrated as examples. The photoelectric conversion unit  101  is constituted by an n-type semiconductor region  121 . Specifically, a photodiode configured using a pn junction of an interface between the n-type semiconductor region  121  and a p-type well region in the periphery thereof corresponds to the photoelectric conversion unit  101 . Charge generated by photoelectric conversion is accumulated in the n-type semiconductor region  121 . Note that a p-type semiconductor region  122  formed to have a relatively high impurity concentration is disposed between the n-type semiconductor region  121  and the surface of the semiconductor substrate  120  on the front surface side. The p-type semiconductor region  122  is a semiconductor region for pinning a surface level on the front surface side of the semiconductor substrate  120  adjacent to the semiconductor region  121 . It is possible to reduce a dark current caused by the surface level of the semiconductor substrate  120  by disposing the semiconductor region  122 . Note that the semiconductor region  122  is an example of a second high impurity concentration region described in the claims. 
     The charge holding unit  102  is constituted by an n-type semiconductor region  124 . The n-type semiconductor region  124  is a region which is configured to have a relatively high impurity concentration and in which charge generated by photoelectric conversion and accumulated in the n-type semiconductor region  121  is held. The charge holding unit  102  constituted by the semiconductor region  124  is referred to as a floating diffusion. The n-type semiconductor region  124  is connected to the image signal generation unit  110  through a wiring layer  143  to be described later. 
     The charge transfer unit  103  is a MOS transistor which is disposed between the n-type semiconductor region  121  constituting the photoelectric conversion unit  101  and the n-type semiconductor region  124  constituting the charge holding unit  102  and transfers charge accumulated in the n-type semiconductor region  121  to the n-type semiconductor region  124 . The charge transfer unit  103  includes an embedded gate  132  that is configured to be embedded in an opening portion  129  formed in the semiconductor substrate  120 . The embedded gate  132  is disposed adjacent to the opening portion  129  of the semiconductor substrate  120  through a gate insulating film  131 . A channel is formed in a well region of the semiconductor substrate  120  along the opening portion  129 . A MOS transistor including such an embedded gate  132  is referred to as a vertical transistor. It is possible to improve the transfer efficiency of charge from the n-type semiconductor region  121  disposed in a relatively deep region of the semiconductor substrate  120 . 
     In addition, a p-type semiconductor region  123  configured to have a relatively high impurity concentration is disposed in the semiconductor substrate  120  adjacent to the opening portion  129 . The p-type semiconductor region  123  is a region for pinning the surface level of the semiconductor substrate  120  of the opening portion  129 . The semiconductor region  123  can be formed by injecting an acceptor such as boron (B) into the semiconductor substrate  120 . Note that the semiconductor region  123  is an example of a high impurity concentration region described in the claims. 
     In addition, a substrate surface gate  133  can be disposed adjacent to the embedded gate  132 . The substrate surface gate  133  is a gate disposed on the front surface side of the semiconductor substrate  120  and is a gate configured to have a shape that covers the opening portion  129 . In addition, the substrate surface gate  133  in the drawing is configured to have a shape that covers the p-type semiconductor region  123 . A gate insulating film  131  is disposed between the substrate surface gate  133  and the front surface side of the semiconductor substrate  120 , similarly to the embedded gate  132 . A channel is formed in the semiconductor substrate  120  immediately below the substrate surface gate  133 . 
     The opening portion  129  can be formed by etching the surface of the semiconductor substrate  120 . As will be described later, the opening portion  129  is configured as a polygon in a plan view. 
     The gate insulating film  131  can be formed of, for example, SiO 2 . The SiO 2  can be formed by oxidizing Si on the surface of the opening portion  129 . 
     The embedded gate  132  and the substrate surface gate  133  can be formed of, for example, polycrystalline silicon or amorphous silicon. The embedded gate  132  can be formed by disposing polycrystalline silicon or the like in the opening portion  129 . As described above, the opening portion  129  is configured as a polygon in a plan view, and thus the external form of the embedded gate  132  disposed in the opening portion  129  is also configured as a polygon in a plan view. In addition, the embedded gate  132  and the substrate surface gate  133  can be formed at the same time. 
     The wiring region  140  is a region where wiring, which is disposed on the front surface side of the semiconductor substrate  120  and transmits signals to the elements of the semiconductor substrate  120 , is disposed. The wiring region  140  includes wiring layers  142  and  143  and an insulating layer  141 . The wiring layers  142  and  143  are wirings that transmit signals to the elements of the semiconductor substrate  120 . The wiring layer  142  or the like can be formed of a metal such as copper (Cu), tungsten (W), or the like. The wiring layer  142  is a wiring which is connected to the gate (substrate surface gate  133 ) of the charge transfer unit  103 , and the wiring layer  143  is a wiring which is connected to the semiconductor region  124  of the charge holding unit  102 . Although not illustrated in the drawing, wiring layers constituting other wirings are also disposed in the wiring region  140 . The insulating layer  141  insulates the wiring layer  142  and the like. The insulating layer  141  can be formed of, for example, SiO 2 . The semiconductor region of the semiconductor substrate  120  constituting the element, the gate of the charge transfer unit  103 , the wiring layer  142 , and the like can be connected to each other by a contact plug  144 . The contact plug  144  is formed of a metal column. 
     The insulating film  150  is a film disposed on the rear surface side of the semiconductor substrate  120  to protect the semiconductor substrate  120 . The insulating film  150  can be formed of, for example, SiO 2 . 
     The color filter  160  is an optical filter that transmits light having a predetermined wavelength in incident light. As the color filter  160 , three types of color filters that transmit, for example, red light, green light, and blue light can be used. In the pixel  100 , one of the three types of color filters  160  is disposed. 
     The protection film  170  is a film that protects the rear surface side of the pixel  100  in which the color filters  160  are disposed. The protection film  170  can be formed of the same material as the on-chip lens  180  to be described later. 
     The on-chip lens  180  is a lens which is disposed for each pixel  100  to focus incident light on the photoelectric conversion unit. The on-chip lens  180  in the drawing is configured in a hemispherical shape to focus incident light. The on-chip lens  180  can be formed of an inorganic material such as silicon nitride (SiN) or an organic material such as an acrylic resin. 
     As described above, the photoelectric conversion unit  101  of the pixel  100  receives incident light from the rear surface side of the semiconductor substrate  120 . The imaging element  1  including such a pixel  100  is referred to as a backside irradiation type imaging element. 
     As described above, the substrate surface gate  133  can be disposed in the charge transfer unit  103 . By disposing the substrate surface gate  133 , it is possible to reduce the influence of a potential barrier formed between the photoelectric conversion unit  101  and the charge transfer unit  103 . In an exposure period, the charge transfer unit  103  is set to be in an electrical non-conduction state to accumulate charge in the semiconductor region  121  of the photoelectric conversion unit  101 . In this case, for example, a negative voltage is applied to the gate (embedded gate  132 ) of the charge transfer unit  103 , and the charge transfer unit  103  has a higher potential than the semiconductor region  121 . When charge of the photoelectric conversion unit  101  is transferred after the exposure period has elapsed, a positive voltage is applied to the gate of the charge transfer unit  103 . The potential of the charge transfer unit  103  becomes lower than that of the semiconductor region  121 , and the charge of the semiconductor region  121  is moved to the charge transfer unit  103  and transferred. 
