EUV Mirror Surface Correction with Vacuum-Index Intermediary
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Solution Overview
Problem
EUV multilayer-film reflective mirrors in EUVL systems face challenges in maintaining accurate surface profiles to minimize wavefront aberrations and prevent surface degradation, which affects optical performance.
Innovation Solution
A multilayer-film reflective mirror design featuring a base with alternating first and second layers, a third layer with a refractive index matching that of a vacuum, and a protective layer to maintain optical characteristics while preventing surface degradation, achieved through localized layer-shaving or addition processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a protective layer is applied to cover the reflective surface of the multilayer-film mirror, then surface degradation is suppressed, but optical characteristics are adversely changed
Solution Approach 1:
The patent introduces an intermediary substance (liquid or gas) with refractive index matching the vacuum to form a third layer between the multilayer film and protective layer. This intermediary layer acts as a mediator that allows the protective layer to exist without directly contacting and degrading the multilayer film surface, while maintaining optimal optical characteristics through refractive index matching.
Solution Approach 2:
The patent changes the refractive index parameter of the intermediate layer to match that of vacuum, thereby optimizing optical performance. By selecting materials with specific refractive indices (e.g., fluorinated liquids or gases close to 1.0), the system achieves both surface protection and maintained optical characteristics.
2Manufacturing precision
If layer-shaving is performed to correct surface figure errors, then wavefront aberration is reduced, but surface vulnerability to degradation increases
Solution Approach 1:
The patent applies protective layers and intermediary substances before the multilayer film surface is exposed to degrading environments. By pre-establishing the protective configuration after layer-shaving correction but before operational use, the system preserves the corrected surface figure while preventing future degradation.
Solution Approach 2:
The intermediary substance creates a protective barrier between the shaved multilayer film surface and the external environment, allowing the precisely corrected surface to maintain its accuracy without direct exposure to degrading factors.
3Manufacturing precision
If no protective layer is used on the multilayer-film mirror, then optical characteristics are maintained, but surface degradation occurs
Solution Approach 1:
The patent inserts an intermediary layer that optically behaves like vacuum (refractive index ≈ 1.0) while providing physical protection. This allows the system to maintain optical characteristics as if no layer were present, while actually gaining surface protection against degradation.
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 solution ensures desired optical performance with reduced wavefront errors and surface degradation, enabling precise EUV light reflection and extended mirror lifespan.
Implementation Method 1
In EUV light reflected from a multilayer-film mirror, the respective phases of multiple fronts of weakly reflected light at layer interfaces are superposed constructively to yield high overall reflectance.
Implementation Method 2
The top surface of the multilayer film has an irregular surface profile and is capable of reflecting incident extreme ultraviolet (EUV) light.
Implementation Method 3
A third layer, covering the top surface of the multilayer film, is formed of a substance having substantially the same refractive index as the refractive index of a vacuum.
Data Source
AI summary
Multilayer-film reflective mirrors are disclosed that exhibit desired optical characteristics and resistance to reflective-surface degradation. An exemplary multilayer-film mirror includes a base and a multilayer film on the base. The multilayer film is made of first and second layers alternatingly laminated at a prescribed period length. The surface of the multilayer film has an irregular surface profile, relative to the surface profile of the base. The multilayer film reflects incident extreme ultraviolet (EUV) light. A third layer, situated on and covering the surface of the multilayer film, is formed of a substance having substantially the same refractive index to EUV light as the refractive index of a vacuum. The third layer has a surface profile substantially the same as the surface contour of the base. The third layer is covered with a protective layer.