Tilted Transfer Gate CMOS Image Sensor Dark Current
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in optimizing white pixel and dark current performance due to surface defects and charge leakage, which affect image quality and dynamic range.
Innovation Solution
The implementation of tilted transfer gates in CMOS image sensor pixel circuits, where a trench is etched into the semiconductor substrate with a tilted side surface, separates the charge transfer path from the blooming path, improving image charge transfer and reducing dark current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional transfer gate design is used, then device complexity is low, but white pixel and dark current performance deteriorates due to surface defects and charge leakage
Solution Approach 1:
The transfer gate is designed with a tilted orientation relative to the photodiode and floating diffusion, transitioning from a conventional planar geometry to a three-dimensional angled structure. This dimensional change allows the gate to simultaneously achieve effective charge transfer and spatial separation from blooming paths, resolving the contradiction between performance improvement and structural complexity
Solution Approach 2:
The pixel circuit is segmented into distinct functional regions with separated charge transfer paths and blooming suppression paths. The tilted transfer gate creates independent conduction channels that prevent charge leakage while maintaining transfer efficiency, addressing the performance-reliability contradiction
2Reliability
If transfer gate separates charge paths, then dark current performance improves, but manufacturing precision requirements increase
Solution Approach 1:
The transfer gate angle is optimized within a specific range (45-60 degrees) to balance dark current suppression effectiveness with manufacturing feasibility. This parameter optimization ensures that the tilted gate achieves sufficient path separation without requiring extreme precision in trench etching, resolving the contradiction between performance and manufacturability
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 white pixel and dark current performance by isolating charge transfer and blooming paths, leading to improved image quality and dynamic range without compromising on performance trade-offs.
Implementation Method 1
The image sensor includes an array of pixel circuits having photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light and generate image charge upon absorption of the image light
Data Source
AI summary
A pixel circuit includes a trench etched into a front side surface of a semiconductor substrate. The trench includes a bottom surface etched along a <100> crystalline plane and a tilted side surface etched along a <111> crystalline plane that extends between the bottom surface and the front side surface. A floating diffusion is disposed in the semiconductor substrate beneath the bottom surface of the trench. A photodiode is disposed in the semiconductor substrate beneath the tilted side surface of the trench and is separated from the floating diffusion. The photodiode is configured to photogenerate image charge in response to incident light. A tilted transfer gate is disposed over at least a portion of the bottom surface and at least a portion of the tilted side surface of the trench. The tilted transfer gate is configured to transfer the image charge from the photodiode to the floating diffusion.