Abstract:
The invention is directed to an image sensor with enhanced blue response and limited cross-talk. The image sensor is made of a photodiode layer. Disposed on one side of the photodiode layer is a substrate layer made out of an oppositely charged semiconductor material. The substrate layer is further defined by two different sub-layers, where the doping densities of the sub-layers differ. This difference in doping creates a deep electric field that inhibits carriers from moving to another sensor. Additionally, the potential of the deep electric field directs these carriers back to the N-P junction formed by the substrate layer and the photodiode layer. Working in conjunction with this, a shallow implant layer is disposed on the opposite side of the photodiode layer. The shallow implant layer creates an electric field between the photodiode layer and the shallow implant layer, directing carriers to the photodiode layer. As such, carriers generated in the shallow areas of the image sensor are discouraged from surface recombination effects.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention is directed to imaging sensors and associated imaging devices. In particular, the invention is directed towards a more efficient image sensor having enhanced blue response and cross-talk suppression based on properly positioned and doped wires of a semiconductor substrate. 
     2. Related Art 
     Charge Coupled Device (CCD) technologies have always customized the fabrication process to properly position the junctions and depletion depths for optimal spectral sensitivities and minimum cross-talk. In the course of using standard CMOS technologies to build image sensors, attention needs to be paid to the location depth of photodiode junctions. 
     The depth of the depletion of the photodiode is also important as well. This, in combination with the depth of the photodiode junctions, determine the spectral sensitivity and optical cross-talk of an imager. 
     Standard CMOS technology indicates the edge of the depletion region, meaning the junction depth plus the depletion depth of a source/drain diode at VDD reversed bias, ranges from 0.25 micron to 0.8 micron. 
     Comparing these depths with the penetration depth of visible light in silicon, it is apparent that for standard CMOS imagers that most photo carriers are generated in the neutral region. Thus, photo carriers cannot be efficiently collected for the imaging process. Further, this allows for the possibility of excessive cross-talk. 
     In standard CMOS imagers, most photo carriers for blue light, however, are generated shallower in the substrate. This shallow generation of blue light photo carriers has the problem of surface recombination. Thus, the blue response in a standard CMOS imager is attenuated by this characteristic. 
     As the CMOS technology is scaled down, this non-optimal carrier collection situation gets worse. As such, present photodiode structures do not allow for enhanced blue response and do not allow for cross-talk suppression between image sensors. 
     Previous solutions employed standard CMOS N+−P− well or P+−N− well photodiode structures. These standard CMOS photodiode structures provide shallow junctions. As such these standard CMOS photodiode structures tend to have a low blue response and generate a potential cross-talk problem. 
     The blue color response in the standard CMOS photodiode structures compares relatively low to green and red color output CMOS photodiode structures. This is primarily due to the loss of photo generated carriers near the diode surface due to surface recombination. 
     Signal cross-talk in standard CMOS photodiode structures is also a problem due to the standard structure of these semiconductor devices. The shallow depletion region allows carrier diffusion to adjacent pixels, allowing for poor cross-talk responses in standard CMOS photodiode structures. 
     Many other problems and disadvantages of the prior art will become apparent to those schooled in the art after comparing such prior art with the present invention described herein. 
     BRIEF SUMMARY OF THE INVENTION 
     In short, the invention is a light sensor on a die. The light sensor is made of a photodiode layer, a substrate layer, and a carrier direction layer. The photodiode layer is made of a semiconductor material having a charge. The substrate layer is disposed on one side of the photodiode layer, and is made of a semiconductor material of an opposite charge than that of the photodiode layer. 
     A carrier direction layer is disposed between the surface of the die and the other side of the photodiode layer, opposite the substrate layer. The carrier direction layer is made of a semiconductor material. The material of the carrier direction layer and that of the photodiode layer produces an electric field between the photodiode layer and the carrier direction layer. In this manner photogenerated carriers produced in the photodiode layer or, in the charge collection layer are directed to the photodiode layer by the electric field. 
     In one embodiment of the invention, the substrate layer is made of P-type semiconductor material. Thus, the photodiode would be made of an N-type semiconductor material. 
     In another embodiment, the carrier direction layer is made of P-type semiconductor material. In this case, the P-type carrier direction layer and the N-type photodiode layer creates an electric field in which electron carriers are directed to the photodiode layer. 
     In another embodiment, the carrier direction layer is made of N-type semiconductor material. In this case the carrier direction layer is made of a heavily doped (N+) semiconductor material while the photodiode would be made of a lighter doped (N−) semiconductor material. The resulting potential would direct holes to the photodiode layer. 