     However, in the charge transfer unit  103 , the p-type semiconductor region  123  for pinning is disposed, and a relatively high potential barrier is formed. When the charge transfer unit  103  is set to be in an electrical conduction state, the potential barrier remains between the charge transfer unit  103  and the photoelectric conversion unit  101  to inhibit the movement of charge. Consequently, the substrate surface gate  133  is disposed to be configured in a shape that covers the p-type semiconductor region  123 . By applying a voltage to the substrate surface gate  133 , a voltage is also applied to the semiconductor region  123  immediately below the substrate surface gate  133 , and a potential barrier can be lowered. The movement of charge is not inhibited, and the transfer efficiency of charge can be improved. 
     In addition, the substrate surface gate  133  is configured in a shape that covers the p-type semiconductor region  123 , and thus it is possible to reduce an electric field intensity of the surface of the semiconductor substrate  120  in the vicinity of the gate of the charge transfer unit  103 . When the size of the pixel  100  is reduced, the charge transfer unit  103  and the charge holding unit  102  approach each other. Since the p-type semiconductor region  123  is configured to have a relatively high impurity concentration, an electric field suddenly changes at an interface between the semiconductor region  123  and the semiconductor region  124  of the charge holding unit  102 , thereby causing a tunnel effect and increasing a leakage current. Consequently, the substrate surface gate  133  is disposed in the vicinity of a boundary of the semiconductor region  123  to apply a voltage, and thus it is possible to alleviate a sudden change in an electric field in the vicinity of the surface of the semiconductor substrate  120 . Accordingly, a leakage current can be reduced. 
     Configuration of Surface of Pixel 
       FIG.  4    is a plan view illustrating a configuration example of the pixel according to the first embodiment of the present disclosure. The drawing is a plan view illustrating a configuration example of the pixel  100  and is a plan view from the front surface side of the semiconductor substrate  120 . Note that  FIG.  3    corresponds to a sectional view along a line A-A′ in the drawing. 
     In the drawing, the semiconductor region  121  of the photoelectric conversion unit  101  is disposed on the upper right side, and the semiconductor region  124  of the charge holding unit  102  is disposed on the lower left side. The charge transfer unit  103  is disposed between the photoelectric conversion unit  101  and the charge holding unit  102 . A solid polygon of the charge transfer unit  103  represents the substrate surface gate  133 . An alternating dotted-dashed polygon represents the opening portion  129 . A dashed polygon represents the embedded gate  132 . The opening portion  129  and the embedded gate  132  in the drawing indicate an example configured to have an octagon in a plan view. Here, the plan view represents a view from a direction perpendicular to the surface of the semiconductor substrate  120 . The opening portion  129  and the like are configured to have a polygonal shape on a surface parallel to the surface of the semiconductor substrate  120 . 
     In addition, the MOS transistors  104  to  106  described in  FIG.  2    are disposed on the lower right side in the drawing. The MOS transistor  104  is constituted by semiconductor regions  125  and  126  and a gate  134 . The semiconductor regions  125  and  126  correspond to a source region and a drain region, respectively. The MOS transistor  105  is constituted by the semiconductor regions  126  and  127  and a gate  135 . The semiconductor regions  126  and  127  correspond to a drain region and a source region, respectively. The MOS transistor  106  is constituted by the semiconductor regions  127  and  128  and a gate  136 . The semiconductor regions  127  and  128  correspond to a drain region and a source region, respectively. Note that the gates  134  to  136  are gates constituted by an electrode disposed on the front surface side of the semiconductor substrate  120 , similarly to the substrate surface gate  133 . 
     The semiconductor region  125  constituting the source region of the MOS transistor  104  and the gate  135  of the MOS transistor  105  are connected to the semiconductor region  124  constituting the charge holding unit  102 . The wiring  109  in the drawing represents a wiring for connecting these and is a wiring constituted by the wiring layer  143  described in  FIG.  3   . In addition, black circles in the drawing represent connection portions with the wiring  109 , the semiconductor region  124 , and the like. A contact plug is disposed in the connection portion. As described above, the MOS transistors  105  and  106  constitute the image signal generation unit  110 . 
     Note that it is possible to adopt a configuration in which the charge holding unit  102  and the MOS transistors  104  to  106  are shared by the plurality of pixels  100 . As illustrated in the drawing, the charge transfer unit  103  and the photoelectric conversion unit  101  are disposed in one of long sides of the semiconductor region  124  having an octagonal shape. It is possible to adopt a configuration in which four pixels  100  share the charge holding unit  102 , the MOS transistor  104 , and the image signal generation unit  110  by disposing the charge transfer unit  103  and the photoelectric conversion unit  101  in each of the other three long sides of the semiconductor region  124 . 
     The opening portion  129  configured into a polygon in a plan view can be formed by recrystallizing Si in an inner wall of the opening portion formed in the semiconductor substrate  120 . Specifically, a circular opening portion is formed on the front surface side of the semiconductor substrate  120  and heated to several hundred degrees. The heating causes migration of Si of the semiconductor substrate  120 . The migrated Si precipitates on the side surface of the opening portion and recrystallizes. At the time of recrystallization, a plane (a 100 plane or a 110 plane) having a specific orientation is grown, and thus the opening portion  129  having a polygonal cross section surrounded by the plane can be formed. 
     The semiconductor substrate can be heated immediately before a step of forming the gate insulating film  131  in the opening portion  129 . As described above, the gate insulating film  131  can be formed by oxidizing the surface of the semiconductor substrate  120  including the opening portion  129 . Since the semiconductor substrate  120  is heated in the oxidation step, it is possible to simplify a step of manufacturing the imaging element  1  by continuously performing the step of forming the inner wall of the opening portion  129  into a polygon and the oxidation step. As a method of oxidizing the semiconductor substrate  120 , radical oxidation and plasma oxidation can be applied. These are oxidation methods for oxidizing the semiconductor substrate  120  with oxygen radicals. 
     Effects of Embedded Gate 
       FIG.  5    is a plan view illustrating a configuration example of the embedded gate according to the first embodiment of the present disclosure. The drawing is an enlarged view of a portion of the embedded gate  132  of the charge transfer unit  103  described in  FIG.  4   . In the drawing, a hatched region represents the gate oxide film  131 . Note that the description of the substrate surface gate  133  is omitted. As described above, the opening portion  129  can be configured in an octagonal shape in a plan view. The gate insulating film  131  formed along the inner wall of the opening portion  129  is also have an octagonal shape, and the external form of the embedded gate  132  is also an octagonal shape. 
     When charge of the semiconductor region  121  of the photoelectric conversion unit  101  is transferred to the semiconductor region  124  of the charge holding unit  102 , the charge is moved along a channel formed on the outer side of the opening portion  129 . Since the channel is formed along the octagonal external form of the opening portion  129 , the charge from the semiconductor region  121  moves along the octagonal external form of the opening portion  129 . A dashed line in the drawing assumes a circle circumscribing the octagonal opening portion  129  and represents the opening portion  129  in a case of being formed in a circular shape in a plan view. In addition, a curved arrow in the drawing represents an example of a charge moving path. 
