CMOS Image Sensor Doping Strategy for Dark Current Reduction
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Solution Overview
Problem
CMOS image sensors face issues with increased dark current and dead-zone generation due to n-type impurity ion doping processes, which affect the transfer transistor's threshold voltage and channeling efficiency.
Innovation Solution
A CMOS image sensor manufacturing method where a first n-type impurity region is formed between the photodiode and the transfer transistor, followed by a second n-type impurity region in the photodiode, using different energy levels for ion injection to minimize dark current and dead-zone formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If n-type impurity ion doping is performed in the photodiode region, then the photodiode's charge collection efficiency is improved, but dark current increases and dead-zone is generated in the transfer transistor
Solution Approach 1:
The patent divides the n-type impurity doping process into two separate stages: first doping the photodiode region, then doping the transfer transistor region. This segmentation allows each region to receive optimized doping doses at appropriate times, preventing the transfer transistor channel from being contaminated by excessive n-type impurities that would cause dead-zone, while still achieving sufficient doping in the photodiode for efficient charge collection
Solution Approach 2:
The patent performs n-type impurity doping in the photodiode region before forming the transfer transistor's source/drain structures. This preliminary action ensures that the photodiode is adequately doped for charge collection before the transfer transistor is completed, preventing subsequent doping steps from contaminating the transistor channel and creating dead-zone
2Productivity
If high energy ion injection is used to form the photodiode n-type impurity region, then doping efficiency is improved, but gate channeling occurs in the transfer transistor
Solution Approach 1:
The patent applies different doping energies to different regions: high energy ion injection is used specifically for the photodiode region to achieve deep and efficient doping, while lower energy doping is used for the transfer transistor region to avoid excessive penetration that would cause gate channeling. This localized quality approach optimizes doping for each specific region's requirements
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 prevents gate channeling and ion damage, ensuring efficient charge transfer and reducing dead-zone generation, thereby enhancing the sensor's performance and reliability.
Implementation Method 1
n-type impurity ions are injected into a semiconductor substrate having a first conductive type to form a first n-type impurity region
Data Source
AI summary
A CMOS image sensor includes a semiconductor substrate with a first conductive type including a photodiode region and a transistor region, a gate electrode formed on the transistor region of the substrate, a first impurity region with a second conductive type formed in a portion of the semiconductor substrate between the photodiode region and the gate electrode, and a second impurity region with the second conductive type formed in the photodiode region of the semiconductor substrate.