PATENT DOCUMENT

Publication Number: US-9041837-B2
Application Number: US-201313785070-A
Country: US
Kind Code: B2

Title: Image sensor with reduced blooming

Abstract:
An image sensor for an electronic device. The image sensor includes a first light sensitive element for collecting charge and having a first saturation value and a well surrounding at least a portion of the first light sensitive element and having a first doping concentration. The image sensor further includes a bridge region defined in the well and in communication with the first light sensitive element and having a second doping concentration and a blooming node in communication with the bridge region and a voltage source. The second doping concentration is less than the first doping concentration and when light sensitive element collects sufficient charge to reach the first saturation value, additional charge received by the light sensitive element travels to the blooming node via the bridge region.

Claims:
What is claimed is: 
     
       1. An image sensor for an electronic device, comprising:
 a first light sensitive element for collecting charge and having a first saturation value; 
 a well surrounding at least a portion of the first light sensitive element and having a first doping concentration; 
 a bridge region defined in the well and in communication with the first light sensitive element and having a second doping concentration; and 
 a blooming node in communication with the bridge region and a voltage source;
 wherein the second doping concentration is less than the first doping concentration; and 
 when the first light sensitive element collects sufficient charge to reach the first saturation value, additional charge received by the first light sensitive element travels to the blooming node via the bridge region. 
 
 
     
     
       2. The image sensor of  claim 1 , wherein the well and the bridge region are both doped with the same type of dopant. 
     
     
       3. The image sensor of  claim 2 , wherein the well and the bridge region are doped with a P-type dopant. 
     
     
       4. The image sensor of  claim 3 , wherein the blooming node is defined by a N-typed doped region. 
     
     
       5. The image sensor of  claim 1 , further comprising a highly doped surface extending over the first light sensitive element, the well, and at least a portion of the bridge region. 
     
     
       6. The image sensor of  claim 5 , wherein the highly doped surface has a third doping concentration, wherein the third doping concentration is greater than both the first doping concentration and the second doping concentration. 
     
     
       7. The image sensor of  claim 6 , wherein the highly doped surface, the well, and the bridge region are all doped with the same dopant type. 
     
     
       8. The image sensor of  claim 1 , further comprising a second light sensitive element having a second saturation value, wherein the second light sensitive element is in communication with the bridge region. 
     
     
       9. The image sensor of  claim 8 , wherein the bridge region extends beneath the blooming node. 
     
     
       10. The image sensor of  claim 1 , wherein the voltage source is pulsed. 
     
     
       11. A camera, comprising:
 a lens; and 
 an image sensor in optical communication with the lens, comprising:
 a first pixel including a first photodiode; 
 a second pixel including a second photodiode; 
 a blooming junction in communication with the first pixel and the second pixel through a bridge region extending between the blooming junction and the first and second photodiodes, wherein the bridge region defines a blooming path between the first photodiode and the second photodiode to the blooming junction, and wherein the bridge region is doped with a first dopant type and the blooming junction is doped with a second dopant type; and 
 a blooming voltage source in communication with the blooming junction,
 wherein the blooming voltage source determines a potential of the blooming junction; 
 wherein the lens transmits light to the image sensor, creating charge within the first photodiode and the second photodiode; and 
 the blooming junction selectively receives charge from the first photodiode and the second photodiode to substantially prevent the first photodiode and the second photodiode from exceeding a saturation value. 
 
 
 
     
     
       12. The camera of  claim 11 , wherein the bridge region is lightly doped. 
     
     
       13. The camera of  claim 11 , wherein the image sensor further comprises: a third pixel having a third photodiode; and a fourth pixel having a fourth photodiode; wherein the blooming junction is in communication with the third photodiode and the fourth photodiode. 
     
     
       14. The camera of  claim 11 , wherein the first photodiode and the second photodiode are substantially surrounded by a well, wherein the well is doped with the first dopant type. 
     
     
       15. The camera of  claim 14 , wherein the well is doped with a P-type dopant and the blooming junction is doped with an N-type dopant. 
     
     
       16. The camera of  claim 14 , wherein the image sensor further comprises a highly doped surface extending over the first photodiode, the second photodiode, the well, and at least a portion of the bridge region. 
     
     
       17. The camera of  claim 16 , wherein the highly doped surface is doped with the first dopant type. 
     
     
       18. The camera of  claim 11 , wherein the blooming voltage source is a pulsed source. 
     
     
       19. A computing device comprising:
 a processor; and 
 an image sensor in communication with the processor, the image sensor comprising: 
 a photodiode having a full well value; 
 a blooming node in communication with the photodiode; 
 a lightly doped region positioned between the blooming node and the photodiode; and 
 a voltage source in communication with the blooming node, wherein the voltage source determines a potential of the blooming node; wherein when the photodiode collects charge exceeding the full well value, excess charge is transmitted to the blooming node through the lightly doped region. 
 