     As illustrated in the drawing, the sides of the octagon are shorter than the circumference of the circumscribed circle, and thus the charge transfer unit  103  including the octagonal opening portion  129  and the embedded gate  132  embedded in the opening portion can have a reduced charge transfer path as compared with a case where the charge transfer unit  103  includes a gate embedded in a circular opening portion. Thereby, it is possible to reduce time required to transfer charge. In addition, a charge transfer path is expanded by forming the opening portion  129  in an octagonal shape. This is because the above-described channel is expanded from the circumference illustrated in the drawing to the position of the side of the octagon. Thereby, it is possible to improve the transfer efficiency of charge in the charge transfer unit  103 . 
     In addition, the opening portion  129  is configured in an octagonal shape in a plan view, and thus it is possible to reduce the area of the inner surface of the opening portion  129 , as compared to a case where the opening portion  129  is configured in a circular shape. Thereby, it is possible to reduce a defect of the semiconductor substrate  120  formed in the opening portion  129 . It is possible to reduce the surface level of the opening portion  129  and reduce the generation of a dark current. 
     Such effects can be obtained by configuring the opening portion  129  in a polygonal shape with six or more sides in a plan view. On the other hand, in a case where the opening portion  129  is configured into a quadrangle or a pentagon in a plan view, an electric field is concentrated on the gate insulating film  131  in a vertex portion of the opening portion  129 , which leads to a possibility that a defect such as breakage will occur. This is because the vertex of the opening portion  129  has a small angle. The angle of the vertex of the opening portion  129  can be set to  120  to  150  degrees. Thereby, it is possible to alleviate the concentration of an electric field on the gate insulating film  131  in the vertex portion of the opening portion  129 . 
     In addition, it is possible to widen an interval with respect to the semiconductor region  121  of the photoelectric conversion unit  101  by configuring the opening portion  129  into a polygon. “D” illustrated in the drawing represents an increase in the interval between the opening portion  129  and the semiconductor region  121  compared with a circular opening portion  129 . By widening an interval with respect to the semiconductor region  121  of the photoelectric conversion unit  101 , it is possible to widen an interval between a boundary of the p-type semiconductor region  123  (not illustrated) in the vicinity of the opening portion  129  and the semiconductor region  121 . As described above, the substrate surface gate  133  is disposed in a shape that covers the semiconductor region  123 . It is possible to reduce the influence of a potential barrier at an interface with the photoelectric conversion unit  101  by the substrate surface gate  133 . It is possible to more reduce the influence of a potential barrier by relatively widening an interval between the boundary of the semiconductor region  123  and the semiconductor region  121 . 
     Similarly, the opening portion  129  is configured into a polygon, and thus it is possible to relatively widening an interval between the boundary of the semiconductor region  123  and the semiconductor region  124  and improve the above-described effect of alleviating an electric field that changes suddenly. 
     The position of the side of the opening portion  129  configured into a polygon is made parallel to the interface with the semiconductor region  121  of the photoelectric conversion unit  101 , and thus an interval between the boundary of the semiconductor region  123  and the semiconductor region  121  can be made widest. This can be performed by forming an orientation plane in a direction parallel to the boundary of the semiconductor region  121 . An orientation plane  129   a  in the drawing represents an orientation plane in a direction parallel to the boundary of the semiconductor region  121 . For example, a 100 plane of Si is grown as the orientation plane  129   a,  and thus the position of the side of the opening portion  129  can be made parallel to the boundary of the semiconductor region  121 . 
     Such an orientation plane  129   a  can be formed, for example, by adjusting the orientation of a wafer-shaped semiconductor substrate  120  on the front surface side and the orientation of an orientation flat plane. For example, by using a wafer of which the front surface side is a 100 plane and configuring an orientation flat plane as a 100 plane, a 100 plane can be formed on the surface of an opening portion perpendicular or parallel to the orientation flat. Then, the boundary of the semiconductor region  121  is disposed in a direction perpendicular or parallel to the orientation flat, and thus a 100 plane which is the orientation plane parallel to the boundary of the semiconductor region  121  can be formed in the opening portion  129  close to the semiconductor region  121 . In this case, an orientation plane of a 110 plane is formed on a surface adjacent to the orientation plane  129   a  configured on a 100 plane of the opening portion  129 . “A” illustrated in the drawing represents an angle formed by these planes. A can be set to 120 to 150 degrees. 
     Similarly, the position of the side of the opening portion  129  configured into a polygon can be made parallel to an interface with the semiconductor region  124  of the charge holding unit  102 . In this case, it is possible to widen an interval between the boundary of the semiconductor region  123  and the semiconductor region  124  and improve the above-described effect of alleviating a change in an electric field. 
     Method of Manufacturing Pixel 
       FIGS.  6  to  9    are diagrams illustrating an example of a method of manufacturing the pixel according to the first embodiment of the present disclosure. First, a p-type well region is formed in the semiconductor substrate  120 . Next, the n-type semiconductor region  121  is formed in the well region (A in  FIG.  6   ). 
     Next, a silicon-based insulating film  401  is formed on the front surface side of the semiconductor substrate  120 . The silicon-based insulating film  401  is an insulating film configured by laminating SiN and SiO 2 , and is a film serving as a mask at the time of forming the opening portion  129  in the semiconductor substrate  120 . The silicon-based insulating film  401  can be formed through chemical vapor deposition (CVD) (B in  FIG.  6   ). 
     Next, an opening portion  402  is formed in the silicon-based insulating film  401  in a region where the opening portion  129  is formed. This can be formed by disposing a resist having an opening portion at the position of the opening portion  402  on the surface of the silicon-based insulating film  401  and performing etching (C in  FIG.  6   ). 
     Next, the opening portion  129  is formed. This can be performed by etching the front surface side of the semiconductor substrate  120  by using the silicon-based insulating film  401  as a mask. Dry etching can be applied to the etching. The formed opening portion  129  is configured, for example, in a circular shape (D in  FIG.  7   ). 
     Next, a sacrificial oxide film  403  is formed on the surface of the silicon-based insulating film  401  and the inner wall of the opening portion  129 . The sacrificial oxide film  403  can be formed as follows. First, the semiconductor substrate  120  is heated while supplying an oxygen (O 2 ) gas and a hydrogen (H 2 ) gas. Oxygen radicals are generated by raising the temperature of the semiconductor substrate  120  to several hundred degrees in a state where gas is supplied. The surface of the semiconductor substrate  120  is oxidized with the oxygen radicals, and the sacrificial oxide film  403  is formed (E in  FIG.  7   ). 
     Next, the semiconductor region  123  is formed. This can be performed by implanting boron (B) ions using the silicon-based insulating film  401  as a mask (F in  FIG.  8   ). 
     Next, the sacrificial oxide film  403  and the silicon-based insulating film  401  are removed using a chemical liquid such as hydrofluoric acid (G in  FIG.  8   ). 
     Next, Si in the inner wall of the opening portion  129  is recrystallized. This can be performed by heating the semiconductor substrate  120 . The semiconductor substrate  120  is heated to several hundred degrees and held, which leads to the migration of Si constituting the semiconductor substrate  120 , and Si is recrystallized on the inner wall of the opening portion  129 . Thereby, it is possible to form the opening portion  129  having a polygonal shape in a plan view. In addition, the opening portion  129  becomes narrow due to the recrystallization of Si (H in  FIG.  8   ). At the time of the recrystallization of Si, it is preferable to supply O 2  gas or O 2  gas mixed with H 2  gas. This is because it can be used in combination with the next radical oxidation step. 
