Patent Publication Number: US-2023164462-A1

Title: Photodetector, solid-state image sensor, and method of manufacturing photodetector

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of PCT International Application No. PCT/JP2021/025919 filed on Jul. 9, 2021, designating the United States of America, which is based on and claims priority of Japanese Patent Application No. 2020-129154 filed on Jul. 30, 2020. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to a photodetector, a solid-state image sensor including the photodetector, and a method of manufacturing the photodetector. 
     BACKGROUND 
     In recent years, photon counting photodetectors utilizing avalanche photodiodes (APDs) have been developed as one type of photodetectors that detect weak light. Patent literatures (PTLs) 1 to 3 each discloses a technique related to a photodiode. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. 2019-180048 
         PTL 2: Japanese Unexamined Patent Application Publication No. 2004-20957 
         PTL 3: Japanese Unexamined Patent Application Publication No. 2010-141358 
       
    
     SUMMARY 
     Technical Problem 
     The present disclosure provides a photodetector, for example, which can be manufactured more simply than before and exhibits a higher light collection efficiency at a photoelectric converter. 
     Solution to Problem 
     A photodetector according to one aspect of the present disclosure includes: a semiconductor substrate; a photoelectric converter in the semiconductor substrate; and a condenser light-transmissive and opposed to the photoelectric converter. The condenser includes: a first inorganic material layer at least partially overlapping the photoelectric converter in a plan view; and a second inorganic material layer covering the first inorganic material layer and having a refractive index lower than a refractive index of the first inorganic material layer. 
     A solid-state image sensor according to one aspect of the present disclosure includes: a pixel array obtained by arranging photodetectors, each being the photodetector described above, in a matrix; and a readout circuit that reads a signal output from the pixel array. 
     A method of manufacturing a photodetector according to one aspect of the present disclosure includes: forming a photoelectric converter and a pixel separator around the photoelectric converter in a semiconductor substrate in a plan view of the semiconductor substrate; and forming a condenser opposed to the photoelectric converter and including a first inorganic material layer and a second inorganic material layer with a refractive index lower than a refractive index of the first inorganic material layer by: forming the first inorganic material layer to overlap the photoelectric converter at least partially in a plan view; and forming the second inorganic material layer to cover the first inorganic material layer. 
     Advantageous Effects 
     The present disclosure provides a photodetector, for example, which can be manufactured more simply and exhibits a higher light collection efficiency at a photoelectric converter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG.  1    is a cross-sectional view showing a photodetector according to an embodiment. 
         FIG.  2    is a plan view showing a condenser according to the embodiment. 
         FIG.  3 A  is a first cross-sectional view showing a method of manufacturing the photodetector according to the embodiment. 
         FIG.  3 B  is a second cross-sectional view showing the method of manufacturing the photodetector according to the embodiment. 
         FIG.  3 C  is a third cross-sectional view showing the method of manufacturing the photodetector according to the embodiment. 
         FIG.  3 D  is a fourth cross-sectional view showing the method of manufacturing the photodetector according to the embodiment. 
         FIG.  4 A  is a graph showing a relationship between a material of an inorganic material layer included in the photodetector according to the present disclosure and the light collection efficiency. 
         FIG.  4 B  is a graph showing a relationship between a width of the inorganic material layer included in the photodetector according to the present disclosure and the light collection efficiency. 
         FIG.  5    is a cross-sectional view showing a photodetector according to Variation 1 of the embodiment. 
         FIG.  6    is a cross-sectional view showing a photodetector according to Variation 2 of the embodiment. 
         FIG.  7    is a cross-sectional view showing a photodetector according to Variation 3 of the embodiment. 
         FIG.  8    is a cross-sectional view showing a photodetector according to Variation 4 of the embodiment  FIG.  9    is a plan view showing a condenser according to Variation 1. 
         FIG.  10    is a plan view showing a condenser according to Variation 2. 
         FIG.  11    is a plan view showing a condenser according to Variation 3. 
         FIG.  12    shows a solid-state image sensor according to an embodiment. 
         FIG.  13    shows a layout of a plurality of photodetectors included in the solid-state image sensor according to the embodiment. 
         FIG.  14    is a cross-sectional view showing an example photodetector included in the solid-state image sensor according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     (Underlying Knowledge Forming Basis of the Present Disclosure) 
     In recent years, photon counting photodetectors have been developed which utilize APDs as photodetectors for detecting weak light. APDs are each a photodiode that multiplies a photocurrent upon application of a predetermined reverse voltage. It is an objective of a photodetector including, as a photodetector, a photodiode such as an APD to improve light collection efficiency at a photoelectric converter. At present, an on-chip lens or a gradient index lens as shown in PLTs 1 to 3 described above is suggested to improve the light collection efficiency at a photoelectric converter. 
     However, these lenses require advanced lithography techniques and microfabrication at a higher aspect ratio and simultaneous formation of a plurality of materials, and are thus difficult to form. Accordingly, a simple manufacturing method is necessary as a technique of improving light collection characteristics of a photodetector with pixels to be further miniaturized. There is thus a room for reviewing the technique of miniaturizing pixels and improving light collection efficiency. 
     Now, an embodiment of photodetectors, for example, will be described in detail with reference to the drawings. The embodiment that will be described below are general or specific examples. The numerical values, shapes, materials, constituent elements, the arrangement and connection of the constituent elements etc. shown in the embodiment below are thus mere examples, and are not intended to limit the scope of the present disclosure. 
     The figures are schematic representations and not necessarily drawn strictly to scale. The same reference signs represent substantially the same configurations in the drawings and redundant description will be omitted or simplified. 
     The drawings used for the detailed description of the following embodiment may show coordinate axes. Out of the coordinate axes, a Z-axis extends, for example, in a stacking direction and in a vertical direction. The positive side of the Z-axis may be referred to as an “upper” side, while the negative side of the Z-axis may be referred to as a “lower” side. In other words, the Z-axis is vertical to the main surface of a semiconductor substrate with a photoelectric converter (i.e., a condenser), and may also be expressed as the “stacking direction” or a “thickness” direction of the semiconductor substrate. X- and Y-axes are orthogonal to each other on a plane (e.g., horizontal plane) vertical to the Z-axis. 
     In the embodiment below, a “plan view” means that the photodetectors are seen along the Z-axis. In other words, in the embodiment below, the “plan view” means that the photodetector is seen along a normal of the main surface of the semiconductor substrate. 
     The present disclosure does not exclude a structure with conductivities inverted from those described below in the embodiment. Specifically, all the p- and n-types described below may be inverted. 
     Embodiment 
     [Configuration of Photodetector] 
     Now, a configuration of a photodetector according to an embodiment will be described in detail with reference to the drawings. 
       FIG.  1    is a cross-sectional view showing photodetectors  10  according to the embodiment.  FIG.  1    shows parts of photodetectors  10  corresponding unit pixels in an enlarged manner. As well as  FIG.  1   ,  FIGS.  3 A to  3 D,  5 , and  6   , which will be described, show three photodetectors including those shown partially on both sides. 
     Photodetectors  10  photoelectrically convert incident light (also referred to as “external light”) into electricity and output the electricity. Photodetectors  10  are mainly targeted at near-infrared light. The “near-infrared light” has a wavelength within a range from 750 nm to 1400 nm, for example. 
     As shown in  FIG.  1   , photodetectors  10  include semiconductor substrate  100 , multilayer  200 , and condenser  300 . 
     Semiconductor substrate  100  is made of silicon (Si), for example. Semiconductor substrate  100  may have a p- or n-type conductivity. 
     In the following description, the upper surface (i.e., the main surface) of semiconductor substrate  100  will also be referred to as a “light incident surface” or a “light receiving surface”. 
     Semiconductor substrate  100  includes photoelectric converters  101  and pixel separators  102  for separating adjacent photoelectric converters  101  into unit pixels. 
