CMOS Image Sensor Self-Aligned Ion Implantation
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Solution Overview
Problem
CMOS image sensors suffer from poor dynamic range and charge transfer efficiency due to fixed pattern noise caused by misalignment and non-uniformity in the overlapping area between the gate and the photodiode region, leading to undesirable image quality.
Innovation Solution
A method involving a self-aligned ion implantation process to form a diode diffusion region and a pocket diffusion region under the gate electrode, with spacers formed on the gate sidewalls, enhancing the overlapping area and distance between the pinning layer and the substrate, thereby improving charge transfer efficiency and reducing fixed pattern noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional non-self alignment method is used to form the pixel sensor, then the overlapping area and distance can be made larger, but fixed pattern noise occurs due to misalignment and non-uniformity
Solution Approach 1:
The method performs preliminary ion implantation to form the diode diffusion region before forming the gate electrode, establishing a self-aligned structure that ensures uniform overlapping area across all pixels without misalignment issues
Solution Approach 2:
The gate electrode serves as a self-aligned mask during the ion implantation process, automatically defining the pocket diffusion region position and ensuring consistent overlapping area without requiring additional alignment steps
2Length of moving object
If the distance between the surface P+ pinning layer and the P-type substrate is too small, then the structure is more compact, but pinch-off occurs resulting in poor charge transfer efficiency and narrow dynamic range
Solution Approach 1:
The method forms the diode diffusion region with appropriate depth before gate electrode formation, pre-establishing the N-type region that will interact with the gate to create the desired charge transfer characteristics without pinch-off
Solution Approach 2:
The method optimizes the ion implantation parameters (dose, energy, angle) to create a diode diffusion region with controlled depth and concentration profile, ensuring the distance B is sufficient to prevent pinch-off while maintaining compact structure
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 method effectively suppresses fixed pattern noise, enhances charge transfer efficiency, and widens the dynamic range of the CMOS image sensor, resulting in improved image quality by aligning the ion implantation processes with the gate electrode and increasing the critical areas for charge transfer.
Implementation Method 1
A self-aligned ion implantation process is performed to form a diode diffusion region of a second conductivity type in the semiconductor substrate within the light-sensing region. An oblique ion implantation process is then performed to form a pocket diffusion region of the second conductivity type in the semiconductor substrate directly under the gate electrode
Data Source
AI summary
This invention provides a CMOS image sensor having a pinned photodiode. A P substrate is provided having thereon a P well. The P well is adjacent to a light-sensing region of the CMOS image sensor. A gate electrode of a transfer transistor of the CMOS image sensor is formed on the P well. A self-aligned implantation is performed to form N-type diode diffusion within the light-sensing region. An oblique ion implantation process is then performed to form N-type pocket diffusion directly under the gate electrode. Spacers are formed on sidewalls of the gate electrode. A surface P+ pinning diffusion region is then formed in the diode diffusion region.