Patent Publication Number: US-2022232151-A1

Title: Imaging element and imaging device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is continuation of U.S. patent application Ser. No. 16/639,243 filed Feb. 14, 2020, which is a national stage application under 35 U.S.C. 371 and claims the benefit of PCT Application No. PCT/JP2018/027865 having an international filing date of 25 Jul. 2018, which designated the United States, which PCT application claimed the benefit of Japanese Patent Application No. 2017-159957 filed 23 Aug. 2017, the entire disclosures of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present technology relates to an imaging element and an imaging device. Specifically, the present technology relates to an imaging element and an imaging device configured to detect image plane phase differences. 
     BACKGROUND ART 
     Hitherto, as imaging elements that are used in cameras configured to perform autofocus, an imaging element in which pixels each including two photoelectric conversion portions, which are separated from each other, are arranged two-dimensionally in a grid pattern, has been used. In such an imaging element, photoelectric conversion is performed in each photoelectric conversion portion of each pixel so that two image signals are generated. In a camera using this imaging element, a phase difference between images generated from these two image signals is detected and a focus position is thus detected. Next, the position of a photographing lens is adjusted on the basis of the detected focus position so that autofocus is performed. The method that detects phase differences for autofocus by pixels arranged in an imaging element as described above is referred to as “image plane phase difference autofocus.” Note that, in imaging after autofocus, the image signals from the two photoelectric conversion portions of the pixel are combined to be output as an image signal. With this, an image corresponding to an object can be obtained. 
     Further, in an imaging element, light from an object substantially vertically enters pixels arranged in the center portion of the imaging element through a photographing lens. Meanwhile, the light from the object diagonally enters pixels arranged in the peripheral portion of the imaging element. Thus, the amount of light that reaches the photoelectric conversion portions is low, resulting in low pixel sensitivity. In order to prevent a reduction in sensitivity of the pixels arranged in the peripheral portion, pupil correction is performed. Here “pupil correction” is a correction method that places a microlens on each pixel (hereinafter referred to as “on-chip lens”) in a manner that the on-chip lens is eccentric to the center of the pixel according to light that diagonally enters the peripheral portion. Further, “on-chip lens” is a lens that is placed on each pixel to concentrate light incident on the pixel on the photoelectric conversion portions. With the eccentric on-chip lens placed on the optical axis of incident light that passes through the center between the photoelectric conversion portions, the amount of light incident on the photoelectric conversion portions can be increased, and a reduction in sensitivity can thus be prevented. 
     Also in the imaging element configured to detect the image plane phase differences described above, a reduction in sensitivity of the pixels arranged in the peripheral portion of the imaging element can be prevented through application of pupil correction. For example, as a surface imaging element in which a wiring layer configured to transmit image signals is formed between a semiconductor substrate in which photoelectric conversion portions are formed and on-chip lenses, an imaging element in which, in a wiring layer, waveguides are formed for every two photoelectric conversion portions (for example, see PTL 1), is used. Here, “waveguide” includes a core configured to transmit light and cladding surrounding the core, and guides incident light through an on-chip lens to a photoelectric conversion portion. 
     In this imaging element, two waveguides, namely, a first waveguide and a second waveguide, are arranged adjacent to each other. The first waveguide has an opening portion in a direction different from the eccentric direction of an on-chip lens with respect to the central axis of a pixel, while the second waveguide has an opening portion in the same direction as the eccentric direction of the on-chip lens. With the on-chip lens placed at a position that allows light that passes through the center of the on-chip lens to be radiated on the boundary portion between the first and second waveguides, phase differences can be detected at pixels arranged in the peripheral portion. 
     However, since light that has penetrated the on-chip lens enters the first and second waveguides at different incident angles, the coupling efficiency of light is different between the first and second waveguides. The reason of this is as follows: the first waveguide has the opening portion in the direction different from the eccentric direction of the on-chip lens and light thus enters the opening portion of the first waveguide at a larger incident angle than that in the second waveguide having the opening portion in the same direction as the eccentric direction of the on-chip lens, and with a large incident angle, light is coupled to a high-order light-guiding mode of a waveguide, resulting in small coupling coefficient in a waveguide having a finite degree. Thus, the sensitivity of a photoelectric conversion portion configured to receive incident light transmitted through the first waveguide is lower than the sensitivity of a photoelectric conversion portion configured to receive incident light transmitted through the second waveguide. 
     In order to prevent this reduction in sensitivity, in the related art described above, the first waveguide has a larger refractive index difference between the core and the cladding than the second waveguide. Alternatively, the first waveguide has a larger core cross-sectional area than the second waveguide. With at least one of the measures applied, the reduction in waveguide coupling efficiency is prevented so that the photoelectric conversion portions configured to receive incident light transmitted through the first and second waveguides have substantially the same sensitivity, with the result that a reduction in detection accuracy of phase differences is prevented. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laid-open No. 2015-152738 
     SUMMARY 
     Technical Problem 
     In the related art described above, the first and second waveguides have different refractive index differences or different core cross-sectional areas according to the incident angles of light. However, in order to achieve a refractive index difference different between the first and second waveguides, the claddings or the cores of these waveguides are required to be made of different materials. Further, in order to achieve a cross-sectional area different between the first and second waveguides, while light that passes through the center of the on-chip lens is required to be radiated on the boundary between the first and second waveguides, the cross-sectional areas of the first and second waveguides are required to be different from each other according to incident angles. In either case, the waveguides are difficult to form, which is a problem. 
     The present technology has been made in view of the problem described above, and it is an object of the present technology to simplify the configuration of pixels in an imaging element configured to detect an image plane phase difference. 
     Solution to Problem 
     The present technology has been made to solve the problem described above, and according to a first aspect of the present technology, there is provided an imaging element including: an on-chip lens that is configured to concentrate incident light on a pixel and placed on each of pixels so as to be shifted from a center of the pixel according to an incident angle of the incident light; a plurality of photoelectric conversion portions arranged in the pixel and configured to perform photoelectric conversion according to the incident light; and a plurality of waveguides arranged for the plurality of respective photoelectric conversion portions in the pixel, the plurality of waveguides each being configured to guide the concentrated incident light so that the incident light enters each of the plurality of photoelectric conversion portions, and being formed into shapes dissimilar to each other on the basis of the shift of the on-chip lens. This provides an action that light incident through the on-chip lens placed by being shifted from the center of the pixel is guided to the plurality of respective photoelectric conversion portions by the plurality of waveguides formed into shapes dissimilar to each other. Light-guiding losses are assumed to be adjusted by the waveguides formed into the dissimilar shapes, and the sensitivity of each of the plurality of photoelectric conversion portions in the case where the on-chip lens is shifted is assumed to be corrected. 
     Further, in the first aspect, the plurality of waveguides may each include a core serving as an optical path and cladding accommodating the core, and may be different from each other in inner surface inclination of the cladding from an entrance to an exit of the incident light in each of the waveguides and thus have dissimilar shapes. This provides an action that incident light is guided by the waveguides including the cladding having the inner surface inclinations different from each other. Losses in light-guiding are assumed to be adjusted by the cladding having the inner surface inclinations different from each other. 
     Further, in the first aspect, the plurality of waveguides may be different from each other in inner surface inclination of the cladding according to the shift of the on-chip lens. This provides an action that incident light is guided by the waveguides including the cladding having the inclinations different from each other according to the shift of the on-chip lens. Losses in light-guiding according to the shift of the on-chip lens are assumed to be adjusted. 
     Further, in the first aspect, the plurality of waveguides may each include the cladding having a plurality of inner surfaces formed of different inclinations. This provides an action that incident light is guided by the waveguide including the cladding having the different inclinations. Losses in light-guiding are assumed to be adjusted by the cladding having the different inclinations. 