     Photoelectric converters  101  are located in relatively upper positions of semiconductor substrate  100  (specifically, near main surface  110  of semiconductor substrate  100 ) and convert incident external light into electricity, in other words, convert light into signal charges. Photoelectric converters  101  are photodiodes. The photodiodes here include avalanche photodiodes. Photoelectric converters  101  are obtained, for example, by implanting ions in the silicon substrate. 
     Pixel separators  102  are provided for separating pixels including photoelectric converters  101  and isolation regions (i.e., insulating regions) arranged alternately with photoelectric converters  101  in semiconductor substrate  100 . Pixel separators  102  are interposed between photoelectric converters  101  included in adjacent photodetectors  10 . For example, in a plan view, pixel separators  102  are located around photoelectric converters  101  in semiconductor substrate  100 . Pixel separators  102  are obtained, for example, by implanting ions in the silicon substrate. 
     Placed on main surface  110  of semiconductor substrate  100  is multilayer  200 . 
     Multilayer  200  is for taking out charges (i.e., signal charges) generated when photoelectric converters  101  convert the external light incident thereon into electricity. Multilayer  200  includes interconnect layers (interconnects)  201 , for example. More specifically, multilayer  200  includes a plurality of interconnect layers (interconnects)  201  and a plurality of interlayer insulating films  202 , a plurality of liner layers  203  including a liner layer (i.e., an uppermost liner layer)  205 , and a plurality of vias  204 . 
     Out of the plurality of liner layers  203 , liner layer  205  is the uppermost liner layer. Specifically, liner layer  205  is the uppermost one of the plurality of liner layers  203  and located above one of interconnect layers  201 . 
     Multilayer  200  has a height (i.e., a height from main surface  110  of semiconductor substrate  100  to the upper surface of multilayer  200 , in other words, to lower layer  310  of condenser  300 ) of 1.0 μm, for example. 
     Formed in interconnect layers  201  are interconnects constituting circuits included in photodetectors  10 , for example. Interconnect layers  201  are for transmitting the charges generated by photoelectric converters  101 , for example, to readout circuit  505  which will be described later or other circuits. Interconnect layers  201  are made of copper (Cu), for example. Alternatively, interconnect layers  201  may be made of metal, such as aluminum (Al) or tungsten (W), other than copper. 
     In a plan view, interconnect layers  201  overlap pixel separators  102 . In this embodiment, interconnect layers  201  are arranged right above pixel separators  102 . 
     Interlayer insulating films  202  are interposed between sets of interconnect layers  201  to insulate interconnect layers  201  from each other. Interlayer insulating films  202  are silicon oxide (SiOx) or carbon-doped silicon oxide (SiOC), for example. 
     Liner layers  203  are interposed between a plurality of interlayer insulating films  202  or on interlayer insulating films  202  so as to stop etching at the time of manufacturing photodetectors  10  and/or reduce diffusion of metal atoms from interconnect layers  201 . In this embodiment, interlayer insulating films  202  and liner layers  203  are stacked alternately. Liner layers  203  are made of silicon oxycarbide (SiCO) or silicon carbonitride (SiCN), for example. 
     Vias  204  are through-electrodes for electrically connecting sets of interconnect layers  201 . Vias  204  are made of copper (Cu), for example. Alternatively, vias  204  may be made of metal, such as aluminum (Al) or tungsten (W), other than copper. 
     Located above multilayer  200  is condenser  300 . In this manner, photodetectors  10  according to this embodiment include semiconductor substrate  100 , multilayer  200 , and condenser  300  stacked in this order. That is, photodetectors  10  are each what is called a “front side illumination (FSI)” photodetector including multilayer  200  of sets of interconnect layers  201  and interlayer insulating films  202  between the sets of interconnect layers  201 , on main surface  110  of semiconductor substrate  100 . 
     Condenser  300  is an optical member that collects external light incident from photodetectors  10  and emits the collected light toward photoelectric converters  101 . Condenser  300  is light-transmissive (i.e., transmissive to light). In this embodiment, photodetectors  10  are for detecting near-infrared external light (e.g., with a wavelength within a range from about 750 nm to about 1400 nm). In this embodiment, condenser  300  is thus transmissive to (e.g., characterized to transmit 90% or more) light with a wavelength within the range from 750 nm to 1400 nm. 
     Condenser  300  includes inorganic material layer (or first inorganic material layer)  301  and inorganic material layer (or second inorganic material layer)  302 . 
     Specifically, condenser  300  are opposed to photoelectric converters  101  and includes inorganic material layers  301 , and inorganic material layers  302  with a refractive index lower than that of inorganic material layers  301 . Inorganic material layers  301  are arranged to overlap photoelectric converters  101  at least partially in a plan view. Inorganic material layers  302  are arranged to cover inorganic material layers  301 . In this embodiment, for example, inorganic material layers  301  each has an outer edge more inward than corresponding one of interconnect layers  201  that overlap pixel separators  102  in a plan view. For example, inorganic material layers  302  each includes groove  350  recessed toward semiconductor substrate  100  above corresponding one of interconnect layers  201  that overlap pixel separators  102  of semiconductor substrate  100  around photoelectric converters  101  in a plan view. 
     For example, in an alignment of a plurality of photodetectors  10 , interconnect layers  201  are located below the gaps between adjacent inorganic material layers  301 . 
     Liner layer (uppermost liner layer)  205  has a refractive index lower than that of inorganic material layers  301  and higher than that of inorganic material layers  302 . 
     In this embodiment, condenser  300  is in the form of a layer (i.e., film) including inorganic material layers  301  on multilayer  200 , and inorganic material layers  302  in the form of a layer to cover inorganic material layers  301 . 
     Inorganic material layers  301  are made of a light-transmissive inorganic material. Inorganic material layers  301  are located above photoelectric converters  101 . Specifically, inorganic material layers  301  overlap photoelectric converters  101  at least partially in a plan view. In this embodiment, entire inorganic material layers  301  overlap photoelectric converters  101  in a plan view. The outer edges of inorganic material layers  301  are located more inward than interconnect layers  201  not to overlap interconnect layers  201  in a plan view. The outer edge of each inorganic material layer  302  includes groove  350  above corresponding one of interconnect layers  201  in a plan view. Inorganic material layers  301  are separated from (i.e., not in contact with) adjacent inorganic material layers  301 . For example, in a plan view, separation grooves  330  for separating inorganic material layers  301  from adjacent inorganic material layers  301  are formed on the outer peripheries of inorganic material layers  301  above interconnect layers  201 . 
     Separation grooves  330  are for separating adjacent inorganic material layers  301 . In this embodiment, inorganic material layers  302  fill separation grooves  330 . 
     Inorganic material layers  302  are made of a light-transmissive inorganic material. Inorganic material layers  302  have a refractive index lower than that of inorganic material layers  301 . Specifically, the inorganic material of inorganic material layers  302  has a lower refractive index than the inorganic material of inorganic material layers  301 . In this embodiment, since photodetectors  10  are for detecting near-infrared external light, and inorganic material layers  302  have a lower refractive index than inorganic material layers  301  with respect to near-infrared external light. 
     Inorganic material layers  301  have a refractive index higher than that of inorganic material layers  302  and fall within a range from 1.6 to 2.5, for example. 
     Inorganic material layers  301  are films containing Si and at least any of O, N, or C, for example, or films containing Ti and O. Specifically, inorganic material layers  301  are made of silicon nitride (SiN), silicon-oxynitride (SiON), SiCN, SiCO, or titanium oxide (TiO x ), where x is one or two. 
     The refractive index of inorganic material layers  302  falls within a range from 1.3 to 1.6, for example. 
     Inorganic material layers  302  are films containing Si and at least any of O or C, for example. Specifically, inorganic material layers  302  are made of SiOx or SiOC, for example, where x is one or two. 
     Inorganic material layers  302  cover inorganic material layers  301 , specifically, the upper and side surfaces of inorganic material layers  301  and arranged in contact with inorganic material layers  301 . 
