Patent Publication Number: US-2005139769-A1

Title: Solid state imaging device

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims priority from Japanese Priority Document No. 2003-411261, filed on Dec. 10, 2003 with the Japanese Patent Office, which document is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a solid state imaging device having a device separation area provided around a pixel area, and an electrode provided on a surface of the device separation area.  
      2. Description of Related Art  
      Conventionally, an imaging apparatus such as a video camera, an electronic digital still camera, and the like has employed a solid state imaging device.  
      In a conventional solid state imaging device  100 , each pixel area  103  was configured with a photo diode  101 , and a register  102 , as shown in  FIGS. 2 and 3 , and a device separation area  105  was further formed around the pixel area  103 .  
      The above mentioned device separation area  105  was formed with a P type diffusion area provided at a predetermined position of a P well  106 , and as the P type diffusion area became an electric potential barrier between the pixel area  103  and the peripheral devices, this P type diffusion area was configured to electrically isolate the pixel area  103  and the peripheral devices.  
      In addition, on the surface of the device separation area  105 , a metal electrode  107  was connected, and a potential of the device separation area  105  was fixed to be a ground potential by connecting the metal electrode  107  to the ground.  
      As described above, in the above mentioned conventional solid state imaging device  100 , the positive holes generated at the photo diode  101  were discharged outside the pixel area  103  by connecting the metal electrode  107  to the ground (see Patent Document: Japanese Laid-Open Patent H11-177078, for example).  
     SUMMARY OF THE INVENTION  
      In the solid state imaging device  100  of this conventional example, after the insulating film  108  was formed on the surface of the device separation area  105 , the surface of the device separation area  105  was partially exposed to outside by etching a predetermined position of the insulating film  108 , and after that, the metal electrode  107  was connected to the exposed surface of the device separation area  105 .  
      However, in this conventional solid state imaging device  100 , the surface of the device separation area  105  was exposed to outside by etching a predetermined position of the insulating film  108  when forming the metal electrode  107  on the surface of the device separation area  105  as described above.  
      On this account, some crystal defects were occurred on the surface of the device separation area  105  due to damages by aforementioned etching, and accordingly, there was the threat that unnecessary electrons caused by the crystal defects would mix in the pixel area  103  as a dark current.  
      Thus, if such dark current gets mixed into the pixel area  103 , not only electrons obtained by photoelectric conversion of an incoming light, but also such unnecessary electrons by the dark current were resultantly accumulated in the photo diode  101  and the register  102 .  
      Accordingly, there was the threat that when the thus accumulated electrons were read out from a transfer electrode as a video signal, the unnecessary electrons by the dark current would appear on an image as noises.  
      According to an embodiment of the present invention, a device separation area is provided around each pixel area, and an electrode is provided on a surface of the device separation area, wherein a drain is provided near the electrode in order to discharge electrons generated at the device separation area  
      According to the embodiment of the present invention, the drain is provided adjacent to the device separation area provided next to a photo diode configuring the pixel area.  
      According to the embodiment of the present invention, the drain is provided next to an ohmic area which performs ohmic junction between the electrode and the device separation area.  
      The embodiment of the present invention has following advantages.  
      According to the first aspect of the present invention, in the solid state imaging device having the device separation area provided around the pixel area and the electrode provided on the surface of the device separation area, the drain for discharging the electrons generated at the device separation area is formed near the electrode. Accordingly, unnecessary electrons generated on the surface of the device separation area due to crystal defects are to be discharged to the drain side without mixing into the pixel area, so that it is able to prevent the generation of dark currents caused by the crystal defects in the device separation area.  
      Further, according to the second aspect of the present invention, the drain is provided adjacent to the device separation area provided next to each photo diode forming the pixel area, so that it is able to prevent the unnecessary electrons generated near the surface of the device separation area from mixing into the photo diode.  
      Still further, according to the third aspect of the present invention, the drain is provided adjacent to the ohmic area for performing the ohmic junction between the electrode and the device separation area, so that it is able to discharge the unnecessary electrons to outside of the pixel area from neighbors of the ohmic area where the crystal defects tend to occur. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an explanatory sectional view for describing one embodiment of a solid state imaging device according to the present invention;  
       FIG. 2  is a top view of the embodiment of the present invention in  FIG. 1 , wherein A-A shows a sectional line for the sectional view in  FIG. 1 ;  
       FIG. 3  is an explanatory sectional view for describing a conventional solid state imaging device; and  
       FIG. 4  is a top view of the conventional solid state imaging device in  FIG. 3 , wherein B-B shows a sectional line for the sectional view in  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      A solid state imaging device of the present invention includes a photo diode for converting an incoming light into electrons and positive holes and for accumulating the electrons for a predetermined time, and a register for reading out the electrons thus accumulated in the photo diode as an electrical video signal. In this case, one pixel area is configured to include the photo diode and the register.  
      A device separation area is further provided around the pixel area in order to electrically isolate the pixel area and peripheral devices.  
      This device separation area is formed with an impurity diffused area provided around the pixel area, and this impurity diffused area becomes an electrical potential barrier between the pixel area and the peripheral devices, so that the pixel area and the peripheral devices are electrically isolated to each other.  
      At a predetermined position of the surface side of the device separation area, an ohmic area in which an impurity is highly diffused is provided, and a metal electrode is provided on the surface of the ohmic area.  
