Abstract:
An image sensor includes a plurality of unit pixels for sensing a light beam to generate an image data. Each of the unit pixels includes, a photoelectric element for sensing a light beam incident thereto and generating photoelectric charges, a transistor including a gate dielectric formed adjacent to the photoelectric element and a gate electrode formed on top of the gate dielectric and a capacitor structure including an insulating film formed on a portion of the photoelectric element and a bottom electrode, wherein the insulating film and the gate dielectric are made of a same material and the bottom electrode and the gate electrode are made of a same material.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an image sensor; and, more particularly, to an image sensor incorporating therein a capacitor structure for improving an optical efficiency of the image sensor.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    As is well known, an image sensor is a semiconductor device for sensing a light reflected from an object to generate an image data. Especially, an image sensor fabricated by using a complementary metal oxide semiconductor (CMOS) technology is called a CMOS image sensor.  
           [0003]    Generally, the CMOS image sensor includes a plurality of unit pixels. Each of the unit pixels also includes a light sensing element and a plurality of transistors. The light-sensing element such as a photodiode senses incident light reflected from an object and accumulates photoelectric charges that are generated due to the incident light. The transistors control a transfer of the photoelectric. charges.  
           [0004]    In FIG. 1, there is shown a prior art image sensor  100  including: a silicon substrate  102 , a photodiode  120  formed into the silicon substrate  102  for sensing a light beam incident thereto and generating photoelectric charges, a capacitor structure  130  formed on the silicon substrate  102  and the photodiode  120 , a diffusion region  106 , a transfer transistor  110  formed on the photodiode  120 , the diffusion region  106  and an isolation region  104 .  
           [0005]    In the image sensor  100 , the transfer transistor  110  is provided with a gate dielectric  112 , a gate electrode  114  and a spacer  116 . The transfer transistor  110  is coupled to a sensing node for transferring the photoelectric charges to the sensing node in response to a transfer control signal. The capacitor structure  130  is provided with an insulating film  132 , a bottom electrode  134 , a capacitor dielectric  136  and a top electrode  138 . In case when the photodiode  120  does rot have sufficient capacitance, a certain portion of the photoelectric charges cannot be stored in the photodiode  120 , which will, in turn, leak out into the silicon substrate  102 , thereby decreasing an overall optical efficiency thereof and making noses in the image sensor  100 . These problems can be eliminated by using the capacitor structure  130  which is capable of providing additional capacitances to the photodiode  120 .  
           [0006]    In FIGS. 2A to  2 D, there are illustrated manufacturing steps involved in manufacturing the conventional image sensor  100 .  
           [0007]    The process for manufacturing the conventional image sensor  100  begins with the preparation of an active matrix having a silicon substrate  102 , a transfer transistor structure  110  formed thereon, an isolation region  104  and a photodiode  120  formed into the silicon substrate  102 , as shown in FIG. 2A. An insulating layer  132 , e.g., made of SiOx, is formed over the entire surface by using a chemical vapor deposition (CVD) technique. The photodiode  120  is capable of converting a light beam incident thereto into photoelectric charges. The transfer transistor  110  includes a gate oxide  112 , a gate electrode  114  and a spacer  116 . The transfer transistor  110  is coupled to a sensing node  106  for transferring the photoelectric charges to the sensing node  106  in response to a transfer control signal. The sensing node  106  can be connected to a transistor such as a reset transistor or an amplification transistor not shown for the sake of simplicity.  
           [0008]    Referring to FIG. 23, an insulating layer  132 , e.g., made of silicon oxide (SiO 2 ), is formed on top of the active matrix by using a method such as CVD. Thereafter, the insulating layer  132  is patterned into a first predetermined configuration to form a contact hole.  
           [0009]    In a next step, a bottom electrode layer  134 , a capacitor dielectric layer  136  and a top electrode layer  138  are formed on the insulating layer  132 , successively, as shown in FIG. 2C.  
           [0010]    Finally, the top electrode layer  138 , the capacitor dielectric layer  136  and the bottom electrode layer  134  are patterned into a second predetermined configuration, thereby obtaining a capacitor structure  130 .  
           [0011]    One of the major shortcomings of the above-described image sensor  100  is that it has complex manufacturing steps to form the capacitor structure  130  on the photodiode  120 .  
         SUMMARY OF THE INVENTION  
         [0012]    It is, therefore, an object of the present. invention to provide an image sensor incorporating therein a capacitor structure for improving an optical efficiency thereof.  
           [0013]    It is another object of the present invention to provide a method for manufacturing an image sensor incorporating therein a capacitor for improving an optical efficiency hereof.  
           [0014]    In accordance with an aspect of the present invention, there is provided an image sensor provided with a plurality of unit pixels, each unit pixel comprising: a photoelectric element for sensing a light beam incident thereto and generating photoelectric charges; a transistor including a gate dielectric formed adjacent to the photoelectric element and a gate electrode formed on top of the gate dielectric; and a capacitor structure including an insulating film formed on a portion of the photoelectric element and a bottom electrode, wherein the insulating film and the gate dielectric are made of a same material and the bottom electrode and the gate electrode are made of a same material.  
