Abstract:
An image sensor using a back-illuminated photodiode and a manufacturing method thereof are provided. According to the present invention, since a surface of the back-illuminated photodiode can be stably treated, the back-illuminated photodiode can be formed to have a low dark current, a constant sensitivity of blue light for all photodiodes, and high sensitivity. In addition, it is possible to manufacture an image sensor with high density by employing a three dimensional structure in which a photodiode and a logic circuit are separately formed on different substrates.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an image sensor, and more particularly, to an image sensor using a back-illuminated photodiode which absorbs light through its bottom side and a method of manufacturing the image sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    A complementary metal-oxide semiconductor (CMOS) image sensor is a device for measuring light intensity. In general, such image sensor has a structure in which a photodiode is formed on a plane where a digital/analog circuit is formed, and a metal wire and an inter-metal dielectric (IMD) layer are formed on a top side of the photodiode. 
         [0005]    In the conventional image sensor, light passes through a micro-lens and a color filter and thereafter proceeds along a pathway of several films before being absorbed by the photodiode. Throughout this process, sensitivity of light, particularly, blue light, deteriorates. 
         [0006]    Furthermore, in the conventional image sensor, after passing through green, red, and blue color filters, light is reflected by a multi-layered metal wire. This affects an adjacent photodiode, resulting in a color crosstalk. Also, an electrical crosstalk occurs in a bottom side of the photodiode due to red light having a long wavelength. 
         [0007]    In the image sensor using the back-illuminated photodiode, instead of passing through the IMD layer laminated on the top side of the photodiode, light is directly absorbed by a silicon substrate on which the photodiode is formed. Thus, sensitivity of light is significantly improved. 
         [0008]    In order to employ a structure of the back-illuminated photodiode, there is a need to remove noise caused by a surface defect, for example, a dangling bond existing on a surface of an epitaxial layer with low concentration. However, there has been a problem in that heat treatment cannot be performed at a high-temperature because of the existing metal wire. 
         [0009]    Therefore, the conventional image sensor using the back-illuminated photodiode cannot solve a noise problem that is resulted from a defected surface of a silicon substrate on which the back-illuminated photodiode is formed. 
         [0010]    To address this problem, laser scanning is generally used for instantaneous high-temperature heat treatment, which is disadvantageously expensive and time consuming. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides an image sensor using a back-illuminated photodiode which absorbs light through its bottom side instead of its top side, restricts the generation of noise and crosstalk by treating a wafer surface, and thus improves sensitivity, and a method of manufacturing the image sensor. 
         [0012]    According to an aspect of the present invention, there is provided an image sensor using a back-illuminated photodiode, comprising: an epitaxial layer (hereinafter, referred to as epi-layer) formed on a specific type of wafer by doping impurities with low concentration; a potential barrier formed on a surface of the epi-layer; a back-illuminated photodiode formed on the epi-layer formed on a back side of the first wafer; a logic circuit formed on a specific region of the wafer so as to be electrically connected to the back-illuminated photodiode by the use of a metal wire; a pad formed on the potential barrier and electrically connected to an external circuit of the image sensor; and a via formed through the epi-layer and a plug which fills the via so that the pad is electrically connected to the logic circuit. 
         [0013]    According to another aspect of the present invention, there is provided an image sensor using a back-illuminated photodiode, comprising: epi-layers formed on first and second wafers each having a specific type by doping impurities with low concentration; a potential barrier formed on a surface of the epi-layer of the first wafer; a back-illuminated photodiode formed on the epi-layer which is formed on a back side of the first wafer; the second wafer on which a logic circuit is formed; a metal bonding portion by which a metal wire connected to the back-illuminated photodiode formed on the first wafer is electrically connected to a metal wire connected to the logic circuit formed on the second wafer; a pad formed on the potential barrier of the first wafer and electrically connected to an external circuit of the image sensor; and a via formed through the epi-layer and a plug which fills the via so that the pad is electrically connected to the logic circuit of the second wafer. 
         [0014]    According to another aspect of the present invention, there is provided a method of manufacturing an image sensor, comprising the steps of: (a) forming an epi-layer with low concentration on a specific type of wafer, and forming a potential barrier on a surface of the epi-layer; (b) removing a back side of the wafer except for the epi-layer; (c) forming a back-illuminated photodiode and a logic circuit on the surface of the epi-layer exposed in (b); (d) forming an inter-metal dielectric (IMD) layer on the back-illuminated photodiode, and forming a metal wire inside the IMD layer to electrically connect the back-illuminated photodiode and the logic circuit; (e) forming a passivation layer, a color filter, and a micro-lens on a surface of the potential barrier; (f) forming a via connected to the logic circuit through the epi-layer from the surface of the potential barrier, and filling a plug into the via; and (g) forming a pad on a surface of the via filled with the plug. 
