Abstract:
An image sensor includes a photosensitive region disposed within a semiconductor layer and a stress adjusting layer. The photosensitive region is sensitive to light incident through a first side of the image sensor to collect an image charge. The stress adjusting layer is disposed over the first side of the semiconductor layer to establish a stress characteristic that encourages photo-generated charge carriers to migrate towards the photosensitive region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a Divisional of U.S. patent application Ser. No. 12/853,803, filed on Aug. 10, 2010. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to image sensors, and in particular but not exclusively, relates to backside illuminated (“BSI”) complementary metal-oxide-semiconductor (“CMOS”) image sensors. 
       BACKGROUND INFORMATION 
       [0003]    CMOS image sensors may generate inaccurate image data due to dark current in the pixels themselves and variation in the level of dark current from pixel to pixel. Dark currents add to the output voltages and degrade the picture provided by the imaging system. Therefore, to generate accurate image data, it is desirable to reduce or eliminate dark currents. For a BSI CMOS image sensor, dark currents may be a particular problem. A typical BSI CMOS image sensor has dark current levels that are over 100 times greater than that of a front side illuminated sensor. This may be caused by the fabrication process to produce the relatively thin substrate layer of the BSI CMOS image sensor. 
         [0004]    The fabrication process for BSI CMOS image sensors produces a relatively thin (e.g., less than 4 micrometers) backside silicon layer with techniques such as chemical mechanical polishing (“CMP”) and chemical etching. The resulting backside silicon surface may suffer from a high number of defects that may exacerbate dark currents by providing leakage paths, resulting in a relatively high number of hot pixels. This surface defect problem may be addressed by ion implantation of P or N type dopants into the backside surface. Backside dopant implantation produces fields that may facilitate the movement of photo generated electric carriers away from the backside surface. For example, for an N type photodiode, the photoelectric effect produces electrons as charge carriers. Therefore, a P type dopant may be implanted on the backside surface to generate a requisite field to reduce the contribution of the electron carriers from the backside surface into the phododiode. For a P type photodiode, the photoelectric effect produces holes as charge carriers. Therefore, an N type dopant may be implanted on the backside surface to generate the requisite field to reduce the contribution of the hole carriers from the backside surface into phododiode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
           [0006]      FIG. 1  is a cross sectional view of backside illuminated image sensor with dopant implantation to address defects of the backside surface. 
           [0007]      FIG. 2A  is a cross sectional view of a partially fabricated backside illuminated image sensor before thinning of the epi layer, in accordance with an embodiment. 
           [0008]      FIG. 2B  is a cross sectional view of a partially fabricated backside illuminated image sensor after thinning of the epi layer, in accordance with an embodiment. 
           [0009]      FIG. 2C  is a cross sectional view of a partially fabricated backside illuminated image sensor showing the deposition of a stress film on the backside surface of the epi layer, in accordance with an embodiment. 
           [0010]      FIG. 2D  is a cross sectional view of a backside illuminated image sensor including a stress film on the backside surface of the epi layer, in accordance with an embodiment. 
           [0011]      FIG. 2E  is a cross sectional view of a backside illuminated image sensor showing the deposition of an additional layer of stress film on a first layer of stress film, in accordance with an embodiment. 
           [0012]      FIG. 2F  is a cross sectional view of a backside illuminated image sensor including two stress films on the backside surface of the epi layer, in accordance with an embodiment. 
           [0013]      FIG. 2G  is a cross sectional view of a backside illuminated image sensor showing the deposition of an additional layer of stress film on the front side of the image sensor, in accordance with an embodiment. 
           [0014]      FIG. 2H  is a cross sectional view of a backside illuminated image sensor including a stress film on the backside surface of the epi layer and a stress film on the front side of the imager, in accordance with an embodiment. 
           [0015]      FIG. 3A  is a cross sectional view of a backside illuminated image sensor with an N type photodiode and further including a compressive film, in accordance with an embodiment. 
           [0016]      FIG. 3B  is a cross sectional view of a backside illuminated image sensor with a P type photodiode and further including a tensile film, in accordance with an embodiment. 