     Inorganic material layers  302  are in contact with (i.e., continuous with) inorganic material layers  302  of adjacent photodetectors  10 . Since there are separation grooves  330 , inorganic material layers  302  have grooves  350  above separation grooves  330 . 
     Inorganic material layers  302  of adjacent photodetectors  10  may be separated. 
     Condenser  300  has a height (i.e., a height from the upper surface of multilayer  200 , in other words, lower layer  310  of condenser  300  to upper surface  320  of condenser  300 ) of 2.0 μm, for example. Inorganic material layers  301  and  302  have a height (i.e., a thickness) of 1.0 μm, for example. 
     Condenser  300  has widths (i.e., lengths along the X- and Y-axes) different between inorganic material layers  301  and  302 . 
     Width A 1  (e.g., the length along the X-axis shown in  FIG.  1   ) of each inorganic material layer  301  is 4.0 μm, for example. 
     Width A 1  of inorganic material layer  301  falls within a range from 57% to 83% of the length (i.e., distance A 3 ) that is a width (i.e., cell size) of each photodetector  10 . Specifically, in a cross-sectional view (e.g., a cross-section along the XZ-plane shown in  FIG.  1   ) along the alignment of photoelectric converters  101  and inorganic material layers  301  (i.e., the Z-axis in this embodiment), width A 1  of the inorganic material layers falls within a range from 57% to 83% of distance A 3  the centers of adjacent pixel separators  102  with photoelectric converters  101  interposed therebetween. On the other hand, in a cross-sectional view (e.g., a cross-section along the XZ-plane shown in  FIG.  1   ) along the alignment of photoelectric converters  101  and inorganic material layers  301  (i.e., the Z-axis in this embodiment), width A 1  of the inorganic material layers may fall within a range from 63% to 77% of distance A 3  between the centers of adjacent pixel separators  102  with photoelectric converters  101  interposed therebetween. 
     In this embodiment, photoelectric converters  101  are rectangular in a plan view. In a plan view, each pixel separator  102  is in the form of a loop surrounding corresponding one of photoelectric converters  101  and having a uniform width and a rectangular periphery. 
     For example, in a cross-sectional view, distance A 4  between sets of interconnect layers  201 , more specifically, distance A 4  that is the shortest distance between interconnect layer  201  right above one of two pixel separators  102  shown in  FIG.  1    and interconnect layer  201  right above the other is longer than width A 1  of inorganic material layers  301 . For example, width A 2  between inorganic material layers  302  is larger than distance A 4  between sets of interconnect layers  201 . 
     Width A 2  of inorganic material layers  302  (e.g., the maximum length along the X-axis shown in  FIG.  1   ) is 6.0 μm, for example. 
     Inorganic material layers  302  are conformal to corresponding inorganic material layers  301  and has a height (i.e., thickness along the Z-axis) substantially identical to a width (i.e., thickness along the X-axis) from inorganic material layer  301 . 
     In this embodiment, as shown in  FIG.  2    which will be described later, inorganic material layers  301  are in a square shape with the same length along the X- and y-axes. 
     The heights of condenser  300  and inorganic material layers  301  and  302  may be any values. 
       FIG.  2    is a plan view showing condenser  300  according to an embodiment. 
     As shown in  FIG.  2   , inorganic material layers  301  and  302  have an area increasing with a decrease in a distance to the top in a plan view. Specifically, inorganic material layers  302 , which are formed on the upper surface of inorganic material layers  301  so as to cover inorganic material layer  301 , have an area larger than that of inorganic material layer  301  in a plan view. 
     In this embodiment, inorganic material layers  301  and  302  are in a rectangular (more specifically, square) shape in a plan view. The centers of inorganic material layers  301  and  302  overlap each other. Although not shown, in photodetectors  10 , the centers of inorganic material layers  301 , inorganic material layers  302 , photoelectric converters  101  overlap each other in a plan view. 
     As described above, condenser  300  may be light-transmissive. For example, if photodetectors  10  is used to detect near-infrared external light, condenser  300  is transmissive to the near-infrared external light. 
     For example, condenser  300  is made of an inorganic material. 
     Here, inorganic material layers  301 , which are the lower layers of condenser  300 , are films with a higher refractive index than that of inorganic material layers  302  which are the upper layers covering inorganic material layers  301 . Specifically, inorganic material layers  301  have a higher refractive index to near-infrared external light than that of inorganic material layers  302 . 
     Inorganic material layers  301  are SiN layers made of SiN, for example. 
     Inorganic material layers  302  are, for example, SiO 2  layers made of tetraethyl orthosilicate (TEOS). 
     Inorganic material layers  302  are rounded at the corners (i.e., stepped surfaces  340  in  FIG.  1   ) of inorganic material layers  301 . That is, the outer edges of the upper surfaces of inorganic material layers  302  are curved. More specifically, the outer edges of the upper surfaces (i.e., stepped surfaces  340 ) of inorganic material layers  302  are curved to project to the outside of condenser  300  and have round corners. 
     With rounded stepped surfaces  340 , condenser  300  is locally (i.e., at the corners) in the same shape as the on-chip lens (e.g., a typical lens with a curved upper surface). 
     Accordingly, if these rounded corners are regarded as a part of a circle, the external light incident on inorganic material layers  302  tends to be refracted at the center of the circle. The rounded corners with a properly set curvature easily orients the external light incident on condenser  300  (more specifically, inorganic material layers  302 ) to photoelectric converters  101 . As a result, the light collection efficiency of photodetectors  10  improves. 
     Condenser  300  with the double layer structure of inorganic material layers  301  and  302  has been described above. The structure is however not limited to the double-layer, and inorganic material layers  301  and  302  may be a multi-layer. 
     [Manufacturing Method] 
     Next, a method of manufacturing photodetectors  10  will be described with reference to  FIGS.  3 A to  3 D .  FIGS.  3 A to  3 D  are cross-sectional views for describing the method of manufacturing photodetectors  10 . 
     First, photoelectric converters  101  and pixel separators  102  are formed in semiconductor substrate  100 . Pixel separators  102  are formed around photoelectric converters  101  in a plan view of semiconductor substrate  100 . Specifically, as shown in  FIG.  3 A , photoelectric converters  101  and pixel separators  102  are formed in semiconductor substrate  100 , and multilayer  200  is formed on semiconductor substrate  100  (specifically, on main surface  110 ). For example, ion implantation is employed for forming photoelectric converters  101  and pixel separators  102 . By ion implantation from main surface  110  of semiconductor substrate  100  made of silicon, photoelectric converters  101  and pixel separators  102  are formed on a relatively upper part inside semiconductor substrate  100  so as to be exposed from main surface  110 , for example. 
     Multilayer  200  is formed as follows. 
     First, a Cu multi-layer interconnect structure is formed on main surface  110  of semiconductor substrate  100  including photoelectric converters  101  and pixel separators  102  by dual damascene (DD) process. In the DD process, after forming the original interconnect layer, liner layers  203  and interlayer insulating films  202  are deposited by chemical vapor deposition (CVD). 
     Then, interconnect grooves (in other words, trenches) and vias (more specifically, through-holes for forming vias  204 ) are patterned by lithography. After that, trenches and vias (i.e., through-holes) are formed in interlayer insulating films  202  by dry etching. 
     Subsequently, a barrier layer that reduces the diffusion of Cu and a Cu seed layer for flowing a current at the time of electroplating are disposed on the inner walls of the trenches and vias (i.e., through-holes) by physical vapor deposition (PVD). After that, a Cu film is embedded in the trenches and via (i.e., through-holes) by Cu electroplating. 
     In addition, the excessive Cu film and barrier layer on the interconnect layers are removed by chemical mechanical polishing (CMP), thereby forming eventual interconnect layers  201  including interconnect layers  201  and vias  204 . After repeating this process, a Cu multi-layer interconnect structure is obtained which includes a predetermined number of interconnect layers  201 . That is, multilayer  200  is formed by DD. 
     Next, as shown in  FIG.  3 B , inorganic material layer  303  for forming condenser  300  are deposited on multilayer  200  by CVD. Condenser  300  is formed as follows. 