     Further, in the first aspect, the imaging element may further include: a pixel circuit that is placed in the pixel and configured to generate an image signal based on photoelectric conversion in the plurality of photoelectric conversion portions; and a wiring layer that is placed on a surface different from a surface for receiving the concentrated incident light, of surfaces of a semiconductor substrate in which the photoelectric conversion portions are formed, the wiring layer being configured to transmit the image signal. This provides an action that the waveguides are formed on a surface different from the surface of the semiconductor substrate on which the wiring layer is formed. Wiring layer interference-free waveguide formation is assumed. 
     Further, in the first aspect, the pixel may include two photoelectric conversion portions and two waveguides. This provides an action that the two waveguides configured to guide incident light to the respective two photoelectric conversion portions are arranged. 
     Further, in the first aspect, the pixels may be arranged two-dimensionally in a grid pattern, and the on-chip lens may be placed by being shifted from the center of the pixel according to the incident angle of the incident light on each of the pixels arranged. This provides an action that the on-chip lens is placed by being shifted with respect to the pixel according to the incident angle of incident light, and the plurality of pixels on each of which light is concentrated by the on-chip lens are arranged two-dimensionally in a grid pattern. 
     Further, in the first aspect, the imaging element may further include: a plurality of the on-chip lenses each of which is placed by being shifted from the center of the pixel according to the incident angle with respect to each lens configured to make light from an object enter an imaging element corresponding to the lens; and a plurality of the pixels each of which includes the plurality of waveguides that are formed into shapes dissimilar to each other on the basis of the shift of each of the plurality of on-chip lenses. This provides an action that the plurality of pixels each including the plurality of waveguides that are formed into the shapes dissimilar to each other according to the incident angle with respect to each lens are arranged. Losses in light-guiding in each pixel according to the plurality of lenses having different incident angles are assumed to be adjusted. 
     Further, according to a second aspect of the present technology, there is provided an imaging device including: an on-chip lens that is configured to concentrate incident light on a pixel and placed on each of pixels so as to be shifted from a center of the pixel according to an incident angle of the incident light; a plurality of photoelectric conversion portions arranged in the pixel and configured to perform photoelectric conversion according to the incident light; a plurality of waveguides arranged for the plurality of respective photoelectric conversion portions in the pixel, the plurality of waveguides each being configured to guide the concentrated incident light so that the incident light enters each of the plurality of photoelectric conversion portions, and being formed into shapes dissimilar to each other on the basis of the shift of the on-chip lens; a pixel circuit that is placed in the pixel and configured to generate an image signal based on photoelectric conversion in the plurality of photoelectric conversion portions; and a processing circuit configured to detect a phase difference on the basis of a plurality of image signals based on photoelectric conversion by the plurality of photoelectric conversion portions. This provides an action that light incident through the on-chip lens placed by being shifted from the center of the pixel is guided to the plurality of respective photoelectric conversion portions by the plurality of waveguides formed into the shapes dissimilar to each other. Light-guiding losses are assumed to be adjusted by the waveguides formed into the dissimilar shapes, and the sensitivity of each of the plurality of photoelectric conversion portions in the case where the on-chip lens is shifted is assumed to be corrected. Moreover, phase difference detection from image signals generated by the plurality of photoelectric conversion portions having corrected sensitivity is further assumed. 
     Advantageous Effect of Invention 
     According to the present technology, there is provided an excellent effect that, in the imaging element configured to detect image plane phase differences, the waveguides, which guide incident light to the plurality of respective photoelectric conversion portions, are formed into shapes dissimilar to each other so that the phase difference pixels on each of which the on-chip lens is placed by being shifted are formed in a simplified manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration example of an imaging element according to a first embodiment of the present technology. 
         FIG. 2  is a diagram illustrating a configuration example of a pixel according to the first embodiment of the present technology. 
         FIG. 3  is a view illustrating configuration examples of pixels according to the first embodiment of the present technology. 
         FIG. 4  is a view illustrating configuration examples of waveguides according to the first embodiment of the present technology. 
         FIG. 5  is a diagram illustrating examples of the characteristics of photoelectric conversion portions in the pixel according to the first embodiment of the present technology. 
         FIG. 6  is a view illustrating an example of an imaging element manufacturing method with respect to the pixel according to the first embodiment of the present technology. 
         FIG. 7  is a view illustrating the example of the imaging element manufacturing method with respect to the pixel according to the first embodiment of the present technology. 
         FIG. 8  is a view illustrating configuration examples of a pixel according to a second embodiment of the present technology. 
         FIG. 9  is a view illustrating configuration examples of a pixel according to a third embodiment of the present technology. 
         FIG. 10  is a view illustrating configuration examples of waveguides according to modified examples of the embodiment of the present technology. 
         FIG. 11  is a block diagram illustrating a schematic configuration example of a camera that is an example of an imaging device to which the present technology can be applied. 
     
    
    
     DETAILED DESCRIPTION 
     Description of Embodiments 
     Next, modes for carrying out the present technology (hereinafter referred to as “embodiment”) will be described with reference to the drawings. In the following drawings, the same or similar parts are denoted by the same or similar reference signs. However, the drawings are schematic, and the dimensional ratios and the like of the respective parts do not necessarily match the actual ones. Further, it goes without saying that the drawings include portions having different dimensional relationships or ratios. Further, the embodiments will be described in the following order.
         1. First Embodiment   2. Second Embodiment   3. Third Embodiment   4. Modified Examples   5. Application Example for Camera       

     First Embodiment 
     Configuration of Imaging Element 
       FIG. 1  is a diagram illustrating a configuration example of an imaging element according to a first embodiment of the present technology. An imaging element  9  in  FIG. 1  includes a pixel array portion  1 , a vertical driving portion  2 , a column signal processing portion  3 , and a control portion  4 . 
     The pixel array portion  1  includes pixels  100  arranged two-dimensionally in a grid pattern. Here, the pixel  100  generates image signals corresponding to radiated light. As described later, the pixel  100  includes photoelectric conversion portions configured to generate charges corresponding to the radiated light. In addition, the pixel  100  also includes a pixel circuit. This pixel circuit generates image signals based on charges generated by the photoelectric conversion portions. Image signal generation is controlled by control signals generated by the vertical driving portion  2  described later. In the pixel array portion  1 , signal lines  91  and  92  are arranged in an X-Y matrix. The signal line  91  is a signal line configured to transmit the control signals for the pixel circuits in the pixels  100 . The signal line  91  is placed in each row of the pixel array portion  1  and connected to the pixels  100  arranged in the corresponding row, in common. The signal line  92  is a signal line configured to transmit the image signals generated by the pixel circuits of the pixels  100 . The signal line  92  is placed in each column of the pixel array portion  1  and connected to the pixels  100  arranged in the corresponding column, in common. The photoelectric conversion portions and the pixel circuits are formed in a semiconductor substrate. 
     In  FIG. 1 , a pixel  100   a  represents the pixel  100  placed in the center portion of the pixel array portion  1 , and a pixel  100   b  represents the pixel  100  placed in the peripheral portion of the pixel array portion  1 . The pixels  100   a  and  100   b  are arranged in a row positioned around the center portion of the pixel array portion  1  and each include two photoelectric conversion portions. The dashed line rectangles illustrated in the pixels  100   a  and  100   b  in  FIG. 1  represent the photoelectric conversion portions. Such pixels each including the two photoelectric conversion portions are used as phase difference pixels. 
