US-12621583-B1 - Image sensor with lateral overflow integrating capacitor (LOFIC) for high dynamic range (HDR) and split pixel for light emitting diode (LED) flicker mitigation

Abstract

A pixel array for CMOS image sensors with split pixel design for producing high dynamic range (HDR) images with LED Flicker Mitigation are disclosed herein. The pixel array is comprised of a plurality of pixel cells, each pixel cell comprising a pixel photodiode region having at least a large photosensitive element (LPD) and a small photosensitive element (SPD), wherein the LPD comprises a lateral overflow integrated capacitor (LOFIC). A method of generating an HDR image includes: exposing a large photosensitive element (LPD) of a pixel cell for a first duration of time, exposing a small photosensitive element (SPD) of the pixel cell for a second duration of time, and combining the resulting readouts into a combined pixel readout (CPR). This CPR is then utilized in combination with a generated LED Flicker Map (LFM) bit, and the resulting readout from the SPD to produce a final corrected pixel readout that mitigates LED flicker in the resulting HDR image.

Inventors

• Ramakrishna Kakarala
• Young Jun Song
• Zhongyang Huang

Assignees

• OMNIVISION TECHNOLOGIES, INC.

Dates

Publication Date

20260505

Application Date

20241106

Claims (19)

1. 1 . A pixel array for a CMOS image sensor, comprising: a plurality of pixel cells formed in a semiconductor substrate, each pixel cell comprising: a pixel photodiode region having at least a large photosensitive element (LPD) and a small photosensitive element (SPD), wherein a size of the LPD is greater than a size of the SPD, and wherein the LPD comprises a lateral overflow integrated capacitor (LOFIC); and a pixel transistor region disposed adjacent to the pixel photodiode region; wherein the LPD is exposed for a first duration of time during each exposure and the SPD is exposed for a second duration of time during each exposure; wherein the first duration of time and the second duration of time are at least partially concurrent; wherein for each exposure: the LPD generates high-conversion gain (HCG), low-conversion gain (LCG) and LOFIC readouts; and the SPD generates a short exposure(S) readout; and wherein the HCG, LCG, LOFIC, and S readouts are combined into a combined pixel readout (CPR) based on determining a first absolute difference between the HCG readout and the S readout, or determining a second absolute difference between the LCG readout and the S readout, or determining a third absolute difference between the LOFIC readout and the S readout.
2. 2 . The pixel array of claim 1 , wherein the first duration of time is less than the second duration of time, and wherein the LPD and SPD begin their respective exposures simultaneously.
3. 3 . The pixel array of claim 2 , wherein the first duration of time is less than 5 ms and the second duration of time is greater than or equal to 11 ms.
4. 4 . The pixel array of claim 1 , wherein the first duration of time is greater than the second duration of time, and wherein the LPD begins its exposure before the SPD begins its exposure.
5. 5 . The pixel array of claim 4 , wherein the first duration of time is greater than or equal to 11 ms and the second duration of time is less than 5 ms.
6. 6 . The pixel array of claim 1 , wherein the pixel cells are arrayed in a split diode tiling arrangement, wherein the arrangement comprises: a plurality of LPDs; and a plurality of SPDs, wherein the plurality of LPDs and the plurality of SPDs are each arranged into rows and columns, wherein the plurality of LPDs are configured adjacent to one another, and wherein the plurality of SPDs is embedded between adjacent LPDs such that the LPDs and SPDs tessellate to share common boundaries with each other.
7. 7 . The pixel array of claim 6 , wherein the plurality of LPDs and the plurality of SPDs each include two green photodiodes, one blue photodiode, and one red photodiode; and wherein the plurality of LPDs and the plurality of SPDs alternate colors by their respective rows and columns.
8. 8 . The pixel array of claim 1 , wherein the CPR is a high dynamic range (HDR) image.
9. 9 . The pixel array of claim 1 , wherein the HCG, LCG, LOFIC, and S readouts are combined into a combined pixel readout (CPR) by following steps: shifting the S readout; assigning a weight for each absolute difference determined; selecting the maximum weight (W) computed for each absolute difference; generating a flicker pixel map; and outputting a corrected pixel readout (C) based upon the CPR, flicker pixel map, and the SPD readout.
10. 10 . A method of generating a high dynamic range (HDR) image, comprising: exposing a large photosensitive element (LPD) of a pixel cell for a first duration of time, wherein the LPD comprises a lateral overflow integrated capacitor (LOFIC), and wherein the LPD generates high-conversion gain (HCG), low-conversion gain (LCG) and LOFIC readouts; exposing a small photosensitive element (SPD) of the pixel cell for a second duration of time, wherein the LPD is larger than the SPD, and wherein the SPD generates a short exposure(S) readout, and wherein the first duration of time and the second duration of time are at least partially concurrent; and combining the HCG, LCG, LOFIC, and S readouts into a combined pixel readout (CPR) based on determining a first absolute difference between the HCG readout and the S readout, or determining a second absolute difference between the LCG readout and the S readout, or determining a third absolute difference between the LOFIC readout and the S readout.
11. 11 . The method of claim 10 , further comprising: shifting the S readout; assigning a weight for each absolute difference determined; selecting the maximum weight (W) computed for each absolute difference; generating a flicker pixel map; and outputting a corrected pixel readout (C) based upon the CPR, flicker pixel map, and the SPD readout.
12. 12 . The method of claim 11 , wherein the corrected pixel readout (C) is calculated by the equation C=(CPR*W)+S*(1−W).
13. 13 . The method of claim 10 , wherein the first duration of time is less than the second duration of time, and wherein the LPD and SPD begin their respective exposures simultaneously.
14. 14 . The method of claim 13 , wherein the first duration of time is less than 5 ms and the second duration of time is greater than or equal to 11 ms.
15. 15 . The method of claim 10 , wherein the first duration of time is greater than the second duration of time, and wherein the LPD begins its exposure before the SPD begins its exposure.
16. 16 . The method of claim 15 , wherein the first duration of time is greater than or equal to 11 ms and the second duration of time is less than 5 ms.
17. 17 . The method of claim 10 , wherein the pixel cells are arrayed in a split diode tiling arrangement, wherein the arrangement comprises: a plurality of LPDs; and a plurality of SPDs, wherein the plurality of LPDs and the plurality of SPDs are each arranged into rows and columns, wherein the plurality of LPDs are configured adjacent to one another, and wherein the plurality of SPDs is embedded between adjacent LPDs such that the LPDs and SPDs tessellate to share common boundaries with each other.
18. 18 . The method of claim 17 , wherein the plurality of LPDs and the plurality of SPDs each include two green photodiodes, one blue photodiode, and one red photodiode; and wherein the plurality of LPDs and the plurality of SPDs alternate colors by their respective rows and columns.
19. 19 . The method of claim 10 , wherein the CPR is a high dynamic range (HDR) image.