     A resist film (not shown) is deposited for forming holes (i.e., separation grooves  330  shown in  FIG.  3 C ) by lithography, and dry etching is performed using the deposited resist film as a mask. 
     Accordingly, as shown in  FIG.  3 C , inorganic material layers  301  in a size of each photodetector  10  and separation grooves  330  for separating inorganic material layers  301  for each pixel (i.e., photodetector  10 ) are formed. As etching gas, for example, carbon fluoride (CF) gas is used. After that, the resist film is removed by ashing. 
     Next, as shown in  FIG.  3 D , inorganic material layers  302  are deposited by CVD using an inorganic material, which has a refractive index lower than that of inorganic material layers  301 , to cover inorganic material layers  301 . 
     Through the process shown in  FIGS.  3 B to  3 D , condenser  300  is formed as follows which is opposed to photoelectric converters  101  and includes inorganic material layers  301 , and inorganic material layers  302  with a refractive index lower than that of inorganic material layers  301 . Inorganic material layers  301  are formed to overlap photoelectric converters  101  at least partially in a plan view so that the outer edges of inorganic material layers  301  are located more inward than interconnect layers  201 , which overlap pixel separators  102 , in a plan view. Inorganic material layers  302  are formed to cover inorganic material layers  301 . Specifically, in addition, multilayer  200  including interconnect layers  201  that overlap pixel separators  102  in a plan view is formed above semiconductor substrate  100 , and condenser  300  is formed above multilayer  200 . 
     By the method of manufacturing photodetectors  10  described above, inorganic material layers  301  and  302  are formed. Inorganic material layers  301  are formed on the uppermost surface (i.e., the upper surface) of multilayer  200  including interconnect layers  201 . Inorganic material layers  301  are divided (i.e., separated) by separation grooves  330  above pixel separators  102  and interconnect layers  201 . Inorganic material layers  302 , which cover inorganic material layers  301  and include grooves  350  above pixel separators  102  and interconnect layers  201 , can be formed on inorganic material layers  301 . 
     [Experimental Results] 
     Now, the experimental results (i.e., simulation results) of the light collection efficiency of photodetectors  10  according to the present disclosure will be described. 
       FIG.  4 A  is a graph showing the light collection efficiency of photodetectors  10  according to the present disclosure. In the graph shown in  FIG.  4 A , the vertical axis represents the following percentage (i.e., light collection efficiency) normalized with the light collection efficiency of a photodetector according to Comparative Example 1 (i.e., an experimental result at the left end of  FIG.  4 A ). The percentage is a ratio of the amount of light that has reached photoelectric converters  101  to the amount of light (i.e., external light) incident in a direction orthogonal to main surface  110  toward condenser  300 . The photodetector according to Comparative Example 1 includes no element above multilayer  200 , that is, includes multilayer  200  in contact with air. A photodetector according to Comparative Example 2 includes a condenser that is a lens made of an organic material, which has been typically used, and having an upper surface in a circular shape as a whole. In other respects, the photodetector according to Comparative Example 2 has the same configuration as the photodetectors according to the present disclosure. 
       FIG.  4 A  also shows the light collection efficiencies of four types of the inorganic materials used for inorganic material layers  301  and  302  included in photodetectors  10 . The experimental results shown in  FIG.  4 A  are, in the order from left: a result of Comparative Example 1; a result of Comparative Example 2; a result of using inorganic material layers  301  made of SiN and inorganic material layers  302  made of TEOS (TEOS/SiN); a result of using inorganic material layers  301  made of TEOS and inorganic material layers  302  made of SiN (SiN/TEOS); a result of using inorganic material layers  301  and  302  made of TEOS (i.e., a TEOS single layer); and a result of using inorganic material layers  301  and  302  made of SiN (i.e., a SiN single layer). 
     In the simulations shown in  FIG.  4 A , SiN has a refractive index of 1.9, while TEOS has a refractive index of 1.46. 
     In the simulations shown in  FIG.  4 A , external light incident on the condenser has a wavelength of 940 nm. 
     Assume that condenser  300  includes inorganic material layers  301  made of SiN and inorganic material layers  302  made of TEOS (“TEOS/SiN” shown in  FIG.  4 A ). In this case, as shown in  FIG.  4 A , such a condenser exhibits a light collection efficiency higher than that of Comparative Example 1, and higher than or substantially equal to that of the condenser according to Comparative Example 2 made of an organic material. 
     On the other hand, assume that the condenser is a single layer (e.g., a “TEOS single layer” and a “SiN single layer” shown in  FIG.  4 A ) or that condenser  300  includes inorganic material layers  301  made of TEOS and inorganic material layers  302  made of SiN (“SiN/TEOS”) shown in  FIG.  4 A ). In these cases, such condensers exhibit light collection efficiencies higher than that of Comparative Example 1 but lower than the light collection efficiency of Comparative Example 2. 
     In this manner, condenser  300  including inorganic material layers  301 , and inorganic material layers  302  with a refractive index lower than that of inorganic material layers  301  can be manufactured simply and exhibits a light collection efficiency higher than or substantially equal to those of the comparative examples. 
       FIG.  4 B  is a graph showing a relationship between width A 1  of inorganic material layers  301  included in photodetector  10  according to the present disclosure and the light collection efficiency. 
     In the graph shown in  FIG.  4 B , the vertical axis represents the following percentage (i.e., light collection efficiency) normalized with the light collection efficiency of the photodetector according to Comparative Example 1 (i.e., the experimental result at the left end of  FIG.  4 B ). The percentage is the ratio of the amount of light that has reached photoelectric converters  101  to the amount of light (i.e., external light) incident in a direction orthogonal to main surface  110  toward condenser  300 . The photodetector according to Comparative Example 1 includes no element above multilayer  200 , that is, includes multilayer  200  in contact with air. The photodetector according to Comparative Example 2 includes the condenser that is the lens made of the organic material, which has been typically used, and having the upper surface in a circular shape as a whole. In other respects, the photodetector according to Comparative Example 2 has the same configuration as the photodetectors according to the present disclosure. 
       FIG.  4 B  shows the light collection efficiencies of inorganic material layers  301  included in photodetectors  10  with seven different widths A 1 . 
     The experimental results shown in  FIG.  4 B  are, in the order from left: a result of Comparative Example 1; a result of Comparative Example 2; a result of using inorganic material layers  301  with width A 1  of 2.6 μm, a result of using inorganic material layers  301  with width A 1  of 3.0 μm, a result of using inorganic material layers  301  with width A 1  of 3.4 μm, a result of using inorganic material layers  301  with width A 1  of 3.6 μm, a result of using inorganic material layers  301  with width A 1  of 3.8 μm, a result of using inorganic material layers  301  with width A 1  of 4.2 μm, a result of using inorganic material layers  301  with width A 1  of 4.6 μm, a result of using inorganic material layers  301  with width A 1  of 4.8 μm, a result of using inorganic material layers  301  with width A 1  of 5.0 μm, and a result of using inorganic material layers  301  with width A 1  of 5.4 μm. 
     In the simulations shown in  FIG.  4 B , inorganic material layers  301  are made of SiN, while inorganic material layers  302  are made of TEOS. In the simulations shown in  FIG.  4 B , SiN has a refractive index of 1.9, while TEOS has a refractive index of 1.46. In the simulations shown in  FIG.  4 B , external light incident on the condenser has a wavelength of 940 nm. 