         [0015]    According to another aspect of the present invention, there is provided a method of manufacturing an image sensor, comprising the steps of: (a) forming epi-layers with low concentration on first and second wafers each having a specific type, and forming a potential barrier on a surface of the epi-layer of the first wafer; (b) removing a back side of the first wafer except for the epi-layer; (c) forming a back-illuminated photodiode on the surface of the epi-layer exposed in (b); (d) forming an inter-metal dielectric (IMD) layer on the back-illuminated photodiode, and forming a metal wire inside the IMD layer; (e) forming a via connected to the metal wire through the epi-layer from a surface of the potential barrier, and filling a plug into the via; (f) forming an IMD layer on the epi-layer formed on the second wafer, and forming a logic circuit including a metal wire inside the IMD layer; (g) connecting a metal wire of the first wafer to a metal wire of the logic circuit of the second wafer by using metal bonding; (h) forming a passivation layer, a color filter, and a micro-lens on a surface of the potential barrier; and (i) forming a pad on a surface of the via filled with the plug of the first wafer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIG. 1  is a cross-sectional view of a structure of an image sensor using a back-illuminated photodiode according to an embodiment of the present invention. 
           [0017]      FIG. 2  is a cross-sectional view of a structure of an image sensor using a back-illuminated photodiode according to another embodiment of the present invention. 
           [0018]      FIG. 3  is a flowchart of a method of manufacturing an image sensor using a back-illuminated photodiode according to an embodiment of the present invention. 
           [0019]      FIG. 4  illustrates cross-sectional views for explaining major processes of  FIG. 3 . 
           [0020]      FIG. 5  is a flowchart of a method of manufacturing an image sensor using a back-illuminated photodiode according to another embodiment of the present invention. 
           [0021]      FIG. 6  illustrates cross-sectional views for explaining major processes of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0023]      FIG. 1  is a cross-sectional view of a structure of an image sensor using a back-illuminated photodiode according to an embodiment of the present invention. The image sensor is formed of a specific type of wafer  10 , an epitaxial layer (or epi-layer)  20 , a potential barrier  30 , a back-illuminated photodiode  40 , an inter-metal dielectric (IMD) layer  50 , a metal wire  60 , a pad  70 , a via &amp; plug  75 , a passivation layer  80 , a color filter  82 , and a micro-lens  84 . 
         [0024]    The potential barrier  30  is formed on a surface of the epi-layer  20  with low concentration by doping impurities having the same type as the epi-layer  20  and having higher concentration than the epi-layer  20 . 
         [0025]    The potential barrier  30  may be formed by using ion implantation, epitaxial growth, or boro silicate glass (BSG) deposition. 
         [0026]    The back-illuminated photodiode  40  is formed on the epi-layer  20  that is formed on a back side of the wafer  10 , that is, a back side of the potential barrier  30 . 
         [0027]    The IMD layer  50  is formed below the back-illuminated photodiode  40 , that is, in a direction opposite to a portion where the potential barrier  30  is formed. 
         [0028]    The metal wire  60  is formed in the IMD layer  50  so that the back-illuminated photodiode  40  is electrically connected to a logic circuit (not shown) formed on a specific region of the wafer  10 . 
         [0029]    The pad  70  is electrically connected to an external circuit and is formed above the potential barrier  30 . The via  75  is formed to be connected to the metal wire  60  of the logic circuit (not shown) through the epi-layer  20 , and thus the pad  70  is connected to the logic circuit (not shown). The plug is filled in the via  75 . 
         [0030]    As a protection layer, the passivation layer  80  is formed on the potential barrier  30 . A plurality of color filters  82  are formed on the passivation layer  80 . The micro-lens  84  is formed on the color filters  82 . 
         [0031]      FIG. 2  is a cross-sectional view of a structure of an image sensor using a back-illuminated photodiode according to another embodiment of the present invention. A back-illuminated photodiode  40  and a logic circuit (not shown) are separately formed on first and second wafers  11  and  12 . Then, the wafers  11  and  12  are bonded with each other by the use of a metal bonding pad  90 . 
         [0032]    Referring to  FIG. 2 , the first wafer  11  is formed of an epi-layer  20 , a potential barrier  30 , a back-illuminated photodiode  40 , an IMD layer  50 , a metal wire  60 , a pad  70 , a via &amp; plug  75 , a passivation layer  80 , a color filter  82 , a micro-lens  84 , and a bonding pad  90 . Further, the second wafer  12  is formed of an epi-layer  20 , an IMD layer  50 , a metal wire  61 , transistors  92  on a logic region, and a metal bonding pad  90 . 
