Rigidly Connected EUV Mirrors for Stiffness
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Solution Overview
Problem
High-aperture microlithographic projection exposure apparatuses face challenges in maintaining optical element stability and reducing parasitic moments or forces, particularly due to the thinness of mirrors used in the EUV range, which affects imaging properties and stiffness.
Innovation Solution
The optical arrangement involves a rigid connection between two mirrors, either monolithic or connected via a glass ceramic ring, to increase stiffness and natural frequency, reducing the impact of parasitic forces and maintaining imaging quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If mirrors are made thinner to reduce mass and allow smaller spacing between mirror and wafer, then the numerical aperture can be increased, but the stiffness and natural frequency of the mirror decrease, leading to increased sensitivity to parasitic forces and vibrations
Solution Approach 1:
The patent merges two mirrors (first mirror and second mirror) into a single rigidly connected unit. This combination increases the overall stiffness and mass of the optical element, thereby raising the natural frequency and reducing sensitivity to vibrations while maintaining the required small spacing to the wafer for high numerical aperture operation
2Length of stationary object
If mirrors are made thinner to reduce mass, then the spacing between mirror and wafer can be reduced, but the mirror becomes more susceptible to parasitic moments and forces from external vibrations
Solution Approach 1:
By rigidly connecting two mirrors into one unit, the patent increases the moment of inertia and structural stability of the optical element. This allows the use of thinner mirrors with smaller spacing to the wafer while maintaining adequate resistance to parasitic moments and external vibrations through the combined structure's higher natural frequency
3Adaptability or versatility
If all mirrors are coupled to a carrier structure by actuators without rigid solid-body connection, then each mirror can be independently actuated, but the overall system complexity increases and the mirrors lack sufficient stiffness
Solution Approach 1:
The patent combines two mirrors into a rigidly connected unit, reducing the number of separate actuation systems needed. This merger simplifies the overall connection structure while maintaining adequate actuation capability through the 6 remaining degrees of freedom of the combined unit, and significantly increases stiffness compared to individually mounted thin mirrors
Data Source
AI summary
The disclosure relates to an optical arrangement in a projection objective of a microlithographic projection exposure apparatus which is designed for operation in EUV. The optical arrangement includes first and second mirrors that are in direct succession to each other along the projection beam direction. The second mirror is rigidly connected to the first mirror.