TiO2-Doped Quartz Substrates for EUV Lithography Thermal Deformation Control
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Solution Overview
Problem
EUV lithography projection lenses face challenges with temperature-dictated deformations due to the stringent requirements for low coefficient of thermal expansion (CTE) in substrates, which can lead to image aberrations and reduced performance.
Innovation Solution
The use of TiO2-doped quartz glass substrates with different zero crossing temperatures and gradient signs in optical elements of a projection lens, adjusted through varying TiO2 proportions and heat treatment processes, allows for individual adaptation to operating conditions, reducing thermal deformations and aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional TiO2-doped quartz glass with positive CTE gradient is used, then manufacturing is simplified, but thermal deformations increase at operating temperatures above zero crossing temperature
Solution Approach 1:
The patent applies parameter changes by inverting the sign of the CTE gradient through controlled modification of TiO2 concentration and heat treatment parameters. By adjusting the TiO2 proportion to specific ranges (8-12% for positive gradient, >12% for negative gradient) and applying specific heat treatment regimes, the CTE gradient is transformed from positive to negative, enabling the substrate to compensate for thermal expansions at operating temperatures.
Solution Approach 2:
The patent directly addresses thermal expansion by engineering substrates with negative CTE gradient that exhibit thermal contraction behavior opposite to conventional materials. This allows the substrate to compensate for thermal expansions of other optical components or to maintain dimensional stability when operating temperatures deviate from the zero crossing temperature, thereby reducing thermal deformations.
2Manufacturing precision
If TiO2 proportion is increased to achieve negative CTE gradient, then thermal deformation compensation improves, but risk of Ti-rich particle crystallization increases
Solution Approach 1:
The patent resolves this contradiction through precise parameter control, establishing specific TiO2 concentration ranges (8-12% for positive gradient, >12% for negative gradient) and corresponding heat treatment parameters. These controlled parameter changes enable achievement of negative CTE gradient while preventing Ti-rich particle crystallization by maintaining TiO2 content below the crystallization threshold and applying appropriate thermal histories.
Solution Approach 2:
The patent employs composite material design by creating TiO2-doped quartz glass with optimized composition ratios. The specific TiO2 concentration ranges are carefully selected to achieve the desired negative CTE gradient while maintaining the glass phase homogeneity and preventing secondary phase formation, effectively creating a composite structure at the molecular level that balances thermal properties with material stability.
3Manufacturing precision
If zero crossing temperature is adjusted to match operating temperature, then thermal expansion is minimized, but CTE gradient becomes positive reducing temperature adaptability
Solution Approach 1:
The patent applies inversion by reversing the conventional approach: instead of accepting a positive CTE gradient and hoping the zero crossing temperature matches operating conditions, the patent inverts the gradient sign to negative. This inversion ensures that the substrate exhibits compensatory thermal contraction behavior when operating temperatures deviate from the zero crossing temperature, thereby enhancing temperature range adaptability while maintaining low thermal expansion at the operating point.
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 enhances the performance of the optical arrangement by minimizing image aberrations and maintaining low CTE gradients, ensuring optimal operation across varying temperatures.
Implementation Method 1
The substrate materials used there typically comprise two constituents, the coefficients of thermal expansion of which have a mutually opposite dependence on temperature, such that the coefficients of thermal expansion mutually compensate for one another virtually completely in the range of the operating temperature.
Implementation Method 2
The derivative of the thermal expansion of zero expansion materials with respect to temperature, i.e. the coefficient of thermal expansion, is approximately linearly dependent on temperature in this range and changes sign at the temperature at which the thermal expansion is extremal, for which reason this temperature is designated as zero crossing temperature.
Implementation Method 3
It is known from US 2011/0048075 A1 that the zero expansion temperature of TiO2-doped quartz glass (without exceeding the softening point) can be set to a specific value within a predefined range of values by means of a final heat treatment step.
Implementation Method 4
through a suitable choice of the temperature/time curve during a heat treatment process for the glass, the fictive temperature of the glass and thus not only the zero crossing temperature but also the gradient of the coefficient of thermal expansion at the zero crossing temperature can be set.
Implementation Method 5
a first optical element, which comprises a surface that reflects EUV radiation
Data Source
AI summary
An optical arrangement, e.g. projection lens, for EUV lithography, provided with: a first optical element (22) having a reflective surface (31a) and a first substrate (32) composed of TiO2-doped quartz glass, which has a temperature-dependent coefficient of thermal expansion having a zero crossing at a first zero crossing temperature (TZC1), and a second optical element (24) having a reflective surface (36a) and a second substrate (37) composed of TiO2-doped quartz glass, which has a temperature-dependent coefficient of thermal expansion having a zero crossing at a second zero crossing temperature (TZC2), which is different from the first. A gradient of the coefficient of thermal expansion of the first substrate (32) at the first zero crossing temperature (TZC1) and/or a gradient of the coefficient of thermal expansion of the second substrate (37) at the second zero crossing temperature (TZC2) have/has a negative sign.