     Here, “phase difference pixel” is a pixel configured to detect, as a phase difference, a shift between images formed by light that has passed through different regions of a photographing lens configured to concentrate light from an object to the pixel array portion  1  of the imaging element  9 , and is a pixel used for autofocus. A plurality of such phase difference pixels is arranged in the row in which the pixels  100   a  and  100   b  are arranged, and the two photoelectric conversion portions are arranged side by side in the direction of the row in which the phase difference pixels are arranged is extended. This can be understood as follows: a phase difference pixel includes two pixels formed of photoelectric conversion portions separated from each other in the same direction as a direction in which phase difference pixels are arranged. Of the pixels separated from each other, the left pixel and the right pixel are referred to as “pixel A” and “pixel B,” respectively. Light that has passed through the right portion of the photographing lens enters the pixel A, and light that has passed through the left portion the photographing lens enters the pixel B. Through detection of a phase difference between an image based on image signals generated by a plurality of the pixels A and an image based on image signals generated by a plurality of the pixels B in the row in which the pixels  100   a  and  100   b  are arranged, the focus position of the photographing lens with respect to an object can be detected. Through adjustment of the position of the photographing lens based on the detected focus position, autofocus can be performed. 
     In this way, light enters the imaging element  9  from an object through the photographing lens, and the light vertically enters the pixel  100   a  placed in the center portion of the pixel array portion  1 . Meanwhile, the light diagonally enters the pixel  100   b  placed in the peripheral portion of the pixel array portion  1 . Specifically, in  FIG. 1 , the light diagonally enters the pixel  100   b , which is placed at the right end of the pixel array portion  1 , from an upper left position in the vertical direction of the pixel array portion  1 . Accordingly, in order to correct this, the pupil correction described above is performed. The details of the configurations of the pixels  100   a  and  100   b  are described later. 
     The vertical driving portion  2  generates the control signals for the pixel circuits of the pixels  100 . The vertical driving portion  2  transmits generated control signals to the pixels  100  through the signal line  91  in  FIG. 1 . The column signal processing portion  3  processes the image signals generated by the pixels  100 . The column signal processing portion  3  processes image signals transmitted from the pixels  100  through the signal line  92  in  FIG. 1 . The processing in the column signal processing portion  3  corresponds to, for example, analog-to-digital conversion of converting analog image signals generated in the pixels  100  into digital image signals. The control portion  4  controls the entire imaging element  9 . The control portion  4  generates and outputs control signals for controlling the vertical driving portion  2  and the column signal processing portion  3 , to thereby control the imaging element  9 . The control signals generated by the control portion  4  are transmitted to the vertical driving portion  2  through a signal line  93 , and are transmitted to the column signal processing portion  3  through a signal line  94 . 
     Configuration of Pixel 
       FIG. 2  is a diagram illustrating a configuration example of the pixel according to the first embodiment of the present technology.  FIG. 2  is a circuit diagram illustrating the configuration of the pixel  100 . The pixel  100  in  FIG. 2  includes photoelectric conversion portions  101  and  102 , a charge holding portion  103 , and MOS transistors  104  to  108 . Further, the signal line  91  in  FIG. 2  includes a signal line TR 1 , a signal line TR 2 , a signal line RST, and a signal line SEL. Further, the pixel  100  is supplied with power supply through a power line Vdd. Note that, in the pixel  100  in  FIG. 2 , a circuit including the charge holding portion  103  and the MOS transistors  104  to  108  corresponds to the pixel circuit described above. 
     The photoelectric conversion portion  101  has an anode grounded and a cathode connected to the source of the MOS transistor  104 . The photoelectric conversion portion  102  has an anode grounded and a cathode connected to the source of the MOS transistor  105 . The MOS transistor  104  has a gate connected to the signal line TR 1 , and the MOS transistor  105  has a gate connected to the signal line TR 2 . The MOS transistor  104  has a drain connected to the drain of the MOS transistor  105 , the source of the MOS transistor  106 , the gate of the MOS transistor  107 , and one end of the charge holding portion  103 . The other end of the charge holding portion  103  is grounded. The MOS transistor  106  has a gate connected to the signal line RST, and a drain connected to the power line Vdd. The MOS transistor  107  has a drain connected to the power line Vdd, and a source connected to the drain of the MOS transistor  108 . The MOS transistor  108  has a gate connected to the signal line SEL, and a source connected to the signal line  92 . 
     The photoelectric conversion portions  101  and  102  perform photoelectric conversion according to light incident on the pixel  100 . As the photoelectric conversion portions  101  and  102 , photodiodes can be used. Charges generated through photoelectric conversion are held by the respective photoelectric conversion portions  101  and  102 . Note that, the photoelectric conversion portion  101  can correspond to the photoelectric conversion portion of the pixel A described above, and the photoelectric conversion portion  102  can correspond to the photoelectric conversion portion of the pixel B described above. 
     The MOS transistors  104  and  105  are MOS transistors configured to transfer, to the charge holding portion  103 , charges generated through photoelectric conversion by the photoelectric conversion portions  101  and  102  to be held. The MOS transistor  104  transfers charges of the photoelectric conversion portion  101 , and the MOS transistor  105  transfers charges of the photoelectric conversion portion  102 . The MOS transistor  104  is controlled by control signals that are transmitted through the signal line TR 1 , and the MOS transistor  105  is controlled by control signals that are transmitted through the signal line TR 2 . 
     The charge holding portion  103  holds charges generated by the photoelectric conversion portions  101  and  102  and transferred by the MOS transistors  104  and  105 . As the charge holding portion  103 , a floating diffusion region formed in the diffusion layer of a semiconductor substrate can be used. 
     The MOS transistor  106  is a MOS transistor for resetting the charge holding portion  103 . The MOS transistor  106  is controlled by control signals that are transmitted through the signal line RST. The MOS transistor  106  electrically connects the charge holding portion  103  and the power line Vdd to each other, to thereby drain charges transferred to the charge holding portion  103  into the power line Vdd. With this, the charge holding portion  103  is reset. After the charge holding portion  103  has been reset, the MOS transistors  104  and  105  transfer charges. 
     The MOS transistor  107  is a MOS transistor configured to generate image signals based on the charges transferred to the charge holding portion  103 . The MOS transistor  107  has a gate connected to the not-grounded terminal of the charge holding portion  103 , and the source to which voltage according to the charges transferred to the charge holding portion  103  to be held is output. The MOS transistor  108  is a MOS transistor configured to output the image signals generated by the MOS transistor  107  to outside the pixel  100 . The MOS transistor  108  is controlled by control signals that are transmitted through the signal line SEL. The MOS transistor  108  electrically connects the source of the MOS transistor  107  and the signal line  92  to each other, to thereby output image signals. 
     When autofocus is performed in the imaging element  9 , generation of image signals based on photoelectric conversion by the photoelectric conversion portion  101  and generation of image signals based on photoelectric conversion by the photoelectric conversion portion  102  are performed alternately. Specifically, reset is performed after a predetermined exposure period has elapsed, and the charges generated and held by the photoelectric conversion portion  101  are transferred to the charge holding portion  103  by the MOS transistor  104 . After that, an image signal is generated by the MOS transistor  107  and output from the pixel  100  through the MOS transistor  108  as an image signal of the pixel A. In a similar manner, charges generated by the photoelectric conversion portion  102  are transferred by the MOS transistor  105 . An image signal is thereby generated and output from the pixel  100  as an image signal of the pixel B. Generation and output of the image signals are alternately performed, and autofocus is executed on the basis of the output image signals (the image signals of the pixel A and the pixel B). 
     Note that, when normal imaging is performed, an image signal that is a combination of the image signals based on the photoelectric conversion by the respective photoelectric conversion portions  101  and  102  is generated. 
