EUVL Substrate TiO2 Gradient CTE Control
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Solution Overview
Problem
Conventional substrates for EUV lithography optical members exhibit non-zero coefficient of linear thermal expansion (CTE) due to temperature differences between the incidence and back surfaces during EUV light irradiation, leading to distortion and deformation.
Innovation Solution
A substrate with a silica glass containing TiO2, where the CTE is controlled to be substantially zero across the entire optical member by varying the Cross-Over Temperature (COT) between opposite surfaces, ensuring a difference of 5°C or more, and optimizing TiO2 concentration and fictive temperature to maintain CTE within 0±50 ppb/°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a conventional TiO2-SiO2 glass substrate with uniform COT is used, then the material has low CTE at controlled temperature, but the temperature difference between incidence and back surfaces during EUV irradiation causes non-zero CTE and dimensional instability
Solution Approach 1:
The patent applies local quality by creating a substrate with non-uniform TiO2 concentration distribution, where the TiO2 concentration varies from the incidence surface to the back surface. This gradient structure causes different regions of the substrate to have different COT values, with the incidence surface region having a higher COT than the back surface region. This local variation in material composition allows each region to compensate for temperature-induced expansion differences, achieving overall dimensional stability despite temperature gradients during EUV irradiation.
2Stability of the object's composition
If the TiO2 concentration is increased to lower CTE, then the CTE becomes more negative at low temperatures, but the temperature region where CTE is substantially zero becomes narrower
Solution Approach 1:
The patent applies parameter changes by systematically varying the TiO2 concentration across different regions of the substrate. Instead of using a uniform high TiO2 concentration that would narrow the zero-CTE temperature region, the invention uses a concentration gradient where TiO2 content increases from the back surface toward the incidence surface. This gradual parameter change allows the substrate to maintain CTE close to zero across a wide temperature range by distributing the thermal expansion compensation function across multiple regions with progressively different 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
The substrate maintains zero CTE across the entire optical member during EUV light irradiation, preventing distortion and deformation, and is suitable for use in EUV lithography tools such as masks and mirrors.
Implementation Method 1
a first principal surface and a second principal surface opposite to each other in the thickness direction, wherein the first principal surface and the second principal surface have a difference in Cross-Over Temperatures of 5°C. or more, wherein the Cross-Over Temperature refers to a temperature at which a coefficient of linear thermal expansion is 0 ppb/°C
Data Source
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AI summary
The present invention relates to a substrate for EUV lithography optical member, comprising a silica glass containing TiO2, in which the substrate has two opposite surfaces, and the substrate has temperatures at which a coefficient of linear thermal expansion (CTE) is 0 ppb/°C (Cross-Over Temperature: COT), and in which the two opposite surfaces have difference in the COTs of 5°C or more.