# Patent

## Title
Photodiode with different electric potential regions for image sensors

## Abstract

An image sensor pixel is disclosed. The pixel may include a photodiode having a first region with a first potential and a second region with a second, higher potential, with the second region being offset in depth from the first region in a semiconductor chip. A storage node may be positioned at substantially the same depth as the second region of the photodiode. A storage gate may be operable to transfer charge between the photodiode and the storage node.

## Background
TECHNICAL FIELD
The present invention relates generally to image sensors, and, more specifically, to a photodiode with different electric potential regions for use in an image sensor
BACKGROUND
Many widely-used image sensors include global-shutter pixels because of their high efficiency and lack of a blur as compared to rolling-shutter pixels. Global shutter pixels typically include a storage node, separate from the photodiode region, where charge generated during a previous integration frame can be stored and subsequently read out. All of the pixels in a global shutter image sensor typically transfer charge generated in their respective photodiodes to their respective storage nodes ‘globally,’ which eliminates the blur caused by the row-by-row exposure and readout in rolling shutter pixels. The global shutter storage nodes can be read out when convenient, such as while the photodiode is integrating charge for a subsequent frame.
The storage node in global shutter pixels is usually located on the same surface of a semiconductor wafer as the photodiode region, and thus typically needs to be shielded in order to maintain the integrity of the charge stored in the storage node. Also, positioning the storage node on the same surface of a semiconductor wafer as the photodiode reduces the amount of surface area of the photodiode that can be exposed to light, and hence reduces the sensitivity of the pixel.
SUMMARY
One example of the present disclosure may take the form of an image sensor pixel. The image sensor pixel may include a photodiode having a first region with a first potential and a second region with a second, higher potential. The second region may be offset in depth from the first region in a semiconductor chip. A storage node may be positioned at substantially the same depth as the second region of the photodiode, and a storage gate may be operable to selectively transfer charge from the photodiode to the storage node.
Another example of the disclosure may take the form of a method of operating an image sensor pixel. The method may include integrating charge in a first region of a photodiode, and funneling charge from the first region of the photodiode to a second region of the photodiode responsive to a potential difference between the first and second regions. The method may also include transferring charge from the second region of the photodiode to a storage node positioned at least partially beneath the first region of the photodiode.
Another example of the disclosure may take the form of a method of manufacturing an image sensor pixel. The method may include forming a storage node and a first region of a photodiode on a first surface of a silicon wafer. The method may also include forming a second region of the photodiode on a second surface of the silicon wafer, the second region of the photodiode having a lower concentration of doping than the first region, and the second region of the photodiode at least partially covering the storage node formed on the first surface of the silicon wafer.



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 simplified schematic 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 a pixel architecture of FIG. 4A illustrating a single pixel.
FIG. 5 is a simplified schematic view of a pixel cell having a global shutter configuration.
FIG. 6A is a simplified schematic cross-section view of one embodiment of an image sensor pixel.
FIG. 6B is a simplified potential profile of the image sensor pixel shown in FIG. 6A.
FIGS. 7A through 11B illustrate the operation of the image sensor pixel shown in FIG. 6A.
FIGS. 12A and 12B illustrate one embodiment of steps for manufacturing the image sensor pixel shown in FIG. 6A.
FIG. 13 is a simplified schematic cross-section view of another embodiment of an image sensor pixel.
FIG. 14 is a simplified schematic cross-section view of another embodiment of an image sensor pixel.
FIG. 15 is a simplified schematic cross-section view of yet another embodiment of an image sensor pixel.