     
     
       20. The computing device of  claim 19 , wherein the image sensor further comprises a pinned transfer gate in communication with the photodiode, wherein the transfer gate selectively enables charge from the photodiode to be transmitted to the processor. 
     
     
       21. The computing device of  claim 19 , wherein the lightly doped region is doped with a first dopant type and the photodiode and the blooming node are doped with a second dopant type.

Description:
TECHNICAL FIELD 
     The present invention relates generally to electronic devices, and more specifically, to image sensors for electronic devices. 
     BACKGROUND 
     Cameras and other image recording devices often use one or more image sensors, such as a charged-coupled device (CCD) sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor. A typical CMOS image sensor may include a two-dimensional array of pixels, where each pixel may include a photo detector or light sensitive element, such as a photodiode, and one or more transistors to activate each pixel. 
     The photodiode or other light detector may capture light which may then be used to create an image of a scene or object. Depending on the light exposed to the image sensor, as well as the configuration of the image sensor, one or more image artifacts may appear in the image. For example, light may impinge on a certain pixel or group of pixels that may exceed the charge capacity of the exposed pixels. The excess charge may then “spill” into adjacent pixels that may not have yet reached capacity. Due to the spillage into adjacent pixels, the captured image may cause certain pixels (e.g., the overflow pixels) to produce inaccurate image data. This light leakage is generally referred to as blooming and may impact the white balance and/or color accuracy of the captured image. 
     The blooming or excessive light exposure may be due to a light source within a captured scene or object, as well as to a color filter that may be used with the image sensor. For example, some image sensors may utilize a Bayer or other color filter, where light is filtered before reaching each pixel. Certain light wavelengths (or colors) may be more dominate in a scene or object or may increase the sensitivity of the pixel. This may result in the image sensor capturing an image where the color accuracy may be affected. For example, pixels with certain color filters may fill up quickly, blooming to adjacent pixels. 
     SUMMARY 
     Examples of the disclosure may an image sensor for an electronic device. The image sensor includes a first light sensitive element for collecting charge and having a first saturation value and a well surrounding at least a portion of the first light sensitive element and having a first doping concentration. The image sensor further includes a bridge region defined in the well and in communication with the first light sensitive element and having a second doping concentration and a blooming node in communication with the bridge region and a voltage source. The second doping concentration is less than the first doping concentration and when light sensitive element collects sufficient charge to reach the first saturation value, additional charge received by the light sensitive element travels to the blooming node via the bridge region. 
     Other examples of the disclosure may take the form of a camera for an electronic device. The camera includes a processor, a display in communication with the processor, a lens, and an image sensor. The image sensor is in communication with the processor and in optical communication with the lens. The image sensor includes a first pixel including a first photodiode, a second pixel including a second photodiode, a blooming junction in communication with the first pixel and the second pixel, and a blooming voltage source in communication with the blooming junction, wherein the blooming voltage source determines a potential of the blooming junction. During operation, the lens transmits light to the image sensor, creating charge within the first photodiode and the second photodiode and the blooming junction selectively receives charge from the first photodiode and the second photodiode to substantially prevent the first photodiode and the second photodiode from exceeding a saturation value. 
     Yet other examples of the disclosure may take the form of a computing device. The computing device includes a processor and an image sensor in communication with the processor. The image sensor includes a photodiode having a full well value, a blooming node in communication with the photodiode, and a voltage source in communication with the blooming junction, wherein the voltage source determines a potential of the blooming node. During operation, when the photodiode collects charge exceeding the full well value, excess charge is transmitted to the blooming node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a front perspective view of an electronic device including one or more cameras. 
         FIG. 1B  is a rear perspective view of the electronic device of  FIG. 1A . 
         FIG. 2  is a simplified block diagram of the electronic device of  FIG. 1A . 
         FIG. 3  is a cross-section view of the electronic device of  FIG. 1A  taken along line  3 - 3  in  FIG. 1A . 
         FIG. 4A  is a simplified diagram of an image sensor architecture for a camera of the electronic device. 
         FIG. 4B  is an enlarged view of the pixel architecture of  FIG. 4A  illustrating a single pixel. 
         FIG. 5  is a simplified schematic view of the pixel of  FIG. 4A . 
         FIG. 6A  is a diagram of a conventional pixel circuit including a blooming pathway to the floating diffusion node. 
         FIG. 6B  is a potential diagram of the pixel of  FIG. 6A . 
         FIG. 7A  is a schematic diagram of a pixel of the image sensor including the blooming node. 
         FIG. 7B  is a potential diagram of the pixel of  FIG. 7A . 
         FIG. 8  is a top plan view of the pixel with a top surface layer hidden for clarity. 
         FIG. 9  is a cross-section of the pixel taken along line  9 - 9  in  FIG. 8 . 
         FIG. 10A  is a top plan view of two pixels sharing a blooming node with a top surface layer hidden for clarity. 
         FIG. 10B  is a cross-section of the pixels illustrated in  FIG. 10A  taken along line  10 B- 10 B in  FIG. 10A . 
         FIG. 11  is a simplified top plan view of four photodiodes having a common blooming node with a top surface layer hidden for clarity. 
     
