Abstract:
A method for transferring charge from a photo sensing region to a charge-coupled device in an image sensor, the method comprises the steps of providing a plurality of pixels including a photo sensing region for collecting photo-generated charge and a charge-coupled device region between which is a transfer region; providing a barrier in the charge-coupled device for causing the transfer of the charge through the charge-coupled device; and providing the transfer region having a lower barrier than the charge-coupled device so that excess charge in the charge-coupled device flows into the photo sensing region.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to the field of image sensors having a photodiode adjacent a charge-coupled device (CCD) and, more particularly, to such image sensors in which the CCD receives its maximum capacity of charge from the photodiode and any excess charge capacity of the photodiode charge flows back into the photodiode. This stored excess charge in the photodiode is subsequently transferred back out to the CCD and added to the charge for that particular photodiode.  
         BACKGROUND OF THE INVENTION  
         [0002]    The typical structure of an interline CCD pixel  5  is shown in FIG. 1. It consists of a photodiode  10  adjacent to a CCD  20 . The photodiode  10  converts incident light photons into charge. The transfer of charge through the CCD  20  is generally controlled by at least two control gates,  40  and  50 . One of the gates  50  has a transfer gate region  30  which controls the transfer of charge from the photodiode  10  to the CCD  20 . The pixels  5  are arranged in a two-dimensional array of at least one row and one column.  
           [0003]    [0003]FIG. 2 shows a cross section A-B of FIG. 1. This cross section shows the additional detail of the pixel  5  including the gate insulator  70  separating the gate  50  from the CCD  20 . There is also a light shield  60  to prevent photo charge from being generated in the CCD  20 . The photodiode  10  charge is cleared by applying a high voltage pulse to the substrate  80 .  
           [0004]    The typical sequence of events in a camera to acquire an image using an external shutter are (a) open external shutter to start the exposure time; (b) wait for the desired exposure time; (c) close the external shutter to end the exposure time; (d) transfer charge from the photodiode  10  to the CCD  20 ; and (e) transfer charge through the CCD  20  by clocking the control gates  40  and  50  to move charge towards a charge measurement structure.  
           [0005]    Sometimes very short exposure times are desired, i.e., much shorter than what can be obtained by an external shutter. In this case, the electronic shuttering capabilities of an interline CCD are used. The typical sequence of events to acquire an image using the electronic shutter are (a) open external shutter; (b) pulse the substrate  80  voltage to clear the photodiode  10 ; (c) wait for the desired exposure time; (d) transfer charge from the photodiode  10  to the CCD  20  to end the exposure time; (e) close the external shutter; (e) transfer charge through the CCD  20  by clocking the control gates  40  and  50  to move charge towards a charge measurement structure. When using the electronic shutter every pixel must transfer its charge from the photodiodes to the CCD simultaneously. The simultaneous charge transfer prevents unwanted artifacts when capturing an image of a fast moving object. The charge capacity of the photodiodes must be designed to be smaller than the charge capacity of the CCD. If the photodiodes contain more charge than the charge capacity of the CCD, the CCD will overflow. This is commonly referred to as blooming image defects.  
           [0006]    [0006]FIG. 3, which is a cross section from point A to point C in FIG. 1, illustrates how blooming occurs in the CCD. The cross section begins in the photodiode  10 , goes through the transfer gate region  30 , and then up the CCD  20 . The CCD in FIG. 3 also shows barrier implants  90  and  95  which are used to modify the potential energy in the CCD channel  20  to control the direction of charge transfer. At time T1 of FIG. 3 the CCD is empty and the photodiode contains a full charge packet. In this case, to illustrate blooming, the charge packet is larger than the capacity of the CCD. At time T2 the gate  50  over the transfer gate region is set to a high voltage level which lowers the barrier between the photodiode and CCD. All charge flows from the photodiode into the CCD. Then at time T3, the gate  50  is returned to its normal low level. Since the charge packet does not fit entirely under gate  50 , charge spills backwards into gate  40 .  
