Microcrystalline TFT Nitrogen Diffusion Suppression
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Solution Overview
Problem
Inverted staggered thin film transistors with silicon nitride gate insulating films face issues with negative threshold voltage offset due to nitrogen impurity diffusion, leading to circuit operation failures and increased optical leak currents, especially when used in liquid crystal display devices with backlight irradiation.
Innovation Solution
A microcrystalline semiconductor layer with a higher oxygen concentration than nitrogen in the vicinity of the nitride film interface is used, and a silicon nitride gate insulating film with an N/Si composition ratio of 1.0 or less is employed to suppress nitrogen diffusion and improve crystallization rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a silicon nitride gate insulating film is used in inverted staggered TFTs, then the manufacturing process is simplified and mass production is improved, but nitrogen impurity diffusion occurs causing negative threshold voltage offset and circuit operation failures
Solution Approach 1:
An oxygen plasma treatment is performed on the gate insulating film surface before depositing the microcrystalline semiconductor layer. This preliminary action introduces oxygen into the interface region, creating oxygen-rich zones that prevent nitrogen diffusion from the silicon nitride gate insulating film into the semiconductor layer, thereby preventing negative threshold voltage offset while maintaining the simplified manufacturing process
Solution Approach 2:
The oxygen concentration in the microcrystalline semiconductor layer is controlled to be higher than nitrogen concentration at the interface with the gate insulating film. This parameter change (oxygen/nitrogen concentration ratio) fundamentally alters the interface chemistry, suppressing nitrogen diffusion and stabilizing the threshold voltage while maintaining device reliability
2Device complexity
If microcrystalline silicon is directly deposited by plasma CVD method, then the manufacturing process is simplified compared to laser annealing, but an amorphous incubation layer forms reducing crystallization rate and deteriorating channel characteristics
Solution Approach 1:
Oxygen plasma treatment is applied to the gate insulating film surface before microcrystalline silicon deposition. This preliminary action modifies the substrate surface to promote direct crystallization, eliminating or reducing the formation of the amorphous incubation layer and enabling direct deposition of high-quality microcrystalline silicon with improved channel characteristics
Solution Approach 2:
The oxygen concentration at the interface is increased through plasma treatment, which changes the deposition conditions and promotes crystallization during the plasma CVD process. This parameter change enables direct formation of microcrystalline structure without requiring subsequent laser annealing, simplifying the manufacturing process while maintaining high crystallization rate
3Reliability
If nitrogen diffusion is suppressed by reducing nitrogen content in gate insulating film, then threshold voltage offset is reduced, but the dielectric constant and insulation performance may be affected
Solution Approach 1:
Oxygen acts as an intermediary substance at the interface between the silicon nitride gate insulating film and the microcrystalline semiconductor layer. The oxygen-rich interface region serves as a barrier that prevents nitrogen diffusion from the gate insulating film into the semiconductor, maintaining threshold voltage stability without requiring reduction of nitrogen content in the gate insulating film itself, thus preserving its dielectric properties
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 effectively suppresses negative threshold voltage offsets and optical leak currents, ensuring reliable electrical characteristics for both pixel and driving transistors, suitable for liquid crystal display devices with backlight irradiation.
Implementation Method 1
the microcrystalline semiconductor layer contains oxygen at a concentration higher than that of contained nitrogen in at least the vicinity of the interface with the nitride film, the nitrogen being diffused from the nitride film
Implementation Method 2
nitrogen being diffused from the nitride film
Implementation Method 3
researches have been conducted to directly deposit microcrystalline silicon by the plasma CVD method
Data Source
AI summary
An exemplary aspect of the present invention is a thin film transistor including: a gate electrode formed on a substrate; a gate insulating film that includes a nitride film and covers the gate electrode; and a semiconductor layer that is disposed to be opposed to the gate electrode with the gate insulating film interposed therebetween, and has a microcrystalline semiconductor layer formed in at least an interface in contact with the nitride film, in which the microcrystalline semiconductor layer contains oxygen at a concentration higher than that of contained nitrogen in at least the vicinity of the interface with the nitride film, the nitrogen being diffused from the nitride film.