    
    
     SPECIFICATION 
     Overview 
     The disclosure may take the form of an image sensor for cameras and other electronic devices. The image sensor may include a blooming node that may remove excess charge from a photodiode or photogate. In some embodiments, the blooming node may define a blooming path for charge to travel away from an overexposed photodiode. The blooming node may have a positive potential and may be connected to the photodiode through a lightly doped region which is typically doped oppositely of the blooming node. As one example, the blooming node may be N-typed doped and the bridge or lightly doped region may be P-typed doped. The lightly doped region may create a low potential barrier between the blooming node and the photodiode. The barrier defined by the lightly doped region may have a lower potential than a well (such as a P-well) surrounding the photodiode. The blooming node may be tied to a voltage source, which may allow the blooming node to have a high potential. The blooming node may receive charge from a blooming pixel through the lightly doped region. 
     In some embodiments, the blooming node or junction may be connected to a control signal (or voltage source). The control signal may be a separate signal for the blooming node, or may be the same source as other components of the image sensor (e.g., source follower gate). Additionally, the image sensor may include a blooming node for each pixel or may include a blooming node for two or more pixels. In other words, each pixel may have its own blooming node or the blooming node may be shared by two or four pixels. 
     The blooming node and lightly doped region may reduce pixel blooming. Additionally, the blooming pathway to the blooming node may be separate from a transfer gate or other components of the pixel, which may allow the transfer gate to be pinned. By providing a blooming pathway, without requiring the transfer gate to be un-pinned, the image sensor may reduce blooming without creating extra dark current or hot pixels. 
     DETAILED DESCRIPTION 
     Turning now to the figures, the image sensor and an illustrative electronic device for incorporating the image sensor will be discussed in more detail.  FIG. 1A  is a front elevation view of an electronic device  100  including the image sensor.  FIG. 1B  is a rear elevation view of the electronic device  100 . The electronic device  100  may include a first camera  102 , a second camera  104 , an enclosure  106 , a display  110 , and an input/output button  108 . The electronic device  100  may be substantially any type of electronic or computing device, such as, but not limited to, a computer, a laptop, a tablet, a smart phone, a digital camera, a printer, a scanner, a copier, or the like. The electronic device  100  may also include one or more internal components (not shown) typical of a computing or electronic device, such as, but not limited to, one or more processors, memory components, network interfaces, and so on. 
     As shown in  FIG. 1A , the enclosure  106  may form an outer surface or partial outer surface and protective case for the internal components of the electronic device  100  and may at least partially surround the display  110 . The enclosure  106  may be formed of one or more components operably connected together, such as a front piece and a back piece, or may be formed of a single piece operably connected to the display  110 . 
     The input member  108  (which may be a switch, button, capacitive sensor, or other input mechanism) allows a user to interact with the electronic device  100 . For example, the input member  108  may be a button or switch to alter the volume, return to a home screen, and the like. The electronic device  100  may include one or more input members  108  and/or output members, and each member may have a single input or output function or multiple input/output functions. 
     The display  110  may be operably connected to the electronic device  100  or may be communicatively coupled thereto. The display  110  may provide a visual output for the electronic device  100  and/or may function to receive user inputs to the electronic device  100 . For example, the display  110  may be a multi-touch capacitive sensing screen that may detect one or more user inputs. 
     The electronic device  100  may also include a number of internal components.  FIG. 2  is a simplified block diagram of the electronic device  100 . The electronic device  100  may also include one or more processors  114 , a storage or memory component  116 , an input/output interface  118 , a power source  120 , and one or more sensors  122 , each will be discussed in turn below. 
     The processor  114  may control operation of the electronic device  100 . The processor  114  may be in communication, either directly or indirectly, with substantially all of the components of the electronic device  100 . For example, one or more system buses  124  or other communication mechanisms may provide communication between the processor  114 , the cameras  102 ,  104 , the display  110 , the input member  108 , the sensors  122 , and so on. The processor  114  may be any electronic device cable of processing, receiving, and/or transmitting instructions. For example, the processor  114  may be a microprocessor or a microcomputer. As described herein, the term “processor” is meant to encompass a single processor or processing unit, multiple processors, or multiple processing units, or other suitably configured computing element. 
     The memory  116  may store electronic data that may be utilized by the electronic device  100 . For example, the memory  116  may store electrical data or content e.g., audio files, video files, document files, and so on, corresponding to various applications. The memory  116  may be, for example, non-volatile storage, a magnetic storage medium, optical storage medium, magneto-optical storage medium, read only memory, random access memory, erasable programmable memory, or flash memory. 