           [0007]    The invention described here overcomes the problem of the photodiodes containing more charge than the charge capacity of the CCD that leads to blooming image artifacts.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for transferring charge from a photo sensing region to a charge-coupled device in an image sensor, the method comprising the steps of (a) providing a plurality of pixels including a photo sensing region for collecting photo-generated charge and a charge-coupled device region between which is a transfer region; (b) providing a barrier in the charge-coupled device for causing the transfer of the charge through the charge-coupled device; and (c) providing the transfer region having a lower barrier than the charge-coupled device so that excess charge in the charge-coupled device flows into the photo sensing region.  
           [0009]    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.  
           [0010]    Advantageous Effect of the Invention  
           [0011]    The present invention has the advantage of preventing blooming artifacts while also preserving all of the original charge in the photodiode for subsequent readout. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a top view of a prior art CCD;  
         [0013]    [0013]FIG. 2 is a side view along line A-B of the prior art CCD of FIG. 1;  
         [0014]    [0014]FIG. 3 is a timing diagram of FIG. 1 illustrating readout;  
         [0015]    [0015]FIG. 4 is a timing diagram of the image sensor of the present invention;  
         [0016]    [0016]FIG. 5 is a flowchart for image capture using the image sensor of the present invention;  
         [0017]    [0017]FIG. 6 is an illustration of a camera for implementing the present invention;  
         [0018]    [0018]FIG. 7 is a flowchart illustrating image capture steps for an exemplary practical application of the present invention;  
         [0019]    [0019]FIG. 8 is an alternative embodiment of the present invention; and  
         [0020]    [0020]FIG. 9 is a flowchart illustrating image capture steps for an exemplary practical application of the alternative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The first embodiment of the invention is shown in FIG. 4. It is instructive to note the difference of the transfer gate region  30  potential from the prior art. The potential in the transfer gate region  30  is adjusted such that it is deeper than the barriers  90  and  95  in the CCD  20 . The deeper transfer gate region barrier will allow excess charge in the CCD to overflow into the photodiode instead of overflowing (blooming) into neighboring CCD gates as in the prior art. At time T1 in FIG. 4 the photodiode is full of charge and ready for transfer into the CCD. At time T2 the gate  50  voltage is set to the high level to lower the barrier between the photodiode and CCD and allow charge to transfer across the transfer gate region  30 . At T3, when the gate  50  voltage is returned to its normal low level excess charge flows back through the transfer gate region into the photodiode.  
         [0022]    Another advantage to this embodiment is the charge that overflows back into the photodiode may still be used. After the charge in the CCD is transferred out of the CCD and measured, the excess charge in the photodiode may be transferred back into the now empty CCD. The excess charge is then transferred out of the CCD and measured. The two measurements are added together to recover the total amount of charge that was originally in the photodiode. The result is that the entire photodiode charge was measured even though it exceeded the CCD charge handling capacity. This is analogous to a person emptying a large bucket of water with a smaller bucket by filling the small bucket multiple times. In the invention, the smaller charge capacity CCD empties the larger photodiode by transferring small amounts of charge from the photodiodes multiple times.  
         [0023]    [0023]FIG. 5 is a flow chart showing the complete image capture sequence of a camera  98  employing the first embodiment of the invention. The camera  98  shown in FIG. 6 consists of a lens  100 , an external shutter  110 , and the interline CCD image sensor  120 . The image capture sequence of FIG. 5 begins by opening the external shutter S 2  to expose the sensor to the image formed by the camera lens. The entire array of photodiodes is cleared by employing the electronic shuttering capabilities of the interline CCD S 4 . This marks the start of the image integration time. At the end of the integration time S 6 , transfer gate region  30  is turned on and held in the on state S 8 . After the charge is transferred to the CCD from the photodiode, the electronic shutter is turned back on and held on to prevent further accumulation of charge in the photodiodes S 10 . The electronic shutter is turned off S 14  only after the external shutter has fully closed S 112 . Now the transfer gate region  30  is turned off S 16 . If the photodiodes contained more charge than what will fit in the CCD, then the excess is allowed to overflow back into the photodiodes S 118 . Now the image is read out of the CCD S 20 . If any pixel of the image contained enough charge to fill the CCD then there is a possibility that there is still some charge remaining in the photodiode for that pixel S 22 . The photodiodes are read out a second time S 24  to obtain the charge that overflowed from the CCD back into the photodiodes. This process is repeated until the photodiodes have all been emptied. If the photodiodes had to be read out more than once, then the second (and if needed third images) are added together S 26  to reconstruct the final image S 28 .  
