Vertical Overflow Drain and Transfer Transistor for Image Sensor Crosstalk
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Solution Overview
Problem
Electrical crosstalk remains a significant issue in image sensor technologies, affecting the performance and accuracy of image sensors by causing unwanted charge flow between pixels.
Innovation Solution
The integration of a vertical overflow drain combined with a vertical transfer transistor in a backside illuminated image sensor, along with a buried photodiode, helps to reduce crosstalk and improve performance metrics such as blooming and global reset, while also managing image charge saturation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional image sensor architectures are used, then device complexity is reduced, but electrical crosstalk between pixels increases
Solution Approach 1:
The pixel structure is segmented into distinct functional regions including a photodiode region, a first overflow drain region, a second overflow drain region, and a transfer transistor region. This segmentation isolates charge generation and overflow paths, preventing electrical crosstalk between adjacent pixels while maintaining manageable device complexity through modular design
Solution Approach 2:
Transfer transistors are introduced as intermediary elements between the photodiode and readout circuitry. These transistors control and isolate charge transfer, acting as mediators that prevent direct electrical crosstalk between pixels while enabling controlled charge movement when needed for reset or overflow management
2Measurement precision
If photodiode size is increased to improve light sensitivity, then image quality improves, but blooming effect increases
Solution Approach 1:
Overflow drain regions are positioned adjacent to the photodiode to provide preliminary charge diversion paths before blooming can occur. When the photodiode approaches saturation, excess charge is preemptively directed to the overflow drains, preventing the blooming effect while allowing the photodiode to maintain a larger active area for improved light sensitivity
Solution Approach 2:
The overflow drain regions are strategically positioned at specific locations around the photodiode perimeter where charge overflow is most likely to occur. This localized approach provides targeted anti-blooming protection at critical points while maintaining the overall photodiode area for light sensitivity, rather than uniformly reducing the photodiode size
3Object-affected harmful factors
If vertical overflow drain is added to reduce crosstalk and blooming, then harmful factors are reduced, but device complexity increases
Solution Approach 1:
The overflow drain functionality is merged with the existing pixel structure by integrating drain regions directly into the photodiode substrate and using the same charge collection mechanisms. This combining approach adds anti-blooming and anti-crosstalk functionality without requiring completely separate additional components, thereby reducing the net increase in device complexity
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 effectively minimizes electrical crosstalk, reduces blooming, and enhances global reset capabilities, allowing for improved image quality and reduced pulse time in image sensors.
Implementation Method 1
each photosensitive element absorbs a portion of incident image light. Photosensitive elements included in the image sensor, such as photodiodes, each generate image charge upon absorption of the image light
Data Source
AI summary
An image sensor pixel includes a photodiode disposed in a semiconductor material to generate image charge in response to light incident on a backside of the semiconductor material, and a pinning layer disposed in the semiconducting material and coupled to the photodiode. The pixel also includes a vertical overflow drain disposed in the semiconductor material and coupled to the pinning layer such that the pinning layer is disposed between the vertical overflow drain and the photodiode. A floating diffusion disposed in the semiconductor material proximate to the photodiode, and a vertical transfer transistor is disposed in part in the semiconductor material and coupled to the photodiode to transfer the image charge from the photodiode to the floating diffusion in response to a transfer signal applied to the gate terminal of the vertical transfer transistor.