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,
Direct imaging of an Earth-like exoplanet requires starlight suppression with a contrast ratio on the order of 1 × 10 −10 at small angular separations of 100 milliarcseconds or less in visible light with more than 50 nm bandwidth. To our knowledge, the technology needed to achieve the contrast and stability has not been demonstrated as of January 2019. The science requirements for near future National Aeronautics and Space Administration (NASA) missions such as James Webb Space Telescope (JWST)'s Near Infrared Camera (NIRCam) coronagraph and Wide-Field InfraRed Survey Telescope (WFIRST) Coronagraph Instrument (CGI) are at least 10 times short. To investigate and guide the technology to reach this capability, we built a high contrast coronagraph testbed at NASA's Jet Propulsion Laboratory (JPL). Titled the Decadal Survey Testbed (DST), this state-of-art testbed is based on the accumulated experience of JPL's High Constrast Imaging Testbed (HCIT) team. Currently, the DST hosts a Hybrid Lyot Coronagraph (HLC) with an unobscured, circular pupil. The DST also has two deformable mirrors and is equipped with the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to sense and correct the dynamic wavefront disturbances. In this paper, we present up-to-date progress of the testbed demonstration. As of January 2019, we repeatedly obtain convergence below 4 × 10 −10 mean contrast with 10% broadband light centered at 550 nm in a 360 degrees dark hole with a working angle between 3 λ/D and 8 λ/D. We show the key elements used in the testbed and the performance results with associated analysis.
Aluminum fluoride (AlF3) is a low refractive index material with promising optical applications for ultraviolet (UV) wavelengths. An atomic layer deposition process using trimethylaluminum and anhydrous hydrogen fluoride has been developed for the deposition of AlF3 at substrate temperatures between 100 and 200 °C. This low temperature process has resulted in thin films with UV-optical properties that have been characterized by ellipsometric and reflection/transmission measurements at wavelengths down to 200 nm. The optical loss for 93 nm thick films deposited at 100 °C was measured to be less than 0.2% from visible wavelengths down to 200 nm, and additional microstructural characterization demonstrates that the films are amorphous with moderate tensile stress of 42–105 MPa as deposited on silicon substrates. X-ray photoelectron spectroscopy analysis shows no signature of residual aluminum oxide components making these films good candidates for a variety of applications at even shorter UV wavelengths.
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