     Cross-Sectional Configuration of Pixel 
       FIG. 3  is a view illustrating configuration examples of the pixels according to the first embodiment of the present technology.  FIG. 3  is a schematic sectional view illustrating the configurations of the pixels  100 , and is a view illustrating the configurations of the pixels  100   a  and  100   b , which are described with reference to  FIG. 1 . 
     The pixel  100   a  includes an on-chip lens  191 , a color filter  192 , a planarization film  193 , waveguides  110  and  120 , a semiconductor substrate  181 , a wiring layer  185 , and an insulating layer  184 . Further, the pixel  100   b  includes a waveguide  130  instead of the waveguide  110 . The pixel  100   a  placed in the center portion of the pixel array portion  1  and the pixel  100   b  placed in the peripheral portion thereof can have the same configuration except for waveguide shapes and the positions of the on-chip lens  191  with respect to the pixel  100 . The pixel array portion  1  includes the pixels  100   a  and  100   b.    
     The semiconductor substrate  181  is a semiconductor substrate in which the semiconductor portions of the pixels  100 , such as the photoelectric conversion portions and the pixel circuits described with reference to  FIG. 2 , are formed. In  FIG. 3 , of those, the photoelectric conversion portions  101  and  102  are illustrated. For convenience, it is assumed that the semiconductor substrate  181  in  FIG. 3  is formed as a P-type well region. The photoelectric conversion portion  101  has an N-type semiconductor region  182  and a P-type well region surrounding the N-type semiconductor region  182 . At a PN junction formed on the interface between the N-type semiconductor region  182  and the P-type well region, photoelectric conversion according to incident light is performed, and charges generated through this photoelectric conversion are held by the N-type semiconductor region  182 . In a similar manner, the photoelectric conversion portion  102  has an N-type semiconductor region  183  and a P-type well region surrounding the N-type semiconductor region  183 . On the basis of the charges generated through photoelectric conversion by the photoelectric conversion portions  101  and  102 , image signals are generated by the pixel circuit, which is not illustrated. 
     The wiring layer  185  is a wiring line configured to transmit the image signals generated in the pixels  100  and the control signals for controlling the pixel circuits. The wiring layer  185  corresponds to the signal lines  91  and  92  described with reference to  FIG. 1 . Further, portions of the wiring layer  185  are insulated from each other by the insulating layer  184 . Note that, the imaging element  9  including the pixel array portion  1  in  FIG. 3  is a back-illuminated imaging element in which the wiring layer  185  is placed on a surface different from the light incident surface of the semiconductor substrate  181 . 
     The on-chip lens  191  is a lens configured to concentrate incident light on the photoelectric conversion portions  101  and  102 . Further, the on-chip lens  191  is placed on each of the pixels  100  so as to be shifted from the center of the pixel  100  according to the incident angle of light. The details of placement of the on-chip lens  191  are described later. 
     In the lower layer of the on-chip lens  191 , the color filter  192  and the planarization film  193  are arranged. The color filter  192  is an optical filter, and is a filter configured to transmit light having a predetermined wavelength, for example, red light of, light that has penetrated the on-chip lens  191 . The planarization film  193  is placed between the color filter  192  and the waveguide  110  and the like, which are described later, and is a film for making a surface on which the color filter  192  is to be formed flat. With the planarization film  193 , the color filter  192  having a uniform thickness can be formed. 
     The waveguides  110  and  120  guide, to the photoelectric conversion portions  101  and  102 , light incident through the on-chip lens  191 . The waveguide  110  guides incident light to the photoelectric conversion portion  101 , and the waveguide  120  guides incident light to the photoelectric conversion portion  102 . The waveguides each include a core serving as an optical path and cladding accommodating the core. Specifically, the waveguide  110  includes a core  115  and claddings  111  and  112 , and the waveguide  120  includes a core  125  and claddings  121  and  122 . Note that, the cladding  111  and the like each correspond to the inner surface of the cladding in contact with the core. The cladding has the plurality of surfaces. Further, the cladding  111  and the like each correspond to the surface of an opening portion in a cladding member  199 , the opening portion being extended from the surface of the cladding member  199  to the semiconductor substrate  181 . With a core material  198  placed in the opening portion, the core  115  or the like is formed. The details of the configuration of the cladding are described later. 
     As the core material  198 , a material having a higher refractive index than the cladding member  199  is employed, and light incident on the core  115  is totally reflected by the interface between the core  115  and the cladding  111  and the like. With this, light incident on the waveguide  110  or the like is guided from the entrance of the waveguide  110  or the like to the surface of the photoelectric conversion portion  101  or the like placed at the exit. As the core  115  and the like, insulators having light transparency, for example, silicon nitride (SiN), can be used. Further, as the cladding member  199 , an insulator having a smaller refractive index than the core  115  and the like, for example, silicon oxide (SiO2), can be used. 
     As described above, in the pixel  100   a , the waveguides  110  and  120  are arranged. The waveguides  110  and  120  are formed into a shape having an exit narrower than an entrance. In other words, the inner surfaces of the cladding  111  and the like each have a shape with a predetermined inclination (tapered shape). Further, the waveguides  110  and  120  are formed into symmetrical shapes. In other words, the claddings  111  and  112  have symmetrical shapes to the claddings  121  and  122 . Thus, losses in incident light guiding in the waveguides  110  and  120  are substantially the same. Further, the photoelectric conversion portions  101  and  102  are formed into substantially equivalent shapes. 
     The arrows in  FIG. 3  represent light incident on the pixels  100   a  and  100   b , and the light substantially vertically enters the pixel  100   a  placed in the center portion of the pixel array portion  1 . The vertical incident light is divided by the waveguides  110  and  120  to be guided to the respective photoelectric conversion portions  101  and  102 . Since the losses in the waveguides  110  and  120  are substantially the same as described above, the photoelectric conversion portions  101  and  102  have substantially the same sensitivity with respect to the incident light. Light incident on the pixel  100  through the center of the on-chip lens  191  is hereinafter referred to as “principal ray.” 
     Meanwhile, in the pixel  100   b , the waveguides  130  and  120  are arranged. The waveguide  130  includes a core  135  and claddings  131  and  132 . Of those, the cladding  132  is formed into a shape substantially equivalent to that of the cladding  112 . Meanwhile, the cladding  131  is formed at a smaller inclination angle than the cladding  111 . Thus, the waveguide  130  has a smaller exit-to-entrance area ratio than the waveguides  110  and  120 . A narrower exit increases a loss. Thus, in the pixel  100   b , losses in the two waveguides  130  and  120  are different from each other, that is, the loss in the waveguide  130  placed closer to the center of the pixel array portion  1  is larger. As a result, in the pixel  100   b , the sensitivity of the photoelectric conversion portion  101  is lower than the sensitivity of the photoelectric conversion portion  102 . 
     As illustrated in  FIG. 3 , light enters the pixel  100   b  diagonally. In  FIG. 3 , it is assumed that light enters the pixel  100   b  at an incident angle of 15°. In other words, the incident light is shifted to the left by 15° from the vertical direction on the figure. In order to compensate for this, the pupil correction described above is performed, and the on-chip lens  191  is placed by being shifted toward the center of the pixel array portion  1  with the center of the pixel  100   b  serving as a reference. In other words, the centers of the on-chip lens  191  and the pixel  100   b  are located at different positions. Here, the on-chip lens  191  is placed at a position that allows principal rays to reach the boundary between the waveguides  130  and  120 . In other words, in the pixel  100   b , principal rays enter the top portion of the cladding member  199  placed at the boundary between the waveguides  130  and  120 . With this, incident light having smaller incident angles than principal rays is guided to the photoelectric conversion portion  101 , and incident light having larger incident angles than principal rays is guided to the photoelectric conversion portion  102 . A cross point, which is described later, can be set as a principal ray incident angle, and image signals according to focus shifts can be obtained. Note that, the cladding member  199  placed at the boundary between the waveguides  130  and  120  has a cross-sectional shape that is a triangle shape having two sides inclined at substantially the same angle. 
