Abstract:
A monolithic backside-sensor-illumination (BSI) image sensor has a sensor array is tiled with a multiple-pixel cells having a first pixel sensor primarily sensitive to red light, a second pixel sensor primarily sensitive to red and green light, and a third pixel sensor having panchromatic sensitivity, the pixel sensors laterally adjacent each other. The image sensor determines a red, a green, and a blue signal comprising by reading the red-sensitive pixel sensor of each multiple-pixel cell to determine the red signal, reading the sensor primarily sensitive to red and green light to determine a yellow signal and subtracting the red signal to determine a green signal. The image sensor reads the panchromatic-sensitive pixel sensor to determine a white signal and subtracts the yellow signal to provide the blue signal.

Description:
FIELD 
     The present application relates to the art of semiconductor array photosensors, and in particular to the art of backside-sensor-illumination photosensor arrays as used on image sensor integrated circuits. 
     BACKGROUND 
     Photosensor arrays are commonly used in electronic cameras, including both still and video cameras. These photosensor arrays are often incorporated as components of image sensor integrated circuits along with circuitry for reading images from the photosensor array. Typically, these devices are integrated circuit die having a rectangular array of pixel sensors, where each pixel sensor includes at least one photodiode or phototransistor adapted for detecting light, together with circuitry for sensing the sensors to generate an electronic signal representing light detected by the sensors, and for exporting that signal to off-chip circuitry. Most such photosensors are of the topside-illumination type, designed to receive light into the pixel sensors through the same die surface into which controlling transistors, including transistors of the sensing and signal exporting circuitry, are built. 
     While photosensor arrays may be of the “black and white” type, often used for security cameras, in 2012 most video and still camera applications demand color. 
     A common type of color photosensor array has color filters deposited over topside-illuminated pixel sensors. These filters are often in a four-pixel, three-color, pattern that is repeated, or tiled, throughout the array; the filters in such arrays are typically colored such that-one filter admits red light to a first sensor, another admits green light to a second sensor, another admits blue light to a third pixel sensor, and the fourth filter of each pattern admits one of red, green, or blue light to a fourth pixel sensor. 
     In many camera systems, outputs of the pixel sensors from the patterns are processed to provide traditional red, green, and blue (RGB) color signals, such as may be used in an additive color display system to provide a full-color image. Red-Green-Blue has become the standard for color electronic cameras and color computer monitor video. 
     In recent times, backside-illuminated (BSI) photosensor arrays have been developed. These photosensor arrays typically are built on thinned die, with controlling transistors on a first surface of the die, but designed to receive light through a second, or backside, surface of the die opposite the first surface. 
     While some BSI photosensors use a pattern of filters printed onto the backside surface to selectively admit red, green, or blue light to sensors of each tiled pattern, it has been found that pixel sensors can be designed to have color response determined by junction profiles and depths in the sensors. In FIG. 1 of US published patent application PCT/US01/29488, a color photosensor array is described having a pixel sensor with three photodiode junctions stacked vertically, one on top of each other, with one deep junction, of depth about 2 microns, sensing red light, another intermediate-depth junction, of depth about 0.6 microns, sensing green light, and another shallow junction, of depth about 0.2 microns, sensing blue light. 
     SUMMARY 
     A monolithic backside-sensor-illumination (BSI) image sensor has a sensor array tiled with-multiple-pixel cells having a first pixel sensor primarily sensitive to red light, a second pixel sensor primarily sensitive to both red and green (known herein as “yellow light”) light, and a third pixel sensor having panchromatic sensitivity, the pixel sensors laterally adjacent each other. In a particular embodiment, the primary spectral sensitivity of each pixel sensor is determined by junction depths of the photodiode portion of each sensor. The image sensor determines a red, a green, and a blue signal comprising by reading the red-sensitive pixel sensor of each multiple-pixel cell to determine the red signal, reading the sensor primarily sensitive to red and green light to determine a yellow signal and subtracting the red signal to determine a green signal. The image sensor reads the panchromatic-sensitive pixel sensor to determine a white signal and subtracts the yellow signal to provide the blue signal. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic cross-sectional diagram showing a red, a yellow, and a white-sensitive pixel sensor of a photosensor array. 
