Abstract:
A semiconductor based image sensor having a plurality of pixels formed on a surface of the image sensor, such that each of the pixels has a photodetector configured to collect majority carriers created from incident light; a region within each of the photodetectors that is narrowed, the narrowed region of the photodetector being electrically coupled to a drain for the majority carriers; a reset means; a transistor for converting photo-charge to voltage or current. The narrowed region provides a path for excess photoelectrons in the photodetector to the drain. The narrow regions path to the drain, in the preferred embodiment, is that of the drain used for the adjacent transistor.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of solid state photo-sensors and imagers, and more specifically to imagers referred to as Active Pixel Sensors, (APS). 
     BACKGROUND OF THE INVENTION 
     APS are solid state imagers where each pixel contains a photo-sensing means, reset means, a charge transfer means, a charge to voltage conversion means, and all or part of an amplifier. One undesirable phenomenon that can occur with solid state image sensors is referred to as blooming. This occurs when an extremely bright region in an image produces excess photoelectrons that can traverse the isolation regions that surround the photodetector in the pixel in which they were created and end up in other pixels or charge transfer regions within the image sensor and corrupt the photoelectron packet that was created by the irradiance incident upon the other pixels. Prior art APS devices have provided protection against blooming by keeping either the transfer gate and/or reset gate “off-level” electrostatically deeper than zero volts so that if more photoelectrons are generated than that which can be held by the photodetector, the excess photoelectrons can spill over the transfer gate and/or reset gate into the floating diffusion and/or reset supply. This approach has the disadvantage of diminishing the amount of photoelectrons that can be held by the photodetector, and in the case of a transfer gated pixel architecture, does not provide any blooming protection during read out of the sensor. Blooming protection can also be provided by inclusion of a lateral overflow drain (LOD) and lateral overflow gate (LOG) or a vertical overflow drain (VOD) within each pixel. Use of LOD and LOG has the disadvantage of decreasing fill factor. Inclusion of a VOD requires a more complex and thus a higher cost process. 
     In addition, APS devices have been operated in a manner where each line or row of the imager is reset, integrated and read out at a different time interval than each of the remaining lines or rows. In other words, the image capture for each row is done sequentially with the image capture for each row temporally displaced from every other row, with each row having the same integration time. Hence if one were reading out the entire imager, each line would have captured the scene at a different point in time. Since illumination conditions can and do vary temporally, and since objects in the scene may also be moving, this method of read out can produce line artifacts in the resulting representation of the image. This limits the usefulness of APS devices in applications where high quality motion or still images are required. In U.S. Pat. No. 5,986,297, entitled COLOR ACTIVE PIXEL SENSOR WITH ELECTRONIC SHUTTER, ANTIBLOOMING AND LOW CROSS-TALK, BY Robert M. Guidash, et al., disclosed is a means to mitigate these artifacts by performing frame integration, followed by row at a time readout. Since the readout time in this mode of operation can be as long as 30 msec. for the last row of pixels being read out, it is even more important to provide blooming protection during read out. 
     From the discussion above, it is evident that there is a need to provide blooming protection during integration and read out that does not adversely affect the fill factor of the pixel. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. It provides a means for blooming protection during integration and read out that does not affect the fill factor of the pixel. Briefly summarized, according to one aspect of the present invention, a semiconductor based image sensor having a plurality of pixels formed on the surface of the image sensor, each of the pixels further comprising: a photodetector configured to collect majority carriers created from incident light; at least one transistor configured to convert photocharge to a voltage or current; a drain for majority carriers; a region in between the photodetector drain that has no separate control gate, the region having an electrostatic potential that is shallower than the photodetectors but deep enough to provide a path for photoelectrons to preferentially flow from the photodetector to the drain. 