     Here, light enters the waveguide  130  at a smaller incident angle than that in the waveguide  120 . Specifically, light enters the waveguide  130  at an angle relatively close to the vertical direction. This achieves a high incident-light-and-waveguide coupling efficiency. Meanwhile, since the waveguide  120  is placed in the end portion of the on-chip lens  191 , light enters the waveguide  120  at a relatively large angle. In such a case, the light is coupled to a high-order light-guiding mode of the waveguide, resulting in low incident-light-and-waveguide coupling efficiency. 
     Accordingly, as described above, a difference in coupling efficiency between the waveguides  130  and  120  is compensated for through adjustment of the losses in the waveguides  130  and  120  with the waveguides  130  and  120  having the different cladding shapes. In other words, the waveguide  130  having the high coupling efficiency includes the cladding  131  formed at the smaller inclination angle than that in the waveguide  120  to increase the loss. With this, the quantity of light that reaches the photoelectric conversion portion  101  from the on-chip lens  191  through the waveguide  130  can be substantially the same as the quantity of light that reaches the photoelectric conversion portion  102  through the waveguide  120 . In this way, with the waveguides  130  and  120  having the dissimilar cladding shapes according to the shift of the on-chip lens  191 , the quantities of light that passes through the waveguides  130  and  120  can be adjusted. In the pixel  100   b , the photoelectric conversion portions  101  and  102  can have the same sensitivity. 
     As described above, the imaging element  9  in  FIG. 3  has a back-illuminated configuration. Since the wiring layer  185  and the insulating layer  184  are formed on a surface different from the surface on which the waveguide  110  and the like are formed of the semiconductor substrate  181 , the waveguide  110  and the like can be easily arranged, and the shapes of the cladding  131  and the like of each of the pixels  100  can be easily adjusted on the basis of incident angles. With the waveguide  130  and the like having the different cladding inner surface shapes, the sensitivity of the pixel A and that of the pixel B in the phase difference pixel can be adjusted. Further, the N-type semiconductor regions  182  and  183  of the photoelectric conversion portions  101  and  102  in the semiconductor substrate  181  can have the same shape in all the pixels  100  of the pixel array portion  1 , with the result that formation of the diffusion layer of the semiconductor substrate  181  can be simplified. 
     Note that, the imaging element  9  can be a front-illuminated imaging element. Specifically, in the imaging element  9  in  FIG. 3 , the cladding member  199  is used as the insulating layer  184  and the wiring layer  185  is embedded in the region of the cladding member  199  of the waveguide  110  and the like. A front-illuminated imaging element can be formed in this way. In this case, the wiring layer  185  is required to be placed in consideration that the waveguide  130  and the like are to have different cladding inner surface shapes. As described in a second embodiment described later, when the cladding members  199 , each of which is placed at the boundary between two waveguides in the pixel  100 , are formed into shapes greatly different from each other, a measure such that the wiring layer  185  is not placed at positions corresponding to the cladding members  199  placed at the boundaries or other measure is required. 
     Configuration of Waveguide 
       FIG. 4  is a view illustrating configuration examples of the waveguides according to the first embodiment of the present technology.  FIG. 4  is a top view illustrating the configurations of the waveguides in the pixels  100 , and is a top view illustrating the shape of the cladding member  199 . In  FIG. 4 , a illustrates the waveguide configuration of the pixel  100   a , and  b  illustrates the waveguide configuration of the pixel  100   b . In  FIG. 4 , the dotted line represents a ridge line  197  of the cladding member  199 . The rectangles formed by the ridge line  197  represent the entrances of the waveguide  110  and the like. Further, the long dashed double-short dashed line represents the on-chip lens  191 . Note that, the cross-sectional configurations along the line A-A′ in a and b of  FIG. 4  correspond to the configurations of the pixels  100   a  and  100   b  in  FIG. 3 . 
     The pixel  100   a  illustrated in a of  FIG. 4  includes the waveguides  110  and  120 . The waveguide  110  includes cladding having four rectangular (trapezoid) inner surfaces with a predetermined inclination. In other words, the waveguide  110  includes claddings  111  to  114 . At the exit of the waveguide  110 , namely, in the bottom portions of the claddings  111  to  114 , the semiconductor substrate  181  is placed. The claddings  111  to  114  are inclined at substantially the same angle. In a similar manner, the waveguide  120  includes claddings  121  to  124 . The claddings  111  to  114  are inclined at substantially the same angle. 
     The pixel  100   b  illustrated in b of  FIG. 4  includes the waveguide  130  instead of the waveguide  110  of the pixel  100   a . The waveguide  130  includes claddings  131  to  134 . The claddings  132 ,  133 , and  134  are inclined at the same angles as the claddings  112 ,  113 , and  114  in a of  FIG. 4 , respectively. Meanwhile, the cladding  131  is inclined at a smaller inclination angle than the claddings  132  to  134 . Thus, the exit of the waveguide  130  is narrower than those of the waveguides  110  and  120 . As described above, the waveguide  130  has a smaller exit-to-entrance area ratio. 
     Characteristic of Photoelectric Conversion Portion 
       FIG. 5  is a diagram illustrating examples of the characteristics of the photoelectric conversion portions in the pixel according to the first embodiment of the present technology.  FIG. 5  is a diagram illustrating the incident-angle and sensitivity relationships of the two photoelectric conversion portions arranged in the pixel  100 . In  FIG. 5 , the horizontal axis indicates the incident angle (unit: degree) of light incident on the pixel  100 , and the vertical axis indicates the sensitivity. Here, “sensitivity” is the ratio of an image signal to the quantity of incident light. In  FIG. 5 , a illustrates the incident-angle and sensitivity relationships in the pixel  100   a , and  b  illustrates the incident-angle and sensitivity relationships in the pixel  100   b.    
     The graphs  201  and  202  in a of  FIG. 5  indicate the incident-angle and sensitivity relationships of the photoelectric conversion portions  101  and  102  in the pixel  100   a . In other words, the graph  201  indicates the sensitivity of the pixel A at the position of the pixel  100   a  and the graph  202  indicates the sensitivity of the pixel B at the position of the pixel  100   a . In the pixel  100   a , the waveguides  110  and  120  are formed symmetrically. Thus, the graphs  201  and  202  cross each other with an incident angle of zero, and the graphs  201  and  202  have symmetrical shapes. In the pixel  100   a , since principal rays enter vertically (0°), an incident angle shift when the focus position of the photographing lens is what is generally called a front focus or a rear focus can be detected from a phase difference between the image signals of the pixel A and the pixel B. Note that, the point at which the graphs  201  and  202  cross each other is referred to as “cross point.” 
     In b of  FIG. 5 , the graphs  203  and  204  indicate the incident-angle and sensitivity relationships of the photoelectric conversion portions  101  and  102  in the pixel  100   b . In the pixel  100   b  in b of  FIG. 5 , the principal ray incident angle is 15°. Thus, pupil correction is performed so that the graphs  203  and  204  cross each other with the incident angle of 15°. In other words, the on-chip lens  191  is placed by being shifted from the center of the pixel  100   b  as described with reference to  FIG. 3 . Here, of the waveguides  130  and  120 , the cladding shape of the waveguide  130 , which is placed in the shift direction of the on-chip lens  191 , is adjusted so that the photoelectric conversion portions  101  and  102  have the same incident-angle and sensitivity relationship. With this, the incident-angle and sensitivity relationships of the photoelectric conversion portions  101  and  102  in the pixel  100   b  can be adjusted as indicated by the graphs  203  and  204  illustrated in b of  FIG. 5 . In other words, the cross point is formed at the principal ray incident angle of 15° so that the incident-angle and sensitivity relationships can have symmetrical shapes. With this, also in the pixel  100   b  placed in the peripheral portion of the pixel array portion  1 , the focus position of the photographing lens can be detected from a difference between the image signals of the pixel A and the pixel B. 
