Low-Compaction Glass Sheet for LTPS TFT Displays
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Solution Overview
Problem
Glass substrates for low-temperature polysilicon TFT displays experience undesirable compaction when rapidly cooled and reheated, affecting pixel registry and TFT performance, as existing glass sheets made by fusion down-draw processes are not suitable for LTPS technology due to excessive compaction.
Innovation Solution
A process involving a glass material with an anneal point of at least 765° C. and a controlled cooling rate of at least 5° C./s from 1.0×10^10 to 1.0×10^15 poise viscosity, resulting in a glass sheet with minimal compaction (|CM675|≦175 ppm), suitable for LTPS TFT displays without the need for secondary annealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If glass melt is rapidly cooled from high temperature, then production efficiency is improved, but compaction occurs when reheated to intermediate temperatures
Solution Approach 1:
The patent applies parameter changes by modifying the cooling rate parameter to be at least 5°C/s through the critical viscosity range (1.0×10^10 to 1.0×10^15 poise). This specific parameter change in the cooling process transforms the glass structure to prevent subsequent compaction when reheated, while maintaining high production efficiency.
Solution Approach 2:
The patent implements preliminary action by performing the controlled rapid cooling at least 5°C/s during the initial glass sheet formation process. This preliminary cooling action sets the glass in a state that resists compaction during subsequent LTPS processing, eliminating the need for secondary annealing steps.
2Ease of manufacture
If glass substrate temperature is raised to 600°C or more for LTPS processing, then TFT fabrication is enabled, but substantial compaction occurs without intermediate annealing
Solution Approach 1:
The patent changes the thermal history parameters of the glass by implementing rapid cooling at ≥5°C/s through the critical viscosity range. This parameter change modifies the glass structure to be stable at LTPS processing temperatures (600°C or more), enabling TFT fabrication without intermediate annealing and maintaining pixel registry.
Solution Approach 2:
The patent converts the potential harm of rapid cooling (which could cause stress) into a benefit by controlling the cooling rate through the critical viscosity range. This transforms the rapid cooling process into a method that actually prevents compaction during subsequent high-temperature LTPS processing, enabling direct fabrication without annealing.
3Productivity
If glass sheets are made by conventional fusion down-draw processes, then production is economical, but compaction is too large for LTPS technology
Solution Approach 1:
The patent applies parameter changes to the cooling rate parameter in the fusion down-draw process, specifying a minimum cooling rate of 5°C/s through the critical viscosity range. This parameter modification maintains the economical production benefits of fusion down-draw while achieving the low compaction required for LTPS technology.
Solution Approach 2:
The patent makes the fusion down-draw process multi-functional by adjusting the cooling rate parameter to simultaneously achieve economical production and low compaction. This modified process serves both the economic production requirement and the technical requirement for LTPS compatibility, eliminating the need for separate annealing operations.
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 process produces glass sheets with low compaction, suitable for LTPS TFT displays, enabling economic volume production and maintaining pristine surface quality without additional annealing or surface finishing, thus addressing the compaction issues in existing glass substrates.
Implementation Method 1
when a rapidly quenched glass is reheated to intermediate temperatures, the thermally-populated vibrational states allow for relaxation of atoms into positions that better satisfy their individual and collective bonding requirements. Since this is typically accompanied by an irreversible decrease in the physical dimensions of a bulk piece of glass, thermal relaxation upon reheating is said to produce compaction of the glass.
Implementation Method 2
When glass melt is cooled rapidly from high temperature, the movement of atoms within the cooling liquid slows down with decreasing temperature and eventually diminishes to oscillations about fixed positions due to normal thermal population of vibrational states.
Data Source
AI summary
A process for making glass sheet with low compaction suitable for high temperature applications, such as low-temperature polysilicon-based TFT displays, and glass sheets thus made. The glass sheet desirably has an anneal point of at least 765° C., a CTE at most 42×10−7/° C. The process involves cooling the glass melt form a temperature corresponding to a viscosity of 1.0×1010 poise to a temperature corresponding to a viscosity of 1.0×1015 poise at a cooling rate CR, where CR≧5° C./second. The absolute value of the measured compaction of the glass sheet desirably is at most 175 ppm upon being re-heated to 675° C. for a period of time.