Abstract:
An image sensor includes a two-dimensional array of image sensor pixels, which are formed in a semiconductor layer. Each image sensor pixel is formed in a substrate having a corresponding semiconductor region therein. Each semiconductor region contains at least first and second photoelectric conversion elements, which are disposed at side-by-side locations therein. An electrically insulating isolation region is also provided, which extends at least partially through the semiconductor region and at least partially between the first and second photoelectric conversion elements, which may be configured respectively as first and second semiconductor regions of first conductivity type (e.g., N-type). At least one optically reflective region is also provided, which extends at least partially through the semiconductor region and surrounds at least a portion of at least one of the first and second photoelectric conversion elements. A semiconductor floating diffusion (FD) region (e.g., N-type region) is provided within the semiconductor region.

Description:
REFERENCE TO PRIORITY APPLICATION 
       [0001]    This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2015-0112536, filed Aug. 10, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The inventive concepts relate to an image sensor and, more particularly, to an image sensor with improved image quality. 
         [0003]    Image sensors are semiconductor devices capable of converting an optical image into electrical signals. Image sensors may be categorized as any one of charge coupled device (CCD) type image sensors and complementary metal-oxide-semiconductor (CMOS) type image sensors. 
         [0004]    As semiconductor devices have been highly integrated, image sensors have also been highly integrated and sizes of pixels have been reduced. Thus, various researches are conducted for an image sensor capable of reducing crosstalk and improving sensitivity in a fine area. 
       SUMMARY 
       [0005]    Embodiments of the inventive concepts may provide an image sensor capable of reducing or minimizing crosstalk. In some of these embodiments, an image sensor includes a two-dimensional array of image sensor pixels, which are formed in a semiconductor layer. In some of these embodiments, each image sensor pixel is formed in a substrate having a corresponding semiconductor region therein. Each semiconductor region contains at least first and second photoelectric conversion elements, which are disposed at side-by-side locations therein. An electrically insulating isolation region is also provided, which extends at least partially through the semiconductor region and at least partially between the first and second photoelectric conversion elements, which may be configured respectively as first and second semiconductor regions of first conductivity type (e.g., N-type). At least one optically reflective region is also provided, which extends at least partially through the semiconductor region and surrounds at least a portion of at least one of the first and second photoelectric conversion elements. A semiconductor floating diffusion (FD) region (e.g., N-type region) is provided within the semiconductor region. According to some embodiments of the invention, the FD region extends between the first and second photoelectric conversion elements and opposite the electrically insulating isolation region. In particular, the electrically insulating isolation region may extend between a back surface of the semiconductor region, which is configured to receive incident light thereon, and the floating diffusion region. 
         [0006]    According to additional embodiments of the invention, the semiconductor region may have a trench therein that surrounds at least uppermost portions of the first and second photoelectric conversion elements on four sides thereof. In some of these embodiments, the optically reflective region may at least partially fill the trench and surround the uppermost portions of the first and second photoelectric conversion elements when viewed in a direction normal to a surface of the semiconductor region. In still further embodiments of the invention, the electrically insulating isolation region may include optically reflective material therein. The optically reflective region and the optically reflective material may be metals selected from a group consisting of tungsten (W), copper (Cu) and aluminum (Al). According to additional embodiments of the invention, the optically reflective region extends entirely through the semiconductor region and surrounds the first and second photoelectric conversion elements on four sides thereof. The optically reflective region may also be electrically isolated from the semiconductor region by an electrically insulating material. 
         [0007]    According to further embodiments of the invention, an image sensor may include a semiconductor layer, a first isolation layer disposed in the semiconductor layer to define a unit pixel region of the semiconductor layer, a first photoelectric conversion element and a second photoelectric conversion element that are disposed in the semiconductor layer of the unit pixel region, and a second isolation layer disposed in the semiconductor layer of the unit pixel region and disposed between the first photoelectric conversion element and the second photoelectric conversion element. The first isolation layer may surround the first photoelectric conversion element and the second photoelectric conversion element, and the first isolation layer may include a vertical reflective layer. 