     The input/output interface  118  may receive data from a user or one or more other electronic devices. Additionally, the input/output interface  118  may facilitate transmission of data to a user or to other electronic devices. For example, in embodiments where the electronic device  100  is a phone, the input/output interface  118  may be used to receive data from a network, or may be used to send and transmit electronic signals via a wireless or wired connection (Internet, WiFi, Bluetooth, and Ethernet being a few examples). In some embodiments, the input/output interface  118  may support multiple network or communication mechanisms. For example, the network/communication interface  118  may pair with another device over a Bluetooth network to transfer signals to the other device, while simultaneously receiving data from a WiFi or other network. 
     The power source  120  may be substantially any device capable of providing energy to the electronic device  100 . For example, the power source  120  may be a battery, a connection cable that may be configured to connect the electronic device  100  to another power source such as a wall outlet, or the like. 
     The sensors  122  may include substantially any type of sensor. For example, the electronic device  100  may include one or more audio sensors (e.g., microphones), light sensors (e.g., ambient light sensors), gyroscopes, accelerometers, or the like. The sensors  122  may be used to provide data to the processor  114 , which may be used to enhance or vary functions of the electronic device  100 . 
     With reference again to  FIGS. 1A and 1B , the electronic device  100  may also include one or more cameras  102 ,  104  and optionally a flash  112  or light source for the cameras.  FIG. 3  is a simplified cross-section view of one camera  102 , taken along line  3 - 3  in  FIG. 1A . Although  FIG. 3  illustrates the first camera  102 , it should be noted that the second camera  104  may be substantially similar to the first camera  102 . In some embodiments one camera may include a global shutter configured image sensor and one camera may include a rolling shutter configured image sensor. In other examples, one camera may have an image sensor with a higher resolution than the image sensor in the other camera. With reference to  FIG. 3 , the cameras  102 ,  104  may include a lens  126  in optical communication with an image sensor  130 . The lens  126  may be operably connected to the enclosure  106  and positioned above the image sensor  130 . The lens  126  may direct or transmit light  128  within its field of view on to a photodiode layer (discussed in more detail below) of the image sensor  130 . 
     The image sensor  130  may be supported beneath the lens  126  by a substrate  132  or other support structure. The image sensor  130  may convert light  128  into electrical signals that may represent the light from the captured scene. In other words, the image sensor  130  captures the light  128  optically transmitted via the lens  126  into electrical signals. 
     Image Sensor Architecture 
     An illustrative architecture for the image sensor  130  will now be discussed in more detail.  FIG. 4A  is a simplified schematic of an architecture for the image sensor  130 .  FIG. 4B  is an enlarged view of a pixel of the pixel architecture of  FIG. 4A .  FIG. 5  is a simplified schematic view of the pixel of  FIG. 4A . As will be discussed in more detail below, with reference to  FIG. 7A , the architecture of  FIG. 4A-5  may be implemented with the blooming node, although the blooming node is not shown in  FIG. 5 . 
     With reference to  FIGS. 4A-5 , the image sensor may include an image processing component  150  and a pixel architecture  134  or pixel array. This architecture defines one or more pixels  136  and/or groups of pixel cells  138  (e.g., groups of pixels  136  grouped together to form a Bayer pixel or other set of pixels). The pixel architecture  134  may be in communication with a column select  140  through one or more column output lines  146  and a row select  144  through one or more row select lines  148 . 
     The row select  144  and/or the column select  140  may be in communication with an image processor  142 . The image processor  142  may process data from the pixels  136  and provide that data to the processor  114  and/or other components of the electronic device  100 . It should be noted that in some embodiments, the image processor  142  may be incorporated into the processor  114  or separate therefrom. The row select  144  may selectively activate a particular pixel  136  or group of pixels, such as all of the pixels  136  on a certain row. The column select  140  may selectively receive the data output from select pixels  136  or groups of pixels  136  (e.g., all of the pixels with a particular column). 
     With reference to  FIG. 5 , each pixel  136  may include a transistor array  152  or control circuitry and a photodiode  154 . The photodiode  154  may be in optical communication with the lens  126  to receive light transmitted therethrough. The photodiode  154  may absorb light and convert the absorbed light into an electrical signal. The photodiode  154  may be an electron-based photodiode or a hole based photodiode. Additionally, it should be noted that the term photodiode as used herein is meant to encompass substantially any type of photon or light detecting component, such as a photogate or other photon sensitive region. The photodiode  154  is coupled to a transfer gate  158 , the transfer gate  158  selectively connects the photodiode  154  to the remaining control circuitry  152  of the pixel  136 . 
     The transfer gate  158  is coupled to a reset gate  156  and a source follower gate  160 . A reset gate  162  and the source follower gate  160  are coupled to a reference voltage node  164  which connects the two gates to a reference voltage source (Vdd)  166 . The row select gate  162  is coupled to a row select line  148  for the pixel  136 . A floating diffusion node  163  including a charge storage component  168  may be coupled between the transfer gate  158  and the reset gate  156  and source follower gate  160 . The control circuitry  152  (or transistor array) may include additional gates other than those shown in  FIG. 5 . For example, an anti-blooming gate may be in communication with the photodiode  154  to drain charge in excess of saturation level from the photodiode. 
