Magnesium fluoride (MgF2) thin films are useful for many different optics applications. In particular, they are useful for ultraviolet anti-reflective and protective coatings. However, in the far UV, one needs a very small, controllable amount of material to get the best optical performance. That is difficult to achieve with conventional methods. Atomic layer deposition (ALD) is an ideal UV-compatible thin-film deposition technique due to its ability to deposit uniform, pin-hole free films with angstrom-level thickness control. Therefore, it is an ideal technique to use to deposit protective thin films in the 2-nm thickness range. However, conventional ALD-MgF2 reactions are very unpredictable due to the low reactivity and volatility of the precursors.
A new chemistry was developed to deposit these films based on Mg(EtCp)2 and HF. This reaction proceeds readily, as the two constituents are very well behaved during the deposition process.
The compound Mg(EtCp)2 is evaporated by heating in a stainless steel bubbler. The material is then carried into the deposition chamber by pushing with an inert carrier gas (in this case, argon). Once a saturated coverage is obtained, the Mg(EtCp)2 is purged from the gas phase in the system and a puff of HF is introduced. The fluorine in the HF reacts with the Mg to make MgF2, and the hydrogen reacts to form 2 HEtCp molecules. The reaction is complete when all the Mg has been converted to MgF2. This series of exposures is repeated until an MgF2film of the desired thickness has been grown.
This novel reaction is the first demonstration of an MgF2 ALD deposition method that is compatible with deposition on ALD aluminum metal with simple and volatile precursors. The use of HF also ensures a very clean thin film.
This work was done by Harold F. Greer, Shouleh Nikzad, Michael C. Lee, and Wesley A. Traub of Caltech; and Steven George and Matthew Beasley of the University of Colorado for NASA’s Jet Propulsion Laboratory. NPO-48678
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Overview
The document discusses advancements in the development and characterization of high-reflectivity mirror coatings using Atomic Layer Deposition (ALD) technology, specifically focusing on magnesium fluoride (MgF₂) thin films. Conducted at NASA's Jet Propulsion Laboratory (JPL), the research aims to create mirror coatings suitable for space telescopes that can support both ultraviolet (UV) astrophysics and optical exoplanet science.
The project utilized an Oxford OpAL machine and a conventional electron-beam evaporation chamber to deposit aluminum as a reflective layer and MgF₂ as a protective overcoat. The study highlights the challenges associated with the existing ALD chemistry for MgF₂, prompting the invention of a new chemical reaction pathway to improve film quality. X-ray Photoelectron Spectroscopy (XPS) and cross-sectional Transmission Electron Microscopy (TEM) were employed to analyze the composition and morphology of the films, revealing that the ALD MgF₂ was amorphous, which is advantageous for moisture barrier applications.
Optical characterization of the films was performed using spectroscopic ellipsometry and UV-VIS spectrometry. Results indicated that the ALD MgF₂ films exhibited superior performance compared to bare evaporated aluminum mirrors at shorter wavelengths, although some data interpretation challenges were noted. The study emphasizes the importance of understanding the initial ALD nucleation on aluminum, as it significantly impacts mirror performance.
The project objectives include achieving high reflectivity across a wavelength range of 100 to 1000 nm and minimizing s-p phase shifts at visible wavelengths. The successful development of these coatings could enhance the capabilities of future space missions by providing better reflectance and durability, as well as improved control over polarization effects, which are critical for high-contrast imaging applications.
The document concludes that further research is warranted to optimize the ALD aluminum process and explore the potential for an all-ALD solution for mirror coatings. The findings hold promise for advancing technologies that can meet the needs of both the UV astrophysics and exoplanet communities, ultimately contributing to the development of a shared telescope for future strategic missions.
