Using thermal atomic layer deposition (ALD), with trimethylaluminum (TMA) and water (H2O), it was found that coatings of aluminum oxide (AlOx) deposited on silver-based telescope mirrors operate as viable means to both protect the mirrors from corrosion and reap the benefits of the mirrors while only minimally impacting the mirror’s performance. However, as effective as AlOx coatings are in high temperature/high humidity (HTHH) testing, the mirrors tend to catastrophically fail after an extended period of time during the HTHH test. In order to further improve long-term resilience, the use of Pure Ozone (PO) as an oxygen precursor in replacing H2O is proposed and investigated. While the initial reflectance of the samples prepared with PO is slightly lower than those prepared with H2O, they exhibit better resilience during the HTHH test. In this paper, the study on progressive structural changes that occur in silver-based telescope mirrors protected by AlOx is described over several different stages during cyclic HTHH tests (i.e., temperature is cycled between two, high and low, temperatures). Two types of samples are prepared using either H2O or PO as an oxidation precursor and compared. The study reveals that the initial drop in spectral reflectance of the PO samples during the first HTHH cycle is associated with the surface reconfiguring itself in the presence of the extra energy which effectively tempers the surface to gain higher stability.
Although silver-based telescope mirrors excel over other materials such as gold and aluminum in the visible-infrared spectral range, they require robust protective coatings to overcome their inherent low durability. Our research shows that a single-layer of aluminum oxide (AlOx) deposited through thermal atomic layer deposition (ALD) using trimethylaluminum (TMA) and water (H2O) at low temperatures (~60°C) serves as an acceptable protection coating without adversely impacting the optical performance of the mirrors. While the use of TMA and H2O as precursors in thermal ALD offers AlOx that performs decently in the field, it degrades quickly in environmental tests under high-humidity at high-temperature conditions, suggesting that there is room to improve. In this paper, two approaches by which ALD processes of AlOx protection coatings can be improved are investigated: exploring another precursor for oxygen and implementing a pre-deposition conditioning. The study is carried out by introducing two new processes – the use of Pure Ozone (PO) and Ozone-Ethylene Radical (OER) in comparison to thermal ALD with TMA and H2O. Our study shows that samples prepared by PO have the initial spectral reflectance lower than that of those prepared by the thermal ALD; however, reflectance of the PO samples remains nearly constant 1.6 times longer in the environmental test, suggesting promising characteristics of AlOx prepared with PO.
Depositing thin films is often limited to a specific deposition process by which precursors are transported and reacted in a deposition environment. In other words, a deposition environment in which two deposition processes are unified should offer a new perception of devising a thin film structure, which galvanizes our combining atomic layer deposition (ALD) and magnetron sputtering (SPU) in a single chamber – sputtering atomic layer augmented deposition (SALAD). The SALAD system offers advantages of both ALD capable of delivering precursor precisely and accurately and SPU versatile in choosing chemical elements. In this paper, the SALAD system is employed to deposit nanocomposites consisting of multiple pairs of an aluminum oxide thin film deposited by ALD and a copper thin film deposited by SPU. Optical properties collected from the nanocomposites show distinctive dispersion features to which the effective medium approximation does not seem to simply apply.
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