     Generally, in operation, when one of the cameras  102 ,  104  is actuated to take a picture by a user, the reference voltage  166  is applied to the reset gate  156  and the transfer gate  158 . When the transfer gate  158  is open, the charge within the photodiode  154  is drained to deplete the photodiode. In some embodiments, the cameras  102 ,  104  may not include a shutter over the lens  126 , and so the image sensor  130  may be constantly exposed to light. In these embodiments, the photodiode  154  may have to be reset or depleted before a desired image is to be captured. Once the charge from the photodiode  154  has been depleted, the transfer gate  158 , and the reset gate  156  may be turned off, isolating the photodiode  154 . The photodiode  154  may then begin integration and collecting light  128  transmitted to the image sensor  130  from the lens  126 . As the photodiode  154  receives light, it starts to collect charge (e.g., a depletion region reduces as electrons from the light are received). However, the charge within the photodiode  154  may remain within a well of the photodiode  154  because the transfer gate  158  (connecting the photodiode  154 ) to the control circuitry  150  and other gates is off. As will be discussed in more detail with respect to  FIGS. 7A and 7B , in instances where the photodiode may be over-exposed, the excess charge may bloom to a blooming junction or node. 
     Once integration is complete and the photodiode  154  has collected light  128  from the lens  126 , the reset gate  152  may be turned on to reset the floating diffusion node  163 . Once the floating diffusion  163  has been reset, the reset gate  156  may be turned off and the transfer gate  158  may be turned on. The charge from the photodiode  154  can then be transferred to the floating diffusion node  163  and be stored in the storage component  168 . To read out the charge from the photodiode  154  (here, via the floating diffusion  163 ), the row select gate  152  and the source follower gate  160  may be activated, and the source follower gate  160  amplifies the charge within the floating diffusion  163  and through the row select gate  162 , the signal or charge is provide to the column output line  146 . 
     In a rolling shutter operation, photodiodes  154  in different rows may be exposed at different times. Accordingly, if one or more objects within a scene are moving, a first row may capture a different position of the image than a second row as they are exposed sequentially, which may cause motion artifacts in the sensed image. In a global shutter operation, additional storage nodes may be added to store charge from the photodiode  154 . In the global shutter operation, each row within the pixel architecture  134  may be reset and exposed at substantially the same time. Each pixel may also simultaneously transfer the charge from the photodiode  154  to a storage node, and then each pixel  136  may be read out row by row. 
     In some instances, a scene or object to be captured may include one or more bright light sources or may include a higher percentage of certain light wavelengths than others. In these instances, one or more pixels may exceed their charge limitations and may bloom.  FIG. 6A  is a diagram of a conventional pixel circuit including a blooming pathway to the floating diffusion node.  FIG. 6B  is a potential diagram of the pixel of  FIG. 6A . With reference to  FIGS. 6A and 6B , in conventional image sensors, the photodiode  254 , floating diffusion  263 , and transfer gate  158  may be surrounded by a P-well  252 . A charge path  264  is defined by the transfer gate  258  to the floating diffusion  263 . In this manner, the floating diffusion  263  may act to collect overflow charge from the photodiode  254 , such as excess charge due to blooming. 
     In order to allow charge to transfer between the photodiode  254  and the floating diffusion  263  when the transfer gate  258  is not activated (e.g., off), a barrier  270  needs to be lowered. For example, as shown in  FIG. 6B , the barrier  270  may have a potential of P1. In operation, once charge from the lens  126  has filled the entire photodiode  254 , the charge may flow into the floating diffusion  263  due to the lowered potential P1 of the barrier  270 . 
     In the conventional pixels illustrated in  FIGS. 6A and 6B , the potential P1 of the transfer gate  258  may not be sufficiently negative to pin a surface of the transfer gate  258 . In other words, to define a path for excess charge, the transfer gate is typically unpinned (i.e., the transfer gate does not include a shallow doped layer at the surface, where the potential of the surface is pinned to the substrate potential). Therefore, in conventional image sensors, the unpinned transfer gate may create dark current, as well as hot pixels. This is because typically the surface doped layer of a pinned gate masks traps that can be one of the main sources of dark current. In other words, the fully pinned transfer gate surface can protect traps (which may be due to foreign objects or particles in the substrate) from receiving electrons that could then be recombined or released to form dark current or hot pixels. Generally, any trap surrounding the transfer gate may be a potential source of dark current and hot pixels and by pinning the transfer gate, the traps may be protected. Additionally, by being unpinned, the transfer gate may increase the chance of hot pixels (i.e., pixels that unnaturally bright). 
     Blooming Node 