         FIG. 2  is a schematic plan view illustrating a four-pixel cell having one red, one yellow, and one white pixel sensor, and one pixel sensor selected from red, yellow, and white. 
         FIG. 3A  illustrates a 4-pixel tiling pattern having two red, one yellow, and one white-sensitive sensors. 
         FIG. 3B  illustrates a 4-pixel tiling pattern having one red, one yellow, and two white-sensitive sensors. 
         FIG. 3C  illustrates a 4-pixel tiling pattern having one red, two yellow, and one white-sensitive sensors. 
         FIG. 3D  illustrates a 9-pixel tiling pattern having two red, one yellow, and six white-sensitive sensors. 
         FIG. 3E  illustrates a 16-pixel tiling pattern. 
         FIG. 4  is a block diagram of a backside-illuminated image sensor circuit having tiled cells according to  FIGS. 1 and 2  in its photosensor array. 
         FIG. 5  is a block diagram of a color-recovery unit for providing red, green, and blue intensities for each tiling unit of the type illustrated in  FIGS. 3B and 2 . 
         FIG. 6  is a block diagram of further color-recovery for providing individual red, green, and blue intensities for each pixel of a 4-pixel tiling pattern. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Structure of the Photosensor Array 
     As light enters a surface of a silicon photosensor, shorter wavelengths tend to be absorbed closer to the surface, with longer wavelengths at greater depths in the surface. This means that blue light is absorbed closest to the surface, with green and yellow at greater depths, and red absorbed at deeper levels. 
     A photosensor array  98 , optimized for backside illumination, is constructed in a semiconductor wafer, in a particular embodiment the semiconductor wafer is a silicon wafer. Specific photosensors, without associated electronics, of this array are illustrated in cross section in  FIG. 1 ;  FIG. 2  illustrates a top view of a repeating, or tiled unit, portion of this array. The photosensor array has a first or top surface  100  into which diffused areas  102 ,  103  forming parts of silicon-gate metal-oxide-semiconductor (MOS) transistors are formed. On this first surface is grown a gate oxide, and on the gate oxide is deposited gate material  104 , which in an embodiment is polysilicon gate material; thresholds of these MOS transistors are adjusted as known in the art with an ion implant. In an embodiment, both N and P channel transistors are formed. In a particular embodiment, P-channel transistors are formed having P-type source and drain implanted regions  102  in N well  106 ; and N-channel transistors are formed having N-type source and drain implanted regions  103  in P wells  108 . On this top surface  100 , is also deposited one or more layers of dielectric oxide  110 , together with one or more layers of interconnect metal  112 . Vias  114 , providing patterned interconnects between metal  112  layers, and contacts  116 , providing patterned interconnects between lower metal  112  and diffused areas  102 ,  103 , are also provided within dielectric  110  on top surface  100 , as known in the art of multilayer-metal, silicon-gate, CMOS, integrated circuit fabrication. The transistors are used for decoding, driving, precharging or resetting, sensing, and reading photosensors of the photosensor array, and for other purposes 
     A portion of photosensor array  98  is allocated to photosensors  152 ,  154 ,  156 . Each photosensor is formed in a P− epitaxial layer of the first surface  100  of the semiconductor wafer by forming an N− region  122 ,  124 ,  126  that serves as a photodiode. The N− region is capped with a surface P+ cap region  128 , and may have P+ isolation sidewalls  127  for isolating it from adjacent N− regions and areas having transistors. Each N− photodiode region is also associated with an adjacent selection-transistor gate  129 , a P well region  131  provided for threshold adjustment, and a drain diffusion  132 . Drain diffusion  132  may in some embodiments be coupled to a column sense line  133 . 
     The semiconductor wafer is thinned to permit at least some light  153 ,  155 ,  157  incident upon the second or backside surface  135  of the wafer to reach the N− photodiode regions  122 ,  124 ,  126 . 
