Photodiode Mesa Trench and Implant Isolation for Cross-Talk Reduction
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Solution Overview
Problem
Conventional pixel designs in photodiode arrays suffer from high dark current and capacitance, leading to noise in camera systems, particularly in fine pitch and low dark current detector arrays, and face challenges with cross-talk and modulation transfer function.
Innovation Solution
The design includes a photodiode with a cap layer and mesa trench surrounding a pixel diffusion area, where the mesa trench defines a floor at the absorption layer surface and extends with opposed sidewalls, and an implant aligned with the mesa trench to isolate pixels, reducing cross-talk and improving modulation transfer function.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional pixel designs are used in photodiode arrays, then the pixel structure is simple and easy to manufacture, but the dark current and capacitance are high leading to noise in camera systems
Solution Approach 1:
The pixel structure is segmented into distinct regions including a pixel diffusion area, mesa trench, and implant region. The mesa trench physically divides adjacent pixels, separating the pixel diffusion areas and reducing electromagnetic coupling between them. This segmentation lowers capacitance and dark current while maintaining manufacturability through standard semiconductor fabrication processes.
Solution Approach 2:
The implant is selectively positioned in specific locations within the pixel structure, particularly in the mesa trench region and adjacent to the pixel diffusion area. This localized doping creates regions with specific electrical properties that reduce dark current generation at critical interfaces while leaving other regions optimized for photon detection. The local quality modification addresses harmful effects without requiring complete structural redesign.
2Area of stationary object
If fine pitch detector arrays are used to increase pixel density, then the area per pixel is reduced, but cross-talk between adjacent pixels increases
Solution Approach 1:
The mesa trench acts as a physical barrier that segments adjacent pixels, preventing optical and electrical cross-talk. Even as pixel pitch is reduced to increase density, the mesa trench maintains effective isolation between pixels. The trench depth and width are optimized to provide sufficient isolation while minimizing the area consumed by isolation structures, allowing fine pitch arrays to achieve high pixel density without excessive cross-talk.
Solution Approach 2:
The implant serves as an intermediary element positioned between adjacent pixel diffusion areas. This doped region creates a potential barrier that electrically isolates neighboring pixels, reducing carrier diffusion and electrical cross-talk. The implant acts as a mediator that enables closer pixel spacing while maintaining signal integrity and minimizing interference between adjacent photodetectors.
3Measurement precision
If pixel diffusion area extends into the absorption layer to increase charge collection, then the signal detection is improved, but the capacitance and dark current increase
Solution Approach 1:
The pixel diffusion area is selectively extended into the absorption layer only in regions where photon absorption and charge generation occur, rather than uniformly across the entire pixel structure. The mesa trench and implant create localized regions with different electrical properties, confining the high-capacitance diffusion area to necessary locations while isolating it from adjacent pixels. This local quality approach maximizes charge collection where needed while minimizing overall capacitance and dark current.
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 configuration reduces pixel-to-pixel cross-talk and enhances modulation transfer function, leading to improved signal-to-noise ratios and reduced system noise in camera systems.
Implementation Method 1
an absorption layer. A cap layer is disposed on a surface of the absorption layer... to receive a charge generated from photons therefrom
Implementation Method 2
A respective implant is aligned with each respective mesa trench as described above so the mesa trenches and implants isolate the pixel diffusion areas from one another
Data Source
AI summary
A photodiode includes an absorption layer. A cap layer is disposed on a surface of the absorption layer. A pixel diffusion area within the cap layer extends beyond the surface of the absorption layer and into the absorption layer to receive a charge generated from photons therefrom. A mesa trench is defined through the cap layer surrounding the pixel diffusion area, wherein the mesa trench defines a floor at the surface of the absorption layer and opposed sidewalls extending away from the surface of the absorption layer. An implant is aligned with the mesa trench and extends from the floor of the mesa trench through the absorption layer surrounding a portion of the absorption layer proximate the pixel diffusion area.