SPECIFICATION
Overview
In some embodiments disclosed herein, apparatuses and methods for transferring charge from one region of a photodiode to another region of the photodiode in an image sensor are disclosed. The charge may be funneled from a first region to a second region due to different electric potentials in the respective first and second regions, with charge generally flowing to the region with the higher potential. Having two or more regions of a photodiode on a single semiconductor chip may allow for an image pixel to be formed on two sides of the semiconductor chip, with the charge funneling phenomenon being used to transfer charge from one side of the chip (e.g., a backside illuminated photodiode) to another side of the chip (e.g., with transfer transistors and circuitry), thus taking advantage of both sides of the semiconductor chip and increasing the efficiency and size of the pixels without increasing the absolute size of the overall image sensor.
Turning now to the figures, an 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 one or more image sensors. FIG. 1B is a rear elevation view of the electronic device 100. The electronic device 100 may include any or all of 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. Examples of such internal components will be discussed with respect to FIG. 2.
As shown in FIG. 1, the enclosure 106 may form an 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(s).
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 102, 104. FIG. 3 is a simplified cross-section view of the first 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 other. Likewise, it should be appreciated that the structure shown in FIG. 3 is but one possible structure for either of the first and second cameras.
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 onto a photodiode (discussed in more detail below) of the image sensor 130. 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 one possible 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. With reference to FIGS. 4A-5, the electronic device 100 may include an image processing component having a pixel architecture defining 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 the simplified schematic of one embodiment of a pixel 136 illustrated in FIG. 5, each pixel 136 may include 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 photo-sensitive region.
The photodiode 154 may be coupled to a storage node SN 192 through a storage gate SG 190. The storage node 192 may store charge from the photodiode 154 to allow a global shutter operation, and may in some examples be electrically and/or optically shielded so as to prevent stray charge and/or light from corrupting the contents of the storage node 192. The storage node 192 may be coupled to a floating diffusion node FD 163 through a transfer gate TX 158. The floating diffusion node 163 is provided as the gate input to a source follower gate SF 160. A row select gate 162 and the source follower gate 160 may be coupled to a reference voltage source (Vdd) node 166. The row select gate 162 may further be coupled to a row select line (e.g., 148 in FIG. 4B) for the pixel 136. The control circuitry for the pixel 136 may additionally or alternatively include one or more other gates. For example, an anti-blooming gate 194 may be in communication with the photodiode 154 to drain excess charge from the photodiode 154—such as when the photodiode 154 is not integrating charge.
In some embodiments, the photodiode 154 and the gates 194, 190, 158, 156, 160, 162 of the pixel 136 may all be positioned on a single semiconductor chip or wafer, whereas in other embodiments, some components of the pixel 136 may be on one semiconductor chip with other components on a second chip. For example, the photodiode 154, the storage gate 190, the storage node 192 may be positioned on one chip, while the floating diffusion node 163, reset gate 156, and so forth positioned on another chip, with the transfer transistor 158 vertically extending between the two chips. As another example, the photodiode 154, along with the storage gate 190, the transfer gate 158, the source follower gate 160, the reset gate 156, and the row select gate 162 may be positioned on one chip, with further readout circuitry positioned on another chip. In general, the components of the pixel 136 may be spread across one or a plurality of chips.
In operation, when one of the cameras 102, 104 is actuated to capture an image, the reference voltage 166 is applied to the reset gate 156, the transfer gate 158, and the storage gate 190 in order to deplete charge from the photodiode 154. 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 need to be reset or depleted before a desired image is to be captured. In other embodiments, an anti-blooming gate 194 may be used for a similar purpose. Once the charge from the photodiode 154 has been depleted, the storage gate 190, the transfer gate 158, and the reset gate 156 may be turned off, isolating the photodiode 154. The photodiode 154 may then begin collecting light transmitted to the image sensor 130 from the lens 126 and integrating charge derived therefrom. As the photodiode 154 receives light, it starts to collect charge generated by the incident photons. The charge remains in the photodiode 154 because the storage gate 190 connecting the photodiode 154 to the storage node 192 is turned off, as is the anti-blooming gate 194.
