Patent Publication Number: US-2023133421-A1

Title: Imaging device and imaging system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/083,376, filed on Oct. 29, 2020, which is a continuation of International Patent Application No. PCT/JP2019/041830, filed on Oct. 25, 2019, which claims the benefit of foreign priority of Japanese patent application No. 2018-216499 filed on Nov. 19, 2018, the contents all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to imaging devices and imaging systems. 
     2. Description of the Related Art 
     In the field of light sensors, there is a demand for performing sensing in a limited wavelength region. In such an application, it is common practice to capture images based on a desired wavelength by disposing a wavelength-limiting optical filter in front of an imaging unit. For example, the specification of U.S. Patent Application Publication No. 2013/0089237 discloses a camera module in which an optical filter is disposed above an image sensor. 
     In the technique described in the specification of U.S. Patent Application Publication No. 2013/0089237, imaging to be performed is switched between imaging in a visible region and imaging in an infrared region by disposing the optical filter. As illustrated in FIG. 10 of the specification of U.S. Patent Application Publication No. 2013/0089237, a band pass filter may be used for filtering a particular wavelength component. Here, as the band pass filter, an interference-type optical filter is generally used. In this case, such an optical filter has a high dependence on an incident angle of light and therefore is unsuitable for image capturing at a wide angle. 
     On the other hand, Japanese Unexamined Patent Application Publication No. 2012-099797 discloses a photodetector having a multilayer structure including a first photodiode formed of amorphous silicon and a second photodiode formed in the form of a layer of crystalline silicon such as polycrystalline silicon. The first photodiode is located above the second photodiode and detects visible light, while the second photodiode detects infrared light transmitted through the first photodiode. In other words, the first photodiode in this photodetector also functions as an optical filter for reducing visible light incident on the second photodiode. 
     Meanwhile, Japanese Unexamined Patent Application Publication No. 2015-037121 discloses a solid imaging device in which second and third photoelectric converters are provided respectively on a front surface and a back surface of a single crystal Si substrate in which a first photoelectric converter is formed. The third photoelectric converter includes at part thereof an organic layer formed of quinacridone, while the second photoelectric converter includes at part thereof a silicon germanium layer. These photoelectric converters are stacked in the order of the third photoelectric converter, the first photoelectric converter, and the second photoelectric converter from the light incident side. This multilayer structure makes it possible to individually obtain a signal relating to a green wavelength region component, a signal relating to a blue wavelength region component, and a signal relating to a red wavelength region component in incident light. 
     SUMMARY 
     In one general aspect, the techniques disclosed here feature an imaging device including a semiconductor substrate including a first surface that receives light from outside, and a second surface opposite to the first surface; a first transistor located on the second surface; and a photoelectric converter that faces the second surface and that receives light transmitted through the semiconductor substrate. The semiconductor substrate is a silicon substrate or a silicon compound substrate, and the photoelectric converter includes a first electrode electrically connected to the first transistor, a second electrode, and a photoelectric conversion layer that is located between the first electrode and the second electrode and that contains a material which absorbs light having a first wavelength longer than or equal to 1.1 μm, and the material has a quantum nanostructure. 
     It should be noted that general or specific embodiments may be implemented as an element, a device, an apparatus, a system, an integrated circuit, a method, or any combination of computer programs. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic sectional view illustrating a device structure of pixels of an imaging device according to a first embodiment of the present disclosure; 
         FIG.  2    is a diagram illustrating an exemplary spectral sensitivity curve regarding a main photoelectric conversion material contained in a photoelectric conversion layer; 
         FIG.  3    is a diagram illustrating a relationship between a wavelength and a transmittance in a Si substrate; 
         FIG.  4    is a schematic plan view illustrating an example of a pixel electrode shape; 
         FIG.  5    is a schematic plan view illustrating another example of a pixel electrode shape; 
         FIG.  6    is a schematic plan view illustrating still another example of a pixel electrode shape; 
         FIG.  7    is a schematic plan view illustrating still another example of a pixel electrode shape; 
         FIG.  8    is a schematic plan view illustrating still another example of a pixel electrode shape; 
         FIG.  9    is a schematic sectional view illustrating a modification example of the imaging device according to the first embodiment of the present disclosure; 
         FIG.  10    is a diagram schematically illustrating an exemplary circuit configuration of each pixel; 
         FIG.  11    is a diagram illustrating both an example of energy bands for a material in the form of a bulk, and an example of energy bands for a material in the form of quantum dots; 
         FIG.  12    is an exemplary energy diagram for a photoelectric converter; 
         FIG.  13    is a diagram schematically illustrating a typical spectral sensitivity curve for a first layer of a photoelectric conversion film illustrated in  FIG.  12   ; 
         FIG.  14    is a diagram schematically illustrating an example of a spectral sensitivity curve for a second layer of the photoelectric conversion film illustrated in  FIG.  12   ; 
         FIG.  15    is a diagram schematically illustrating a spectral sensitivity curve for the photoelectric conversion film illustrated in  FIG.  12   ; 
         FIG.  16    is a schematic sectional view for explaining an exemplary method of manufacturing an imaging device according to the first embodiment of the present disclosure; 
         FIG.  17    is a schematic sectional view for explaining the exemplary method of manufacturing an imaging device according to the first embodiment of the present disclosure; 
         FIG.  18    is a schematic sectional view for explaining the exemplary method of manufacturing an imaging device according to the first embodiment of the present disclosure; 
         FIG.  19    is a schematic sectional view for explaining the exemplary method of manufacturing an imaging device according to the first embodiment of the present disclosure; 
         FIG.  20    is a schematic sectional view for explaining the exemplary method of manufacturing an imaging device according to the first embodiment of the present disclosure; 
         FIG.  21    is a schematic sectional view for explaining the exemplary method of manufacturing an imaging device according to the first embodiment of the present disclosure; 
         FIG.  22    is a schematic sectional view illustrating a device structure of pixels of an imaging device according to a second embodiment of the present disclosure; 
         FIG.  23    is a diagram schematically illustrating an exemplary circuit configuration of each pixel in the imaging device illustrated in  FIG.  22   ; 
         FIG.  24    is a diagram schematically illustrating another exemplary circuit configuration of each pixel in the imaging device illustrated in  FIG.  22   ; 
         FIG.  25    is a schematic sectional view illustrating a device structure of pixels in an imaging device according to a modification example of the second embodiment; 
         FIG.  26    is a schematic top view illustrating an example of a layout of microlenses, a color filter, and pixel electrodes; 
         FIG.  27    is a schematic sectional view illustrating a device structure of pixels in an imaging device according to another modification example of the second embodiment; 
         FIG.  28    is a schematic sectional view for explaining a modification example of a waveguide structure; 
         FIG.  29    is a schematic sectional view illustrating a device structure of an imaging device according to a third embodiment of the present disclosure; 
         FIG.  30    is a schematic block diagram illustrating an overall configuration of an imaging system according to a fourth embodiment of the present disclosure; 
         FIG.  31    is a flowchart illustrating an example of an operation of an imaging system; 
         FIG.  32    is a flowchart illustrating another example of an operation of the imaging system; 
         FIG.  33    is a schematic sectional view illustrating another modification example of a device structure of pixels; 
         FIG.  34    is a schematic sectional view illustrating still another modification example of a device structure of pixels; 
         FIG.  35    is a schematic sectional view illustrating an example of a lens disposed in front of microlenses on a semiconductor substrate; 
         FIG.  36    is a schematic sectional view illustrating another example of an optical member having an infrared transmissive film on a light incident surface thereof; 
         FIG.  37    is a schematic sectional view illustrating still another example of a device structure of pixels; 
         FIG.  38    is a schematic sectional view illustrating still another example of a device structure of pixels in an imaging device according to an embodiment of the present disclosure; 
         FIG.  39    is a schematic sectional view for explaining an exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  40    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  41    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  42    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  43    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  44    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  45    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  46    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; 
         FIG.  47    is a schematic sectional view for explaining the exemplary method of manufacturing the imaging device illustrated in  FIG.  38   ; and 
         FIG.  48    is a schematic sectional view illustrating still another example of a device structure of pixels in an imaging device according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     An outline of an embodiment of the present disclosure is as follows. 
     Item 1 
     An imaging device according to an item 1 of the present disclosure includes:
     a semiconductor substrate including a first surface that receives light from outside, and a second surface opposite to the first surface;   a first transistor located on the second surface; and   a photoelectric converter that faces the second surface and that receives light transmitted through the semiconductor substrate, in which   the semiconductor substrate is a silicon substrate or a silicon compound substrate,   the photoelectric converter includes
       a first electrode electrically connected to the first transistor,   a second electrode, and   a photoelectric conversion layer that is located between the first electrode and the second electrode and that contains a material which absorbs light having a first wavelength longer than or equal to 1.1 μm,   
       the first electrode is located between the second surface and the photoelectric conversion layer, and   a spectral sensitivity of the material in a wavelength region of longer than or equal to 1.0 μm and shorter than 1.1 μm is within a range of more than or equal to 0% and less than or equal to 5% of the maximum value of a spectral sensitivity of the material in a wavelength region of longer than or equal to 1.1 μm.   

     According to the structure in the item 1, since light having a wavelength shorter than 1.1 μm is absorbed by the semiconductor substrate, a component transmitted through the semiconductor substrate and having a wavelength longer than or equal to 1.1 μm enters the photoelectric converter. Since the photoelectric conversion layer may have spectral sensitivity characteristics demonstrating an absorption peak on a longer wavelength side than the wavelength of 1.1 μm, the imaging device is able to perform near-infrared narrow-band imaging without needing a band pass filter for a narrow band. In addition, having an ability to reduce noise due to light in a wavelength region of longer than or equal to 1.0 μm and shorter than 1.1 μm, the imaging device is particularly advantageous for narrow-band imaging in an infrared region. 
     Item 2 
     In the imaging device according to item 1, the material may have a quantum nanostructure. 
     With the structure in item 2, the photoelectric conversion film can be configured to demonstrate almost no absorption of the wavelength region of longer than or equal to 1000 nm and shorter than 1100 nm, which makes it possible to achieve an imaging device having a higher signal-to-noise ratio (SNR) for an infrared narrow band in particular. 
     Item 3 
     In the imaging device according to item 2, the material may be carbon nanotubes. 
     With the structure in item 3, the peak of the spectral sensitivity curve of the photoelectric conversion layer has a sharp shape due to a discrete density of states, and therefore it is possible to achieve a pixel having a specifically high sensitivity for a particular wavelength. 
     Item 4 
     In the imaging device according to item 2, the material may contain at least one selected from the group consisting of a Group III element, a Group IV element, a Group V element, and a Group VI element. 
     With the structure in item 4, the peak of the spectral sensitivity curve of the photoelectric conversion layer has a sharp shape due to a discrete density of states, and therefore it is possible to achieve a pixel having a specifically high sensitivity for a particular wavelength. 
     Item 5 
     In the imaging device according to any one of items 2 to 4, the material may absorb light having a second wavelength shorter than 1.1 μm, and the semiconductor substrate may absorb the light having the second wavelength. 
     With the structure in item 5, since the semiconductor substrate absorbs the light having the second wavelength, the sensitivity for the second wavelength can be effectively eliminated and accordingly be prevented from appearing as the sensitivity of the imaging device. 
     Item 6 
     In the imaging device according to any one of items 1 to 5, the semiconductor substrate may have a thickness of greater than or equal to 30 μm and less than or equal to 800 μm. 
     With the structure in item 6, a component having a wavelength shorter than 1.1 μm can be sufficiently attenuated by absorption in the semiconductor substrate. 
     Item 7 
     The imaging device according to any one of items 1 to 6 may further include a microlens facing the first surface. 
     Item 8 
     The imaging device according to item 7 may further include a conductive structure that is located between the photoelectric converter and the semiconductor substrate, and that electrically connects the first electrode to the first transistor, and a connecting portion between the first electrode and the conductive structure may be located outside an outer edge of the microlens when viewed in a direction normal to the first surface. 
     With the structure in item 8, since the connecting portion is located outside the outer rim of the microlens, the light blocking by the conductive structure can be avoided. 
     Item 9 
     The imaging device according to any one of items 1 to 8 may further include an insulating layer that is located between the photoelectric converter and the semiconductor substrate and that includes a waveguide structure. 
     With the structure in item 9, infrared light transmitted through the semiconductor substrate can be efficiently guided to the photoelectric converter. 
     Item 10 
     In the imaging device according to any one of items 1 to 9, the first electrode may have a transmittance of more than or equal to 80% for the light having the first wavelength. 
     With the structure in item 10, there is no need to provide an aperture, for passing through light, to the first electrode, so that a higher aperture ratio can be achieved easily. 
     Item 11 
     In the imaging device according to any one of items 1 to 10, the second electrode may have a reflectance of more than or equal to 80% for the light having the first wavelength. 
     With the structure in item 11, since light having passed through the photoelectric conversion layer can be reflected by the second electrode toward the semiconductor substrate, more light can be made to contribute to photoelectric conversion. In short, the exhibition of higher sensitivity can be expected. 
     Item 12 
     The imaging device according to any one of items 1 to 11 may further include a sealing film that covers the photoelectric converter. 
     The structure in item 12 is capable of producing an effect of preventing an oxygen from entering the photoelectric converter. 
     Item 13 
     The imaging device according to any one of items 1 to 12 may further include a pixel and the pixel may include the photoelectric converter and the first transistor. 
     Item 14 
     The imaging device according to item 13 may further include one or more photodiodes that are disposed in the semiconductor substrate and that include a first photodiode, and the pixel may further include the first photodiode. 
     With the structure in item 14, a component in a visible region in incident light can be photoelectrically converted by the first photodiode. 
     Item 15 
     The imaging device according to item 14 may further include a second transistor that is located on the second surface and that is electrically connected to the first photodiode. 
     With the structure in item 15, charges generated by the first photodiode are read by the second transistor, and thereby information on the intensity of the component in the visible region in the incident light can also be obtained. 
     Item 16 
     In the imaging device according to item 14 or 15, the one or more photodiodes may include a plurality of photodiodes and the pixel may include the plurality of photodiodes. 
     Item 17 
     The imaging device according to any one of items 13 to 16 may further include a color filter that faces the first surface, and the color filter may transmit substantially no light in a wavelength region of longer than or equal to 0.75 μm and shorter than 1.1 μm. 
     The structure in item 17 is capable of suppressing generation of unnecessary charges due to photoelectric conversion of infrared light by the first photodiode, and therefore preventing the white balance from being lost. 
     Item 18 
     The imaging device according to any one of items 1 to 16 may further include a band stop filter that faces the first surface and that transmits substantially no light in a wavelength region of longer than or equal to 0.75 μm and shorter than 1.1 μm. 
     The structure in item 18 is capable of suppressing generation of unnecessary charges due to photoelectric conversion of infrared light by the first photodiode, and therefore preventing the white balance from being lost. 
     Item 19 
     The imaging device according to any one of items 1 to 12 may further include a plurality of pixels, each of the plurality of pixels may include the photoelectric converter and the first transistor, and the photoelectric conversion layer included in the plurality of pixels may be a continuous single layer. 
     Item 20 
     An imaging system according an item 20 of the present disclosure includes
     the imaging device according to any one of items 1 to 19, and   a light source that emits light in a wavelength region of longer than or equal to 1.1 μm.   

