Centrally Symmetric Vertical Transfer Gate for Faster Pixel Charge Transfer
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Solution Overview
Problem
Image sensors face challenges with incomplete charge transfer from photodiodes, leading to image lag and deterioration in readout information, especially as pixel size increases, and require improved charge transfer speed for indirect time-of-flight applications.
Innovation Solution
The integration of a vertically symmetric transfer gate with a photodiode region having different doping concentrations and wells, where the vertical transfer gate is centrally symmetric to the photodiode region, enhances charge transfer efficiency by reducing diffusion distance and improving uniformity of charge transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the pixel size is increased, then the photodiode area is increased, but the charge transfer speed becomes insufficient
Solution Approach 1:
The patent introduces a vertical transfer gate that extends in the vertical dimension (depth direction) to create a three-dimensional charge transfer path. This vertical dimension allows charges to be transferred more efficiently from the photodiode to the floating diffusion, solving the speed limitation that occurs when simply increasing the horizontal photodiode area.
2Device complexity
If a conventional transfer gate is used, then the structure is simple, but charge transfer is incomplete causing image lag
Solution Approach 1:
The vertical transfer gate extends in the depth direction (vertical dimension) to create a three-dimensional charge transfer path. This vertical dimension allows charges to be transferred more efficiently from the photodiode to the floating diffusion, solving the speed limitation that occurs when simply increasing the horizontal photodiode area.
Solution Approach 2:
The patent implements non-uniform doping concentrations within the vertical transfer gate structure, with different doping levels at different depths. This local variation in doping quality optimizes the electric field distribution specifically in the charge transfer region, enhancing transfer efficiency without affecting other parts of the device.
3Area of stationary object
If the photodiode size is increased, then the light capture area is increased, but the charge transfer uniformity deteriorates
Solution Approach 1:
The vertical transfer gate extends in the depth direction (vertical dimension) to create a three-dimensional charge transfer path. This vertical dimension allows charges to be transferred more efficiently from the photodiode to the floating diffusion, solving the speed limitation that occurs when simply increasing the horizontal photodiode area.
Solution Approach 2:
The patent employs asymmetric doping concentrations within the vertical transfer gate, with higher doping near the photodiode interface and lower doping toward the floating diffusion. This asymmetric doping profile creates an optimized electric field distribution that ensures uniform charge transfer across the entire photodiode area, even when the photodiode is large.
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 enhances carrier transfers, reduces image lag, and improves time resolution and frame rates in image sensors by ensuring more uniform and efficient charge transfer across the pixel area.
Implementation Method 1
the photodiode region is configured to accumulate charge photogenerated in the photodiode region in response to incoming light
Implementation Method 2
the vertical transfer gate is configured to transfer the photogenerated charge from the photodiode region to a planar transfer gate
Data Source
AI summary
This application describes systems and methods related to vertical transfer gates. An example system includes a photodiode region disposed in a substrate, wherein: the photodiode region is configured to accumulate charge photogenerated in the photodiode region in response to incoming light, the photodiode region comprises a top surface and a bottom surface, the top surface being smaller than the bottom surface, the photodiode region comprises at least two doping concentrations, and a first doping concentration of the two doping concentrations that is closer to the top surface is higher than a second doping concentration of the two doping concentrations that is closer to the bottom surface; and a vertical transfer gate in the substrate, wherein: the vertical transfer gate is above the top surface of the photodiode region and is centrally symmetric to the top surface of the photodiode region, and the vertical transfer gate is configured to transfer the photogenerated charge from the photodiode region to a transfer gate.