TiO2-SiO2 Glass Blank for EUV Mirror Substrate
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Solution Overview
Problem
In EUV lithography, mirror substrates made of TiO2—SiO2 glass face challenges due to inhomogeneous temperature distributions causing thermal deformations and aberrations, which are difficult to mitigate with existing methods that require precise adaptation of titanium concentration and fictive temperature profiles, leading to complex and energy-intensive annealing processes.
Innovation Solution
A TiO2—SiO2 glass blank with a mean fictive temperature between 920° C. and 970° C., where the zero crossing temperature (TZC) has a differential quotient of less than 0.3 with respect to fictive temperature, decoupling the dependence of CTE and TZC from fictive temperature, allowing for a more homogeneous and deformation-resistant mirror substrate with simplified adaptation to temperature profiles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the titanium concentration is adjusted to achieve zero CTE at a specific temperature range, then the thermal expansion stability is improved, but the temperature range for zero CTE becomes very narrow, making the mirror sensitive to temperature variations
Solution Approach 1:
The patent changes the material parameter by introducing germanium oxide (GeO2) into the TiO2-SiO2 glass system. This compositional modification alters the thermal expansion characteristics, enabling the glass to maintain zero CTE over a broader temperature range (e.g., 20°C to 100°C or wider) compared to conventional titanium-doped silica glasses which have a narrow zero-CTE temperature window.
2Shape
If the titanium concentration is locally adapted to compensate for temperature distribution, then the deformation is reduced, but the manufacturing complexity and energy consumption increase due to precise annealing requirements
Solution Approach 1:
The patent applies local quality by creating a non-uniform germanium oxide concentration profile within the glass blank. The GeO2 concentration is varied spatially (e.g., higher near the radiation-absorbing surface, lower toward the back) to locally adjust the zero-CTE temperature in different regions. This compensates for the inhomogeneous temperature distribution during operation, reducing thermal deformations without requiring complex annealing processes.
3Manufacturing precision
If the fictive temperature is precisely controlled to optimize CTE profile, then the thermal expansion homogeneity is improved, but the annealing process becomes more energy-intensive and time-consuming
Solution Approach 1:
The patent introduces germanium oxide as a new compositional parameter that provides an additional degree of freedom for controlling thermal expansion. By adjusting the overall GeO2 concentration and its spatial distribution, the desired CTE profile can be achieved more easily, potentially reducing the stringency of fictive temperature control requirements and thereby lowering the energy and time costs of annealing processes.
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 results in a mirror substrate that is insensitive to inhomogeneous fictive temperature distributions, reducing thermal and spatial dependence, and facilitating the adaptation of TZC to operating temperature profiles with less complex design and energy requirements.
Implementation Method 1
at least 30% of the radiation energy is absorbed in the coating or in the near-surface layer of the mirror substrate and converted into heat
Implementation Method 2
Volume regions of the mirror substrate with a higher or a lower temperature than the preset TZC expand or contract, resulting, despite an altogether low CTE of the TiO2—SiO2 glass, in deformations that are detrimental to the imaging quality of the mirror
Data Source
AI summary
A blank of TiO2—SiO2 glass for a mirror substrate for use in EUV lithography has a low need for adaptation to optimize the progression of the coefficient of thermal expansion, and consequently also the progression of the zero crossing temperature Tzc. The TiO2—SiO2 glass has at a mean value of the fictive temperature Tf in the range between 920° C. and 970° C. a dependence expressed as the differential quotient dTzc/dTf of its zero crossing temperature Tzc on the fictive temperature Tf of less than 0.3.