     The image sensor  130  of the present disclosure may include a blooming node that may receive excess charge from the photodiode, as well as allow the transfer gate to be pinned, reducing dark current and hot pixels.  FIG. 7A  is a schematic diagram of a pixel of the image sensor including the blooming node.  FIG. 7B  is a potential diagram of the pixel of  FIG. 7A . In some embodiments, the image sensor  130  may include a blooming node  300  in communication with the photodiode  154 . The blooming node  300  may be in communication with a blooming voltage source  165 . As shown in  FIG. 7A , the blooming voltage source may be a separate voltage source  165 ; however, in other embodiments, the blooming voltage source may be Vdd  166 . Accordingly, it should be noted that the blooming node  300  may be connected to its own control voltage signal or another control signal. For example, the blooming node  300  may be connected to the floating diffusion node  163 , pixel output, or the like. As another example, the blooming node may be connected to a separate blooming node voltage source that may provide power specifically to the blooming node. As yet another example, the blooming node may be connected to the same voltage source as the other components of the pixel. 
     The blooming node  300  node may be positioned on an opposite side of the photodiode  154  from its connection to the transfer gate  158 . However, in other embodiments the blooming gate may be otherwise connected to the pixel to provide a blooming path for the pixel. 
     With reference to  FIG. 7B , the blooming node  300  may define a blooming path  302  through a bridge region  304 . As will be discussed in more detail below, the bridge region  304  may have a potential P2 that may be defined to allow charge to transfer from the photodiode  154  to the blooming node  300 . The bridge region  304  reduces the barrier between the photodiode  154  and the blooming node  300 , which will allow extra charge from the photodiode  154  to follow the blooming path  302  to the blooming node  300 . Because the blooming node  300  is connected to Vdd  165  (or another potential source), the blooming node  300  may continue to receive charge from the photodiode  154  without reaching capacity. The blooming node  300  may only receive charges from the photodiode  154  when the potential in the photodiode  154  is higher than the barrier between the photodiode and the blooming node. Operation of the blooming node  300  will be discussed in more detail below. 
     The blooming node  300  and the bridge region  304  may be defined by varying the doping profile of a substrate of the image sensor. One or more pixels  152  of the image sensor  130  may be formed in a semiconductor substrate that may be doped with certain elements to create components of the image sensor. For example, the semiconductor substrate may be silicon doped with arsenic, boron, antimony, arsenic, aluminum, selenium, germanium, or the like, depending on the particular semiconductor. In particular, the silicon substrate may be doped with a P-type dopant such as boron or gallium and/or N-type dopant such as phosphorous or arsenic. The type of dopant, as well as the concentration, may be varied depending on the desired characteristics of the image sensor and pixel. 
     The structure for one or more pixels  136  of the image sensor  130  will now be discussed in more detail.  FIG. 8  is a top plan view of the pixel  136  with a top surface layer hidden for clarity.  FIG. 9  is a cross-section of the pixel taken along line  9 - 9  in  FIG. 8 . With reference to  FIGS. 8 and 9 , the pixel  136  includes a substrate  310  defining the photodiode  154  substantially surrounded by a well  306 . The substrate  310  may be a semiconductor material, such as silicon and the photodiode  154  and other components in the substrate  310  may be defined by varying the concentration of one or more dopants within the substrate  310 . 
     In some embodiments, the photodiode  154  may be N-type doped and the well  306  may be P-type doped or a P-well. The well  306  may surround the photodiode  154  and the transfer gate  158 . A path to the floating diffusion node  163  may be defined beneath the transfer gate  158  (e.g., when the transfer gate is activated), allowing charge to travel from the photodiode  154  to the floating diffusion  163 . 
     With reference to  FIG. 9 , a surface of the pixel  136  or protective layer may be more strongly doped than the well  306 . For example, the surface  308  may be P+ doped (e.g., a doping concentration ranging between 10 18 /cm 3  to 10 19 /cm 3 ) whereas the well  306  may be P or P− doped (e.g., a doping concentration ranging between 10 15 /cm 3  to 10 17 /cm 3 ) and the photodiode  154  may be buried beneath the surface  308 . By increasing the dopant level in the surface  306 , the photodiode  154  may be pinned. The pinned photodiode  154  reduces surface defect noise such as dark current. As an example, because of the highly doped surface, a negative voltage may be applied to the transfer gate  158  to pin the transfer gate  158  surface, suppressing dark current. As one implementation, a voltage of negative 1.2V may be applied to the transfer gate  158 , pinning the transfer gate  158 . 
     With reference again to  FIGS. 8 and 9 , the bridge region  304  and blooming node  300  may be formed against one portion of the photodiode  154 . In some embodiments, the bridge region  304  and the blooming node  300  may be at least partially surrounded by the well  306 . The blooming node  300  may be an N-typed doped region or may otherwise have a positive potential. For example, the blooming node  300  may be doped with an N-typed dopant in a concentration ranging between 10 17 /cm 3  to 10 20 /cm 3 . 
