EUV Mirror Substrate Composite Base Body Microroughness
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Solution Overview
Problem
EUV projection exposure apparatuses face challenges in achieving high reflectivity and maintaining low microroughness in mirror substrates, especially at elevated temperatures, due to limitations in existing reflective materials and coating technologies.
Innovation Solution
A mirror substrate comprising a particulate composite base body with a polishing layer and a highly reflective layer, where the base body is made of a particulate composite with ceramic dispersoids, and a polishing layer such as nickel-phosphorus or nickel-boron is used, along with an adhesion-promoter layer to ensure a stable and smooth reflecting surface, achieving low microroughness and high structural strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional mirror substrates are used for EUV lithography, then manufacturing is simpler, but microroughness increases and reflectivity decreases
Solution Approach 1:
The patent employs a composite substrate structure consisting of a particulate composite base body (aluminum matrix with silicon carbide particles) and a metallic polishing layer (nickel-phosphorus or nickel-boron). This composite construction enables the substrate to achieve microroughness values below 0.5 nm RMS while maintaining manufacturing feasibility through established composite material fabrication techniques.
2Productivity
If mirrors operate at elevated temperatures, then processing efficiency improves, but microroughness stability deteriorates
Solution Approach 1:
The patent utilizes the phase transition behavior of nickel-phosphorus alloys, specifically leveraging the fact that these materials remain in an amorphous state below their glass transition temperature (approximately 800-900°C). This allows the mirrors to operate at elevated temperatures for improved processing efficiency while maintaining microroughness stability, as the amorphous structure prevents grain growth and surface degradation that would occur in crystalline materials at similar temperatures.
3Illumination intensity
If highly reflective coatings are applied to improve reflectivity, then optical performance improves, but intensity loss from stray radiation increases
Solution Approach 1:
The patent converts the typically harmful effect of stray radiation into a beneficial outcome by using the highly reflective nickel-phosphorus or nickel-boron coating to redirect stray EUV radiation back toward the intended optical path. The coating's exceptional reflectivity (exceeding 70% in the 13.5 nm wavelength range) transforms scattered radiation that would normally represent energy loss into useful illumination, thereby improving both reflectivity and reducing net intensity loss.
4Adaptability or versatility
If multiple mirror elements are used in EUV projective devices, then optical functionality improves, but overall reflectivity decreases due to cumulative losses
Solution Approach 1:
The patent achieves record-high reflectivity values (exceeding 70% at 13.5 nm wavelength) through precise control of the nickel-phosphorus or nickel-boron coating parameters, including phosphorus content (8-13 wt%), layer thickness (50-200 nm), and deposition conditions. This high per-mirror reflectivity minimizes cumulative losses in multi-mirror EUV optical systems, allowing complex optical functionality to be achieved while maintaining sufficient overall reflectivity for practical lithography applications.
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 provides mirrors with high reflectivity and stable microroughness over long periods, even at elevated temperatures, ensuring minimal intensity loss and imaging aberrations, suitable for EUV lithography applications.
Implementation Method 1
a polishing layer which has been deposited without external current, for example nickel-phosphorus or nickel-boron layers
Implementation Method 2
a highly reflective layer on the polishing layer... highly reflective coatings which are adapted to the respective operating wavelength
Data Source
Figure 1a~1b
Figure 2a~2b
Figure 3
AI summary
Substrates suitable for mirrors used at wavelengths in the EUV wavelength range comprising a base body (2) made of a particulate composite with a metallic or non-metallic matrix. In addition, a highly reflective layer (6) is provided on the polishing layer (3) of the substrate (1) of the EUV mirror (5).