Once integration is complete and the photodiode 154 has collected light 128 from the lens 126, the reset gate 156 may be turned on to reset the floating diffusion node 163 and/or the transfer gate 158 may be turned on to reset the storage node 192. The storage gate 190 may then be activated and the charge from the photodiode 154 may be transmitted to the storage node 192. The charge from the photodiode 154 may be held at the storage node 192 until the pixel 136 is ready to be read out. In the global shutter operation, each row within the pixel architecture 134 may be reset and exposed (i.e., integrate charge generated by light transmitted through the lens 126) at substantially the same time. Each pixel 136 may 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 embodiments. When the pixel 136 is to be read out row by row, the transfer gate 158 may be activated to transfer the charge from the storage node 192 to the floating diffusion node 163. Once the charge is stored in the floating diffusion node 163, the row select gate 162 may be activated, and the SF gate 160 amplifies the charge in the floating diffusion node 163 and provides a signal indicative thereof through the row select gate 162.
Pixel Structure
With reference now to FIG. 6A, a simplified schematic cross section for one embodiment of a pixel 136 is shown. The pixel 136 includes a photodiode 154 with at least two regions 154a, 154b. The two regions 154a, 154b are coupled together but have different electric potentials. The first region 154a has a first electric potential, and the second region 154b has a second electric potential, with the second electrical potential being greater than the first. As described herein, the electric potential of a region may refer to the potential of the region before and/or after the region is depleted during operation, as described above.
As described in more detail below with reference to FIGS. 12A and 12B, the electric potential of the regions 154a, 154b may be different because the regions 154a, 154b were doped differently during manufacture of the pixel 136, thereby causing more or fewer ionized atoms to be present in the regions 154a, 154b compared to one other. For example, the first region 154a may have been very lightly doped (shown as “n−−” in FIG. 6A), whereas the second region 154b may have been lightly doped (shown as “n−” in FIG. 6A). Alternatively, or additionally, the first region 154a may have been doped with one type of implant, and the second region 154b may have been doped with a different type of implant, for example an implant with a different energy. Alternatively, or additionally, the first and second regions 154a, 154b may have had different thermal treatments applied.
The second region 154b of the photodiode 154 may be offset in depth from the first region 154a of the photodiode 154a within a single semiconductor chip 170, one example of which is illustrated in FIG. 6A. The first region 154a may have a larger cross-section than the second region 154b, and the electric potential of the second region 154b may be higher than the electric potential of the first region 154a. In this manner, as described below, charge that is generated in the first region 154a may be funneled to the second region 154b because of the potential difference between the two regions 154a, 154b. 
The thickness (in depth) of the two regions 154a, 154b of the photodiode 154 may vary among different embodiments. In one embodiment, the first region 154a may be substantially thicker than the second region 154b. In other embodiments, the second region 154b may be thicker than the first region 154a. In one particular embodiment, the thickness of the first region 154a may be determined by the expected depth at which most or all incident light will be converted into electrical charge within the photodiode 154. In other words, in one embodiment, the first region 154a may be engineered so that few, if any, photons are not converted into electron-hole pairs within the first region 154a, and, consequently, so that few, if any, photons proceed to the storage node 192. Alternatively, or in addition to having a thick first region 154a, a shielding layer 182, 183 may be used in some embodiments, as described below.
In some embodiments, the photodiode 154 may include only two regions 154a, 154b, whereas in other embodiments, the photodiode 154 may include more than two regions. For example, the photodiode may include a transition region (not shown in FIG. 6A) between the first and second regions, with the electric potential of the transition region gradually increasing. The transition region may be formed naturally as a result of the doping and implantation manufacturing processes, or may be deliberately formed. In other embodiments, however, the doping concentration, and thus the potential, of the two regions 154a, 154b may be stepped, or may transition abruptly from one concentration/potential to the other concentration/potential.
As illustrated in FIG. 6A, in some embodiments, the photodiode 154 may substantially extend between a first surface 172 and a second surface 174 of a semiconductor chip 170, with the first region 154a of the photodiode 154 positioned proximate the first surface 172 and the second region 154b of the photodiode 154 positioned proximate the second surface 174 of the semiconductor chip 170. The first surface 172 of the semiconductor chip 170 may be the “backside” 172 of the chip 170, and the second surface 174 of the semiconductor chip 170 may be the “frontside” 174 of the chip 170. As described below, this may allow the entire first side 172 of the semiconductor chip 170 for pixel 136 to be used for illumination of the photodiode 154 (specifically, the first region 154a of the photodiode 154), while the second surface 174 of the semiconductor chip 170 can be used for transferring charge to the storage node 192 and to the floating diffusion node 163. In this manner, no storage node or charge transfer circuitry may be present on the first surface 172 of the semiconductor chip 170 such that nearly all of the first surface 172 of the semiconductor chip 170 can be used for photodiodes 154 for various pixels 136, thus increasing the sensitivity of the pixels 136 as compared to prior art pixels with charge storage nodes and charge transfer circuitry on the same surface as the photodiode. In other examples, however, the photodiodes 154 may not fill the entire first surface 172 of the semiconductor chip 170 so that other circuitry or nodes can be present on the first surface 172 of the semiconductor 170.