     In the simulations shown in  FIG.  4 B , distance A 3  shown in  FIG.  1    is 6.0 μm. Width A 1  of 2.6 μm is about 0.43 times (i.e., 43%) distance A 3 . Width A 1  of 3.0 μm is about 0.5 times (i.e., 50%) distance A 3 . Width A 1  of 3.4 μm is about 0.57 times (i.e., 57%) distance A 3 . Width A 1  of 3.6 μm is about 0.6 times (i.e., 60%) distance A 3 . Width A 1  of 3.8 μm is about 0.63 times (i.e., 63%) distance A 3 . Width A 1  of 4.2 μm is about 0.7 times (i.e., 70%) distance A 3 . Width A 1  of 4.6 μm is about 0.77 times (i.e., 77%) of distance A. Width A 1  of 4.8 μm is about 0.8 times (i.e., 80%) distance A 3 . Width A 1  of 5.0 μm is about 0.83 times (i.e., 83%) distance A 3 . Width A 1  of 5.4 μm is about 0.9 times (i.e., 90%) distance A 3 . 
     With width A 1  higher than or equal to 4.8 μm, inorganic material layers  301  overlap interconnect layers  201  partially in a plan view. 
     As shown in  FIG.  4 B , a condenser including inorganic material layers  301  and  302  exhibits a light collection efficiency higher than that of Comparative Example 1. A condenser with width A 1  within a range from 3.4 μm to 5.0 μm secures a light collection efficiency of 0.9 times (i.e., 90%) the light collection efficiency of Comparative Example 2. A condenser with width A 1  within a range from 3.8 μm to 4.6 μm exhibits a light collection efficiency higher than or substantially equal to that of Comparative Example 2. This may be because, in a condenser with width A 1  of 4.8 μm or more, inorganic material layers  301  overlap interconnect layers  201  partially in a plan view and light is thus reflected or absorbed by interconnect layers  201 . In a condenser with width A 1  of 3.6 μm or lower, inorganic material layers  301  has an area smaller than the width (i.e., the cell size) of photodetectors  10  in a plan view, which is believed to cause deterioration of the light collection efficiency. 
     As described above, inorganic material layers  301  are arranged so that the outer edges of inorganic material layers  301  are located more inward than interconnect layers  201 , which are arranged right above pixel separators  102 , in a plan view. This arrangement causes less deterioration of the light collection efficiency. 
     For example, a condenser with width A 1  within a range from 0.57 times (i.e., 57%) to 0.83 times (i.e., 83%) distance A 3  achieves a light collection efficiency of 90% or more the light collection efficiency of Comparative Example 2. For example, a condenser with width A 1  within a range from 0.63 times (i.e., 63%) to 0.77 times (i.e., 77%) distance A 3  achieves a light collection efficiency higher than or substantially equal to that of Comparative Example 2. 
     [Advantages] 
     As described above, photodetectors  10  include semiconductor substrate  100 ; photoelectric converters  101  in semiconductor substrate  100 ; condenser  300  light-transmissive and opposed to photoelectric converters  101 . Condenser  300  includes inorganic material layers  301  that overlap photoelectric converters  101  at least partially in a plan view, inorganic material layers  302 , which cover inorganic material layers  301  and has a refractive index lower than that of inorganic material layers  301 . 
     With this configuration, inorganic material layers  301  and  302  improve the light collection efficiency. Simply by forming inorganic material layers  302  to cover inorganic material layers  301 , the light collection efficiency improves. Accordingly, condenser  300  can be formed more simply than a typical lens (i.e., condenser) made of an organic material with an upper surface in a circular shape as a whole. In addition, photodetectors  10  exhibit an improved light collection efficiency at photoelectric converters  101 . 
     A typical lens made of an organic material has poor temperature characteristics due to the organic material. Upon application of a temperature of 200° C., for example, lens characteristics deteriorate, which limits the manufacturing process after the formation of the lens. By contrast, condenser  300  is made of an inorganic material and thus has excellent temperature characteristics. This allows application of a high-temperature manufacturing process even after the formation of the lens. Accordingly, options in the manufacturing process increase and the reliability of the lens characteristics improves. 
     For example, in a plan view, the outer edges of inorganic material layers  301  are located more inward than interconnect layers  201  that overlap pixel separators  102  arranged around photoelectric converters  101  in semiconductor substrate  100 . 
     As described with reference to  FIG.  4 B , inorganic material layers  301  are arranged so that the outer edges of inorganic material layers  301  are located more inward than interconnect layers  201  arranged right above pixel separators  102  in a plan view. This arrangement causes less deterioration in the light collection efficiency. 
     For example, in a cross-sectional view along the alignment of photoelectric converters  101  and inorganic material layers  301 , inorganic material layers  301  has a width within a range from 57% to 83% of the distance between the centers of adjacent pixel separators  102  with photoelectric converters  101  interposed therebetween. 
     With such a configuration, as described with reference to  FIG.  4 B , photodetectors  10  achieve a light collection efficiency of 90% or more the light collection efficiency of a typical lens made of an organic material. 
     For example, in a cross-sectional view along the alignment of photoelectric converters  101  and inorganic material layers  301 , inorganic material layers  301  has a width within a range from 63% to 77% the distance between the centers of adjacent pixel separators  102  with photoelectric converters  101  interposed therebetween. 
     With such a configuration, as described with reference to  FIG.  4 B , photodetectors  10  achieve a light collection efficiency higher than or substantially equal to that of a typical lens made of an organic material. 
     For example, photodetectors  10  include multilayer  200  including interconnect layers  201 . In this embodiment, semiconductor substrate  100 , multilayer  200 , and condenser  300  are stacked in this order. 
     That is, photodetectors  10  according to this embodiment may serve as FSI photodetectors. 
     For example, the refractive index of inorganic material layers  302  falls within a range from 1.3 to 1.6. 
     Inorganic material layers  302  are located in contact with air. Air has a refractive index of about 1. The refractive index of inorganic material layers  302  is set to be closer to the refractive index of air, for example, within a range from 1.3 to 1.6, thereby reducing the reflection of light at the interface between inorganic material layers  302  and the air. Such a configuration thus improves the light collection efficiency of photodetectors  10 . 
     For example, inorganic material layers  302  are films containing Si and at least any of O or C, specifically, SiOx or SiOC, for example. 
     Accordingly, the refractive index of inorganic material layers  302  may fall within a range from 1.3 to 1.6, for example. Such a configuration thus improves the light collection efficiency of photodetectors  10 . 
     For example, inorganic material layers  301  has a refractive index within a range from 1.6 to 2.5. 
     Accordingly, inorganic material layers  302  tend to emit incident external light toward photoelectric converters  101  due to the difference in the refractive index from inorganic material layers  301 . In addition, inorganic material layers  301  with a refractive index of 2.2 or lower reduces the reflection of external light at the interface between inorganic material layers  301  and  302 . Such a configuration thus improves the light collection efficiency of photodetectors  10 . 
     For example, inorganic material layers  301  are films containing Si and at least any of O, N, or C, or films containing Ti and O, specifically, SiN, SiON, SiCN, SiCO, or TiO x , for example. 
     Accordingly, for example, the refractive index of inorganic material layers  301  falls within a range from 1.6 to 2.5. Such a configuration thus improves the light collection efficiency of photodetectors  10 . 
     For example, condenser  300  is transmissive to near-infrared external light. More specifically, inorganic material layers  301  and  302  are transmissive to near-infrared external light. 
     Specifically, photodetectors  10  are, for example, assumed to image an object by detecting near-infrared external light. In this case, the light detected by photodetectors  10  has a wavelength (wavelength range) within a range from 750 nm to 1400 nm, which is the wavelength range of near-infrared external light longer than that of visible light. That is, even designed (manufactured) in a large size, condenser  300  with a structure for collecting near-infrared external light can collect near-infrared external light more efficiently than a condenser with a structure for collecting visible light. Accordingly, photodetectors  10  can be manufactured more simply. Since such photodetectors can be achieved by a simply manufacturing process, condenser  300  can be manufactured simply in a smaller pixel size to improve the light collection efficiency. 
     For example, the outer edges of the upper surfaces of inorganic material layers  302  are curved. 
     Accordingly, less light is reflected at the outer edges than in the case where the upper surfaces of inorganic material layers  302  have sharp outer edges. With the outer edges with a properly set curvature, light (i.e., external light) incident on inorganic material layers  302  can be refracted properly toward photoelectric converters  101 . With such a configuration, the light collection efficiency of the photodetectors further improves. 
