Global shutter correction

A pixel circuit includes a photodiode disposed in a semiconductor material to accumulate image charge in response to incident light directed into the photodiode, and a transfer transistor coupled to the photodiode. The circuit also includes a noise correction circuit coupled to receive a transfer control signal and the noise correction circuit is coupled to selectively enable or disable the transfer transistor from receiving the transfer control signal. A storage transistor is coupled to the transfer transistor, and the transfer transistor is coupled to selectively transfer the image charge accumulated in the photodiode to the storage transistor for storage in response to the transfer control signal if the transfer transistor is enabled to receive the transfer control signal.

TECHNICAL FIELD

This disclosure relates generally to image sensor operation and in particular but not exclusively, relates to global shutters.

BACKGROUND INFORMATION

The process of shuttering consists of exposing an image sensor to light at a rate equal to (or faster than) a frame rate. The goal of this process is to reduce blurring effects from motion within an image frame.

There are several different types of shuttering including rolling shuttering and global shuttering. A rolling shutter exists where a line of pixels, or a group of several lines of pixels, is read out while other lines in the image sensor are exposed to image light. Readout times for rolling shutter image sensors vary depending on frame rate and architecture, but can be as high as several hundred microseconds. Accordingly, due to the delay between reading each line of pixels, moving image subjects can cause optical distortion and blur within the image.

A global shutter, unlike a rolling shutter, exposes all photodiodes in the image sensor at the same time. This results in little or no image blur because there is no delay between integration of individual pixel lines during image acquisition. However, global shutters generally require an additional pixel storage element which allows the pixels to store previously acquired image charge to be read out while the next image frame is captured. This additional storage element generally enlarges the footprint of individual pixels on a wafer, and can be a source of problems such as light absorption, cross talk, etc.

DETAILED DESCRIPTION

Throughout this specification, several terms of art are used. These terms are to take on their ordinary meaning in the art from which they come, unless specifically defined herein or the context of their use would clearly suggest otherwise. It is worth noting that specific elements of circuitry may be substituted for logically equivalent or analogous circuitry.

FIG. 1is an illustration of pixel circuit101. In the depicted example, pixel circuit101includes: photodiode103, transfer transistor105, noise correction circuit111, storage transistor107, output transistor109, global shutter transistor121, amplifier transistor115, reset transistor113, and row select transistor117. Photodiode103is disposed in semiconductor material104to accumulate image charge in response to incident light directed into photodiode103. Transfer transistor105is coupled to photodiode103, and noise correction circuit111is coupled to the control terminal of transfer transistor105. Global shutter transistor121is also coupled to photodiode103. Transfer transistor105is coupled to storage transistor107, and storage transistor107is coupled to output transistor109. The control terminal of amplifier transistor115is coupled to an output of output transistor109, and reset transistor113is also coupled to the output of output transistor109as well as the control terminal of amplifier transistor115. In one example, amplifier transistor115includes a source follower coupled transistor. Row select transistor117is coupled between an output of amplifier transistor115and a bit line output of pixel circuit101. In one example, a floating diffusion may be disposed between the second terminal of output transistor109and the control terminal of amplifier transistor115. In another or the same example, the floating diffusion is disposed in semiconductor material104.

In the depicted example, optical shield119is disposed proximate to storage transistor107in order to shield storage transistor107from incident light. In one example, optical shield119includes a metal, such as copper or aluminum. However in another example, optical shield119may include a metal oxide or semiconductor oxide. Optical shield119prevents formation of unwanted image charge in storage transistor107since, in one example, the active region of storage transistor107may be disposed in semiconductor material104.