In some but not all embodiments, the photodiode 154 may be pinned to the first and/or second surfaces 172, 174 of the semiconductor chip 170 (illustrated by the shallow p+ regions on the surfaces of the photodiode 154 in FIG. 6A). Pinning one or both surfaces 172, 174 of the photodiode 154 may reduce dark current, and thus improve signal to noise ratios in some embodiments.
Referring still to FIG. 6A, the pixel 136 also includes a storage node 192, at least a portion of which may be positioned at substantially the same depth in the semiconductor chip 170 as at least a portion of the second region 154b of the photodiode 154, and may be positioned proximate the second surface 174 of semiconductor chip 170. The storage node 192 may be positioned at least partially beneath the first region 154a of the photodiode 154. In some embodiments, the storage node 192 is positioned entirely beneath the first region 154a of the photodiode 154 so that the first region 154a effectively shields the storage node 192 from light incident on the back surface 172 of the semiconductor chip 170. This positioning may allow for high shutter efficiency for the pixel 136.
The storage node 192 may have an electric potential that is higher than the second region 154b of the photodiode 154, so that charge that accumulates in the second region of the photodiode 154b can be transferred to the storage node 192 when the storage gate 190 is activated. The storage node 192 may have a higher potential than the second region 154b of the photodiode 154 because, for example, the storage node 192 was doped with a higher concentration of dopant (shown as “n” in FIG. 6A) during manufacturing as compared with the second region 154b. 
The storage node 192 may in some embodiments be less thick (in depth) than, as deep as (in depth), or more thick (in depth) than the second region 154b of the photodiode 154 in different embodiments. In embodiments where the thickness in depth of the second region 154b of the photodiode 154 is less than the thickness in depth of the storage node 192, charge may be transferred from the second region 154b of the photodiode 154 to the storage node 192 during operation, as described in more detail below. In some embodiments, the storage node 192 may be pinned to one surface 174 of the semiconductor chip 170, for example, the same surface 174 of the semiconductor chip 170 to which the second region 154b of the photodiode 154 is pinned.
One or more storage gates 190 may be operable to transfer charge from the photodiode 154 to the storage node 192. With reference to FIG. 6A, the storage gate 190 may be formed on the second surface 174 of the semiconductor chip 170 and may be coupled between the second region 154b of the photodiode 154 and the storage node 192. The storage gate 190 may thus be configured to form a channel between the photodiode 154 and the storage node 192 responsive to an applied voltage, with charge being transferable over the formed channel. Also, the pixel 136 may include a floating diffusion node 163, and a transfer gate 158 coupling the storage node 192 to the floating diffusion node 163, the operation of which is described above.
In some embodiments, the pixel 136 may include one or more shielded regions 182. With reference to FIG. 6A, one shielded region 182 may be an electrical shield, and may be configured to shield the storage node 192 from charge that is generated in the first region 154a of the photodiode 154. The shielded region 182 may, for example, be a layer that is p-doped.
In some examples, the shielded region 182 may laterally extend the entire width and length of the storage node 192 in order to shield the storage node 192 from light incident from the first region 154a of the photodiode 154 (i.e., that was not converted to electron-hole pairs in the photodiode). In another example, the shielded region may also or alternatively extend in depth between the second region 154b of the photodiode 154 and the storage node 192. Also, in those examples where a floating diffusion node 163 is proximate the second surface 174 of the semiconductor chip 170, the shielded region 182 may extend partially or fully over the floating diffusion node 163.
With reference now to the potential profile 186 illustrated in FIG. 6B, representative of the electric potential of the pixel 136 in FIG. 6A along dotted line 185 in FIG. 6A, the relative electric potentials of the first and second photodiode regions 154a, 154b, the storage node 192, and the floating diffusion node 163 are shown. As illustrated, the electric potential for the second region 154b of the photodiode 154 is greater than the electric potential of the first region 154a. Also, the electric potential of the storage node 192 is greater than the electric potential of the second region 154b of the photodiode 154, and the electric potential of the floating diffusion node 163 is greater than the electric potential of the storage node 192. In this manner, and as described now with reference to FIGS. 7A through 11B, charge generated in the first region 154a of the photodiode 154 from incident photons may be transferred to the floating diffusion node 163 upon the proper signaling of the storage and transfer gates 190, 158.
