Shared Floating Diffusion Pixel Group for CMOS Image Sensor Sensitivity
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Solution Overview
Problem
CMOS image sensors face challenges in maintaining image quality and sensitivity due to the interference between highly integrated pixels, leading to noise and reduced light sensing, especially as pixel sizes decrease, and they need to perform additional functions beyond image capture.
Innovation Solution
A pixel group with a shared common floating diffusion region and symmetric vertical transfer gates, along with trench structures extending through the semiconductor substrate, is implemented to enhance sensitivity and reduce cross-talk between pixels, allowing for independent driving of vertical transfer gates for improved dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixel size is decreased to increase resolution, then the number of pixels increases, but sensing sensitivity deteriorates and noise increases due to interference between highly integrated elements
Solution Approach 1:
The pixel array is divided into multiple pixel groups, where each group shares a common floating diffusion region. This segmentation allows independent optimization of each pixel group while reducing interference between adjacent pixels, thereby maintaining sensing sensitivity even as overall pixel density increases.
Solution Approach 2:
Multiple pixels within each pixel group share a common floating diffusion region, merging their signal processing functions. This reduces the total number of floating diffusion regions needed, decreases interference between highly integrated elements, and maintains sensing sensitivity by consolidating readout paths.
2Productivity
If pixel size is decreased to increase resolution, then more pixels fit in the sensor area, but noise increases due to interference between highly integrated elements
Solution Approach 1:
The sensor is segmented into multiple pixel groups with shared floating diffusion regions, creating isolated processing zones that prevent interference noise from propagating across the entire sensor array, thereby reducing overall noise levels despite high pixel density.
Solution Approach 2:
By merging the floating diffusion regions of multiple pixels into a shared common region within each pixel group, the design reduces the total number of interfaces between pixels, thereby minimizing the sources of interference noise while maintaining high integration density.
3Adaptability or versatility
If additional functions are added beyond image capture, then functionality increases, but device complexity increases
Solution Approach 1:
The pixel group structure with shared floating diffusion regions and symmetric vertical transfer gates is designed to perform multiple functions including image capture, high dynamic range imaging, and other advanced functions. This universal design reduces the need for separate specialized structures for each function, thereby increasing versatility without proportionally increasing complexity.
4Reliability
If symmetric vertical transfer gates are used to enhance sensitivity, then sensing sensitivity improves, but device complexity increases
Solution Approach 1:
While the transfer gates within each pixel are symmetric, the overall pixel group structure uses asymmetric arrangements of shared floating diffusion regions and transfer gate pairs. This allows optimization of sensitivity through symmetry where needed while reducing overall structural complexity through asymmetric sharing across the pixel group.
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 configuration enhances image quality by increasing sensing sensitivity, reducing noise, and enabling efficient high dynamic range imaging by minimizing interference and optimizing light collection.
Implementation Method 1
A pixel array included in a CMOS image sensor may include a photoelectric conversion element such as a photodiode in each pixel. The photoelectric conversion element generates an electrical signal that varies based on the quantity of incident light.
Data Source
AI summary
A pixel group of an image sensor includes first through fourth unit pixels in a matrix form of two pixels rows and two pixel columns, and a common floating diffusion region in a semiconductor substrate at a center of the pixel group and shared by the first through fourth unit pixels. Each of the first through fourth unit pixels includes a photoelectric conversion element in the semiconductor substrate, and a pair of vertical transfer gates in the semiconductor substrate and extending in a vertical direction perpendicular to a surface of the semiconductor substrate. The pair of vertical transfer gates transfer photo charges collected by the photoelectric conversion element to the common floating diffusion region. Image quality is enhanced by increasing sensing sensitivity of the unit pixel through the shared structure of the floating diffusion region and the symmetric structure of the vertical transfer gates.