CMOS Image Sensor Pixel Isolation Structure for Dark Current Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
CMOS image sensors face challenges in increasing photoelectric conversion efficiency while minimizing dark current and white spots due to the increased electric field and impurity concentration in the signal charge accumulation region.
Innovation Solution
A solid-state image pickup device with a two-dimensionally arrayed pixel structure, utilizing a first and second isolation diffusion layer to increase the charge accumulation region, where the inner peripheral part of the first isolation diffusion layer projects into the charge accumulation region, and impurities of both layers are mixed, reducing the electric field and enhancing photoelectric conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the STI side wall layer has a certain width to suppress dark current, then dark current is reduced, but the effective area of the signal charge accumulation region decreases
Solution Approach 1:
The invention introduces a third isolation diffusion layer at a different depth dimension below the first isolation diffusion layer. This vertical layering approach allows the third layer to provide additional electrical isolation without increasing the lateral width of the STI side wall layer, thereby maintaining the effective area of the signal charge accumulation region while enhancing dark current suppression through multi-layer isolation architecture.
Solution Approach 2:
The third isolation diffusion layer is nested within the well region beneath the first isolation diffusion layer, creating a hierarchical isolation structure. This nested configuration enables the system to achieve enhanced isolation functionality without proportionally increasing the overall footprint or consuming additional lateral space that would reduce the photodiode's effective area.
2Quantity of substance
If the impurity concentration of the n+ layer and surface p+ layer is increased to increase the amount of electric charge, then the saturation signal is increased, but the electric field increases causing white spots
Solution Approach 1:
The invention segments the isolation function into multiple distinct layers: the first isolation diffusion layer provides primary lateral isolation, while the third isolation diffusion layer provides secondary deep isolation. This segmentation allows the system to achieve enhanced charge storage capacity through optimized impurity distribution without concentrating excessive impurity in a single region that would create harmful electric fields and white spots.
Solution Approach 2:
The third isolation diffusion layer is positioned specifically in the deep region beneath the signal charge accumulation region, providing localized electrical isolation where needed. This local quality enhancement allows for optimized impurity concentration distribution - sufficient to increase charge storage capacity in the photodiode while maintaining appropriate isolation to prevent excessive electric field buildup that causes white spots.
3Productivity
If the impurity concentration is increased to increase the amount of electric charge handled per unit area, then photoelectric conversion efficiency is improved, but dark current and white spots increase due to increased electric field
Solution Approach 1:
The invention transitions from a single-layer isolation approach to a multi-layer vertical isolation architecture. The third isolation diffusion layer operates in the vertical dimension beneath the first layer, enabling the system to achieve high photoelectric conversion efficiency through optimized impurity concentration in the photodiode while the distributed vertical isolation structure prevents excessive electric field concentration, thereby suppressing both dark current and white spots.
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 structure increases the charge accumulation area, improving photoelectric conversion efficiency and reducing the generation of dark current and white spots by minimizing excessive impurity concentration and electric field strength.
Implementation Method 1
a first isolation diffusion layer that electrically isolates the pixels under the element isolation insulating film; and a second isolation diffusion layer that electrically isolates the pixels under the first isolation diffusion layer
Implementation Method 2
a light-receiving part of such a CMOS image sensor is composed of an embedded photodiode including the hole accumulation layer 3 provided on the top face of the substrate and the signal charge accumulation region 4 provided under the hole accumulation layer 3
Data Source
AI summary
A solid-state image pickup device includes an element isolation insulating film electrically isolating pixels on the surface of a well region; a first isolation diffusion layer electrically isolating the pixels under the element isolation insulating film; and a second isolation diffusion layer electrically isolating the pixels under the first isolation diffusion layer, wherein a charge accumulation region is disposed in the well region surrounded by the first and second isolation diffusion layers, the inner peripheral part of the first isolation diffusion layer forms a projecting region, an impurity having a conductivity type of the first isolation diffusion layer and an impurity having a conductivity type of the charge accumulation region are mixed in the projecting region, and a part of the charge accumulation region between the charge accumulation region and the second isolation diffusion layer is abutted or close to the second isolation diffusion layer under the projecting region.