CMOS Image Sensor Shallow Trench Isolation Optical Grating
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Solution Overview
Problem
CMOS time-of-flight image sensors face challenges in achieving high modulation contrast and optical sensitivity, particularly at near-infrared wavelengths, due to limitations in optical energy redistribution and dark current issues.
Innovation Solution
The integration of shallow trench isolation (STI) structures within the photodetector area of the CMOS image sensor, which acts as an optical grating to redistribute incident light and serves as a physical barrier to reduce dark current, enhancing modulation contrast and optical sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If shallow trench isolation structures are integrated into the photodetector area, then optical sensitivity and modulation contrast are improved, but device complexity increases
Solution Approach 1:
The patent combines the STI structures with the photodetector area, making the STI structures serve dual purposes: electrical isolation and optical redistribution. The STI structures are integrated within the photodetector area rather than being separate components, thereby improving optical sensitivity while minimizing additional complexity
Solution Approach 2:
The STI structures perform multiple functions simultaneously: they provide electrical isolation between photodetectors and act as optical gratings to redistribute incident light. This multi-functionality allows the same structure to address both electrical and optical requirements, improving measurement precision without proportionally increasing device complexity
2Measurement precision
If shallow trench isolation structures are integrated into the photodetector area, then modulation contrast is improved, but manufacturing precision requirements increase
Solution Approach 1:
The STI structures are formed using standard CMOS fabrication processes that are already part of the manufacturing flow. By integrating the optical grating function into the existing STI formation steps, the patent avoids adding separate manufacturing stages, thereby reducing the burden on manufacturing precision while achieving improved modulation contrast
3Object-generated harmful factors
If dopant is used to passivate the shallow trench isolation structure, then dark current is reduced, but manufacturing process complexity increases
Solution Approach 1:
The dopant used for passivating the STI structures is the same dopant already present in the semiconductor layer as part of the pixel design. The heating process causes this existing dopant to diffuse toward the STI walls, eliminating the need for additional dopant introduction steps. This self-service approach reduces dark current while avoiding significant increases in manufacturing process complexity
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 STI structures improve optical sensitivity by 10-15% and increase modulation contrast, enabling better depth quality and a larger depth operation range without significant increases in cost or complexity, while maintaining low dark current levels.
Implementation Method 1
The shallow trench isolation structure acts as an optical grating that modifies incident light by reflection, deflection and/or diffraction at its interface
Implementation Method 2
The shallow trench isolation structure acts as an optical grating that modifies incident light by reflection, deflection and/or diffraction at its interface
Implementation Method 3
The photosensitive semiconductor layer converts incident light into electrical charges
Implementation Method 4
Heating of the substrate causes the dopant to diffuse toward the walls of the shallow trench isolation structures
Data Source
Figure 1A
Figure 1B~1C
Figure 1D~1G
AI summary
A CMOS image sensor pixel has an integrated shallow trench isolation structure, resulting in higher optical sensitivity in general, and specifically for long wavelengths (red, near infrared, infrared). The shallow trench isolation structure acts as an optical grating that reflects and diffracts light so that an increased optical energy (photo generation) is observed in the photosensitive semiconductor layer of the pixel. An increase in dark current is avoided by passivating the shallow trench isolation structure with dopant which was implanted within the photosensitive semiconductor layer. Annealing in a standard CMOS process causes the dopant to diffuse toward the shallow trench isolation structure. The pixel can be configured as a time-of-flight sensor. The shallow trench isolation structure acts as a physical barrier for electrical charge motion, resulting in a higher modulation contrast pixel. Further, front side or backside illumination can be used.