     For example, inorganic material layers  302  include grooves  350  recessed toward semiconductor substrate  100  above interconnect layers  201  that overlap pixel separators  102  around photoelectric converters  101  in semiconductor substrate  100  in a plan view. 
     In such a configuration, for example, the outer edges of inorganic material layers  302  are curved like stepped surfaces  340  due to grooves  350 , which easily orients the external light toward photoelectric converters  101 . 
     For example, in an arrangement of a plurality of photodetectors  10 , interconnect layers  201  are located below gaps between adjacent inorganic material layers  301 . 
     In such a configuration, the entry of the external light into photoelectric converters  101  is less hindered by interconnect layers  201 . 
     For example, photodetectors  10  further include multilayer  200  including interconnect layers  201  and liner layer  205  above interconnect layers  201 . In this case, for example, liner layer  205  has a refractive index lower than that of inorganic material layers  301  and higher than that of inorganic material layers  302 . 
     In such a configuration, less light is reflected at the interfaces between inorganic material layers  301  and liner layer  205 . Accordingly, further more light is collected by condenser  300  and incident on photoelectric converters  101 . 
     A method of manufacturing photodetectors  10  according to an embodiment includes: forming photoelectric converters  101  in semiconductor substrate  100 ; and forming condenser  300  that is opposed to photoelectric converters  101  and includes inorganic material layers  301 , and inorganic material layers  302  with a refractive index lower than that of inorganic material layers  301  by: forming inorganic material layers  301  to overlap photoelectric converters  101  at least partially in a plan view; and forming inorganic material layers  302  to cover inorganic material layers  301 . 
     Accordingly, photodetectors  10  with a light collection efficiency improved by inorganic material layers  301  and  302  can be manufactured simply. 
     For example, the method of manufacturing photodetectors  10  according to the embodiment further includes forming, above semiconductor substrate  100 , multilayer  200  including interconnect layers  201  to overlap pixel separators  102  in a plan view; and forming condenser  300  above multilayer  200 . 
     Accordingly, FSI photodetectors  10  with a light collection efficiency improved by inorganic material layers  301  and  302  can be manufactured simply. 
     [Variations of Photodetector] 
     Now, variations of the photodetectors will be described. In the following variations, differences from photodetectors  10  will be described mainly. Detailed description of the same configurations as photodetectors  10  may be omitted or simplified. 
     [Variation 1] 
       FIG.  5    is a cross-sectional view showing photodetectors  10   a  according to Variation 1 of the embodiment. 
     As shown in  FIG.  5   , photodetector  10   a  include waveguides  400  in addition to the configuration of photodetectors  10 . 
     In photodetectors  10 , condenser  300  is formed on the uppermost surface of multilayer  200  including metal interconnects. 
     Condenser  300  may be formed on the uppermost surfaces of waveguides  400  in multilayer  200   a.    
     Multilayer  200   a  is a multilayer film that corresponds to multilayer  200  included in photodetectors  10  and further includes grooves for arranging waveguides  400 . 
     Photodetectors  10   a  shown in  FIG.  5    include waveguides  400  penetrating multilayer  200   a  between photoelectric converters  101  and condenser  300  so as to introduce light into photoelectric converters  101 . 
     Waveguides  400  are light-transmissive optical waveguides for introducing incident light into photoelectric converters  101 . A material employed for waveguides  400  is, for example, silicon nitride, silicon oxynitride, silicon carbonitride, carbon-added silicon oxide, or silicon oxide. 
     Waveguides  400  are interposed between photoelectric converters  101  and condenser  300 . Specifically, waveguides  400  penetrate multilayer  200   a  between photoelectric converters  101  and condenser  300 . In other words, multilayer  200   a  includes waveguides  400  between photoelectric converters  101  and condenser  300 . 
     In this variation, waveguides  400  penetrate multilayer  200   a  and are in contact with photoelectric converters  101 . Alternatively, interlayer insulating films  202  may be interposed between waveguides  400  and photoelectric converters  101 . 
     The three-dimensional shape of waveguides  400  is, for example, a truncated quadrangular pyramid. The radius (or width) of waveguides  400  in a cross-sectional view increases with an increasing distance from photoelectric converters  101  in the stacking direction. For example, the radius of waveguides  400  is about 3.6 μm at the bottom, which is the closest to photoelectric converters  101 , and 4.0 μm at the top, which is the farthest from photoelectric converters  101 . 
     Waveguides  400  are formed as follows. For example, multilayer  200  is formed and then patterned by lithography and dry etching to form multilayer  200   a . After that, an inorganic material, such as silicon nitride, silicon oxynitride, silicon carbonitride, or carbon-added silicon oxide, with a higher refractive index, or a silicon oxide film is deposited by CVD. 
     A material employed for waveguides  400  may have the same refractive index as condenser  300  (more specifically, inorganic material layers  301 ). Specifically, waveguides  400  and inorganic material layers  301  may be made of the same material. 
     Condenser  300  and waveguides  400  may be in contact with each other. That is, condenser  300  and waveguides  400  may be continuous. In other words, condenser  300  and waveguides  400  may be integral with each other. 
     In this configuration, waveguides  400  and inorganic material layers  301  have the same refractive index and are directly in contact with each other with no other member interposed therebetween. Less light is reflected between condenser  300  and waveguides  400 . Accordingly, the light collection efficiency of photodetectors  10   a  improves. 
     In this manner, in addition to the configuration of photodetectors  10  (i.e., the FSI structure including semiconductor substrate  100 , multilayer  200   a , and condenser  300  stacked in this order), photodetectors  10   a  further include waveguides  400 . Waveguides  400  penetrate multilayer  200   a  between photoelectric converters  101  and condenser  300  and are for introducing light into photoelectric converters  101 . 
     Accordingly, waveguides  400  efficiently guide the external light collected by condenser  300  to photoelectric converters  101 . As a result, the light collection efficiency of photodetectors  10   a  further improves. 
     [Variation 2] 
       FIG.  6    is a cross-sectional view showing photodetectors  10   b  according to Variation 2 of the embodiment. 
     Photodetectors  10  shown in  FIG.  1    are what are called “FSI photodetectors” including multilayer  200 , which includes interconnect layers  201  and other layers, on main surface  110  of semiconductor substrate  100 . The photodetectors according to the present disclosure are not limited thereto. 
     For example, photodetectors  10   b  may be what are called “back side illumination (BSI) photodetectors including multilayer  200 , which includes interconnect layers  201  (more specifically, interlayer insulating films  202  between sets of interconnect layers  201 ) on back surface  120  which is opposite to main surface  110   a  of semiconductor substrate  100 . That is, photodetectors  10   b  according to this variation include multilayer  200 , semiconductor substrate  100   a , and condenser  300  stacked in this order. 
     In this embodiment, interconnect layers  201  are located right below pixel separators  102 . 
     Photodetectors  10   b  according to Variation 2 of the embodiment include, for example, semiconductor substrate  100   a , multilayer  200 , condenser  300 , and support substrate  401 . 
     Semiconductor substrate  100   a  includes photoelectric converters  101  and pixel separators  102 . 
     Formed on main surface  110   a  of semiconductor substrate  100   a  are interlayer insulating films  202 . That is, in photodetectors  10   b , condenser  300  is formed on main surface  110   a , which is the upper surfaces of interlayer insulating films  202  included in semiconductor substrate  100   a . Accordingly, semiconductor substrate  100   a  and condenser  300  are electrically insulated from each other. 
     Support substrate  401  is for supporting multilayer  200 . A material employed for support substrate  401  is not particularly limited. Support substrate  401  may be a ceramic substrate or a semiconductor substrate. 
     [Variation 3] 
       FIG.  7    is a cross-sectional view showing photodetectors  10   g  according to Variation 3 of the embodiment. 
     Photodetectors  10   g  include wavelength selector  601  above condenser  300 . Specifically, wavelength selector  601  is located above the upper surface of condenser  300 . 
