Elevated Floating Diffusion Structure for Low-Leakage Image Sensors
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Solution Overview
Problem
CMOS image sensors face issues with floating diffusion leakage, leading to high dark current, sensor noise, and reduced image quality due to highly doped floating diffusion regions causing gate induced drain leakage and Trap-Assisted-Tunneling leakage.
Innovation Solution
An elevated floating diffusion region is formed by separating a doped floating diffusion region from the semiconductor substrate with an undoped or lightly doped intervening layer, reducing leakage by avoiding abrupt junctions and silicon damage, and providing an ohmic contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a highly doped floating diffusion region is used, then the photosensitivity and charge collection efficiency are improved, but gate induced drain leakage and junction leakage increase
Solution Approach 1:
The floating diffusion region is segmented into multiple doping zones with different doping concentrations. A lightly doped or undoped region is positioned between the highly doped floating diffusion and the substrate, creating a gradient structure that reduces abrupt junctions and associated leakage while preserving photosensitivity.
Solution Approach 2:
Different regions of the floating diffusion structure are assigned different doping qualities: the surface region maintains high doping for charge collection, while the deeper region transitions to lighter doping to reduce leakage. This local differentiation resolves the contradiction between photosensitivity and leakage suppression.
2Productivity
If device geometries are scaled down, then fabrication costs decrease and device integration density increases, but defects become more apparent and image quality deteriorates
Solution Approach 1:
The doping concentration parameter is changed spatially within the floating diffusion region, transitioning from uniform high doping to a graded structure with lighter doping deeper in the substrate. This parameter modification reduces leakage defects that become more prominent at scaled dimensions while maintaining the high integration density.
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 minimizes diffusion leakage, enhancing image quality, yield, and speed by suppressing gate induced drain leakage and junction leakage, thereby improving the performance of CMOS image sensors.
Implementation Method 1
Photosensitive elements included in the image sensor, such as photodiodes, each generate image charge upon absorption of the image light
Data Source
AI summary
A pixel cell with an elevated floating diffusion region is formed to reduce diffusion leakage (e.g., gate induced drain leakage, junction leakage, etc.). The floating diffusion region can be elevated by separating a doped floating diffusion region from the semiconductor substrate by disposing an intervening layer (e.g., undoped, lightly doped, etc.) on the semiconductor substrate and beneath the doped floating diffusion region. For instance, the elevated floating diffusion region can be formed by stacked material layers composed of a lightly or undoped base or intervening layer and a heavy doped (e.g., As doped) “elevated” layer. In some examples, the stacked material layers can be formed by first and second epitaxial growth layers.