CMOS Image Sensor Dark Current Reduction via P+ Doping Barrier
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Solution Overview
Problem
Conventional CMOS image sensors suffer from increased dark current due to ion diffusion into the isolation layer, leading to reduced charge-storing capability and performance, especially in low illumination conditions.
Innovation Solution
A CMOS image sensor design that includes a P+-type doping area and a thermal oxide layer between the isolation layer and the active area, preventing ion diffusion and thereby minimizing dark current. This is achieved through specific fabrication steps involving the formation of a P+-type doping area, a thermal oxide layer, and an isolation layer, which are strategically placed to isolate the active area from the isolation layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If ion implantation is performed to form source/drain regions in the active area, then the transistor functionality is achieved, but ions diffuse into the isolation layer causing dark current increase
Solution Approach 1:
A P-type diffusion area is introduced as an intermediary region between the N-type photodiode diffusion area and the isolation layer. This P-type area acts as a barrier that prevents ions from the active area from diffusing into the isolation layer, thereby eliminating dark current while maintaining transistor functionality.
Solution Approach 2:
The patent applies different doping types in different regions: N-type doping for the photodiode active area, P-type doping for the isolation boundary area, and appropriate doping for transistor source/drain regions. This local differentiation of material properties prevents harmful ion diffusion while maintaining device functionality.
2Reliability
If the isolation layer is placed directly adjacent to the active area, then device isolation is achieved, but ion diffusion occurs at the boundary causing performance degradation
Solution Approach 1:
The P-type diffusion area serves as a buffer zone between the N-type photodiode region and the isolation layer. This intermediary P-type region prevents direct contact and ion diffusion between the active area and isolation layer, maintaining both isolation effectiveness and boundary integrity.
3Productivity
If photodiode and transistor are integrated in close proximity, then pixel density is increased, but dark current from the isolation layer boundary increases
Solution Approach 1:
The patent implements local P-type doping specifically at the boundary regions where photodiodes interface with isolation layers, while maintaining N-type doping in the photodiode active areas. This localized differentiation allows high pixel density integration while preventing dark current generation at critical boundaries.
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 solution effectively reduces dark current at the boundary area between the photodiode and the isolation layer, enhancing the reliability and performance of the CMOS image sensor by preventing defects and leakage currents.
Implementation Method 1
Conventional CMOS image sensors suffer from increased dark current due to ion diffusion into the isolation layer
Implementation Method 2
The photodiodes convert optical signals into electric signals by outputting charges
Data Source
AI summary
A CMOS image sensor and method for fabricating the same, wherein the CMOS image sensor has minimized dark current at the boundary area between a photodiode and an isolation layer. The present invention includes a first-conductivity-type doping area formed in the device isolation area of the substrate, the first-conductivity-type doping area surrounding the isolation area and a dielectric layer formed between the isolation layer and the first-conductivity-type doping area, wherein the first-conductivity-type doping area and the dielectric layer are located between the isolation layer and a second-conductivity-type diffusion area.