Development of High-Reflectivity Optical Coatings for the Vacuum Ultraviolet and Verification on a Sounding Rocket Flight

We worked to develop new thin film (< 20 nm) coatings of fluorides to utilize the high intrinsic reflectivity of aluminum (Al). Highly controllable thickness of fluorides can be attained through Atomic Layer Deposition (ALD). We will utilize ALD to create highly transmissive protective thin film coating layers to improve Al reflectance (R > 60 %) at least to wavelengths as short as 90 nm. Our more ambitious goal is to enhance the reflectance down to 80 nm. In addition to this new deposition technique we will explore material combinations and possible deposition processes for effective coatings. Finally, Atomic Layer Etching of a pre-deposited Al films can also be an avenue to minimizing surface oxide between the Al and metal fluoride overcoat layers. Currently, our most promising coating candidate is a thin layer of MgF2 (AlF3 or LiF) deposited by ALD. I will utilized the VUV optical test facilities at the University of Colorado to test the reflectance of the new coatings in a laboratory environment. We strived to raise the TRL of these new laboratory tested coatings to a level sufficient for use on future UV/Visible astrophysics and/or solar physics NASA missions. This can be achieved by using the coatings on the optics of a sounding rocket. We have produced ALD AlF3 films on Si wafers and explored their surface roughness, UV-Vis-IR reflectance at two angles of incidence (10° and 45°, in segments), polarization, deposition temperature and environmental storage sensitivities. Thicker layers (d > 20 nm) have performed well in each of these investigations and have proven to be model-able for λ > 120 nm. We have also developed a process that can remove native aluminum oxide. The ALE removal of Al2O3 also improve the surface roughness over ALD AlF3 process alone. The subsequent ALE of Al2O3 and capping with ALD of AlF3 has been proven to enhance the reflectance for λ < 200 nm. This is another breakthrough. The improvement of this process and scalability of ALD and ALE open an array of possibilities not limited to mirror coatings. I now summarize the main aspects of this UV coatings project and the first half of my PhD research. 1. An atomic layer etch of Al2O3procedure has been modified (with LiF preconditioning) to create mirrors capped with ALD AlF3. 2. E-beam Al + ALE of Al2O3+ ALD of ~2 nm of AlF3 has been proven to improve the reflectance over e-beam Al + ALD of ~2 nm of AlF3 from 90 - 200 nm. Increases from ~8% up to ~30% near 95 nm, from ~5% up to ~30% near 100 nm and from ~10% up to ~40% near 105 nm were observed. These results prove that the ALE removal of Al2O3 and subsequent coating of ultra-thin layers of AlF3 by ALD can mitigate absorption loses in the LUV and FUV. 3. High quality reflectance measurements were performed via the refurbishing of the LASP CTE3 chamber. 4. The NIST SURF reflectometer was commissioned, systematic effects uncovered and improvements were made to conduct quality unpolarized reflectance measurements from 80 - 500 nm. 5. The CASA Square Tank system was characterized, modified, used to perform initial project and degradation study measurements. 6. ALD AlF3 film surface roughness on Si wafers are consistent with ~15% increase in RMS as indicated by AFM. 7. Unpolarized reflectance measurements from 90 - 800 nm have proven that ALD AlF3 films are consistent with extracted optical constant calculations, which are similar to values listed in literature from thermal evaporation (90 - 124 nm) and magnetron sputtering (190 - 700 nm) except for higher than predicted reflectance near 110 - 120 nm. The absorption band of our films appears to occur at shorter wavelengths than calculations based upon Bridou optical constants predict. 8. ALD AlF3 has good Optical properties for deposition temperatures between 100 - 200 °C. Films deposited at different temperatures of 100 °C and 200 °C show small differences in UV reflectance, but still within measurement uncertainties. 9. Polarization sensitivity was explored via ellipsometric angles. Polarization changes induced can be beneficial depending on the optical system and the film substrate choice. SE measurement agreement with optical constant calculations from 200 - 800 nm at multiple angles of incidence (θ = 45°, 55° and 65°) confirm favorable optical characteristics of the ALD AlF3 films. Furthermore, the optical predictability attained will allow ALD AlF3 implementation in optical designs to modify the polarization states induced by uncoated mirror properties. 10. Our ALD AlF3 on Si films exhibit minimal UV reflectance sensitivity to the MDL `glove; box storage conditions of ~40 °C and 1% relative humidity for 2 months, as compared to samples measured immediately. With continued development, ALD AlF3 can be a viable coating material candidate for many optical systems, and not limited to smooth mirror surfaces and conformal diffraction grating coatings which have motivated our initiation of this project. The long term objective is to development of ultra-thin (~few nm) layers of metal fluoride films. Future results should include further analysis of film structures (crystalline vs. amorphous) via measurements such as X-ray Diffraction (XRD). Surface roughness vs. film deposition temperature derived from AFM. Film surface structure and composition from X-ray Photoelectron Spectroscopy (XPS), Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and/or TOF Electron Recoil Detection Analysis (TOF-ERDA).