Image Sensor Pixel Structure for Full Well Capacity and Blooming Control
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Solution Overview
Problem
Image sensors face challenges in scaling while maintaining sufficient full well capacity (FWC) and preventing the blooming phenomenon, where electric charges overflow from one pixel to adjacent pixels, due to limited space and potential increases that can lead to inefficiencies in charge transfer.
Innovation Solution
The image sensor design incorporates a substrate with specific conductivity types and impurity concentrations in isolation regions and transfer gates, allowing for efficient charge transfer and reduced blooming by forming distinct potential gradients and channel regions, with the first and second transfer gates having different bottom surface levels and impurity concentrations, and the third element isolation region having a lower potential than the off potentials of the channel regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the image sensor is scaled down to reduce pixel size, then the device complexity and manufacturing cost are reduced, but the full well capacity decreases and blooming phenomenon increases
Solution Approach 1:
The patent applies local quality by creating different impurity concentration zones within the pixel structure. Specifically, the first channel region has a first impurity concentration while the second channel region has a second impurity concentration that is lower than the first. This local differentiation allows each region to be optimized for its specific function: the first channel region for efficient charge transfer and the second channel region for increased charge storage capacity, thereby maintaining full well capacity despite pixel scaling.
Solution Approach 2:
The patent changes physical parameters by varying the impurity concentration in different channel regions. The first channel region maintains a higher impurity concentration for effective charge transfer, while the second channel region uses a lower impurity concentration to increase its charge storage capability. This parameter variation resolves the contradiction between miniaturization and maintaining full well capacity.
2Speed
If the channel region impurity concentration is increased to improve charge transfer efficiency, then the charge transfer speed increases, but the blooming phenomenon worsens due to insufficient potential barriers
Solution Approach 1:
The patent applies local quality by creating distinct impurity concentration zones: the first channel region has a higher impurity concentration optimized for charge transfer efficiency, while the second channel region has a lower impurity concentration that creates sufficient potential barriers to prevent blooming. This spatial differentiation of impurity concentrations allows simultaneous optimization of both charge transfer speed and blooming prevention.
Solution Approach 2:
The element isolation region acts as an intermediary structure with a third impurity concentration that is lower than both channel regions. This intermediate zone creates potential barriers that prevent charge overflow to adjacent pixels (blooming) while allowing efficient charge transfer within the pixel, thus mediating between the conflicting requirements of transfer efficiency and blooming prevention.
3Object-generated harmful factors
If the potential barrier is increased to prevent blooming, then the blooming phenomenon is reduced, but the charge transfer efficiency decreases due to higher potential obstacles
Solution Approach 1:
The patent resolves this contradiction by applying local quality through differentiated impurity concentrations. The first channel region maintains higher impurity concentration for efficient charge transfer, while the element isolation region uses lower impurity concentration to create potential barriers for blooming prevention. This localized optimization allows both functions to coexist without compromising either performance metric.
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 design enhances the full well capacity and mitigates the blooming phenomenon by allowing electric charges to overflow from the first photoelectric conversion unit to the second transfer gate across the third element isolation region, effectively preventing charge transfer to adjacent pixels and improving overall image sensor performance.
Implementation Method 1
forming distinct potential gradients and channel regions
Implementation Method 2
a potential of the third element isolation region is less than the first off potential and less than the second off potential
Data Source
AI summary
An image sensor includes a substrate, a first isolation region defining a unit pixel, a first photoelectric conversion region in the unit pixel, a second photoelectric conversion region in the unit pixel, the second photoelectric conversion region spaced apart from the first photoelectric conversion region, a floating diffusion region, the floating diffusion region adjacent to the first surface of the substrate, a first transfer gate on the first surface of the substrate, the first transfer gate between the first photoelectric conversion region and the floating diffusion region, and a second transfer gate on the first surface of the substrate, the second transfer gate between the second photoelectric conversion region and the floating diffusion region. At least a part of the first transfer gate is buried in the substrate, and a bottom surface of the first transfer gate is different in height from a bottom surface of the second transfer gate.