     Wavelength selector  601  is an optical member for selectively allowing the entry of light with a predetermined wavelength to photoelectric converters  101 . Specifically, wavelength selector  601  blocks light with at least some wavelengths out of external light by absorption or reflection, and causes light with the predetermined wavelength to pass. Wavelength selector  601  is a color filter, for example. A material of the color filter is, for example, an organic resin that blocks light with at least some wavelengths (e.g., light in a visible range) out of external light and causes light with a predetermined wavelength to pass. 
     Wavelength selector  601  may be a photonic filter. The photonic filter has a periodic multilayer structure obtained by alternately stacking a material with a low refractive index and a material with a high refractive index in a period corresponding to a wavelength. The photonic filter blocks light within a specific wavelength range determined by a structure parameter. 
     Provided right below wavelength selector  601  is planarization layer  602  to fill the gaps of grooves  350  in condenser  300 . 
     Planarization layer  602  has a flat upper surface to place wavelength selector  601  properly. A material employed for planarization layer  602  only needs to be light-transmissive and may be any suitable material, such as a resin material or a glass material. 
     In this variation, the pixels (e.g., plurality of photodetectors  10   g  of a solid-state image sensor including the plurality of photodetectors  10   g ) may have the same selectivity or different selectivities of wavelength. 
     [Variation 4] 
       FIG.  8    is a cross-sectional view showing photodetectors  10   h  according to Variation 4 of the embodiment. 
     Photodetectors  10   h  include wavelength selector  601  between condenser  300  and interconnect layers  201 . Specifically, wavelength selector  601  is located below condenser  300  and above interconnect layers  201 . 
     In order to reduce processing damages of wavelength selector  601  at the time of forming inorganic material layers  301 , protective film  603  is located right on wavelength selector  601 . 
     Protective film  603  is for reducing processing damages of wavelength selector  601  at the time of forming inorganic material layers  301 . A material employed for protective film  603  only needs to be light-transmissive and may be any suitable material, such as a resin material or a glass material. 
     In this variation as well as in Variation 3, the pixels (e.g., plurality of photodetectors  10   h  of a solid-state image sensor including the plurality of photodetectors  10   h ) may have the same selectivity or different selectivities of wavelength. 
     Condenser  300  is located on the uppermost layer of the multilayer structure of photodetectors  10   h . After collecting external light, the wavelength of the collected external light can be selected by wavelength selector  601 , which reduces color mixtures between the pixels. 
     [Variations of Condenser] 
     Now, variations of the condenser included in photodetectors will be described. In the following variations, differences from condenser  300  included in photodetectors  10  will be described mainly. Detailed description of the same configurations as condenser  300  may be omitted or simplified. 
     [Variation 1] 
       FIG.  9    is a plan view showing condenser  300   a  according to Variation 1. 
     Condenser  300   a  is in a circular shape in a plan view. More specifically, inorganic material layers  301   a  and inorganic material layers  302   a  included in condenser  300   a  are in a circular shape in a plan view and the centers thereof are substantially in the identical position. 
     [Variation 2] 
       FIG.  10    is a plan view showing condenser  300   b  according to Variation 2. 
     Condenser  300   b  is in an oval shape in a plan view. More specifically, inorganic material layers  301   b  and inorganic material layers  302   b  included in condenser  300   b  are in an oval shape in a plan view and the centers thereof are substantially in the identical position. 
     [Variation 3] 
       FIG.  11    is a plan view showing condenser  300   c  according to Variation 3. 
     Condenser  300   c  is in a hexagonal (more specifically, regular hexagonal) shape in a plan view. More specifically, inorganic material layers  301   c  and inorganic material layers  302   c  included in condenser  300   c  are in a hexagonal shape in a plan view and the centers thereof are substantially in the identical position. 
     As described above in Variations 1 to 3, condenser  300  shown in  FIG.  2    is, for example, in a quadrilateral (specifically, rectangular, and more specifically, square) shape in a plan view. The shape of the condenser included in the photodetectors according to the present disclosure in a plan view is not limited thereto. 
     Note that the shape of photoelectric converters  101  in a plan view is not particularly limited but may be substantially identical to the shape of condenser in a plan view. Accordingly, the external light collected by the condenser can be incident on photoelectric converters  101  more efficiently. 
     [Configuration of Solid-State Image Sensor] 
     The present disclosure may be implemented as a line sensor obtained by arranging a plurality of photoelectric converters  101  in a line in semiconductor substrate  100 . Alternatively, the present disclosure may be implemented as a solid-state image sensor obtained by arranging a plurality of photoelectric converters  101  in a matrix in semiconductor substrate  100 . 
       FIG.  12    shows solid-state image sensor  500  according to an embodiment. 
     As shown in  FIG.  12   , solid-state image sensor  500  includes pixel array  502  of a plurality of pixels  501 , vertical scanning circuit  503 , horizontal scanning circuit  504 , readout circuit  505 , and buffer amplifier (amplifier circuit)  506 . Pixel array  502  is obtained by arranging photoelectric converters  101  in a matrix along the XY plane in photodetectors  10 ,  10   a , or  10   b . In the example of  FIG.  12   , photoelectric converters  101  are avalanche photodiodes which will also be referred to as “APDs”. Readout circuit  505  reads out signals output by pixel array  502 . 
     Each pixel  501  includes pixel circuit PC including APD, transfer transistor TRN, reset transistor RST, floating diffusion region FD, amplification transistor SF, selection transistor SEL, and overflow transistor OVF. 
     In this embodiment, MOS transistors (MOSFETs) will be simply referred to as “transistors”. However, transistors constituting the pixel circuit of solid-state image sensor  500  are not limited to MOS transistors but may be junction transistors (JFETs), bipolar transistors, or a combination of these transistors. 
     The signal charges detected by the APD are transferred via transfer transistor TRN to floating diffusion region FD. Signals corresponding to the amount of the signal charges detected by sequentially selected pixel  501  by vertical scanning circuit  503  and horizontal scanning circuit  504  are transmitted via amplification transistor SF to readout circuit  505 . 
     The signals obtained by pixels  501  are output from readout circuit  505  via buffer amplifier  506  to a signal processing circuit (not shown). After being subjected to signal processing, such as white balance, by the signal processing circuit (not shown), the signals are transferred to a display (not shown) or a memory (not shown) to create an image. 
     Overflow transistor OVF is a protective element, at which a current starts flowing once a potential of APD reaches a certain value. That is, overflow transistor OVF limits a voltage to be applied to APD. Once the APD detects light at a high multiplication, a current starts flowing at overflow transistor OVF before the voltage of the APD exceeds the breakdown voltage of transfer transistor TRN. Also, once the APD detects intense light so that the voltage at the time of reset becomes negative, a current starts flowing at overflow transistor OVF before the voltage of the APD exceeds the breakdown voltage of transfer transistor TRN. That is, solid-state image sensor  500  with overflow transistor OVF can be designed so that the voltage of the APD does not reach the breakdown voltage. The upper limit of the voltage applied to the APD is controllable by the threshold voltage of overflow transistor OVF, a voltage applied to the gate of overflow transistor OVF, or drain voltage (V OVF ) of overflow transistor OVF. 
     In pixel circuit PC shown in  FIG.  12   , peripheral circuits (i.e., vertical scanning circuit  503 , horizontal scanning circuit  504 , readout circuit  505 , and buffer amplifier  506 ) are added to pixel array  502 . Solid-state image sensor  500  does not necessarily include such peripheral circuits. 
     Pixel circuit PC includes five transistors (i.e., transfer transistor TRN, reset transistor RST, amplification transistor SF, selection transistor SEL, and overflow transistor OVF) and floating diffusion region FD. Pixel circuit PC is not limited to such a configuration. Solid-state image sensor  500  may include more or less transistors as long as being operatable. 
     The circuit configuration of pixel circuit PC is an example. Pixel circuit PC may have any other circuit configuration capable of reading signal charges stored in the APDs. 