           [0017]      FIG. 4A  is a cross sectional view of a partially fabricated backside illuminated image sensor showing the deposition of a buffer film on the backside surface of the epi layer, in accordance with an embodiment. 
           [0018]      FIG. 4B  is a cross sectional view of a partially fabricated backside illuminated image sensor including a buffer film on the backside surface of the epi layer, in accordance with an embodiment. 
           [0019]      FIG. 4C  is a cross sectional view of a partially fabricated backside illuminated image sensor showing a photo printing process that creates a pattern on the buffer film, in accordance with an embodiment. 
           [0020]      FIG. 4D  is a cross sectional view of a partially fabricated backside illuminated image sensor including a buffer film that has a pattern, in accordance with an embodiment. 
           [0021]      FIG. 4E  is a cross sectional view of a partially fabricated backside illuminated image sensor showing the deposition of a stress film on a patterned buffer film, in accordance with an embodiment. 
           [0022]      FIG. 4F  is a cross sectional view of a backside illuminated image sensor including a stress film that has a pattern, in accordance with an embodiment. 
           [0023]      FIG. 4G  is a cross sectional view of a partially fabricated image sensor including a buffer film that has an alternate pattern, in accordance with an embodiment. 
           [0024]      FIG. 4H  is a cross sectional view of a backside illuminated image sensor including a stress film that has an alternate pattern, in accordance with an embodiment. 
           [0025]      FIG. 5  is a block diagram illustrating a backside illuminated imaging system, in accordance with an embodiment of the invention. 
           [0026]      FIG. 6  is a circuit diagram illustrating pixel circuitry of two 4T pixels within a backside illuminated imaging system, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Embodiments of a system and method of fabrication for a BSI imaging system with black reference pixels that more faithfully produce a black reference level for offsetting active imaging pixels are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
         [0028]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of a given embodiment may be combined in any suitable manner with any of the other embodiments. It is anticipated that the features described below in connection with the various figures may be combined in a variety of different combinations. 
         [0029]      FIG. 1  illustrates an example of a BSI image sensor pixel  100  including a photodiode (“PD”) region  105  (more generically referred to as a photosensitive region) disposed within an epitaxial (“epi”) layer  110 . BSI image sensor  100  is photosensitive to light incident upon the backside of the sensor die. Pixel circuitry for operation of the BSI image sensor is formed over a P well  115 . The illustrated example of pixel  100  also includes a surface treatment layer  101 , a P+ dopant layer  116 , N+ source/drain diffusions  125 , pixel circuitry (only transfer transistor T 1  and reset transistor T 2  are illustrated), a microlens  130 , a color filter layer  135 , an antireflective (“AR”) film  140 , an inter-metal dielectric layer  120 , a P+ passivation or pinning layer  147 , and a metal stack  150 . The metal stack  150  includes multiple metal layers (e.g., M 1 , M 2 , etc.) separated by inter-metal dielectrics  120 , and interconnected with vias (e.g., V 1 , V 2 , V 3 , etc.). Pixel  100  includes one or more shallow trench isolations (STI), at some of which are used to separate pixel  100  from adjacent pixels in a pixel array (not illustrated). On top of the metal stack  150  is an isolation layer  160 . The isolation layer  160  isolates the metal stack  150  from harmful environmental elements such as pollutants or moisture. The isolation layer  160  may be composed of materials such as silicon nitride. 
         [0030]    The dopant layer  116  creates a sharp dopant gradient which provides a vertical force that pushes photo generated electrons vertically towards the collecting depletion region of the photodiode region  105 . Additionally, the dopant layer  116  passivates the backside surface of the epi layer  110 , thereby reducing the leakage current from the photodiode region  105 . However, the relative thinness of the backside epi layer  110  causes problems during the fabrication process. First, it can be difficult to control the relatively shallow implant depth of the dopant layer  116 . The lack of control of the implant depth may cause the implanted dopant to penetrate too deep into the epi layer  110  towards the photodiode region  105 , thus harming the quantum efficiency of the device. Second, it can be difficult to keep the temperature in check when the dopant is annealed by a laser following the implantation step. A relatively high annealing temperature may overheat the epi layer  110 , thereby damaging the front structures such as T 1 , T 2 , M 1 , M 2 , V 1 , V 2  and V 3 . 
