Patent Publication Number: US-2006017830-A1

Title: Active pixel sensor with isolated photo-sensing region and peripheral circuit region

Description:
BACKGROUND OF INVENTION  
      1. Field of the Invention  
      The present invention relates to an active pixel sensor with an isolated photo-sensing region and peripheral circuit region, and more particularly, to an active pixel sensor which can reduce dark current leakage and increase the fill factor.  
      2. Description of the Prior Art  
      A complementary metal-oxide-semiconductor (CMOS) image sensor is a common solid-state image sensor. Since a CMOS image sensor device is produced by conventional semiconductor techniques, the CMOS image sensor has advantages of low cost and reduced device size. In addition, the CMOS image sensor further has advantages of high quantum efficiency and low read-out noise. The CMOS image is therefore commonly used in photoelectric products, such as PC cameras and digital cameras.  
      Please refer to  FIG. 1  and  FIG. 2 .  FIG. 1  is a diagram of the prior art active pixel sensor  10  of CMOS image sensor device.  FIG. 2  is the circuit of the active pixel sensor  10  of  FIG. 1 . Active pixel sensor  10  comprises a photo diode D 1  for sensing the light, and three metal-oxide semiconductor (MOS) transistors M 1 ˜M 3 , including a row selector M 1 , a source follower M 2 , and a reset MOS M 3 . The photo diode D 1  induces photo current according to the light received from the photo-sensing region. The row selector M 1  is used to select whether to output the voltage signal integrated by the photo diode D 1  or not. The output of the source follower M 2  is modulated according to the charge of the photo diode D 1 . The reset MOS M 3  is used to reset the photo diode D 1 ; that is, when the reset MOS M 3  is “on”, the voltage of the photo diode D 1  is retained at a constant voltage, which does not change with the light received from the photo-sensing region. On the other hand, when the reset MOS M 3  is “off”, the voltage of the photo diode D 1  changes with the light received from the photo-sensing region.  
      The active pixel sensor  10  is produced by conventional semiconductor techniques. It has advantages of low cost and reduced device size. However, the drawbacks are that current leakage occurs in the high slope area of the diffusion region between the reset MOS M 3  and the photo diode D 1  and that the fill factor is reduced. Generally, a high fill factor represents higher photo-sensitivity. The equation of fill factor is as follows:  
       ff   =       Av   A     ×   100   ⁢           ⁢   %         
 
      where ff represents the fill factor;  
      A represents the entire area of the active pixel sensor; and  
      Av represents the area of the photo-sensing region.  
      The current leakage occurs at the connection between the depletion region and the isolation region. The problems of current leakage and reducing the fill factor are discussed as follows.  
      Please refer to  FIG.3 .  FIG. 3  is a cross sectional diagram along line  3 - 3  ′ of the active pixel sensor  10  of  FIG. 1 . The prior art photo diode D 1  includes a P-type substrate  12 , an N− doped region  16 , an N+ doped region  18 , and a shallow trench isolation (STI)  20 . There is a depletion region  14  between the P-type substrate  12  and the N− doped region  16 . When the depletion region  14  contacts the flat area of the STI  20 , current leakage is small due to well oxide surface control. As shown in  FIG. 3 , current leakage is small at the part along line  3 - 3 ′ of the active pixel sensor  10 .  
      Please refer to  FIG. 4 .  FIG. 4  is a cross sectional diagram along line  4 - 4  ′ of the active pixel sensor  10  of  FIG. 1 . One end of the depletion region  14  contacts the N+ doped region  18  and the other end of the depletion region  14  contacts the flat area of the STI  20 . The current leakage is small because the depletion region  14  does not contact the high slope area of the STI  20 .  
      Please refer to  FIG. 5  and  FIG. 6 .  FIG. 5  is a cross sectional diagram along line  5 - 5 ′ of the active pixel sensor  10  of  FIG. 1 .  FIG. 6  is a three dimensional diagram of the active pixel sensor  10  of  FIG. 5 . As shown in  FIG. 6 , one end of the depletion region  14  strides across the high slope area of the STI  20  (as the high slope area  15  in  FIG. 5 ), a PN junction therefore is formed. The markable current leakage occurs in the PN junction.  