         [0008]    In a further embodiment, the first isolation layer may include first patterns extending in one direction and second patterns disposed between the first patterns so as to be connected to the first patterns. The second isolation layer may be connected to the first patterns of the first isolation layer and may be spaced apart from the second patterns of the first isolation layer. The second isolation layer may include an additional vertical reflective layer. The additional vertical reflective layer may be connected to the vertical reflective layers included in the first patterns of the first isolation layer. 
         [0009]    In an additional embodiment, the first isolation layer may include first patterns extending in one direction and second patterns disposed between the first patterns so as to be connected to the first patterns. The second isolation layer may be spaced apart from the first patterns and the second patterns of the first isolation layer. The second isolation layer may also include an additional vertical reflective layer and an insulating layer disposed between the additional vertical reflective layer and the semiconductor layer. 
         [0010]    In an additional embodiment, the first isolation layer may further include a vertical insulating layer covering a surface of the vertical reflective layer. The vertical insulating layer may include the same material as the second isolation layer. The first isolation layer may further include an air gap disposed in the vertical reflective layer. And, a width of the first isolation layer may be greater than that of the second isolation layer. 
         [0011]    In an additional embodiment, the semiconductor layer may include a first surface on which light is incident, and a second surface opposite to the first surface. A distance between a bottom surface of the first isolation layer and the second surface of the semiconductor layer may be smaller than a distance between a bottom surface of the second isolation layer and the second surface of the semiconductor layer. 
         [0012]    In an additional embodiment, the semiconductor layer may include a first surface on which light is incident, and a second surface opposite to the first surface. The first isolation layer may penetrate the semiconductor layer, and a bottom surface of the second isolation layer may be spaced apart from the second surface of the semiconductor layer. 
         [0013]    In an additional embodiment, the image sensor may further include a floating diffusion region disposed in the semiconductor layer of the unit pixel region. The floating diffusion region may be disposed between the first photoelectric conversion element and the second photoelectric conversion element, and the floating diffusion region may vertically overlap with the second isolation layer but may not vertically overlap with the first isolation layer. 
         [0014]    In an embodiment, an image sensor may include a semiconductor layer, a first photoelectric conversion element and a second photoelectric conversion element that are disposed in the semiconductor layer, a first isolation layer disposed in the semiconductor layer and surrounding the first and second photoelectric conversion elements, a second isolation layer disposed in the semiconductor layer and isolating the first and second photoelectric conversion elements from each other, and a color filter vertically overlapping with both the first photoelectric conversion element and the second photoelectric conversion element when viewed from a plan view. The first isolation layer may include a vertical reflective layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The inventive concepts will become more apparent in view of the attached drawings and accompanying detailed description. 
           [0016]      FIG. 1  is a schematic plan view illustrating an image sensor according to embodiments of the inventive concepts. 
           [0017]      FIGS. 2A and 2B  are enlarged views of a portion ‘A’ of  FIG. 1  to illustrate image sensors according to embodiments of the inventive concepts. 
           [0018]      FIG. 3  is a cross-sectional view taken along a line I-I′ of  FIG. 2A or 2B  to illustrate an image sensor according to embodiments of the inventive concepts. 
           [0019]      FIG. 4  is a cross-sectional view taken along the line I-I′ of  FIG. 2A or 2B  to illustrate an image sensor according to embodiments of the inventive concepts. 
           [0020]      FIG. 5  is a cross-sectional view taken along the line I-I′ of  FIG. 2A or 2B  to illustrate an image sensor according to embodiments of the inventive concepts. 
           [0021]      FIG. 6  is a cross-sectional view taken along the line I-I′ of  FIG. 2A or 2B  to illustrate an image sensor according to embodiments of the inventive concepts. 
           [0022]      FIGS. 7 and 8  are enlarged views of the portion ‘A’ of  FIG. 1  to illustrate image sensors according to embodiments of the inventive concepts. 
           [0023]      FIG. 9  is a cross-sectional view taken along a line II-II′ of  FIG. 7 or 8  to illustrate an image sensor according to embodiments of the inventive concepts. 