Pixel Operation
In FIG. 7A and the corresponding potential profile 186 of the pixel 136 in FIG. 7B, the pixel 136 is in integration mode and electron-hole pairs are generated in the first region 154a of the photodiode 154 as a result of incident light. As also illustrated in the corresponding potential profile 186 in FIG. 7B, the charge carriers (electrons here) initially are located in the first region 154a of the photodiode 154. Of course, in some examples, some charge may be generated in the second region 154b of the photodiode as well (e.g., for photons that are not absorbed in the first region 154a. In any event, because of the potential difference between the first and second regions 154a, 154b of the photodiode 154, the charge from the first region 154a is funneled or swept to the second region 154b soon after it is generated, as illustrated by FIGS. 8A and 8B. Because the potential of the second region 154b is greater than the potential of the first region 154a, most of the charge generated in the photodiode 154 will be stored in the second region 154b of the photodiode 154.
After integration is complete, or even during integration in some embodiments, the storage gate 190 may be activated (e.g., by providing a high voltage to its gate terminal), which may cause a channel to form proximate the second surface 174 between the second region 154b of the photodiode 154 and the storage node 192. As illustrated in FIGS. 9A and 9B, activating the storage gate 190 may cause the charge stored in the second region 154b of the photodiode to be transferred through the channel formed by the storage gate 190 to the storage node 192. As discussed above, in global shutter pixels 136, the integration and transfer of generated charge from the photodiode 154 to the storage node 192 may occur at substantially the same time for all pixels 136 in an image sensor.
After the charge has been transferred to the storage node 192, the storage gate 190 is deactivated, and the charge is isolated in the storage node 192 as illustrated in FIGS. 10A and 10B. Because the storage node 192 is positioned at least partially beneath the first region 154a of the photodiode 154, the storage node 192 may be electrically and/or optically shielded from charge or photons from the photodiode 154, so that the charge stored in the storage node 192 across an image sensor can be read out one row and column at a time, without incurring the blurring and noise typically associated with a rolling shutter pixel architecture. As illustrated in FIGS. 11A and 11B, the charge stored in the storage node 192 may be transferred to a floating diffusion node 163 by activating a transfer gate 158. In some examples, correlated double sampling (CDS) may be used because the storage node 192 and/or the floating diffusion node 163 may be sampled both before and after charge is stored therein.
Pixel Manufacturing
With reference now to FIGS. 12A and 12B, one embodiment of manufacturing the pixel 136 illustrated in FIG. 6A will now be described. In FIG. 12A, the second surface 174 (e.g., the front side) of the semiconductor chip 170 is processed by forming the second region 154b of the photodiode 154, the storage node 192, and, in some embodiments, forming the shielding region 182, the floating diffusion node 163, the storage gate 190, and the transfer gate 158. These components of the pixel 136 may be formed using conventional semiconductor device lithographic and implantation processes. In some examples, the second region 154b and the storage node 192 may be formed using the same type of dopant, such as an n-type dopant. The concentration of the dopant used to form the second region 154b, however, may be less than the concentration of the dopant used to form the storage node 192 so that, as described above, when the storage gate 190 is activated, charge will flow from the second region 154b to the storage node 192. In other examples, the second region 154b and the storage node 192 may be formed using different types of dopants, with similar or different concentrations, such that the potential of the storage node 192 is greater than the second region 154b during operation of the pixel 136. In still other examples, voltages or other techniques may be used to increase the potential of the second region 154b relative to the potential of the storage node 192.
After the second surface 174 has been processed, the first surface 172 of the semiconductor chip 170 may be processed by forming the first region 154a of the photodiode 154. The first region 154a of the photodiode 154 may be formed such that it at least partially overlaps with the second region 154b in depth so that charge generated in the first region 154a may be funneled to the second region 154b. Also, in some embodiments, the second surface may be processing using blanket implants, since the second regions 154b of the pixels 136 may substantially define the pixels 136. Using blanket implants may allow one or more lithographic or other semiconductor device processing steps to be skipped when processing the first surface 172, thereby reducing manufacturing and processing costs.
Although the pixel 136 has been described in FIGS. 12A and 12B as being manufactured by processing the second surface 174 before processing the first surface 172, in another embodiment, the first surface 172 is processed before the second surface 174. In still other embodiments, the first or second surfaces 172, 174 may be only partially processed before proceeding to the other, or may be processed at substantially the same time. Also, as illustrated in FIGS. 12A and 12B, one or more portions of the first and second surfaces 172, 174 may be pinned by forming a shallow p+ region in, for example, both surfaces of the photodiode 154 and the sole exposed surface of the storage node 192.