      Please refer to  FIG. 3  and  FIG. 4  again. The cross sectional diagram of the part above line  5 - 5 ′ is shown as  FIG. 3  while the cross sectional diagram of the part below line  5 - 5 ′ is shown as  FIG. 4 . Therefore, the PN junction is formed in the high slope area  15  of the STI  20  in the left side of  FIG. 5  to cause large current leakage.  
      Large dark current leakage will induce a large fixed pattern noise (FPN) in low light condition and suffer the image quality.  
      Please refer to  FIG. 7  and  FIG. 8 .  FIG. 7  is a diagram of the active pixel sensor  30  that can improve the dark current leakage.  FIG. 8  is a cross sectional diagram along line  8 - 8 ′ of the active pixel sensor  30  of  FIG. 7 . In  FIG. 8 , the two ends of the depletion region  14  both contact the N+ doped region  18  so that the depletion region  14  does not stride across the high slope area of the STI  20  to form the PN junction. This avoids generating markable current leakage, but reduces the fill factor. Please refer to  FIG. 7 . The two ends of the depletion region  14  both contact the N+ doped region  18 . Therefore, the area of the photo-sensing region of  FIG. 7  surrounded by the dotted line is smaller than that of  FIG. 1 . In other words, the fill factor of  FIG. 7  is smaller than that of  FIG. 1 . That is, the fill factor of the active pixel sensor  30  is reduced. Although the active pixel sensor  30  improves dark current leakage, the fill factor of the active pixel sensor  30  is reduced.  
      As mentioned above, the fill factor of the active pixel sensor  10  of  FIG. 1  is larger. However, there is a large current leakage occurring in the diffusion region between the reset MOS M 3  and the photo diode D 1 , as shown in  FIG. 5 . The active pixel sensor  30  solves the problem of current leakage, but the fill factor of the active pixel sensor  30  is reduced. Therefore, a solution is needed to solve the problem of the current leakage while promoting the fill factor.  
     SUMMARY OF INVENTION  
      It is therefore a primary objective of the claimed invention to provide an active pixel sensor with isolated photo-sensing region and peripheral circuit region to solve the above-mentioned problem.  
      The present invention discloses an active pixel sensor including a substrate, a photo-sensing region, a peripheral circuit region, and an isolation region. The photo-sensing region and the peripheral circuit region are formed on the substrate. The isolation region is formed between the photo-sensing region and the peripheral circuit region for isolating the photo-sensing region and the peripheral circuit region. The photo-sensing region induces photo current according to the received light. The peripheral circuit region includes a first, second, and third transistors. The first transistor has a source connected to a bit line. The second transistor has a gate connected to the photo-sensing region, a source connected to the drain of the first transistor, and a drain connected to a voltage source. The third transistor has a source connected to the photo-sensing region and a drain connected to the voltage source. The first transistor is used to select whether to output data stored in the photo-sensing region or not. The third transistor is used to reset the photo-sensing region.  
      These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a diagram of a prior art active pixel sensor of a CMOS image sensor device.  
       FIG. 2  is the circuit of the active pixel sensor of  FIG. 1  .  
       FIG. 3  is a cross sectional diagram along line  3 - 3 ′ of the active pixel sensor of  FIG. 1 .  
       FIG. 4  is a cross sectional diagram along line  4 - 4 ′ of the active pixel sensor of  FIG. 1 .  
       FIG. 5  is a cross sectional diagram along line  5 - 5 ′ of the active pixel sensor of  FIG. 1 .  
       FIG. 6  is a three dimensional diagram of the active pixel sensor of  FIG. 5 .  
       FIG. 7  is a diagram of the active pixel sensor that can solve the problem of current leakage.  
       FIG. 8  is a cross sectional diagram along line  8 - 8 ′ of the active pixel sensor of  FIG. 7 .  
       FIG. 9  is a diagram of the active pixel sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION  
      In order to solve the prior art problems, the present invention re-designs the layout of the active pixel sensor of CMOS image sensor device. Please refer to  FIG. 9 .  FIG. 9  is a diagram of the active pixel sensor  40  according to the present invention corresponding to the circuit of  FIG. 2 . The active pixel sensor  40  includes a substrate  12 , a photo-sensing region  46 , a peripheral circuit region  44 , and an isolation region  48 . The photo-sensing region  46 , the peripheral circuit  44  and the isolation region  48  are formed on the substrate  12 . The isolation region  48  is formed between the photo-sensing region  46  and the peripheral circuit region  44  for isolating the photo-sensing region  46  and the peripheral circuit region  44 .  