           [0024]      FIGS. 10A to 10D  are cross-sectional views taken along the line I-I′ of  FIG. 2A or 2B  to illustrate a method of manufacturing an image sensor according to embodiments of the inventive concepts. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    The inventive concepts will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the inventive concepts are shown. The advantages and features of the inventive concepts and methods of achieving them will be apparent from the following exemplary embodiments that will be described in more detail with reference to the accompanying drawings. It should be noted, however, that the inventive concepts are not limited to the following exemplary embodiments, and may be implemented in various forms. Accordingly, the exemplary embodiments are provided only to disclose the inventive concepts and let those skilled in the art know the category of the inventive concepts. In the drawings, embodiments of the inventive concepts are not limited to the specific examples provided herein and are exaggerated for clarity. The same reference numerals or the same reference designators denote the same elements throughout the specification. 
         [0026]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular terms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0027]    In addition, exemplary embodiments are described herein with reference to cross-sectional views and/or plan views that are idealized exemplary views. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Accordingly, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an etching region illustrated as a rectangle will, typically, have rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
         [0028]      FIG. 1  is a schematic plan view illustrating an image sensor according to embodiments of the inventive concepts.  FIGS. 2A and 2B  are enlarged views of a portion ‘A’ of  FIG. 1  to illustrate image sensors according to embodiments of the inventive concepts.  FIG. 3  is a cross-sectional view taken along a line I-I′ of  FIG. 2A or 2B  to illustrate an image sensor according to embodiments of the inventive concepts. 
         [0029]    Referring to  FIGS. 1, 2A, 2B, and 3 , an image sensor may include unit pixels P arranged in a matrix form. At least one photoelectric conversion element  110  may be disposed in each of the unit pixels P. The image sensor may include a light receiving part P 1 , an interconnection part P 2 , and a light filter part P 3 . The light receiving part P 1  may include a semiconductor layer  100 , the photoelectric conversion elements  110 , and a first isolation layer  104  defining unit pixel regions PX of the semiconductor layer  100 . In an embodiment, the semiconductor layer  100  may be a single-crystalline semiconductor substrate. In an embodiment, the semiconductor layer  100  may be an epitaxial layer formed by an epitaxial growth process. The semiconductor layer  100  may include a back surface  101   a  and a front surface  101   b.  The back surface  101   a  of the semiconductor layer  100  may be a surface on which light is incident. 
         [0030]    The photoelectric conversion elements  110  may be disposed in the semiconductor layer  100 . The photoelectric conversion elements  110  may be two-dimensionally arranged in the semiconductor layer  100  to constitute a two-dimensional array. The photoelectric conversion elements  110  may be doped with, for example, N-type dopants. The photoelectric conversion elements  110  may be more adjacent to the front surface  101   b  of the semiconductor layer  100 . 
         [0031]    The photoelectric conversion element  110  may include a first photoelectric conversion element PD 1  and a second photoelectric conversion element PD 2  that are disposed in each of the unit pixel regions PX. In other words, two photoelectric conversion elements may be disposed in one unit pixel region PX. Each of the first and second photoelectric conversion elements PD 1  and PD 2  may independently collect light incident upon and passing through the back surface  101   a  of the semiconductor layer  100 . 
         [0032]    A floating diffusion region FD may be disposed in the semiconductor layer  100 . The floating diffusion region FD may be disposed in each of the unit pixel regions PX. In an embodiment, the floating diffusion region FD may be disposed between the first photoelectric conversion element PD 1  and the second photoelectric conversion element PD 2  in each of the unit pixel regions PX. In an embodiment, the floating diffusion region FD may be doped with N-type dopants. 
         [0033]    The first isolation layer  104  and a second isolation layer  106  may be disposed in the semiconductor layer  100 . In an embodiment, the first isolation layer  104  may surround the first and second photoelectric conversion elements PD 1  and PD 2  when viewed from a plan view. The first isolation layer  104  may include first patterns PT 1  extending in one direction and second patterns PT 2  disposed between the first patterns PT 1  so as to be connected to the first patterns PT 1  when viewed from a plan view. The first isolation layer  104  may not vertically overlap with the floating diffusion region FD. 
         [0034]    The first isolation layer  104  may include a multi-layer. In an embodiment, the first isolation layer  104  may include a vertical insulating layer  104   a  and a vertical reflective layer  104   b.  The vertical reflective layer  104   b  may surround the first and second photoelectric conversion elements PD 1  and PD 2  disposed in each of the unit pixel regions PX when viewed from a plan view. The vertical insulating layer  104   a  may cover sidewalls and a bottom surface of the vertical reflective layer  104   b.  For example, the vertical insulating layer  104   a  may include at least one of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a hafnium oxide layer. For example, the vertical reflective layer  104   b  may include a metal material (e.g., tungsten, copper, and/or aluminum). 