      By attaching the metal electrode on the surface of the ohmic area where the impurity is highly diffused, the ohmic area and the metal electrode are connected by the ohmic junction.  
      In addition, the metal electrode is grounded to fix the potential of the device separation area to the ground potential, so that it is so arranged as to discharge the positive holes generated at the photo diode to outside of the pixel area,  
      Further, a drain is provided near the metal electrode, and this drain has a conduction type opposite to the conduction type of the impurity diffused area which forms the device separation area.  
      As described above, by providing the drain near the metal electrode, it becomes possible to discharge the unnecessary electrons generated at the surface of the device separation area due to the crystal defects to the drain side.  
      It is clear that this technology dose not depend on a fabrication process for CCD, CMOS, etc., and the present invention is applicable to a solid state imaging device which at least includes a photo diode, a device separation area, and a metal electrode on a surface thereof.  
      Thereby, the unnecessary electrons generated near the surface of the device separation area do not get mixed into the pixel area as dark current, so that it is able to prevent noises caused by the dark current from appearing on the display image before happens.  
      In addition, the drain is provided adjacent to the device separation area formed next to the photo diode to accumulate only necessary electrons obtained by converting the incoming light, so that it is able to make the quality of the display image much better.  
      Further, the drain is provided adjacent to the ohmic area so as to be able to discharge, to outside of the pixel area, the unnecessary electrons from an area near the junction between the ohmic area and the metal electrode where the crystal defects tend to occur, so that it is able to prevent the noises caused by the dark current from appearing on the display image before happens.  
      Hereinafter, one embodiment of a solid state imaging device of the present invention is described with reference to the attached drawings.  
      A solid state imaging device  1  includes, as shown in  FIG. 1  and  FIG. 2 , a pixel area  4  formed by a photo diode  2  and a register  3 , a device separation area  5  provided around the pixel area  4 , a metal electrode  6  connected to the device separation area  5 , and a drain  7  provided near the metal electrode  6 .  
      The photo diode  2  is formed with a P well  9  of a P type diffusion area provided on the surface of the N type semiconductor substrate  8 , an N type diffusion layer  10  provided at the surface side of the P well  9 , and a P type diffusion layer  11  provided on a surface of the N type diffusion layer.  
      When a light of an object is incident on the photo diode  2 , the light is converted into an electric signal by the photo diode  2 , electron-positive hole pairs are generated at a depletion layer of a PN junction in the photo diode  2 , and only the electrons of the electron-positive hole pairs are accumulated for a predetermined time in the photo diode  2 .  
      On the contrary, the positive holes have a characteristic to move to a low potential, so that the generated positive holes are discharged to outside of the pixel area  4  via a later described metal electrode  6 .  
      The register  3  is formed with a readout electrode  13  and a transfer electrode  14  both being provided on a predetermined position at a surface of the P well  9  via the insulating film  12 , and an N type buried channel  15  provided at the P well  9  immediately beneath the transfer electrode  14 .  
      Then, by applying a voltage to the readout electrode  13 , the electrons accumulated in the photo diode  2  are transferred to the N type buried channel  15 , and thus transferred electrons to this N type buried channel  15  are arranged to be outputted from the transfer electrode  14  as an electrical image signal.  
      The device separation area  5  is formed with the P type diffusion area having a conduction type opposite to those of the N type diffusion layer  10  of the photo diode  2 , and the N type buried channel  15  of the register  3 , and is able to electrically isolate the pixel area  4  from the peripheral devices, because the P type diffusion area serves as an electrical potential barrier between the pixel area  4  and the peripheral devices.  
      A surface of the device separation area  5  is exposed to outside by etching a predetermined portion of an insulating film  12  provided on the surface of the device separation area  5 , and the metal electrode  6  is configured to attach on the exposed surface of the device separation area  5 .  
      Further, near the surface of the device separation area  5  bonding with the metal electrode  6 , an ohmic area  16  to which a P type impurity is highly diffused higher than the device separation area  5  is provided, so that the device separation area  5  and the metal electrode  6  are connected by the ohmic junction.  
      In this case, the metal electrode  6  is connected to ground to fix the potential of the device separation area  5  to the ground potential, thereby the positive holes generated at the photo diode  2  are discharged to outside of the pixel area  4 .  
      The drain  7  is configured with the N type diffuision area provided near the junction between the device separation area  5  and the metal electrode  6 , and is arranged to discharge to outside of the pixel area  4  the unnecessary electrons generated regardless of the light incident on the solid state imaging device  1  such as electrons generated due to the crystal defects of the junction between the device separation area  5  and the metal electrode  6 .  
      That is, the unnecessary electrons generated due to the crystal defects of the junction between the device separation area  5  and the metal electrode  6  are not able to exceed the electrical potential barrier of the device separation area  5 , and do not get mixed into the pixel area  4 , so that it is arranged to discharge these unnecessary electrons to the drain  7  side where there is no electrical potential barrier.  
      For the sake, the unnecessary electrons does not get mixed into the pixel area  4  as the dark current, and accordingly, it is able to prevent the noises caused by the dark current from appearing on the display image before happens.  
      Further, the drain  7  is provided adjacent to the ohmic area  16  at the device separation area  5  provided next to the photo diode  2 , so that it is possible to intensively discharge, to outside of the pixel area  4 , the unnecessary electrons near the junction between the ohmic area  16  and the metal electrode  6  where the crystal defects tend to occur. Accordingly, it is able to prevent the noises caused by the dark current from appearing on the display image before happens.