       FIG.  13    shows a layout of a plurality of photodetectors included in solid-state image sensor  500  according to an embodiment.  FIG.  14    is a cross-sectional view showing example photodetectors included in solid-state image sensor  500  according to the embodiment. In  FIG.  13   , the plurality of photodetectors included in pixel array  502  are represented by rectangles, the centers of respective photoelectric converters  101  of the photodetectors by circles, and the centers of respective first inorganic material layers (i.e., inorganic material layers  301 ) of the photodetectors by crosses. 
     As shown in  FIG.  13   , the plurality of photodetectors of solid-state image sensor  500  are arranged in a matrix in a plan view. In  FIG.  13   , some of the plurality of photodetectors are not shown. 
     Here, the arrangements of the respective components of the plurality of photodetectors of solid-state image sensor  500  are not necessarily identical. For example, in a plan view, the photodetector at the center of pixel array  502  and a photodetector at each end may have different positional relationships between the centers of photoelectric converter  101  and inorganic material layer  301 . 
     As shown in  FIG.  13   , for example, solid-state image sensor  500  includes photodetector  10  at the center of pixel array  502  and photodetector  10   c  at an end (more specifically, the positive end of the X-axis in pixel array  502 ) in a plan view. 
     As shown in  FIGS.  1  and  13   , in photodetector  10 , the centers of photoelectric converter  101  and inorganic material layers  301  overlap each other in a plan view. Specifically, in photodetector  10 , the centers of photoelectric converter  101 , inorganic material layer  301 , and inorganic material layer  302  overlap each other in a plan view. 
     On the other hand, as shown in  FIGS.  13  and  14   , in photodetector  10   c , the centers of photoelectric converter  101  and inorganic material layer  301  are shifted from each other in a plan view. Specifically, in photodetector  10   c , center C 2  of inorganic material layer  301  is shifted from center C 1  of photoelectric converter  101  to the positive side of the X-axis. More specifically, in photodetectors  10   c , center C 2  of inorganic material layer  301  and the center of inorganic material layer  302  are shifted from center C 1  of photoelectric converter  101  to the positive side of the X-axis. Center C 2  of inorganic material layer  301  and the center of inorganic material layer  302  overlap each other in a plan view. 
     Similarly, in pixel array  502 , in photodetector  10   d  at the positive end of the Y-axis, the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  to the positive side of the Y-axis. In pixel array  502 , photodetector  10   e  at the negative end of the X-axis, the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  to the negative side of the X-axis. In pixel array  502 , photodetector  10   f  at the negative end of the Y-axis, the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  to the negative side of the Y-axis. 
     In this manner, for example, in the photodetector at the center of pixel array  502 , the centers of photoelectric converter  101  and inorganic material layer  301  overlap each other in a plan view. On the other hand, in the photodetector at each end of pixel array  502 , the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  away from the center of the array. More specifically, in a photodetector at an end away from the center of pixel array  502  in a predetermined direction, the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  away from the center of the array in the predetermined direction. 
     Note that the centers described above may be, for example, the centers of gravity, or the centers of n-time rotations, where n is an integer of two or more. 
     For example, the center and ends described above may be set freely. For example, photodetectors on the outermost periphery of pixel array  502  in a plan view may be referred to as the “photodetectors at the ends”, and the others may be referred to as the “photodetectors at the center”. Alternatively, for example, assume that pixel array  502  includes N×M photodetectors, where N and M are each an integer of three or more. In this case, N/2×M/2 photodetectors closer to the center of pixel array  502  in a plan view may be referred to as the “photodetectors at the center”, and the others may be referred to as the “photodetectors at the ends”. For example, if N and M are each odd numbers, decimals may be rounded down. 
     The amount of the shift between the centers of photoelectric converter  101  and inorganic material layer  301  in a plan view may be set freely. For example, the amount of the shift between the centers of photoelectric converter  101  and inorganic material layer  301  may be determined in accordance with the distance from the center of the array in a plan view. For example, the amount of the shift between the centers of photoelectric converter  101  and inorganic material layer  301  may be set larger, with an increasing distance from the center of the array in a plan view. 
     Although not shown, in adjacent ones of the plurality of photodetectors of solid-state image sensor  500 , interconnect layers  201  that connect the photodetectors and readout circuit  505  are located below the gaps between inorganic material layers  301 . 
     As described above, solid-state image sensor  500  according to an embodiment includes pixel array  502  of the photodetectors described above (e.g., photodetectors  10 ,  10   a , or  10   b ) arranged in a matrix, and readout circuit  505  that reads the signals output by pixel array  502 . 
     Accordingly, like the photodetectors described above (e.g., photodetectors  10 ,  10   a , or  10   b ), solid-state image sensor  500  can be manufactured more simply than before, and exhibit an improved light collection efficiency at photoelectric converters  101 . 
     Note that solid-state image sensor  500  may include, as the photodetectors, photodetectors  10 ,  10   a , or  10   b . Alternatively, solid-state image sensor  500  may include any two or more types of photodetectors  10 ,  10   a , and  10   b.    
     In this embodiment, in a plan view, in pixel array  502 , the photodetector (e.g., photodetector  10  in  FIG.  13   ) at the center and the photodetectors (e.g., photodetectors  10   c  to  10   f  shown in  FIG.  13   ) at the ends have different positional relationships between the centers of photoelectric converter  101  and inorganic material layer  301 . More specifically, in this embodiment, in the photodetector at the center, the centers of photoelectric converter  101  and inorganic material layer  301  overlap each other in a plan view. On the other hand, in the photodetector at each end, the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  away from the center of the array. 
     For example, in pixel array  502 , the photodetectors closer to the ends are more prone to the following problem. Once external light enters the light-receiving surface (e.g., main surface  110 ) obliquely (e.g., in a direction orthogonal to the Z-axis in this embodiment), the external light enters photoelectric converters  101  less properly. That is, at the ends of pixel array  502 , the photodetectors have poor incident angle characteristics. To address the problem, the photodetectors at the ends of pixel array  502 , the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  away from the center of the array. More specifically, in a photodetector at an end away from the center of pixel array  502  in a predetermined direction, the center of inorganic material layer  301  is shifted from the center of photoelectric converter  101  away from the center of the array in the predetermined direction. Accordingly, for example, even being incident obliquely from the normal of main surface  110  at an end of pixel array  502 , the external light easily enters inorganic material layers  301 . This causes less deterioration in the incident angle characteristics of the photodetector and improves the light collection efficiency of solid-state image sensor  500 . 
     For example, interconnect layers  201  that connect photodetectors  10  and readout circuit  505  are located below the gaps between inorganic material layers  301  of adjacent photodetectors  10 . 
     In such a configuration, the entry of the external light into photoelectric converters  101  is less hindered by interconnect layers  201 . 
     OTHER EMBODIMENTS 
     The photodetectors, for example, according to the embodiment and variations have been described above. The present disclosure is however not limited to the embodiment and variations described above. 
     For example, numerical values used in the embodiment described above for explanations are mere examples for specific description of the disclosure. The present disclosure is not limited to the numerical values used at the examples. 
     The embodiment and variations described above may be combined freely. 
     The example main materials of the layers of the multilayer structure of the photodetectors have been described above in the embodiment. The layers of the multilayer structure of the photodetectors may include other materials as long as achieving the same or similar functions as the multilayer structure described above in the embodiment. In the drawings, the corners and sides of the components are shown linearly. For manufacturing reasons, the present disclosure may include rounded corners and sides. 
     The present disclosure may include forms obtained by various modifications to the foregoing embodiment that can be conceived by those skilled in the art or forms achieved by freely combining the constituent elements and functions in the foregoing embodiment without departing from the scope and spirit of the present disclosure. For example, the present disclosure may be implemented as a method of manufacturing a photodetector. 
     INDUSTRIAL APPLICABILITY 
     The photodetector according to the present disclosure is useful as a photodetector with a high light collection efficiency.