         [0035]    Alternatively, as illustrated in  FIG. 4 , the first isolation layer  104  may include an air gap AG. The air gap AG may be disposed in the vertical reflective layer  104   b.  In an embodiment, the air gap AG may extend along the vertical reflective layer  104   b  when viewed from a plan view. In an embodiment, the air gap AG may not extend along the vertical reflective layer  104   b  when viewed from a plan view. For example, the air gap AG may exist in a portion of the vertical reflective layer  104   b  but may not exist in another portion of the vertical reflective layer  104   b.    
         [0036]    Referring again to  FIG. 3 , the second isolation layer  106  may be disposed in the semiconductor layer  100  of each of the unit pixel regions PX. The second isolation layer  106  may isolate the first and second photoelectric conversion elements PD 1  and PD 2  from each other in one unit pixel region PX. In an embodiment, as illustrated in  FIG. 2A , the second isolation layer  106  may be spaced apart from the first patterns PT 1  and the second patterns PT 2  of the first isolation layer  104  and may be parallel to the second patterns PT 2 . In an embodiment, as illustrated in  FIG. 2B , the second isolation layer  106  may be in contact with the first patterns PT 1  of the first isolation layer  104  and may be spaced apart from the second patterns PT 2  of the first isolation layer  104 . The second isolation layer  106  may be parallel to the second patterns PT 2  of the first isolation layer  104 . For example, the second isolation layer  106  may be in contact with the vertical insulating layer  104   a  included in the first patterns PT 1  of the first isolation layer  104 . The second isolation layer  106  may vertically overlap with the floating diffusion region FD. 
         [0037]    In an embodiment, the second isolation layer  106  may include the same material as the vertical insulating layer  104   a.  Alternatively, the second isolation layer  106  may include a different material from the vertical insulating layer  104   a.  For example, the second isolation layer  106  may include at least one of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a hafnium oxide layer. 
         [0038]    A width W 1  of the first isolation layer  104  may be different from a width W 2  of the second isolation layer  106 . In an embodiment, the width W 1  of the first isolation layer  104  may be greater than the width W 2  of the second isolation layer  106  (W 1 &gt;W 2 ). A depth of the first isolation layer  104  may be substantially equal to that of the second isolation layer  106 . In other words, a bottom surface of the first isolation layer  104  may be disposed at the substantially same level as a bottom surface of the second isolation layer  106 . The bottom surface of the first isolation layer  104  and the bottom surface of the second isolation layer  106  may be disposed within the semiconductor layer  100 . The bottom surface of the first isolation layer  104  and the bottom surface of the second isolation layer  106  may be spaced apart from the front surface  101   b  of the semiconductor layer  100 . 
         [0039]    The interconnection part P 2  may be disposed on the front surface  101   b  of the semiconductor layer  100 . The interconnection part P 2  may include a plurality of stacked insulating layers and conductive patterns  202  disposed between the insulating layers. In an embodiment, the interconnection part P 2  may include transfer gates TG. The transfer gates TG may be disposed on the front surface  101   b  of the semiconductor layer  100 . In an embodiment, two transfer gates TG may be disposed to correspond to the first photoelectric conversion element PD 1  and the second photoelectric conversion element PD 2  included in one unit pixel region PX, respectively. 
         [0040]    The light filter part P 3  may be disposed on the back surface  101   a  of the semiconductor layer  100 . The light filter part P 3  may include an insulating layer  302 , color filters  304 , and micro-lenses  308 . 
         [0041]    The insulating layer  302  may be disposed on the back surface  101   a  of the semiconductor layer  100 . The insulating layer  302  may cover the back surface  101   a  of the semiconductor layer  100 , a top surface of the first isolation layer  104 , and a top surface of the second isolation layer  106 . In an embodiment, the insulating layer  302  may function as an anti-reflection layer. For example, the insulating layer  302  may include at least one of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a hafnium oxide layer. 
