Stacked Pixel Structure With Isolated Floating Diffusion for HDR
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Solution Overview
Problem
Existing CMOS image sensors face challenges in achieving high resolution with high dynamic range due to limitations in pixel density and conversion gain, particularly from increased capacitance from wiring connections to secondary dies.
Innovation Solution
A two- or three-device layer system is implemented, where the floating diffusion node is isolated from wiring to the second die in high conversion gain mode, with DCG transistor and source follower on the first die, and remaining photodetector circuit components on the second die, allowing for high pixel density and dynamic range through shared floating diffusion regions and reduced capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photodetector circuit components are integrated on a second die to increase pixel density, then resolution is improved, but capacitance from wiring connections increases and reduces conversion gain
Solution Approach 1:
The photodetector circuit is segmented into two parts: critical components (photosensitive area, transfer gate, floating diffusion node, source follower, and DCG transistor) remain on the first die, while non-critical components (row select transistor, reset transistor) are moved to the second die. This segmentation minimizes wiring connections and associated capacitance, preserving conversion gain while still achieving increased pixel density through the stacked architecture.
Solution Approach 2:
The DCG transistor and source follower are extracted and kept on the first die, isolated from the second die wiring. This extraction eliminates the capacitance burden that would result from connecting these high-impedance nodes to the second die, thereby maintaining high conversion gain in high conversion gain mode while allowing the second die to be used for increasing pixel density.
2Adaptability or versatility
If floating diffusion node is connected to second die wiring, then circuit functionality is improved, but noise increases and dynamic range is reduced
Solution Approach 1:
The system dynamically switches between two operational modes: high conversion gain mode for low-light conditions where the floating diffusion node remains isolated from second die wiring to minimize noise and maximize dynamic range, and low conversion gain mode for bright-light conditions where the node can be connected to second die components. This dynamic adaptation allows the system to optimize performance for different lighting conditions while maintaining full circuit functionality.
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 structure enables a large difference in conversion gain modes, increasing dynamic range and reducing noise, while maintaining high pixel density and resolution.
Implementation Method 1
A photodetector includes a photodiode in a first die
Data Source
AI summary
An image sensor achieves high pixel density, and therefore high resolution, by offloading portions of a photodetector circuit to a separate device layer from the photodiodes. The photodetector uses a lateral overflow integration capacitor and a dual conversion gain transistor to increase dynamic range. The dynamic range is further increased by providing a high conversion gain mode in which the floating diffusion node is isolated from the second device layer and from the wiring that extends to the second device layer. This is accomplished by disposing the DCG transistor and the source follower in the first device layer which has the photodiodes, the transfer gates, and the floating diffusion regions. Isolating the floating diffusion node from the wiring to the second device layer in the high conversion gain mode reduces the capacitance of the floating diffusion node in the high conversion gain mode, and so increases the dynamic range.