     Next, the gate insulating film  131  is formed by oxidizing Si on the front surface of the semiconductor substrate  120 . As described above, radical oxidation can be applied to the oxidation of Si. Similarly to the above-described formation of the sacrificial oxide film  403 , the gate insulating film  131  can be formed by heating the semiconductor substrate  120  to several hundred degrees while supplying O 2  gas or O 2  gas mixed with H 2  gas (I in  FIG.  8   ). 
     Next, a polycrystalline silicon film  404  is disposed on the front surface side of the semiconductor substrate  120 . In this case, the polycrystalline silicon film  404  is also disposed in the opening portion  129 . This can be performed through CVD (J in  FIG.  9   ). 
     Next, the polycrystalline silicon film  404  in a region other than the region of the substrate surface gate  133  on the front surface side of the semiconductor substrate  120  is removed. This can be performed by etching the polycrystalline silicon film  404 . Thereby, the substrate surface gate  133  and the embedded gate  132  can be formed (K in  FIG.  9   ). 
     Next, the semiconductor regions  122  and  124  are formed in the semiconductor substrate  120 . This can be performed by ion implantation (L in  FIG.  9   ). 
     Next, the wiring region  140  is formed on the front surface side of the semiconductor substrate  120 . Next, the rear surface side of the semiconductor substrate  120  is ground to thin the semiconductor substrate  120 . Next, the insulating film  150 , the color filter  160 , the protection film  170 , and the on-chip lens  180  are sequentially formed on the rear surface side of the semiconductor substrate  120 . Thereby, the imaging element  1  can be manufactured. 
     Note that the sacrificial oxide film  403  and the gate insulating film  131  can be formed by plasma oxidation for generating oxygen radicals using oxygen plasma to oxidize Si. In addition, the recrystallization of Si on the inner wall of the opening portion  129  can also be performed at the time of forming the sacrificial oxide film  403 . 
     Modification Example 
     In the charge transfer unit  103  mentioned above, the gate insulating film  131  and the embedded gate  132  are disposed in the opening portion  129  having a regular octagonal shape in a plan view, but opening portions  129  having other shapes can also be applied. 
       FIG.  10    is a plan view illustrating another configuration example of the pixel according to the first embodiment of the present disclosure. The drawing is a diagram illustrating the shapes of the opening portion  129  and the like of the charge transfer unit  103 , similarly to  FIG.  5   . The opening portion  129  in the drawing differs from the opening portion  129  in  FIG.  5    in that the opening portion is configured in a flat octagonal shape. In a case where the opening portion of the semiconductor substrate  120  before the recrystallization of Si is performed is configured in an elliptic shape in a plan view, the opening portion  129  having a flat polygonal shape illustrated in the drawing can be formed. The embedded gate  132  having a flat octagonal shape can be formed by disposing the embedded gate  132  in the opening portion  129 . A charge transfer path can also be reduced in the embedded gate  132  having a flat octagonal shape. 
     As described above, in the imaging element  1  according to the first embodiment of the present disclosure, the embedded gate  132  of the charge transfer unit  103  constituted by a vertical transistor is configured into a polygon in a plan view, and thus a charge transfer path of the charge transfer unit  103  can be reduced. Thereby, it is possible to improve the transfer efficiency of charge in the charge transfer unit  103 . 
     2. Second Embodiment 
     In the above-described imaging element  1  of the first embodiment, the substrate surface gate  133  is disposed in the charge transfer unit  103  of the pixel  100 . On the other hand, an imaging element  1  of a second embodiment of the present disclosure differs from that in the above-described first embodiment in that the substrate surface gate  133  is omitted. 
     Configuration of Pixel 
       FIG.  11    is a cross-sectional view illustrating a configuration example of a pixel according to the second embodiment of the present disclosure. The drawing is a schematic cross-sectional view illustrating a configuration example of a pixel  100 , similarly to  FIG.  3   . This pixel differs from the pixel  100  described in  FIG.  3    in that the substrate surface gate  133  of the charge transfer unit  103  is omitted. 
     The charge transfer unit  103  in the drawing controls the transfer of charge from a photoelectric conversion unit  101  to a charge holding unit  102  by an embedded gate  132 . Also in the drawing, an opening portion  129  and the embedded gate  132  are configured in a polygonal shape in a plan view. 
     A configuration of the imaging element  1  other than the above-described configuration is the same as the configuration of the imaging element  1  described in the first embodiment of the present disclosure, and thus description thereof will be omitted. 
     As described above, in the imaging element  1  of the second embodiment of the present disclosure, a charge transfer path can be reduced also in a case where the substrate surface gate  133  of the charge transfer unit  103  is omitted. 
     3. Third Embodiment 
     In the above-described imaging element  1  of the first embodiment, the photoelectric conversion unit  101  of the pixel  100  is disposed in the vicinity of the front surface side of the semiconductor substrate  120 . On the other hand, an imaging element  1  of a third embodiment of the present disclosure differs from that in the above-described first embodiment in that a photoelectric conversion unit  101  is disposed in a deep portion of the semiconductor substrate  120 . 
     Configuration of Pixel 
       FIG.  12    is a cross-sectional view illustrating a configuration example of a pixel according to the third embodiment of the present disclosure. The drawing is a schematic cross-sectional view illustrating a configuration example of the pixel  100 , similarly to  FIG.  3   . The pixel  100  differs from the pixel  100  described in  FIG.  3    in that a semiconductor region  121  of the photoelectric conversion unit  101  is disposed on the rear surface side of the semiconductor substrate  120 . 
     The semiconductor region  121  of the photoelectric conversion unit  101  in the drawing is not disposed on the front surface side of the semiconductor substrate  120 . For this reason, a semiconductor region  124  of a charge holding unit  102  disposed on the front surface side of the semiconductor substrate  120  can be disposed at a position overlapping the semiconductor region  121  of the photoelectric conversion unit  101 . Thereby, it is possible to miniaturize the pixel  100 . 
     A charge transfer unit  103  in the drawing transfers charge generated by the photoelectric conversion unit  101  in the thickness direction of the semiconductor substrate  120 . An opening portion  129  and an embedded gate  132  of the charge transfer unit  103  are configured into a polygon in a plan view, similarly to the charge transfer unit  103  in  FIG.  3   . Thereby, it is possible to reduce the area of an inner surface of the opening portion  129  and reduce the surface level of the semiconductor substrate  120  in the opening portion  129 . In addition, it is possible to suppress the generation of a dark current. 
     A configuration of the imaging element  1  other than the above-described configuration is the same as the configuration of the imaging element  1  described in the first embodiment of the present disclosure and thus description thereof will be omitted. 
     As described above, in the imaging element  1  of the third embodiment of the present disclosure, charge generated by the photoelectric conversion unit  101  disposed on the rear surface side of the semiconductor substrate  120  is transferred in the thickness direction of the semiconductor substrate  120  by the charge transfer unit  103 . Even in such a case, the opening portion  129  and the embedded gate  132  of the charge transfer unit  103  are configured in a polygonal shape, and thus it is possible to reduce a surface level formed in the opening portion  129  and reduce the generation of a dark current. 
     Note that the opening portion  129  and the embedded gate  132  in  FIG.  10    can be applied to other embodiments. Specifically, the opening portion  129  and the embedded gate  132  in  FIG.  10    can be applied to the charge transfer unit  103  in  FIGS.  11  and  12   . 
