Image Sensor Pixel Dark Current Calibration via Subtraction Circuit
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Solution Overview
Problem
Conventional image sensors face challenges in accurately calibrating dark current due to non-uniform temperature distribution and process defects across the chip, leading to inaccuracies and lack of real-time calibration performance when using fixed dark pixel arrays for calibration.
Innovation Solution
An image sensor with a pixel array structure that includes both photosensitive and dark shielding portions, isolated by deep trench isolation and covered with a metal dark shielding layer, allows for real-time dark current calibration through a subtraction circuit that generates light and non-light ambient voltage signals for precise calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If dark pixel arrays are used at fixed positions on chip edge for calibration, then calibration structure is simple, but calibration accuracy deteriorates due to non-uniform temperature distribution and process defects across the chip
Solution Approach 1:
The pixel array is divided into photosensitive pixels and dark pixels, with dark pixels distributed across multiple regions including chip edges and interior areas. This segmentation allows calibration at multiple temperature zones simultaneously, improving accuracy without excessive complexity
Solution Approach 2:
Different regions of the pixel array are assigned different functions: dark pixels at chip edges for edge region calibration and dark pixels in interior regions for central area calibration. This local differentiation addresses the non-uniform temperature distribution across the chip, ensuring accurate calibration for each region's specific thermal characteristics
2Device complexity
If conventional line-by-line readout method is used, then readout process is simple, but real-time calibration performance deteriorates because each pixel experiences different readout environment
Solution Approach 1:
The readout circuits for photosensitive pixels and dark pixels are merged into a unified readout system. Both pixel types share common readout pathways and control logic, enabling simultaneous calibration and imaging operations without requiring separate complex readout mechanisms
Solution Approach 2:
The system implements periodic calibration cycles where dark pixel readings are interleaved with photosensitive pixel readings. This periodic action ensures that calibration data is continuously updated under the same temporal and environmental conditions as imaging data, achieving real-time calibration performance
3Ease of manufacture
If dark shielding layer above third layer metal is used, then metal interconnection limitation is satisfied, but light leakage occurs causing dark current potential to be higher than real dark environment
Solution Approach 1:
Multiple shielding layers are nested within the pixel structure: the dark pixel's photodiode is surrounded by a first dark shielding layer, which is in turn surrounded by a second dark shielding layer formed by the substrate. This nested configuration creates effective light blocking without requiring access to upper metal layers, maintaining both manufacturability and measurement accuracy
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 approach enables direct, accurate, and real-time dark current and noise calibration within each pixel, reducing calibration errors and improving the accuracy of dark current measurements by ensuring uniform dark current and noise levels across the chip.
Implementation Method 1
the surface of the pixel dark shielding portion is covered with a dark shielding layer for shielding external incident light
Implementation Method 2
An image sensor is a detector that senses light intensity and outputs an intensity map
Data Source
AI summary
The present invention discloses an image sensor for real-time calibration of dark current, including a pixel array comprises at least a pixel unit, the pixel unit includes a pixel photosensitive portion, a pixel dark shielding portion and a subtraction circuit, photodiodes in the pixel photosensitive portion and the pixel dark shielding portion are isolated by deep trench isolations, the pixel dark shielding portion are covered by a dark shielding layer; both of the pixel photosensitive portion and the pixel dark shielding portion adopt a same voltage and sequential control, a light ambient voltage signal and a non-light ambient voltage signal are generated and connected to both ends of a subtraction circuit to realize subtraction and dark current calibration. The present invention discloses an image sensor for real-time calibration of dark current, which can make the dark current calibration completed directly within the pixel, and can better cover the dark pixel part, so as to make calibration value of dark current and dark noise more accurate.