Stacked Photodiode Pixel Structure for Low-Noise Charge Transfer
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Solution Overview
Problem
The reduction in pixel size of photoelectric conversion devices leads to a decrease in saturation signal amount and signal-to-noise ratio due to increased dark current at the interface between the photodiode and dielectric, making it difficult for the transfer gate to effectively manage potential differences across the photodiode, resulting in reduced signal transfer efficiency.
Innovation Solution
A photoelectric conversion device design featuring a first photodiode with a region for signal charge accumulation on the main surface, a second photodiode in the bulk semiconductor layer overlapping the first, and a transfer mechanism using a third region of opposite conductivity type to control potential and enhance signal transfer, thereby increasing saturation signal amount and suppressing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel size is decreased to improve resolution, then the number of pixels increases, but saturation signal amount decreases and dark current increases
Solution Approach 1:
The invention transitions from a two-dimensional surface arrangement to a three-dimensional bulk structure by arranging photodiodes in multiple layers within the semiconductor substrate. This vertical stacking enables increased pixel density without proportionally reducing the active area of each photodiode, thereby maintaining saturation signal amount while improving resolution.
Solution Approach 2:
The semiconductor substrate is divided into multiple distinct photodiode regions (first photodiode region, second photodiode region) separated by isolation regions. This segmentation allows each photodiode to maintain adequate signal accumulation capacity while fitting more pixels into the overall pixel array, addressing both resolution and signal amount requirements.
2Measurement precision
If pixel size is decreased, then resolution improves, but dark current at the interface between photodiode and dielectric increases
Solution Approach 1:
The invention extracts and removes the problematic interface between photodiode and dielectric by eliminating the dielectric layer entirely. Instead, insulating films are formed only where needed for electrical isolation, removing the source of dark current generation at photodiode-dielectric interfaces while maintaining necessary electrical separation.
Solution Approach 2:
The invention converts the potential harm of reduced isolation distance into a benefit by using the closer proximity to improve charge collection efficiency. The reduced distance between photodiode and collection electrodes enhances the electric field strength, improving signal charge transfer while the selective formation of insulating films prevents dark current generation.
3Ease of operation
If transfer gate is arranged on the main surface to correspond to the end portion of the bulk photodiode, then signal transfer is enabled, but the electric field generated is insufficient to overcome the potential difference
Solution Approach 1:
The invention introduces an intermediary potential control mechanism by forming insulating films with specific potentials between the transfer gate and the photodiode regions. This intermediary potential distribution creates additional electric field components that augment the field generated by the transfer gate alone, enabling effective signal charge transfer across the potential barrier.
Solution Approach 2:
The invention changes the potential parameter distribution within the pixel structure by forming insulating films with controlled potentials in different regions. This parameter modification creates favorable potential gradients that enhance the electric field strength in the bulk photodiode regions, enabling effective signal transfer without requiring the transfer gate to generate sufficient field alone.
4Ease of operation
If the potential dip between end portion and center portion of the bulk photodiode is reduced to enable signal transfer, then transfer efficiency improves, but saturation signal amount decreases
Solution Approach 1:
The invention applies local quality differentiation by forming insulating films with specific potentials only in certain regions (first insulating film in the first photodiode region, second insulating film in the second photodiode region) rather than uniformly throughout. This localized potential control maintains adequate potential dips for signal accumulation in bulk photodiodes while enabling efficient transfer at specific locations where insulating films are positioned.
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 saturation signal amount and reduces noise, achieving a higher signal-to-noise ratio and dynamic range compared to conventional buried photodiode structures, while maintaining sensitivity and minimizing dark current noise.
Implementation Method 1
a first photodiode including a first region of a first conductivity type where signal charges are accumulated; a second photodiode arranged between the first region and the second main surface so as to at least partially overlap the first region in an orthogonal projection with respect to the first main surface, and including a second region of the first conductivity type where signal charges are accumulated
Implementation Method 2
signal charges accumulated in the second region are transferred to the first region via a transfer portion formed in the third region by controlling a potential of the first electrode
Data Source
AI summary
A photoelectric conversion device including pixels arranged in a semiconductor layer including first and second main surfaces is provided. Each of the pixels includes a first photodiode including a first region of a first conductivity type where signal charges are accumulated, a second photodiode arranged between the first region and the second main surface so as to at least partially overlap the first region, and including a second region of the first conductivity type where signal charges are accumulated, a third region of a second conductivity type arranged between the first and second region, and a first electrode arranged on the first main surface so as to cover the first region. Signal charges accumulated in the second region are transferred to the first region via a transfer portion formed in the third region by controlling a potential of the first electrode.