     Portions of the photosensor array integrated circuit that have decoding, driving, sense-amplifier, multiplexing, and other CMOS circuitry may be shielded from ambient light  153 ,  155 ,  157  by a patterned opaque coating  136 . This coating has openings to permit incident light  153 ,  155 ,  157  to reach the surface in portions having photosensors, and is present to define, and prevent light from reaching, regions where it is undesirable for light to reach the photosensor array because such light may affect circuit performance, such as in decoders, sense amplifiers, and analog or digital-signal processors that may be on the same die as the photosensor array. Microlenses  138  may be formed on second surface  135  to concentrate light reaching second surface  135  on photodiode regions  122 ,  124 ,  126 . 
     The N− photodiode regions  122 ,  124 ,  126  are each of depth selected from a shallow, red-sensitive absorber depth, such as region  122 , a deeper red-and-green (yellow) sensitive absorber depth such as region  124 , and a still-deeper, white or panchromatic-sensitive absorber region depth, such as region  126 . 
     Additional P+ regions, such as guardrings or diode-contact regions  140 , may be provided to isolate transistor circuitry regions from photosensor regions of the array, and there may also be a buried P+ impedance-reduction region  142 . 
     Photosensor arrays as herein described do not need a color filter array printed on the backside surface  135  having different light transmission and absorption properties for one or more pixels of the array than for other pixels of the array. Such color filter arrays printed on the backside surface, the side from which light is admitted to the array, are required on photosensor arrays where each photosensor has the same optical properties. 
     Fabrication by Implantation of N− Photodiode Absorber Region 
     In an embodiment the N− photodiode absorber region  122 ,  124 ,  126 , has depth determined through an ion implant, where depth is controlled through selection of ion beam energy provided by an ion implanter, this energy is measured in effective acceleration voltage. In an embodiment where the absorber regions  122 ,  124 ,  126  are fabricated by ion implantation, for each particular depth of the absorber region, a photoresist is applied to the first surface of the wafer, exposed, and developed to provide a layer of photoresist patterned with openings where the second diffused region is desired at that particular depth; the wafer is then implanted by exposing its surface to an ion beam of an energy determined to produce an absorber region extending to that particular depth in the ion implanter as known in the art of integrated circuit manufacture, and the remaining resist is removed. The steps of applying a photoresist, exposing and developing the photoresist, and implanting are then repeated with different implanter beam energy to produce additional second diffused regions at different depths. 
     In a particular embodiment, peak implant energies used are 1 MV for panchromatic or white-sensitive, 500 kV for yellow-sensitive, and 250 kV for red-sensitive implants in a silicon substrate, with the 250 kV implants producing an absorber regions at a depth of less than half a micron, and the 500 kV implant producing an absorber region at a depth of less than two microns. Since high-voltage implants may send most ions deep enough to leave the surface un-inverted while creating an inverted N− region below the surface, implants may be superimposed. For example a deep absorber region  126  may receive implants at both 1 MV and 250 kV, or at all three energies 1 MV, 500 kV, and 250 kV, or at additional energies between zero and 1 MV, as necessary to produce a desired doping profile and may extend the absorber region from that depth towards the first surface to the surface or to a base of an overlying cap diffusion  109 . Similarly, an intermediate absorber region  124  may receive implants at 250 kV as well as at 500 kV, or at additional energies between zero and 500 kV. For purposes of this document, an absorber region extending to a depth below the first surface is one that has been a boundary between an N or P type of the absorber region and a complimentary P or N type of surrounding material at that depth, and may have the same N or P type extending from that depth to a point at or near the first surface. Photosensors as herein described may also be fabricated in other semiconductor materials such as silicon carbide, gallium arsenide, or germanium, but will require different beam energies and junction depths than those beam energies used for silicon. 
     The term wavelength-determining implant as used herein shall mean the implant that determines depth of the lowest part of the absorber region  122 ,  124 ,  126 , and therefore a depth of the active photodiode region. 
     Fabrication by Epitaxy and Dopant Application 
     In alternative embodiments, depth of the absorber region is determined in alternative ways, in a particular alternative embodiment where the lightly-doped absorber regions are grown epitaxially on a substrate, this epitaxial growth being interrupted first to apply a dopant to particular areas at a deep depth to form a deep diffused region  126 , then the epitaxial growth is continued and interrupted second to apply a dopant to particular areas at an intermediate depth to form an intermediate diffused region  124 , then the epitaxial growth is continued and interrupted again to apply a dopant to particular areas at a shallow depth to form a shallow buried diffused region  122 . 
