Symmetric Floating Sensing Nodes for Image Sensor Sensitivity Uniformity
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Solution Overview
Problem
CMOS image sensors face challenges in maintaining consistent sensitivity across different chip ray angles, leading to variations in color detection, particularly for long-wavelength light, due to the arrangement of floating sensing nodes relative to the photodiode.
Innovation Solution
The implementation of symmetrically arranged floating sensing nodes adjacent to the photodiode, with a photo gate applying a negative voltage during photocharge transfer, enhances sensitivity uniformity and transfer efficiency by isolating the pixel from adjacent conductivity patterns and optimizing light incidence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If floating sensing nodes are arranged asymmetrically relative to the photodiode, then device complexity is reduced, but sensitivity uniformity deteriorates causing color detection variations
Solution Approach 1:
The patent applies asymmetry in reverse - it deliberately creates symmetric arrangement of floating sensing nodes around the photodiode to counteract the asymmetric sensitivity variations caused by chip ray angle. This symmetric configuration ensures that sensitivity differences in particular colors are compensated, resolving the measurement precision issue while maintaining manageable device complexity.
2Productivity
If photo gate is positioned in the light path, then photocharge transfer efficiency is improved, but light absorption by the photo gate increases reducing overall sensitivity
Solution Approach 1:
The patent changes the electrical parameter of the photo gate by applying a negative voltage during photocharge transfer. This parameter modification enhances the photocharge transfer efficiency to the floating sensing nodes while the gate remains transparent to light, thus improving productivity without compromising reliability.
3Measurement precision
If multiple sensing nodes are added to compensate sensitivity differences, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent segments the sensing function by separating the photodiode from multiple floating sensing nodes. This segmentation allows the photodiode to convert light to photocharge while multiple sensing nodes independently detect the photocharge, enabling sensitivity compensation through their symmetric arrangement without excessive complexity in the overall structure.
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 configuration reduces sensitivity differences for long-wavelength light detection, improves photocharge transfer efficiency, and enhances the lag characteristic of the image sensor, ensuring consistent performance across varying pixel positions.
Implementation Method 1
a photodiode configured to convert an optical signal into photocharge
Implementation Method 2
a photo gate configured to apply a negative voltage to the photodiode when the photocharge is transferred from the photodiode to the at least two sensing nodes
Data Source
AI summary
A pixel and pixel array for use in an image sensor are provided. The image sensor includes floating sensing nodes symmetrically arranged with respect to a photodiode in each pixel.