     On an occasion where, for example, nightfall approaches or the like, the structure in item 20 performs imaging while causing the light source to emit infrared light and thereby is capable of more surely obtaining a bright image based on infrared light. 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that each of the embodiments described below illustrates a general or specific example. The numeric values, shapes, materials, constituent elements, layout and connection forms of the constituent elements, steps, the order of the steps, and so on described in the following embodiments are just examples and are not intended to limit the present disclosure. Any two or more of the various embodiments described in the present description may be combined with each other as long as no contradiction arises. Among the constituent elements described in the following embodiments, the constituent elements not described in the independent claim specifying the highest concept are described as optional constituent elements. In the following description, the same reference sign denotes constituent elements having substantially the same function, and the explanation thereof is omitted in some cases. In addition, some elements are omitted from illustration to avoid excessive complication of the drawings. 
     First Embodiment 
       FIG.  1    illustrates a device structure of pixels in an imaging device according to a first embodiment of the present disclosure.  FIG.  1    schematically illustrates a cross section of a part taken from an imaging device  100 A according to the first embodiment of the present disclosure. As illustrated, the imaging device  100 A includes multiple pixels Px two-dimensionally disposed, for example. Here, the number and layout of the pixels Px are not limited to the example illustrated in  FIG.  1    but may be any desired ones. If an imaging device includes at least one pixel Px, the imaging device can be used as a photodetector element. If the pixels Px are disposed one-dimensionally, the imaging device  100 A may be used as, for example, a line sensor. 
     The imaging device  100 A mainly includes a semiconductor substrate  110 A in which a well  110   w  is formed, and a photoelectric converter  120 . The semiconductor substrate  110 A includes a first surface  110   f  that is a light incident surface, and a second surface  110   b  located opposite to the first surface  110   f,  and the photoelectric converter  120  is located at the second surface  110   b  side of the semiconductor substrate  110 A as schematically illustrated in  FIG.  1   . An insulating layer  130 A is located between the semiconductor substrate  110 A and the photoelectric converter  120 , and the photoelectric converter  120  is disposed on the insulating layer  130 A. 
     The insulating layer  130 A typically has a multilayer structure including multiple insulating interlayers. Each of the layers constituting the insulating layer  130 A is formed of, for example, silicon dioxide. The insulating layer  130 A internally includes a multilayer wire  131 . The multilayer wire  131  is formed of, for example, Cu or the like and includes multiple wiring layers  134 , vias  136  that connect wires in the different layers to each other, and so on. 
     As illustrated in  FIG.  1   , the photoelectric converter  120  includes multiple pixel electrodes  121 , a counter electrode  122 , and a photoelectric conversion layer  123  sandwiched between the counter electrode  122  and the multiple pixel electrodes  121 . In the structure illustrated in  FIG.  1   , the counter electrode  122  is provided in the form of a single electrode continuous over the multiple pixels Px in the photoelectric converter  120 , and the photoelectric conversion layer  123  is also provided in the form of a single layer continuous over the multiple pixels Px as well. In contrast, one pixel electrode  121  is provided corresponding to each of the pixels Px. In other words, each pixel Px includes the pixel electrode  121 , a part of the counter electrode  122 , and a part of the photoelectric conversion layer  123 . In the example illustrated in  FIG.  1   , multiple microlenses  190  are disposed at the first surface  110   f  side of the semiconductor substrate  110 A so as to correspond to the respective pixels Px. 
     Each of the pixels Px further includes a part of the aforementioned semiconductor substrate  110 A. In the embodiment of the present disclosure, a substrate mainly containing silicon is used as the semiconductor substrate  110 A. Here, “mainly containing silicon” means to exclude the case where silicon is contained as a dopant. Examples of the substrate mainly containing silicon are substrates each containing silicon or a silicon compound as a main component, that is, a Si substrate, a so-called SiGe substrate containing SiGe mixed crystals, and so forth. 
     As schematically illustrated in  FIG.  1   , multiple first transistors  111  are formed in the semiconductor substrate  110 A. These first transistors  111  are typically signal detection transistors each formed in the form of an electric field-effect transistor (FET) on the second surface  110   b  side of the semiconductor substrate  110 A. Each of the multiple first transistors  111  forms, for example, a source follower, and thereby outputs an image signal. In the following description, a P-type Si substrate is described as an example of the semiconductor substrate  110 A. In this case, the multiple first transistors  111  are typically N-channel MOSFETs. 
     Each of the pixels Px includes the aforementioned first transistor  111 . The first transistor  111  in each pixel Px has an electric connection with the corresponding pixel electrode  121  of the photoelectric converter  120 . In this example, the first transistor  111  is electrically connected to the corresponding pixel electrode  121  via a conductive structure  132  located between the semiconductor substrate  110 A and the photoelectric converter  120 . The conductive structure  132  is a structure composed of a part of the aforementioned multilayer wire  131  including parts of the wiring layers  134  and the vias  136 . In the semiconductor substrate  110 A, an element isolation region that electrically isolates the first transistor  111  for each pixel Px is also formed although not illustrated in  FIG.  1   . 
     The pixel electrode  121  provided in the photoelectric converter  120  for each pixel Px may be formed of any of Al, Cu, Ti, TiN, Ta, TaN, Mo, Ru, Pt, and so on. In this example, an aperture Ap is formed at a center portion of the pixel electrode  121  such that light can pass through the pixel electrode  121  to the photoelectric conversion layer  123 . As described above, the conductive structure  132  that electrically connects the pixel electrode  121  of each pixel Px to the corresponding first transistor  111  is formed of a material that basically blocks light such as Cu. For this reason, the conductive structure  132  is disposed at a position not covering this aperture Ap when viewed in a direction normal to the semiconductor substrate  110 A. Similarly, the first transistor  111  is also typically disposed at a position not covering this aperture Ap. 
     Instead, the pixel electrode  121  may be formed of a material that transmits infrared rays such as ITO, as described later. Also in this case, the conductive structure  132  is disposed at a position where the conductive structure  132  does not block the progress of light traveling toward the photoelectric conversion layer  123  through the semiconductor substrate  110 A, for example, at a position off the center of the pixel Px. Note that, in the present description, all electromagnetic waves including infrared rays and ultraviolet rays are referred to as “light” for convenience. The pixel electrode  121  is located between the photoelectric conversion layer  123  and the second surface  110   b  of the semiconductor substrate  110 A. 
     The counter electrode  122  of the photoelectric converter  120  is located father from the semiconductor substrate  110 A than the pixel electrodes  121  are. In other words, the counter electrode  122  is located opposite to the pixel electrodes  121  across the photoelectric conversion layer  123 . 
     In the structure illustrated in  FIG.  1   , the photoelectric conversion layer  123  includes a photoelectric conversion film  124  and an electron blocking layer  125 . The electron blocking layer  125  is disposed between the photoelectric conversion film  124  and the multiple pixel electrodes  121 . The photoelectric conversion layer  123  receives incident light to generate positive and negative charge pairs in the photoelectric conversion layer  123 . For example, when a predetermined voltage is applied to the counter electrode  122  to make the potential of the counter electrode  122  higher than the potential of the pixel electrode  121 , the pixel electrode  121  can collect the positive charge of the charge pair. 
     In the embodiment of the present disclosure, a material that absorbs light with a first wavelength longer than or equal to 1.1 μm is selected as a material for the photoelectric conversion film  124 . As described in detail later, the photoelectric conversion film  124  contains a material that exhibits quantum confinement, such as carbon nanotubes or quantum dots. The material such as carbon nanotubes or quantum dots has characteristics demonstrating an absorption peak specific to each particular wavelength. Therefore, the photoelectric conversion film formed using such a material as a photoelectric conversion material demonstrates spectral sensitivity characteristics having sharp absorption peaks, unlike broad absorption characteristics demonstrated by a device formed in semiconductor crystals such as an embedded photodiode. 
     However, even if carbon nanotubes or quantum dots with discrete absorption peaks are used as a main photoelectric conversion material to be involved in the generation of charges for use as signal charges, an absorption peak due to a resonance corresponding to higher energy may exist in the wavelength region of shorter than 1.1 μm. In addition, when the photoelectric conversion layer contains an acceptor or a donor for charge separation, light having a wavelength shorter than 1.1 μm may be absorbed by the material for charge separation. Such unintended absorption in the wavelength region of shorter than 1.1 μm can be a factor that lowers the SNR in the case of performing narrow-band imaging in the wavelength region of longer than or equal to 1.1 μm. 
     However, in the structure illustrated in  FIG.  1   , the semiconductor substrate  110 A is interposed between the photoelectric converter  120  and the microlenses  190 . The first surface  110   f  of the semiconductor substrate  110 A constitutes a light-receiving surface that receives light from outside, and therefore the photoelectric conversion layer  123  receives light transmitted through the semiconductor substrate  110 A. As is well known, silicon absorbs light having a wavelength shorter than 1.1 μm corresponding to its band gap and transmits light having a wavelength longer than or equal to 1.1 μm. Therefore, the semiconductor substrate  110 A functions as a long pass filter, so that the photoelectric conversion layer  123  receives a component with the wavelength longer than or equal to 1.1 μm transmitted through the semiconductor substrate  110 A out of the light incident on the first surface  110   f  of the semiconductor substrate  110 A. 
     Furthermore, as described above, the photoelectric conversion layer  123  contains a material that exhibits quantum confinement, and therefore has spectral sensitivity characteristics that demonstrate a sharp absorption peak on a longer wavelength side than the wavelength of 1.1 μm, which is equivalent to the cut-on wavelength of the long pass filter. Therefore, when light is incident on the photoelectric conversion layer  123  via the semiconductor substrate  110 A, it is possible to obtain spectral sensitivity characteristics that specifically absorb light having a certain wavelength of more than or equal to 1.1 μm. In other words, it is possible to perform imaging for light with a specific wavelength, for example, in the near infrared region, without needing a narrow-band pass filter. 
     Here, in the technique described above in Japanese Unexamined Patent Application Publication No. 2012-099797, the first photodiode formed of amorphous silicon also functions as the optical filter. However, the silicon in the amorphous form does not demonstrate sufficient absorption in a wavelength region of 700 nm to 800 nm. More specifically, in narrow-band imaging in a wavelength region of, for example, longer than or equal to 900 nm, amorphous silicon hardly functions as a filter that cuts infrared light. As a result, even light in a wavelength region of 700 nm to 800 nm, which is not necessary for narrow-band imaging, is photoelectrically converted, leading to an increase in noise. Moreover, in the technique described in Japanese Unexamined Patent Application Publication No. 2012-099797, the second photodiode for detecting infrared light is formed in the form of a crystalline silicon layer. The crystalline silicon absorbs light up to a wavelength region of about 1100 nm. For this reason, when the crystalline silicon is used for narrow-band imaging in the wavelength region of longer than or equal to 900 nm, light with a wavelength longer than the target wavelength is also photoelectrically converted. In other words, such a broad sensitivity covering even a longer wavelength side adversely works and leads to an increase in noise. 
     In contrast, in the embodiment of the present disclosure, although light is made incident on the photoelectric conversion layer via the semiconductor substrate mainly containing silicon, a material that exhibits quantum confinement is used as the material for the photoelectric conversion film. In this case, a material that demonstrates almost no absorption in a wavelength region of, for example, longer than or equal to 1000 nm and shorter than 1100 nm can be selected as the material for the photoelectric conversion film. Thus, even if light having a wavelength longer than or equal to 1000 nm and shorter than 1100 nm is incident on the photoelectric conversion film via the semiconductor substrate, it is possible to avoid the generation of noise due to such a component. In other words, it is possible to provide an imaging device achieving a high SNR particularly for an infrared narrow band. 
     Hereinafter, detailed description will be given of the device structure of the pixels Px and the structure of the photoelectric conversion layer  123 . 
       FIG.  2    presents an exemplary spectral sensitivity curve regarding the main photoelectric conversion material contained in the photoelectric conversion layer  123 . In the example presented in  FIG.  2   , the spectral sensitivity curve of the photoelectric conversion layer  123  has a first peak Pk 1  at a first wavelength λ 1  that is a wavelength longer than or equal to 1.1 μm. The first wavelength λ 1  is a wavelength within a range of, for example, longer than or equal to 1.1 μm and shorter than or equal to 2 μm. The peak in the spectral sensitivity curve, in other words, the sensitivity peak is synonymous with the absorption peak which is the local maximum value in the absorption spectrum. The first peak Pk 1  is one of the peaks appearing in the spectral sensitivity curve, and is specified as the sensitivity peak appearing in the wavelength region of longer than or equal to 1.1 μm. 
     The first peak Pk 1  is relatively sharp, and the full width at half maximum FWHM of the first peak Pk 1  is typically shorter than or equal to 200 nm. In the embodiment of the present disclosure, the main photoelectric conversion material to be involved in the generation of charges for use as signal charges is such that the spectral sensitivity in the wavelength region of longer than or equal to 1.0 μm and shorter than 1.1 μm may be within a range of more than or equal to 0% and less than or equal to 5% of the maximum value of the spectral sensitivity in the wavelength region of longer than or equal to 1.1 μm. The above range of the spectral sensitivity in the wavelength region of longer than or equal to 1.0 μm and shorter than 1.1 μm in the main photoelectric conversion material to be involved in the generation of signal charges is advantageous particularly for narrow-band imaging in the infrared region because noise due to light in the wavelength region of longer than or equal to 1.0 μm and shorter than 1.1 μm can be reduced. A typical example of the material demonstrating such spectral sensitivity characteristics is a material that exhibits a quantum confinement effect such as carbon nanotubes or quantum dots. The above-described first peak Pk 1  may be an absorption peak corresponding to the E11 transition of carbon nanotubes, for example. Also in the case where quantum dots are used as the photoelectric conversion material for the photoelectric conversion film  124 , similar spectral sensitivity characteristics can be obtained because the density of states is discrete as in the case of carbon nanotubes. 
     In the case where a material that can exhibit quantum confinement is used as the material for the photoelectric conversion film  124 , the photoelectric conversion layer  123  can easily achieve a relatively low spectral sensitivity of about 50% or less, in particular, on a longer wavelength side than the first wavelength λ 1  at which the first peak Pk 1  appears. In other words, use of a material that can exhibit quantum confinement as the photoelectric conversion material for the photoelectric conversion film  124  brings about an advantage of no needing a filter that cuts light on the longer wavelength side than the first wavelength λ 1 . Focusing on the shorter wavelength side than the first wavelength λ 1 , the spectral sensitivity of the photoelectric conversion layer  123  as a whole may not decrease as much as in the region on the longer wavelength side than the first wavelength λ 1  due to the influence of the second optical transition or the absorption by the acceptor or donor in the photoelectric conversion layer and the absorption in the ultraviolet region by the electron blocking layer  125 . On the shorter wavelength side than the first wavelength λ 1 , it is possible to produce a region having a relatively low spectral sensitivity, more specifically, a spectral sensitivity lower than the first peak Pk 1 . 
     The spectral sensitivity curve particularly regarding the material to be involved in the generation of signal charges in the photoelectric conversion layer  123  may have multiple peaks including the first peak Pk 1 . In the example presented in  FIG.  2   , the spectral sensitivity curve has a second peak Pk 2  at a second wavelength λ 2  in addition to the first peak Pk 1 . The second wavelength λ 2  is, for example, a wavelength corresponding to the second optical transition. In this case, the first peak Pk 1  corresponds to the first optical transition of the material to be involved in the generation of signal charges, and is located at the longest wavelength position among the multiple peaks. In other words, the second peak Pk 2  is a second absorption peak among the multiple peaks when counted from the long wavelength side. Here, the first optical transition is the optical transition at the lowest energy allowed in the selection rule of the optical transition, and the second optical transition is the optical transition at the second lowest energy. 
