Deep Trench Isolation Layout for High-Resolution CIS Pixels
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Solution Overview
Problem
As CMOS image sensor pixels shrink in size, cross-talk between adjacent photodetector pixels becomes a significant concern, necessitating a method to reduce pixel size while maintaining sensitivity and minimizing cross-talk.
Innovation Solution
A back side isolation structure is formed by etching a grid of deep trenches in the semiconductor substrate using a cyclic process with an etch-resistant coating, allowing the trenches to narrow at their bottoms, resulting in a narrower isolation grid that increases the area available for photodetectors and enhances full well capacity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel size is reduced to increase pixel density, then the number of pixels per unit area increases, but cross-talk between adjacent photodetector pixels increases
Solution Approach 1:
The isolation structure is segmented into multiple depth segments (first depth segment, second depth segment, etc.) with different widths. The upper portions have wider isolation width while the lower portions have narrower isolation width, allowing effective cross-talk blocking at the pixel boundaries while maintaining sufficient space for photodetector active areas, thus reducing cross-talk without limiting pixel density improvement
Solution Approach 2:
The isolation structure implements local quality variation by having different widths at different depths. The upper isolation portions are wider to provide strong isolation where photodetector edges are most vulnerable to cross-talk, while the lower isolation portions are narrower to maximize the area available for photodetector pixels, enabling high pixel density while maintaining effective cross-talk reduction
2Area of moving object
If the isolation grid width is reduced to increase photodetector area, then the available area for photodetectors increases, but the isolation effectiveness may be compromised
Solution Approach 1:
The isolation grid is segmented vertically into multiple depth segments with varying widths. The upper depth segments maintain wider isolation width to ensure effective electrical isolation and cross-talk blocking, while the lower depth segments are narrowed to maximize photodetector area, achieving both isolation effectiveness and area optimization simultaneously
Solution Approach 2:
The isolation structure transitions from a uniform two-dimensional grid to a three-dimensional segmented structure with varying widths at different depths. This dimensional variation allows the isolation to be effective where needed (upper portions) while minimizing area occupation (lower portions), resolving the contradiction between isolation effectiveness and photodetector area
3Object-affected harmful factors
If deep trenches are etched to form the isolation structure, then cross-talk reduction is improved, but the manufacturing complexity increases
Solution Approach 1:
The trench formation process is performed periodically in multiple cycles, with each cycle depositing a layer of etch-resistant coating and then etching a specific depth segment. This periodic action allows precise control over the segmented depth structure, creating effective cross-talk isolation through multiple narrow lower segments while managing manufacturing complexity through systematic repetitive processing
Solution Approach 2:
An etch-resistant coating is deposited on the trench walls before each etching step to prevent excessive lateral etching and substrate damage. This preliminary protective action ensures clean, controlled trench formation with precise width control, reducing manufacturing complexity by preventing defects that would require rework or additional process steps
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 method achieves a 10-20% reduction in isolation grid width, leading to a 30-40% increase in full well capacity, with a more than linear impact on available area, effectively reducing cross-talk and improving quantum efficiency.
Implementation Method 1
lining the trenches with an etch-resistant coating before beginning a next cycle; The etch-resistant coating remains in the upper parts of the trenches to limit lateral etching and substrate damage
Data Source
AI summary
Trenches in which to form a back side isolation structure for an array of CMOS image sensors are formed by a cyclic process that allows the trenches to be kept narrow. Each cycle of the process includes etching to add a depth segment to the trenches and coating the depth segment with an etch-resistant coating. The following etch step will break through the etch-resistant coating at the bottom of the trench but the etch-resistant coating will remain in the upper part of the trench to limit lateral etching and substrate damage. The resulting trenches have a series of vertically spaced nodes. The process may result in a 10% increase in photodiode area and a 30-40% increase in full well capacity.