Polygonal STI Structure for Low-Dark-Current CMOS Image Sensors
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Solution Overview
Problem
Current shallow trench isolation (STI) techniques in CMOS image sensors face challenges in maintaining effective device isolation as pixel size decreases, leading to increased dark current due to trap-assisted-tunneling and junction leakage, especially in high-temperature environments like automotive applications.
Innovation Solution
A two-step etching process is employed to form a shallow trench isolation structure with a polygonal cross-section, combining dry plasma etching to create the trench and a wet etching process to cure damage and achieve smoother sidewalls, reducing trap sites and improving isolation between photodiode and pixel transistor regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If shallow trench isolation (STI) is used to isolate pixels and devices, then device isolation is achieved, but trap-assisted-tunneling increases and dark current rises
Solution Approach 1:
The patent applies parameter changes by modifying the trench geometry from a conventional rectangular cross-section to a polygonal cross-section with specific width variations at different depths. The trench width transitions from a first width at the top to a second width at an intermediate depth, creating a unique profile that reduces trap site formation while maintaining effective device isolation. This geometric parameter modification directly addresses the dark current issue without sacrificing isolation effectiveness.
2Measurement precision
If pixel size is reduced to increase the number of pixels on the wafer, then resolution is improved, but device isolation becomes more difficult
Solution Approach 1:
The patent applies local quality by creating a non-uniform trench width profile where the trench is narrower at certain depths and wider at others. This localized variation in trench geometry provides enhanced isolation effectiveness in critical regions while allowing for reduced pixel dimensions elsewhere. The polygonal cross-section creates different local isolation characteristics that maintain reliability even as overall pixel size decreases.
3Ease of manufacture
If plasma dry etching is used to form STI trenches, then trench formation is achieved, but silicon dangling bonds are created on trench walls
Solution Approach 1:
The patent converts the harmful effect of plasma etching by using the polygonal trench geometry to control where and how dangling bonds form. The specific width variations at different depths create a trench profile that minimizes the surface area exposed to plasma damage while maintaining etching effectiveness. This geometric approach transforms the inherent limitation of plasma etching into a manageable characteristic through clever structural design.
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 electrical isolation, reduces dark current, and maintains high image sensor resolution by minimizing trap sites and junction leakage, even in high-temperature conditions.
Implementation Method 1
a wet etching process to cure damage and achieve smoother sidewalls
Implementation Method 2
combining dry plasma etching to create the trench
Data Source
AI summary
A shallow trench isolation (STI) structure and method of fabrication includes forming a shallow trench isolation (STI) structure having a polygonal shaped cross-section in a semiconductor substrate of an image sensor includes a two-step etching process. The first step is a dry plasma etch that forms a portion of the trench to a first depth. The second step is a wet etch process that completes the trench etching to the desired depth and cures damage caused by the dry etch process. A CMOS image sensor includes a semiconductor substrate having a photodiode region and a pixel transistor region separated by a shallow trench isolation (STI) structure having a polygonal shaped cross-section.