Image Sensor Deep Well Crosstalk Reduction
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Solution Overview
Problem
MOS image sensors experience crosstalk between pixels due to charge transfer from one photodiode to an adjacent one, leading to image distortion, poor tint, and blooming, especially with long-wavelength light, which affects the resolution and quality of both black and white and color image sensors.
Innovation Solution
An image sensor design featuring a substrate with an active pixel sensor region and a peripheral region, including a first conductivity-type deep well electrically connected to a guard ring and positioned to receive positive voltage, which reduces electrical crosstalk by draining negative charges away from adjacent photodiodes, and a guard ring surrounding the active pixel sensor array to prevent charge transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If photodiodes are arranged closely to increase pixel density, then productivity and resolution are improved, but electrical crosstalk between adjacent pixels increases
Solution Approach 1:
A deep well structure with opposite conductivity type is introduced between adjacent photodiodes of the same conductivity type. This deep well acts as an intermediary barrier that repels minority carriers (electrons in P-type photodiodes, holes in N-type photodiodes) generated by incident light, preventing them from migrating to adjacent pixels and causing electrical crosstalk, while allowing the photodiodes to be closely spaced for high pixel density.
2Reliability
If deep well structures are added to reduce crosstalk, then image quality is improved, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The deep well structure is merged with the existing photodiode formation process. The deep well and photodiodes are created using the same ion implantation and thermal diffusion steps, integrating the crosstalk reduction function into the standard CMOS image sensor fabrication process without requiring separate manufacturing stages, thereby limiting the increase in device complexity.
3Reliability
If deep well structures are added to reduce crosstalk, then manufacturing precision requirements increase
Solution Approach 1:
The deep well is designed with specific parameter ranges: depth between 0.5-2.0 micrometers and doping concentration between 1×10^19 to 1×10^21 atoms/cm³. These parameter specifications provide a tolerance window that accommodates normal manufacturing variations while still achieving effective crosstalk reduction, thereby balancing reliability improvement with manageable manufacturing precision requirements.
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
The solution effectively reduces electrical crosstalk, improving image reproduction characteristics by minimizing image distortion, poor tint, and blooming, while maintaining high sensitivity to incident light.
Implementation Method 1
a first conductivity-type first deep well formed in the active pixel sensor region, in a location which does not include the plurality of first conductivity type photodiodes. The first deep well is electrically connected to a positive voltage
Implementation Method 2
The pixel of the MOS image sensor photoelectrically converts incident light to accumulate charges corresponding to the amount of the light in a photodiode
Data Source
AI summary
An image sensor includes a substrate having an active pixel sensor region defined therein, a plurality of first conductivity type photodiodes formed in the active pixel sensor region and a first conductivity-type first deep well formed in the active pixel sensor region in a location which does not include the plurality of the first conductivity-type photodiodes. Moreover, the first deep well is electrically connected to a positive voltage.