Air Gap Shallow Trench Isolation for CMOS Image Sensor Crosstalk
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Solution Overview
Problem
Conventional shallow trench isolation technologies, such as LOCOS and existing shallow trench isolation, face limitations in reducing isolation layer width and lead to degradation of optical device characteristics in high-integration CMOS image sensors due to dark current and crosstalk issues.
Innovation Solution
A shallow trench isolation structure is developed with an air gap formed between the trench and the liner, sealed by a buffer layer, which suppresses photon injection and dark current generation, and includes a method for manufacturing CMOS image sensors using this structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the integration degree of semiconductor devices is increased, then the device density is improved, but the optical device characteristics are degraded due to dark current and crosstalk
Solution Approach 1:
The isolation structure is segmented into multiple functional layers: trench structure for physical separation, inner wall oxide layer for surface passivation, liner layer for structural support, and air gap for enhanced isolation. This segmentation allows each layer to address specific aspects of the contradiction, enabling high integration while maintaining optical device characteristics.
Solution Approach 2:
The air gap acts as an intermediary medium between adjacent photodiodes, providing enhanced optical and electrical isolation. This intermediary structure prevents photon injection and dark current generation from neighboring pixels, thereby maintaining optical device characteristics while allowing high device density.
2Length of moving object
If the width of isolation layer is reduced, then the device integration is improved, but the isolation effect is insufficient leading to dark current and crosstalk
Solution Approach 1:
Different regions of the isolation structure have different properties optimized for their specific functions: the trench provides physical separation, the inner wall oxide layer provides surface passivation with high-quality interface, the liner provides structural integrity, and the air gap provides enhanced isolation. This local quality optimization allows reduced isolation layer width while maintaining effective isolation against dark current and crosstalk.
Solution Approach 2:
The isolation structure uses a composite of multiple materials: silicon dioxide for the trench filling and inner wall oxide layer, nitrogen-containing material for the liner, and air for the gap. This composite structure provides superior isolation performance compared to single-material structures, enabling reduced width while maintaining effectiveness against dark current and crosstalk.
3Ease of manufacture
If conventional LOCOS technology is used, then the manufacturing process is simple, but the isolation layer width cannot be reduced for submicron devices
Solution Approach 1:
The inner wall oxide layer is formed preliminarily on the trench structure before filling with the liner and oxide materials. This preliminary action of growing the oxide layer on the trench walls provides a high-quality interface that prevents dark current, enabling the trench isolation structure to achieve both reduced width and effective isolation for submicron devices.
Solution Approach 2:
The isolation approach transitions from planar LOCOS to a three-dimensional trench structure with vertical walls and an air gap. This dimensional change from two-dimensional surface isolation to three-dimensional subsurface isolation enables reduced isolation layer width while maintaining or improving isolation effectiveness for submicron scale devices.
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 air gap structure enhances the insulating effect, reduces dark current, and achieves high-quality images by preventing photon interference and ensuring uniform doping profiles in the CMOS image sensor.
Implementation Method 1
the air gap to suppress the photons injected from neighboring pixels
Implementation Method 2
a buffer layer to seal the air gap... electrons are prevented from diffusing toward the surface
Data Source
AI summary
Disclosed is a shallow trench isolation structure having an air gap for suppressing the dark currents and cross-talk which occur in CMOS image sensors. The shallow trench isolation structure suppresses photons injected from neighboring pixels and dark current, so that high-quality images are obtained. Since impurities are removed from a p type ion implantation region for a photodiode when an inner wall oxide layer is etched to form the air gap, the p type ion implantation region has a uniform doping profile, thereby suppressing the diffusion of electrons towards the surface and achieving an image having a high quality.