           [0015]    In accordance with another aspect of the present invention, there is provided a method for manufacturing an image sensor, the method comprising the steps of: a) preparing a silicon substrate; b) forming a first dielectric layer and a first conductive layer, successively; c) patterning the first dielectric layer and the first conductive layer to obtain an insulating film and a bottom electrode of a capacitor structure and a gate dielectric and a gate electrode of a transistor, simultaneously; d) implanting a first type of dopants into a portion of the silicon substrate which is not covered with the insulating film and the gate dielectric and placed therebetween, thereby forming a photoelectric element; e) forming a second dielectric layer; f) removing a portion of the second dielectric layer which is located on top of the photoelectric element, thereby forming a contact hole; g) forming a second conductive layer on top of the second dielectric layer and the contact hole; and h) removing portions of the second conductive layer and the second dielectric layer which are placed on top of the gate electrode and the remaining portion of the photoelectric element. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is a cross-sectional view showing a prior art CMOS image sensor;  
         [0018]    [0018]FIGS. 2A to  2 D provide cross-sectional views presenting a prior art method for manufacture of the CMOS image sensor shown in FIG. 1;  
         [0019]    [0019]FIG. 3 illustrates a cross-sectional view representing a CMOS image sensor in accordance with a first preferred embodiment of the present invention;  
         [0020]    [0020]FIGS. 4A to  4 E are schematic cross-sectional views illustrating a method for the manufacture of a CMOS image sensor shown in FIG. 3;  
         [0021]    [0021]FIG. 5 is a cross-sectional view setting forth a CMOS imager sensor in accordance with a second preferred embodiment of the present invention; and  
         [0022]    [0022]FIGS. 6A to  6 E show schematic cross-sectional views depicting a method for the manufacture of a CMOS image sensor in accordance with a second preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    There are provided in FIGS. 3, 4A to  4 E,  5  and  6 A to  6 E a cross sectional views of CMOS image sensors  200 ,  300  and cross sectional views setting forth methods for the manufacture thereof in accordance with preferred embodiments of the present invention.  
         [0024]    In FIG. 3, there is provided a cross sectional view of the inventive image sensor  200  comprising a silicon substrate  202 , a photo-sensing element  212 , an isolation region  208 , a transfer transistor  210  and a capacitor structure  230 . The photo-sensing element  212  includes an N-type conducting region that is formed in the silicon substrate  202 . The conducting region forms a P-N junction with the P-type material of the silicon substrate  202  to collect photoelectric charges. Therefore, the photo-sensing element  212  is capable of converting a light beam impinging thereon into photoelectric charges. The transfer transistor  210  includes a gate oxide  205 , a gate electrode  207  and a spacer  211 . The transfer transistor  210  is coupled to a sensing node  203 . The sensing node  203  is implanted with N+ dopants for transferring the photoelectric charges to the sensing node  203  in response to a transfer control signal. Although the other devices, e.g., a reset transistor or an amplification transistor, are not shown for the sake of the simplicity, the sensing node can be connected to the other devices.  
         [0025]    The capacitor structure  230  includes an insulating film  231 , a bottom electrode  233 , a spacer  232 , a capacitor dielectric  234  and a top electrode  235 . In the preferred embodiment, the insulating film  231  is made of a material, e.g., SiOx or the like, used for the gate oxide  205 . The bottom electrode  233  is also made of a material, e.g., doped polysilicon or the like, used for the gate electrode  207 . It is possible that the gate oxide  205  and the insulating film  231  can be made of a high K dielectric material such as Ta 2 O 5 .  
         [0026]    In FIGS. 4A to  4 E, there are illustrated manufacturing steps involved in manufacturing the image sensor  200  in accordance with a first preferred embodiment of the present invention.  
         [0027]    The process for manufacturing the image sensor  200  begins with the preparation of a silicon substrate  202  provided with an isolation region  208  and a sensing node  203  formed therein. Thereafter, a first dielectric layer  204 , e.g., made of SiO 2 , is formed on the silicon substrate  202  by using a method such as a chemical vapor deposition (CVD). A first conductive layer  206 , e.g., made of doped polysilicon, formed on top of the first dielectric layer  204  by using a method such as CVD. In order to define a conducting region, a transfer transistor and a capacitor structure, a first photoresist layer is formed on top of the first conductive layer  206  and patterned into a predetermined configuration, thereby obtaining a patterned photoresist layer  209 , as shown in FIG. 4A. It is preferable that the silicon substrate  202  is prepared with forming a P-type epitaxial layer on a P-type substrate, wherein an impurity concentration of the D-type epitaxial layer is lower than that of the P-type substrate.  