     These and other aspects, objects, features, and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     Advantageous Effect of the Invention 
     The present invention has the advantages of providing blooming protection during both integration and readout, electronic frame integration, and high fill factor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a diagram of a prior art pixel; 
     FIG. 1 b  is another diagram of a prior art pixel; 
     FIG. 1 c  is another diagram of a prior art pixel; 
     FIG. 1 d  is top view of the prior art pixel shown in FIG. 1 c;    
     FIG. 2 a  is a diagram of a pixel as envisioned by the present invention; 
     FIG. 2 b  is a top view of a pixel architecture as envisioned by the present invention 
     FIG. 3 a  is a diagram illustrating the electrostatic potential within a wide width photodetector section; 
     FIG. 3 b  is a diagram illustrating the electrostatic potential within a narrow width photodetector section; 
     FIG. 3 c  is a diagram comparing the electrostatic potential between wide and narrow regions of a photodetector within a pixel as envisioned by the present invention; 
     FIG. 4 a  is a diagram illustrating another embodiment of the lateral overflow region as envisioned by the present invention; 
     FIG. 4 b  is a cross sectional diagram along the line AA′ of FIG. 4 a  with a corresponding electrostatic potential diagram; 
     FIG. 4 c  is a cross sectional diagram along the line BB′ of FIG. 4 a  with a corresponding electrostatic potential diagram; 
     FIGS. 5 a  and  5   b  are diagrams illustrating another embodiment of the lateral overflow region as envisioned by the present invention with a corresponding electrostatic potential diagram. 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Typical prior art APS pixels are shown in FIGS. 1 a  and  1   b . The pixel in FIG. 1 a  comprises a photodetector  12 , typically a photodiode (PD), a reset transistor  14  with a reset gate  15  (RG), a row select transistor  16  with a row select gate  17  (RSG), and signal transistor  18  (SIG). Light incident upon the photodetector creates free electrons that are confined to the photodetector by an isolation region, or region, surrounding the photodetector. The isolation regions typically comprise a field oxide region surrounding the photodetector, and a transfer gate or reset gate that when appropriately biased confine photocharge to the photodetector. Blooming protection is to provided by keeping the off-level of the reset gate  15  at an electrostatic potential (the conduction band minima) deeper than zero volts so that excess photoelectrons can flow through the region under the reset gate  15  into VDD, which is a drain for the photoelectrons. This is depicted by inclusion of the electrostatic potential diagram for the pixel cross section. The electrostatic potential is shown by the dotted line in FIG. 1 a . The electrostatic potential of the isolation regions are typically 0 volts In the pixel design shown in FIG. 1 a , blooming protection is essentially inherent, but this pixel architecture cannot perform frame integration without use of a mechanical shutter. The pixel in FIG. 1 b  comprises a photodetector  12 , that can be either a photodiode or photogate, a transfer gate  11 , a floating diffusion  13 , a reset transistor  14  with a reset gate  15 , a row select transistor  16  with a row select gate  17 , and signal transistor  18 . Blooming protection can be done in the same manner as described for the pixel in FIG. 1 a . The transfer gate  11  and reset gate  15  off-levels are kept at an electrostatic potential deeper than zero volts so that excess photoelectrons flow through the region under transfer gate  11  into the floating diffusion  13 , through the region under reset gate  15  and into VDD. Once again this is depicted by the dotted line diagram of electrostatic potential for the pixel cross section. This method will provide blooming protection only during integration. It will not provide blooming protection during read out, (i.e. when one is sampling the signal stored on the floating diffusion  13 ). 
     Referring to FIG. 1 c , the problem discussed above relating to blooming protection during readout is addressed by the inclusion of lateral overflow gate (LOG)  9  and lateral overflow drain (LOD)  8 . Now excess photoelectrons can flow through the region under LOG  9  and into the LOD  8  so that the excess photoelectrons cannot corrupt the signal on the floating diffusion  13  during read out. Inclusion of an LOG  9  and LOD  8  within the pixel reduces fill factor leading to inferior sensitivity. This is also shown in top view in FIG. 1 d.    
     This invention provides a means to provide blooming protection during integration and read out, without impacting fill factor, for a pixel architecture that can be used to perform electronic frame integration. Some physical embodiments of the new pixel architecture are shown in FIGS. 2 a  through FIG.  5 . While other specific physical embodiments will be realizable, these are chosen for illustration because they represent the most preferred embodiments of the present invention. FIG. 2 a  illustrates a pixel comprising: a photodetector  22  (either a photodiode or a photogate); a transfer gate (TG)  21 ; a floating diffusion (FD)  23 ; a reset transistor  24  with a reset gate (RG)  25 ; a row select transistor  26  with a row select gate (RSG)  27 ; a signal transistor (SIG)  28 ; and a lateral overflow region (LOR)  39 . It is the LOR  39  that provides a path to reach the VDD of an adjacent pixel  38 . The lateral overflow region  39  is realized by using 2-dimensional narrow width effects to provide a region that has an electrostatic potential that is deeper than the off potential used on TG  21 , but shallower than the collection potential for photodetector  22 . The electrostatic potential is shown by the dotted line for the pixel cross section provided in FIG. 2 a . The LOR  39  is placed between the photodetector  22  and a VDD  38  for an adjacent pixel. It is envisioned that the, VDD of the pixel itself can be used to remove charge from photodetector  12 , however, for the purposes of a preferred embodiment, the best mode takes into account the overall architectural design of the image sensor. Accordingly, issues such as fill factor dictate that it is preferred that VDD  38  be the drain VDD of another pixel, or pixels. 
