Backside Illumination Image Sensor Reflective Grid Isolation
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Solution Overview
Problem
Backside illumination (BSI) image sensors face challenges with cross-talk between neighboring pixel sensors and reduced light collection due to their decreasing size, leading to lower sensitivity, especially in low light environments.
Innovation Solution
Incorporating a reflective grid buried in the isolation structure within the substrate to act as a light guide or mirror, directing light back to the pixel sensors and reducing cross-talk, while a hybrid isolation structure provides both electrical and optical isolation between neighboring sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If pixel sensors are reduced in size to increase integration density, then device complexity and manufacturing cost are reduced, but light collection area decreases leading to lower sensitivity
Solution Approach 1:
The patent introduces a vertical dimension by embedding reflective structures (metal layers or dielectric layers with high refractive index) within the substrate depth. This allows light that would otherwise be lost laterally to be reflected back to the photodiode, effectively increasing light collection without increasing pixel area, thus resolving the contradiction between high integration density and light collection efficiency.
Solution Approach 2:
The patent employs intermediate layers (isolation structures, reflective layers, or dielectric layers) positioned between adjacent pixel sensors. These intermediaries serve dual purposes: providing electrical isolation and optical reflection. The reflective intermediary captures stray light and redirects it to the photodiode, while the isolation intermediary prevents cross-talk, thereby maintaining sensitivity despite reduced pixel size.
2Area of stationary object
If pixel sensors are placed closer together to increase array density, then device area is reduced, but cross-talk between neighboring pixels increases
Solution Approach 1:
The patent divides the substrate into isolated pixel regions using deep trench isolation structures filled with dielectric material. These isolation trenches physically segment the continuous substrate into discrete pixel units, preventing electrical and optical interference between adjacent pixels. This segmentation allows pixels to be placed closer together without increasing cross-talk, thus reducing overall sensor area while maintaining pixel independence.
Solution Approach 2:
The patent introduces intermediary isolation structures (deep trench isolation, metal isolation layers, or dielectric isolation layers) between neighboring pixel sensors. These intermediaries act as barriers that block electrical signals and reflect optical signals laterally, preventing cross-talk. The presence of these intermediaries enables higher pixel density without compromising signal integrity, as each pixel is electromagnetically isolated from its neighbors.
3Device complexity
If conventional isolation structures are used without reflective properties, then device complexity is reduced, but light absorption efficiency decreases
Solution Approach 1:
The patent designs isolation structures that perform multiple functions simultaneously: electrical isolation, optical reflection, and mechanical support. By integrating reflective properties (through metal layers or high refractive index dielectric layers) into the isolation structure, a single component achieves what would otherwise require separate elements. This multi-functionality increases light absorption efficiency without proportionally increasing device complexity, as the same structural element serves multiple purposes.
Solution Approach 2:
The patent employs composite isolation structures combining different materials with complementary properties. For example, a deep trench isolation filled with dielectric material is combined with metal reflective layers or low-refractive-index dielectric layers. This composite structure provides both electrical isolation and optical reflection, enhancing quantum efficiency while maintaining manageable device complexity through systematic material integration.
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 approach enhances the performance and sensitivity of BSI image sensors by reducing cross-talk and increasing light absorption, thereby improving their quantum efficiency and angular response.
Implementation Method 1
Incorporating a reflective grid buried in the isolation structure within the substrate to act as a light guide or mirror, directing light back to the pixel sensors and reducing cross-talk
Implementation Method 2
each of the pixel sensors includes a photodiode disposed in the substrate and configured to accumulate charge from incident light
Data Source
AI summary
A BSI image sensor includes a substrate including a front side and a back side opposite to the front side, a plurality of pixel sensors, an isolation grid disposed in the substrate and separating the plurality of pixel sensors from each other, a reflective grid disposed over the isolation grid on the back side of the substrate, an a low-n grid disposed over the back side of the substrate and overlapping the reflective grid from a top view. A width of the low-n grid is greater than a width of the reflective grid.