         [0031]    A BSI image sensor&#39;s backside surface stress may affect its dark current level. The present application discloses utilizing structures and methods to adjust the stress on a CMOS image sensor&#39;s backside silicon surface, thereby reducing the dark current effect by facilitating the movement of photo generated charge carriers away from the backside surface. 
         [0032]    Stress on a backside silicon surface may be adjusted by forming a stress loaded layer on the surface. A stress loaded layer may include materials such as metal, organic compounds, inorganic compounds, or otherwise. For example, the stress loaded layer may include a silicon oxide (SiO 2 ) film, a silicon nitride (SiNx) film, a silicon oxynitride (SiOxNy) film, or a combination thereof. In the CMOS manufacturing art, the abovementioned films may be formed by a process such as chemical vapor deposition (“CVD”) or physical vapor deposition (“PVD”). A CVD process may include a plasma enhanced chemical vapor deposition (PECVD) process, a high density plasma-enhanced chemical vapor deposition (HDPCVD) process, etc. 
         [0033]    Depending on factors such as gas pressure, temperature, film thickness, material composition and annealing, a film may possess a tensile stress or a compressive stress of various values, or may be stress neutral. In order to use PECVD or HDPCVD to form a film that achieves a desirable stress, a manufacturer may vary several process parameters such as the relative abundance of particular precursors, power level, plasma drive frequency, temperature, post-deposition processing, etc. For example, a SiO 2  film may be formed by a plasma deposition of precursors SiH 4  and O 2 . Increasing the O 2 :SiH 4  ratio decreases the film&#39;s compressive stress, whereas increasing the rf chuck power increases the film&#39;s compressive stress. In another example, a SiNx film may be formed by a plasma deposition of precursors SiH 4  and N 2 . Increasing the N 2 :SiH 4  ratio increases the film&#39;s compressive stress, whereas increasing the rf chuck power decreases the film&#39;s compressive stress. Yet in another example, a SiOxNy film may be formed by a plasma deposition of precursors SiH 4  and N 2 O. Here, an increased N 2 O:SiH 4  ratio generally increases the film&#39;s compressive stress. Yet in another example, as a SiNx film is formed by a PECVD process involving precursors SiH 4 , NH 3  and N 2 , a conventional 13.56 MHz frequency 17 W power tends to produce a tensile stress in the film, whereas adding a less than 1 MHz low frequency 400 W power component may result in high energy ion bombardment of the growing SiNx film, thereby changing the film&#39;s stress state from tensile to compressive. Yet in another example, periodically alternating high and low frequency power sources may produce a film that has a substantially low level of stress. Yet in another example, a ultra-violet assisted thermal processing step after the formation of a SiNx film may break Si—H and N—H bonds in the film, resulting in hydrogen removal, thereby increasing the film&#39;s stress. 
         [0034]    By employing various film forming techniques such as PECVD and HDPCVD, and by controlling various process parameters such as precursor ratio, power level, and power frequency, one may form a film or films of desirable stress types and stress levels on a backside silicon surface. These stress adjustment films may influence the stress conditions of the backside surface and/or the front side surface, thereby affecting the behavior of photo generated charge carriers, resulting in a change of the dark current level. 
         [0035]      FIG. 2A  shows a partially fabricated BSI CMOS image sensor pixel  200   a  including a photodiode region  105 , a P well  115 , a metal stack  150 , and a relatively thick backside substrate layer  108  and epi layer  110 . With the exception of the relatively thick backside substrate layer  108 , pixel  200   a  has substantially the same back-end-of-the-line components (e.g. metal stack  150 ) as pixel  100  in  FIG. 1 . The backside substrate layer  108  may be removed and epi layer  110  substantially thinned to produce a relatively thin epi layer  110 , thereby producing pixel  200   b,  as shown in  FIG. 2B . The thinning may be achieved by a combination of chemical mechanical polishing, chemical etching, or otherwise. In one embodiment, the thinned epi layer  110  may have a thickness of less than 4 micrometer. In some embodiments, the thickness may be between 1.5 to 3 micrometers. 