     4. Example of Application to Camera 
     The technology according to the present disclosure (the present technology) can be applied to various products. For example, the present technology may be realized as an imaging element mounted on an imaging device such as a camera. 
       FIG.  13    is a block diagram illustrating a schematic configuration example of a camera which is an example of an imaging device to which the present technology is applicable. A camera  1000  in the drawing includes a lens  1001 , an imaging element  1002 , an imaging control unit  1003 , a lens driving unit  1004 , an image processing unit  1005 , an operation input unit  1006 , a frame memory  1007 , a display unit  1008 , and a recording unit  1009 . 
     The lens  1001  is an imaging lens of the camera  1000 . The lens  1001  focuses light from a subject, causes the light to be incident on the imaging element  1002 , which will be described later, and forms an image of the subject. 
     The imaging element  1002  is a semiconductor element that images the light from the subject focused by the lens  1001 . The imaging element  1002  generates an analog image signal corresponding to emitted light, converts the analog image signal into a digital image signal, and outputs the digital image signal. 
     The imaging control unit  1003  controls imaging in the imaging element  1002 . The imaging control unit  1003  controls the imaging element  1002  by generating a control signal and outputting the control signal to the imaging element  1002 . In addition, the imaging control unit  1003  can perform auto-focus in the camera  1000  based on an image signal output from the imaging element  1002 . Here, the auto-focus is a system that detects a focal position of the lens  1001  and automatically adjusts the focal position. As the auto-focus, a method of detecting an image surface phase difference according to phase difference pixels disposed in the imaging element  1002  to detect a focal position (image surface phase difference auto-focus) can be used. In addition, a method of detecting a position at which the contrast of an image is maximized as a focal position (contrast auto-focus) can also be applied. The imaging control unit  1003  adjusts the position of the lens  1001  through the lens driving unit  1004  based on the detected focal position and performs auto-focus. Meanwhile, the imaging control unit  1003  can be configured as, for example, a digital signal processor (DSP) provided with firmware. 
     The lens driving unit  1004  drives the lens  1001  based on the control of the imaging control unit  1003 . The lens driving unit  1004  can drive the lens  1001  by changing the position of the lens  1001  using a built-in motor. 
     The image processing unit  1005  processes an image signal generated by the imaging element  1002 . This processing corresponds to, for example, demosaicing for generating an image signal of an insufficient color among image signals corresponding to red, green, and blue for each pixel, noise reduction for removing noise in an image signal, image signal encoding, and the like. The image processing unit  1005  can be constituted by, for example, a microcomputer provided with firmware. 
     The operation input unit  1006  receives an operation input from a user of the camera  1000 . For example, a push button or a touch panel can be used as the operation input unit  1006 . An operation input received by the operation input unit  1006  is transmitted to the imaging control unit  1003  and the image processing unit  1005 . Thereafter, processing corresponding to the operation input, for example, processing such as imaging of a subject is started. 
     A frame memory  1007  is memory that stores a frame which is an image signal corresponding to one screen. The frame memory  1007  is controlled by the image processing unit  1005  and holds frames during image processing. 
     The display unit  1008  displays an image processed by the image processing unit  1005 . For example, a liquid crystal panel can be used as the display unit  1008 . 
     The recording unit  1009  records an image processed by the image processing unit  1005 . For example, a memory card or a hard disk can be used as the recording unit  1009 . 
     A camera to which the present disclosure can be applied has been described above. The present technology can be applied to the imaging element  1002  among the components described above. Specifically, the imaging element  1  illustrated in  FIG.  1    can be applied to the imaging element  1002 . Since the transfer efficiency of charge in the charge transfer unit  103  is improved by applying the imaging element  1  to the imaging element  1002 , it is possible to increase the speed of imaging of the camera  1000 . Note that the image processing unit  1005  is an example of a processing circuit recited in the claims. The camera  1000  is an example of an imaging device described in the claims. 
     5. Example of Application to Endoscopic Operation System 
     The technology according to the present disclosure can be applied to various products. For example, the technology according to the present disclosure may be applied to an endoscopic operation system. 
       FIG.  14    is a diagram illustrating an example of a schematic configuration of an endoscopic operation system to which the technique according to the present disclosure is applicable. 
       FIG.  14    illustrates a state where an operator (doctor)  11131  is performing a surgical operation on a patient  11132  on a patient bed  11133  by using the endoscopic operation system  11000 . As illustrated, the endoscopic operation system  11000  includes an endoscope  11100 , other operation tools  11110  such as a pneumoperitoneum tube  11111  or an energy treatment tool  11112 , a support arm device  11120  supporting the endoscope  11100 , and a cart  11200  on which various devices for an endoscopic operation are mounted. 
     The endoscope  11100  includes a body tube  11101  of which a region with a predetermined length is inserted from a distal end into a body cavity of the patient  11132  and a camera head  11102  connected to a base end of the body tube  11101 . In the illustrated example, the endoscope  11100  configured as a so-called hard mirror having a hard body tube  11101  is illustrated, but the endoscope  11100  may be configured as a so-called soft mirror having a soft body tube. 
     At the distal end of the body tube  11101 , an opening portion into which an objective lens is inserted is provided. A light source device  11203  is connected to the endoscope  11100 , light generated by the light source device  11203  is guided to the distal end of the body tube by a light guide extended to the inside of the body tube  11101 , and the light is radiated to an observation target in the cavity of the patient  11132  through the objective lens. The endoscope  11100  may be a direct-viewing mirror, an oblique-viewing mirror, or a side-viewing mirror. 
     An optical system and an imaging element are provided inside the camera head  11102  and light (observation light) reflected from the observation target is focused on the imaging element by the optical system. The observation light is photoelectrically converted by the imaging element and an electric signal corresponding to the observation light, that is, an image signal corresponding to an observation image, is generated. The image signal is transmitted to a CCU (Camera Control Unit)  11201  as RAW data. 
     The CCU  11201  is configured of a central processing unit (CPU), a graphics processing unit (GPU) or the like, and comprehensively controls operations of the endoscope  11100  and a display device  11202 . Further, the CCU  11201  receives the image signal from the camera head  11102  and performs various image processing such as development processing (demosaic processing) on the image signal for displaying an image based on the image signal. 
     The display device  11202  displays an image based on an image signal having been subjected to image processing by the CCU  11201  under the control of the CCU  11201 . 
     The light source device  11203  includes a light source such as a light emitting diode (LED) and supplies the endoscope  11100  with irradiation light for imaging an operation part or the like. 
     The input device  11204  is an input interface for the endoscopic operation system  11000 . A user can input various kinds of information or instructions to the endoscopic operation system  11000  through the input device  11204 . For example, the user inputs an instruction or the like to change imaging conditions (a kind of irradiation light, a magnification, a focal distance, and the like) for the endoscope  11100 . 
     A treatment tool control device  11205  controls driving of the energy treatment tool  11112  for tissue cautery or incision, blood vessel sealing, or the like. A pneumoperitoneum device  11206  sends a gas into the cavity via the pneumoperitoneum tube  11111  to inflate the cavity of the patient  11132  in order to guarantee a visual field for the endoscope  11100  and guarantee a working space of the operator. A recorder  11207  is a device capable of recording various kinds of information regarding surgery. A printer  11208  is a device capable of printing various kinds of information regarding operation in various forms of text, images, graphs, or the like. 