     The LOR  39  can be formed in various ways. A top view of one method is shown in FIG. 2 b . In this case the active area region and implants or diffusions used to form the photodetector  22 , are used to create a narrow region for the LOR  39  that is essentially the same as the photodetector  22  only significantly narrower and situated between the photodetector and a VDD region  38 . 
     FIGS. 3 a - 3   c  illustrates the electrostatic potential of a narrow vs. wide photodetector region showing how narrow width effects can be used to create a region with an electrostatic potential that is deeper than zero volts but shallower than the photodetector potential. FIG. 3 a  illustrates the electrostatic potential for the wide region used to create the photodetector  22 . FIG. 3 b  illustrates the electrostatic potential for the narrow region used to create the LOR  39 . Again referring to FIG. 2 a , as photoelectrons begin to fill the photodetector  22 , excess photoelectrons will flow through the LOR  39  and into VDD. The electrostatic potential level of LOR  39  is lower than the electrostatic potential for transfer gate  21  in the off-state, accordingly, excess photoelectrons cannot flow over transfer gate  21  onto the floating diffusion  23  and corrupt the signal level during read out. Since there have been no added components placed within the pixel  20 , the fill factor of pixel  20  is left unchanged. 
     Another means for producing an LOR for a photogate based pixel  40  architecture is shown in FIG.  4 . In this case the active area and photogate regions  42  are used to create the LOR  49 . A separate p-type implant could also be used in the narrow active area region underneath photogate  42  to make the effective threshold voltage of that region higher than that of the photogate photodetector region. In either case the result is that when photogate  42  is biased in depletion in order to collect photoelectrons, the electrostatic potential of the LOR  49  region is shallower than that of the photogate  42  photodetector region but deeper that that of TG  43  in its off-state. 
     FIG. 4 b  is a cross sectional diagram along the line AA′ of FIG. 4 a  with a corresponding electrostatic potential diagram that illustrates the electrostatic potential of LOR  49  with respect to VDD and the photogate  42 . FIG. 4 c  is a cross sectional diagram along the line BB′ of FIG. 4 a  with a corresponding electrostatic potential diagram illustrating the electrostatic potential of the LOR  49  with respect to the field oxide region on either side of LOR  49 . The electrostatic potentials is represented by the dotted lines in FIGS. 4 a  and  4   b  and the overall operation is similar to that described for FIGS. 2 a  and  2   b.    
     In addition to using narrow active area regions and implants to create the LOR, a narrow field oxide region can be used. This is seen in pixel  50  as shown in FIG.  5 . By making the field oxide region in between the photodetector  52  and VDD of the adjacent pixel narrow, the depletion regions from the photodetector and VDD begin to merge. As a result the electrostatic potential of the field region is pulled deeper than 0 volts. By properly designing the width of this narrow field oxide region  59 , its electrostatic potential can be adjusted to be deeper than the off potential of the transfer gate, the reset gate, and the electrostatic potential of the field oxide isolation regions, but shallower than the electrostatic potential of the photodetector. Hence, blooming of photoelectrons is prevented. Operation is similar to that already described. 
     Another method for forming an LOR is envisioned by selective blocking of the field threshold adjust implant. This implant is used in CMOS processes and comprises a boron field threshold adjust implant. It is typically employed in the field oxide regions to prevent depletion and inversion of the field oxide region. This implant can be selectively blocked in a narrow region in between the photodetector and VDD to create a LOR. 
     The foregoing description of the preferred embodiments has detailed the best modes envisioned by the inventor, obvious variations will be readily apparent to those skilled in the relevant art, accordingly, the scope of the present invention should be measured by the appended claims. 
     Parts List 
       8  lateral overflow drain 
       9  lateral overflow gate 
       10  pixel 
       11  transfer gate 
       12  photodetector 
       13  floating diffusion 
       14  reset transistor 
       15  reset gate 
       16  row select transistor 
       17  row select gate 
       18  signal transistor 
       20  pixel 
       21  transfer gate 
       22  photodetector 
       23  floating diffusion 
       24  reset transistor 
       25  reset gate 
       26  row select transistor 
       27  row select gate 
       28  signal transistor 
       38  VDD used as a drain for LOR 
       39  lateral overflow region (LOR) 
       40  pixel 
       42  photogate 
       49  Lateral overflow region 
       50  pixel 
       52  photodetector 
       59  narrow field oxide region 
     VDD Drain of majority carriers (power source)