Composite Backside Illumination Structure for Pixel Crosstalk Reduction
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Solution Overview
Problem
CMOS image sensors using monocrystalline silicon substrates have low absorption efficiency for high wavelength radiation and reduced optical performance due to small pixel pitch, which exacerbates challenges in image sensing applications.
Innovation Solution
The implementation of composite backside illuminated (CBSI) structures in image sensors, comprising trench isolation and absorption enhancement structures with angled sidewalls, enhances radiation absorption and reduces crosstalk by utilizing total internal reflection and reflective materials with lower refractive indices than the substrate, thereby improving quantum efficiency.
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 absorption efficiency for high wavelength radiation deteriorates
Solution Approach 1:
The patent implements a backside illuminated structure where radiation enters through the backside of the substrate rather than the front, changing the dimensional approach to radiation interaction. This allows radiation to traverse the full thickness of the substrate before reaching photodetectors, significantly improving absorption efficiency for high wavelength radiation while maintaining compatibility with standard monocrystalline silicon manufacturing processes
Solution Approach 2:
The patent divides the substrate into multiple functional regions including pixel regions with photodetectors, isolation regions with trenches, and circuit regions. This segmentation allows optimization of each region's function while maintaining overall manufacturing efficiency and absorption performance
2Productivity
If pixel pitch is reduced to increase pixel density, then device integration is improved, but optical performance deteriorates
Solution Approach 1:
The patent introduces trench isolation structures that segment the substrate into individual pixel regions. These trenches prevent optical crosstalk between adjacent pixels, allowing reduced pixel pitch while maintaining optical performance. The isolation structures create optical boundaries that contain radiation within each pixel region even at high densities
Solution Approach 2:
By implementing backside illumination, the patent changes the optical path dimension, allowing radiation to enter from the rear and travel through the full substrate thickness. This provides increased absorption path length that compensates for the reduced pixel area resulting from smaller pixel pitch
3Reliability
If trench isolation structures are implemented, then crosstalk reduction is improved, but manufacturing complexity increases
Solution Approach 1:
The patent combines multiple functions into the trench isolation structures: they serve as both optical isolation elements to reduce crosstalk and as structural support elements. The trenches are integrated into the substrate fabrication process rather than being added as separate post-processing steps, reducing overall manufacturing 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 CBSI structures increase quantum efficiency by reflecting unabsorbed radiation back to photodetectors, enhancing absorption and reducing crosstalk, particularly for high wavelength radiation, thus improving image sensor performance.
Implementation Method 1
enhances radiation absorption and reduces crosstalk by utilizing total internal reflection and reflective materials with lower refractive indices than the substrate
Implementation Method 2
The CBSI structures increase quantum efficiency by reflecting unabsorbed radiation back to photodetectors
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.