     The blooming node  300  or the N-type node (NN) may be formed through the surface  308  of the pixel  136 . In other words, in some embodiments, the blooming node  300  may not be buried beneath a P+ doped area or may include at least one contact or portion exposed through the surface. This is because the blooming node  300  may be biased externally (e.g., by the blooming node voltage source  165 ) and thus includes a contact exposed on the surface to connect to the biasing source. Additionally, the blooming node  300  may extend from a top of the surface  308  into the bridge region  304 . In some embodiments, the blooming node  300  may have a depth greater than the highly doped surface  308 . By extending the blooming node  300  into the bridge region  304 , extra blooming charge can drain from the photodiode  154  to the blooming node through the bridge region  304 . In some embodiments, the blooming node  300  may be on the surface of the substrate  310  to reduce the depth of the doping required to define the node  300 . 
     With reference to  FIGS. 7A and 8 , the blooming node  300  may be connected to a separate positive bias (blooming voltage source  165 ) or may be connected to the shared pixel power supply  166 . The blooming node  300  may rely on a blooming voltage source or positive bias to deplete charge from the node  300  as it is received. In other words, the blooming node  300  may define an extra junction in the pixel circuitry and may clear out blooming charge without requiring a separate gate (such as an anti-blooming gate). This may reduce the complexity of the pixel circuitry, as well as reduce the required size of the image sensor. By connecting the blooming node  300  to a potential source, excess charge from the photodiode  154  that reaches the blooming node  300  may be pulled out. 
     In some embodiments, the potential of the blooming node  300  may be substantially any voltage source that has a potential higher than the potential of the photodiode  154 . Additionally, as will be discussed in more detail below, the voltage source connected to the blooming node  300  may be a pulsed or variable source or may be substantially continuous. 
     The bridge region  304  may connect the blooming node  300  to the photodiode  154 . The bridge region  304  may be a lightly doped P-type region (LDP). For example, the bridge region  304  may be doped to be P− (e.g., a P-type dopant concentration of 10 15 /cm 3  to 10 17 /cm 3 ). In this example, the bridge region  304  has a lower P-type dose than the P-type concentration in the well  306 . The lower doping concentration of the bridge region  304  creates a lower potential barrier between the photodiode  154  and the blooming node  300  as compared to other areas of the well  306  region. In other words, a barrier under the bridge region  304  may be lower than the remaining areas of the well  306 , forming a pathway for charge to travel from the photodiode  154  to the blooming node  300 . In some embodiments, charge may travel through the bridge region  304  to the blooming node  300  and the pathway may be defined through the bridge region. It should be noted (as shown in  FIG. 9 ), that the surface  310  may include the protective doping layer P+ above the bridge region  304 . This highly doped surface  308  protects the silicon surface and reduces dark current and hot pixels. 
     Operation of the blooming node  300  to remove excess charge from the photodiode  154  will now be discussed in more detail. With reference to  FIGS. 7A-9 , the photodiode  154  may be reset prior to beginning integration. As discussed above with respect to  FIG. 5 , the photodiode  154  may be reset by activating the transfer gate  158  and the reset gate  156 . Once the photodiode  154  has been reset, the photodiode  154  may begin integration and receive light form the lens  126 . As the photodiode  154  is exposed to light, it may begin collecting charge carriers. During integration, if the photodiode  154  collects too much charge or otherwise exceeds its well capacity (indicated by dashed line  312 ), the excess charge may following the blooming path  302  defined by the potential P2 through the bridge region  304  to the blooming node  300 . Because the bridge region  304  is lightly doped, the barrier between the photodiode  154  and the blooming node  300  may be lower than the well  306  region, allowing the charge to bloom to the blooming node  300 . For example, the bridge region  304  may have a potential that is slightly higher than the potential of the photodiode  154 . In a specific implementation, the bridge region  304  may have a potential greater than the photodiode  154  by about 200 mV. 
     The blooming node  300  junction is tied to a voltage source  165 , therefore the excess charge received into the blooming node  300  will be pulled into the potential of the Vdd  165  (see  FIG. 7B ). As the voltage source Vdd  165  may have a controlled potential level, the potential “well” of the blooming node  300  may remain the same although excess charge is dumped into the node  300 . In other words, the blooming node  300  may not have a “well” that may fill to capacity, because the junction of the blooming node  300  is tied to a controlled voltage source that may absorb the excess current without changing the potential at the node. Thus, the blooming node  300  may pull excess charge from the photodiode  154  without reaching a full well capacity. This may prevent the pixel from blooming, even if the photodiode  154  is exposed to a substantial level of excess light. 
     In embodiments where the voltage source  165  connected to the blooming node  300  may be separate from the voltage source Vdd  166 , the blooming node voltage source may be pulsed. For example, the voltage source of the blooming node may be configured to go high once every frame. In these embodiments, excess charge from the photodiode  154  may be pulled every time the voltage source goes high, which may be selected to be pulsed once every frame. However, in other embodiments, the voltage source for the blooming node  300  may be activated before (or at the same time) as integration. In this manner, the blooming node  300  may collect charge initially, rather than during a set time during integration. As another example, the voltage source may be pulsed multiple times during integration. 