         [0036]    Following the thinning, a film formation process, such as a plasma enhanced chemical vapor deposition process, may be applied to the backside surface of epi layer  110 , as shown in  FIG. 2C . The process forms a stress film  210  on the backside surface of the epi layer  110 , as shown in  FIG. 2D . For example, utilizing a PECVD process and with precursors SiH 4  (at a 73 standard cubic centimeters per minute, or sccm flow rate), NH 3  (at a 122 sccm flow rate) and N 2  (at a 1000 sccm flow rate), one may form a SiNx film with a 0.45 GPa compressive stress value by applying a 400 W rf chuck power at a 380 kHz frequency, and maintaining a 250 deg. C. substrate temperature and a 750 mTorr gas pressure. In another example, utilizing a PECVD process and with precursors SiH 4  (at a 30 sccm flow rate), NH 3  (at a 50 sccm flow rate) and N 2  (at a 1960 sccm flow rate), one may form a SiNx film with a 0.36 GPa tensile stress value by applying a 17 W rf chuck power at a 13.65 MHz frequency, and maintaining a 250 deg. C. substrate temperature and a 900 mTorr gas pressure. Yet in another example, one may form a SiO 2  film by using precursors SiH 4  and O 2 , or a SiOxNy film by using precursors SiH 4  and N 2 O. In other various embodiments, one of ordinary skill in the art may use materials and methods as disclosed above to form films of desirable stress characteristics. By controlling various parameters such as power level, power frequency, precursor ratio, temperature, pressure, etc., one may form tensile, compressive, or stress neutral films with desirable stress characteristics. 
         [0037]      FIG. 2E  illustrates a BSI CMOS image sensor pixel  200   e  that includes stress film  210  on the backside surface of the epi layer  110 . Pixel  200   e  may be subjected to one or several additional film formation processes, such as a plasma enhanced chemical vapor deposition process. The result is a pixel  200   f  with multiple layers of stress films  210 ,  215 , as shown in  FIG. 2F . The multiple layers of stress films may have similar or different stress characteristics. Their combination may facilitate “fine tuning” of the overall stress characteristics of the backside surface. For example, if a single layer of compressive film is not enough to produce a desirable level of compressive stress on the backside surface, then multiple layers of compressive films may be formed to produce a combined stress level that satisfies the design goal. In one embodiment, the stress films  210  and  215  may impart counteracting stresses (e.g., one tensile and the other compressive). In one embodiment, stress film  215  may be patterned and removed in selected areas, while retaining stress film  210  in those selected areas. 
         [0038]      FIG. 2G  shows a BSI CMOS image sensor pixel  200   g  that includes a stress film  210  on the backside surface of the epi layer  110 . Pixel  200   g  may be subjected to one or several additional film formation processes, such as a plasma enhanced chemical vapor deposition process, at the front side of the pixel  200   g.  The result is a pixel  200   h  with one or several isolation layers  160  on top of the metal stack  150 , as shown in  FIG. 2H . The isolation layer  160  may be stress adjusting in nature, and may be formed with materials or methods as disclosed above. Compared with the backside stress film  210 , the isolation layer  160  may have similar or different stress characteristics. The isolation layer  160  may possess stress characteristics that allow it to add or subtract stress from the front side in cooperation with the stress film  210  at the backside surface of pixel  200   h.  For example, the combined effect of the isolation layer  160  and the stress film  210  may allow one to “fine tune” the overall stress characteristics of pixel  200   h,  such as “tightening” or “loosening” the stress profile throughout pixel  200   h.    
