BSI Image Sensor Overflow Drains Reduce Blooming
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Solution Overview
Problem
Backside-illuminated image sensors face challenges with blooming due to the lack of a highly-doped bulk region to recombine extra photoelectrons, limited space for anti-blooming features in small pixel cells, and insufficient vertical space for overflow drains in the silicon substrate.
Innovation Solution
Incorporating overflow drains at the front side of the silicon substrate, which can be grounded or slightly positive biased, and using a doped bridge to act as an overflow drain, allowing excess charge carriers to be diverted from neighboring pixels, thereby reducing blooming and enabling electrical shutter functionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If BSI sensor structure is used to improve fill factor and quantum efficiency, then optical performance is improved, but blooming issues worsen due to lack of highly-doped bulk region
Solution Approach 1:
The patent introduces overflow drains extending vertically from the front surface through the substrate to the back surface, utilizing the third dimension (depth) to provide blooming control. This vertical drainage structure allows excess photoelectrons to be collected along the depth axis rather than relying on lateral bulk recombination regions, thereby resolving the blooming issue inherent in thin-substrate BSI sensors while preserving optical performance.
2Productivity
If pixel size is reduced to 1.75 micron and below to enable scaling, then sensor scalability is improved, but space for anti-blooming features is lost
Solution Approach 1:
By transitioning from lateral anti-blooming structures to vertical overflow drains that extend through the substrate depth, the invention provides effective blooming control within the constrained lateral footprint of sub-1.75 micron pixels. The vertical dimension offers sufficient space for drain structures without compromising lateral pixel dimensions, enabling continued sensor scaling.
Solution Approach 2:
The overflow drains are strategically positioned at specific locations within each pixel (such as at corners or edges), providing localized blooming control where it is most needed. This selective placement allows effective anti-blooming functionality without occupying excessive lateral space, maintaining pixel scalability while addressing blooming in critical regions.
3Measurement precision
If substrate thickness is reduced to 2 microns to enable BSI operation, then optical performance is improved, but vertical space for overflow drains is insufficient
Solution Approach 1:
The overflow drains are implemented as highly-doped regions with concentrated charge carrier collection capability, providing effective blooming control within the limited 2-micron substrate thickness. By optimizing the doping profile and drain geometry locally, the invention achieves sufficient electron collection efficiency despite the reduced vertical space, maintaining both optical performance and blooming control.
Solution Approach 2:
The invention modifies the electrical parameters (doping concentration, potential well depth) and geometric parameters (drain dimensions, spacing) of the overflow drains to maximize their effectiveness within the constrained 2-micron substrate. By tuning these parameters, sufficient electron collection capability is achieved despite the reduced vertical dimension, resolving the contradiction between thin substrate and adequate drain functionality.
4Object-affected harmful factors
If overflow drains are added to reduce blooming, then blooming control is improved, but pixel performance may be compromised
Solution Approach 1:
The overflow drains are positioned at specific locations within the pixel (such as corners or edges) rather than occupying the central photosensitive region. This localized placement ensures that blooming control is provided where excess electrons tend to accumulate, while the main pixel area remains optimized for photon detection, preserving pixel performance.
Solution Approach 2:
The overflow drains act as intermediary structures that provide a controlled pathway for excess photoelectrons to be collected and removed from the pixel. By providing this dedicated electron collection channel, the drains prevent electron spillover into adjacent pixels (blooming) while maintaining proper charge collection within the pixel, thus improving blooming control without compromising pixel performance.
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
Significantly reduces blooming between neighboring pixels without compromising pixel performance and allows for the use of electrical shutters instead of mechanical ones, improving image accuracy and enabling cost-effective shutter implementation in various devices.
Implementation Method 1
Incorporating overflow drains at the front side of the silicon substrate, which can be grounded or slightly positive biased, and using a doped bridge to act as an overflow drain, allowing excess charge carriers to be diverted from neighboring pixels
Implementation Method 2
The optical elements include at least an array of individual pixels to capture light incident on the image sensor
Data Source
AI summary
Embodiments of a pixel including a photosensitive region formed in a surface of a substrate and an overflow drain formed in the surface of the substrate at a distance from the photosensitive area, an electrical bias of the overflow drain being variable and controllable. Embodiments of a pixel including a photosensitive region formed in a surface of a substrate, a source-follower transistor coupled to the photosensitive region, the source-follower transistor including a drain, and a doped bridge coupling the photosensitive region to the drain of the source-follower transistor.