     In another embodiment, the substrate layer is made of a first layer and a second layer. The first and second layers produce an electric field directing carriers to the photodiode layer. In this case, the first layer could be made of a lightly doped P-type (P−) semiconductor material, and the second layer is made of a more heavily doped P-type (P+) semiconductor material. The first layer is disposed between the photodiode layer and the second layer. 
     In another embodiment, the light sensor contains a photodiode layer, a substrate layer, and a carrier direction layer, as described above. In this embodiment, the electric field created between the photodiode layer and the carrier direction layer serves to inhibit surface recombination of photogenerated carriers. 
     In yet another embodiment, the light sensor contains a photodiode layer, a substrate layer, and a carrier direction layer, as described above. The substrate layer is made of two layers of different doping densities. The substrate layers create a deep electric field that serves to inhibit cross-talk of carriers. 
     In another embodiment, the invention is a light imager having a plurality of light sensors and control circuitry. The control circuitry controls the output of the plurality of light sensors. 
     The plurality of light sensors are made of a photodiode layer, a substrate layer, and a charge direction layer, as described previously. The plurality of light sensors may take all the forms of the above mentioned embodiments of the light sensor. 
    
    
     Other aspects of the present invention will become apparent with further reference to the drawings and specification that follow. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a structural block diagram of an active image sensor element with enhanced blue response and signal cross-talk suppression according to the invention. 
     FIG. 2 is a structural diagram of an embodiment of the image sensor with enhanced blue response and signal cross-talk suppression of FIG.  1 . 
     FIG. 3 is a potential diagram of the image sensor with enhanced blue response and signal cross-talk suppression of FIG. 2 in a direction through the layers making up the image sensor. 
     FIG. 4 is a structural diagram of an alternative embodiment of the image sensor with enhanced blue response and signal cross-talk suppression of FIG.  1 . 
     FIG. 5 is a potential diagram of the image sensor with enhanced blue response and signal cross-talk suppression of FIG. 4 in a direction through the layers making up the image sensor. 
     FIG. 6 is a schematic block diagram of a light imager employing the light sensor of FIGS. 1,  2 , and  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a structural block diagram of an active image sensor element with enhanced blue response and signal cross-talk suppression according to the invention. A die  100  contains an image sensor element. 
     The die is made of a doped semiconductor substrate  135 , containing two different doping density layers of semiconductor material. A pixel element sensor is disposed on the doped semiconductor substrate  135 . 
     The pixel element sensor has a photodiode  110 . The photodiode  110  is made out of a differently doped semiconductor material from the semiconductor substrate  135 . For example, if the semiconductor substrate were made of a P-type semiconductor material, the photodiode would be made of a different type doped material, such as an N-type material. 
     The pixel element sensor also comprises a shallow implant region  120 . The surface implant region  120  is disposed on the photodiode  110 , between the photodiode  110  and the surface of the die  100 . 
     The surface implant region  120  can be made of a differently doped material than the photodiode  110 . For example, if the photodiode  110  is made of a N-type material, the surface implant region would be made of a P-type material. 
     Or, the surface implant region  120  can be made of a material that is the same doping type as the photodiode  110 , but at a different doping density. For example, if the photodiode region  110  is made of an N- (lightly doped N-type) semiconductor material, the surface implant region  120  would be made of a N+(highly doped N-type) semiconductor material. 
     As such, the surface implant region  120  repels photo-generated minority carriers away from the surface towards the N-P junction formed between the photodiode  110  and the substrate  135  to be collected. Thus, the photo-generated carriers generated in the shallow regions of the image sensor structure are directed towards the collection junction. As such, the loss of sensor response due to surface recombination of the photo-generated carrier in the shallow regions near the diode surface is substantially diminished. 
     Additionally, the substrate layer surrounding the photodiode can be made of two different layers, a first substrate layer  130  and a second substrate layer  140 . The first substrate layer  130  should be made out of an oppositely charged semiconductor material than that of the photodiode  110 . This is necessary to create the N-P semiconductor junction necessary for the image sensor to work properly. For example, should the photodiode area  110  be made of an N-type material, the first substrate layer  130  should be made out of a P-type layer, and vice versa. 
     In an embodiment of the invention, a second substrate layer  140  is also present. This second substrate layer  140  is of the same semiconductor type material as the first substrate layer  130 , but at a different doping density. As such, a deep level potential gradient is created within the die  100  due to the different doping densities of the first substrate layer  130  and the second substrate layer  140 . The resulting built-in electric field deep in the detector substrate caused by the differences in the doping of the first substrate layer  130  and.the second substrate layer  140  can assist in repelling photo-generated charges back into the active region. 