     Once integration is complete, the voltage source  165  of the blooming node  300  may turn off (or go to a low potential) and the transfer gate  158  may be activated. In some embodiments, the voltage source for the transfer gate  158  may turn on separately from the blooming node  300  to encourage charge transfer from the photodiode  154  to the floating diffusion  163  via the transfer gate  158 . In other words, by deactivating the voltage source of the blooming node  300  during read out, the potential of the blooming node  300  may be increased, such that charge within the photodiode  154  may travel to the floating diffusion  163  (and not the blooming node). The charge may then be transferred to the floating diffusion  163  for readout. The processor may then use the collected light data to create an image that may be displayed on the display and/or stored in memory. 
     Shared Blooming Node Configurations 
     In some embodiments, the blooming node  300  may be shared by two or more pixels.  FIG. 10A  is a top plan view of two pixels sharing a blooming node with a top surface layer hidden for clarity.  FIG. 10B  is a cross-section of the pixels illustrated in  FIG. 10A  taken along line  10 B- 10 B in  FIG. 10A . With reference to  FIGS. 10A and 10B , two photodiodes  154 ,  354  may be defined in the same substrate  310  and the well  306  may surround (or substantially surround) both photodiodes  154 ,  354 . It should be noted that transfer gate and floating diffusion are hidden for clarity as well in  FIGS. 10A and 10B . 
     The blooming node  300  may be connected to the first photodiode  154  through the bridge region  304  and to the second photodiode  354  by a second bridge region  314 . With reference to  FIG. 10B , both bridge regions  304 ,  314  may be formed in the same area of the substrate and may extend beneath the blooming node  300 . Additionally, the highly doped surface  308  may extend over both photodiodes  154 ,  354  to pin both photodiodes. 
     As another example, the blooming node  300  may be shared by a group or cluster of pixels.  FIG. 11  is a simplified top plan view of four photodiodes having a common blooming note  300  with a top surface layer hidden for clarity. With reference to  FIG. 11 , each of the photodiodes  154 ,  354 ,  356 ,  358  may be in communication with the blooming node  300  through a bridge region  304  extending between each photodiode and the blooming node  300 . As with the embodiment illustrated in  FIGS. 8 and 9 , the bridge region  304  may be a lightly doped region, such that the substrate  310  may have a lower barrier beneath the bridge regions  304 , allowing charge to transfer from the photodiodes to the blooming node. 
     With reference to  FIGS. 10A and 11 , each of the blooming nodes  300  may receive overflow charge from two or more photodiodes. However, because the blooming nodes  300  may be connected to a voltage source to control their potential, the blooming nodes  300  may receive the overflow charge from each of the photodiodes without overfilling their capacity. That is, the blooming node  300  may receive substantially an infinite amount of bloom charge due to its connection to a voltage source  166 . 
     Embodiments of the present disclosure may relate to an image sensor including a blooming junction or node. The blooming node may be connected to the photodiodes through a bridge or lightly doped region that may have a reduced barrier through the substrate, allowing charge to flow from the photodiodes to the blooming node. The blooming node may be tied to a blooming voltage source, allowing the blooming node to receive excess charge without reaching a full well capacity. The blooming node may be controlled by its own voltage source (e.g., to allow for specific adjustments in voltage based on desired characteristics), or may be tied to one or more voltage sources of the pixels (e.g., pixel power supply, the floating diffusion supply, or the like). Additionally due to the structure of the blooming node, the extra charge from the photodiode may be prevented from “blooming” to adjacent pixels, but without requiring a tradeoff increase in dark current or hot pixels. 
     CONCLUSION 
     The foregoing description has broad application. For example, while examples disclosed herein may focus on a blooming node for CMOS image sensors, it should be appreciated that the concepts disclosed herein may equally apply to other types of image sensors, such as CCD image sensors. Similarly, although depth sensing system may be discussed with respect to image sensors, the devices and techniques disclosed herein are equally applicable to other types of sensors or semiconductor devices. Moreover, although row select gates are described with respect to the pixel architecture, the embodiments disclosed herein may be used in image sensor pixel architectures that do not include row select pixels, as well as other variations of pixel architecture. Accordingly, the discussion of any embodiment is meant only to be exemplary and is not intended to suggest that the scope of the disclosure, including the claims, is limited to these examples.

Metadata:
Filing Date: 20130305
Publication Date: 20150526
Grant Date: 20150526
Priority Date: 20130305
Inventors: LI XIANGLI
Assignee: APPLE INC
CPC Classifications: [{"code": "H04N25/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N25/622", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04N25/622", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/813", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/802", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/194", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/194", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/813", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10F39/802", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L27/14603", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L27/14641", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/3592", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L27/14672", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04N5/335", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 51487410