High-Performance Aluminum Mirrors for Far-Ultraviolet Applications
Instrumentation
High-Performance Aluminum Mirrors for Far-Ultraviolet Applications (GSC-TOPS-367)
A novel reactive PVD process for enhanced reflectivity and stability
Overview
Aluminum (Al) is known for its high reflectance in the 90 – 2500 nm wavelengths and is commonly used for mirrors in optical applications within this range e.g., for NASA’s proposed Large UV/Optical/IR (LUVOIR) surveyor observatory. However, in order to be used in the far-ultraviolet (FUV) spectral range between 90 – 130 nm, an optical thin film overcoat is needed to protect the Al and prevent the formation of an Al2O3 layer, which occurs upon exposure to oxygen. Lithium fluoride (LiF) has been a long-standing solution for this purpose, particularly for wavelengths below 120 nm. LiF coatings are opaque between 90 – 102 nm and even generate residual absorption above 102 nm, limiting reflectance at these short wavelengths. LiF coatings also suffer from hygroscopic degradation, leading to a reduction in reflectivity over time when exposed to humid environments.
To address these challenges, innovators at NASA Goddard Space Flight Center have developed an advanced method to produce Al mirrors protected with a LiF overcoat using a novel room-temperature reactive Physical Vapor Deposition (rPVD) process. This method introduces a fluorine-containing xenon di-fluoride (XeF2) gas treatment that enhances the durability and stability of the LiF coating while achieving record-high reflectivity levels in the FUV spectrum.
The Technology
This innovative NASA process begins with the deposition of an Al layer onto an optically smooth glass substrate using a high-vacuum PVD system. Unlike conventional methods where the Al surface is immediately exposed to air or coated with LiF, this new technique entails an initial exposure of the fresh Al film to XeF2 gas. This step chemically passivates the Al surface, forming a thin (~2.5-3.2 nm) protective layer of aluminum fluoride (AlF3). This barrier prevents oxidation and contamination of the aluminum before the LiF layer is applied, preserving its reflectivity.
Following the XeF2 treatment, a flash-evaporated LiF overcoat is deposited onto the Al surface using a conventional PVD process. Deposition is performed at a high rate to increase the density of the LiF layer, enhancing its environmental stability and optical performance. Immediately after the LiF deposition, the mirror undergoes a second XeF2 exposure, further passivating the surface. This dual XeF2 treatment is key to the improved durability of the mirrors, as it effectively seals the interfaces and prevents degradation over time. One of the most significant advantages of this process is that it is performed entirely at room temperature, eliminating the need for high-temperature deposition techniques conventionally used for such coatings.
Extensive testing of these new mirrors has demonstrated their performance and durability. Testing of prototypes optimized for 121.6 nm demonstrated 92.6% reflectivity, surpassing all previously reported values for Al/LiF coatings. Long-term environmental testing has shown that these mirrors maintain their high reflectance even after months of storage in moderate humidity conditions. Further stress testing in environments with 50-60% humidity for three weeks resulted in a reflectance reduction of about 2%, demonstrating high environmental stability. This NASA invention is available for patent licensing to industry.
Benefits
- Unprecedented FUV reflectivity: Mirrors created using this process optimized for reflectance at 121.6 nm demonstrated a 92.6% reflectance at that wavelength, surpassing all previously reported Al/LiF coatings.
- Enhanced durability: Improved resistance to humidity and environmental degradation compared to conventional Al/LiF mirrors.
- Room-temperature processing: Eliminates the need for high-temperature deposition, reducing complexity and minimizing potential substrate damage.
- Improved surface smoothness: This NASA innovation results in a denser and smoother coating, reducing light scattering and enhancing optical performance.
- Broad spectral range: This novel method produces mirrors with high reflectivity from 90 nm to 2500 nm or higher.
- Scalability: The process is adaptable for large-area optical components, making it viable for use in next-generation space telescopes and instrumentation.
Applications
- Ground-based & satellite telescopes
- Laser optics (FUV mirrors, laser beam lines, F2 lasers operating at 157 nm, Xe lasers operating at 172 nm, higher order harmonic generation, plasma checking)
- Photolithography
- Batteries (as a passivation mechanism for Al in Li-containing batteries)
- Laboratory instrumentation (spectrometers, monochromators, gratings)
- High frequency wiring
- Vacuum ultraviolet (VUV) plasma analysis tools
- Wafer inspection tools
Technology Details
Instrumentation
GSC-TOPS-367
GSC-18899-1
“Advanced Al Mirrors Protected with LiF Overcoat to Realize Stable Mirror Coatings for Astronomical Telescopes,” Manuel A. Quijada, Luis Rodriguez de Marcos, Javier del Hoyo, Edward Wollack, 8/29/2022
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