Stacked Metal Oxide Pixel Isolation for BSI Image Sensors
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Solution Overview
Problem
Backside illuminated (BSI) image sensor devices suffer from issues such as cross-talk and blooming due to insufficient isolation between neighboring pixels, leading to optical and electrical defects like dark current and white pixels, which worsen as pixel sizes shrink.
Innovation Solution
The implementation of a gradient high-k metal oxide film with alternating layers of materials like aluminum oxide, hafnium oxide, and tantalum oxide, along with deep-trench isolation structures, to enhance electrical isolation and reduce refractive index mismatch between radiation-sensing and isolation regions, thereby mitigating dark current and optical cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel size is reduced to increase sensor resolution, then sensor detail capability is improved, but isolation between neighboring pixels deteriorates leading to cross-talk and blooming
Solution Approach 1:
The patent employs a composite isolation structure combining multiple materials: silicon oxide (SiO2) as the base isolation layer, silicon nitride (Si3N4) as the trench isolation material, and aluminum oxide (Al2O3) as the fixed charge film. This multi-material composite approach provides superior electrical and optical isolation between pixels compared to single-material solutions, effectively addressing cross-talk and blooming issues in high-resolution sensors.
Solution Approach 2:
The isolation structure is segmented into multiple distinct functional layers: deep trenches filled with silicon nitride for primary electrical isolation, silicon oxide layers for additional isolation and as a platform for the fixed charge film, and aluminum oxide specifically deposited to provide fixed negative charges. This segmentation allows each layer to optimize its specific isolation function while working collectively to prevent pixel interference.
2Ease of manufacture
If conventional isolation structures are used, then manufacturing simplicity is maintained, but dark current and white pixel defects increase
Solution Approach 1:
The aluminum oxide fixed charge film acts as an intermediary layer between the silicon substrate and the photodiode. This intermediate layer provides fixed negative charges that repel minority carriers (electrons in P-type substrate), preventing them from reaching the photodiode and causing dark current or white pixel defects. The film is integrated into the existing isolation fabrication process, maintaining manufacturing simplicity while adding this protective intermediary function.
Solution Approach 2:
The patent changes the electrical parameter of the isolation structure by introducing a layer with fixed negative charges (aluminum oxide) rather than using conventional neutral dielectric materials. This parameter change in charge distribution fundamentally alters the carrier behavior at the isolation-photodiode interface, suppressing dark current generation while the deposition process compatible with CMOS technology maintains ease of manufacture.
3Reliability
If deep-trench isolation is implemented to improve pixel isolation, then cross-talk is reduced, but manufacturing complexity increases
Solution Approach 1:
The multi-layer isolation structure serves multiple functions simultaneously: silicon nitride deep trenches provide primary electrical isolation, silicon oxide layers provide additional isolation and serve as a deposition platform, and aluminum oxide provides fixed charges for carrier suppression. This multi-functionality is achieved within a single integrated fabrication sequence that combines with standard CMOS processes, avoiding the need for separate complex manufacturing steps and effectively managing device 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 solution significantly reduces dark current, white pixel defects, and improves quantum efficiency by effectively isolating pixels and reducing optical cross-talk, enhancing the overall performance of the image sensor device.
Implementation Method 1
a third film comprising aluminum oxide is formed over the second film... the third film comprising aluminum oxide is to generate negative fixed charges
Implementation Method 2
reduce refractive index mismatch between the radiation-sensing region and the isolation region, thereby reducing optical cross-talk
Data Source
AI summary
An image sensor device is disclosed. The image sensor device includes: a substrate having a front surface and a back surface; a radiation-sensing region formed in the substrate; an opening extending from the back surface of the substrate into the substrate; a first metal oxide film including a first metal, the first metal oxide film being formed on an interior surface of the opening; and a second metal oxide film including a second metal, the second metal oxide film being formed over the first metal oxide film; wherein the electronegativity of the first metal is greater than the electronegativity of the second metal. An associated fabricating method is also disclosed.