CMOS Pixel Peninsula Transfer Gate Reduces Image Lag
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Solution Overview
Problem
As pixel cell size decreases, the full well capacity (FWC) of CMOS image sensors decreases due to smaller photodiode implants, leading to potential loss of electron transfer efficiency and increased image lag, especially with the encroachment of isolation p regions and channel stops affecting the transfer gate's efficiency.
Innovation Solution
The implementation of a peninsula-shaped transfer gate in CMOS pixels, which extends beyond the typical transfer gate to form a 'T' shape, effectively reducing the potential barrier between the photodiode and the floating drain, thereby enhancing electron transfer and reducing image lag by increasing the physical distance between the transfer gate and shallow trench isolation (STI) structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixel cell size is decreased to increase resolution, then the number of pixels per unit area increases, but the full well capacity decreases due to smaller photodiode implants
Solution Approach 1:
The transfer gate is extended in a specific dimensional direction to form a peninsula shape that protrudes into the photosensitive region. This dimensional change allows the transfer gate to reach closer to the center of the photodiode, effectively increasing the collection area and full well capacity without increasing the overall pixel cell footprint.
2Productivity
If photodiode size is decreased to fit more pixels, then pixel density increases, but electron transfer efficiency decreases due to encroachment of isolation structures
Solution Approach 1:
The transfer gate is designed with an asymmetric peninsula shape that extends preferentially in one direction toward the photosensitive region rather than symmetrically in all directions. This asymmetric configuration allows the transfer gate to effectively reach the photodiode center while maintaining proper isolation from adjacent pixels in other directions.
Solution Approach 2:
The peninsula-shaped transfer gate acts as an intermediary structure that bridges the gap between the isolation structures and the photosensitive region. By extending into the photosensitive region, it mediates the conflict between isolation requirements and electron collection efficiency, providing a pathway for electrons to be transferred without being blocked by isolation structures.
3Quantity of substance
If n-doping concentration is increased to compensate for smaller photodiode size, then full well capacity increases, but image lag increases
Solution Approach 1:
Instead of increasing doping concentration to increase full well capacity, the invention changes the geometric dimension of the transfer gate by extending it into the photosensitive region. This dimensional change increases the effective collection area and full well capacity without altering the doping concentration, thereby avoiding the harmful side effect of increased image lag.
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 maintains or increases the full well capacity and reduces image lag by minimizing the potential barrier, ensuring more electrons are transferred during integration periods and preventing carry-over charge that affects subsequent images.
Implementation Method 1
a photodiode (i.e., a photo-sensitive element responsible for collecting electromagnetic energy and converting the collected electromagnetic energy into electrons)
Data Source
AI summary
A pinned photodiode structure with peninsula-shaped transfer gate which decrease the occurrence of a potential barrier between the photodiode and the floating drain, prevents loss of full well capacity (FWC) and decreases occurrences of image lag.