Rolling Double Reset Timing for Image Sensor Global Storage
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Solution Overview
Problem
Global storage imagers face issues with increased dark current and leakage charge due to surface generation and process-induced damage, leading to degraded image quality, especially in low light conditions with reduced frame rates.
Innovation Solution
Implementing a rolling double reset timing method that minimizes the time between resets of the storage node and maintains a constant reset time across rows, reducing dark current and leakage charge by sweeping residue charge before charge transfer from the storage node to the floating diffusion region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If global storage is implemented to enable rolling shutter operation, then the imager can perform continuous capture and readout, but dark current and leakage charge accumulate in the storage node between resets, degrading image quality
Solution Approach 1:
A rolling shutter reset is performed on the storage node before the global transfer of charge from photosensors to storage nodes. This preliminary reset action clears residue charge and minimizes dark current accumulation before new charge is transferred, thereby preventing degradation of image quality while maintaining continuous capture capability
Solution Approach 2:
The storage node undergoes periodic resetting through the rolling shutter reset mechanism at predetermined intervals before each global transfer operation. This periodic action ensures that dark current and leakage charge are continuously cleared, maintaining image quality during continuous capture operation
2Object-affected harmful factors
If the storage node is reset globally before charge transfer, then dark current accumulation is reduced, but the reset time varies across rows, causing non-uniform shutter efficiency
Solution Approach 1:
The global reset operation is segmented into row-by-row rolling shutter resets. Each row is reset individually in sequence, ensuring that the reset time is synchronized and uniform across all rows. This segmentation eliminates variations in shutter efficiency while still achieving dark current reduction through the preliminary reset action
3Object-affected harmful factors
If rolling shutter reset is implemented, then dark current is reduced, but the complexity of timing control increases
Solution Approach 1:
The rolling shutter reset is implemented as a periodic timing sequence that automatically repeats for each frame. The controller executes the same predetermined timing pattern for each row reset and global transfer operation, which simplifies control logic despite the multiple operations. The periodic nature allows for standardized timing control rather than requiring complex adaptive timing for each operation
Data Source
AI summary
An apparatus for and a method of operating an array of pixels of an image sensor, where each pixel includes at least a photosensor, an associated storage device and a floating diffusion region and the array of pixels is configured in a plurality of rows and columns. The photosensors associated with the pixels are reset and charges are accumulated in the photosensor. The accumulated charges are then globally transferred to storage devices associated with the pixels. A rolling double reset is used to reduce the deleterious effects on the accumulated charges stored in the storage devices. The accumulated charges stored in the storage devices are transferred to floating diffusion regions associated with the pixels and the charges residing in the floating diffusion region are read out. In a second embodiment the storage device is eliminated and the rolling double reset is used to reduce the deleterious effects on the accumulated charges stored in the floating diffusion region.