     Thinning 
     Once the photosensors, associated circuitry, and other circuitry has been formed in the wafer on the first surface, but prior to forming opaque mask regions  136  and microlenses  138 , the opposing or second surface  135  of the wafer is thinned to permit light arriving at the second, or backside, surface to reach the photodiodes of the array as known in the art of backside-illuminated (BSI) silicon array photosensors. Pixels, or photosensors,  152  having a shallow absorber region  122  will primarily respond to red light  153 , since shorter-wavelength light (such as blue light) is absorbed in that portion of the wafer that lies between the shallow absorber region  122  and the second surface  135 . Similarly, pixels  154  having an intermediate-depth absorber region  124  will primarily respond to red and yellow light  155 , because blue light is absorbed in that portion of the wafer that lies between the absorber region  122  and the second surface  135 . Finally, pixels  156 ,  158  having deep absorber regions  126  respond to all wavelengths of light  157 , including blue light, and are considered white-light sensitive. Photosensors sensitive to all wavelengths of visible light are also known as panchromatic sensors. 
     An infrared-absorbing filter, or other filter having uniform absorption characteristics for all pixels or photosensors of the array, may in some embodiments be deposited on the second surface  135 ; in some embodiments this filter lies between second surface  135  and the microlenses  138 , and in other embodiments it lies on top of microlenses  138 . 
     Tiling Patterns 
     Photosensors of the array are organized in a repeating, or tiled, cell having four or more photosensors; an embodiment having a four-photosensor tiled cell is illustrated in  FIG. 2 . In this tiled cell, at least one of each red-sensitive  152 , yellow-sensitive  154 , and white-sensitive  156  pixel photosensors are positioned laterally adjacent each other. A fourth photosensor is provided in the cell and in embodiments is selected from a photosensor of the red, yellow, or white-sensitive type. For optimum low-light sensitivity, the fourth photosensor  158  is an additional photosensor of the white-sensitive type. Column  160  and row  162  circuitry, using one or more transistors formed in first surface of the wafer, is provided for addressing each photosensor through row lines  164  and interfacing the photosensors to column lines  166  as known in the art of array photosensors. In some embodiments, separate column lines are provided for each photosensor of the tiled cell to permit reading all four photosensors simultaneously. In some embodiments having a row memory for color recovery, column lines may be shared between rows of the tiled cell. 
       FIG. 2  illustrates just one of several possible tiling patterns. Four-pixel tiling patterns are illustrated in  FIGS. 3A ,  3 B, and  3 C.  FIG. 3A  illustrates a 4-pixel tiling pattern having two red, one yellow, and one white-sensitive sensors.  FIG. 3B  illustrates a 4-pixel tiling pattern having one red, one yellow, and two white-sensitive sensors.  FIG. 3C  illustrates a 4-pixel tiling pattern having one red, two yellow, and one white-sensitive sensors. 
     Many image sensors are used in television applications, where an effective bandwidth or resolution of color information is often much less than a bandwidth or resolution of luminance—or black and white information. For these applications, tiling patterns having more than four pixels may suffice, so long as each repeated pattern has at least one sensor of each of the red, yellow, and white-sensitive types. For example,  FIG. 3D  illustrates a 9-pixel tiling pattern having two red, one yellow, and six white-sensitive sensors, and  FIG. 3E  illustrates a 16-pixel tiling pattern. 
     An image sensor integrated circuit  200  ( FIG. 3 ), has a photosensor array  202  tiled with a pattern of pixel sensors as described above with reference to  FIG. 1  and  FIG. 2 . The image sensor circuit  200  includes scan and exposure control circuitry  204 , which includes row and column counters for addressing pixel sensors of the array in a determined sequence. An output of the row counter of scan and exposure control  204  is decoded by row logic  206  to provide row selection for the photosensor array  202 . Photosensors of each selected row are coupled to column sense amplifiers and multiplexors  208 , such that signals representing light received by a sequence of pixels, or a sequence of tiling patterns, is provided; these signals incorporate red, yellow, and white information. 
