Possible mirror coatings for POLLUX Luca Teriaca, Udo Schühle Max Planck Institute for Solar System Research POLLUX Workshop Marseille, 9. – 10. October 2017

VUV mirrors and coatings Possible contribution to the UV Spectropolarimeter by Max Planck Institute for Solar System Research: VUV mirrors and coatings

Overview and Requirements of POLLUX mirror coatings shortest wavelength far below 120 nm highest efficiency in the VUV range low polarization under normal incidence stable under laboratory and space environments POLLUX WS, Marseille 9.-10. OCT 2017

Mirror Coating Options bare aluminium coating without oxide layer alumium with thin top layer coating protected aluminium coatings with MgF2, LiF, AlF3, or others Three options of alumiunium mirror coatings POLLUX WS, Marseille 9.-10. OCT 2017

bare aluminium coating without oxide layer bare aluminium gives highest broad-band reflectance, but degrades fast in contact with oxygen 70 80 90 Bare aluminium is the only material with high reflectance for VUV-NUV-Vis-IR range. It‘s reflectance degrades fast with exposure to oxygen. Possible solutions: deposit fresh aluminium in space remove protective coating in space: e. g., by plasma etching of a-SiC, mechanical removal of polymer coating) POLLUX WS, Marseille 9.-10. OCT 2017

VUV reflectance of thin boron carbide (B4C) coating 2. aluminium with thin top layer coating VUV reflectance of thin boron carbide (B4C) coating 10 nm thickness of B4C Boron carbide and silicon carbide provide broad-band reflectance but reduce drastically the longer wavelengths Measured at PTB

2. aluminium with thin top layer coating thin top layer coating of SiC or B4C mirrors with very thin top layers have been fabricated and space qualified by optiXfab (and MPS). absorption is not negligible…only justified for wavelengths below 100 nm! POLLUX WS, Marseille 9.-10. OCT 2017

2. aluminium with thin top layer coating Al mirrors with top layers optimized for 106 nm at 45° angle of incidence for example, several top coatings have been investigated for 106 nm wavelength at 45° angle of incidence (Taracheva, Yulin, Feigl, Kaiser, Proc. SPIE 6705, 2007) POLLUX WS, Marseille 9.-10. OCT 2017

3. Aluminium mirrors protected with MgF2, LiF, AlF3 - is presently the baseline for LUVOIR Strategy to avoid aluminium oxidation: deposit protective layer with transparent materials (MgF2, LiF, AlF3) apply immediately after aluminium deposition avoid transfer between deposition chambers apply protective coating fast, with high deposition rate make layer as thin as possible to minimize absorption study deposition processes study thickness dependence study deposition temperature dependence protected aluminium mirrors have been produced with high-band gap materials: magnesium fluoride, lithium fluoride, aluminium tri-fluoride. POLLUX WS, Marseille 9.-10. OCT 2017

Protected aluminium coatings with MgF2, LiF, AlF3 Coating designs: Coatings with bi-layer protective coatings have been fabricated with combinations of MgF2, LiF, AlF3, e. g., Al/LiF/MgF2…Al/AlF3/MgF2…Al/LiF/AlF3… Coatings with single-layer protective coatings have been fabricated with better performance Deposition processes: Resistive evaporation e-beam evaporation Ion beam sputter deposition Evaporation at high substrate temperature (300 °C)  more uniform layer, no pin holes, lower thickness needed Atomic Layer Deposition (ALD)  more uniform layer, even lower thickness needed, slower deposition - Protective coatings with bi-layers of fluorides and single layers of fluorides have been investigated. Many deposition techniques are being used high substrate temperature provides better protection layers ALD = fairly new deposition technique, promises ultra-thin protection layer POLLUX WS, Marseille 9.-10. OCT 2017

Protected aluminium coatings with MgF2, LiF, AlF3 First studies have investigated degradation (stability), surface roughness, thickness dependence, and deposition speed POLLUX WS, Marseille 9.-10. OCT 2017

Mirror coatings with protected Aluminium optimized for Lyman-a hot deposition at 300 °C Al/MgF2 (Jan. 16) Al/AlF3/MgF2 (Feb. 15) Al/AlF3 (Feb. 15) Mirrors produced for Solar Orbiter by optiXfab: high temperature of the substrate provides better protective coating (better homogeneous coating) below 110 nm Al/AlF3 promises to be the best choice cross over at 110 nm Measured at PTB

Protected aluminium coatings with AlF3 Al/AlF3 - the most promising candidate for reflectors below 110 nm Deposition of AlF3 at 250°C substrate T (Quijada et al., Proc. SPIE, 10372, 2017) Dependence of degradation on Al deposition speed Dependence on layer thickness of ALD AlF3 deposition Results recently published by Manuel Quijada (NASA-GSFC) with AlF3 coating of 25 nm thickness hot-deposited at 250 °C aluminium degrades less at higher deposition speed with ALD AlF3 coating already 3 nm thickness provides protection POLLUX WS, Marseille 9.-10. OCT 2017 (Hennessy et al., JATIS 2(4), 2016) (Balasubramanian et al., Proc. SPIE 9602, 2015)

Protected aluminium coatings with AlF3 - state of the art - ALD AlF3/Al at various thicknesses - thinner layer gives higher reflectance Study by Hennessy at al. shows already that lower thickness coating of ALD ALF3 provides best performance (Hennessy et al., JATIS 2(4), 2016) POLLUX WS, Marseille 9.-10. OCT 2017

- prospective performance - Protected aluminium coatings with AlF3 - prospective performance - Predicted performance of Al mirrors protected by ultra-thin ALD AlF3 using the optical models in comparison to the theoretical performance of ideal (unoxidized) Al and to existing demonstrations. Curve 1: 3 nm of ALD AlF3 assuming 3 Å of interfacial Al oxide. Curve 2: 2 nm of ALD AlF3 assuming no oxide Theoretical performance to be expected from future studies: higher than R = 50% can be achieved (Hennessy et al., JATIS 2(4), 2016) POLLUX WS, Marseille 9.-10. OCT 2017

Protected aluminium coatings with AlF3 - state of the art - Aluminium tri-fluoride as a Protection Layer Results recently published by Manuel Quijada (NASA-GSFC) with AlF3 coating of 25 nm thickness, compared with predicted ultra-thin (3 nm) coating Al/AlF3 coating shows stability under laboratory conditions POLLUX WS, Marseille 9.-10. OCT 2017 (Quijada et al., Proc. SPIE Vol. 10398, 2017)

Protected aluminium coatings Conclusions: AlF3 is a dielectric material with a low refractive index and wide band gap > 10 eV. Because it has a slightly larger band gap (than MgF2 ), the AlF3 gives access to lower wavelengths on FUV reflectors. It also has high transmission at infrared (IR), ultraviolet (UV) and deep UV wavelengths. AlF3 is less hygroscopic than LiF, so it will be a more environmentally stable coating. The low surface roughness, even at high deposition rates, makes AlF3 well suited for use in protected and enhanced FUV Al mirrors. Highest reflectance at shortest wavelengths will be achieved with AlF3 coating immediately after aluminium depostion. The viability of using AlF3 as a protection layer to realize high FUV reflectance for Al mirrors has been proven. Future systematic studies are to be made to optimise deposition processes and stability of thin coatings. wrap up! POLLUX WS, Marseille 9.-10. OCT 2017

additional material POLLUX WS, Marseille 9.-10. OCT 2017