     As schematically presented in  FIG.  2   , the second wavelength λ 2  at which the second peak Pk 2  appears may be shorter than 1.1 μm. Here, in the embodiment of the present disclosure, light transmitted through the semiconductor substrate mainly containing silicon, typically a Si substrate, is incident on the photoelectric conversion layer  123 . In addition, as described above, the silicon absorbs light having a wavelength shorter than 1.1 μm. For this reason, the light having the second wavelength λ 2  that is shorter than 1.1 μm in the light incident on the imaging device is attenuated while passing through the semiconductor substrate, and scarcely reaches the photoelectric conversion layer  123 . Even when the material for the photoelectric conversion film  124  has an absorption peak at the second wavelength λ 2 , components at and around the second wavelength λ 2  are blocked by the semiconductor substrate and do not contribute to the generation of signal charges through photoelectric conversion. Therefore, it is possible to selectively obtain a low-noise image signal based on the component having the first wavelength λ 1  in the incident light. 
       FIG.  3    presents a relationship between the wavelength and the transmittance in a Si substrate. In  FIG.  3   , curves G 1 , G 2 , G 3 , and G 4  present relationships between the wavelength and the transmittance in the cases where the thicknesses of the Si substrates are 10 μm, 30 μm, 100 μm, and 775 μm, respectively. From  FIG.  3   , it can be seen that the Si substrate having a thickness of about 30 μm or more can demonstrate a sufficient filtering function for light having a wavelength shorter than 1.1 μm. In addition, it can also be seen that even when the Si substrate has a thickness of greater than or equal to 30 μm, the Si substrate transmits nearly 100% of light having a wavelength of, for example, 1130 nm. 
     From the viewpoint of sufficiently attenuating the component having the wavelength shorter than 1.1 μm, the semiconductor substrate  110 A has a thickness of, for example, greater than or equal to 30 μm. The thickness of the semiconductor substrate  110 A is not particularly limited, but industrial Si substrates have a thickness in the range of 725±20 μm for 8 inches and 775±20 μm for 12 inches. Therefore, the semiconductor substrate  110 A may also have a thickness of less than or equal to 800 μm. As described above, the insulating layer  130 A is formed of a silicon oxide such as silicon dioxide, a silicon nitride such as silicon nitride, or the like. Even having a thickness of about 2 μm, these materials may demonstrate a transmittance of more than or equal to 90% for light in the wavelength region of longer than or equal to 1 μm. 
     Here, refer to  FIG.  1    again. As schematically illustrated in  FIG.  1   , the pixel electrodes  121  of the photoelectric converter  120  are located between the semiconductor substrate  110 A and the photoelectric conversion film  124  of the photoelectric converter  120 . In the case where the pixel electrodes  121  are formed of a material that does not transmit infrared rays such as TiN, light transmitted through the semiconductor substrate  110 A is blocked by the pixel electrodes  121  if the photoelectric conversion film  124  is covered with the pixel electrodes  121 . For this reason, in this example, the aperture Ap is provided in each pixel electrode  121 . 
       FIGS.  4  and  5    illustrate examples of the shapes of the pixel electrodes  121 .  FIGS.  4  and  5    schematically illustrate the examples of the layout of the pixel electrodes  121  and the shapes of the respective pixel electrodes  121  when viewed in the direction normal to the second surface  110   b  of the semiconductor substrate  110 A. In  FIGS.  4  and  5   , the positions of the outer rims of the microlenses disposed on the first surface  110   f  side of the semiconductor substrate  110 A corresponding to the respective pixels Px are also depicted by chain double-dashed lines. 
     Each pixel electrode  121 A illustrated in  FIG.  4    has a roughly rectangular outer shape, and has a rectangular aperture Ap in the center. In such a structure, the ratio of the area of the aperture Ap to the area of the pixel Px in plan view determines the aperture ratio of the pixel Px. As described in reference to  FIG.  1   , the pixel electrode  121 A of each pixel Px has a connection with the conductive structure  132  electrically connected to the first transistor  111 . In each pixel Px, a connecting portion between the pixel electrode and the conductive structure  132  is located outside the outer rim of the microlens  190 . The conductive structure  132  typically includes the via  136  extending in the direction normal to the semiconductor substrate  110 A between the semiconductor substrate  110 A and the photoelectric converter  120 . Thus, when the connecting portion of the conductive structure  132  connected to the pixel electrode in particular is located outside the outer rim of the microlens  190 , it is possible to avoid the blocking of light by the conductive structure  132 . 
     For example, when the pixel electrode has a rectangular outer shape, the connecting portion between the pixel electrode and the conductive structure  132  may be provided at a position in any one of the four corners of the rectangular shape. In this example, the pixel electrode  121 A includes an extended portion  121   d  that extends toward the outer edge of the pixel Px, and a connecting portion  121   c  between the pixel electrode  121 A and the conductive structure  132  is provided in a region on the extended portion  121   d.  As schematically illustrated in  FIG.  4   , the connecting portion  121   c  is formed outside the outer rim of the microlens  190 . 
     Each pixel electrode  121 B illustrated in  FIG.  5    has a roughly annular shape with an aperture Ap provided in the center. Also in this example, the connecting portion  121   c  is located outside the outer rim of the microlens  190 . Needless to say, the shape and the layout of the pixel electrodes  121  are not limited to the examples illustrated in  FIGS.  4  and  5   . The shape of the pixel electrode  121  may be a polygonal shape such as a triangle or hexagon, and the outer shape of the pixel electrode  121  and the shape of the aperture Ap may not necessarily have to be the same. However, from the viewpoint of preventing only a part of the pixel electrode from concentrically collecting charges generated by photoelectric conversion, it is advantageous that the pixel electrode  121  has a highly symmetrical shape. The multiple pixel electrodes  121  may be disposed in a triangular lattice layout. 
     In the case where the pixel electrode  121  is formed of a material that transmits light in the infrared region, the aperture Ap does not have be provided in the pixel electrode  121 .  FIGS.  6  to  8    illustrate other examples of the shapes of the pixel electrodes  121 . Each pixel electrode  121 C illustrated in  FIG.  6    includes a disk-shaped electrode portion  121 Ce and an extended portion  121   d  extending from the electrode portion  121 Ce. Each pixel electrode  121 D illustrated in  FIG.  7    includes a rectangular electrode portion  121 De and an extended portion  121   d  extending from the electrode portion  121 De. In these examples, the connecting portion  121   c  is disposed in a region on the extended portion  121   d.    
     Each pixel electrode  121 E illustrated in  FIG.  8    includes a rectangular electrode portion  121 Ee but does not include an extended portion  121   d.  As compared with the pixel electrode  121 D illustrated in  FIG.  7   , the electrode portion  121 Ee of the pixel electrode  121 E has a larger area than the electrode portion  121 De of the pixel electrode  121 D has. In the structure illustrated in  FIG.  8   , the four corners of the rectangular electrode portion  121 Ee are located outside the outer rim of the microlens  190 , and the connecting portion  121   c  is provided at a position on one of the corners of the electrode portion  121 Ee. In the case where the pixel electrode  121  is formed of a material that transmits light in the infrared region, the ratio of the area of the pixel electrode  121  to the area of the pixel Px in plan view determines the aperture ratio of the pixel Px. Thus, it is advantageous to make the area of the electrode portion as large as possible from the viewpoint of improving the SNR. When the material that transmits light in the infrared region is applied to the material for the pixel electrode  121 , the aperture Ap for passing through light does not have to be provided, so that a higher aperture ratio can be obtained easily. 
     Each of the aforementioned electrode portions  121 Ce,  121 De, and  120 Ee has a transmittance of, for example, higher than or equal to 80% for light having a wavelength longer than or equal to 1.1 μm, for example, light having the first wavelength λ 1 . Examples of the materials that transmit light in the infrared region are transparent conducting oxides (TCO) represented by ITO and ZnO. For example, in the case where the pixel electrode  121  is formed in the form of an ITO film, the transmittance of the pixel electrode  121  for infrared light can be adjusted by adjustment of the carrier density. When the carrier density is decreased, an ITO film demonstrating a transmittance of about 80% for light having a wavelength of, for example, 1500 nm can be obtained. Instead, when ZnO is formed into a thin film having a thickness of about 200 nm, for example, a transmittance of higher than or equal to 90% for light having a wavelength of 1500 nm can be obtained. In the present description, “transparent” means to transmit at least part of light in a wavelength range to be detected, and does not mean that it is essential to transmit light over the entire wavelength range of visible light. 
     On the other hand, the counter electrode  122  is not required to have a transmittance for light incident on the photoelectric converter  120 . The counter electrode  122  may be formed of a metal such as Al, TiN, or Ag. The counter electrode  122  may be formed of a material that reflects light incident on the photoelectric converter  120 . The counter electrode  122  may have a reflectance of higher than or equal to 80% for light having the first wavelength λ 1  longer than or equal to 1.1 μm. Examples of a material for the counter electrode  122  demonstrating a reflectance of higher than or equal to 80% for light having a wavelength longer than or equal to 1.1 μm are Al and Au. The Al film and the Au film demonstrate a reflectance of about 90% for light having a wavelength longer than or equal to 1.1 μm at normal incidence. When the counter electrode  122  demonstrates a high reflectance of, for example, higher than or equal to 80% for light having a wavelength longer than or equal to 1.1 μm, in particular, having the first wavelength λ 1 , the light having passed through the photoelectric conversion layer  123  can be reflected to the semiconductor substrate  110 A by the counter electrode  122 , so that a larger amount of light may contribute to the photoelectric conversion. In short, the exhibition of a higher sensitivity can be expected. 
     In the structure illustrated in  FIG.  1   , the surface of the photoelectric converter  120  on the opposite side to the semiconductor substrate  110 A, that is, the surface on the counter electrode  122  side is covered with a sealing film  140 . The sealing film  140  is formed of, for example, Al 2 O 3 , SiN, SiON (silicon oxynitride), glass, or the like, and typically has a thickness of greater than or equal to 500 nm. When the surface of the photoelectric converter  120  is covered with the sealing film  140 , it is possible to produce an effect of preventing oxygen from entering the photoelectric converter  120 , in particular, the photoelectric conversion film  124 . 
     The sealing film  140  may be a film obtained by depositing a metal such as Au, Pt, Ti, TiN, or Al in a thickness of, for example, greater than or equal to 200 nm. Containing a metal, the sealing film  140  can suppress not only the penetration of moisture into the photoelectric converter  120  but also the generation of noise due to the incidence of light from the counter electrode  122  side and the generation of noise due to heat by radiation. The sealing film  140  may be formed by depositing SiON, a metal, and SiON in this order on the counter electrode  122 . 
     In the example illustrated in  FIG.  1   , the imaging device  100 A includes a supporting substrate  150  that supports the structure including the semiconductor substrate  110 A, the insulating layer  130 A, the photoelectric converter  120 , and the sealing film  140  described above. Here, the supporting substrate  150  is not an essential element, but is provided in the imaging device  100 A as needed. For example, when the semiconductor substrate  110 A has a thickness sufficient to support the structure, the supporting substrate  150  may be omitted. 
     When the supporting substrate  150  is provided in the imaging device and the photoelectric conversion layer  123  is interposed between the insulating layer  130 A and the supporting substrate  150  as in the example illustrated in  FIG.  1   , the photoelectric conversion layer  123  can be disposed closer to the center in the thickness direction of the pixel Px in the cross sectional view. With such disposition of the photoelectric conversion layer  123 , stress applied to the photoelectric conversion layer  123  from the insulating layer  130 A can be canceled out by stress applied to the photoelectric conversion layer  123  from the supporting substrate  150 . A material for the supporting substrate  150  is not particularly limited, but a Si substrate, a silicon oxide film, a silicon nitride film, or a polymer film may be used as the supporting substrate  150 . 
       FIG.  9    schematically illustrates a modification example of the imaging device according to the first embodiment of the present disclosure. As compared with the imaging device  100 A described in reference to  FIG.  1   , an imaging device  100 K illustrated in  FIG.  9    includes a Peltier element  142  disposed on the sealing film  140 . When the Peltier element  142  is disposed on the sealing film  140  and the photoelectric converter  120  is cooled by the Peltier element  142  during the operation of the imaging device  100 K, the generation of carriers due to thermal excitation can be suppressed. As a result, the generation of dark current that may cause noise can be suppressed. In this example, the photoelectric converter can be cooled from a position closer to the photoelectric converter than in the case of a general cooling-type infrared image sensor, and therefore the photoelectric converter can be cooled more efficiently. As illustrated, the Peltier element  142  is provided for some of the multiple pixels Px. 
       FIG.  10    illustrates a typical example of a circuit configuration of each pixel Px. As schematically illustrated in  FIG.  10   , the pixel Px mainly includes the photoelectric converter  120  and a detection circuit  10  including the aforementioned first transistor  111 . In this example, the detection circuit  10  includes a selection transistor  113  and a reset transistor  115  in addition to the first transistor  111  as a signal detection transistor. The selection transistor  113  and the reset transistor  115  are typically N-channel MOSFETs formed in the second surface  110   b  of the semiconductor substrate  110 A as is the case with the first transistor  111 . 
     As schematically illustrated in  FIG.  10   , the counter electrode  122  of the photoelectric converter  120  is connected to a voltage supply circuit not illustrated in  FIG.  10   , and thereby is configured to be capable of receiving a predetermined voltage V E  applied during operation. On the other hand, the pixel electrode  121  of the photoelectric converter  120  is connected to the gate of the first transistor  111 . During operation, the drain of the first transistor  111  is supplied with a power supply voltage AVDD from a power supply not illustrated in  FIG.  10   , and thereby the first transistor  111  functions as a source follower that outputs a signal depending on the potential at a floating diffusion node FD. Here, the floating diffusion node FD is a node that electrically connects the photoelectric converter  120  and the first transistor  111  to each other. The floating diffusion node FD includes, at a portion thereof, the aforementioned conductive structure  132  and any one of the source region and the drain region of the reset transistor  115 . Here, in  FIG.  10   , a capacity of the floating diffusion node FD is depicted in the form of a capacity element C F . 
     The selection transistor  113  is connected between the source of the first transistor  111  and the corresponding one of multiple output signal lines  12  provided to respective columns of multiple pixels Px. The gate of the selection transistor  113  is connected to a vertical scanning circuit not illustrated through an address control line, and a potential Φ S  at the address control line is controlled for on/off control of the selection transistor  113 . Typically, a column signal processing circuit  14  including an analog-digital conversion circuit is connected to the output signal line  12 . The analog-digital conversion circuit may be a circuit formed in the semiconductor substrate  110 A. The column signal processing circuit  14  may further include a circuit that performs noise suppression signal processing typified by correlated double sampling. 
     In this example, the detection circuit  10  includes the reset transistor  115 . One of the source and the drain of the reset transistor  115  has a connection with the floating diffusion node FD. The other one of the source and the drain of the reset transistor  115  is connected to a reset voltage source not illustrated in  FIG.  10   , and thereby is supplied with a predetermined reset voltage V R  during operation of the imaging device. The gate of the reset transistor  115  is connected to, for example, the vertical scanning circuit through a reset control line. The vertical scanning circuit performs on/off control of the reset transistor  115  by controlling a potential Φ R  at the reset control line. 
     As can be understood from the above, the principle of signal detection in the imaging devices  100 A and  100 K can be almost the same as the principle in a so-called stacked type imaging device. Examples of the basic structure and operation of the stacked type imaging device are described in, for example, Japanese Unexamined Patent Application Publication No. 2017-188917. For reference, the entire content disclosed in Japanese Unexamined Patent Application Publication No. 2017-188917 is incorporated in the present description. 
     Next, the structure of the photoelectric conversion layer  123  is described in more detail. As can be understood from the above description in reference to  FIG.  2   , the photoelectric conversion layer  123  is capable of absorbing light with the first wavelength λ 1  longer than or equal to 1.1 μm. Here, the expression that a photoelectric conversion layer “absorbs” light with a certain wavelength does not necessarily mean that the photoelectric conversion layer has an absorptance of 100% for the light with that wavelength. 
     In the structures illustrated in  FIGS.  1  and  9   , the photoelectric conversion layer  123  includes the electron blocking layer  125  and the photoelectric conversion film  124 . The electron blocking layer  125  is formed of, for example, a metal oxide such as NiO or an organic material such as TAPC represented by the following formula (1). 
     