Additional Pixel Structures
With reference to FIG. 13, a pixel 136 is shown that is substantially similar to the pixel 136 shown in FIG. 6A, except that an additional shielding layer 183 may be used. The shielding layer 183 may be an optical shield, and may be used in addition to or in place of the electrical shield 182 described with reference to FIG. 6A. The optical shielding layer 183 may include a light reflective material (e.g., a shiny metal) that reflects any light that is not converted into charge in the first region 154a of the photodiode 154 so that the light does not corrupt the charge stored in the storage node 192. The optical shielding layer 183 may be formed during manufacturing by, for example, depositing the material and epitaxially growing semiconductor material around the optical shielding layer 183, form which the other components of the pixel 136 may subsequently be formed. In those embodiments where an optical shielding layer 183 is used, the first region 154a of the photodiode 154 may be thinner than in those embodiments without an optical shielding layer 183 because there may be less of a chance of light corrupting the charge stored in the storage node 192.
In FIG. 14, another embodiment of a pixel 136 is shown, in which the pixel 136 includes a plurality of storage nodes 192a, 192b. In general, the pixel 136 may include any number of storage nodes (e.g., 1, 2, 3, 4, 5, 6, etc.). In some embodiments, a plurality of storage nodes such as the storage nodes 192a, 192b illustrated in FIG. 14 may be used simultaneously to store charge (e.g., they together may be equivalent to the single storage node 192 illustrated in FIG. 6A), whereas in other embodiments, the plurality of storage nodes may be used sequentially to store charge corresponding to different frames of data. For example, one storage node 192a may be used to store charge from a first frame of the image sensor, and the other storage node 192b may be used to store charge from a second frame of the image sensor. Such sequential use of a plurality of storage nodes 192a, 192b may allow for various image sensor techniques to be used, including for example high-dynamic range (HDR) imaging, pixel sharing, buffering, burst-mode readout, and so forth. As illustrated in FIG. 14, for each storage node 192a, 192b, the pixel 136 may include a corresponding storage gate 190a, 190b, transfer gates 158a, 158b, floating diffusion nodes 163a, 163b, shielding layers 182a, 182b, and so forth. However, in other embodiments (not shown in FIG. 14), multiple storage nodes 192a, 192b may be coupled to and transfer charge to a common floating diffusion node.
In FIG. 15, yet another embodiment of an image sensor pixel 136 is shown. In FIG. 15, the storage gate 190 is coupled to the second region 154b of the photodiode 154 (as opposed to the surface 174 of the semiconductor chip 170 between the second region 154b and the storage node 192, as in FIG. 6A) and may be configured to selectively increase the potential of the second region 154b responsive to an applied voltage. In this configuration, the inherent potential of the second region 154b of the photodiode 154 may or may not be greater than the potential of the first region 154a, but, in any event, the storage gate 190 may pump any charge generated in the first region 154a towards the second surface 174 of the semiconductor chip 170 so that it can be swept into the storage node 192. As illustrated in FIG. 15, the storage node 192 may be bifurcated into two differently doped regions, and may also be pinned, such that a virtual barrier is created for charge until the voltage applied to the storage gate 190 decreases in order to allow the charge to flow into the storage node 192. In this embodiment, the second region 154b of the photodiode 154 may not be pinned to the second surface 174. In still another embodiment (not shown in the figures), and as briefly mentioned above, a charge pumping gate, similar to the gate 190 in FIG. 15, may be used to alter the potential of one or both of the first and/or second regions 154a, 154b of the photodiode 154 illustrated in FIG. 6A so that charge is funneled from the first region 154a to the second region 154b during integration.
Conclusion
The foregoing description has broad application. For example, while examples disclosed herein may focus on particular architectures of image sensors (e.g., photodiode, global shutter, CMOS sensors, etc.), it should be appreciated that the concepts disclosed herein may equally apply to substantially any other type of image sensor with or without appropriate modifications as would be appreciated by one skilled in the art of image sensors. Moreover, although certain examples have been described with reference to particular dopants (e.g., a storage node 192 in FIG. 6A that is doped with n-type material), it will be understood that other dopants are also within the scope of this disclosure and the appended claims. For example, referring back to FIG. 6A, the storage node 192 may be doped with p-type dopant in an n-type substrate.
Furthermore, the various embodiments described herein may find application in many different implementations. For example, although the funneling of charge has been described with reference to two regions 154a, 154b of a photodiode 154, in other embodiments, charge may be transferred between two regions of a storage node, or between a storage node and another type of node using different regions with different potentials.
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.

