EUV Multilayer Mirror with Phase-Shifting Layer Group
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Solution Overview
Problem
EUV mirrors used in high numerical aperture optical systems for EUV microlithography face challenges in maintaining consistent reflectance over a large angle-of-incidence range, and existing broadband multilayer mirror designs are difficult to produce with high precision due to the need for varying layer thicknesses and potential crystallization issues with thick layers.
Innovation Solution
A multilayer EUV mirror design featuring a periodic first layer group near the radiation entrance, a periodic second layer group closer to the substrate, and a phase-shifting third layer group between them, with optimized period thicknesses and layer constructions to minimize reflectivity variation and simplify production by avoiding large individual layer thicknesses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a broadband multilayer mirror design with varying layer thicknesses is used to reduce reflectivity variation over angle of incidence, then reflectivity consistency is improved, but manufacturing precision deteriorates due to difficulty in producing layers with precisely controlled varying thicknesses
Solution Approach 1:
The patent applies parameter changes by systematically varying the period thickness of layer pairs across different layer groups (first, second, and third layer groups) to optimize reflectivity consistency. By changing the period thickness parameter across groups while maintaining periodic structure within each group, the design achieves broadband reflectivity without requiring complex aperiodic structures that would be difficult to manufacture.
Solution Approach 2:
The multilayer arrangement is segmented into multiple layer groups (first, second, and third layer groups) with different period thicknesses. Each layer group contains a specific number of layer pairs with uniform period thickness, allowing independent optimization of each segment. This segmentation enables control over reflectivity characteristics while simplifying manufacturing within each segment.
2Reliability
If thick layers are used in multilayer mirrors to achieve desired optical performance, then reflectivity is improved, but reliability deteriorates due to crystallization issues in thick layers
Solution Approach 1:
The patent segments the total optical path into multiple thin layer pairs organized in groups, rather than using fewer thick layers. By distributing the required optical functionality across many thin layers with controlled period thicknesses, the design achieves desired reflectivity while keeping individual layer thicknesses below the crystallization threshold.
Solution Approach 2:
The patent changes the period thickness parameter across different layer groups to optimize optical performance without increasing individual layer thickness. By adjusting the period thickness (sum of high and low refractive index layer thicknesses) rather than individual layer thickness, the design maintains thin layers that avoid crystallization while achieving the required optical characteristics.
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 design achieves reduced reflectivity variation over a wide angle-of-incidence range, enhancing the usability of EUV mirrors with more stable reflectivity values and simplifying production by avoiding thick layers that can lead to crystallization issues, while maintaining optical performance.
Implementation Method 1
a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range and comprising a large number of layer pairs comprising alternately relatively low refractive index and relatively high refractive index layer material
Implementation Method 2
The reflectivity or reflectance of multilayer mirrors is greatly dependent on the angle of incidence and on the wavelength of the impinging EUV radiation
Implementation Method 3
a phase-shifting third layer group between them, with optimized period thicknesses and layer constructions to minimize reflectivity variation
Implementation Method 4
One class of EUV mirrors operates at relatively high angles of incidence of the incident radiation, that is to say with grazing incidence according to the principle of total reflection
Data Source
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AI summary
An EUV mirror comprises a substrate and a multilayer arrangement applied on the substrate, which multilayer arrangement has a reflective effect for radiation having a wavelength λ from the extreme ultraviolet range (EUV) and comprises a multiplicity of layer pairs having alternating layers comprising a high refractive index layer material and a low refractive index layer material. The multilayer arrangement comprises: a periodic first layer group (LG1) having a first number N1 > 1 of first layer pairs, which are arranged in the vicinity of a radiation entrance side of the multilayer arrangement and have a first period thickness P1; a periodic second layer group (LG2) having a second number N2 > 1 of second layer pairs, which are arranged between the first layer group and the substrate and have a second period thickness P2; and a third layer group (LG3) having a third number N3 of third layer pairs, which are arranged between the first layer group and the second layer group. The first number N1 is greater than the second number N2. The third layer group has an mean third period thickness P3 which deviates from an average period thickness PM = (P1 + P2)/2 by a period thickness difference ΔΡ, wherein the period thickness difference ΔΡ substantially corresponds to the quotient of the optical layer thickness (λ/4) of a quarter-wave layer and the product of the third number N3 and cos(AOIM), wherein AOIM is the mean angle of incidence for which the multilayer arrangement is designed.