       
         
         
             
             
         
       
     
     The photoelectric conversion film  124  contains a material having quantum nanostructures having a quantum confinement effect. The quantum nanostructure refers to a structure that exhibits a quantum size effect one-dimensionally, two-dimensionally, or three-dimensionally. An example of a material having quantum nanostructures is carbon nanotubes. The carbon nanotube has a structure like a rolled graphene, and has a seamless substantially-cylindrical shape with a diameter in a nanometer range. The structure with one cylinder is called a single-walled carbon nanotube, and the structure with multiple cylinders nested is called a multi-walled carbon nanotube. Many of the electronic and optical properties of the single-walled carbon nanotubes are determined by the chirality specified by the chiral indexes, and the single-walled carbon nanotubes exhibit metallic or semiconducting properties depending on the chirality. 
     The energy of electrons in the single-walled carbon nanotube is specified only by the wavenumber in the axial direction of the tube due to the periodicity attributed to the cylindrical shape of the tube. That is, the state of electrons in the single-walled carbon nanotube is one-dimensional. The band structure of the single-walled carbon nanotube is unique in that energy levels (subbands) at which the density of states diverges discretely appear. Such singularities in the density of states are called Van Hove singularities. 
     The absorption spectrum of the single-walled carbon nanotubes demonstrates a sharp peak at the wavelength corresponding to the energy between these subbands. In the carbon nanotubes, the first optical transition between subbands counted from the Fermi level is the first optical transition, and the second optical transition between subbands counted from the Fermi level is the second optical transition. The second peak Pk 2  described in reference to  FIG.  2    may be an absorption peak corresponding to the E22 transition that is the second optical transition. 
     The single-walled carbon nanotubes have degrees of freedom called the chirality, and demonstrate resonance absorption at wavelengths that vary depending on the chirality as illustrated in Table 1. For example, semiconductor-type carbon nanotubes having the chiral indexes (9, 8) demonstrate resonance absorption at wavelengths around 800 nanometers and around 1.41 micrometers. Semiconductor-type carbon nanotubes having the chiral indexes (7, 6) demonstrate resonance absorption at wavelengths around 650 nanometers and around 1.13 micrometers. Here, the values of the wavelengths presented in Table 1 are merely examples, and an error on the order of 50 nanometers may occur between these values and their actually measured values. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Chirality 
                 (7, 6) 
                 (8, 7) 
                 (9, 8) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Wavelength corresponding to first 
                 1130 
                 nm 
                 1280 
                 nm 
                 1410 
                 nm 
               
               
                 optical transition (equivalent to E11 
               
               
                 transition) 
               
               
                 Wavelength corresponding to second 
                 650 
                 nm 
                 730 
                 nm 
                 800 
                 nm 
               
               
                 optical transition (equivalent to E22 
               
               
                 transition) 
               
               
                   
               
            
           
         
       
     
     A Si substrate having a thickness of greater than or equal to 30 μm transmits almost 100% of light with a wavelength of 1.41 micrometers at which the semiconductor-type carbon nanotubes having the chiral indexes (9, 8) demonstrate resonance absorption, but transmits only less than or equal to 0.1% of light with a wavelength around 800 nanometers corresponding to the second optical transition. Meanwhile, the Si substrate having the thickness of greater than or equal to 30 μm transmits almost 100% of light with a wavelength of 1.13 micrometers at which the semiconductor-type carbon nanotubes having the chiral indexes (7, 6) demonstrate resonance absorption, but transmits only less than or equal to 0.1% of light with a wavelength around 650 nanometers corresponding to the second optical transition. 
     The semiconductor-type carbon nanotubes having the chiralities presented as examples in Table 1 demonstrate almost no absorption of light in a wavelength region of around 1000 nm at which the transmittance of the silicon sharply decreases. Thus, when the photoelectric conversion film  124  is formed by using single-walled carbon nanotubes having the chirality depending on the wavelength of light to be detected, for example, it is possible to obtain pixels having high sensitivity specific to the particular wavelength. The carbon nanotubes are a photoelectric conversion material advantageous to exhibit specifically high sensitivity at a particular wavelength because a sharp absorption peak can be obtained more easily than in the case of quantum dots to be described later. The photoelectric conversion film  124  may contain multiple kinds of carbon nanotubes with different chiralities. 
     As the quantum nanostructures, it is also possible to apply nanorods, nanowires, crystals or superlattices with quantum wells, or quantum dots. As a raw material for forming the quantum nanostructures such as nanorods, nanowires, crystals or superlattices with quantum wells, and quantum dots, it is possible to select a V group semiconductor such as Si or Ge, a IV-VI group semiconductor such as PbS, PbSe, or PbTe, a III-V group semiconductor such as InAs or InSb, or a ternary mixed crystal such as HgCdTe or PbSnTe. A metal may also be applied as a raw material for forming quantum nanostructures such as quantum dots. 
     The nanorods or nanowires having a diameter of about several tens of nanometers or less and a length of several hundreds of nanometers or more demonstrate a one-dimensional quantum confinement effect. The diameter of the nanorods and nanowires varies depending on a raw material used, and is smaller than or equal to 18 nanometers in the case of PbS, smaller than or equal to 29 nanometers in the case of InAs, or smaller than or equal to 61 nanometers in the case of InSb. The length of nanorods and nanowires may have a larger value than that of the Bohr radius of a raw material used. 
     The crystals or superlattices with quantum wells demonstrate a two-dimensional quantum confinement effect. The layer thickness of crystals and the layer thickness of superlattices with quantum wells are adjusted to values equal to or smaller than the Bohr radius of a raw material used. The quantum dots demonstrate a three-dimensional quantum confinement effect. The quantum dot is a nanocrystal having a diameter of about 2 to 10 nanometers and is composed of about several tens of atoms. The surface of the quantum dot may be modified with a dispersant or a ligand. The quantum dot may be a quantum dot having a perovskite structure represented by a chemical formula APbX 3 . Here, in the chemical formula APbX 3 , A represents one selected from the group consisting of cesium, methyl ammonium, and formamidinium, and X represents chlorine, bromine, or iodine. 
     When the particle diameter of particles is equal to or smaller than approximately the Bohr radius of excitons, the excitons and electrons are confined in spaces three-dimensionally, and the density of states is discrete unlike in the case of bulk. In addition, as the particle diameter becomes smaller, this quantum confinement effect increases and accordingly a gap is enlarged as schematically illustrated in  FIG.  11   . Therefore, even if the same raw material is used, the form of quantum dots can achieve a larger energy gap than the energy gap in the bulk and the energy gap can be adjusted according to the particle diameter of the quantum dots. 
     In the quantum dots, the width of the absorption peak at the first optical transition may vary largely depending on a raw material and the particle diameter thereof. Therefore, when quantum dots are selected as a photoelectric conversion material for constituting the photoelectric conversion film  124 , for example, the wavelength at which the photoelectric conversion film  124  specifically demonstrates absorption can be preset by adjusting a raw material and the particle diameter thereof. 
     Note that the photoelectric conversion film  124  may contain an organic compound as an acceptor or an organic compound as a donor in addition to the material with the quantum confinement effect.  FIG.  12    presents an exemplary energy diagram regarding the photoelectric converter. The example illustrated in  FIG.  12    depicts a structure in the case where positive charges among positive and negative charges generated by photoelectric conversion are collected by the pixel electrode  121 . 
     In  FIG.  12   , a thick horizontal line to the left of three rectangles represents the Fermi level of the pixel electrode  121 , and a thick horizontal line to the right of the three rectangles represents the Fermi level of the counter electrode  122 . In the leftmost rectangle of the three rectangles consecutively arranged in  FIG.  12   , the bottom side represents an energy level of the highest occupied molecular orbital (HOMO) of the electron blocking layer  125 , and the side opposite to the bottom side represents an energy level of the lowest unoccupied molecular orbital (LUMO) thereof. In the structure illustrated in  FIG.  12   , a photoelectric conversion layer  123 A includes a photoelectric conversion film  124 A including a first layer  124   p  and a second layer  124   a.  The center rectangle and the right rectangle in  FIG.  12    schematically represent the heights of the energy levels of the HOMO and the LUMO of the first layer  124   p  and the second layer  124   a  in the photoelectric conversion film  124 A. 
     The first layer  124   p  in the photoelectric conversion film  124 A is a layer mainly containing a material that specifically absorbs light with the first wavelength λ 1  and is substantially formed of a material with a quantum confinement effect. On the other hand, the second layer  124   a  in the photoelectric conversion film  124 A is a layer mainly containing an acceptor organic compound and has a function to suppress deactivation of excitons due to recombination. As the acceptor organic compound, it is possible to use any organic compound as long as the organic compound has electron-accepting properties. Representative examples of the acceptor organic compound are C 60  fullerene represented by the following formula (2) and phenyl C 61  butyric acid methyl ester (PCBM) represented by the following formula (3). The values of ionization potential and electron affinity of the C 60  fullerene are about 6.4 eV and 4.0 eV, respectively. The values of ionization potential and electron affinity of the PCBM are about 6.1 eV and 3.7 eV, respectively. 
     
       
         
         
             
             
         
       
     
