Split Pixel Cell Light Attenuation Layer for LED Flicker Reduction
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Solution Overview
Problem
High dynamic range (HDR) image sensors face challenges such as LED flickering, reduced image quality due to defects like high dark current and crosstalk, and difficulties in miniaturizing pixel size for higher resolution, which affect image quality and production costs.
Innovation Solution
The implementation of a split pixel cell design with a light attenuation layer over the small photodiode, formed prior to the metal optical isolation grid structure, to reduce photodiode sensitivity and prevent saturation, allowing for longer integration times and improved dynamic range, while maintaining sensitivity for low light intensity sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If HDR image sensor uses large full well capacity structures to accommodate large dynamic ranges, then dynamic range is improved, but lag, white pixels, and dark current increase
Solution Approach 1:
The pixel is divided into two separate photodiodes: a first photodiode with large full well capacity for capturing high intensity light, and a second photodiode with small full well capacity for capturing low intensity light. This segmentation allows each photodiode to operate in its optimal range, preventing saturation and reducing harmful effects like white pixels and lag.
Solution Approach 2:
Different regions of the pixel (first and second photodiodes) are given different properties - the first photodiode has larger area and higher full well capacity for bright light, while the second photodiode has smaller area and lower full well capacity for dim light. This local differentiation optimizes performance across the entire dynamic range.
2Productivity
If pixel size is reduced to achieve higher resolution, then productivity and device integration density are improved, but crosstalk and diagonal flare increase
Solution Approach 1:
The pixel is segmented into multiple photodiodes with specialized functions, allowing for more efficient space utilization and reduced interference between adjacent pixels, thereby minimizing crosstalk and diagonal flare effects.
3Device complexity
If conventional image sensor uses single photodiode design, then device complexity is low, but LED flickering cannot be reduced and dynamic range is limited
Solution Approach 1:
The image sensor employs a split pixel design with multiple photodiodes having different full well capacities, enabling the sensor to capture both bright and dim light simultaneously, thereby reducing LED flickering artifacts and expanding dynamic range.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces LED flicker and enhances image sensor dynamic range by desensitizing the small photodiode, enabling longer integration times and preventing saturation, thereby improving image quality and resolution without degrading subsequent lithography processes.
Implementation Method 1
a light attenuation layer over the small photodiode... to reduce photodiode sensitivity and prevent saturation
Implementation Method 2
Photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light and generate image charge upon absorption of the image light
Data Source
AI summary
A pixel array for a high definition (HD) image sensor is disclosed. The pixel array includes a number of split pixel cells each including a first photodiode and a second photodiode that is more sensitive to incident light than the first photodiode. The first photodiode can be used to sense bright or high intensity light conditions, while the second photodiode can be used to sense low to medium intensity light conditions. In the disclosed pixel array, the sensitivity of one or more photodiodes is reduced by application of a light attenuation layer over the first photodiode of each split pixel cell. In accordance with methods of the disclosure, the light attenuation layer can be formed prior to the formation of a metal, optical isolation grid structure. This can lead to better control of the thickness and uniformity of light attenuation layer.