## Claims

1. An image sensor pixel, comprising:
a photodiode having a first region with a first potential and a second region with a second, higher potential;
a storage node positioned at substantially the same depth as the second region of the photodiode; and
a storage gate operable to selectively transfer charge from the photodiode to the storage node wherein:
the second region is offset in depth from the first region in a semiconductor;
the first region is doped with a first concentration of dopant; and
the second region is doped with a second concentration of dopant that is greater than the first concentration.

2. The image sensor pixel of claim 1, wherein the dopant is n-type.

3. The image sensor pixel of claim 1, wherein the photodiode substantially extends between first and second surfaces of the semiconductor chip and the storage node is positioned proximate the second surface of the semiconductor chip beneath at least a portion of the first region of the photodiode.

4. The image sensor pixel of claim 3, wherein the photodiode is pinned at both the first and second surfaces of the semiconductor chip.

5. The image sensor pixel of claim 1, wherein the storage node is a first storage node, and further comprising a second storage node positioned at substantially the same depth as the second region of the photodiode.

6. The image sensor pixel of claim 1, wherein the thickness in depth of the second region of the photodiode is less than the thickness in depth of the storage node.

7. The image sensor pixel of claim 1, further comprising a shielded region coupled between the first region of the photodiode and the storage node.

8. The image sensor pixel of claim 7, wherein the shielded region is operable to electrically shield the storage node.

9. The image sensor pixel of claim 7, wherein the shielded region is operable to optically shield the storage node.

10. The image sensor pixel of claim 1, wherein the storage gate is coupled to the second region and is configured to selectively increase the potential of the second region in response to an applied voltage.

11. The image sensor pixel of claim 1, wherein the storage gate is coupled between the photodiode and the storage node and is configured to form a channel between the photodiode and the storage node responsive to an applied voltage.

12. The image sensor pixel of claim 1, wherein the photodiode further comprises a transition region between the first and second regions, and a potential of the transition region gradually increases from the first region to the second region.

13. A method of operating an image sensor pixel, comprising:
integrating charge in a first region of a photodiode having a first potential and a first concentration of dopant;
funneling charge from the first region of the photodiode to a second region of the photodiode responsive to a potential difference between the first and second regions, the second region having a second concentration of dopant greater than the first concentration of dopant; and
transferring charge from the second region of the photodiode to a storage node positioned at least partially beneath the first region of the photodiode and at substantially the same depth as the second region.

14. The method of claim 13, wherein charge is funneled to the second region in response to a voltage applied to a storage gate.

15. The method of claim 13, wherein charge is transferred from the second region of the photodiode to the storage node in response to a voltage applied to a storage gate.

16. The method of claim 13, wherein the second region of the photodiode is below the first region.

17. A method of manufacturing an image sensor pixel, comprising:
forming a storage node and a second region of a photodiode on a second surface of a silicon wafer; and
forming a first region of the photodiode on a first surface of the silicon wafer, the first region of the photodiode having a lower concentration of doping than the second region, and the first region of the photodiode at least partially covering the storage node formed on the second surface of the silicon wafer.

18. The method of claim 17, further comprising pinning the second region of the photodiode to the second surface of the silicon wafer and pinning the first region of the photodiode to the first surface of the silicon wafer.

19. The method of claim 17, further comprising forming a plurality of first regions of photodiodes on the first surface of the silicon wafer by blanket implants.