     The first layer  124   p  in the photoelectric conversion film  124 A generates charge pairs by absorbing light in the infrared region and thereby contributes to the generation of signal charges based on the intensity of the light in the infrared region.  FIG.  13    schematically presents a typical spectral sensitivity curve of the first layer  124   p  in the photoelectric conversion film  124 A. 
     On the other hand,  FIG.  14    schematically presents an example of a spectral sensitivity curve of the second layer  124   a  in the photoelectric conversion film  124 A. The fullerene and the PCBM do not demonstrate absorption in the infrared region, and the second layer  124   a  formed of such a material has no sensitivity in the infrared region. Instead, as can be seen from  FIG.  14   , the second layer  124   a  may have a sensitivity for visible light. The charges generated by the second layer  124   a  and collected by the pixel electrode  121  may cause noise. 
       FIG.  15    schematically presents a spectral sensitivity curve of the photoelectric conversion film  124 A including the first layer  124   p  and the second layer  124   a.  When the photoelectric conversion film contains a first material with a quantum confinement effect and a second material different from the first material, such for example as an organic compound as an acceptor or an organic compound as a donor, the spectral sensitivity characteristics of the photoelectric conversion film are a synthesis of the spectral sensitivity characteristics of the first material and the spectral sensitivity characteristics of the second material as schematically presented in  FIG.  15   . For example, the organic compound as an acceptor or the organic compound as a donor typically does not absorb light with a wavelength longer than 0.9 μm. In this example, the absorption by the second material is in a wavelength region of shorter than 1.1 μm at which the semiconductor substrate  110 A demonstrates absorption. Therefore, even if the photoelectric conversion film contains a second material, such as fullerene and PCBM, other than the material that contributes to photoelectric conversion in the infrared region, light absorption by the second material does not appear in the effective sensitivity of the photoelectric converter  120 . 
     In this way, in the embodiment of the present disclosure, the photoelectric conversion film may further contain a second material such as an acceptor, a donor, or a dispersant in addition to the first material with the quantum confinement effect. Here, although  FIG.  12    illustrates the photoelectric conversion film  124 A having the multilayer structure of the first layer  124   p  and the second layer  124   a,  the photoelectric conversion film may be formed in the form of a bulk heterojunction structure layer in which the first material with the quantum confinement effect and the second material different from the first material are mixed. Also in this case, the light absorption by the second material does not appear in the effective sensitivity of the photoelectric converter  120  as is the case with the above example. The bulk heterojunction structure is described in detail in Japanese Patent No. 5553727. For reference, the entire content disclosed in Japanese Patent No. 5553727 is incorporated in the present description. A hole blocking layer may be further disposed between the photoelectric conversion film and the counter electrode  122 . 
     Since the density of states is discrete in carbon nanotubes and quantum dots, the spectral sensitivity curve of the photoelectric conversion layer formed of any of these materials has peaks in sharp shapes, and a clear valley appears between two peaks. Two adjacent peaks are separated by a wavelength equivalent to a difference between a wavelength corresponding to the energy gap size at a certain optical transition (for example, the E22 transition) and a wavelength corresponding to the energy gap size at a different optical transition (for example, the E11 transition). For this reason, when a valley portion with relatively low sensitivity between the peaks is located at 1.1 μm that is a wavelength at which the transmittance of the silicon sharply decreases, the sensitivity due to absorption peaks other than the first peak Pk 1  can be substantially eliminated. Thus, the imaging devices  100 A and  100 K do not effectively demonstrate the sensitivity at the second wavelength λ 2  corresponding to the second peak Pk 2 . 
     As described above, even if the material of the photoelectric conversion film  124  has multiple absorption peaks, the positions of peaks other than the first peak Pk 1  are adjusted to a wavelength region of shorter than 1.1 μm, so that the sensitivity at the wavelengths corresponding to these absorption peaks can be effectively eliminated and thus prevented from appearing in the sensitivity of the imaging device. In the example illustrated in  FIG.  2   , the first peak Pk 1  is higher than the second peak Pk 2 . However, these peaks are just examples, and it does not matter that the second peak Pk 2  is higher than the first peak Pk 1 . 
     Meanwhile, focusing on a longer wavelength side than the first wavelength λ 1 , the imaging devices  100 A and  100 K do not effectively demonstrate the sensitivity for light with a wavelength exceeding a certain wavelength λ d  that is longer than the first wavelength λ 1  because the first peak Pk 1  has a sharp shape. Here, the wavelength λ d  is a wavelength determined by the spectral sensitivity characteristics of the main photoelectric conversion material contained in the photoelectric conversion layer  123 . 
     In other words, the imaging devices in the embodiment of the present disclosure selectively have the sensitivity in a particular wavelength region including the first wavelength λ 1 . In this way, according to the embodiment of the present disclosure, the main photoelectric conversion material to be involved in the generation of charges for use as signal charges, in particular, among the materials constituting the photoelectric conversion film that receives light transmitted through the semiconductor substrate has a sharp absorption peak, so that narrow-band imaging can be achieved without needing an optical filter such as a band pass filter. In particular, without employing filtering using interference, the imaging device is enabled to exhibit a preset filtering function even for oblique light. 
     Exemplary Method of manufacturing Imaging Device  100 A 
     Here, in reference to  FIGS.  16  to  21   , description will be given of an exemplary method of manufacturing the imaging device  100 A illustrated in  FIG.  1   . First, as illustrated in  FIG.  16   , a semiconductor substrate  110 S mainly containing silicon is prepared. Here, an example is described in which a P-type Si substrate is used as the semiconductor substrate  110 S. 
     Next, by ion implantation from the second surface  110   b  side, which is one of the main surfaces of the semiconductor substrate  110 S, a well  110   w,  N-type impurity regions  110   n  as the source regions or the drain regions of the transistors, an element isolation region, and so on are formed on the second surface  110   b  side as schematically illustrated in  FIG.  17   . Subsequently, the gate electrodes of the first transistor  111 , the selection transistor  113 , and the reset transistor  115  constituting the detection circuit  10  are disposed on predetermined areas of the second surface  110   b.  In  FIG.  17   , the illustration of these gate electrodes is omitted for the purpose of avoiding the complication of the drawing. 
     Then, as illustrated in  FIG.  18   , the insulating layer  130 A covering the second surface  110   b  of the semiconductor substrate  110 S is formed. The insulating film is formed by, for example, chemical vapor deposition (CVD). Typically, an insulating material such as silicon dioxide or silicon nitride is deposited in multiple steps and the multilayer wire  131  including the address control lines, the reset control lines, and the output signal lines  12  described above is formed inside the insulating layer  130 A by applying a Damascene process. In this process, contacts with circuits on the semiconductor substrate  110 S are also formed by patterning the insulating film. 
     Then, as schematically illustrated in  FIG.  19   , the photoelectric converter  120  is disposed on the insulating layer  130 A. In this example, the pixel electrodes  121  are formed near the upper surface of the insulating layer  130 A such that the pixel electrodes  121  are embedded in the insulating layer  130 A, and are connected to the conductive structures  132 . Thus, the photoelectric converter  120  includes the pixel electrodes  121  having connections with the conductive structures  132 . 
     The photoelectric converter  120  can be formed in the same manner as the photoelectric converter in the stacked-type imaging device, and any known semiconductor process can be applied to the formation of the photoelectric converter  120 . According to the embodiment of the present disclosure, the photoelectric converter  120  is disposed on the insulating layer  130 A covering the semiconductor substrate. For this reason, unlike CMOS image sensors in the related art, a material different from a semiconductor such as the silicon constituting the semiconductor substrate can be selected as a material for the photoelectric conversion film  124 . Thus, it is possible to achieve imaging at a desired wavelength irrespective of the absorption characteristics of the material for the semiconductor substrate that supports the photoelectric converter  120 . 
     Here, as illustrated in  FIG.  20   , the sealing film  140  is subsequently formed on the counter electrode  122  of the photoelectric converter  120 . Any of chemical vapor deposition and physical vapor deposition can be applied to the formation of the sealing film  140 . In this example, the supporting substrate  150  is further bonded on the sealing film  140 . 
     After that, typically, the thickness of the semiconductor substrate  110 S is reduced. For example, as schematically illustrated in  FIG.  21   , the semiconductor substrate  110 A including the first surface  110   f  and the second surface  110   b  is obtained by grinding or polishing the semiconductor substrate  110 S from the main surface side opposite to the second surface  110   b.  Thereafter, the microlenses  190  are disposed on the first surface  110   f  side of the semiconductor substrate  110 A. A protective layer for smoothing may be interposed between the microlenses  190  and the first surface  110   f  of the semiconductor substrate  110 A. The imaging device  100 A illustrated in  FIG.  1    can be obtained through the steps described above. Here, the supporting substrate  150  may be removed after the thinning of the semiconductor substrate  110 S. 
     Second Embodiment 
       FIG.  22    schematically illustrates a device structure of pixels of an imaging device according to a second embodiment of the present disclosure. As compared with the imaging device  100 A described in reference to  FIG.  1   , an imaging device  100 B illustrated in  FIG.  22    includes a semiconductor substrate  110 B in place of the semiconductor substrate  110 A. In addition, the imaging device  100 B further includes a color filter  180  between a first surface  110   f  of the semiconductor substrate  110 B and microlenses  190 . 
     As schematically illustrated in  FIG.  22   , one or more photodiodes  110 P are formed in the semiconductor substrate  110 B. These photodiodes  110 P can be formed by ion implantation or the like before the formation of the insulating layer  130 A covering the semiconductor substrate  110 B. Here, the first transistors  111  illustrated in  FIG.  22    are located in front of or behind the sheet surface (that is, the cross section) with respect to the photodiodes  110 P. 
     In the example illustrated in  FIG.  22   , one photodiode  110 P is provided for each pixel Px. The photodiode  110 P in each pixel Px is spatially and electrically isolated by an element isolation region  110   s.  The element isolation region  110   s  is formed by, for example, shallow trench isolation (STI) or ion implantation. In the present description, the “pixel Px” means a unit structure including the pixel electrode  121  and the first transistor  111 . 
     As the color filter  180 , a filter that transmits light in a wavelength region of longer than or equal to 1.1 μm is used. Thus, light having passed through the color filter  180  is incident on the photodiode  110 P in each pixel Px. Of the incident light, a component having a wavelength longer than or equal to 1.1 μm is transmitted through the semiconductor substrate  110 B, and reaches a portion of the photoelectric converter  120  below the photodiode  110 P. In other words, the photoelectric converter  120  detects the intensity of light having a first wavelength Xi longer than or equal to 1.1 μm in the incident light. 
     On the other hand, the photodiode  110 P in each pixel Px generates charges with incidence of a component having a wavelength shorter than 1.1 μm in the light having passed through the color filter  180 . As will be described later, the photodiode  110 P in each pixel Px has a connection with a second transistor not illustrated in  FIG.  22   . The second transistor is a signal detection transistor that reads charges generated by the photodiode  110 P. According to the second embodiment of the present disclosure, the second transistor reads the charges generated by the photodiode  110 P, so that it is possible to also obtain information on the intensity of a component in a visible region in the incident light. 
       FIG.  23    illustrates an example of a circuit configuration of each pixel Px of the imaging device  100 B. In the configuration illustrated in  FIG.  23   , each pixel Px includes a second detection circuit  20  connected to a second output signal line  22  in addition to the detection circuit  10 . As illustrated, the second detection circuit  20  includes a second transistor  112 , a second selection transistor  114 , and a second reset transistor  116 . In this example, the second detection circuit  20  further includes a transfer transistor  118 . This transfer transistor  118  is connected between the photodiode  110 P and the second transistor  112  as the signal detection transistor. The second transistor  112 , the second selection transistor  114 , the second reset transistor  116 , and the transfer transistor  118  are typically N-channel MOSFETs, and are formed in the second surface  110   b  of the semiconductor substrate  110 B as is the case with the transistors in the detection circuit  10 . 
     The gate of the second transistor  112  is connected to the photodiode  110 P via the transfer transistor  118 . When the drain of the second transistor  112  is supplied with a predetermined power supply voltage AVDD 2 , the second transistor  112  functions as a source follower and outputs a signal depending on the voltage applied to the gate to the second output signal line  22  through the second selection transistor  114 . The power supply voltage AVDD 2  may be a voltage different from or same as the power supply voltage AVDD supplied to the drain of the first transistor  111 . Here, the gate electrode of the second transistor  112  may be formed concurrently in the step of forming the gate electrode of the first transistor  111  described above. In other words, the gate electrode of the second transistor  112  may be in the same layer as the gate electrode of the first transistor  111 . 
     The operation of the transfer transistor  118  is controlled by a potential Φ T  at a transfer control line connected to the gate of the transfer transistor  118 . The potential Φ T  at the transfer control line is controlled by, for example, a vertical scanning circuit. Similarly, a potential Φ S   2  at a second address control line connected to the gate of the second selection transistor  114  may also be controlled by the vertical scanning circuit. In other words, the vertical scanning circuit can perform on/off control of the transfer transistor  118  and on/off control of the second selection transistor  114 . 
     One of the source and the drain of the second reset transistor  116  is electrically connected to the gate of the second transistor  112 , and a predetermined reset voltage V R   2  is applied to the other one of the source and the drain during operation. A potential Φ R   2  at a second reset control line connected to the gate of the second reset transistor  116  may also be controlled by the vertical scanning circuit. 
     In this example, the first transistor  111  is connected to the output signal line  12  and the second transistor  112  is connected to the second output signal line  22 . Thus, a signal from the first transistor  111  and a signal from the second transistor  112  can be read independently. More specifically, it is possible to individually obtain a signal relating to the intensity of light in the infrared region, in particular, having the first wavelength λ 1  and a signal relating to the intensity of visible light. 
     In the configuration illustrated in  FIG.  23   , column signal processing circuits each including an analog-digital conversion circuit may be connected to the output signal line  12  and the second output signal line  22  independently. The signal relating to the intensity of light in the infrared region, in particular, having the first wavelength λ 1  obtained by the photoelectric converter  120  and the signal relating to the intensity of visible light obtained by the photodiode  110 P may be converted to digital signals at different timings. 
       FIG.  24    illustrates another example of a circuit configuration of each pixel Px of the imaging device  100 B. In the configuration illustrated in  FIG.  24   , each pixel Px includes a detection circuit  10 B. The detection circuit  10 B has a form in which the transfer transistor  118  is further added to the aforementioned detection circuit  10 . In this example, the photodiode  110 P is further connected to the floating diffusion node FD to which the photoelectric converter  120  is connected. 
     According to the circuit configuration illustrated in  FIG.  24   , it is possible to individually obtain a signal relating to the intensity of light having the first wavelength λ 1  and a signal relating to the intensity of visible light while suppressing an increase in the size of the entire detection circuit. However, from the viewpoint of a flame rate, the circuit configuration illustrated in  FIG.  23    is more advantageous because each of the signals can be read independently. 
     MODIFICATION EXAMPLE 1 
       FIG.  25    schematically illustrates a device structure of pixels of an imaging device according to a modification example of the second embodiment. An imaging device  100 C illustrated in  FIG.  25    includes a semiconductor substrate  110 C located above the photoelectric converter  120 . The imaging device  100 C is the same as the imaging device  100 B illustrated in  FIG.  22    in that multiple photodiodes  110 P are formed in the semiconductor substrate  110 C, and second transistors  112  are connected to the photodiodes  110 P. Therefore, also in this example, it is possible to individually obtain a signal relating to the intensity of light having the first wavelength λ 1  and a signal relating to the intensity of visible light. When the example described in reference to  FIG.  22    and the imaging device  100 C illustrated in  FIG.  25    are compared, the imaging device  100 C in the example illustrated in  FIG.  25    includes multiple photodiodes  110 P for each pixel Px. 
       FIG.  26    illustrates an example of the layout of the microlenses  190 , the color filter  180 , and the pixel electrode  121  when viewed in a direction normal to the semiconductor substrate  110 C. In the structure illustrated in  FIG.  26   , the pixel Px includes four cells CE arrayed in two rows and two columns. Here, each of these cells CE has a structure including one of the aforementioned photodiodes  110 P. 