     As a result, the sensitivity of the sensor increases and the cross-talk decreases. As shown, the invention can be manufactured to minimize carrier surface recombination. This allows for enhanced blue response, since the blue generated photo carriers are generated nearer the surface than the wave lengths of green or red. Thus, the structure of the active image sensor of FIG. 1, by repelling the photo-generated carriers back into the area in which they may be collected, provides for a better and enhanced blue response in a image sensor. The deep electric field prevents cross-talk. As such, a pixel with greater response and efficiency is depicted. 
     FIG. 2 is a structural diagram of an embodiment of the image sensor with enhanced blue response and signal cross-talk suppression of FIG. 1. A die  200  contains an image sensor made of several layers of semiconductor materials. 
     First, a heavily doped P-type material (P+) makes up the deepest substrate level  250  of a image sensor with enhanced blue response and signal cross-talk suppression in a die  200 . A lightly doped P-type (P−) substrate layer  240  is disposed on the heavily doped P+ type substrate layer  250 . These structures thus form an electric field between the P− substrate layer  240  and the P+substrate layer  250  in the image sensor. As such, a potential gradient is formed by the two substrate layers  240  and  250 . 
     A photodiode  230  is then disposed on the P− substrate layer  240 . The photodiode is made of a lightly doped N-type material (N− material). The junction between the photodiode  230  and the P− substrate layer  240  forms the N− P junction required for collection in the functioning of the sensor. 
     A shallow implant layer  220  is disposed on the surface of the photodiode layer  230 . This shallow implant layer  220  is made of heavily doped P-type (P+) material. The P+ shallow implant layer  220  pins the surface potential of the image sensor. This surface implant layer  220  repels the photo-generated minority carriers, in this case electrons, away from the surface and toward the junction to be collected. Thus greatly aids in the enhancement of a shallow generated blue signal. Additionally, surface recombination of the carriers is diminished. 
     Secondly, the potential gradient formed by the P+ substrate  250  layer and the P− substrate layer  240  enhances charge collection efficiency in the image sensor. The deeper electric field formed by the P+ substrate layer  250  and the P− substrate layer  240  also serves to decrease cross talk between image sensors. This occurs since the photo generated carriers are swept by the field created by the P+ substrate layer  250  and the P− substrate layer  240  towards the N-P junction  245  defined by the photodiode layer  230  and the P− substrate layer  240 . 
     FIG. 3 is a potential diagram of the image sensor with enhanced blue response and signal cross-talk suppression of FIG. 2 in a direction through the layers making up the image sensor. The functionality of the present invention may be clearly shown by the potential profile of the photodetector of FIG.  2 . 
     The horizontal axis of FIG. 3 corresponds to the orientation arrow Y of FIG.  2 . The dashed line  310  represents the depth into the die  200  of FIG. 2 at which junction between the P+ surface implant layer  220  and the photodiode layer  230  is. The dashed line  320  of FIG. 3 corresponds to the depth; into the die  200  of FIG. 2 at which the photodiode layer  230  and the first P-type layer  240  junction is created. Likewise the dashed line  330  in FIG. 3 corresponds to the depth into the die  200 , FIG. 2, corresponding to the junction of the P− substrate layer  240  and the P+ substrate layer  250 . The vertical axis of the potential diagram of FIG. 3 corresponds to the potential at the corresponding depth into the die  200  of FIG.  2 . 
     Thus as shown, a very high potential gradient exists between the P+ surface implant layer  220  and the photodiode layer  230  of FIG.  2 . As can be clearly shown by the potential profile of the image sensor of FIG. 2, the photo-generated carriers (electrons, in this case) will clearly be directed back away from the surface and towards the N-P junction formed by the photodiode  230  and the first P− substrate layer, where they will be collected. 
     Additionally, the potential created by the layering of the P− substrate layer  240  and the P+ substrate layer  250  also create a similar effect, directing photo-generated charges back into the active region. Also, photo-generated charges are discouraged from leaving the active region because of the potential between the substrate layers  240  and  250 . Thus, the cross-talk due to the migration of photo-generated charges is discouraged or reduced on other nearby sensors. 
