Attenuated Phase Shift Mask for Deep Focus TFT Exposure
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Solution Overview
Problem
The challenge in producing thin film transistors (TFTs) on glass base plates for display panels is the difficulty in achieving a deep enough depth of focus during the exposure process due to significant variations in glass plate thickness, leading to issues with forming perfectly penetrating contact holes and maintaining resolution.
Innovation Solution
The use of an attenuated type phase shift mask with specific numerical aperture and coherence factor values, along with a projection optical system of equal magnification, to achieve a deeper depth of focus suitable for glass base plates with great thickness variations, allowing for the formation of rectangular or oval-shaped contact holes that penetrate the photoresist layer effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the resolution of the exposure equipment is raised to form finer TFT patterns, then the manufacturing precision is improved, but the depth of focus becomes shallower
Solution Approach 1:
The patent changes the optical parameters of the exposure system by using an attenuated type mask with specific transmittance (1-10%) and controlling the numerical aperture (NA) and coherence factor (σ) to specific ranges. This allows achieving both high resolution and sufficient depth of focus by optimizing the interaction between mask transmittance, NA, and σ parameters
Solution Approach 2:
The attenuated type mask serves as an intermediary element between the light source and the photoresist. By controlling the mask's light attenuation properties, it mediates the trade-off between resolution and depth of focus, enabling both requirements to be satisfied simultaneously
2Ease of manufacture
If a conventional exposure method is used on glass base plates with thickness variation, then the process is simple, but the photoresist cannot be properly exposed due to depth of focus issues
Solution Approach 1:
The patent modifies the exposure parameters by using an attenuated type mask with controlled transmittance (1-10%) and specific NA and coherence factor ranges. This allows the exposure process to accommodate glass base plate thickness variations while maintaining contact hole formation quality
Solution Approach 2:
The attenuated type mask provides beforehand cushioning by pre-controlling the light distribution and exposure intensity. This compensates for the thickness variation of the glass base plate before the exposure actually occurs, ensuring uniform contact hole formation across the entire substrate
3Reliability
If the photoresist film thickness is increased to accommodate depth of focus requirements, then the depth of focus is improved, but the resolution capability deteriorates
Solution Approach 1:
The patent changes the optical parameters (NA and coherence factor σ) in conjunction with the attenuated type mask transmittance to achieve both adequate depth of focus and high resolution, eliminating the need to increase photoresist thickness
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 enables the formation of contact holes that perfectly penetrate the photoresist layer, even on glass base plates with significant thickness variations, thereby improving the yield ratio of TFT production and maintaining high resolution.
Implementation Method 1
attenuated type phase shift mask
Implementation Method 2
attenuated type mask (to be called an 'ATT mask' herein)
Implementation Method 3
projection optical system of equal magnification
Data Source
AI summary
A method of producing a thin film transistor comprises irradiating a resist on a glass base plate with a ray from a light source through a mask and, thereafter, developing the resist to form contact holes, using an i-ray as the ray.