Amorphous Oxide Semiconductor Crystallization for Stable Transistors
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Solution Overview
Problem
The electric conductivity of oxide semiconductors varies due to deviations from stoichiometric composition and the presence of hydrogen or moisture, leading to unstable electric characteristics in transistors.
Innovation Solution
A semiconductor device is manufactured with an amorphous oxide semiconductor layer containing excess oxygen, which is crystallized using heat treatment between a silicon oxide film and an aluminum oxide film, resulting in a crystalline oxide semiconductor layer with c-axis alignment, reducing oxygen vacancies and impurity entry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an oxide semiconductor layer is formed with stoichiometric composition, then the electric conductivity is stable, but oxygen vacancies and impurity entry still occur leading to variation in electric characteristics
Solution Approach 1:
The patent applies preliminary action by forming an amorphous oxide semiconductor layer with excess oxygen content before crystallization. This excess oxygen is intentionally introduced in advance to compensate for oxygen vacancies that will occur during subsequent crystallization and device operation, thereby preventing electric characteristic variation before it happens.
Solution Approach 2:
The patent changes the oxygen content parameter from stoichiometric ratio to excess oxygen state. By controlling the oxygen content to be higher than the stoichiometric composition ratio (e.g., InGaZnO4 with x>4 in InGaZnOx), the material properties are modified to provide oxygen reservoir that compensates for oxygen loss during device operation, improving reliability.
2Reliability
If heat treatment is performed to crystallize the oxide semiconductor layer, then the electric characteristics stability improves, but hydrogen and moisture may enter the layer causing variation
Solution Approach 1:
The patent applies inert atmosphere by performing heat treatment in a controlled atmosphere (e.g., rare gas atmosphere like nitrogen or argon) to prevent hydrogen and moisture from entering the oxide semiconductor layer during crystallization. This inert environment protects the material from harmful impurities while allowing the desired crystallization process to occur.
Solution Approach 2:
The patent forms a protective film (aluminum oxide or nitrogen-containing oxide) over the oxide semiconductor layer before heat treatment. This preliminary protective layer acts as a barrier against hydrogen and moisture during the subsequent heat treatment process, preventing impurity entry while allowing the crystallization to proceed.
3Reliability
If an amorphous oxide semiconductor layer with excess oxygen is formed, then oxygen vacancies are reduced, but the layer lacks crystalline structure which affects performance
Solution Approach 1:
The patent applies phase transition by heating the amorphous oxide semiconductor layer to transform it from an amorphous state to a crystalline state (CAAC-oxide semiconductor). This phase transition occurs at temperatures between 400°C and 700°C, where the material reorganizes into a c-axis aligned crystalline structure while maintaining the excess oxygen content, thereby combining the benefits of both amorphous and crystalline structures.
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 stabilizes the electric characteristics of transistors by maintaining excess oxygen content and preventing hydrogen and moisture entry, thereby reducing variation in threshold voltage and enhancing reliability.
Implementation Method 1
heat treatment is performed so that at least part of the amorphous oxide semiconductor layer is crystallized and an oxide semiconductor layer which includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer is formed
Data Source
AI summary
In a process of manufacturing a transistor including an oxide semiconductor layer, an amorphous oxide semiconductor layer which includes a region containing excess oxygen as compared to a stoichiometric composition ratio of an oxide semiconductor in a crystalline state is formed over a silicon oxide film, an aluminum oxide film is formed over the amorphous oxide semiconductor layer, and then heat treatment is performed so that at least part of the amorphous oxide semiconductor layer is crystallized and an oxide semiconductor layer which includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer is formed.