     The light source device  11203  that supplies the endoscope  11100  with irradiation light at the time of imaging of an operation part can be constituted by, for example, an LED, a laser light source, or a white light source configured in combination thereof. When the white light source is configured in combination of an RGB laser light source, an output intensity and an output timing of each color (each wavelength) can be controlled with high accuracy. Therefore, the light source device  11203  can adjust white balance of a captured image. Further, in this case, by irradiating an observation target with laser light from the RGB laser light source chronologically and controlling driving of the imaging element of the camera head  11102  in synchronization with the irradiation timing, it is also possible to capture images corresponding to RGB chronologically. According to this method, it is possible to obtain a color image even when color filters are not provided in the imaging element. 
     The driving of the light source device  11203  may be controlled such that the intensity of light to be output is changed at each predetermined time. By controlling the driving of the imaging element of the camera head  11102  in synchronization with a change timing of the intensity of the light, acquiring images chronologically, and combining the images, it is possible to generate an image with a high dynamic range in which there are no so-called black spots and white spots. 
     The light source device  11203  may be configured to be able to supply light with a predetermined wavelength band corresponding to special light observation. In the special light observation, for example, so-called narrow band imaging in which a predetermined tissue such as a blood vessel of a mucosal surface layer is imaged with high contrast through irradiation with light in a narrower band than irradiation light (that is, white light) at the time of normal observation using a dependence of absorption of light in a body tissue on a wavelength is performed. Alternatively, in the special light observation, fluorescence observation in which an image is obtained using fluorescence generated through excitation light irradiation may be performed. The fluorescence observation can be performed by emitting excitation light to a body tissue and observing fluorescence from the body tissue (autofluorescence observation), or locally injecting a reagent such as indocyanine green (ICG) to a body tissue and emitting excitation light corresponding to a fluorescence wavelength of the reagent to the body tissue to obtain a fluorescence image. The light source device  11203  may be configured to be able to supply narrow band light and/or excitation light corresponding to such special light observation. 
       FIG.  15    is a block diagram illustrating an example of functional configurations of the camera head  11102  and the CCU  11201  illustrated in  FIG.  14   . 
     The camera head  11102  includes a lens unit  11401 , an imaging unit  11402 , a driving unit  11403 , a communication unit  11404 , and a camera head control unit  11405 . The CCU  11201  includes a communication unit  11411 , an image processing unit  11412 , and a control unit  11413 . The camera head  11102  and the CCU  11201  are connected to be able to communicate with each other via a transmission cable  11400 . 
     The lens unit  11401  is an optical system provided in a connection unit with the body tube  11101 . Observation light received from the distal end of the body tube  11101  is guided to the camera head  11102  and is incident on the lens unit  11401 . The lens unit  11401  is configured to a plurality of lenses including a zoom lens and a focus lens in combination. 
     The imaging unit  11402  is constituted by an imaging element. The imaging element constituting the imaging unit  11402  may be one element (so-called single plate type) or a plurality of elements (so-called multi-plate type). When the imaging unit  11402  is configured as a multi-plate type, for example, image signals corresponding to RGB are generated by the imaging elements, and a color image may be obtained by synthesizing the image signals. Alternatively, the imaging unit  11402  may be configured to include a pair of imaging elements for respectively acquiring right-eye image signals and left-eye image signals corresponding to 3D (dimensional) display. By performing the 3D display, the operator  11131  can ascertain the depth of a body tissue in an operation part more accurately. When the imaging unit  11402  is configured as a multiple-plate, a plurality of systems of the lens unit  11401  may be provided to correspond to each imaging element. 
     In addition, the imaging unit  11402  may not necessarily be provided in the camera head  11102 . For example, the imaging unit  11402  may be provided immediately after the objective lens inside the body tube  11101 . 
     The driving unit  11403  is constituted by an actuator and the zoom lens and the focus lens of the lens unit  11401  are moved by a predetermined distance along an optical axis under the control of the camera head control unit  11405 . Thereby, it is possible to appropriately adjust the magnification and focus of a captured image by the imaging unit  11402 . 
     The communication unit  11404  is constituted by a communication device for transmitting or receiving various information to or from the CCU  11201 . The communication unit  11404  transmits an image signal obtained from the imaging unit  11402  to the CCU  11201  as raw data via the transmission cable  11400 . 
     In addition, the communication unit  11404  receives a control signal for controlling driving of the camera head  11102  from the CCU  11201  and supplies the control signal to the camera head control unit  11405 . The control signal includes, for example, information regarding imaging conditions such as information indicating designation of a frame rate of a captured image, information indicating designation of an exposure value at the time of imaging, and/or information indicating designation of the magnification and focus of the captured image. 
     Note that imaging conditions such as the foregoing frame rate, exposure value, magnification, and focus may be designated appropriately by the user or may be set automatically by the control unit  11413  of the CCU  11201  based on the acquired image signal. In the latter case, a so-called auto exposure (AE) function, an auto focus (AF) function, and an auto white balance (AWB) function are provided to the endoscope  11100 . 
     The camera head control unit  11405  controls the driving of the camera head  11102  based on the control signal from the CCU  11201  received via the communication unit  11404 . 
     The communication unit  11411  is constituted by a communication device for transmitting or receiving various information to or from the camera head  11102 . The communication unit  11411  receives an image signal transmitted via the transmission cable  11400  from the camera head  11102 . 
     In addition, the communication unit  11411  transmits a control signal for controlling the driving of the camera head  11102  to the camera head  11102 . The image signal or the control signal can be transmitted through electric communication, optical communication, or the like. 
     The image processing unit  11412  applies various kinds of image processing to the image signal which is the raw data transmitted from the camera head  11102 . 
     The control unit  11413  performs various kinds of control on imaging of an operation part or the like by the endoscope  11100  and display of a captured image obtained through imaging of an operation part or the like. For example, the control unit  11413  generates a control signal for controlling driving of the camera head  11102 . 
     In addition, the control unit  11413  causes the display device  11202  to display the captured image in which the operation part or the like is shown, based on the image signal subjected to the image processing in the image processing unit  11412 . At this time, the control unit  11413  may recognize various objects in the captured image using various image recognition technologies. For example, the control unit  11413  can recognize an operation tool such as forceps, a specific biological part, bleeding, or mist or the like at the time of use of the energy treatment tool  11112  by detecting the shape, color, or the like of the edge of an object included in the captured image. The control unit  11413  may superimpose various kinds of operation support information on the image of the operation part for display using the recognition result when the display device  11202  is caused to display the captured image. By superimposing and displaying the operation support information and presenting the operation support information to the operator  11131 , it is possible to reduce a burden on the operator  11131  or allow the operator  11131  to perform an operation reliably. 
     The transmission cable  11400  connecting the camera head  11102  and the CCU  11201  to each other is an electric signal cable that supports electric signal communication, an optical fiber that supports optical communication, or a composite cable thereof. 
     Here, in the example illustrated in the drawing, communication is performed in a wired manner using the transmission cable  11400 , but communication between the camera head  11102  and the CCU  11201  may be performed in a wireless manner. 
     An example of the endoscopic operation system to which the technique according to the present disclosure can be applied has been described above. The technology according to the present disclosure may be applied to the imaging unit  11402  of the camera head  11102  among the configurations described above. Specifically, the imaging element  1  in  FIG.  1    can be applied to the imaging unit  10402 . High-speed imaging can be performed by applying the technology according to the present disclosure to the imaging unit  10402 . 
