Low Expansion Glass Substrate for EUVL Mask Distortion Control
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Solution Overview
Problem
Current EUVL systems operating at or below 120 nm face challenges with substrate distortion due to thermal expansion issues, as existing low expansion materials like ULE and ZERODUR glass have limitations in maintaining zero coefficient of thermal expansion (CTE) across the mask thickness, leading to pattern distortion under EUV radiation exposure.
Innovation Solution
A low expansion glass substrate comprising titania and silica with a thermal expansivity average gradient less than 1 ppb/°C/°C and a zero-CTE crossover greater than 30°C is developed, minimizing CTE gradient across the mask thickness, thereby reducing distortion. The substrate's composition, with titania content between 7.4 wt% to 8.5 wt%, and optional additional dopants, ensures a near-zero CTE at the surface and through the mask thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If conventional low expansion materials like ULE or ZERODUR glass are used for EUVL mask substrates, then the substrate maintains low overall thermal expansion, but the CTE gradient through the mask thickness causes distortion of the written surface under thermal gradients
Solution Approach 1:
The patent applies local quality by creating a glass composition with non-uniform dopant distribution through controlled cooling rates. The glass exhibits different CTE properties at different depths: the surface region has CTE ≤ 20 ppb/°C while the bulk has CTE ≤ 50 ppb/°C. This gradient structure compensates for thermal distortion at the critical surface region where the mask pattern is written, while the bulk provides overall dimensional stability.
Solution Approach 2:
The patent changes the CTE parameter distribution through controlled variation of cooling rates during glass fabrication. By adjusting the cooling rate from 10°C/hr to 1000°C/hr, the patent achieves different CTE gradients in the glass structure. This parameter change allows optimization of the CTE profile to minimize surface distortion under thermal gradients while maintaining low overall expansion.
2Manufacturing precision
If the glass composition is optimized for near-zero CTE at the surface, then surface distortion is minimized, but achieving uniform composition throughout the bulk becomes difficult
Solution Approach 1:
The patent deliberately creates local quality differences in the glass composition through controlled cooling. The surface region develops different dopant concentration and CTE properties compared to the bulk. This non-uniform composition is achieved by controlling the cooling rate to create a gradient structure where the surface has optimized CTE ≤ 20 ppb/°C while the bulk maintains stability with CTE ≤ 50 ppb/°C.
Solution Approach 2:
The patent applies preliminary action by controlling the cooling rate during the glass fabrication process to pre-establish the desired CTE gradient before the mask is even used. The glass is cooled at controlled rates (10-1000°C/hr) to create the optimal CTE profile in advance, ensuring surface stability is built into the material structure before deployment in the EUVL system.
3Stability of the object's composition
If higher titania content is used to reduce CTE, then the CTE decreases, but the glass becomes more difficult to manufacture with controlled composition
Solution Approach 1:
The patent changes the manufacturing parameters by controlling the cooling rate (10-1000°C/hr) to achieve the desired CTE gradient without requiring excessive titania content. This allows the surface region to develop the necessary low CTE ≤ 20 ppb/°C through controlled phase separation and dopant redistribution during cooling, rather than requiring uniformly high titania content throughout the bulk, which would complicate manufacturing.
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 low expansion glass substrate effectively minimizes thermal-induced distortion of the mask pattern by maintaining a low CTE gradient, ensuring precise pattern replication on silicon wafers even under significant thermal gradients, thus enhancing the reliability of EUVL systems at extreme ultraviolet wavelengths.
Implementation Method 1
it is important that the material used for the substrate has a low coefficient of thermal expansion so that the substrate does not distort under exposure to EUV radiation
Implementation Method 2
having a zero-CTE crossover greater than 30°C such that the CTE at or near the surface of a mask/optic is less than 20 ppb/°C and the CTE gradient across the thermally non-uniform dimension is less than 1 ppb/°C/°C
Implementation Method 3
To minimize distortion of the written surface, it is desirable to minimize the thermal gradient through the mask thickness during exposure
Implementation Method 4
the thermal gradient through the mask thickness during exposure
Data Source
AI summary
A low expansion glass substrate includes titania and silica and has a thermal expansivity with an average gradient less than 1 ppb/° C./° C. in a temperature range of 19° C. to 25° C.