     Note that, photographing lenses having standard exit pupil distances (EPDs) are assumed for the principal ray incident angle (15° in  FIG. 5 ) in the pixel  100   b  placed in the peripheral portion of the pixel array portion  1 . In other words, in the imaging element  9 , pupil correction suitable for the photographing lenses having standard EPDs is employed. 
     Waveguide Manufacturing Method 
       FIG. 6  and  FIG. 7  are views illustrating an example of an imaging element manufacturing method with respect to the pixel according to the first embodiment of the present technology.  FIG. 6  and  FIG. 7  are views illustrating a waveguide manufacturing process in the manufacturing process of the imaging element  9 . The manufacturing process is described by taking the waveguides  130  and  120  in the pixel  100   b  of the pixel array portion  1  as examples. 
     First, on the rear surface of the semiconductor substrate  181  having formed thereon the insulating layer  184  and the like, a film of a cladding material  301  is formed. This can be formed by, for example, CVD (Chemical Vapor Deposition) (a of  FIG. 6 ). Next, a resist  302  is formed on the cladding material  301  (b of  FIG. 6 ). The resist  302  is formed into the same shape as the cladding member  199  described with reference to  FIG. 3 . In other words, in the waveguide  130 , the resist  302  having a shape with the same inclination as the claddings  131  and  132  is formed. Such an inclination can be formed as follows, for example: a photosensitive resist is applied, and the photosensitive resist is formed by being exposed with the use of a grayscale mask and being developed. Here, “grayscale mask” is a mask having gradation of shade formed thereon, and is a mask in which the shade of the gradation is gradually and continuously changed according to an inclination. The exposure amount can be changed according this shade of the gradation. Thus, the developed resist has a thickness according to the shade of the gradation. The resist  302  having a predetermined inclination can be formed in this way. 
     Note that, due to the pupil correction, the cladding shapes (inclinations) of the waveguides in the pixels  100  are different from each other according to positions from the center of the pixel array portion  1 . Thus, exposure can sequentially be performed with the use of the grayscale mask in which the shade of the gradation is gradually changed from the pixels  100  in the center portion of the pixel array portion  1  toward the peripheral portion. With this, a resist having different inclinations between pixel positions in the pixel array portion  1  can be formed. 
     Next, dry etching is performed with the resist  302  being a mask. As dry etching, anisotropic etching is used. With this, the cladding material  301  is etched, and the cladding member  199  including the cladding  131  and the like can thus be formed (c of  FIG. 6 ). Next, the film of a core material  303  is formed. This can be formed by, for example, CVD (d of  FIG. 7 ). Next, the surface of the core material  303  is made flat by polishing. The surface can be made flat by, for example, dry etching or chemical mechanical polishing (CMP). With this, the core material  198  can be formed, and the waveguide  130  and the like can thus be formed (e of  FIG. 7 ). After that, the planarization film  193 , the color filter  192 , and the on-chip lenses  191  are sequentially formed. The imaging element  9  can be manufactured in this way. 
     As described above, in the imaging element  9  according to the first embodiment of the present technology, with the plurality of waveguides having the cladding shapes dissimilar to each other, the sensitivity of each of the plurality of photoelectric conversion portions in the phase difference pixel is adjusted. Here, the sensitivity of the photoelectric conversion portions can be adjusted without being affected by the shifted position of the on-chip lens  191 , which is due to pupil correction, with the result that waveguide formation can be simplified. 
     2. Second Embodiment 
     The imaging element  9  of the first embodiment described above uses the phase difference pixels assuming the photographing lenses having standard EPDs. In contrast, the imaging element  9  according to a second embodiment of the present technology is different from the first embodiment in including a plurality of phase difference pixels supporting photographing lenses having different EPDs. 
     Cross-Sectional Configuration of Pixel 
       FIG. 8  is a view illustrating configuration examples of a pixel according to the second embodiment of the present technology.  FIG. 8  is a view illustrating examples of a phase difference pixel supporting principal rays having incident angles different from that in the phase difference pixels (assuming the incident angle of 15°) described with reference to  FIG. 3 . In  FIG. 8 , a illustrates a configuration example of the pixel  100   b  assuming an incident angle of 10°, and b illustrates a configuration example of the pixel  100   b  assuming an incident angle of 20°. The pixels  100   b  are arranged in the pixel array portion  1  separately from the pixel  100   b  described with reference to  FIG. 3 , and are used as phase difference pixels when photographing lenses having EPDs different from the standard EPDs are used. Note that, in  FIG. 8 , the illustrations of the insulating layer  184  and the wiring layer  185  are omitted. 
     The pixel  100   b  in a of  FIG. 8  includes waveguides  140  and  150  instead of the waveguides  130  and  120  of the pixel  100   b  described with reference to  FIG. 3 . The on-chip lens  191  in a of  FIG. 8  is placed by being shifted from the center of the pixel  100   b  toward the center portion of the pixel array portion  1 . Specifically, the on-chip lens  191  is placed at a position that allows a principal ray having an incident angle of 10° to enter the top portion of the cladding member  199  placed at the boundary between the waveguides  140  and  150  so that pupil correction is performed. The waveguide  140  includes claddings  141  and  142 , and the waveguide  150  includes claddings  151  and  152 . 
     The cladding member  199  placed at the boundary between the waveguides  140  and  150  has a cross-sectional shape that is a triangle shape with a vertex inclined in the shift direction of the on-chip lens  191 . In other words, the cladding  151  forming a side on a side different from the shift direction of the on-chip lens  191  is formed at a smaller inclination angle than the cladding  142  forming a side on the same side as the shift direction of the on-chip lens  191 . Meanwhile, the cladding  141  of the waveguide  140  has two inclinations with different inclination angles. Since the inclination of the cladding is changed in the middle of the waveguide, a loss in the waveguide  140  is larger than that in a waveguide including cladding having a single inclination. In this way, in the waveguide  140 , the cladding  142  is formed at the inclination angle close to the vertical direction, and the cladding  141  has an inclination angle that is changed step-by-step, and is formed at a relatively small inclination angle in the vicinity of the exit of the waveguide  140 . With this, the loss in the waveguide  140  with respect to the waveguide  150  is adjusted. 
     The pixel  100   b  in b of  FIG. 8  includes waveguides  160  and  170  instead of the waveguides  130  and  120 . Similarly to a of  FIG. 8 , the on-chip lens  191  in b of  FIG. 8  is placed at a position that allows a principal ray having an incident angle of 20° to enter the top portion of the cladding member  199  placed at the boundary between the waveguides  160  and  170 . The waveguide  160  includes claddings  161  and  162 , and the waveguide  170  includes claddings  171  and  172 . 
     The cladding member  199  placed at the boundary between the waveguides  160  and  170  has a cross-sectional shape that is a triangle shape with a vertex inclined in a direction different from the shift direction of the on-chip lens  191 . The incident angle of a principal ray in b of  FIG. 8  is 20°, and is an incident angle larger than that in the pixel  100   b  in a of  FIG. 8 . Thus, the cladding member  199  placed at the boundary between the waveguides  160  and  170  is formed into the cross-sectional shape that is the triangle shape with the vertex inclined in the direction different from the shift direction of the on-chip lens  191 . With this, while principal rays enter the boundary between the waveguides  160  and  170 , the area of the exit of the waveguide  170  is made substantially equivalent to that of the exit of the waveguide  150  in a of  FIG. 8 . 