         [0042]    Color filters  304  may be disposed on the insulating layer  302 . The color filters  304  may correspond to the unit pixel regions PX, respectively. In an embodiment, one color filter  304  may vertically overlap with the first photoelectric conversion element PD 1 , the second photoelectric conversion element PD 2 , and the second isolation layer  106  which are disposed in one unit pixel region PX. 
         [0043]    The color filters  304  may include green filters Gb and Gr of  FIG. 1 , blue filters B of  FIG. 1 , and red filters R of  FIG. 1 . In  FIG. 1 , the color filters  304  may be arranged in a Bayer pattern. In the Bayer pattern, a half of the total pixels may be the green filters Gb and Gr which are the most sensitive to human eyes. 
         [0044]    The micro-lenses  308  may be disposed on the color filters  304 . For example, the micro-lenses  308  may be disposed on the color filters  304 , respectively. 
         [0045]    A planarization layer  306  may be disposed between the color filters  304  and the micro-lenses  308 . The planarization layer  306  may cover top surfaces of the color filters  304 . In an embodiment, the planarization layer  306  may include at least one of a silicon oxide layer, a silicon nitride layer, or a silicon oxynitride layer. In an embodiment, the planarization layer  306  may include an organic layer. 
         [0046]      FIGS. 5 and 6  are cross-sectional views taken along the line I-I′ of  FIG. 2A or 2B  to illustrate image sensors according to embodiments of the inventive concepts. In the embodiments of  FIGS. 5 and 6 , the same elements as described in the embodiment of  FIG. 3  will be indicated by the same reference numerals or the same reference designators, and the descriptions to the same elements as in the embodiment of  FIG. 3  will be omitted or mentioned briefly for the purpose of ease and convenience in explanation. 
         [0047]    Referring to  FIG. 5 , a first isolation layer  104  and a second isolation layer  106  may be disposed in the semiconductor layer  100 . In the present embodiment, the first isolation layer  104  and the second isolation layer  106  may have depths different from each other. In an embodiment, a distance between a bottom surface of the first isolation layer  104  and the front surface  101   b  of the semiconductor layer  100  may be smaller than a distance between a bottom surface of the second isolation layer  106  and the front surface  101   b  of the semiconductor layer  100 . For example, the vertical reflective layer  104   b  may penetrate the semiconductor layer  100 . In this case, the vertical insulating layer  104   a  may cover the sidewalls of the vertical reflective layer  104   b.  The second isolation layer  106  may not penetrate the semiconductor layer  100 . 
         [0048]    Referring to  FIG. 6 , a distance between a bottom surface of the second isolation layer  106  and the front surface  101   b  of the semiconductor layer  100  may be smaller than a distance between a bottom surface of the first isolation layer  104  and the front surface  101   b  of the semiconductor layer  100 . In other words, the bottom surface of the second isolation layer  106  may be disposed at a lower level than the bottom surface of the first isolation layer  104 . 
         [0049]      FIGS. 7 and 8  are enlarged views of the portion ‘A’ of  FIG. 1  to illustrate image sensors according to embodiments of the inventive concepts.  FIG. 9  is a cross-sectional view taken along a line II-II′ of  FIG. 7 or 8  to illustrate an image sensor according to embodiments of the inventive concepts. In the embodiments of  FIGS. 7 to 9 , the same elements as described in the embodiments of  FIGS. 2A, 2B, and 3  will be indicated by the same reference numerals or the same reference designators, and the descriptions to the same elements as in the embodiments of  FIGS. 2A, 2B, and 3  will be omitted or mentioned briefly for the purpose of ease and convenience in explanation. 
         [0050]    Referring to  FIGS. 7 to 9 , a first isolation layer  104  and a second isolation layer  106  may be disposed in the semiconductor layer  100 . In an embodiment, the first isolation layer  104  may define unit pixel regions PX of the semiconductor layer  100  and may surround first and second photoelectric conversion elements PD 1  and PD 2  disposed in each of the unit pixel regions PX. In an embodiment, the first isolation layer  104  may include a multi-layer. The first isolation layer  104  may include a first vertical insulating layer  104   a  and a first vertical reflective layer  104   b.    
