Vertical Trench Transfer Gate for Image Sensor Fill Factor
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Solution Overview
Problem
Conventional image sensor pixels face challenges in efficiently capturing incident light due to reduced pixel sizes, leading to image lag and issues like fixed pattern noise and color shading, primarily caused by the overlap-induced limitations in transfer gate design and asymmetry in photodiode regions.
Innovation Solution
The implementation of a vertical trench transfer gate design that increases the fill factor and symmetry of pixels, allowing for more efficient electron transfer and reduced image lag by using a vertical field effect device as the transfer gate between the photodiode and floating drain, with a shared floating diffusion and trench isolation to enhance light collection and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If conventional transfer gate design is used, then pixel size can be reduced, but light capture efficiency deteriorates and image lag increases
Solution Approach 1:
The transfer gate is reconfigured from a conventional planar structure to a vertical trench structure that extends downward into the substrate. This dimensional change allows the transfer gate to be positioned below the photodiode region, eliminating overlap issues and enabling independent optimization of photodiode area for light capture while maintaining effective transfer gate functionality for charge transfer.
2Length of moving object
If conventional transfer gate design is used, then pixel size can be reduced, but image quality deteriorates due to fixed pattern noise and color shading
Solution Approach 1:
The transfer gate function is extracted from the planar region above the photodiode and relocated to a vertical trench structure. This separation removes the source of asymmetry and overlap-induced variations that cause fixed pattern noise and color shading, allowing the photodiode region to maintain uniform symmetry across all pixels.
Solution Approach 2:
The vertical trench transfer gate design creates a symmetric photodiode region by removing the asymmetric overlap with the transfer gate. All pixels in the array can now have identical photodiode geometries, eliminating the asymmetry-induced variations that manifest as fixed pattern noise and color shading artifacts.
3Device complexity
If overlap-induced transfer gate design is used, then device complexity is reduced, but fill factor decreases
Solution Approach 1:
The transfer gate is moved from the horizontal plane to a vertical dimension, extending as a trench into the substrate below the photodiode. This allows the photodiode to occupy the full planar area for light capture while the transfer gate operates in the vertical dimension, effectively eliminating the trade-off between fill factor and transfer gate functionality.
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 design enhances the fill factor and symmetry of pixels, improving light collection efficiency, reducing image lag, and minimizing fixed pattern noise and color shading, thereby improving the quality of electronic images captured by image sensors.
Implementation Method 1
capture incident light that illuminates the sensing array
Implementation Method 2
using a vertical field effect device as the transfer gate between the photodiode and floating drain
Data Source
AI summary
An image sensor provides high scalability and reduced image lag. The sensor includes a first imaging pixel that has a first photodiode region formed in a substrate of the image sensor. The sensor also includes a first vertical transfer transistor coupled to the first photodiode region. The first vertical transfer transistor can be used to establish an active channel. The active channel typically extends along the length of the first vertical transfer transistor and couples the first photodiode region to a floating diffusion.