         [0039]      FIG. 3A  shows an embodiment of a BSI CMOS image sensor pixel  300   a  with a compressive film  320  positioned on the pixel&#39;s backside surface. Here, pixel  300   a  includes an N type photodiode  305 , a P type epi layer  310 , and a P well  315 . Upon photoelectric activation, the N type photodiode  305  collects electrons as photo generated charge carriers. The compressive film  320  may exert a compressive stress influence on the epi layer  310 . This stress effect may facilitate the movement of carriers away from the backside surface by providing a substantially vertical force to push the carriers towards the collecting depletion region of the photodiode  305 . Here, the compressive film  320  causes a stress effect that may simulate the effect similarly achieved by a dopant implantation process (e.g., if a P type dopant were to be implanted on the backside surface of the epi layer  310 , then a similar vertical force effect might be achieved). In addition to the possibility that the compressive film  320  may produce an effect similar to that of a P type dopant implantation, the compressive film  320  may provide a number of beneficial effects. First, the absence of dopant eliminates the potential problem of the implanted dopant penetrating too deep and harming the quantum efficiency of the device. Second, the absence of dopant obviates the need for a high temperature laser annealing step. By keeping the process temperature relatively low, heat damage to the device may be substantially reduced or eliminated. However, it should be appreciated that embodiments of compressive film  320  may also be implemented in conjunction with a backside dopant implant as illustrated in  FIG. 1 . 
         [0040]      FIG. 3B  shows an embodiment of a BSI CMOS image sensor pixel  300   b  with a tensile film  321  positioned on the pixel&#39;s backside surface. Here, pixel  300   b  includes a P type photodiode  306 , an N type epi layer  311 , and an N well  316 . Upon photoelectric activation, the P type photodiode  306  produces holes as photo generated charge carriers. The tensile film  321  may exert a tensile stress influence on the epi layer  311 . This stress effect may facilitate the movement of carriers away from the backside surface by providing a substantially vertical force to push the carriers towards the collecting depletion region of the photodiode  306 . If an N type dopant were to be implanted on the backside surface of the epi layer  311 , then a similar vertical force effect might be achieved. In addition to the possibility that the tensile film  321  may produce a similar effect as an N type dopant implantation, the tensile film  321  may provide a number of beneficial effects including the lack of harm from dopant penetration, and the avoidance of overheating, as discussed above. However, it should be appreciated that embodiments of tensile film  321  may also be implemented in conjunction with a backside dopant implant as illustrated in  FIG. 1 . 
         [0041]    In certain situations, it may be desirable to increase the effect of dark currents. For example, one may utilize dark current to measure properties such as temperature. For such applications, stress films may be applied differently than the structures and methods disclosed above. For example, for an N type photodiode, a tensile film may be formed on the backside surface. In another example where a photodiode is of P type, a compressive film may be formed on the backside surface. 
         [0042]    For certain applications, it may be desirable to employ processes in addition to the stress film formation. For example, before the stress film formation, one may apply various processes to the backside surface of the epi layer  110 , such as dopant implantation, laser annealing, etc. 
         [0043]    Typically, a BSI CMOS image sensor pixel includes one or several antireflective (“AR”) coatings, such as the AR film  140  as shown in  FIG. 1 . An AR coating may include a layer of silicon oxide under another layer of silicon nitride or silicon oxynitride. By utilizing the above-disclosed techniques to build desirable stress characteristics into an AR coating, one may imbue such an AR coating with a stress-adjusting property in addition to an antireflection property. 
         [0044]    A stress film may be selectively applied to designated areas of an image sensor to achieve a desirable stress adjusting effect. For example, a stress film may be applied to a portion of a pixel or a pixel array in order to achieve a desirable stress pattern on the backside surface. The following embodiments disclose a stress pattern application using a BSI CMOS pixel as an example.  FIG. 4A  shows a partially fabricated BSI CMOS image sensor pixel  400   a  including a photodiode region  105 , a P well  115 , a metal stack  150 , and a relatively thin backside epi layer  110 . Pixel  400   a  may be subjected to a buffer film deposition process, as shown in  FIG. 4A . The buffer film deposition may be accomplished by physical vapor deposition, chemical vapor deposition, sputtering, etc. As a result, a buffer film  410  may form on the backside surface of the epi layer  110 , as shown in  FIG. 4B . The buffer film  410  may be an oxide, a nitride, an oxynitride, a metal, an organic material, etc. The stress characteristic of buffer film  410  may be compressive, tensile, or neutral. In the present example embodiment, buffer film  410  is stress neutral. 
