Back Side Illumination Image Sensor Extended Transfer Gate
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in precisely controlling the overlap between implants and transfer gates, leading to variations in performance across columns, wafers, and lots, and existing solutions either increase leakage current or degrade quantum efficiency.
Innovation Solution
A non-self aligned back side illumination (BSI) CMOS image sensor with an extended transfer gate is developed, which increases the overlap between the n-type implant and the transfer gate, reducing surface dark current and coupling capacitance, while maintaining optical fill factor and enhancing red response through reflection layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the n-type implant edge is extended into the transfer gate area toward the floating diffusion, then the second overlap between the n-type implant and the transfer gate becomes sufficiently large, but the effective transfer gate length becomes shorter, thereby increasing leakage current or even resulting in punch through
Solution Approach 1:
The patent transitions from front side illumination to back side illumination architecture, fundamentally changing the spatial arrangement of components. This dimensional reconfiguration allows the transfer gate to extend over the photodiode region without compromising the photodiode's light-receiving area, as the light enters from the opposite side. The extended transfer gate can now provide sufficient overlap with the n-type implant for stable charge transfer while maintaining adequate effective length to prevent punch-through and leakage current issues.
2Manufacturing precision
If the edge of the transfer gate is extended toward the photodiode, then the second overlap between the n-type implant and the transfer gate becomes sufficiently large, but the effective area for receiving incoming light becomes smaller, thereby degrading quantum efficiency
Solution Approach 1:
By switching to back side illumination, the patent enables the transfer gate to be extended in the lateral direction to achieve sufficient overlap with the n-type implant without encroaching on the photodiode's light-receiving area. The light enters through the back side of the substrate, allowing the photodiode's top surface to be fully utilized for charge transfer operations while maintaining maximum optical fill factor and quantum efficiency.
3Area of moving object
If the pixel size is reduced to meet high resolution requirements, then the form factor is reduced, but the second overlap becomes tiny and is easily affected by other implants, thereby complicating the process optimization
Solution Approach 1:
The back side illumination architecture provides additional spatial freedom that allows the transfer gate to achieve sufficient overlap with the n-type implant even in reduced-size pixels. The extended transfer gate design, enabled by the BSI structure, ensures stable second overlap parameters across varying pixel dimensions, reducing sensitivity to implant variations and simplifying process optimization for high-resolution applications.
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 extended transfer gate design improves charge transfer, reduces overlap variation, and boosts red response, resulting in consistent performance and increased conversion gain, especially in advanced 3D CIS structures.
Implementation Method 1
BSI red response may be boosted by forming reflection layers by the extended transfer gate poly on top of PD
Data Source
AI summary
A process and structure of a back side illumination (BSI) image sensor are disclosed. An n-type doped region is formed in a substrate, and a transfer gate is formed on top of the semiconductor substrate. A p-type doped region is formed in the n-type doped region either using the transfer gate as a mask or is non-self aligned formed.