Although not depicted inFIG. 1, in one or more examples, other pieces of device architecture may be present in/on pixel circuit101. For example, transistors (in addition to storage transistor107) in pixel circuit101may be disposed proximate to an optical shield to protect them from incident light. Further, other layers of device architecture may be formed on semiconductor material104such as encapsulation layers, color filters, and microlenses. In one example, a color filter layer and a microlens layer are disposed proximate to semiconductor material104such that they are optically aligned with photodiode103. In one example, the color filter layer includes red, green, and blue color filters which may be arranged into a Bayer pattern, EXR pattern, X-trans pattern, or the like. However, in a different or the same example, the color filter layer may include infrared filters, ultraviolet filters, or other light filters that isolate invisible portions of the EM spectrum. In the same or a different example, a microlens layer is formed on the color filter layer. The microlens layer may be fabricated from a photo-active polymer that is patterned on the surface of the color filter layer. Once rectangular blocks of polymer are patterned on the surface of the color filter layer, the blocks may be melted (or reflowed) to form the dome-like structure characteristic of microlenses. Additionally, in one example, pixel circuit101may be entirely disposed, in/on semiconductor material104and the internal components of pixel circuit100may be surrounded by electrical and/or optical isolation structures. This may help to reduce noise in pixel circuit101. Electrical isolation may be accomplished by etching isolation trenches in semiconductor material104which may then be filled with semiconductor material, oxide material, or the like. Alternatively, optical isolation structures may be formed by constructing a reflective grid on the surface of semiconductor material104disposed beneath the color filter layer.

In operation, noise correction circuit111is coupled to receive a transfer control signal (TX), and is also coupled to selectively enable or disable transfer transistor105from receiving the transfer control signal. In one example, noise correction circuit111may be implemented as an AND gate, with one input coupled to receive the transfer signal and the other input coupled to receive an enable signal. The AND gate may take the form of a NAND gate coupled to an inverter. Transfer transistor105is coupled to selectively transfer image charge accumulated in photodiode103to storage transistor107for storage in response to the transfer control signal, if transfer transistor105is enabled (in response to the enable signal) to receive the transfer control signal. Output transistor109may be coupled to selectively output an image charge signal responsive to the image charge stored in storage transistor107provided transfer transistor105is enabled to receive the transfer control signal from noise correction circuit111. Output transistor109may also be coupled to selectively output a parasitic signal (noise) responsive to a photoelectric charge (noise) accumulated in storage transistor107provided transfer transistor105is disabled (in response to the enable signal) from receiving the transfer control signal from noise correction circuit111. In other words, output transistor109will output a noise signal when transfer transistor105is disabled from receiving the transfer control signal. It should be noted that the noise charge stored in storage transistor107is representative of light induced noise charge accumulated within storage transistor107. Despite the presence of optical shields, ambient light may leak into storage transistor107. Since pixel circuit101selectively outputs both an image charge signal and a noise signal, the noise signal can be removed from the final image in real time. Accordingly, final image quality and shutter efficiency can be enhanced.

FIG. 2Ais a block diagram illustrating one example of an imaging system including the pixel circuit (e.g., pixel circuit101) ofFIG. 1. Imaging system200includes pixel array205, control circuitry221, readout circuitry211, and function logic215. In one example, pixel array205is a two-dimensional (2D) array of photodiodes, or image sensor pixels (e.g., pixels P1, P2. . . , Pn). As illustrated, photodiodes are arranged into rows (e.g., rows R1to Ry) and columns (e.g., column C1to Cx) to acquire image data of a person, place, object, etc., which can then be used to render a 2D image of the person, place, object, etc.

In one example, after each image sensor photodiode/pixel in pixel array205has acquired its image data or image charge, the image data is readout by readout circuitry211and then transferred to function logic215. Readout circuitry211may be coupled to readout image data from the plurality of photodiodes in pixel array205. In various examples, readout circuitry211may include amplification circuitry, analog-to-digital (ADC) conversion circuitry, or otherwise. Function logic215may simply store the image data or even manipulate the image data by applying post image effects (e.g., crop, rotate, remove red eye, adjust brightness, adjust contrast, or otherwise). In the depicted example, function logic215is coupled to the readout circuitry211, and function logic215is coupled to cancel noise from the image acquired from pixel array205in response to the noise signals readout from pixel array205. In one example, readout circuitry211may readout a row of image data at a time along readout column lines (illustrated) or may readout the image data using a variety of other techniques (not illustrated), such as a serial readout or a full parallel readout of all pixels simultaneously.