      The photo-sensing region  46  includes a first diffusion region  16  formed on the substrate  12 , a second diffusion region  18  formed above the first diffusion region  16 , and a depletion region  14  formed between the first diffusion region  16  and the substrate  12  for receiving light to induce photo current. The doping concentration of the second diffusion region  18  is greater than the doping concentration of the first diffusion region  16 .  
      The peripheral circuit region  44  includes a first transistor M 1 , a second transistor M 2 , and a third transistor M 3 . The first transistor M 1  has a source connected to a bit line. The second transistor M 2  has a gate connected to the photo-sensing region  46 , a source connected to the drain of the first transistor M 1 , and a drain connected to a voltage source VDD. The third transistor M 3  has a source connected to the photo-sensing region  46  and a drain connected to the voltage source VDD. The first transistor M 1  is used to select whether to output data stored in the photo-sensing region  46  or not. The third transistor M 3  is used to reset the photo-sensing region  46 . The operation of the three transistors M 1 -M 3  is described above thereby omitted herein.  
      Since the present invention isolates the photo-sensing region  46  and the peripheral circuit region  44 , a metal conductor  42  is used for connecting the photo diode D 1  to the gate of the second transistor M 2  and connecting the photo diode D 1  to the source of the third transistor M 3 . In other words, the gate of the second transistor M 2  is connected to the second diffusion region  18  of the photo-sensing region  26  through the metal conductor  42 , and the source of the third transistor M 3  is connected to the second diffusion region  18  of the photo-sensing region  26  through the metal conductor  42 . Compared to the prior art, the prior art uses diffusion connection to connect the third transistor M 3  and the photo diode D 1 , causing current leakage to occur. The present invention uses the metal conductor  42  to connect the source of the third transistor M 3  to the second diffusion region  18  of the photo-sensing region  46 . Thus, the present invention avoids forming the PN junction in  FIG. 5  that generates current leakage.  
      Please refer to  FIG. 1 ,  FIG. 7  and  FIG. 9  again. According to the new layout of the active pixel sensor  40  of the present invention, the entire area of the photo-sensing region  46  of the photo diode D 1  (dotted line in  FIG. 9 ) is greater than those of  FIG. 1  and  FIG. 7 . The present invention therefore can promote the fill factor further to improve the resolution.  
      In addition, the substrate  12  of the active pixel sensor  40  of the present invention is a P-type substrate; the first and second diffusion regions  16 ,  18  of the photo-sensing region  46  are N-type regions; and the three transistors M 1 ˜M 3  of the peripheral circuit region  44  are NMOS. Please refer to  FIG. 2  and  FIG. 9 . Due to the layout of  FIG. 9 , the drain of the first transistor M 1  and the source of the second transistor M 2  coexist in the same doped region, and the drain of the second transistor M 2  and the drain of the third transistor M 3  coexist in the same doped region. The isolation region  48  formed between the photo-sensing region  46  and the peripheral circuit region  44  is a shallow trench isolation layer (STI) or a field oxide layer (FOX) for isolating the photo-sensing region  46  and the peripheral circuit region  44 . In addition to the isolation region  48  between the photo-sensing region  46  and the peripheral circuit region  44 , there are isolation regions, which are shallow trench isolation layer or field oxide layer, surrounding the photo-sensing region  46  and the peripheral circuit region  44 . Moreover, the embodiment of the present invention uses NMOS for the three transistors M 1 ˜M 3 . However, the present invention can take other materials such as PMOS for the three transistors M 1 ˜M 3  for modifications and alterations.  
      Compared to the prior art, the present invention isolates the photo-sensing region  46  and the peripheral circuit region  44 . In other words, the present invention isolates the transistor M 3  and the photo diode D 1  to solve the prior art current leakage occurring in the diffusion region between the transistor M 3  and the photo diode D 1  because there is no the PN junction formed between the depletion region and the high slope area of the STI to generate current leakage, as shown in  FIG. 5  and  FIG. 6 . Furthermore, the present invention can enormously promote the fill factor to improve the resolution because the diffusion region is completely within the depletion region.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.