     In this example, the color filter  180  located above the photoelectric converter  120  in a certain pixel Px includes one first filter area  180   r,  two second filter areas  180   g,  and one third filter area  180   b.  The first filter area  180   r  selectively transmits light in a red wavelength region. The second filter areas  180   g  each selectively transmit light in a green wavelength region, and the third filter area  180   b  selectively transmits light in a blue wavelength region. These first filter area  180   r,  second filter areas  180   g,  and third filter area  180   b  respectively cover the corresponding photodiodes  110 P right below them among the multiple photodiodes  110 P not illustrated in  FIG.  26   . This structure is capable of obtaining, for each pixel Px, a signal relating to the intensity of infrared light and additionally, for example, a signal relating to the intensity of red light, a signal relating to the intensity of green light, and a signal relating to the intensity of blue light. 
     In this example, each pixel Px includes four microlenses  190  corresponding to the four cells CE included in the pixel Px. In the structure illustrated in  FIG.  26   , a pixel electrode  121 F coves the four cells CE and includes one aperture Ap at its center. However, the number of apertures Ap is not limited to one, and four apertures in total may be provided corresponding to the four cells CE included in the pixel Px. 
     In the example illustrated in  FIG.  26   , a connecting portion  121   c  between the pixel electrode and the conductive structure  132  is disposed at the cell CE located at the lower right position in  FIG.  26    among the four cells CE. However, the connecting portion  121   c  may be located at any position as long as the connecting portion  121   c  is not located on an optical path in the insulating layer  130 A. In the structure in which each pixel Px includes multiple cells CE as in the example illustrated in  FIG.  26   , the connecting portion  121   c  may be located at the center of the pixel Px. Also in this case, the connecting portion  121   c  is located outside the outer rims of the microlenses  190  as in the examples described in reference to  FIGS.  4  to  8   . 
     MODIFICATION EXAMPLE 2 
       FIG.  27    schematically illustrates a device structure of pixels of an imaging device according to another modification example of the second embodiment. A main difference between the examples described above and an imaging device  100 D illustrated in  FIG.  27    is that the structure illustrated in  FIG.  27    is provided with an insulating layer  130 B in which waveguide structures  135 A are formed in place of the insulating layer  130 A. 
     The waveguide structures  135 A located between the semiconductor substrate  110 C and the photoelectric converter  120  are formed of, for example, an organic material such as polyimide in which TiO is dispersed, a silicon nitride, or the like, and have a higher refractive index than the other part of the insulating layer  130 B has. The range of the refractive index of the waveguide structures  135 A varies depending on a wavelength desired to be detected through the photoelectric converter  120  but is typically equal to or higher than 1.6. 
     The waveguide structures  135 A can be formed in the following way, for example. First, as in the example described in reference to  FIG.  18   , an insulating interlayer covering the second surface  110   b  of the semiconductor substrate  110 C and a multilayer wire are formed. Then, multiple recessed portions are formed in the insulating interlayer by reactive ion etching (RIE) or the like such that the portion of the insulating interlayer where the wire is disposed can remain. These recessed potions are provided at positions corresponding to the photodiodes  110 P formed in the semiconductor substrate  110 C. The depth of the recessed portions formed in this process is within a range of, for example, about 400 nm to 600 nm. Then, a material having a higher refractive index than a material for the insulating interlayer has is deposited. As a result of depositing the high refractive index material, the inside of the recessed portions formed in the insulating interlayer is filled with the high refractive index material. The upper surface of the film of the high refractive index material is smoothed if necessary. The inside of the recessed portions may be filled with the high refractive index material after a film of an infrared reflective material such as a metal is formed on the surfaces of the recessed portions. 
     As can be seed from  FIG.  27   , these waveguide structures  135 A are provided independently of each other in the insulating layer  130 B corresponding to the respective photodiodes  110 P formed in the semiconductor substrate  110 C. The various control lines such as the address control lines, the output signal lines  12 , the second output signal lines  22 , and the conductive structures  132  described above are located in areas of the insulating layer  130 B other than the areas where the waveguide structures  135 A are formed. Protruding portions of the insulating layer  130 B that cover the wiring portions of the multilayer wire and the waveguide structures  135 A located in the recessed portions formed between these protruding portions function as claddings and cores. The structure illustrated in  FIG.  27    is capable of efficiently guiding infrared light transmitted through the semiconductor substrate  110 C to the photoelectric converter. 
     An insulating layer including waveguide structures similar to the foregoing waveguide structures  135 A may be interposed between the color filter  180  and the first surface  110   f  of the semiconductor substrate (the semiconductor substrate  110 B or the semiconductor substrate  110 C) in which the photodiodes  110 P are formed. In this case, a wire to be involved in reading signals from the photodiodes  110 P may be disposed at an area, other than the areas provided with the waveguide structures, inside the insulating layer located between the color filter  180  and the first surface  110   f.  When the waveguide structures are provided inside the insulating layer located between the color filter  180  and the first surface  110   f,  a component in a visible region in light incident on the imaging device  100 B can be efficiently guided to the photodiodes  110 P. 
     A method of forming a waveguide in an insulating interlayer is described in Japanese Unexamined Patent Application Publication Nos. 2012-114155 and 2011-243753, and others. For reference, the entire content disclosed in Japanese Unexamined Patent Application Publication No. 2012-114155 and the entire content disclosed in Japanese Unexamined Patent Application Publication No. 2011-243753 are incorporated in the present description. 
     As illustrated in  FIG.  28   , the waveguide structures  135 B may be formed by forming multiple recessed portions in the insulating interlayer covering the second surface  110   b  of the semiconductor substrate, and thereafter stacking an insulating film, a metal film, and so on inside these recessed portions. When the waveguide structure  135 B is formed in the form of the stacked films of the insulating film, the metal film, and so on, the waveguide structure  135 B can be equipped with a function as an interference-type band pass filter. The waveguide structure  135 B having a band pass filter function can complement the long pass filter function of the semiconductor substrate. 
     In an imaging device  100 E illustrated in  FIG.  28   , an element isolation region  110   s  that isolates multiple photodiodes  110 P from each other is formed in the form of a deep trench inside a semiconductor substrate  110 D. In this case, each photodiode  110 P surrounded by the element isolation region  110   s  may additionally function as a waveguide structure, so that an effect of improving the sensitivity for the visible region can be expected. In addition, light incident on a certain cell CE can be efficiently guided to an area of the photoelectric converter  120  corresponding to that cell CE, so that the sensitivity for the infrared region can also be improved. Furthermore, the effect of compensating for spot spreading due to the aberration of the microlens can be expected. 
     Such an element isolation region  110   s  may be formed by, for example, forming a through hole in the semiconductor substrate and thereafter filling the inside of the through hole with tetraethoxysilane (TEOS). With this structure, light incident on the semiconductor substrate  110 D can be reflected at the position of the side surface defining the through hole. After the formation of the through hole, for example, a SiN film may be formed on the side surface defining the through hole. As another option, the surface of the element isolation region  110   s  may be covered with a metal material that reflects visible light and/or infrared light such as Au, Al, or Ti by, for example, physical vapor deposition, so that the even higher effect can be expected. Such a structure is particularly advantageous for a structure in which one photodiode  110 P is provided for each pixel Px as in the imaging device  100 B illustrated in  FIG.  22   . 
     As schematically illustrated in  FIG.  28   , the element isolation region  110   s  may extend from the second surface  110   b  to an area deeper than the well  110   w  in the semiconductor substrate  110 D. In  FIG.  28   , the element isolation region  110   s  is depicted such that the element isolation region  110   s  is tapered from the second surface  110   b  toward the first surface  110   f  in the semiconductor substrate  110 D. Instead, the element isolation region  110   s  may have a shape tapered from the first surface  110   f  toward the second surface  110   b  in the semiconductor substrate  110 D. 
     Third Embodiment 
       FIG.  29    schematically illustrates a device structure of pixels of an imaging device according to a third embodiment of the present disclosure. An imaging device  100 F illustrated in  FIG.  29    has almost the same structure as the imaging device  100 B described in reference to  FIG.  22    except that the imaging device  100 F further includes a band stop filter  170  located on the first surface  110   f  side of the semiconductor substrate  110 B. In this example, one photodiode  110 P is disposed for each pixel Px, but it goes without saying that each pixel Px may include multiple photodiodes  110 P. 
     The band stop filter  170  has spectral transmission characteristics transmitting substantially no light in a wavelength region of longer than or equal to 0.75 μm and shorter than 1.1 μm, and transmitting light having a wavelength longer than or equal to 1.1 μm. Here, “transmitting substantially no light” means that the transmittance to light having a concerned wavelength is within a range of less than or equal to 5%. For example, a filter commercially available under the name of a band stop filter is typically capable of cutting light having a particular wavelength down to about 1% to 5%. In other words, the band stop filter  170  selectively blocks a component within a band which silicon can absorb in infrared light. 
     As the band stop filter  170 , an absorption-type filter, a film with a surface on which a dielectric multilayer film is formed (for example, a dichroic filter), or the like can be used. In addition, some of optical filters commercially available under the names of short-pass filters or IR-cut filters transmit a component having a wavelength longer than or equal to 1.1 μm. Such an optical filter can also be used as the band stop filter  170  if the optical filter has a property of transmitting substantially no light in a wavelength region of longer than or equal to 0.75 μm and shorter than 1.1 μm. The absorption-type filter is advantageous for wide angle imaging because it does not have a dependence on the light incident angle. On the other hand, when the band stop filter  170  is formed in the form of a dielectric multilayer film, it is easy to obtain a sharp step-like change between a region at which high transmittance is demonstrated and a region at which almost no transmittance is demonstrated. 
     As schematically illustrated with a thick arrow L 1  in  FIG.  29   , a component having a wavelength longer than or equal to 0.75 μm and shorter than 1.1 μm in light incident on the imaging device  100 F is blocked by the band stop filter  170 . Meanwhile, a component having a wavelength shorter than 0.75 μm and a component having a wavelength longer than or equal to 1.1 μm in the light incident on the imaging device  100 F are transmitted through the band stop filter  170  and reach the semiconductor substrate  110 B. Of these components, the former component (schematically illustrated with a thick arrow L 2  in  FIG.  29   ) is photoelectrically converted by the photodiodes  110 P in the semiconductor substrate  110 B. Then, of these components, the latter component (schematically illustrated with a thick arrow L 3  in  FIG.  29   ), that is, infrared light having a wavelength longer than or equal to 1.1 μm is transmitted even through the semiconductor substrate  110 B and the insulating layer  130 A and reaches the photoelectric converter  120 . 
     According to the third embodiment, the light in the wavelength region of longer than or equal to 0.75 μm and shorter than 1.1 μm at which silicon demonstrates absorption in the infrared light is cut by the band stop filter  170  before reaching the photodiodes  110 P. This enables suppression of the generation of unnecessary charges due to photoelectric conversion of infrared light by the photodiodes  110 P, and accordingly avoidance of inclusion of noise into signals relating to the intensity of visible light. In other words, it is possible to prevent the white balance from being lost due to photoelectric conversion of infrared light by the photodiodes  110 P. The disposition of the band stop filter  170  is not limited to the example illustrated in  FIG.  29   , and the band stop filter  170  may be disposed between the color filter  180  and the microlenses  190 . 
     Instead of disposing the band stop filter  170  on the first surface  110   f  side of the semiconductor substrate, an absorption-type filter that transmits substantially no light in the wavelength region of longer than or equal to 0.75 μm and shorter than 1.1 μm may be used as the color filter  180 . A general color filter is produced by curing a material in which a dye is dispersed in an organic material. For example, when a phthalocyanine-based near-infrared absorbing dye or the like is further dispersed in an organic material, a color filter can be obtained which selectively transmits light in a wavelength region of shorter than 0.75 μm and light in a wavelength region of longer than or equal to 1.1 μm. Instead of disposing the band stop filter  170  on the first surface  110   f  side of the semiconductor substrate, the application of such a color filter also produces the same effect of preventing the white balance from being lost as in the foregoing example. 
     Fourth Embodiment 
       FIG.  30    schematically illustrates an imaging system according to a fourth embodiment of the present disclosure. An imaging system  200  illustrated in  FIG.  30    includes an imaging device  100 , a lens optical system  210 , a signal processing circuit  220 , a system controller  230 , and a light source  240 . As the imaging device  100 , any of the foregoing imaging devices  100 B,  100 C,  100 D,  100 E, and  100 F may be applied. In the structure illustrated in  FIG.  30   , the imaging device  100  includes a photoelectric converter  120 , a semiconductor substrate  110 B including one or more photodiodes  110 P, a detection circuit  10 , and a second detection circuit  20 . 
     The lens optical system  210  includes, for example, an auto-focus lens, a zoom lens, and an aperture stop. The lens optical system  210  focuses light on an image plane of the imaging device  100 . The signal processing circuit  220  processes output signals from the detection circuit  10  and the second detection circuit  20 . The signal processing circuit  220  performs processing such for example as gamma correction, color interpolation processing, spatial interpolation processing, and automatic white balancing. Depending on the use purpose of the imaging system  200 , the signal processing circuit  220  also executes processing such as distance measurement calculation and wavelength information separation. The signal processing circuit  220  may be implemented by a digital signal processor (DSP), an image signal processor (ISP), a field-programmable gate array (FPGA), or the like. The signal processing circuit  220  may include one or more memories. 
     The system controller  230  controls the entire imaging system  200 . In this example, in particular, the system controller  230  causes the light source  240  and the imaging device  100  to operate in synchronization with each other. The system controller  230  may be implemented by, for example, a microcontroller. The system controller  230  may include one or more memories. 
     The light source  240  is an infrared light source that emits light in a wavelength region of longer than or equal to 1.1 μm. An infrared laser may be used as the light source  240 . In particular, a light source that emits light in a wavelength region around 1.4 μm, called an eye safe region, may be advantageously used as the light source  240 . As described above, the light source  240  and the imaging device  100  operate in synchronization with each other according to instructions from the system controller  230 . In some application of the imaging system  200 , the light source  240  emits illumination light R 1  toward a subject  300 , and the imaging device  100  performs imaging based on reflected light R 2  from the subject  300 . 
     As schematically illustrated in  FIG.  30   , the detection circuit  10  has a connection with the photoelectric converter  120 , thereby outputting signals based on signal charges generated by the photoelectric converter  120 , while the second detection circuit  20  has a connection with the photodiodes  110 P, thereby outputting signals based on signal charges generated by the photodiodes  110 P. That is, the imaging system  200  can obtain an image based on light with a particular wavelength in an infrared region and an image based on visible light. In the following description, the image based on the light in the particular wavelength in the infrared region is simply referred to as the “infrared image” and the image based on the visible light is simply referred to as the “visible image”. In  FIG.  30   , the semiconductor substrate  110 B, the detection circuit  10 , and the second detection circuit  20  are drawn separately from each other, but the detection circuit  10  and the second detection circuit  20  may be formed on the semiconductor substrate  110 B. The detection circuit  10 B illustrated in  FIG.  24    may be applied instead of a set of the detection circuit  10  and the second detection circuit  20 . 
       FIG.  31    presents an example of an operation of the imaging system  200 .  FIG.  31    is the example in which the imaging system  200  operates according to an event-driven program. 
     In this example, first at step S 11 , the detection circuit  10  captures an infrared image by reading signal charges from the photoelectric converter  120 . The image signal obtained in this step is a signal representing an image based on light having the first wavelength λ 1 . If necessary, the imaging may be executed by causing the light source  240  to emit light having the first wavelength λ 1  and operating the photoelectric converter  120  in synchronization with the light source  240 . The light source  240  does not have to be always turned on, but has to be turned on only during the execution of imaging. 
     Next, at step S 12 , the obtained image signal based on the infrared light is analyzed to determine whether to capture a visible image. For example, a preset threshold and a pixel value may be compared, and the processing may be advanced to step S 13  if the pixel value exceeds the threshold, or be returned to step S 11  if the pixel value is equal to or less than the threshold. Instead, a difference from the infrared image captured in the previous flame may be obtained or pattern matching may be executed to detect a moving object such as a person. Then, if such an object is detected, the processing may be advanced to step S 13 . The distance to the subject may be calculated based on a signal from the detection circuit  10 , and the processing may be advanced to step S 13  if the distance does not exceed a predetermined threshold. Such determination may be executed by the signal processing circuit  220 . 
