Abstract:
A CMOS image sensor and a method of fabricating the same are provided. The CMOS image sensor includes: a semiconductor substrate of a first conductivity type having a photodiode region and a transistor region defined therein; a gate electrode formed above the transistor region of the semiconductor substrate with a gate insulating layer interposed therebetween; a first impurity region formed of the first conductivity type in the semiconductor substrate below the gate electrode and having a higher concentration of first conductivity type ions than the semiconductor substrate; and a second impurity region formed of a second conductivity type in the photodiode region of the semiconductor substrate.

Description:
RELATED APPLICATION 
   This application claims the benefit under 35 U.S.C. §119(e), of Korean Patent Application Number 10-2005-0076800 filed Aug. 22, 2005, which is incorporated herein by reference in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor and a method for fabricating the same. 
   BACKGROUND OF THE INVENTION 
   Generally, an image sensor is a semiconductor device that converts an optical image to an electric signal. An image sensor is typically classified into a charge coupled device (CCD) or a complementary metal oxide silicon (CMOS) image sensor. 
   The CMOS image sensor includes a photodiode and a metal oxide semiconductor (MOS) transistor in a unit pixel, and sequentially detects electrical signals of unit pixels in a switching manner to realize an image. 
     FIG. 1  is a photograph view of a punch-through of a CMOS image sensor according to the prior art. 
   In the CMOS image sensor according to the prior art, a wide depletion region is formed due to the low impurity doping concentration of a N − -type diffusion region of a photodiode and the low impurity doping concentration of an epitaxial layer formed on a semiconductor substrate. 
     FIG. 1  shows a punch-through ‘A’ occurring. The punch-through ‘A’ is a phenomenon where a photodiode region  69  contacts a floating diffusion region  72  due to the wide depletion area of the photodiode formed between an N − -type diffusion region of the photodiode  69  and an N + -type floating diffusion region  72 , which is the drain region of a transistor  32 . When the punch-through ‘A’ occurs, a dark current increases, which decreases characteristics of the photodiode. 
   Also, the CMOS image sensor according to the prior art generates noise because of the increase of dark current. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a CMOS image sensor and method for fabricating the same that addresses and/or substantially obviates one or more problems, limitations, and/or disadvantages of the prior art. 
   An object of the present invention is to provide a CMOS image sensor and a method for fabricating the same, capable of improving characteristics of the image sensor by preventing or reducing dark current. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a CMOS image sensor including: a semiconductor substrate of a first conductivity type having a photodiode region and a transistor region defined therein; an impurity region of a second conductivity type formed in the photodiode region of the semiconductor substrate; a gate electrode formed on a portion of the transistor region of the semiconductor substrate with a gate insulating layer interposed between the gate electrode and the substrate; and an impurity region of the first conductivity type formed in a portion of the semiconductor substrate that is located below the gate electrode, wherein the impurity region of the first conductivity type has a higher concentration of impurities than that of the semiconductor substrate. 
   In another aspect of the present invention, there is provided a method for fabricating a CMOS image sensor including: preparing a semiconductor substrate of a first conductivity type having a photodiode region and a transistor region defined therein; forming an impurity region of the first conductivity type in a portion of the transistor region by implanting first conductivity type impurity ions at a higher concentration than that of the semiconductor substrate; forming a gate electrode above the impurity region of the first conductivity type with a gate insulating layer interposed therebetween; and forming an impurity region of the second conductivity type in the photodiode region. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a cross-sectional photo showing a punch-through of a CMOS image sensor according to the prior art; and 
       FIGS. 2 through 8  are cross-sectional views showing a method of fabricating a CMOS image sensor according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   Hereinafter, a CMOS image sensor and a method for fabricating the same according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     FIGS. 2 through 8  are sectional views illustrating a method of fabricating a CMOS image sensor according to an embodiment of the present invention. 
   Although a P-type semiconductor substrate  161  will be used to describe the methods of fabricating a CMOS image sensor, the present invention is not limited to the P-type semiconductor substrate. 
   An embodiment according to the present invention will be described for a 4T-type CMOS image sensor having one photodiode and four transistors. However, the present invention can be also applied to other types of CMOS image sensors such as a 3T-type CMOS image sensor having one photodiode and three transistors and a 5T-type CMOS image sensor having one photodiode and five transistors. 
   The 4T-type CMOS image sensor according to the present invention can have four transistors such as a reset transistor, a transfer transistor, a drive transistor, and a select transistor. 
   Referring to  FIG. 2 , a low concentration P − -type epitaxial layer  162  can be formed by performing an epitaxial process on a high concentration P ++ -type semiconductor substrate  161 . 
   An active region and a device isolation region can be defined in the semiconductor substrate  161 . A device isolation layer  163  can be formed in the device isolation region using, for example, a shallow trench isolation (STI) process or a local oxidation of silicon (LOCOS) process. 
   Referring to  FIG. 3 , a first photoresistive layer  160  can be formed on an entire surface of the P − -type epitaxial layer  162  including where the device isolation layer  163  is formed. The first photoresistive layer  160  can be patterned using an exposure process and a development process to cover a photodiode region and source/drain regions and expose a transfer gate electrode region of the transfer transistor. 
   A P-type impurity region  159  can be formed by implanting P-type impurity ions into the exposed transfer gate electrode region using the patterned first photoresistive layer  160  as a mask. The P-type impurity region has a higher impurity ion concentration than that of the P − -type epitaxial layer  162   
   The P-type impurity region  159  can be formed to a depth that has none or negligible influence on a threshold voltage (V th ) of the transistor. When the p-type impurity region  159  is adjacent to a gate insulating layer  164 , mobility of electrons generated in the photodiode region that move to the transistor region is decreased by increasing the threshold voltage of the transistor. 