     Here, although the endoscopic operation system has been described as an example, the technology according to the present disclosure may be applied to other, for example, a microscopic operation system. 
     6. Example of Application to Moving Body 
     The technology according to the present disclosure can be applied to various products. For example, the technology of the present disclosure may be implemented as a device mounted in any type of moving body such as an automobile, an electric automobile, a hybrid electric automobile, a motorbike, a bicycle, a personal mobility, an airplane, a drone, a ship, and a robot. 
       FIG.  16    is a block diagram illustrating a schematic configuration example of a vehicle control system, which is an example of a moving body control system to which the technique according to the present disclosure can be applied. 
     A vehicle control system  12000  includes a plurality of electronic control units connected to each other via a communication network  12001 . In the example illustrated in  FIG.  16   , the vehicle control system  12000  includes a drive system control unit  12010 , a body system control unit  12020 , a vehicle external information detection unit  12030 , a vehicle internal information detection unit  12040 , and an integrated control unit  12050 . Further, as functional constituents of the integrated control unit  12050 , a microcomputer  12051 , a sound and image output unit  12052 , and an on-vehicle network I/F (Interface)  12053  are illustrated. 
     The drive system control unit  12010  controls an operation of a device related to a drive system of a vehicle according to various programs. For example, the drive system control unit  12010  functions as a control device of a driving force generator for generating a driving force of a vehicle such as an internal combustion engine or a driving motor, a driving force transmission mechanism for transmitting a driving force to wheels, a steering mechanism for adjusting a turning angle of a vehicle, a braking device that generates a braking force of a vehicle and the like. 
     The body system control unit  12020  controls operations of various devices equipped in a vehicle body in accordance with various programs. For example, the body system control unit  12020  functions as a control device of a keyless entry system, a smart key system, a power window device, or various lamps such as a head lamp, a back lamp, a brake lamp, a turn signal, or a fog lamp. In this case, radio waves transmitted from a portable device that substitutes for a key or signals of various switches can be input to the body system control unit  12020 . The body system control unit  12020  receives inputs of these radio waves or signals and controls a door lock device, a power window device, a lamp, and the like of the vehicle. 
     The vehicle external information detection unit  12030  detects information on the outside of the vehicle having the vehicle control system  12000  mounted thereon. For example, an imaging unit  12031  is connected to the vehicle external information detection unit  12030 . The vehicle external information detection unit  12030  causes the imaging unit  12031  to capture an image outside the vehicle and receives the captured image. The vehicle external information detection unit  12030  may perform object detection processing or distance detection processing for people, cars, obstacles, signs, and letters on a road based on the received image. 
     The imaging unit  12031  is an optical sensor that receives light and outputs an electrical signal according to the amount of received light. The imaging unit  12031  can also output the electrical signal as an image and ranging information. In addition, light received by the imaging unit  12031  may be visible light, or may be invisible light such as infrared light. 
     The vehicle internal information detection unit  12040  detects information inside the vehicle. For example, a driver state detection unit  12041  that detects a state of a driver is connected to the vehicle internal information detection unit  12040 . The driver state detection unit  12041  includes, for example, a camera that captures an image of the driver, and the vehicle internal information detection unit  12040  may calculate a degree of fatigue or concentration of the driver or may determine whether or not the driver is dozing based on detection information input from the driver state detection unit  12041 . 
     The microcomputer  12051  can calculate a control target value of the driving force generation device, the steering mechanism, or the braking device based on information inside and outside the vehicle acquired by the vehicle external information detection unit  12030  or the vehicle internal information detection unit  12040 , and output a control command to the drive system control unit  12010 . For example, the microcomputer  12051  can perform cooperative control for the purpose of realization of functions of an advanced driver assistance system (ADAS) including collision avoidance or impact mitigation of a vehicle, follow-up traveling based on an inter-vehicle distance, vehicle speed keeping traveling, a vehicle collision warning, a vehicle lane departure warning, and the like. 
     Further, the microcomputer  12051  can perform cooperative control for the purpose of automated driving or the like in which automated travel is performed without depending on operations of the driver by controlling the driving force generator, the steering mechanism, the braking device, and the like based on information regarding the surroundings of the vehicle acquired by the vehicle external information detection unit  12030  or the vehicle internal information detection unit  12040 . 
     In addition, the microcomputer  12051  can output a control command to the body system control unit  12020  based on the information outside the vehicle acquired by the vehicle external information detection unit  12030 . For example, the microcomputer  12051  can perform cooperative control for the purpose of preventing glare, such as switching from a high beam to a low beam, by controlling the headlamp according to the position of a preceding vehicle or an oncoming vehicle detected by the vehicle external information detection unit  12030 . 
     The sound and image output unit  12052  transmits an output signal of at least one of audio and an image to an output device capable of visually or audibly notifying a passenger or the outside of the vehicle of information. In the example of  FIG.  16   , an audio speaker  12061 , a display unit  12062 , and an instrument panel  12063  are illustrated as the output device. The display unit  12062  may include, for example, at least one of an on-board display and a heads-up display. 
       FIG.  17    is a diagram illustrating an example of an installation position of the imaging unit  12031 . 
     In  FIG.  17   , a vehicle  12100  includes imaging units  12101 ,  12102 ,  12103 ,  12104 , and  12105  as the imaging unit  12031 . 
     The imaging units  12101 ,  12102 ,  12103 ,  12104 , and  12105  are provided at, for example, positions of a front nose, side mirrors, a rear bumper, a back door, an upper portion of a vehicle internal front windshield, and the like of the vehicle  12100 . The imaging unit  12101  provided on a front nose and the imaging unit  12105  provided in an upper portion of the vehicle internal front windshield mainly acquire images in front of the vehicle  12100 . The imaging units  12102  and  12103  provided on the side mirrors mainly acquire images on the lateral side of the vehicle  12100 . The imaging unit  12104  provided on the rear bumper or the back door mainly acquires images in the rear of the vehicle  12100 . The front view images acquired by the imaging units  12101  and  12105  are mainly used for detection of preceding vehicles, pedestrians, obstacles, traffic signals, traffic signs, lanes, and the like. 
     Note that  FIG.  17    illustrates an example of imaging ranges of the imaging units  12101  to  12104 . An imaging range  12111  indicates an imaging range of the imaging unit  12101  provided in the front nose, imaging ranges  12112  and  12113  indicate imaging ranges of the imaging units  12102  and  12103  provided in the side mirror, respectively, and an imaging range  12114  indicates an imaging range of the imaging unit  12104  provided in the rear bumper or back door. For example, a bird&#39;s-eye view image of the vehicle  12100  from above can be obtained by superimposing image data captured by the imaging units  12101  to  12104 . 
     At least one of the imaging units  12101  to  12104  may have a function for acquiring distance information. For example, at least one of the imaging units  12101  to  12104  may be a stereo camera constituted by a plurality of imaging elements or may be an imaging element having pixels for phase difference detection. 