In one example, control circuitry221is coupled to pixel array205to control operation of the plurality of photodiodes in pixel array205. For example, control circuitry221may generate a shutter signal for controlling image acquisition. In the depicted example, the shutter signal is a global shutter signal for simultaneously enabling all pixels within pixel array205to simultaneously capture their respective image data during a single acquisition window. In another example, image acquisition is synchronized with lighting effects such as a flash. In the depicted example, control circuitry221is coupled to set noise correction circuits included in a first portion of the plurality of pixel circuits (i.e., circuits associated with pixels P1, P2. . . , Pn) to output image data of an image, and control circuitry221is coupled to set noise correction circuits included in a second portion of the plurality of pixel circuits (i.e., circuits associated with pixels P1, P2. . . , Pn) to output noise data concurrently with the first portion of the plurality of pixel circuits outputting image data of the image.

In one example, imaging system200may be included in a digital camera, cell phone, laptop computer, or the like. Additionally, imaging system200may be coupled to other pieces of hardware such as a processor, memory elements, output (USB port, wireless transmitter, HDMI port, etc.), lighting/flash, electrical input (keyboard, touch display, track pad, mouse, microphone, etc.), and/or display. Other pieces of hardware may deliver instructions to imaging system200, extract image data from imaging system200, manipulate image data supplied by imaging system200, or reset image data in imaging system200.

FIG. 2Bis an example illustration of the pixel array205inFIG. 2A. In the depicted example, the pixel circuit ofFIG. 1(e.g., pixel circuit101) is one of a plurality of pixel circuits arranged in pixel array205and disposed in a semiconductor material (e.g., semiconductor material104). Noise correction circuits are included in a first portion of the plurality of pixel circuits202(shown as circles) and are set to output image data of the image. Noise correction circuits included in a second portion of the plurality of pixel circuits201(shown as stars) are set to output noise data concurrently with the first portion of the plurality of pixel circuits202outputting image data of the image. In the depicted example, second portion of the plurality of pixel circuits201are interspersed among the first portion202of the plurality of pixel circuits in the pixel array. In one example, the first202and second201portions of the plurality of pixel circuits are coupled to be dynamically selected within the pixel array for each image acquisition of pixel array205. In other words, the pixels that output image data of the image and the pixels that output noise data may change. In one example, the location of pixels that output image data and the location of pixels that output noise data may change as a function of time, in response to light/image conditions, in response to user input (e.g., in a photography, cell phone, or automotive application), or randomly. However, in another example, the pixels that output image data of the image and the pixels that output noise data may be static (i.e., remain the same for every exposure period).

FIG. 3shows an example image correction process using the pixel circuit ofFIG. 1. In the depicted example, image301is collected by photodiodes in a pixel array (e.g., pixel array205) via a global shuttering process. Thus all photodiodes in the pixel array are capturing light (to generate image charge) at the same time. During the image acquisition process, storage transistors (e.g., storage transistor107) may collect unwanted ambient light/noise charge that—when read out of storage transistors at the same time as the image charge collected in the photodiodes—degrades the quality of the final image. Accordingly, in the depicted example, some of the pixels in the pixel array will read out image charge from their respective photodiodes (along with the noise charge generated in their respective storage transistors) to form image301, while other pixels will only read out the noise charge generated in their respective storage transistors to form storage transistor stored image303. Both image301and storage transistor stored image303are sent to image correction logic305. Image correction logic305may correct image301by subtracting the noise charge signal (i.e., storage transistor stored image303) from image301. Thus, image correction logic305produces corrected image307, and may output corrected image307to a display, memory, etc.

In one example, the pixels that only read out noise charge from their storage transistors achieve this feat by having a noise correction circuit coupled to the control terminal of their transfer transistors. In these pixels, the transfer transistor is disposed between the photodiode and the storage transistor. When the noise correction circuit disables the transfer transistor, charge from the photodiode is unable to be transferred to the storage transistor. As a result, image301is formed from the pixels that read out image data from their photodiodes, and storage transistor stored image303is formed from pixels that read out data from their storage transistors. In one example, the pixels in the array that form image301and the pixels in the array that form storage transistor stored image303may change between subsequent image acquisition windows. Depending on use case, a user of the imaging system (in a camera, phone, automobile, etc.) may choose how many pixels read out charge from their photodiodes and how many pixels read out charge solely from their storage transistors. This may allow the user to alter image quality based on personal preference. Additionally, the image sensor may select the number and spatial configuration of pixels that read out image charge from their photodiodes vs. those that only read out charge from their storage transistors. This number may change depending on lighting conditions, image subject location, calibration measurements, etc.