CMOS Image Sensor Embedded Photodiode Aperture Ratio
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Solution Overview
Problem
CMOS image sensors face a drop in aperture ratio and light sensitivity due to the embedding of N-type photodiode regions below transistor formation regions, leading to increased threshold voltage and junction capacitance, which affects charge transfer efficiency and uniformity across pixels.
Innovation Solution
A CMOS image sensor design where the photodiode region is embedded below a well region containing reset and source follower transistors, avoiding formation below the floating diffusion region, allowing for reduced impurity concentration and increased aperture ratio, and shared transistors between pixels to maintain signal uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the photodiode region is embedded below the transistor formation region, then the aperture ratio is improved, but the threshold voltage increases and charge transfer efficiency decreases
Solution Approach 1:
The patent applies local quality by creating different impurity concentration zones within the semiconductor substrate. Specifically, a first impurity concentration zone is formed below the floating diffusion region with a lower impurity concentration than surrounding areas. This localized variation in impurity concentration allows the photodiode region to extend below the transistor formation region (improving aperture ratio) while maintaining low threshold voltage and high charge transfer efficiency in the critical area below the floating diffusion.
2Area of moving object
If the photodiode region is embedded below the transistor formation region, then the aperture ratio is improved, but the junction capacitance increases
Solution Approach 1:
The patent reduces junction capacitance by implementing a localized low impurity concentration zone directly below the floating diffusion region. This first impurity concentration zone has a lower impurity concentration than the second impurity concentration zone that surrounds it. The reduced impurity concentration in this specific area decreases the junction capacitance between the photodiode region and the substrate, allowing the photodiode to extend deeper without suffering from excessive capacitance penalties.
3Reliability
If the impurity concentration is reduced below the floating diffusion region, then the threshold voltage is lowered and charge transfer efficiency is improved, but the photodiode region cannot be extended below the transistor formation region
Solution Approach 1:
The patent resolves this contradiction by creating a spatially varying impurity concentration distribution. A first impurity concentration zone with low impurity concentration is formed specifically below the floating diffusion region to ensure low threshold voltage and high charge transfer efficiency. Surrounding this first zone is a second impurity concentration zone with higher impurity concentration that provides adequate electrical isolation. This localized quality variation allows the photodiode region to extend below the transistor formation region while maintaining excellent charge transfer characteristics in the critical area.
Solution Approach 2:
The patent segments the semiconductor substrate into distinct impurity concentration zones. The first impurity concentration zone below the floating diffusion region is separated from the surrounding second impurity concentration zone by a third impurity concentration zone. This segmentation allows different functional requirements to be met in different areas: low impurity concentration for charge transfer efficiency in the first zone, and higher impurity concentration for electrical isolation in the surrounding areas.
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 design enhances the aperture ratio and light sensitivity by lowering the threshold voltage and junction capacitance, while ensuring uniform detection signals across pixels through shared transistors and proper photodiode placement.
Implementation Method 1
a photodiode as an O/E (Opto/Electric) conversion element and extracts incident light intensity as an electrical signal by reading the amount of electrical charge that has accumulated in the photodiode
Data Source
AI summary
An image sensor in which a plurality of pixels having at least a photodiode, a reset transistor, and source follower transistor are formed, wherein each pixel comprises an electrical-charge transfer gate transistor between the photodiode and reset transistor, and a floating diffusion region constituting a node connecting the reset transistor and transfer gate transistor is connected to the gate of the source follower transistor. Further, a photodiode region is embedded below a well region in which the reset transistor and source follower transistor of each pixel are formed. In addition, the photo diode region is not formed below at least a partial region of the floating diffusion region.