         [0033]    The metal wires  60  and  61  are respectively formed on the first and second wafers  11  and  12  and are electrically connected with each other by metal bonding. 
         [0034]    In  FIG. 2 , the metal wire  60  connected to the back-illuminated photodiode  40  of the first wafer  11  is electrically connected to the metal wire  61  connected to the logic circuit (not shown) including the transistors  92  on the logic region of the second wafer  12  by the use of the metal bonding pad  90 . 
         [0035]    Unlike the image sensor of  FIG. 1 , the image sensor of  FIG. 2  can be formed to have high density since the logic circuit (not shown) and the back-illuminated photodiode  40  are formed on the different wafers  11  and  12 . 
         [0036]      FIG. 3  is a flowchart of a method of manufacturing an image sensor using a back-illuminated photodiode according to an embodiment of the present invention. The method includes the steps of: forming a potential barrier (step  110 ); removing a back side (step S 120 ); forming a back-illuminated photodiode (step S 130 ); forming a metal wire (step S 140 ); forming a color filter (step S 150 ); forming a via &amp; plug (step S 160 ); and forming a pad (step S 170 ). 
         [0037]    In step  110 , an epi-layer is formed on a specific type of wafer by doping impurities with low concentration, and a potential barrier is then formed on a surface of the epi-layer. 
         [0038]    The potential barrier may be formed using at least one method selected from a group consisting of ion implantation, epi-growth, and BSG deposition. In order to stabilize the surface of the epi-layer on which the potential barrier is formed, heat treatment may be performed at a specific temperature. 
         [0039]    In step S 120 , the back side of the wafer is removed except for the epi-layer. Only the epi-layer remains on the wafer after this process is performed. 
         [0040]    In some cases, a semiconductor substrate, that is, a wafer, may become too thin after its back side is removed in step S 120 , and thus a subsequent process cannot be performed. To ensure a thickness sufficient to perform the subsequent process, a first quartz may be bonded to the surface of the epi-layer before or after the back side is removed. 
         [0041]    In step S 130 , a back-illuminated photodiode is formed on the exposed surface of the epi-layer by using a typical image sensor manufacturing process. A logic circuit is also formed in this step. 
         [0042]    In step S 140 , an IMD layer is formed on the epi-layer surface where the back-illuminated photodiode is formed. Further, a metal wire is formed so that the back-illuminated photodiode formed inside the IMD layer is electrically connected to the logic circuit. 
         [0043]    In step S 150 , a passivation layer, a color filter, and a micro-lens are formed on a surface of the potential barrier formed in step S 110 . 
         [0044]    A second quartz may be bonded to the IMD layer in the presence of the first quartz, and the color filter or the like may be formed after removing the first quartz. 
         [0045]    In step S 160 , a via is formed to be connected to the logic circuit. The via is formed through the epi-layer from the surface of the potential barrier after step S 150 , and a plug is filled in the via. 
         [0046]    In step S 170 , a pad is formed on a surface of the via filled with the plug, and thus the via is electrically connected to an external circuit through the pad. 
         [0047]      FIG. 4  illustrates cross-sectional views for explaining of major processes of  FIG. 3 . A manufacturing process of the image sensor of  FIG. 1  will now be described with reference to  FIG. 4 . 
         [0048]    In step  1 , as shown in the first figure, a potential barrier  30  is formed over an entire surface of an epi-layer  20  with low concentration by doping impurities having the same type as the epi-layer  20  by using ion implantation or the like. Then, the surface of the epi-layer  20  is stabilized by using heat treatment or the like. 
         [0049]    In step  2 , as shown in the second figure, a first quartz  95  is bonded to the epi-layer  20 . In step  3 , as shown in the third figure, a wafer  10  is polished or etched to obtain a desired thickness of the epi-layer  20 . 
         [0050]    After steps  1  to  3  are completed, in step  4 , the wafer  10  is turned over. Then, by using a typical image sensor manufacturing process, a logic circuit including a metal wire  60  and a back-illuminated photodiode  40  are formed on the remaining portion of the epi-layer  20  after etching. 
         [0051]    Thereafter, in order to form a pad  70  through a back side, in step  5 , a via &amp; plug  75  is formed to pass through the epi-layer  20 , and thus the metal wire  60  can be connected to the back side. 
         [0052]    Thereafter, a second quartz  96  is bonded to an IMD layer  50  where the logic circuit is formed. In step  6 , the first quartz  95  on the back side is removed by etching. In step  7 , a pad  70 , a color filter  82 , and a micro-lens  84  are formed. Accordingly, the manufacture of the image sensor using a back-illuminated photodiode and having high sensitivity is completed. 
