Dummy Pixel Guard Band for Image Sensor Edge Effect Reduction
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Solution Overview
Problem
Image sensors face challenges in achieving uniform dark current and edge effect reduction, particularly in peripheral pixels, which affect image quality due to variations in diode ideality and resistance-area product, and the need for dummy pixels that mimic clear and optical black pixels without perturbing electrical characteristics.
Innovation Solution
The formation of dummy pixels with identical electrical characteristics to clear and optical black pixels, using a separate Optical Black layer and soft reset mode for the reset transistor, along with guard band regions to isolate sub-arrays and prevent edge effects, ensures minimal impact on existing pixel performance and maximizes image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dummy pixels are added to reduce edge effects, then image quality is improved, but device complexity increases
Solution Approach 1:
The pixel array is segmented into distinct regions: clear pixels for imaging, optical black pixels for reference, and dummy pixels for edge effect reduction. This segmentation allows each region to serve its specific function while maintaining overall system performance.
Solution Approach 2:
Different pixel types are assigned to different locations within the array. Dummy pixels are strategically placed at edges and corners where edge effects are most pronounced, while clear pixels occupy the central imaging region. This local differentiation optimizes performance for each spatial zone.
2Reliability
If optical black pixels are used as reference pixels, then black level stability is improved, but manufacturing precision requirements increase
Solution Approach 1:
All pixels including optical black pixels are formed using the same fabrication process steps, ensuring uniform electrical characteristics across the array. The optical black layer is applied uniformly to specific pixel regions, maintaining consistency in material properties and reducing manufacturing variability.
3Reliability
If dummy pixels are formed with identical electrical characteristics to clear pixels, then edge effect reduction is improved, but process complexity increases
Solution Approach 1:
The same fabrication process and pixel structure serve multiple functions: clear pixels for imaging, optical black pixels for reference measurements, and dummy pixels for edge effect compensation. This universal approach simplifies manufacturing by using identical processes for all pixel types rather than requiring separate specialized processes.
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 effectively minimizes edge effects and maintains uniformity in key photodetector characteristics, enhancing image quality by ensuring identical Shockley currents and reduced dark current generation, thus optimizing imaging sensor performance across various applications.
Implementation Method 1
The total current in clear, dummy and an OB pixel is the sum of each diode's Shockley-Read current, photocurrent and the electrical bias current
Data Source
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AI summary
A method and apparatus for forming dummy pixels exhibiting electrical characteristics virtually identical to the clear pixels of the imaging array. Arrays of such dummy pixels are used to form regions that isolate the main imaging array and sub-arrays of optical black pixels while preventing edge effects. The dummy pixels are preferably clear but can also be covered with optical black. By setting quiescent operation in soft reset, the dummy pixels exhibit the diode ideality and R0A product that are typical of any of the pixels in the entire array.