     Next, the second detection circuit  20 , which has been turned off, is turned on at step S 13 , and captures a visible image by reading signal charges from the photodiodes  110 P at step S 14 . After capturing the visible image, the second detection circuit  20  is again turned off at step S 15 . The imaging system  200  may include a circuit that determines whether to operate the photodiodes  110 P based on an analysis result of the image signal based on infrared light as described above. The circuits on the photodiode  110 P side are activated selectively when capturing of a visible image is determined at step S 12 , which leads to saving of power consumption. 
     Thereafter, whether a turning-off command is inputted by a user is determined at step S 16 , and the processing is terminated if the turning-off of the imaging system  200  is selected. If the command to turn off the imaging system  200  is not given, the processing is returned to step S 11 , and the above steps S 12  to S 16  are iterated. 
     In the example illustrated in  FIG.  31   , the photoelectric converter  120  and the detection circuit  10  are used as an infrared sensor, and whether to capture a visible image is determined according to an output therefrom. In this way, the circuits on the photodiode  110 P side are operated as needed based on information on the distance to the subject or information on a movement of the subject, which is obtained by the photoelectric converter  120  and the detection circuit  10 , so that the imaging system  200  can capture an image containing information on colors when necessary while saving power consumption. 
     Such an application is particularly useful for nighttime surveillance. The imaging system may be further equipped with a light source that emits visible light, and may turn on the light source when determining to capture a visible image. Turning on the light source that emits light in the visible region makes it possible to capture a clearer visible image containing information on colors or to give a warning to a suspicious person. The captured visible image may be transmitted to an external server. Here, the capturing of the infrared image at step S 11  may be executed in certain cycles such as every minute or every hour. 
     Here, if carbon nanotubes having different chiralities are mixed in the photoelectric conversion film  124  of the photoelectric converter  120 , the wavelength region of light to be emitted from the light source  240  may be selected depending on the chiralities of the carbon nanotubes used in the photoelectric conversion film  124 . For example, if carbon nanotubes having an absorption peak at a wavelength of 1.2 μm and carbon nanotubes having an absorption peak at a wavelength of 1.4 μm are mixed in the photoelectric conversion film  124 , any one of 1.2 μm and 1.4 μm may be selected as the wavelength of light to be emitted from the light source  240 . In the case of indoor imaging or nighttime imaging, artificial illumination does not basically contain light in a wavelength region of longer than or equal to 1 μm. For this reason, even when the photoelectric conversion film  124  absorbs both light at the wavelength of 1.2 μm and light at the wavelength of 1.4 μm, the light source  240  is set to emit light at the wavelength of, for example, 1.2 μm, so that the generation of charges due to the absorption of light at the wavelength of 1.4 μm can be avoided and thereby a decrease in the SNR can be avoided. 
     The determination on whether to capture a visible image may be executed outside the imaging system  200 . For example, the data of the captured infrared image may be transmitted to an external server or the like, and the external server may execute the determination on whether to capture a visible image. If the imaging system  200  is installed in a vehicle such as an automobile, a visible image may be captured when a headway (time headway) to an obstacle, a preceding vehicle, a person, or the like falls down below a predetermined threshold. 
       FIG.  32    presents another example of an operation of the imaging system  200 . The operation presented in  FIG.  32    is an example where visible images are captured in a normal operation and an infrared image is captured if necessary. 
     In this example, first at step S 21 , the second detection circuit  20  reads signal charges from the photodiodes  110 P. In other words, a visible image is captured herein. The capturing of the visible image is executed, for example, in certain cycles. 
     Next, at step S 22 , whether to capture an infrared image is determined by analyzing the obtained image signals based on the visible light. For example, a preset threshold and a pixel value are compared, and the processing is advanced to step S 23  if the pixel value falls below the threshold, or is returned to step S 21  if the pixel value is equal to or more than the threshold. In the case where such a criterion is applied, capturing of an infrared image is determined, for example, when the brightness of the image decreases over all due to approaching nightfall. Such determination may be executed by the signal processing circuit  220  or the system controller  230 . 
     Next, at step S 23 , the light source  240  is turned on and the detection circuit  10 , which has been turned off, is turned on. When the light source  240  is turned on, the photoelectric converter  120  receives infrared light emitted from the light source  240  and then reflected by a subject. Since the detection circuit  10  is turned on, an infrared image based on a signal from the photoelectric converter  120  is captured at step S 24 . After the infrared image is captured, the processing is advanced to step S 25 , at which the light source  240  and the detection circuit  10  are again turned off. 
     Thereafter, whether a turning-off command is inputted by a user, for example, is determined at step S 26 . If the command to turn off the imaging system  200  is not given, the processing is returned to step S 21 . 
     In such an operation, when the brightness of the visible image decreases, the imaging system  200  is transitioned into an infrared imaging mode, so that a bright image based on infrared light can be more surely captured. In addition, unless capturing of an infrared image is determined at step S 22 , the circuits on the photoelectric converter  120  side and the light source  240  are not turned on. This enables saving of power consumption. Such an application is useful for switching between daytime and nighttime imaging modes in a surveillance camera. Thus, the imaging system  200  may include a control circuit or the like that determines whether to operate the light source  240  based on outputs from the photodiodes  110 P. 
     Other Modification Examples 
       FIG.  33    schematically illustrates another modification example of a device structure of pixels. An imaging device  100 G illustrated in  FIG.  33    includes the aforementioned pixels Px and additionally a pixel Dx provided with a light-shielding film  196  on the first surface  110   f  side of the semiconductor substrate  110 A. The light-shielding film  196  is typically formed of a metal material that does not transmit visible light such as Ti, or a metal compound material that does not transmit visible light such as TiN. Here, instead of the semiconductor substrate  110 A, the semiconductor substrate  110 B,  110 C, or  110 D including the photodiodes  110 P may be applied to the pixels Px and the pixel Dx. 
     For example, when the imaging device  100 G includes multiple pixels Px two-dimensionally arrayed, the pixel Dx is disposed outside an imaging area formed of the two-dimensional array of the pixels Px. As illustrated, the pixel Dx basically has the same photoelectric conversion structure as the pixels Px except that the light-shielding film  196  is disposed on the first surface  110   f  in place of the microlens  190 . For this reason, the first transistor  111  of the pixel Dx is capable of outputting a signal according to an amount of signal charges collected by the pixel electrode  121  of the pixel Dx. Since the light-shielding film  196  is disposed on an area for the pixel Dx in the first surface  110   f  of the semiconductor substrate  110 A, the imaging device provided with the pixel Dx is enabled to obtain a signal level under dark conditions from the first transistor  111  of the pixel Dx. 
     In the structure illustrated in  FIG.  33   , a multilayer wire  131 G disposed in the insulating layer  130 A includes, at a portion thereof, a light-shielding portion  138  that substantially covers the pixel electrode  121  of the pixel Dx. The light-shielding portion  138  is provided at a position at least partially overlapping the light-shielding film  196  in plan view. Here, “substantially covers the pixel electrode  121 ” means to cover, in plan view, a portion of the pixel electrode  121  located above an area of the photoelectric conversion film  124  which contributes to the generation of signal charges. For example, as in the example illustrated in  FIG.  33   , when the pixel electrode  121  includes an aperture Ap, the light-shielding portion  138  may be located right above the aperture Ap of the pixel electrode  121  of the pixel Dx. 
     In the example illustrated, the light-shielding portion  138  is composed of a part of a wiring layer  134 , which is one layer above the pixel electrode  121 . When the pixel electrode  121  is formed of a material that transmits light in the infrared region as in the example described in reference to  FIGS.  6  to  8   , the light-shielding portion  138  is formed in the insulating layer  130 A such that the light-shielding portion  138  covers the area of the pixel electrode  121  excluding the extended portion  121   d  provided with the connecting portion  121   c  to which the conductive structure  132  is connected. 
     As described above, the multilayer wire  131 G is formed of, for example, Cu, and the light-shielding portion  138 , which is a part of the multilayer wire  131 G, does not transmit infrared light transmitted through the semiconductor substrate  110 A. As in this example, the structure in which the light-shielding portion  138  is preliminarily disposed between the photoelectric converter  120  and the semiconductor substrate located in front of the photoelectric converter  120  makes it possible to suppress incidence of the infrared light onto a portion of the photoelectric converter  120  inside the pixel Dx even when the shielding of the infrared light by the light-shielding film  196  is insufficient. The suppression of incidence of infrared light makes it possible to prevent the signal level under dark conditions from varying due to the generation of charge pairs attributable to the incidence of infrared light. 
     The light-shielding portion  138  may be a part of the conductive structure  132  or a part of the other wire, or may be electrically floating by being isolated from the other wire and the like. The light-shielding portion  138  is not limited to one composed of a single member, but may include multiple portions disposed so as to substantially cover the pixel electrodes  121  as a whole in plan view. 
       FIG.  34    schematically illustrates still another modification example of a device structure of pixels. An imaging device  100 H illustrated in  FIG.  34    includes, for example, one or more pixels Px two-dimensionally arrayed, and a lens  212  dispose in further front of microlenses  190  provided corresponding to the respective pixels Px. The lens  212  may be one of lenses in a group constituting the lens optical system  210  (see  FIG.  30   ). 
     In the structure illustrated in  FIG.  34   , each of the pixels Px has the same structure as the pixels Px of the imaging device  100 B illustrated in  FIG.  22   . More specifically, each of the pixels Px includes the photodiode  110 P formed in the semiconductor substrate  110 B at a portion thereof, and is configured to be capable of obtaining information on the intensity of the component in the visible region in incident light though, for example, the second transistor  112  not illustrated in  FIG.  34    (see  FIG.  23   , for example). 
     As is well known, the focus position of parallel light incident on a lens in parallel to the optical axis differs depending on the wavelength. In other words, chromatic aberration occurs. Here, the “focus position” means a position of a screen on which a beam has a minimum diameter. Due to the axial chromatic aberration, the longer the wavelength of light, the farther from the lens the position at which the light converges. More specifically, infrared light having a wavelength longer than or equal to 1100 nanometers is focused on a deeper position in the pixel Px than visible light (for example, light having a wavelength of 400 nanometers to 700 nanometers) is. 
     As described above, according to the second embodiment of the present disclosure, it is possible to obtain information on the intensity of the component in the infrared region in incident light through the first transistor  111  having the connection with the photoelectric converter  120  and additionally obtain information on the intensity of the component in the visible region in the incident light through the second transistor  112 . In a typical embodiment of the present disclosure, both of the component in the infrared region and the component in the visible region in incident light enter the photoelectric converter  120  or the photodiodes  110 P through the common lens optical system and microlenses. At this time, the gap between the focus position of the visible light and the focus position of the infrared light may be longer than or equal to 1 micrometer. In an embodiment to be described later in which the substrate in which the photodiodes  110 P are formed and the substrate on which the photoelectric converter  120  is stacked are different substrates, the distance between the focus position of the visible light and the focus position of the infrared light may be longer than or equal to 30 micrometers. 
     The lens  212  is designed such that the component in the visible region in incident light is focused on the photodiodes  110 P and the component in the infrared region is focused on the photoelectric conversion film  124  located farther from the first surface  110   f  of the semiconductor substrate  110 B. For example, the lens  212  may be designed to converge light having a wavelength longer than or equal to 1.1 μm at a position apart from the focus position of visible light by longer than or equal to 1 μm. 
     In an embodiment of the present disclosure, the photoelectric converter  120  that generates signals relating to the intensity of infrared light is located farther from a subject than the photodiodes  110 P that detect visible light. With the structure in which the photoelectric converter  120  is disposed at a deeper position in the pixel Px than the photodiodes  110 P are as described above, the axial chromatic aberration is utilized rather advantageously such that the focus position of visible light can be set inside the photodiodes  110 P, while the focus position of infrared light can be set inside the photoelectric conversion film  124  as schematically illustrated in  FIG.  34    or inside the waveguide structures in the insulating layer  130 B (see  FIGS.  27  and  28   ). 
     The lenses in the group including the lens  212  in the lens optical system may be each formed of glass or a polymer. The lens optical system may include a combination of an optical member formed of glass and an optical member formed of a polymer. With the structure in which the photoelectric converter  120  is disposed at the deeper position in the pixel Px than the photodiodes  110 P are, it is possible to select a combination of materials having a small difference in refractive index as materials for different optical members in the lens optical system. This contributes to reductions in design cost and manufacturing cost of the lens optical system. In addition, since the band of infrared light is broader than the band of visible light, the focus position of infrared light may spread in a wider range than in the case of visible light. For this reason, the disposition of the photoelectric converter  120  at the deeper position in the pixels Px also produces an effect of giving a larger allowance in the optical design of the microlenses  190  regarding chromatic aberration. Note that the optical members in the lens optical system may be disposed apart from each other or some or all of them may be cemented together. 
       FIG.  35    illustrates an example of a lens structure of a lens disposed in front of the microlenses  190 . A lens  212 A illustrated in  FIG.  35    is an example of the lens  212  illustrated in  FIG.  34   , and includes a transparent lens portion  212   t  and an infrared transmissive film  212   s  disposed on a light incident surface of the lens portion  212   t.  The infrared transmissive film  212   s  selectively transmits infrared light while blocking visible light by reflecting or absorbing the visible light. The infrared transmissive film  212   s  is a functional film formed in the form of an infrared transmissive material film or a dielectric multilayer film. 
     As illustrated in  FIG.  35   , the infrared transmissive film  212   s  includes an aperture  212   ap  in the center, and annularly covers the light incident surface of the lens portion  212   t.  In other words, an area around the center of the light incident surface of the lens portion  212   t  is exposed from the infrared transmissive film  212   s.  Since the infrared transmissive film  212   s  reflects or absorbs visible light, the substantial size of the aperture stop for the component in the visible region in incident light is determined by the diameter of the aperture  212   ap , which is indicated by a double-sided arrow Dv in  FIG.  35   . On the other hand, the infrared transmissive film  212   s  transmits infrared light. Thus, when the lens  212 A is applied to the lens optical system, the aperture stop for the component in the infrared region in incident light is substantially an open aperture. In other words, use of the lens  212 A makes it unnecessary to prepare individual optical systems respective for visible light and infrared light in the wavelength region of, for example, longer than or equal to 1.1 μm, and enables different F values to be obtained for these types of light with a static structure. 
     The structure capable of obtaining different F values for visible light and infrared light as in this example is particularly useful for application in an imaging system or the like including an active light source  240  that emits infrared light to a subject in synchronization with the imaging device as illustrated in  FIG.  30   . For example, when the lens  212 A is applied, the F value obtained for infrared light is smaller than that for visible light, and accordingly a brighter image can be captured, which is advantageous for long-distance sensing. Here, it is generally known that the diffraction limit is proportional to the wavelength and the magnitude of the F value. The wavelength of infrared light is longer than that of visible light. Therefore, even though the aperture stop for the component in the infrared region is substantially the open aperture, it can be said that the influence on a decrease in the diffraction limit is small. 
     The larger the aperture stop, in other words, the smaller the F value, the smaller the depth of field in close-range imaging where the distance to a subject is short. However, imaging with infrared light is often used for long-range sensing rather than close-range imaging. In the imaging in which the focus is set for a long distance in advance, the brightness may be prioritized over the resolution. For this reason, it is acceptable that the depth of field becomes small due to the small F value. If a light source that emits eye-safe light is used as the light source  240 , a relatively large output can be easily achieved, and thus a brighter image can be easily obtained. From this viewpoint, imaging with infrared light is advantageous for long-range imaging. 