         [0053]      FIG. 5  is a flowchart of a method of manufacturing an image sensor using a back-illuminated photodiode according to another embodiment of the present invention. The method includes the steps of: forming a potential barrier (step  210 ); removing a back side (step S 220 ); forming a back-illuminated photodiode (step S 230 ); forming a metal wire (step S 240 ); forming a via &amp; plug (step S 250 ); forming a logic circuit (step S 260 ); bonding a wafer (step S 270 ); forming a color filter (step S 280 ); and forming a pad (step S 290 ). 
         [0054]    In step S 210 , epi-layers are formed on first and second wafers by doping impurities of opposite types. A potential barrier is formed on a surface of the epi-layer of the first wafer. 
         [0055]    As previously described with reference to  FIG. 3 , in  FIG. 5 , the potential barrier may also be formed by using at least one method selected from a group consisting of ion implantation, epi-growth, and BSG deposition. In order to stabilize the surface of the epi-layer on which the potential barrier is formed, heat treatment may be performed after the potential barrier of the first wafer is formed. 
         [0056]    In step S 220 , a back side of the first wafer is removed except for the epi-layer. 
         [0057]    In step S 230 , a back-illuminated photodiode is formed on the epi-layer surface exposed when the back side of the first wafer is removed in step S 220 . 
         [0058]    In step S 240 , an IMD layer is formed on the back-illuminated photodiode, and a metal wire is formed inside the IMD layer. 
         [0059]    In step S 250 , a via is formed to be connected to the metal wire, and the via is formed through the epi-layer from the surface of the potential barrier. A plug is filled in the via. 
         [0060]    In step S 260 , an IMD layer is formed on the epi-layer of the second wafer, and a logic circuit including a metal wire is formed inside the IMD layer. 
         [0061]    In step S 270 , the metal wired formed on the first wafer is connected to the metal wire of the logic circuit formed on the second wafer by metal bonding. 
         [0062]    In step S 280 , a passivation layer, a color filter, and a micro-lens are formed on a surface of the potential barrier. 
         [0063]    In step S 290 , a pad is formed on a surface of the via filled with the plug of the first wafer. 
         [0064]    As previously described with reference to  FIG. 3 , in  FIG. 5 , in order to ensure a thickness sufficient to perform a subsequent process, a first quartz may be bonded to the surface of the epi-layer before or after the back side is removed in step S 220 . In the case of boding the first quartz, the first quartz may be removed prior to step S 280 . 
         [0065]      FIG. 6  illustrates cross-sectional views for explaining major processes of  FIG. 5 . A manufacturing process of the image sensor of  FIG. 2  will now be described with reference to  FIG. 6 . 
         [0066]    Steps  1  to  3  are the same as steps  1  to  3  of  FIG. 4 . In step  4 , a photodiode and a control transistor are individually formed by using a typical image sensor manufacturing process. In step  5 , a via &amp; plug  75  is formed to pass through the epi-layer  20 , and thus a metal bonding pad is electrically wired to a logic circuit formed on a separate second wafer  12 . 
         [0067]    After the first wafer  11  on which the photodiode is formed is electrically connected to the separate second wafer  12  on which the logic circuit is formed in step  6  by the use of a metal bonding pad, in step  7 , a first quartz  95  formed on a substrate where the photodiode is formed is etched. In step  8 , a pad  70 , a color filter  82 , and a micro-lens  84  are formed. Accordingly, the manufacture of an image sensor using a back-illuminated photodiode and having high sensitivity and high density is completed. 
         [0068]    In the present invention, light can be absorbed through a bottom side of the photodiode rather than a top side of the photodiode. Thus, a step coverage is negligible from the photodiode to the color filter, thereby increasing efficiency of the photodiode even when a transmission depth is small. In addition, since light does not affect another photodiode after passing through respective color filters, color crosstalk can be avoided. 
         [0069]    Furthermore, since a potential barrier is formed by doping impurities with high concentration on a surface of the epi-layer before the image sensor is manufactured in a conventional method in a state that no pattern is formed on a semiconductor substrate, heat treatment can be easily performed while restricting temperature, which has been an obstacle in the conventional method. Thus, a surface defect can be easily removed, and thus noise caused by a defected surface can be significantly restricted. 
         [0070]    In a method of manufacturing an image sensor using a back-illuminated photodiode according to the present invention, a back side of a silicon substrate can be stability treated. Therefore, a noise problem of a back-illuminated photodiode can be prevented. 
         [0071]    In addition, since light is absorbed through a bottom side of the back-illuminated photodiode, sensitivity is significantly improved, and crosstalk can be restricted. 
         [0072]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.