Description

BACKGROUND INFORMATION Field of the Disclosure This disclosure relates generally to image sensors, and in particular but not exclusively, relates to image sensors, such as high dynamic range (HDR) image sensors, that mitigate the effects of light emitting diode (LED) flicker in images. Background CMOS image sensors (CIS) have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as medical, automobile, and other applications. The typical image sensor operates in response to image light reflected from an external scene being incident upon the image sensor. The image sensor includes an array of pixels having photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light and generate image charge upon absorption of the image light. The image charge of each of the pixels may be measured as an output voltage of each photosensitive element that varies as a function of the incident image light. In other words, the amount of image charge generated is proportional to the intensity of the image light, which is utilized to produce a digital image (i.e., image data) representing the external scene. The typical image sensor operates as follows. Image light from an external scene is incident on the image sensor. The image sensor includes a plurality of photosensitive elements such that each photosensitive element absorbs a portion of incident image light. Photosensitive elements included in the image sensor, such as photodiodes, each generate image charge upon absorption of the image light. The amount of image charge generated is proportional to the intensity of the image light. The generated image charge may be used to produce an image representing the external scene. Integrated circuit (IC) technologies for image sensors are constantly being improved, especially with the constant demand for higher resolution and lower power consumption. Such improvements frequently involve scaling down device geometries to achieve lower fabrication costs, higher device integration density, higher speeds, and better performance. But as the miniaturization of image sensors progresses, defects within the image sensor architecture become more readily apparent and may reduce the image quality of the image. For example, excess current leakage within certain regions of the image sensor may cause high dark current, sensor noise, white pixel defects, and the like. These defects may significantly deteriorate the image quality from the image sensor, which may result in reduced yield and higher production costs. High dynamic range (HDR) image sensors may present other challenges. For example, some HDR image sensor layouts are not space efficient and are difficult to miniaturize to a smaller pitch to achieve higher resolutions. Accordingly, systems and methods for improved HDR are still needed. BRIEF DESCRIPTION OF THE DRAWINGS Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. FIG. 1 is a block diagram illustrating an example image sensor in accordance with an embodiment of the present technology. FIG. 2 is an illustrative schematic of one example of a pixel cell in accordance with an embodiment of the present disclosure. FIGS. 3A-3B are examples of a pixel array and its associated timing diagram, respectively, in accordance with an embodiment of the present disclosure. FIGS. 4A-4C are embodiments of pixel arrays in accordance with the present disclosure. FIG. 5A is a timing diagram in accordance with an embodiment of the present disclosure. FIG. 5B is a graph of a Signal-to-Noise Ratio (SNR) related to the timing diagram of FIG. 5A. FIG. 5C is a timing diagram in accordance with an embodiment of the present disclosure. FIG. 5D is a graph of a Signal-to-Noise Ratio (SNR) related to the timing diagram of FIG. 5C. FIG. 6 is a schematic of signal processing in accordance with an embodiment of the present disclosure. Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present disclosure. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present disclosure. DETAILED DESCRIPTION Examples of an apparatus and method for producing HDR images with CMOS image sensors using lateral overflow integrating capacitors (LOFIC) and LED flicker mitigation are d