Back-illuminated CMOS Image Sensor Interface Passivation
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Solution Overview
Problem
Back-illuminated image sensors face challenges with high dark current and reduced quantum efficiency due to interface issues between the sensor layer and insulating layer, particularly in the blue light spectrum, and conventional passivation techniques are affected by subsequent processing steps.
Innovation Solution
A method where n-type dopants with a high segregation coefficient accumulate in the sensor layer side of the interface between the backside of the sensor layer and the insulating layer, passivating the interface and preventing the formation of electrostatic potential wells, while the well or buried well can be biased to direct photo-generated charges into photodetectors, and a doping gradient steers these charges effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional passivation techniques are used at the interface between sensor layer and insulating layer, then dark current is reduced, but subsequent CMOS fabrication steps cause dopant diffusion that creates electrostatic potential wells, increasing dark current and reducing quantum efficiency
Solution Approach 1:
The patent applies preliminary action by forming the doping profile in the sensor layer before subsequent CMOS fabrication steps. The doping structure is prepared in advance with a gradient that will maintain its integrity through thermal processing, preventing the formation of electrostatic potential wells that would otherwise trap photo-induced charge carriers and reduce quantum efficiency.
Solution Approach 2:
The patent changes the doping concentration parameter by implementing a graded doping profile in the sensor layer, with doping concentration varying from the interface toward the bulk. This parameter change creates a monotonic doping profile that prevents electrostatic potential well formation while maintaining effective passivation of the interface, thereby reducing dark current and preserving quantum efficiency simultaneously.
2Reliability
If p-type dopants are used in the sensor layer, then interface passivation is achieved, but thermal diffusion during processing creates electrostatic potential wells that trap charge carriers
Solution Approach 1:
The patent changes the doping concentration parameter by implementing a graded doping profile in the sensor layer, with doping concentration varying from the interface toward the bulk. This parameter change creates a monotonic doping profile that prevents electrostatic potential well formation while maintaining effective passivation of the interface, thereby reducing dark current and preserving quantum efficiency simultaneously.
3Stability of the object's composition
If indium is used instead of boron to reduce thermal diffusion, then dopant diffusion is minimized, but dark field bright point defects increase
Solution Approach 1:
The patent changes the doping concentration parameter by implementing a graded doping profile in the sensor layer, with doping concentration varying from the interface toward the bulk. This parameter change creates a monotonic doping profile that prevents electrostatic potential well formation while maintaining effective passivation of the interface, thereby reducing dark current and preserving quantum efficiency simultaneously.
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 approach minimizes thermal diffusion of dopants, reduces dark current, and enhances quantum efficiency by preventing the formation of potential wells at the interface, effectively directing photo-generated charges towards photodetectors and improving image sensor performance.
Implementation Method 1
n-type dopants with a high segregation coefficient accumulate in the sensor layer side of an interface between a backside of a sensor layer and an insulating layer
Implementation Method 2
light-sensitive photodetectors that convert incident light into electrical signals
Data Source
Figure 1~2
Figure 3~4
Figure 5~6(A)
AI summary
A back-illuminated image sensor includes a sensor layer disposed between an insulating layer and a circuit layer electrically connected to the sensor layer. An imaging area includes a plurality of photodetectors is formed in the sensor layer and a well that spans the imaging area. The well can be disposed between the backside of the sensor layer and the photodetectors, or the well can be a buried well formed adjacent to the backside of the sensor layer with a region including formed between the photodetectors and the buried well. One or more side wells can be formed laterally adjacent to each photodetector. The dopant in the well has a segregation coefficient that causes the dopant to accumulate on the sensor layer side of an interface between the sensor layer and the insulating layer.