         [0045]    Following its formation, buffer film  410  may be patterned by utilizing various processes as known in the semiconductor art. For example, buffer film  410  may be patterned by utilizing a photolithography process, as shown in  FIG. 4C . The photolithography process may include steps such as application of a photoresist mask  415  that substantially covers the entire backside surface of the buffer film  410 , exposure and developing, etching of the photoresist mast  415 , and removal of the photoresist mask  415 . The resulting pixel  400   d  may include a buffer film  410  that has a desirable pattern, as shown in  FIG. 4D . A subsequent stress film formation process, such as a chemical vapor deposition step, may follow, as shown in  FIG. 4E . A stress film  420  is formed, resulting in a pixel  400   f,  as shown in  FIG. 4F . Pixel  400   f  includes a stress film  420  that covers parts of the backside of the epi layer  110 , and the patterned buffer film  410 . Due to the stress neutral characteristics of the buffer film  410 , the stress film  420  exerts its stress adjusting effect onto the portion of the epi layer  110  that is directly exposed to the stress film  420 . For the part of the pixel  400   f  that is “protected” by the buffer film  410 , it is relatively stress free. 
         [0046]    Depending on the particular pattern of the buffer film  410 , various stress patterns may be achieved on the backside surface of a pixel. For example, the buffer film  410  may take on a different pattern (due to a different photo mask used in the photolithography process), as shown in  FIG. 4G . Here, pixel  401  contains a different buffer film pattern from pixel  400   d,  as shown in  FIG. 4D . After stress film formation, the resulting pixel  402 , in  FIG. 4H , has a different backside surface stress pattern from pixel  400   f  in  FIG. 4F . 
         [0047]    Similar to the above disclosed formation of a patterned stress film on an image sensor, a patterned isolation layer may be selectively applied to designated areas of an image sensor to achieve a desirable stress adjusting effect. For example, an isolation layer may be applied to a portion of a pixel or a pixel array in order to achieve a desirable stress pattern on the front side surface of the metal stack. The materials and methods of forming a desirable front side stress pattern may be similar to the materials and methods as disclosed above. 
         [0048]    By varying the stress characteristics (compressive stress, tensile stress, or neutral stress at their desirable values) of the buffer film and the stress film (and/or the isolation layer), and by employing various buffer film or stress film (and/or isolation layer) patterns, one may achieve a wide variety of stress patterns on the backside surface (and/or the front side surface) of a BSI image sensor. 
         [0049]    For example, in the case of an N type photodiode and P type epi layer  110 , a compressive stress may be applied to the epi layer  110  immediately below each photodiode using a compressive film, while a buffer film may be positioned under P-well  115  or between adjacent pixels so as not to promote cross-talk between adjacent pixels. Additionally (or alternatively), a tensile film may be positioned beneath P well  115  or between adjacent pixels (e.g., the region below STI separating adjacent pixels) to attract stray charge carriers and further impede lateral crosstalk between adjacent pixels, while a compressive film is applied beneath the photodiode regions. In the case of a P type photodiode and N type epi layer  110 , a tensile stress may be applied to the epi layer  311  immediately below each photodiode  306  using a tensile film, while a buffer film may be positioned under N-well  316  or between adjacent pixels so as not to promote cross-talk between adjacent pixels. Additionally (or alternatively), a compressive film may be positioned beneath N well  316  or between adjacent pixels to attract stray charge carriers and further impede lateral crosstalk between adjacent pixels, while a tensile film is applied beneath the photodiode regions. Other combination tensile/compressive patterns may be implemented. 