     Further, since the cladding  162  is formed at a relatively small inclination angle, with the cladding  161  formed at a large inclination angle, a loss in the waveguide  160  with respect to the waveguide  170  can be adjusted. 
     In this way, the cladding members  199 , each of which is placed at the boundary between two waveguides arranged in the pixel  100   b , have different shapes depending on the EPD of a corresponding photographing lens to support pupil correction, and the claddings of the respective waveguides have different inclinations, on the side different from the waveguide boundary. With this, waveguide losses can be adjusted. 
     Note that, the configuration of the imaging element  9  according to the second embodiment of the present technology is not limited to this example. For example, the cladding  141  in a of  FIG. 8  can have a single inclination. 
     The remaining configuration of the imaging element  9  is similar to the configuration of the imaging element  9  according to the first embodiment of the present technology, and hence description thereof is omitted. 
     As described above, in the imaging element  9  according to the second embodiment of the present technology, the phase difference pixels supporting the incident angles of a plurality of principal rays are arranged in the pixel array portion  1 . With this, in the case where photographing lenses having different EPDs are used, focus positions can be detected by the phase difference pixels. 
     3. Third Embodiment 
     The imaging element  9  of the first embodiment described above uses, as the cladding member  199  placed at the boundary between the two waveguides arranged in the pixel  100 , the cladding member  199  having the triangle shape with the vertex in section. On the other hand, the imaging element  9  according to a third embodiment of the present technology uses the cladding member  199  having different vertex shapes. 
     Cross-Sectional Configuration of Pixel 
       FIG. 9  is a view illustrating configuration examples of the pixel according to the third embodiment of the present technology. The pixel  100   b  in  FIG. 9  is different from the pixel  100   b  described with reference to  FIG. 3  in the shape of the top portion of the cladding member  199  placed at the boundary between two waveguides. Note that, in the pixel  100   b  in  FIG. 9 , the illustrations of the on-chip lens  191 , the color filter  192 , the planarization film  193 , the insulating layer  184 , and the wiring layer  185  are omitted. 
     In a of  FIG. 9 , as the cladding member  199  placed at the boundary between two waveguides, the cladding member  199  having a flat top portion  196  is used. Further, in b of  FIG. 9 , as the cladding member  199  placed at the boundary between two waveguides, the cladding member  199  having a top portion  195  having an arc shape in section is used. Either shape makes it possible to easily form the cladding member  199  compared to the cladding member  199  having an acute top portion. Note that, when pupil correction is performed, the on-chip lens  191  is placed by being shifted to a position that allows principal rays to enter the top portion of the cladding member  199 . 
     The remaining configuration of the imaging element  9  is similar to the configuration of the imaging element  9  according to the first embodiment of the present technology, and hence description thereof is omitted. 
     As described above, the imaging element  9  according to the third embodiment of the present technology uses the cladding member  199  having the flat top portion or the top portion having the arc shape in section, and hence waveguide formation can further be simplified. 
     4. Modified Examples 
     The imaging element  9  of the first embodiment described above includes the two waveguides arranged adjacent to each other in the lateral direction of the pixel array portion  1 . On the other hand, in modified examples of the embodiment of the present technology, the waveguide arrangement is changed. 
     Cross-Sectional Configuration of Pixel 
       FIG. 10  is a view illustrating configuration examples of waveguides according to the modified examples of the embodiment of the present technology.  FIG. 10  is a view illustrating the configuration examples of the waveguides in arrangements different from those of the waveguides described with reference to  FIG. 4 . In  FIG. 10 , a illustrates an example of the pixel  100  in which two waveguides formed into a triangle shape are arranged adjacent to each other in the diagonal direction. In this case, the bottom sides of the two waveguides are adjacent to each other. Further, b of  FIG. 10  illustrates an example of the pixel  100  in which two waveguides formed into a rectangular shape are arranged adjacent to each other in the longitudinal direction. In this case, the long sides of the two waveguides are adjacent to each other. Note that, photoelectric conversion portions in such pixels  100  are formed into substantially the same shapes as the waveguides and are each placed at a position corresponding to the exit of the waveguide. 
     Each of a plurality of the pixels  100  including the waveguides in a of  FIG. 10  is arranged in the pixel array portion  1  in the diagonal direction and used for focus position detection. Meanwhile, each of the plurality of pixels  100  including the waveguides in b of  FIG. 10  is arranged in the pixel array portion  1  in the longitudinal direction and used for focus position detection. 
     As described above, for focus position detection, phase difference of images based on image signals from the pixels A and the pixels B of a plurality of phase difference pixels is required to be detected. Here, depending on objects, it is sometimes difficult to detect an image phase difference by the pixels A and the pixels B in the phase difference pixels arranged in the lateral direction, such as the pixels  100   a  and  100   b  described with reference to  FIG. 2 . For example, in a case where an object has little change in brightness in the lateral direction, images from the pixel A and the pixel B are images substantially equivalent to each other, and a phase difference thus becomes difficult to detect. Accordingly, phase difference is detected with the phase difference pixels arranged also in the diagonal direction or the longitudinal direction in the pixel array portion  1 . With this, object phase difference can be detected from a plurality of directions, and the detection accuracy of focus positions can be enhanced. 
     Further, c of  FIG. 10  illustrates an example of the pixel  100  including four waveguides, and d of  FIG. 10  illustrates an example of the pixel  100  including 16 waveguides. Note that, irrespective of the number of waveguides, two photoelectric conversion portions are arranged in each of the pixels  100 . In such pixels, waveguide losses can be adjusted by individually changing the shapes of the waveguides. For example, in the example in c of  FIG. 10 , the waveguides in which the inclination angles of the cladding are different from each other according to the arrangement positions in the pixel array portion  1  can be arranged. In other words, in a pixel placed in the center portion of the pixel array portion  1 , the cladding inclination of the waveguide is not changed, while in a pixel placed in the peripheral portion, the cladding inclinations of all the waveguides are changed. Further, in the pixel  100  placed between the center portion and the peripheral portion of the pixel array portion  1 , of the two waveguides, which are provided for each photoelectric conversion portion, the cladding inclination of one waveguide is changed. In this way, waveguide losses can be adjusted according to the positions of the pixels  100  in the pixel array portion  1 . 
     Note that, the configuration of the imaging element  9  according to the modified examples of the embodiment of the present technology is not limited to the examples. For example, phase difference pixels each including four or more photoelectric conversion portions can be arranged. Specifically, in c of  FIG. 10 , the phase difference pixels each can include a photoelectric conversion portion provided for every four waveguides. 
     The remaining configuration of the imaging element  9  is similar to the configuration of the imaging element  9  according to the first embodiment of the present technology, and hence description thereof is omitted. 
     5. Application Example for Camera 
     The present technology can be applied to various products. For example, the present technology may be realized as an imaging element that is mounted on an imaging device such as a camera. 
       FIG. 11  is a block diagram illustrating a schematic configuration example of a camera that is an example of an imaging device to which the present technology can be applied. A camera  1000  in  FIG. 11  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 a photographing lens of the camera  1000 . The lens  1001  collects light from an object and makes the light enter the imaging element  1002  described later to form an image of the object. 
     The imaging element  1002  is a semiconductor element configured to image light from an object collected by the lens  1001 . The imaging element  1002  generates analog image signals corresponding to radiated light, converts the analog image signals into digital image signals, and outputs the digital image signals. 