   In one embodiment, the P-type impurity region  159  can be formed to a depth spaced about 0.15 to about 0.3 μm from the gate insulating layer  164 . 
   In another embodiment, the P-type impurity region  159  can be formed by implanting ions using implanting energy of about 100˜about 150 KeV. 
   In a further embodiment, the P-type impurity region  159  can be formed by implanting ions to a depth spaced about 0.15˜about 0.3 μm from the gate insulating layer  164  using implanting energy of about 100 to about 150 KeV. 
   Referring to  FIG. 4 , after the first photoresistive layer  160  is removed, a gate insulating layer and a conductive layer can be sequentially deposited on the entire surface of the P − -type epitaxial layer  162 . In a specific embodiment, the conductive layer can be a high concentration poly-crystal silicon layer. Then, the transfer gate electrode  165  can be formed by selectively removing the conductive layer and the gate insulating layer. 
   The transfer gate electrode  165  can be formed to have a larger width than that of the P-type impurity region  159 . When the CD (Critical Dimension) of the P-type impurity region  159  is very large, the number of electrons generated in the photodiode region are decreased due to a narrowed photodiode region. In addition, when the width of the P-type impurity region  159  is very narrow, the P-type impurity region  159  does not properly function as a punch stopper. 
   Accordingly, in an embodiment for a CD (Critical Dimension) of a transistor of about 0.35 μm, a P-type impurity region  159  having a width less than the CD of the transistor and greater than or equal to about 0.15 μm can serve as a punch stopper. 
   Referring to  FIG. 5 , a second photoresistive layer  166  can be formed on an entire surface of the semiconductor substrate  161  including the transfer electrode  165 , and can be patterned by an exposure process and a development process to cover the photodiode region and expose source/drain regions of the transfer transistor. 
   A N − -type diffusion region  167  can be formed by implanting N-type impurity ions at a low concentration into the exposed source/drain regions using the patterned the second photoresistive layer  166  as a mask. In one embodiment, a portion of the gate electrode  165  can be exposed by the exposure and development processes, and the exposed portion of the gate electrode  165  can also be used as the mask. 
   Referring to  FIG. 6 , after the second photoresistive layer  166  is removed, a third photoresistive layer  168  can be formed on an entire surface of the semiconductor substrate  161 , and can be patterned by an exposure process and a development process. 
   A N − -type diffusion region  169  can be formed on the photodiode region by implanting N-type impurity ions into the P − -type epitaxial layer  162  using the patterned third photoresistive layer  168  as a mask. The impurity ion implantation for forming the N − -type diffusion region  169  can be implanted with a higher ion implantation energy than that of the N − -type diffusion region  167  in order to form the N − -type diffusion region  169  of the photodiode region more deeply than the N − -type diffusion region  167  of the source/drain regions. The N − -type diffusion region  169  is deeply formed by the high energy ion implantation energy so as to improve the sensitivity of the image sensor. 
   The N − -type diffusion region  169  formed on the photodiode region can be prevented from a having wide spread of a depletion area by the P-type impurity region  159  formed below the transfer gate electrode  165 . Thus, it is preferable that the P-type impurity region  159  is formed deeper than the N − -type diffusion region  169 . 
   Referring to  FIG. 7 , after the third photoresistive layer  168  is removed, an insulating layer can be deposited on an entire surface of the semiconductor substrate  161 , and insulating layer sidewalls  170  can be formed on both sides of the transfer gate electrode  165  by an etch-back process. 
   A fourth photoresistive layer  171  can be formed on an entire surface of the semiconductor substrate  161 . The fourth photoresistive layer  171  can be patterned by an exposure process and a development process to cover the photodiode region and expose source/drain regions of the transfer transistor. 
   An N + -type floating diffusion region  172  can be formed by implanting N-type impurity ions into the exposed source/drain regions using the patterned fourth photoresistive layer  171  as a mask. 
   Referring to  FIG. 8 , after the fourth photoresistive layer  171  is removed, a fifth photoresistive layer  173  can be formed on an entire surface of the semiconductor substrate  161  and patterned by an exposure process and a development process to expose the photodiode regions. 
   A P o -type diffusion region  174  can be formed in a surface of the semiconductor substrate by implanting P o -type impurity ions into the photodiode region including the N − -type diffusion region  169  using the patterned fifth photoresistive layer  173  as a mask. 
   The CMOS image sensor and the method for fabricating the same according to embodiments of the present invention forms a P-type impurity region by increasing the P-type doping concentration below the gate electrode of the transfer transistor. Therefore, the present invention prevents the depletion area from being widely spread by the N − -type diffusion region  169  and can adjust the critical voltage of the transfer transistor. 
   Thus, the present invention can prevent the punch-through between the N − -type diffusion region  169  of the photodiode and the N + -type floating diffusion region  172  that is defined as the drain region of the transfer transistor. Also, the present invention can improve characteristics of the photodiode by decreasing an increase of dark current. 
   The CMOS image sensor and the method for fabricating the same according to the present invention have the following effect. 
   First, the present invention improves sensitivity of an image sensor by preventing electrons generated by light in the photodiode from leakage before turn-on of the transistor. 
   Second, the present invention improves a low light performance by preventing electrons generated by light from leakage before operation. 
   Third, the present invention can improve the characteristics of the photodiode by decreasing an increase of dark current generated before turn-on of the transistor. 
   Fourth, the present invention extends space for storing electrons at each region of the photodiode region and the floating diffusion region by increasing a capacity of the photodiode region and the floating diffusion region by a narrowed depletion area of the photodiode region and the floating diffusion region. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.