CMOS Image Sensor Trench Isolation for Light Leakage Suppression
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Solution Overview
Problem
The issue with existing CMOS image sensors is the reduction in quantum efficiency and increase in dark current due to light leakage through silicon oxide trench isolations used for electrical isolation, which affects the performance of both the image sensor and logic circuit on the same wafer.
Innovation Solution
The implementation of trench isolations with a dielectric layer configuration that includes a first silicon oxide layer and a second silicon oxynitride layer with higher refractive index, designed to reflect light and prevent it from entering adjacent pixel units, while maintaining electrical isolation between photo-detecting portions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If silicon oxide trench isolations are used for electrical isolation between pixel units, then electrical isolation is achieved, but light leakage occurs through the trench isolations reducing quantum efficiency and increasing dark current
Solution Approach 1:
The patent applies composite materials by combining multiple dielectric layers with different optical properties (silicon oxide, silicon oxynitride, and fluorinated silicon oxide) to create a trench isolation structure that simultaneously achieves electrical isolation and light reflection. Each layer contributes different characteristics: silicon oxide provides electrical isolation, silicon oxynitride provides high refractive index for light reflection, and fluorinated silicon oxide provides low refractive index to enhance the optical contrast and reflection efficiency, thereby resolving the contradiction between electrical isolation and quantum efficiency.
Solution Approach 2:
The patent changes the optical parameters of the trench isolation structure by introducing dielectric layers with specifically engineered refractive indices. The silicon oxynitride layer (high refractive index) and fluorinated silicon oxide layer (low refractive index) create an optical mirror effect that reflects light back to its origin pixel unit. This parameter change in refractive index distribution transforms the trench isolation from a light-transmissive structure to a light-reflecting structure, maintaining electrical isolation while preventing light leakage.
2Reliability
If silicon oxide trench isolations are used for electrical isolation, then transistor performance is maintained, but dark current increases due to light leakage
Solution Approach 1:
The composite dielectric structure maintains the electrical isolation function provided by silicon oxide while adding optical reflection capabilities through silicon oxynitride and fluorinated silicon oxide layers. This composite approach preserves the transistor performance benefits of silicon oxide trench isolations while eliminating the harmful dark current generated by light leakage, as the optical mirror effect prevents photons from reaching adjacent pixel units where they would generate unwanted dark current signals.
3Ease of manufacture
If a single-layer silicon oxide trench isolation is used, then manufacturing is simple, but light reflection capability is insufficient
Solution Approach 1:
The patent employs composite materials to achieve superior light reflection capability. The multi-layer dielectric structure with alternating high and low refractive indices creates constructive interference for reflected light, significantly enhancing the optical mirror effect compared to a single-layer structure. While the manufacturing process becomes more complex with multiple deposition steps, the dramatic improvement in light reflection capability justifies the additional processing, as it directly addresses the quantum efficiency and dark current issues.
Solution Approach 2:
The patent changes the optical parameters of the trench isolation by introducing layers with specifically controlled refractive indices and thicknesses. The silicon oxynitride layer (high refractive index) and fluorinated silicon oxide layer (low refractive index) are engineered to create optimal optical interference conditions for light reflection. By adjusting these parameters, the structure achieves maximum reflection efficiency, transforming the trench isolation from a light-transmissive to a light-reflecting structure.
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 enhances quantum efficiency and reduces dark current, improving the overall performance of the CMOS image sensor by minimizing light interference and maintaining transistor performance.
Implementation Method 1
a second silicon oxynitride layer with higher refractive index, designed to reflect light and prevent it from entering adjacent pixel units
Data Source
AI summary
A semiconductor structure includes: an epitaxial layer; photo-detecting portions disposed in the epitaxial layer and spaced apart from each other, each of the photo-detecting portions including a p-n junction; and trench isolations disposed in the epitaxial layer, each of the trench isolations being disposed to separate two adjacent ones of the photo-detecting portions from each other. Each of the trench isolations includes a first dielectric layer having a first refractive index and a first thickness, and a second dielectric layer having a second refractive index that is different from the first refractive index, and a second thickness that is different from the first thickness. The first dielectric layer and the second dielectric layer are arranged to prevent a light incident to one of the photo-detecting portions from entering an adjacent one of the photo-detecting portions.