Maskless Laser Crystallization for Polysilicon TFT Grain Alignment
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Solution Overview
Problem
Current methods for crystallizing amorphous silicon into polycrystalline silicon for thin film transistors in flat panel displays face challenges in achieving high electron mobility and uniformity, particularly in organic light emitting diode (OLED) displays, due to non-uniform grain boundaries and the complexity and cost of using masks in laser-based crystallization processes.
Innovation Solution
A directional crystallization method using a laser beam with a small irradiation width and length, eliminating the need for masks, which forms polycrystalline silicon with grain boundaries parallel to the crystal growth direction, resulting in improved surface roughness and reduced defects, thereby enhancing the electrical characteristics of thin film transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If mask-based laser crystallization is used, then crystallization can be achieved, but process complexity and cost increase
Solution Approach 1:
The patent removes the mask component from the laser crystallization process. By using a maskless laser crystallization method, the process complexity and cost are reduced while maintaining crystallization quality through direct laser irradiation on the amorphous silicon layer.
Solution Approach 2:
The amorphous silicon layer undergoes self-crystallization through direct laser irradiation without requiring external mask structures to define the crystallization regions. The laser beam directly induces crystallization in the targeted areas, eliminating the need for mask-based patterning.
2Quantity of substance
If conventional laser crystallization is used, then polycrystalline silicon can be formed, but grain boundaries are non-uniform and perpendicular to crystal growth direction
Solution Approach 1:
The patent changes the laser irradiation parameters, specifically using a linear scanning pattern with controlled beam width and scanning speed. This parameter modification causes grain boundaries to form parallel to the crystal growth direction instead of perpendicular, improving grain boundary uniformity and reducing defects.
Solution Approach 2:
The laser beam is scanned in a periodic linear motion across the amorphous silicon layer, creating uniform grain boundaries parallel to the scanning direction. This periodic scanning action ensures consistent crystal growth and uniform grain structure throughout the crystallized region.
3Reliability
If high temperature crystallization is used, then polycrystalline silicon can be formed, but glass substrate cannot endure the temperature
Solution Approach 1:
The patent utilizes rapid phase transition of the laser beam from continuous to pulsed mode, delivering high energy density in short durations. This allows localized melting and crystallization of the amorphous silicon layer at temperatures that the glass substrate can tolerate, achieving polycrystalline silicon formation without substrate damage.
Solution Approach 2:
The laser is operated in pulsed mode with periodic on-off cycles, allowing the substrate to cool between pulses. This periodic heating prevents excessive temperature accumulation on the glass substrate while still providing sufficient energy for amorphous silicon to polycrystalline silicon transformation.
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 method produces polycrystalline silicon with large grains and minimal surface roughness, ensuring high electron mobility and uniformity, simplifying the process and reducing costs by eliminating the need for masks, while maintaining excellent electrical characteristics in thin film transistors and OLED displays.
Implementation Method 1
a directional crystallization method using a laser beam with a small irradiation width and length
Implementation Method 2
crystallize the amorphous silicon into polycrystalline silicon having a grain boundary parallel to a crystal growth direction
Implementation Method 3
directly irradiating a laser beam having a specific width and length on amorphous silicon to crystallize the amorphous silicon
Data Source
AI summary
A thin film transistor and a method of fabricating the same include: a semiconductor layer having a grain boundary disposed in a crystal growth direction and having a variation in height of a top surface of 15 nm or less formed by a thin beam directional crystallization method. Also, an organic light emitting diode (OLED) display device comprising the thin film transistor is provided and has excellent characteristics fabricated by a simple process. Also, a flat panel display device and a method of fabricating the same are provided and include: a polycrystalline silicon layer in a pixel region; and a polycrystalline silicon layer in a peripheral region formed by the thin beam directional crystallization method. Also, a semiconductor device and a method of fabricating the same include: an intrinsic region of a semiconductor layer in the photodiode region formed by the thin beam directional crystallization method.