     FIG. 4 is a structural diagram of an alternative embodiment of the image sensor with enhanced blue response and signal cross-talk suppression of FIG.  1 . Similar to the embodiment as envisioned in FIG. 3, this embodiment of the invention also contains a P+ substrate layer  450  as a base layer in the die  400 . Again, as in FIG. 2, a P− substrate layer  440  is formed and disposed on the P+ substrate layer  450 . 
     A photodiode layer  430  is formed with a junction to the P− substrate layer  440 . The photodiode  430  is made of an N-type material, in this case a lightly doped N+ material, thus providing for the P-N junction necessary in the operation of a semiconductor photo collection device. 
     A surface implant layer  420  is formed and is disposed on the photodiode  430 . The surface implant layer  420  is made of a heavily-doped N-type material (N+). This structure uses the shallow implant layer  420  and the photodiode layer  430  to create a built-in electric field near the detector surface. The shallow formed photogenerated minority carriers, holes in this case, are directed by this built-in electric field into the photodiode  420 , and towards the active region of the sensor. As such, these photogenerated minority carriers are repelled away from the surface and towards the P-N junction formed by the photodiode layer  430  and the P− substrate layer  440  to be collected. 
     This resulting electric field directs the photogenerated minority carriers back to the active region of the image sensor, and inhibits surface recombination for carriers generated in the shallow regions of the die  400 . As such, the blue response of the image sensor as envisioned in FIG. 4 is enhanced. 
     FIG. 5 is a potential diagram of the image sensor with enhanced blue response and signal cross-talk suppression of FIG. 4 in a direction through the layers making up the image sensor. The functionality of the present invention may be clearly shown by the potential profile of the photodetector of FIG.  4 . 
     The horizontal axis of FIG. 5 corresponds to the orientation arrow Y of FIG.  4 . The dashed line  510  represents the depth into the die  400  of FIG. 4 at which junction between the N+ surface implant layer  420  and the photodiode layer  430  is. The dashed line  520  of FIG. 5 corresponds to the depth into the die  400  of FIG. 4 at which the junction between the photodiode layer  430  and the first P-type layer  440  is. Likewise the dashed line  530  in FIG. 5 corresponds to the depth into the die  400 , FIG. 4, corresponding to the junction of the P− substrate layer  440  and the P+ substrate layer  450 , in FIG.  4 . The vertical axis of the potential diagram of FIG. 5 corresponds to the potential at the corresponding depth into the die  400  of FIG.  4 . 
     Thus as shown, a high potential gradient exists between the N+ surface implant layer  420  and the photodiode layer  430  of FIG.  4 . As can be clearly shown by the potential profile of the image sensor of FIG. 4, the photo-generated carriers (holes, in this case) will clearly be directed back away from the surface and towards the N-P junction formed by the photodiode  430  and the first P− substrate layer, where they will be collected. 
     Additionally, the potential created by the layering of the P− substrate layer  440  and the P+ substrate layer  450  also aids in diminishing cross-talk. 
     Additionally, the actual doping types of layers may be further used to enhance the sensitivity of an image sensor made with the invention disclosed. For example, doping with different materials may lead to better responses. Smaller mass dopants may be implanted further into a substrate. Thus, the active region in the image sensor may be increased by using these smaller mass dopants, leading to a greater efficiency in photo generated charge collection. 
     A high doping concentration of the photodiode region relative to the doping density of the substrate can cause a poor response due to a decrease of the active region. Using a lightly doped region may further enhance the characteristics of the invention due to the increase of the active area. Thus, the combination of doping concentrations and species of dopants can be used concurrently to enhance the response of the image sensor. 
     Thus, the invention shows an image sensor with enhanced blue response and signal cross-talk suppression. This is made possible by the interweaving of heavily and lightly doped layers within the sensor, as well as layers made of different semiconductor types. This creates a mechanism by which photocarriers generated in a neutral region may be channeled or directed back into the active region for more efficient collection. Additionally, the deep electric potentials created by materials of the deeper substrates enhance the charge collection, and discourage cross-talk of photogenerated carriers. 
     It should be noted that several technologies exist for the manufacture of the light sensors as described above. The light sensors of FIGS. 1,  2 , and  4  may presently be made with standard semiconductor fabrication technologies. 
     A light imager  600  is made of a control circuitry  610  and a plurality of light sensors  620   a-n  are disposed on a die  630 . The plurality of light sensors are manufactured in accordance with the structures described above. The control circuitry  610  directs and controls the outputs of the plurality of light sensors  620 a-n. 
     In view of the above detailed description of the present invention and associated drawings, other modifications and variations will now become apparent to those skilled in the art. It should also be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the present invention as set forth in the claims which follow.