Composite BSI Pixel Structure for High-Wavelength Quantum Efficiency
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Solution Overview
Problem
CMOS image sensors using monocrystalline silicon substrates have low quantum efficiency for high wavelength radiation due to low absorption coefficients, and reducing pixel pitch to increase density degrades optical performance.
Innovation Solution
The implementation of composite backside illuminated (CBSI) structures in image sensors, which include trench isolation and absorption enhancement structures with angled sidewalls, allows for unimpeded radiation entry and internal reflection, reducing crosstalk and enhancing radiation absorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If monocrystalline silicon substrates are used in CMOS image sensors, then manufacturing cost and integration are improved, but quantum efficiency for high wavelength radiation deteriorates due to low absorption coefficients
Solution Approach 1:
The patent transitions from front-side illumination to backside illumination, changing the dimensional approach of radiation entry. By illuminating the backside of the substrate, radiation can enter directly without passing through metal interconnect layers, significantly improving quantum efficiency for high wavelength radiation while maintaining compatibility with monocrystalline silicon substrates
Solution Approach 2:
The patent introduces angled sidewalls (e.g., 45-degree angles) in trenches and absorption enhancement structures. These curved/angled surfaces facilitate total internal reflection of radiation, increasing the optical path length and absorption probability within the photodetector, thereby improving quantum efficiency without changing the substrate material
2Productivity
If pixel pitch is reduced to increase pixel density, then productivity and device integration are improved, but optical performance deteriorates due to increased crosstalk and reduced light absorption
Solution Approach 1:
The patent divides the pixel structure into segmented components including trench isolation structures that physically separate adjacent pixels. This segmentation prevents crosstalk between neighboring pixels, allowing reduced pixel pitch while maintaining optical performance. The trench structures create electrical and optical isolation between densely packed pixels
Solution Approach 2:
The patent introduces absorption enhancement structures with angled sidewalls as intermediary elements between incident radiation and the photodetector. These structures act as optical mediators that guide and concentrate radiation onto the photodetector active area, improving light absorption efficiency even in densely packed pixel configurations
3Reliability
If backside illuminated structures are implemented, then radiation absorption is improved, but device complexity increases due to additional manufacturing steps
Solution Approach 1:
The patent merges the formation of trenches and absorption enhancement structures into a unified backside processing sequence. By combining multiple functions (isolation, light guiding, and absorption enhancement) into integrated structures formed through coordinated etching and deposition steps, the manufacturing complexity is managed while achieving superior radiation absorption
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 CBSI structures increase quantum efficiency by reflecting and re-absorbing radiation, improving image sensor performance for high wavelength radiation and maintaining optical performance at higher pixel densities.
Implementation Method 1
The CBSI structures increase quantum efficiency by reflecting and re-absorbing radiation
Implementation Method 2
absorption enhancement structures with angled sidewalls, allows for unimpeded radiation entry and internal reflection
Data Source
AI summary
Various embodiments of the present application are directed towards image sensors including composite backside illuminated (CBSI) structures to enhance performance. In some embodiments, a first trench isolation structure extends into a backside of a substrate to a first depth and comprises a pair of first trench isolation segments. A photodetector is in the substrate, between and bordering the first trench isolation segments. A second trench isolation structure is between the first trench isolation segments and extends into the backside of the substrate to a second depth less than the first depth. The second trench isolation structure comprises a pair of second trench isolation segments. An absorption enhancement structure overlies the photodetector, between the second trench isolation segments, and is recessed into the backside of the semiconductor substrate. The absorption enhancement structure and the second trench isolation structure collectively define a CBSI structure.