Composite Grid Structure for Crosstalk Reduction in BSI Image Sensors
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Solution Overview
Problem
Back side illumination (BSI) image sensors face challenges with cross talk between neighboring pixel sensors, reducing quantum efficiency, angular response, and signal-to-noise ratio (SNR) under oblique incident light, and have limited light collection due to their small size, which worsens in low light environments.
Innovation Solution
A semiconductor structure with improved optical isolation and light collection, featuring a composite grid with a metal grid and a low refractive index (low-n) grid, and a deep trench isolation (DTI) region, which isolates neighboring pixel sensors and directs light to color filters, increasing their effective size and reducing cross talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional BSI image sensor structure is used, then manufacturing cost and integration are improved, but cross talk between neighboring pixel sensors increases
Solution Approach 1:
The patent divides the optical isolation function into multiple segments: a metal grid layer segmented into individual pixel-aligned structures, a low-n grid layer segmented into corresponding pixel structures, and a DTI region segmented between neighboring photodiodes. This segmentation allows each layer to independently contribute to cross talk reduction while maintaining manufacturing simplicity through standardized repetitive patterns across the sensor array.
Solution Approach 2:
The patent employs a composite grid structure combining metal grid and low-n grid materials, each with distinct optical properties. The metal grid provides strong light blocking, while the low-n grid material (with refractive index 1.4-1.6) provides optical isolation through refractive index contrast. This composite approach addresses cross talk more effectively than single-material solutions while remaining compatible with conventional semiconductor manufacturing processes.
2Area of stationary object
If the sensor size is reduced, then device size and integration are improved, but light collection capability deteriorates
Solution Approach 1:
The patent applies local quality optimization by configuring the metal grid and low-n grid structures to extend beyond the photodiode boundaries, creating localized light guiding regions. The grids are positioned and dimensioned to specifically address light collection at the pixel edges where cross talk and light loss are most problematic, rather than uniformly across the entire sensor. This allows small sensors to maintain effective light collection area through optimized local structures.
3Object-affected harmful factors
If optical isolation structures are added to reduce cross talk, then cross talk reduction is improved, but device complexity increases
Solution Approach 1:
The patent merges multiple functions into the composite grid structure: the metal grid simultaneously provides light blocking and structural support, the low-n grid provides optical isolation through refractive index contrast, and the DTI region provides substrate-level isolation. These merged structures are integrated into the conventional BSI fabrication process flow, avoiding the need for separate complex isolation steps and maintaining manufacturing simplicity despite enhanced functionality.
4Quantity of substance
If the color filter size is increased to improve light collection, then light collection is improved, but cross talk between neighboring pixels increases
Solution Approach 1:
The patent introduces the composite grid structure as an intermediary element between neighboring color filters and photodiodes. The metal grid and low-n grid act as mediating structures that guide light from larger color filters toward the photodiode while blocking lateral light propagation that would cause cross talk. This intermediary structure enables larger color filters to be used without proportionally increasing cross talk, as the grids manage the light distribution between adjacent pixels.
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 solution enhances quantum efficiency, angular response, and SNR under oblique incident light while improving optical isolation near the upper surface of the semiconductor substrate, effectively addressing cross talk and light collection issues.
Implementation Method 1
the low-n grid has a refractive index less than a refractive index of the color filters... the low-n grid isolates neighboring color filters and directs light to the color filters
Implementation Method 2
A composite grid includes a metal grid and a low-n grid overlying the metal grid... the metal grid blocks light, thereby isolating neighboring color filters
Implementation Method 3
A deep trench isolation (DTI) region isolates neighboring pixel sensors... improving optical isolation near the upper surface of the semiconductor substrate
Data Source
AI summary
A semiconductor structure for back side illumination (BSI) pixel sensors is provided. Photodiodes are arranged within a semiconductor substrate. A metal grid overlies the semiconductor substrate and is made up of metal grid segments that surround outer perimeters of the photodiodes, respectively, such that first openings within the metal grid overlie the photodiodes, respectively. A low-n grid is made up of low-n grid segments that surround the respective outer perimeters of the photodiodes, respectively, such that second openings within the low-n grid overlie the photodiodes, respectively. Color filters are arranged in the first and second openings of the photodiodes and have a refractive index greater than a refractive index of the low-n grid. A substrate isolation grid extends into the semiconductor substrate and is made up of isolation grid segments that surround outer perimeters of the photodiodes, respectively. A method for manufacturing the BSI pixel sensors is also provided.