Enhanced atomic layer etching of native aluminum oxide for ultraviolet optical applications

Abstract

We report on the development and application of an atomic layer etching (ALE) procedure based on alternating exposures of trimethylaluminum and anhydrous hydrogen fluoride (HF) implemented to controllably etch aluminum oxide. Our ALE process utilizes the same chemistry previously demonstrated in the atomic layer deposition of aluminum fluoride thin films, and can therefore be exploited to remove the surface oxide from metallic aluminum and replace it with thin fluoride layers in order to improve the performance of ultraviolet aluminum mirrors. This ALE process is modified relative to existing methods through the use of a chamber conditioning film of lithium fluoride, which is shown to enhance the loss of fluorine surface species and results in conformal layer-by-layer etching of aluminum oxide films. Etch properties were explored over a temperature range of 225 to 300 °C with the Al2O3 etch rate increasing from 0.8 to 1.2 Å per ALE cycle at a fixed HF exposure of 60 ms per cycle. The effective etch rate has a dependence on the total HF exposure, but the process is shown to be scalable to large area substrates with a post-etch uniformity of better than 2% demonstrated on 125 mm diameter wafers. The efficacy of the ALE process in reducing interfacial native aluminum oxide on evaporated aluminum mirrors is demonstrated with characterization by x-ray photoelectron spectroscopy and measurements of ultraviolet reflectance at wavelengths down to 120 nm. IntroductionAluminum possesses a higher ultraviolet (UV) reflectance than other metals like gold and silver due to material properties such as a higher plasma frequency and the absence of strong interband transitions at UV wavelengths. These properties of Al are employed in classical UV optical components like mirrors, 1,2 reflective diffraction gratings, 3,4 and multilayer UV bandpass filters. 5,6 Al is also gaining interest as a plasmonic material for UV applications. The same material properties lead to a wider spectral tunability range for plasmonic effects in Al, 7 and nanostructured Al is being considered for applications like UV resonant biochemical sensors,

Atomic Layer Deposition of Lithium Fluoride Optical Coatings for the Ultraviolet

“…Following the ALE procedure, Al encapsulation was performed by the LiF ALD procedure, described in previous sections, as the substrate temperature was reduced to 100 • C. The reduced temperature relative to the majority of the optical characterization described previously was chosen for the possible benefits of reduced surface roughness; it also allowed for more rapid subsequent encapsulation of the deposited LiF layer with thin AlF 3 . Previous experiments have indicated a significant chemical interaction between the precursors required for AlF 3 deposition and LiF at temperatures significantly greater than 100 • C [30].…”

“…Samples were allowed to oxidize to completion over the course of several days. The surface oxide layer was then removed with an atomic layer etching (ALE) procedure that has been described elsewhere [30]. Removal of the native oxide is critical for achieving high reflectance at FUV wavelengths.…”

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“…These predictions are based in experimental data and demonstrate that AlF 3 has good reflectance in the FUV and even extends the cutoff wavelength over traditional PVD techniques [183]. Further experimental work has demonstrated the scalability of this approach and its environmental stability [184] through incorporating an atomic layer etching procedure [185].…”

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“…This oxidation is similar to that experience by Al mirror systems which have been investigated with similar ALD processes at JPL. 12,13 We have explored atomic layer etching (ALE) processes to remove this oxide inside the ALD chamber prior to encapsulation with ALD fluorides, 14 and initial demonstrations have been promising. 15 We will incorporate this same ALE method into our MDDIF development to reduce the influence of oxidation on each metal layer in the stack.…”