     The imaging control unit  1003  controls imaging in the imaging element  1002 . The imaging control unit  1003  controls the imaging element  1002  by generating control signals and outputting the control signals to the imaging element  1002 . Further, the imaging control unit  1003  can perform autofocus in the camera  1000  on the basis of image signals output from the imaging element  1002 . Here, “autofocus” is a system that detects the focus position of the lens  1001  and automatically adjusts the focus position. As this autofocus, a method in which an image plane phase difference is detected by phase difference pixels arranged in the imaging element  1002  to detect a focus position (image plane phase difference autofocus) can be used. Further, a method in which a position at which the contrast of an image is highest is detected as a focus position (contrast autofocus) can also be applied. The imaging control unit  1003  adjusts the position of the lens  1001  through the lens driving unit  1004  on the basis of the detected focus position, to thereby perform autofocus. Note that, the imaging control unit  1003  can include, for example, a DSP (Digital Signal Processor) equipped with firmware. 
     The lens driving unit  1004  drives the lens  1001  on the basis of 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 image signals generated by the imaging element  1002 . This processing includes, for example, demosaicing that generates image signals of lacking color among image signals corresponding to red, green, and blue for each pixel, noise reduction that removes noise of image signals, and encoding of image signals. Note that, the image processing unit  1005  can include, for example, a microcomputer equipped with firmware. 
     The operation input unit  1006  receives operation inputs from a user of the camera  1000 . As the operation input unit  1006 , for example, a push button or a touch panel can be used. An operation input received by the operation input unit  1006  is transmitted to the imaging control unit  1003  and the image processing unit  1005 . After that, processing corresponding to the operation input, for example, the processing of imaging an object or the like, is started. 
     The frame memory  1007  is a memory configured to store frames that are image signals for one screen. The frame memory  1007  is controlled by the image processing unit  1005  and holds frames in the course of image processing. 
     The display unit  1008  displays images 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 images processed by the image processing unit  1005 . As the recording unit  1009 , for example, a memory card or a hard disk can be used. 
     The camera to which the present invention can be applied has been described above. The present technology can be applied to the imaging element  1002  among the configurations described above. Specifically, the imaging element  9  described with reference to  FIG. 1  can be applied to the imaging element  1002 . The imaging control unit  1003  detects image plane phase difference on the basis of the image signals generated by the phase difference pixels arranged in the imaging element  9 , and controls the lens driving unit  1004  to adjust the position of the lens  1001 , thereby being capable of executing autofocus. With this, focus positions can be detected on the basis of image signals generated by the phase difference pixels in which pupil correction has been performed, enabling the camera  1000  to obtain sharp images. Note that, the imaging control unit  1003  is an example of a processing circuit described in the scope of claims. The camera  1000  is an example of an imaging device described in the scope of claims. 
     Note that, although the camera has been described as an example here, the technology according to the present invention may be applied to other devices such as monitoring devices, for example. 
     Finally, the description of each embodiment above is an example of the present technology, and the present technology is not limited to the above-mentioned embodiments. For this reason, it is a matter of course that various modifications, other than the above-mentioned embodiments, can be made according to the design and the like as long as they do not depart from the technical idea according to the present technology. 
     Further, the process procedures described in the above-mentioned embodiments may be regarded as a method including the series of procedures. Alternatively, the process procedures may be regarded as a program for causing a computer to execute the series of procedures or a recording medium storing the program. As this recording medium, for example, a CD (Compact Disc), a DVD (Digital Versatile Disk), a memory card, or the like can be used. 
     Note that, the present technology can also take the following configurations. 
     (1) An imaging element, including: 
     an on-chip lens that is configured to concentrate incident light on a pixel and placed on each of pixels so as to be shifted from a center of the pixel according to an incident angle of the incident light; 
     a plurality of photoelectric conversion portions arranged in the pixel and configured to perform photoelectric conversion according to the incident light; and 
     a plurality of waveguides arranged for the plurality of respective photoelectric conversion portions in the pixel, the plurality of waveguides each being configured to guide the incident light concentrated so that the incident light enters each of the plurality of photoelectric conversion portion, and being formed into shapes dissimilar to each other on the basis of the shift of the on-chip lens. 
     (2) The imaging element according to (1), in which 
     the plurality of waveguides each include a core serving as an optical path and cladding accommodating the core, and are different from each other in inner surface inclination of the cladding from an entrance to an exit of the incident light in each of the waveguides and thus have dissimilar shapes. 
     (3) The imaging element according to (2), in which 
     the plurality of waveguides are different from each other in inner surface inclination of the cladding according to the shift of the on-chip lens. 
     (4) The imaging element according to (2), in which 
     the plurality of waveguides each include the cladding having a plurality of inner surfaces formed of different inclinations. 
     (5) The imaging element according to any one of (1) to (4), further including: 
     a pixel circuit that is placed in the pixel and configured to generate an image signal based on photoelectric conversion in the plurality of photoelectric conversion portions; and 
     a wiring layer that is placed on a surface different from a surface for receiving the concentrated incident light, of surfaces of a semiconductor substrate on which the photoelectric conversion portions are formed, the wiring layer being configured to transmit the image signal. 
     (6) The imaging element according to any one of (1) to (5), in which 
     the pixel includes two photoelectric conversion portions and two waveguides. 
     (7) The imaging element according to any one of (1) to (6), in which 
     the pixels are arranged two-dimensionally in a grid pattern, and 
     the on-chip lens is placed by being shifted from the center of the pixel according to the incident angle of the incident light on each of the pixels arranged. 
     (8) The imaging element according to any one of (1) to (7), further including: 
     a plurality of the on-chip lenses each of which is placed by being shifted from the center of the pixel according to the incident angle with respect to each lens configured to make light from an object enter an imaging element corresponding to the lens; and 
     a plurality of the pixels each of which includes the plurality of waveguides formed into shapes dissimilar to each other on the basis of the shift of each of the plurality of on-chip lenses. 
     (9) An imaging device, including: 
     an on-chip lens that is configured to concentrate incident light on a pixel and placed on each of pixels so as to be shifted from a center of the pixel according to an incident angle of the incident light; 
     a plurality of photoelectric conversion portions arranged in the pixel and configured to perform photoelectric conversion according to the incident light; 
     a plurality of waveguides arranged for the plurality of respective photoelectric conversion portions in the pixel, the plurality of waveguides each being configured to guide the incident light concentrated so that the incident light enters each of the plurality of photoelectric conversion portion, and being formed into shapes dissimilar to each other on the basis of the shift of the on-chip lens; 
     a pixel circuit that is placed in the pixel and configured to generate an image signal based on photoelectric conversion in the plurality of photoelectric conversion portions; and 
     a processing circuit configured to detect a phase difference on the basis of a plurality of image signals based on photoelectric conversion by the plurality of photoelectric conversion portions. 
     REFERENCE SIGNS LIST 
     
         
           2  Vertical driving portion 
           3  Column signal processing portion 
           4  Control portion 
           9  Imaging element 
           100 ,  100   a ,  100   b  Pixel 
           101 ,  102  Photoelectric conversion portion 
           110 ,  120 ,  130 ,  140 ,  150 ,  160 ,  170  Waveguide 
           111 - 114 ,  121 - 124 ,  131 - 134 ,  141 ,  142 ,  151 ,  152 ,  161 ,  162 ,  171 ,  172  Cladding 
           115 ,  125 ,  135  Core 
           181  Semiconductor substrate 
           184  Insulating layer 
           185  Wiring layer 
           191  On-chip lens 
           198  Core material 
           199  Cladding member 
           1000  Camera 
           1001  Lens 
           1002  Imaging element 
           1003  Imaging control unit 
           1004  Lens driving unit