CMOS Pixel Sense Node Design for Charge Transfer Speed
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Solution Overview
Problem
CMOS imagers face limitations in charge transfer speed due to thermal diffusion, leading to high pixel cross-talk and charge diffusion clouds that exceed pixel sizes, affecting performance in high-resolution imaging applications.
Innovation Solution
The implementation of a CMOS pixel design with a semiconductor substrate, a floating sense node, and a transfer gate, along with substrate biasing techniques to enhance charge transfer efficiency, reduce pixel cross-talk, and improve modulation transfer function (MTF) performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional CMOS pixel design is used, then manufacturing simplicity is maintained, but charge transfer speed is limited due to thermal diffusion
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: a sense node region for charge collection, a transfer gate region for controlled charge transfer, and a photodiode region for light detection. This segmentation allows optimized charge transfer paths that minimize thermal diffusion while maintaining manufacturing feasibility through standard CMOS processes.
Solution Approach 2:
The patent implements local quality by creating a specialized sense node region with specific doping characteristics and geometry optimized for rapid charge collection. The transfer gate is positioned and dimensioned to create localized electric fields that accelerate charge transfer in critical areas while maintaining overall pixel performance.
2Measurement precision
If thicker epitaxial layers are used to improve quantum efficiency, then light absorption is enhanced, but charge diffusion cloud size increases
Solution Approach 1:
The sense node is positioned closer to the front surface of the epitaxial layer, and the transfer gate is configured to activate early in the charge transfer sequence. This preliminary action ensures that charges generated deep in thick epitaxial layers are rapidly collected and transferred before thermal diffusion can significantly increase cloud size, thereby maintaining quantum efficiency without sacrificing charge confinement.
3Ease of manufacture
If standard transfer gate design is used, then manufacturing is simplified, but pixel cross-talk increases
Solution Approach 1:
The transfer gate and sense node are designed with asymmetric geometry where the sense node extends preferentially in the direction away from neighboring pixels. This asymmetric configuration creates an electric field distribution that directs charge transfer away from adjacent pixels, reducing cross-talk while maintaining compatibility with standard CMOS fabrication processes.
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 achieves faster charge transfer, reduces charge diffusion, and enhances image quality by minimizing transit time and diffusion cloud size, thereby improving the signal-to-noise ratio and quantum efficiency, especially in thick epitaxial layer CMOS imagers.
Implementation Method 1
at least one photodiode formed about the transfer gate
Implementation Method 2
a transfer gate formed about the sense node
Data Source
AI summary
A CMOS pixel is disclosed. The CMOS pixel includes a semiconductor substrate; a sense node formed in the semiconductor substrate and positioned substantially in the center of the CMOS pixel; a transfer gate formed about the sense node; and at least one photodiode formed about the transfer gate. A reset transistor, a source follower transistor, and a row select transistor are located substantially to one side of the CMOS pixel substantially adjacent to the photodiode. The sense node is operable to be floating. An implant may be formed about the photodiode configured to step potential in a direction toward the sense node.