         [0051]    The second isolation layer  106  may be disposed in each of the unit pixel regions PX. In an embodiment, the second isolation layer  106  may isolate the first and second photoelectric conversion elements PD 1  and PD 2  from each other in each of the unit pixel regions PX. In an embodiment, the second isolation layer  106  may include a multi-layer. The second isolation layer  106  may include a second vertical insulating layer  106   a  and a second vertical and optically reflective layer  106   b,  which operates to confine and channel light incident the image sensor. The second vertical insulating layer  106   a  may cover sidewalls and a bottom surface of the second vertical reflective layer  106   b.    
         [0052]    In an embodiment, as illustrated in  FIG. 7 , the second isolation layer  106  may be spaced apart from the first isolation layer  104 . Thus, the second vertical reflective layer  106   b  may also be spaced apart from the first vertical reflective layer  104   b.  In an embodiment, as illustrated in  FIG. 8 , the second isolation layer  106  may be in contact with the first isolation layer  104 . Thus, the second vertical reflective layer  106   b  may also be in contact with the first vertical reflective layer  104   b.  In other word, the second vertical reflective layer  106   b  may intersect the first vertical reflective layer  104   b  included in the first patterns PT 1  of the first isolation layer  104 . 
         [0053]    Referring again to  FIGS. 7 to 9 , a width W 1  of the first isolation layer  104  may be equal to a width W 2  of the second isolation layer  106  (W 1 =W 2 ). In addition, a depth of the first isolation layer  104  may be equal to a depth of the second isolation layer  106 . Even though not shown in the drawings, the widths and the depths of the first and second isolation layers  104  and  106  may not be limited to these descriptions according to the present embodiment but may be variously modified as described with reference to  FIGS. 3 to 6 . 
         [0054]      FIGS. 10A to 10D  are cross-sectional views taken along the line I-I′ of  FIG. 2A or 2B  to illustrate a method of manufacturing an image sensor according to embodiments of the inventive concepts. 
         [0055]    Referring to  FIGS. 2A, 2B, and 10A , a semiconductor layer  100  may be provided. In an embodiment, the semiconductor layer  100  may be a single-crystalline semiconductor substrate. In an embodiment, the semiconductor layer  100  may be an epitaxial layer formed by an epitaxial growth process. The semiconductor layer  100  may include a back surface  101   a  and a front surface  101   b  opposite to the back surface  101   a.  The back surface  101   a  of the semiconductor layer  100  may be a surface on which light is incident. 
         [0056]    Photoelectric conversion elements  110  may be formed in the semiconductor layer  100 . The photoelectric conversion elements  110  may be formed by performing an ion implantation process through the front surface  101   b  of the semiconductor layer  100 . The photoelectric conversion elements  110  may be doped with, for example, N-type dopants. The floating diffusion regions FD may be formed in the semiconductor layer  100 . In an embodiment, each of the floating diffusion regions FD may be formed between a pair of the photoelectric conversion elements  110  disposed in each of unit pixel regions PX to be defined. The floating diffusion regions FD may be doped with, for example, N-type dopants. 
         [0057]    Transfer gates TG may be formed on the front surface  101   b  of the semiconductor layer  100 . The transfer gates TG may be formed to correspond to the photoelectric conversion elements  110 , respectively. 
         [0058]    An interconnection structure  120  may be formed on the front surface  101   b  of the semiconductor layer  100 . The interconnection structure  120  may include a plurality of stacked insulating layers. In addition, the interconnection structure  120  may further include metal interconnections  202  formed in the stacked insulating layers. The insulating layers of the interconnection structure  120  may cover the transfer gates TG. 
         [0059]    A support substrate  130  may be adhered to a top surface of the interconnection structure  120 . The support substrate  130  may support deposited layers in processes of manufacturing the image sensor. For example, the support substrate  130  may be a silicon substrate or a glass substrate. 
         [0060]    Referring to  FIGS. 2A, 2B, and 10B , the back surface  101   b  of the semiconductor layer  100  may be selectively etched to form first trenches  140   a  and second trenches  140   b  in the semiconductor layer  100 . The first trenches  140   a  may define unit pixel regions PX. The first trenches  140   a  may be formed to surround two photoelectric conversion elements  110  adjacent to each other on all four sides thereof. In other word, the two photoelectric conversion elements  110  may be disposed in each of the unit pixel regions PX. Each of the second trenches  140   b  may be formed in each of the unit pixel regions PX of the semiconductor layer  100 . Each of the second trenches  140   b  may physically separate the two photoelectric conversion elements  110  disposed in each unit pixel region PX from each other. Widths of the first trenches  140   a  may be greater than those of the second trenches  140   b.  Alternatively, the widths of the first trenches  140   a  may be equal to those of the second trenches  140   b.    
