EUV Spectral Purity Optical Element with Absorptive Layer
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Solution Overview
Problem
Conventional reflective grazing-incidence spectral purity filters are ineffective in suppressing 10.6 μm radiation in Laser Produced Plasma (LPP) Extreme Ultraviolet (EUV) sources due to low reflectivity of AR coatings for EUV and the tendency of conventional mirror substrates to reflect rather than absorb this radiation.
Innovation Solution
An optical element comprising a first layer reflective for EUV radiation, a second layer absorptive for 10.6 μm radiation, and a third layer transparent for both, with the first layer positioned upstream to improve spectral purity by absorbing or scattering unwanted radiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional reflective grazing-incidence spectral purity filter with AR coating is used, then EUV radiation is reflected, but 10.6 μm radiation is also reflected instead of being absorbed
Solution Approach 1:
The optical element is segmented into multiple functional layers: a first layer for EUV reflection, a second layer for 10.6 μm absorption, and a third layer for mechanical support. This segmentation allows each layer to perform its specific function optimally without interfering with others.
Solution Approach 2:
Different layers are assigned different material properties tailored to specific wavelength requirements. The first layer has high EUV reflectivity, the second layer has high 10.6 μm absorptivity, and the third layer provides mechanical strength. This local quality differentiation resolves the contradiction by making the optical element selectively interactive with different radiation types.
2Use of energy by moving object
If an AR coating with high EUV reflectivity is used, then EUV radiation is effectively reflected, but the coating has very low reflectivity for 10.6 μm radiation
Solution Approach 1:
The second layer acts as an intermediary between the first and third layers, specifically designed to absorb 10.6 μm radiation that passes through the first layer. This intermediary layer prevents the harmful radiation from reaching the substrate while not affecting the EUV reflection function of the first layer.
Solution Approach 2:
The optical element uses a composite structure combining materials with different optical properties: a reflective material for EUV, an absorptive material for 10.6 μm, and a transparent support material. This composite approach allows simultaneous optimization for multiple wavelength requirements.
3Ease of operation
If conventional mirror substrates are used, then they reflect 10.6 μm radiation, but this reflection is insufficient for spectral purity requirements
Solution Approach 1:
The invention changes the material parameter of the second layer from conventional reflective materials to specifically absorptive materials for 10.6 μm radiation. This parameter change transforms the optical interaction from reflection to absorption, achieving the required spectral purity while maintaining mechanical functionality.
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 significantly enhances spectral purity by reducing reflection of 10.6 μm radiation while maintaining high EUV reflectivity, thereby minimizing unwanted heating of the wafer and improving the overall performance of EUV lithography tools.
Implementation Method 1
a first layer (4) comprising a first material, the first layer being configured to be reflective for radiation of a first wavelength
Implementation Method 2
the first layer being configured to be at least partially transparent for radiation of a second wavelength
Implementation Method 3
a second layer (2) comprising a second material, the second layer being configured to be substantially absorptive for the radiation of the second wavelength
Implementation Method 4
the first layer being located upstream in the optical path of incoming radiation with respect to the second layer in order to improve spectral purity of the radiation of the first wavelength
Data Source
AI summary
An optical element includes a first layer that includes a first material, and is configured to be substantially reflective for radiation of a first wavelength and substantially transparent for radiation of a second wavelength. The optical element includes a second layer that includes a second material, and is configured to be substantially absorptive or transparent for the radiation of the second wavelength. The optical element includes a third layer that includes a third material between the first layer and the second layer, and is substantially transparent for the radiation of the second wavelength and configured to reduce reflection of the radiation of the second wavelength from a top surface of the second layer facing the first layer. The first layer is located upstream in the optical path of incoming radiation with respect to the second layer in order to improve spectral purity of the radiation of the first wavelength.