     Sensing Light 
     In operation, a bias is applied to each photodiode of each sensor during a precharge phase by a reset or precharge device that may be part of the row logic  206 , and may make use of selection gate  129 . Light received through second surface  135  and absorbed in absorber region  122 ,  124 ,  126  causes minority carrier production in those regions that provides leakage across junctions of the photodiode. After an exposure time, remaining charge on the photodiode of each sensor is measured, in a particular embodiment by coupling sensors through devices of row logic  206  through column lines  166  to sense amplifiers, not shown, to generate signals representing light received by each sensor. 
     Signals representing light received by a sequence of pixels, or a sequence of tiling patterns, may in an embodiment be digitized by an analog to digital converter  210  prior to color recovery. In an alternative embodiment, digitization is performed by an analog to digital converter  212  after color recovery to provide a digital image signal for further processing. In either embodiment, a color recovery processor  214  is provided to translate the red, yellow, and white information derived from the photosensors into red, green, and blue information corresponding to that provided by traditional image sensors. 
     Color Recovery 
     A block diagram of a color-recovery processor  214  for providing red, green, and blue information for each tiling pattern as illustrated in  FIG. 2  or  FIG. 3B  is provided in  FIG. 5 . If provided ahead of analog-to-digital converter  212 , this unit is constructed of analog multipliers and summing amplifiers, if provided after analog-to-digital converter  210 , this unit has digital array multipliers and binary adders. Two white signals, W1 and W2, representing light received by photosensors  156  and  158 , are summed by adder or summing amplifier  252  to provide twice an average white level, which is then multiplied by a white scale factor  254  in multiplier  256  to give a scaled white level. The yellow sensor signal Y, representing photosensor  154 , is similarly multiplied by a yellow scale factor  258  in multiplier  260  to give a scaled yellow level, the scaled yellow level is then subtracted from the scaled white level in adder or summer  262  to give a blue signal BLUE. This circuitry implements the equation BLUE=W*(W scale factor)−Y*(Y scale factor). 
     Similarly, the yellow signal Y is multiplied by a second yellow scale factor  264  in multiplier  266  to give a scaled white level. The red sensor signal R, representing photosensor  152 , is similarly multiplied by a red scale factor  268  in multiplier  270  to give a scaled red level, the scaled red level is then subtracted from the scaled yellow level in adder or summer  272  to give a green signal GREEN. This circuitry implements the equation GREEN=Y*(Y scale factor)−R*(R scale factor). 
     The red signal R is then multiplied by a second red scale factor  276  in multiplier  278  to give a red signal RED. This circuitry implements the equation RED=R*(R scale factor) 
     In some embodiments, a second stage of color processing, the RED, GREEN, and BLUE signals may be multiplied by raw white signals W1 and W2 to provide separate pixel red, green, and blue signals RW1, RW2, GW1, GW2, BW1, and BW2 by multipliers  302 ,  304 ,  306 ,  308 ,  310 ,  312  as illustrated in  FIG. 6 . Similarly, individual, artificial red, green, and blue values may be generated for pixels associated with the yellow (RY, GY, BY) and red (RR, GR, BR) photosensors  152 ,  154 . This circuitry implements the equations:
 
 RW 1=RED* W 1
 
 RW 2=RED* W 2
 
 GW 1=GREEN* W 1
 
 GW 2=GREEN* W 2
 
 BW 1=BLUE* W 1
 
 BW 2=BLUE* W 2
 
     In embodiments optimized for intensity resolution at the expense of color resolution, such as may be used for television where historically chrominance is allocated much less bandwidth than luminance, the RED, GREEN, and BLUE signals may optionally be averaged with RED, GREEN, and BLUE signals from adjacent tiling patterns prior to reconstruction of the individual pixel red, green, and blue signals. This average is then used to provide red, green, and blue signals for each pixel of the tiling pattern. 
     Modifications can be made to the invention in light of the above detailed description while remaining within the spirit and scope of this document. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. It is intended that all matter contained in the above description drawings be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are to cover certain generic and specific features described herein.