         [0028]    In an ensuing step, portions of the first conductive layer  206  and the first dielectric layer  204 , which are not covered with the patterned photoresist layer  209 , are removed by using an etching process, thereby obtaining an insulating film  231 , a bottom electrode  233 , a gate dielectric  205  and a gate electrode  207 , as shown in FIG. 4B. Optionally, a spacer  211  can be formed on sides of the gate dielectric  205  and the gate electrode  207 . A spacer  232  also can be formed on sides of the insulating film  231  and the bottom electrode  233 . Thereafter, first N +  dopants are implanted into a sending node  203  and second N +  dopants are implanted into the conducting region  212 , wherein the impurity of the second N +  dopants is deeper than that of the first N +  dopants.  
         [0029]    In a next step, a second dielectric layer  220  is formed on top of the bottom electrode  233  and the gate electrode  207 . A second photoresist layer is formed on top of the second dielectric layer  220  by using a method such as a spin coating and patterned into a preset configuration  228  to define a contact hole, as shown in FIG. 4C. Thereafter, the second dielectric layer  220  is etched by using a chemical, thereby exposing a portion of the conducting region  212 .  
         [0030]    In a following step, a second conductive layer  222 , e.g., made of doped polysilicon, is formed in the contact hole and formed on top of the second dielectric layer  220 . And then, a third photoresist layer is formed on top of the second conductive layer  222  and patterned into a certain configuration  240  to define a capacitor structure, as shown in FIG. 4D.  
         [0031]    Thereafter, portions of the second conductive layer  222  and the second dielectric layer  220  are removed by using a method such as a chemical etching, thereby obtaining the capacitor structure  230 , as shown in FIG. 4E.  
         [0032]    In comparison with the prior art, the present invention can reduce the steps of the manufacturing the image sensor  2000 . This is achieved by forming elements, e.g., the insulating film  231 , of the capacitor structure  230  and elements, e.g., the gate dielectric  205 , of the transfer transistor  210  in the same process.  
         [0033]    Alternatively, in FIG. 5, there is provided a cross sectional view of an image sensor  300  in accordance with a second preferred embodiment of the present invention. The image sensor  300  comprises a silicon substrate  302 , a photo-sensing element  312 , an isolation region  308 , a transfer transistor  310  and a capacitor structure  330 .  
         [0034]    The inventive image sensor  300  is similar to the image sensor  200  shown in FIG. 3 except that the top electrode  334  does not directly contact to the photo-sensing element  312 . In the second preferred embodiment, the top electrode  334  can be electrically connected to the photo-sensing element  312  through a conducting member  340 .  
         [0035]    In FIGS. 6A to  6 E, there are illustrated manufacturing steps involved in manufacturing the image sensor  300  in accordance with the second preferred embodiment of the present invention.  
         [0036]    The process for manufacturing the image sensor  300  begins with the preparation of a silicon substrate  302  provided with an isolation region  308  and a sensing node  303  formed therein. Thereafter, a first dielectric layer  304 , e.g., made of SiO 2 , is formed on the silicon substrate  302  by using a method such as a chemical vapor deposition (CVD). A first conductive layer  306 , e.g., made of doped polysilicon, formed on top of the first dielectric layer  304  by using a method such as CVD. In order to define a conducting region, a transfer transistor and a capacitor structure, a first photoresist layer is formed on top of the first conductive layer  306  and patterned into a predetermined configuration, thereby obtaining a patterned photoresist layer  309 , as shown in FIG. 6A. It is preferable that the silicon substrate  302  is prepared with forming a P-type epitaxial layer on a P-type substrate, wherein an impurity concentration of the P-type epitaxial layer is lower than that of the P-type substrate.  
         [0037]    In an ensuing step, portions of the first conductive layer  306  and the first dielectric layer  304 , which are not covered with the patterned photoresist layer  309 , are removed by using an etching process, thereby obtaining an insulating film  331 , a bottom electrode  333 , a gate dielectric  305  and a gate electrode  307 , as shown in FIG. 6B. Optionally, a spacer  311  can be formed on sides of the gate dielectric  305  and the gate electrode  307 . A spacer  332  also can be formed on sides of the insulating film  331  and the bottom electrode  333 . Thereafter, first N +  dopants are implanted into a sending node  303  and second N +  dopants are implanted into the conducting region  312 , wherein the impurity of the second N +  dopants is deeper than that of the first N +  dopants.  
         [0038]    In a next step, a second dielectric layer  320  is formed on top of the bottom electrode  333  and the gate electrode  307 . A second conductive layer  322 , e.g., made of doped polysilicon, is formed on the second dielectric layer  320 , successively. And then, a second photoresist layer is formed on top of the second conductive layer  322  and patterned into a certain configuration  336  to define a capacitor structure, as shown in FIG. 6C.  
         [0039]    Thereafter, portions of the second conductive layer  322  and the second dielectric layer  320 , which are not covered with the certain configuration  336  of the second photoresist layer, are removed by using a method such as a chemical etching, thereby obtaining the capacitor structure  330 , as shown in FIG. 6D.  
         [0040]    Finally, a conductive member  340  is formed on top of he photo-sensing element  312  with extending over the top electrode  334  of the capacitor structure  330  in such a way that the photo-sensing element  312  is electrically connected to the top electrode  334 .  
         [0041]    While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invent-on as defined in the following claims.