     For example, the microcomputer  12051  can extract, particularly, a closest three-dimensional object on a path through which the vehicle  12100  is traveling, which is a three-dimensional object traveling at a predetermined speed (for example, 0 km/h or higher) in the substantially same direction as the vehicle  12100 , as a preceding vehicle by acquiring a distance to each of three-dimensional objects in the imaging ranges  12111  to  12114  and temporal change in the distance (a relative speed with respect to the vehicle  12100 ) based on distance information obtained from the imaging units  12101  to  12104 . Further, the microcomputer  12051  can set an inter-vehicle distance which should be guaranteed in advance in front of a preceding vehicle and can perform automated brake control (also including following stop control) or automated acceleration control (also including following start control). Thus, it is possible to perform cooperative control for the purpose of, for example, automated driving in which the vehicle autonomously travels without requiring the driver to perform operations. 
     For example, the microcomputer  12051  can classify and extract three-dimensional data regarding three-dimensional objects into other three-dimensional objects such as a two-wheeled vehicle, a normal vehicle, a large vehicle, a pedestrian, and an electric pole based on distance information obtained from the imaging units  12101  to  12104  and can use the other three-dimensional objects to perform automated avoidance of obstacles. For example, the microcomputer  12051  identifies surrounding obstacles of the vehicle  12100  as obstacles which can be viewed by the driver of the vehicle  12100  and obstacles which are difficult to view. Then, the microcomputer  12051  determines a collision risk indicating the degree of risk of collision with each obstacle, and when the collision risk is equal to or higher than the set value and there is a possibility of collision, an alarm is output to the driver through the audio speaker  12061  and the display unit  12062 , forced deceleration and avoidance steering are performed through the drive system control unit  12010 , and thus it is possible to perform driving support for collision avoidance. 
     At least one of the imaging units  12101  to  12104  may be an infrared camera that detects infrared rays. For example, the microcomputer  12051  can recognize a pedestrian by determining whether there is a pedestrian in the captured image of the imaging units  12101  to  12104 . Such recognition of a pedestrian is performed through, for example, a procedure of extracting feature points in the captured images of the imaging units  12101  to  12104  serving as infrared cameras, and a procedure of performing pattern matching processing on a series of feature points indicating a contour of an object to determine whether or not the object is a pedestrian. When the microcomputer  12051  determines that pedestrians are in the images captured by the imaging units  12101  to  12104  and recognize the pedestrians, the sound and image output unit  12052  controls the display unit  12062  such that rectangular contour lines for emphasis are superimposed and displayed on the recognized pedestrians. In addition, the sound and image output unit  12052  may control the display unit  12062  such that icons and the like indicating pedestrians are displayed at desired positions. 
     The example of the vehicle control system to which the technology according to the present disclosure can be applied has been described above. The technique according to the present disclosure may be applied to the imaging unit  12031  and the like among the above-described configurations. Specifically, the imaging element  1  described in  FIG.  1    can be applied to the imaging unit  12031  and the like. High-speed imaging can be performed by applying the technology according to the present disclosure to the imaging unit  12031  and the like. 
     Finally, the descriptions of the above-described embodiments are merely examples of the present disclosure, and the present disclosure is not limited to the above-described embodiments. Therefore, it goes without saying that various changes aside from the above-described embodiments can be made according to the design and the like within a scope that does not depart from the technical spirit of the present disclosure. 
     Additionally, the effects described in the present specification are merely examples, and are not limiting. Other effects may be obtained as well. 
     In addition, the drawings in the above-described embodiments are schematic, and dimensional ratios and the like of respective parts are not necessarily consistent with actual ones. In addition, the drawings of course include parts where dimensional relationships and ratios differ from drawing to drawing. 
     Note that the present technique can also have the following configurations. 
     (1) An imaging element including: 
     a photoelectric conversion unit configured to be disposed on a semiconductor substrate and to generate charge corresponding to incident light by photoelectric conversion; 
     a charge holding unit configured to hold the charge; 
     a charge transfer unit configured to include an opening portion, which is formed in the semiconductor substrate and having a polygonal shape in a plan view, and an embedded gate disposed in the opening portion and to transfer the charge from the photoelectric conversion unit to the charge holding unit; and 
     an image signal generation unit configured to generate an image signal based on the held charge. 
     (2) The imaging element according to (1), wherein the charge transfer unit includes the opening portion having a polygonal shape with six or more sides. 
     (3) The imaging element according to (2), wherein the charge transfer unit includes the opening portion having a polygonal shape in which an interior angle of a vertex is 120 to 150 degrees. 
     (4) The imaging element according to (3), wherein the charge transfer unit includes the opening portion having an octagonal shape. 
     (5) The imaging element according to any one of (1) to (4), wherein the charge transfer unit includes the opening portion having a polygonal shape which is formed by performing recrystallization of a member constituting the semiconductor substrate. 
     (6) The imaging element according to any one of (1) to (5), wherein the photoelectric conversion unit includes a boundary surface parallel to a side of the polygon of the opening portion of the charge transfer unit. 
     (7) The imaging element according to any one of (1) to (6), wherein the charge holding unit includes a boundary surface parallel to a side of the polygon of the opening portion of the charge transfer unit. 
     (8) The imaging element according to any one of (1) to (7), wherein the charge transfer unit further includes a gate insulating film disposed between the semiconductor substrate and the embedded gate. 
     (9) The imaging element according to (8), wherein the charge transfer unit includes the gate insulating film formed by oxidizing the semiconductor substrate. 
     (10) The imaging element according to (9), wherein the charge transfer unit includes the gate insulating film formed by oxidizing the semiconductor substrate with oxygen radicals. 
     (11) The imaging element according to any one of (1) to (10), wherein the semiconductor substrate is formed of silicon. 
     (12) The imaging element according to any one of (1) to (11), wherein the charge transfer unit further includes a high impurity concentration region which is disposed on the semiconductor substrate adjacent to the opening portion and configured to have a high impurity concentration. 
     (13) The imaging element according to (12), wherein the charge transfer unit further includes a substrate surface gate which is adjacent to the embedded gate and configured to have a shape covering the high impurity concentration region on a front surface side of the semiconductor substrate. 
     (14) The imaging element according to any one of (1) to (13), further including a second high impurity concentration region which is adjacent to the photoelectric conversion unit, disposed on a front surface side of the semiconductor substrate, and configured to have a high impurity concentration. 
     (15) An imaging device including: 
     a photoelectric conversion unit configured to be disposed on a semiconductor substrate and to generate charge corresponding to incident light by photoelectric conversion; 
     a charge holding unit configured to hold the charge; 
     a charge transfer unit configured to include an opening portion, which is formed in the semiconductor substrate and having a polygonal shape in a plan view, and an embedded gate disposed in the opening portion and to transfer the charge from the photoelectric conversion unit to the charge holding unit; 
     an image signal generation unit configured to generate an image signal based on the held charge; and 
     a processing circuit configured to process the generated image signal. 
     REFERENCE SIGNS LIST 
       1 ,  1002  Imaging element 
       10  Pixel array portion 
       30  Column signal processing unit 
       100  Pixel 
       101  Photoelectric conversion unit 
       102  Charge holding unit 
       103  Charge transfer unit 
       104  to  106  MOS transistor 
       110  Image signal generation unit 
       120  Semiconductor substrate 
       129  Opening portion 
       131  Gate insulating film 
       132  Embedded gate 
       133  Substrate surface gate 
       403  Sacrificial oxide film 
       1000  Camera 
       1005  Image processing unit 
       10402 ,  12031 ,  12101  to  12105  Imaging unit