Vertical Transfer Gate Image Sensor Pixel Structure
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Solution Overview
Problem
Increasing the size of the N well in monolithic image sensors to enhance sensitivity and reduce size while maintaining charge transfer efficiency and avoiding increased sensor size is challenging.
Innovation Solution
The design incorporates a first and second charge transfer gate with column electrodes surrounded by insulating layers in isolation trenches between pixels, allowing for efficient charge transfer and collection, with a common isolation trench and charge collection nodes to optimize pixel area usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the N well size is increased to enhance sensitivity and reduce sensor size, then sensitivity improves and sensor size reduces, but charge transfer efficiency deteriorates and device complexity increases
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: photodiode region for charge generation, transfer gate region for charge transfer, and isolation trench for electrical separation. This segmentation allows the N well to be optimized for sensitivity while the transfer gate structure handles charge transfer efficiently, resolving the contradiction between sensitivity and transfer efficiency.
Solution Approach 2:
Different regions of the pixel are given different doping concentrations and structural properties. The photodiode region has optimized doping for sensitivity, while the transfer gate region has specific doping profiles for efficient charge transfer. This local optimization allows each region to perform its function at peak efficiency without compromising the other.
2Area of moving object
If the N well size is increased to maximize photodiode area, then charge collection area increases, but transfer transistor area increases and overall sensor size increases
Solution Approach 1:
The transfer gate is positioned vertically within the isolation trench structure, utilizing the third dimension (depth) rather than expanding horizontally. This vertical configuration allows the photodiode area to be maximized in the horizontal plane while the transfer gate operates in the vertical dimension, preventing increase in overall sensor footprint.
Solution Approach 2:
The transfer gate structure is nested within the isolation trench, which itself is integrated into the pixel structure. This nesting allows the transfer gate to occupy space within the existing pixel footprint rather than adding to it, enabling larger photodiode area without increasing overall sensor size.
3Area of stationary object
If the N well size is increased to reduce sensor size, then integration density improves, but charge transfer efficiency deteriorates
Solution Approach 1:
The transfer gate acts as an intermediary structure between the photodiode region and the readout circuitry. It provides a dedicated charge transfer pathway that is optimized for efficiency, allowing compact pixel design without compromising transfer performance. The isolation trench further mediates between adjacent pixels, preventing charge leakage while maintaining efficient transfer within each pixel.
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 increases the sensitivity of the image sensor and reduces its size by maximizing the photodiode area for charge collection while minimizing the transfer transistor area, resulting in improved performance and efficiency.
Implementation Method 1
a first charge transfer gate comprising a first column electrode surrounded by an insulating layer and positioned in an opening of the isolation trench between the first and second pixels, the first column electrode being configured to receive a first transfer voltage signal
Implementation Method 2
A charge accumulated by the photodiode during an integration period can be transferred to the sense node
Data Source
AI summary
An image sensor including a first pixel positioned between second and third pixels, each of the first, second and third pixels comprising a photodiode region surrounded by an isolation trench; a first charge transfer gate comprising a first column electrode surrounded by an insulating layer and positioned in an opening of the isolation trench between the first and second pixels, the first column electrode being configured to receive a first transfer voltage signal; and a second charge transfer gate including a second column electrode surrounded by an insulating layer and positioned in an opening of the isolation trench between the first and third pixels, the second column electrode being configured to receive a second transfer voltage signal.