CMOS Image Sensor Floating Diffusion Leakage Reduction
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Solution Overview
Problem
CMOS image sensors suffer from junction leakage in the floating diffusion region, which affects their performance by causing loss of electrons and deteriorating image sensor characteristics, as the impurity ion concentration in this region is the same as in other transistor regions, leading to uniform leakage current per unit area.
Innovation Solution
The concentration of impurity ions in the floating diffusion region is made lower than in the source/drain regions of other transistors, reducing leakage current and enhancing image sensor characteristics by forming a second diffusion region with a lower impurity concentration specifically in the floating diffusion area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the same ion implantation concentration is used for all n+ diffusion regions (including floating diffusion region), then the manufacturing process is simple and uniform, but the junction leakage in the floating diffusion region increases and deteriorates image sensor characteristics
Solution Approach 1:
The patent applies local quality by implanting impurity ions at different concentrations in different regions: the floating diffusion region receives a first concentration of impurity ions, while other n+ diffusion regions receive a second concentration. This localized differentiation reduces junction leakage in the floating diffusion region while maintaining appropriate doping levels in other regions, thereby improving image sensor characteristics without compromising manufacturing feasibility.
Solution Approach 2:
The patent changes the impurity ion concentration parameter specifically for the floating diffusion region by implanting at a first concentration that differs from the second concentration used in other regions. This parameter modification optimizes the electrical properties of the floating diffusion region, reducing junction leakage and improving overall device performance.
2Reliability
If high concentration impurity ions are implanted uniformly in all active regions, then the transistor source/drain regions achieve proper electrical characteristics, but the floating diffusion region experiences excessive junction leakage
Solution Approach 1:
The patent applies local quality by differentiating the impurity ion concentration between the floating diffusion region and other n+ diffusion regions. The floating diffusion region is implanted with a first concentration optimized to minimize junction leakage, while other regions receive a second concentration optimized for transistor source/drain characteristics. This localized quality control simultaneously achieves proper transistor electrical characteristics and reduces harmful junction leakage.
Solution Approach 2:
The patent modifies the impurity ion concentration parameter specifically for the floating diffusion region by implanting at a first concentration that is optimized to reduce junction leakage, while other regions maintain a second concentration for proper transistor operation. This selective parameter change resolves the contradiction between achieving good transistor characteristics and minimizing leakage current.
3Ease of manufacture
If the floating diffusion region uses the same doping concentration as other regions, then the manufacturing process is straightforward, but electrons diffused from the photodiode are lost due to leakage
Solution Approach 1:
The patent applies local quality by implanting impurity ions at a first concentration in the floating diffusion region that differs from the second concentration used in other n+ diffusion regions. This localized doping optimization reduces junction leakage in the floating diffusion region, preventing loss of electrons diffused from the photodiode while maintaining manufacturing feasibility through a controlled two-concentration implantation process.
Solution Approach 2:
The patent changes the impurity ion concentration parameter for the floating diffusion region to a first concentration that minimizes electron loss due to leakage, while other regions maintain a second concentration. This parameter optimization specifically addresses the electron loss problem in the floating diffusion region without compromising the overall manufacturing process.
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 effectively decreases leakage current in the floating diffusion region, thereby improving the overall performance and characteristics of the CMOS image sensor by varying the impurity ion concentration, specifically in the 4T and 3T type sensors.
Implementation Method 1
n+ diffusion region 36, i.e., source/drain region, is formed in the active region of each transistor by implanting to the same depth n+ impurity ions at high concentration into the active region of each transistor
Implementation Method 2
forming a second diffusion region of a second conductive type in the floating diffusion region by implanting impurity ions into the floating diffusion region at a concentration lower than that of the first diffusion region
Data Source
AI summary
Provided is a CMOS image sensor and method of manufacturing same. The CMOS image sensor includes a photodiode, a transfer transistor, a reset transistor, a drive transistor, and a select transistor. A device isolation layer is formed on a first conductive type substrate. Gate electrodes of the transfer transistor, the reset transistor, the drive transistor, and the select transistor are formed on an active region of the substrate with gate insulating layers interposed therebetween. A first diffusion region is formed of a second conductive type in a first region of the active region, where the first region does not include a floating diffusion region between the transfer transistor and the reset transistor and the photodiode region. A second diffusion region is formed of the second conductive type in the floating diffusion region at a concentration lower than that of the second conductive type first diffusion region.