EUV Mirror Thermal Control via Negative CTE Layer
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Solution Overview
Problem
EUV projection exposure apparatuses face challenges with mirror heating due to EUV radiation, leading to deformations and imaging aberrations, which are difficult to control and can damage the reflective layers, especially when rapid adjustments to the mirror surface profile are needed.
Innovation Solution
Incorporating a mirror arrangement with layers having negative and positive coefficients of thermal expansion, allowing for targeted local heat application to adjust the mirror surface profile without extensive heating, using IR radiation sources for contactless heat input and thermally insulating layers to control heat distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional cooling concepts (cooling fingers or cooling channels) are introduced into the mirrors, then the mirrors can be cooled, but parasitic deformations are introduced into the mirror structure
Solution Approach 1:
The harmful cooling structures (fingers or channels) are extracted from the mirror interior and replaced by an external laser heating system that acts on the mirror surface without penetrating into the mirror structure, thereby eliminating parasitic deformations while still enabling thermal control
Solution Approach 2:
A laser beam is introduced as an intermediary medium to transfer thermal energy to the mirror surface without requiring physical contact or internal structures. The laser heats the reflective layer directly, allowing precise thermal control without mechanical intervention that would cause deformations
2Temperature
If cooling channels are introduced into the mirrors, then cooling is achieved, but vibrations occur due to flow dynamics of the cooling liquid
Solution Approach 1:
The mechanical cooling system (pumps, channels, flowing liquid) is replaced by an optical heating system (laser). This substitution eliminates all mechanical components that could cause vibrations, while still achieving thermal control through non-contact laser heating of the mirror surface
3Shape
If extensive heating is applied to adjust the mirror surface profile, then the surface profile can be changed, but the thermal budget of the projection lens is exceeded and layer damage may occur
Solution Approach 1:
Instead of uniform heating, the laser system applies heat locally to specific regions of the mirror surface. By controlling the laser beam position and intensity distribution, only the required areas are heated to adjust the surface profile, minimizing overall thermal load and preventing layer damage while achieving the desired shape changes
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
Enables rapid adjustment of the mirror surface profile with reduced heat input, minimizing deformations and layer stress, and maintaining the thermal budget of the projection lens, allowing for precise control of the wavefront profile of EUV radiation.
Implementation Method 1
at least one mirror of the plurality of mirrors has at least one layer having a negative coefficient of thermal expansion
Implementation Method 2
heat is locally applied in a targeted manner to the at least one layer having a negative coefficient of thermal expansion
Data Source
Figure 1~2
Figure 3~4c
Figure 5~6b
AI summary
A mirror arrangement for an EUV projection exposure apparatus for microlithography comprises a plurality of mirrors each having a layer (32) which is reflective in the EUV spectral range and to which EUV radiation can be applied, and having a main body (34). In this case, at least one mirror (32) of the plurality of mirrors has at least one layer (36) comprising a material having a negative coefficient of thermal expansion. Moreover, a method for operating the mirror arrangement and a projection exposure apparatus are described. At least one heat source is arranged, in order to locally apply heat in a targeted manner to the at least one layer having a negative coefficient of thermal expansion of the at least one mirror.