Image Sensor Dual Fixed Charge Films for Dark Current and Moisture
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Solution Overview
Problem
High-resolution image sensors face issues with dark current, dark level defects, white spot defects, and blooming due to thermal electrons, and are prone to moisture penetration, which affects their density and reliability.
Innovation Solution
The image sensor incorporates a first hole accumulation layer as an anti-reflection layer and a second hole accumulation layer as a moisture protection layer, using metal oxides like hafnium oxide and aluminum oxide, respectively, with an insulating silicon oxide layer to fill pixel isolation trenches and reduce defect occurrences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a single fixed charge film is used for anti-reflection, then light-receiving efficiency is improved, but moisture penetration occurs causing defects
Solution Approach 1:
The patent divides the protective film structure into two separate fixed charge films: a first fixed charge film (e.g., hafnium oxide) serving as the anti-reflection layer for improved light-receiving efficiency, and a second fixed charge film (e.g., aluminum oxide) serving as the moisture protection layer for enhanced reliability. This segmentation allows each film to specialize in one function without compromising the other.
Solution Approach 2:
The patent employs a composite structure consisting of two different metal oxide materials with distinct properties. The first fixed charge film uses high-k metal oxide (such as hafnium oxide) optimized for anti-reflection characteristics, while the second fixed charge film uses metal oxide (such as aluminum oxide) optimized for moisture barrier properties. This composite approach combines the advantages of different materials to achieve both light-receiving efficiency and moisture protection.
2Measurement precision
If high-resolution pixel density is increased, then image quality is improved, but thermal electron generation increases causing dark current and defects
Solution Approach 1:
The patent applies preliminary anti-action by forming the first fixed charge film with positive fixed charges directly on the pixel region surface before light reception occurs. This pre-established positive charge layer creates an electric field that repels thermally generated electrons (which are negatively charged), preventing them from reaching the photoelectric conversion elements and causing dark current or white spot defects. This proactive measure counteracts the harmful effect of thermal electron generation that increases with higher pixel density.
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 configuration effectively suppresses dark current and moisture penetration, enhancing the image sensor's density and reliability by improving light-receiving efficiency and preventing defects, thus improving photoelectric conversion characteristics.
Implementation Method 1
a first hole accumulation layer between a lower portion of the first fixed charge film and upper portions of the photoelectric conversion parts
Implementation Method 2
a second fixed charge film directly on the first fixed charge film and an inner wall of the pixel isolation trench
Implementation Method 3
a first fixed charge film directly on the surface of the substrate... serving as an anti-reflection layer
Data Source
AI summary
An image sensor includes a substrate including a plurality of photoelectric conversion parts and a pixel isolation trench extending from a surface of the substrate between the photoelectric conversion parts, a first fixed charge film directly on the surface of the substrate, a second fixed charge film directly on the first fixed charge film and an inner wall of the pixel isolation trench, and an insulating layer directly on the second fixed charge film, the insulating layer configured to fill the pixel isolation trench.