Polycrystalline Silicon TFT Layer With Large Grains and Low Roughness
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Solution Overview
Problem
Active matrix organic light emitting display devices using amorphous silicon thin film transistors have low electron mobility, which is improved by replacing them with polycrystalline silicon transistors, but existing methods for manufacturing polycrystalline silicon layers often result in small grain sizes and high surface roughness, affecting transistor characteristics.
Innovation Solution
A method involving the formation of a polycrystalline silicon layer by cleaning and rinsing an amorphous silicon layer with hydrofluoric acid and hydrogenated deionized water, followed by irradiation with a laser beam of specific energy density and wavelength, to achieve large grain sizes and low surface roughness, thereby improving transistor characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If polycrystalline silicon is manufactured by conventional methods (CVD, sputtering, vacuum evaporation, or a-Si deposition followed by crystallization), then the material can be deposited, but the resulting grains have small sizes and high surface roughness, which deteriorates transistor characteristics
Solution Approach 1:
The patent changes the crystallization parameters by using laser irradiation with specific energy density (440-490 mJ/cm²) and wavelength (308 nm) to achieve optimal grain growth. This parameter optimization results in grains with average size of 400-800 nm and surface roughness of 4 nm or less, resolving the contradiction between manufacturing precision and device reliability
Solution Approach 2:
The patent replaces conventional thermal crystallization methods with laser-induced crystallization. The laser beam provides localized energy for crystallization without requiring high-temperature processing, enabling precise control over grain size and surface morphology while improving transistor characteristics
2Reliability
If amorphous silicon TFTs are used in pixel circuits, then the device can be manufactured, but the electron mobility is low (about 1 cm²/Vs or less)
Solution Approach 1:
The patent utilizes the phase transition from amorphous to polycrystalline silicon through laser-induced crystallization. This phase transition transforms the material structure to achieve high electron mobility (greater than 100 cm²/Vs) while maintaining the ease of amorphous silicon deposition processes
Solution Approach 2:
The patent performs preliminary cleaning of the amorphous silicon layer with hydrofluoric acid and hydrogenated deionized water before laser irradiation. This preliminary action removes surface contaminants and oxides, preparing the surface for optimal crystallization and ensuring high electron mobility in the resulting polycrystalline silicon TFTs
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 results in thin film transistors with enhanced electric charge mobility and reduced hysteresis, characterized by large grain sizes and small surface roughness, leading to improved display device performance.
Implementation Method 1
irradiating the amorphous silicon layer with a laser beam having an energy density in a range of about 440 mJ/cm2 to about 490 mJ/cm2 to form the polycrystalline silicon layer
Implementation Method 2
cleaning the amorphous silicon layer with hydrofluoric acid
Data Source
AI summary
A display device may include a thin film transistor disposed on a substrate, and a display element electrically connected to the thin film transistor. The thin film transistor may include an active pattern including polycrystalline silicon, a gate insulation layer disposed on the active pattern, and a gate electrode disposed on the gate insulation layer. An average value of grain sizes of the active pattern may be in a range of about 400 nm to about 800 nm. An RMS value of a surface roughness of the active pattern may be about 4 nm or less. A method of manufacturing a polycrystalline silicon layer may include cleaning an amorphous silicon layer with hydrofluoric acid, rinsing the amorphous silicon layer with hydrogenated deionized water, and irradiating the amorphous silicon layer with a laser beam having an energy density of about 440 mJ/cm2 to about 490 mJ/cm2.