         [0061]    Referring to  FIGS. 2A, 2B, and 10C , an isolation insulating layer  151  may be formed on the back surface  101   a  of the semiconductor layer  100 . In an embodiment, the isolation insulating layer  151  may be conformally formed on inner surfaces of the first trenches  140   a  but may completely fill the second trenches  104   b.  When the widths of the second trenches  140   b  are smaller than those of the first trenches  140   a,  the second trenches  140   b  may be completely filled with the isolation insulating layer  151  while the isolation insulating layer  151  is conformally formed on the inner surfaces of the first trenches  140   a.  Thus, empty regions may remain in the first trenches  140   a  after the formation of the isolation insulating layer  151 . For example, the isolation insulating layer  151  may include at least one of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a hafnium oxide layer. 
         [0062]    A reflective layer  153  may be formed on the isolation insulating layer  151 . The reflective layer  153  may cover a top surface of the isolation insulating layer  151  and may completely fill the remaining empty regions of the first trenches  140   a.  In an embodiment, the reflective layer  153  may not completely fill the first trenches  140   a  partially filled with the isolation insulating layer  151  since the first trenches  140   a  are narrow. In this case, an air gap AG of  FIG. 4  may be formed in the reflective layer  153  in the first trench  140   a.  For example, the reflective layer  153  may include a conductive material (e.g., tungsten, copper, or aluminum). 
         [0063]    Referring to  FIG. 10D , an etching process may be performed on the reflective layer  153  to form first isolation layers  104  in the first trenches  140   a  and second isolation layers  106  in the second trenches  140   b.  The etching process may be performed until the back surface  101   a  of the semiconductor layer  100  is exposed. The etching process may include a chemical mechanical polishing (CMP) process and/or an etch-back process. The first isolation layer  104  may have a multi-layered structure. For example, the first isolation layer  104  may include a vertical insulating layer  104   a  and a vertical reflective layer  104   b.  The second isolation layer  106  may have a single-layered structure. The second isolation layer  106  may include the same material as the vertical insulating layer  104   a.    
         [0064]    Referring again to  FIGS. 2A, 2B, and 3 , an insulating layer  302  may be formed on the back surface  101   a  of the semiconductor layer  100 . The insulating layer  302  may cover the back surface  101   a  of the semiconductor layer  100 , a top surface of the first isolation layer  104 , and a top surface of the second isolation layer  106 . For example, the insulating layer  302  may include at least one of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a hafnium oxide layer. 
         [0065]    Color filters  304  may be formed on the insulating layer  302 . The color filters  304  may correspond to the unit pixel regions PX, respectively. In an embodiment, the color filters  304  may be arranged in the Bayer pattern, as illustrated in  FIG. 1 . A planarization layer  306  may be formed on the color filters  304 . In an embodiment, the planarization layer  306  may include at least one of a silicon oxide layer, a silicon nitride layer, or a silicon oxynitride layer. In an embodiment, the planarization layer  306  may include an organic layer. Micro-lenses  308  may be formed on the planarization layer  306 . The support substrate  130  may be removed after the formation of the micro-lenses  308 . 
         [0066]    As described above, the image sensor according to embodiments of the inventive concepts may include the first isolation layer defining the unit pixel region and including the vertical reflective layer, and the second isolation layer disposed in the unit pixel region to isolate the two photoelectric conversion elements from each other. If light incident on the semiconductor layer is irregularly reflected by the second isolation layer, the irregularly reflected light may be reflected by the vertical reflective layer so as to be incident on the photoelectric conversion element of a desired unit pixel region. Thus, crosstalk between unit pixels may be inhibited or prevented to improve the image quality of the image sensor. 
         [0067]    While the inventive concepts have been described with reference to example embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirits and scopes of the inventive concepts. Therefore, it should be understood that the above embodiments are not limiting, but illustrative. Thus, the scopes of the inventive concepts are to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing description.