         [0050]      FIG. 5  is a block diagram illustrating a BSI imaging system  500 , in accordance with an embodiment of the invention. The illustrated embodiment of imaging system  500  includes a pixel array  505 , readout circuitry  510 , function logic  515 , and control circuitry  520 . The pixels of pixel array  505  may be implemented using any or all of the above described techniques including dopant layer  116  and/or the frontside and/or backside stress adjusting layers. 
         [0051]    Pixel array  505  is a two-dimensional (“2D”) array of backside illuminated imaging sensors or pixels (e.g., pixels P 1 , P 2  . . . , Pn). In one embodiment, each pixel is a complementary metal-oxide-semiconductor (“CMOS”) imaging pixel. As illustrated, each pixel is arranged into a row (e.g., rows R 1  to Ry) and a column (e.g., column C 1  to Cx) to acquire image data of a person, place, or object, which can then be used to render a 2D image of the person, place, or object. 
         [0052]    After each pixel has acquired its image data or image charge, the image data is readout by readout circuitry  510  and transferred to function logic  515 . Readout circuitry  510  may include amplification circuitry, analog-to-digital (“ADC”) conversion circuitry, or otherwise. Function logic  515  may simply store the image data or even manipulate the image data by applying post image effects (e.g., crop, rotate, remove red eye, adjust brightness, adjust contrast, or otherwise). In one embodiment, readout circuitry  510  may readout a row of image data at a time along readout column lines (illustrated) or may readout the image data using a variety of other techniques (not illustrated), such as a serial readout or a full parallel readout of all pixels simultaneously. 
         [0053]    Control circuitry  520  is coupled to pixel array  505  to control operational characteristic of pixel array  505 . For example, control circuitry  520  may generate a shutter signal for controlling image acquisition. In one embodiment, the shutter signal is a global shutter signal for simultaneously enabling all pixels within pixel array  505  to simultaneously capture their respective image data during a single acquisition window. In an alternative embodiment, the shutter signal is a rolling shutter signal whereby each row, column, or group of pixels is sequentially enabled during consecutive acquisition windows. 
         [0054]      FIG. 6  is a circuit diagram illustrating pixel circuitry  600  of two four-transistor (“4T”) pixels within a BSI imaging array, in accordance with an embodiment of the invention. Pixel circuitry  600  is one possible pixel circuitry architecture for implementing each pixel within pixel array  500  of  FIG. 5 . However, it should be appreciated that embodiments of the present invention are not limited to 4T pixel architectures; rather, one of ordinary skill in the art having the benefit of the instant disclosure will understand that the present teachings are also applicable to 3T designs, 5T designs, and various other pixel architectures. 
         [0055]    In  FIG. 6 , pixels Pa and Pb are arranged in two rows and one column. The illustrated embodiment of each pixel circuitry  600  includes a photodiode PD, a transfer transistor T 1 , a reset transistor T 2 , a source-follower (“SF”) transistor T 3 , and a select transistor T 4 . During operation, reset transistor T 2  is coupled between a power rail VDD and the floating diffusion node FD to reset FD (e.g., charge the FD to a preset voltage) under control of a reset signal RST. During an image acquisition window, photo-generated charge carriers (e.g., electrons) accumulate within photodiode PD. Subsequently, transfer transistor T 1  receives a transfer signal TX, which transfers the charge accumulated in photodiode PD to floating diffusion node FD. In one embodiment, floating diffusion node FD may be coupled to an additional storage capacitor for temporarily storing image charges. 
         [0056]    The floating diffusion node FD is coupled to control the gate of SF transistor T 3 . SF transistor T 3  is coupled between the power rail VDD and select transistor T 4 . SF transistor T 3  operates as a source-follower providing a high impedance output from the pixel. Finally, select transistor T 4  selectively couples the output of pixel circuitry  600  to the readout column line under control of a select signal SEL. In one embodiment, the TX signal, the RST signal, and the SEL signal are generated by control circuitry  520 . 
         [0057]    The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
         [0058]    These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.