     On the other hand, imaging based on visible light is usually executed under ambient light. By using a lens including the infrared transmissive film  212   s  as illustrated in  FIG.  35   , it is possible to obtain a large F value for the visible region while obtaining the open aperture for infrared light. In other words, a large diffraction limit for visible light can be maintained. In this case, as cells including the photodiodes  110 P are disposed more densely, the resolution is enhanced. For example, as in the imaging device  100 C described in reference to  FIGS.  25  and  26   , the form in which multiple cells CE are disposed in each pixel Px is advantageous from the viewpoint of achieving a higher resolution for visible light. In the structure illustrated in  FIGS.  25  and  26   , the size of the area of the photoelectric converter  120  that contributes to imaging in the infrared region in each pixel Px is larger than the size of the photodiode  110 P, which is also advantageous for capturing bright images. In this way, the imaging in the infrared region using the photoelectric converter  120  or the imaging in the visible region using the photodiodes  110 P, which can easily achieve relatively high resolution without relying on a particular light source, may be selectively used depending on a distance to a subject. 
     Instead of the lens  212 A, a plate-shaped optical member  212 B including an infrared transmissive film  212   s  on a light incident side as illustrated in  FIG.  36    may be applied to the lens optical system. In the structure illustrated in  FIG.  36   , the infrared transmissive film  212   s  is disposed on a transparent support member  212   u  formed of glass or the like, and is also provided with an aperture  212   ap  in the center as in the example of the lens  212 A illustrated in  FIG.  35   . An outer shape of the optical member  212 B may be any, and may be, for example, a rectangular shape or a circular shape. Even with this structure, it is possible to obtain different F values for visible light and infrared light. For example, the aperture stop for the component in the infrared region in incident light may be substantially set to an open aperture. 
       FIG.  37    schematically illustrates still another modification example of a device structure of pixels. An imaging device  100 I illustrated in  FIG.  37    has almost the same structure as the imaging device  100 B illustrated in  FIG.  22    in that the imaging device  100 I includes a semiconductor substrate  110 B in which one or more photodiodes  110 P are formed, a photoelectric converter  120  disposed on a second surface  110   b  side of the semiconductor substrate  110 B, and an insulating layer  130 A located between the semiconductor substrate  110 B and the photoelectric converter  120 . The imaging device  100 B illustrated in  FIG.  22    and the imaging device  100 I illustrated in  FIG.  37    are mainly different in that the imaging device  100 I further includes a reflective film  139  provided on the second surface  110   b  of the semiconductor substrate  110 B and configured to selectively transmit infrared light while reflecting visible light. 
     The reflective film  139  reflects visible light while transmitting infrared light having a wavelength, for example, longer than or equal to 1.1 μm. When the reflective film  139  is disposed between the second surface  110   b  of the semiconductor substrate  110 B and the photoelectric converter  120 , at least part of a component in the visible region transmitted through the semiconductor substrate  110 B is reflected and returned to the photodiodes  110 P. In other words, an effect of improving an external quantum efficiency for visible light can be expected. In the structure illustrated in  FIG.  37   , the reflective film  139  is disposed on each of areas corresponding to the respective photodiodes  110 P in the second surface  110   b  of the semiconductor substrate  110 B. 
     The reflective film  139  is typically a dielectric multilayer film, and, for example, a thin film for use in an optical member which is commercially available under the name of “cold mirror” may be applied. Before the step of forming the insulating layer  130 A (see  FIG.  18   ), the reflective film  139  to be interposed between the semiconductor substrate  110 B and the insulating layer  130 A may be formed by depositing a material for the reflective film  139  on the second surface  110   b  of the semiconductor substrate  110 S and then patterning the deposited material. Needless to say, the semiconductor substrate  110 C (see  FIG.  25   ) in which multiple photodiodes  110 P are formed in each pixel Px may be applied instead of the semiconductor substrate  110 B, and the insulating layer  130 B internally including the waveguide structures  134 A or  135 B may be applied instead of the insulating layer  130 A. 
       FIG.  38    illustrates still another embodiment of the imaging device according to the embodiment of the present disclosure. A pixel Px of an imaging device  100 J illustrated in  FIG.  38    includes a part of a photoelectric converter  120  and a part of a semiconductor substrate  110 C located in front of the photoelectric converter  120  and in which multiple photodiodes  110 P are formed. 
     In the structure illustrated in  FIG.  38   , the imaging device  100 J includes the semiconductor substrate  110 C, and a semiconductor substrate  110 D internally including multiple through electrodes  133 . Multiple microlenses  190  are disposed, for example, corresponding to the respective multiple photodiodes  110 P above one main surface  110   g  of the semiconductor substrate  110 C opposite to the semiconductor substrate  110 D. In other words, light from a subject is incident on the imaging device  100 J from the one main surface  110   g  side of the semiconductor substrate  110 C. 
     An insulating layer  130 C is located between the semiconductor substrate  110 C and the semiconductor substrate  110 D. The insulating layer  130 C internally includes a multilayer wire  131 C. As schematically illustrated in  FIG.  38   , the multilayer wire  131 C in the insulating layer  130 C has a conductive structure that electrically connects each photodiode  110 P in the semiconductor substrate  110 C to the corresponding through electrode  133  in the semiconductor substrate  110 D. In addition, an insulating layer  130 D in which a multilayer wire  131 D is disposed is located between the semiconductor substrate  110 D and the photoelectric converter  120 . As illustrated, the through electrode  133  electrically connects the conductive structure at the first surface  110   f  side, which is beside the semiconductor substrate  110 C, of the semiconductor substrate  110 D to the multilayer wire  131 D at the second surface  110   b  side opposite to the first surface  110   f.    
     In the example illustrated in  FIG.  38   , multiple first transistors  111  and second transistors  112  are formed in the second surface  110   b  of the semiconductor substrate  110 D. The multilayer wire  131 D in the insulating layer  130 D includes, at portions thereof, a conductive structure  132  that electrically connects each pixel electrode  121  of the photoelectric converter  120  to the corresponding first transistor  111 , and a portion that electrically connects each through electrode  133  to the corresponding second transistor  112 . In this example, the first transistors  111  and the second transistors  112  are formed in the semiconductor substrate  110 D. Alternatively, it does not matter that the detection circuits including the first transistors  111 , the second transistors  112 , and so on are formed in the semiconductor substrate  110 C. 
     As will be described later, roughly speaking, the structure illustrated in  FIG.  38    can be obtained by bonding together a first semiconductor substrate  110 C including multiple photodiodes  110 P and a second semiconductor substrate  110 D supporting a photoelectric converter  120 . It can be said that a portion drawn under the insulating layer  130 C in  FIG.  38    has a structure similar to that of the imaging device  100 A illustrated in  FIG.  1   , except that the through electrodes  133  are formed in the semiconductor substrate  110 D and the second transistors  112  are formed on the second surface  110   b  side. 
     As the semiconductor substrates  110 C and  110 D, semiconductor substrates mainly containing silicon such as Si substrates are used as is the case with the foregoing semiconductor substrates  110 A and  110 B. Out of light incident on the main surface  110   g  of the semiconductor substrate  110 C, a component having a wavelength shorter than 1.1 μm enters the photodiodes  110 P and thereby generates signal charges based on the intensity of the light having the wavelength shorter than 1.1 μm (typically, the intensity of visible light). The second transistors  112  electrically connected to the photodiodes  110 P detect the signal charges based on the intensity of the light having the wavelength shorter than 1.1 μm. 
     Here, the semiconductor substrate  110 C may typically have a thickness smaller than that of the semiconductor substrate  110 B in the structure described in reference to  FIG.  22   . For this reason, the distance between the main surface  110   g  and the photodiodes  110 P in the semiconductor substrate  110 C of the structure illustrated in  FIG.  38    is typically smaller than the distance between the first surface  110   f  and the photodiodes  110 P in the semiconductor substrate  110 B of the structure illustrated in  FIG.  22   . In other words, the photodiodes  110 P of the imaging device  100 J illustrated in  FIG.  38    are located at relatively shallow portions that are closer to the surface of the semiconductor substrate on which the color filter  180  and so on are disposed than in the imaging device  100 B illustrated in  FIG.  22   . Thus, with the structure illustrated in  FIG.  38   , the reduced thickness of the semiconductor substrate makes it possible to suppress attenuation in the amount of light incident on the photodiodes  110 P. In other words, the sensitivity for the visible region can be improved. 
     With the reduced thickness of the semiconductor substrate, a portion passing through the photodiodes  110 P out of light having the wavelength shorter than 1.1 μm may be increased. However, such a component is sufficiently attenuated in the course of passing through the insulating layer  130 C, the semiconductor substrate  110 D, and the insulating layer  130 D before reaching the photoelectric converter  120 . In particular, having a thickness of, for example, about 30 μm or more, the semiconductor substrate  110 D functions as a filter that blocks light having a wavelength shorter than 1.1 μm. 
     On the other hand, out of light incident on the main surface  110   g  of the semiconductor substrate  110 C, a component having a wavelength longer than or equal to 1.1 μm is transmitted through the semiconductor substrate  110 C, the insulating layer  130 C, the semiconductor substrate  110 D, and the insulating layer  130 D and reaches the photoelectric conversion layer  123  through the apertures Ap of the pixel electrodes  121 . The main photoelectric conversion material for the photoelectric conversion layer  123  absorbs light having the first wavelength λ 1 , so that an image signal relating to the intensity, in particular, the first wavelength λ 1  longer than or equal to 1.1 μm can be obtained as in the examples described above. 
     Hereinafter, a method of manufacturing the imaging device  100 J illustrated in  FIG.  38    will be briefly described. First, as illustrated in  FIG.  39   , a semiconductor substrate  110 T mainly containing silicon is prepared. Here, a P-type Si substrate is illustrated as the semiconductor substrate  110 T. 
     Next, by ion implantation or the like from one main surface  110   h  side of the semiconductor substrate  110 T, the well  110 W, the photodiodes  110 P, and the element isolation regions  110   s,  and so on are formed on the main surface  110   h  side as schematically illustrated in  FIG.  40   . In this process, the first transistors  111  and/or the second transistors  112  may be formed on the main surface  110   h  side. 
     Next, the insulating layer  130 C covering the main surface  110   h  of the semiconductor substrate  110 T is formed as illustrated in  FIG.  41    in the same way as in the step of forming the insulating layer  130 A described in reference to  FIG.  18   . In this process, the multilayer wire  131 C having electric connections with the photodiodes  110 P is formed inside the insulating layer  130 C. As in the example of the imaging device  100 I described in reference to  FIG.  37   , a reflective layer that reflects visible light while transmitting infrared light having a wavelength longer than or equal to 1.1 μm may be provided on the main surface  110   h  of the semiconductor substrate  110 T. In this case, the reflective film  139  may be formed on the main surface  110   h  of the semiconductor substrate  110 T before the insulating layer  130 C and the multilayer wire  131 C are formed. 
     Subsequently, the thickness of the semiconductor substrate  110 T is reduced by grinding or polishing the semiconductor substrate  110 T from the main surface side opposite to the main surface  110   h  as schematically illustrated in  FIG.  42   . The thinned semiconductor substrate  110 T, in other words, the semiconductor substrate  110 C may have a thickness smaller than that of the semiconductor substrate  110 D. By thinning the semiconductor substrate  110 T, the above-mentioned main surface  110   g  is formed on the side opposite to the main surface  110   h.  After that, if necessary, a passivation film is formed on the insulating layer  130 C. In this process, for electric connections with the through electrodes  133  to be described later, parts of the passivation film are removed by patterning and parts of the multilayer wire  131 C are exposed from the passivation film. 
     Further, separately from the semiconductor substrate  110 T, a semiconductor substrate  110 U is prepared which has a first surface  110   f  and a second surface  110   b  opposite to the first surface  110   f,  and mainly contains silicon. Here, a P-type Si substrate is illustrated as the semiconductor substrate  110 U. Next, as illustrated in  FIG.  43   , the well, the first transistors  111 , the second transistors  112 , the element isolation regions, and so on are formed on the second surface  110   b  side of the semiconductor substrate  110 U as in the example described in reference to  FIG.  17   . Thus, the aforementioned semiconductor substrate  110 D is obtained from the semiconductor substrate  110 U. In this process, a junction capacitance for temporarily holding signal charges generated by the photoelectric converter  120  may be formed on the second surface  110   b  side of the semiconductor substrate  110 U. 
     Next, as in the example described in reference to  FIG.  18   , the insulating layer  130 D covering the transistors and the like on the second surface  110   b  of the semiconductor substrate  110 D and the multilayer wire  131 D having electric connections with the first transistors  111  and the second transistors  112  are formed on the second surface  110   b  as illustrated in  FIG.  44   . Further, the pixel electrodes  121  are formed in the vicinity of the upper surface of the insulating layer  130 D. 
     Subsequently, the photoelectric conversion layer  123  and the counter electrode  122  are formed in this order on the pixel electrodes  121  as in the example described in reference to  FIG.  19   , so that the photoelectric converter  120  is formed on the insulating layer  130 D as illustrated in  FIG.  45   . Typically, the sealing film  140  covering the counter electrode  122  is further formed in the form of an insulating film or a metal film. 
     Subsequently, as illustrated in  FIG.  46   , the through electrodes  133  are formed in the semiconductor substrate  110 D. A known semiconductor process may be applied to the formation of the through electrodes  133 . Note that the through electrodes  133  may be disposed inside the pixels Px or the through electrodes  133  may be disposed in an area outside an array of multiple pixels Px instead of being provided inside the pixels Px. 
     Next, as illustrated in  FIG.  47   , the structure illustrated in  FIG.  42    and the structure illustrated in  FIG.  46    are united together by bonding. In the bonding, the first surface  110   f  of the semiconductor substrate  110 D is opposed to the insulating layer  130 C, and the multilayer wire  131 C in the insulating layer  130 C is electrically connected to the through electrodes  133  provided in the semiconductor substrate  110 D. Thus, the photodiodes  110 P in the semiconductor substrate  110 C are electrically connected to the corresponding second transistors  112  through the multilayer wire  131 C in the insulating layer  130 C, the through electrodes  133  in the semiconductor substrate  110 D, and the multilayer wire  131 D in the insulating layer  130 D. The bonding of the structure illustrated in  FIG.  42    and the structure illustrated in  FIG.  46    may use any of known methods such as diffusion bonding, normal temperature bonding, or anodic bonding. Here, the electric connections between the photodiodes  110 P in the semiconductor substrate  110 C and the second transistors  112  are not limited to the bonding via the through electrodes  133  but may be connections via Cu bonding or the like. Also, the way of connecting the structure illustrated in  FIG.  42    and the structure illustrated in  FIG.  46    is not limited to a particular method. 
     Typically, the color filter  180  and the microlenses  190  are further disposed on the main surface  110   g  side of the semiconductor substrate  110 C. Through the aforementioned steps, the imaging device  100 J illustrated in  FIG.  38    is obtained. 
     As in an imaging device  100 L illustrated in  FIG.  48   , the pixel Dx having the light-shielding film  196  may be formed on the main surface  110   g  side of the semiconductor substrate  110 C. As in the example described in reference to  FIG.  33   , the light-shielding film  196  has a function to block visible light. 
     In the structure illustrated in  FIG.  48   , in addition to the light-shielding film  196  located above the semiconductor substrate  110 C, a light-shielding portion  138   c  and a light-shielding portion  138   d  substantially covering the pixel electrode  121  of the pixel Dx in plan view are further disposed in the insulating layer  130 C and the insulating layer  130 D, respectively. The light-shielding portion  138   c  is, for example, a part of the multilayer wire  131 C, and the light-shielding portion  138   d  is, for example, a part of the multilayer wire  131 D. The multilayer wires  131 C and  131 D are formed of, for example, Cu, and therefore these light-shielding portions  138   c  and  138   d  neither transmit infrared light transmitted through the semiconductor substrate  110 C nor typically transmit visible light either. 
     When the light-shielding portion(s) is disposed in the insulating layer  130 C and/or the insulating layer  130 D in addition to the light-shielding film  196  located above the semiconductor substrate  110 C, the incidence of infrared light on the portion inside the pixel Dx in the photoelectric converter  120  can be suppressed, which makes it possible to avoid variations in the signal level under dark conditions. When the light-shielding portion  138   c  and the light-shielding portion  138   d  are formed in the insulating layer  130 C and the insulating layer  130 D, respectively, and these light-shielding portions are disposed one on top of the other when viewed in plan view, infrared light transmitted through the semiconductor substrate  110 C can be more effectively prevented from entering the portion inside the pixel Dx in the photoelectric converter  120 .