         [0024]    [0024]FIG. 7 shows the image capture sequence of the first embodiment of the invention for the case of no electronic shuttering. The sequence begins with all of the image sensor photodiodes empty. The external shutter is opened S 30  and closed S 34  to expose the image sensor to the image for a predetermined exposure time S 32 . The exposure time is controlled entirely by the external shutter. The external shutter is used to prevent additional accumulation of charge by the photodiodes during image readout. Next the transfer gate region is turned on S 36  to transfer charge from the photodiode to the CCD. When the transfer gate region is turned off S 38 , a short delay allows excess charge in the CCD to overflow back into the photodiode S 40 . The image in the CCD is then readout and stored S 42 . If any pixel of the image corresponds to a full charge packet in the CCD S 44 , then it is likely that there is still overflow charge left in the photodiodes. The photodiode readout sequence is repeated until most pixels are below the charge capacity of the CCD. At the end of the sequence, the stored images are added together S 46  to reconstruct the final image S 48 . It shall be noted as obvious that the images may be added together while they are being read out of the image sensor so that only enough memory is required to hold just one image.  
         [0025]    An alternative embodiment of the present invention is shown in FIG. 8. It is instructive to note the difference in the image capture sequence as compared to the prior art. The sequence begins with all of the image sensor photodiodes empty. Referring to FIGS. 8 and 9, the external shutter is opened S 50  and closed S 54  to expose the image sensor to the image for a predetermined exposure time S 52 . The exposure time is controlled entirely by the external shutter. The external shutter is used to prevent additional accumulation of charge by the photodiodes during image readout. This portion of the image capture sequence is shown by the potential energy diagram at time T1 of FIG. 8.  
         [0026]    Next at time T2 of FIG. 8, the transfer gate region is turned partially on S 56  to transfer charge from the photodiode to the CCD. The amount that the transfer gate region is turned on is only that which is required to fill the CCD with charge. The photodiode transfer gate is turned off S 58 . At T3, there will still be some charge in the photodiodes that is not transferred to the CCD.  
         [0027]    Next in the image capture sequence of FIG. 9 the first image is read out S 60  of the CCD and stored in memory. Then the transfer gate region is turned on again S 62 . This time the transfer gate region is turned on enough to allow complete transfer of charge from the photodiodes to the CCD. Next the transfer gate region is turned off S 64  and a second image is read out of the CCD. The first and second images are then added together S 66  to reconstruct the final image S 68 .  
         [0028]    It shall be noted as obvious that image capture sequence of FIG. 9 may be extended beyond two photodiode readout steps to three or more readout steps.  
         [0029]    The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.  
       Parts List  
       [0030]    [0030] 5  interline CCD pixel  
         [0031]    [0031] 10  photodiode  
         [0032]    [0032] 20  CCD  
         [0033]    [0033] 30  transfer gate region  
         [0034]    [0034] 40  control gate  
         [0035]    [0035] 50  control gate  
         [0036]    [0036] 60  light shield  
         [0037]    [0037] 70  gate insulator  
         [0038]    [0038] 80  substrate  
         [0039]    [0039] 90  barrier implants  
         [0040]    [0040] 95  barrier implants  
         [0041]    [0041] 98  camera  
         [0042]    [0042] 100  lens  
         [0043]    [0043] 110  external shutter  
         [0044]    [0044] 120  image sensor