Deep Trench Pixel Isolation with Gradient Sidewall Doping
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Solution Overview
Problem
Existing deep trench isolation (DTI) structures in CMOS image sensors face challenges in effectively passivating the sidewall surface, leading to increased dark currents and white pixels due to surface defects and the difficulty in achieving a gradient doping profile without compromising photodiode performance.
Innovation Solution
A hybrid DTI structure with a p-type region having a gradient doping profile along the sidewall surface is introduced, formed without using photolithography processes, to provide adequate passivation and reduce dark currents while maintaining photodiode performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional DTI structures are used to isolate pixels, then optical cross talk is reduced, but dark currents increase due to insufficient passivation of sidewall surface defects
Solution Approach 1:
The patent applies local quality by creating a gradient doping profile in the p-type region where the dopant concentration varies spatially - higher concentration near the sidewall surface for passivation and lower concentration deeper in the substrate to avoid compromising photodiode performance. This localized variation in doping quality resolves the contradiction by providing strong passivation exactly where needed (at the sidewall surface) while maintaining acceptable electrical characteristics in the bulk region.
2Manufacturing precision
If photolithography processes are used to form gradient doping profile, then doping precision is improved, but manufacturing complexity and cost increase
Solution Approach 1:
The patent employs self-service through self-aligned plasma doping where the doping process automatically conforms to the trench geometry without requiring photolithography masks. The plasma doping method inherently creates the gradient profile by self-aligning to the sidewall surface, eliminating the need for complex mask alignment steps while achieving the desired doping precision.
Solution Approach 2:
The patent replaces the mechanical photolithography system with a plasma-based doping system. Instead of using photoresist masks and optical alignment, the invention uses plasma immersion ion implantation that naturally follows the trench contours, substituting a simpler, mask-less process for the complex photolithography approach.
3Measurement precision
If pixel size is reduced to increase resolution, then image quality is improved, but optical cross talk and interference among pixels increase
Solution Approach 1:
The patent applies segmentation by creating a deep trench isolation structure that physically divides and separates adjacent pixels. The trench extends deep into the substrate and is filled with dielectric material, effectively segmenting the continuous silicon substrate into isolated pixel regions. This physical segmentation prevents optical cross talk even when pixels are closely spaced for high resolution.
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 proposed DTI structure effectively reduces dark currents and white pixels by accumulating holes at the sidewall surface, thereby enhancing passivation without compromising other design aspects, and allows for reduced fabrication costs through mask-less processes.
Implementation Method 1
a p-type region (246) having a gradient doping profile along a sidewall surface of the isolation structure
Implementation Method 2
accumulating holes at the sidewall surface, thereby enhancing passivation
Data Source
AI summary
An image sensor device is disclosed. The image sensor device includes a number of pixels and neighboring pixels are isolated by deep trench isolation structures. In an embodiment, a method of forming a deep trench isolation structure includes performing a first etching process to remove a portion of a substrate, thereby forming a first trench in the substrate, performing a first doping process to form a first sidewall doped region along a sidewall surface of the first trench, after the performing of the first plasma doping process, performing a second etching process to extend the first trench, thereby forming a second trench in the substrate, and, after the performing of the second etching process, performing a second doping process to form a second sidewall doped region along a sidewall surface of the second trench, a portion of the second sidewall doped region overlaps with the first sidewall doped region.