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

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

3. 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 OCT 2017

4. 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 OCT 2017

5. bare aluminium coating without oxide layer
bare aluminium gives highest broad-band reflectance, but degrades fast in contact with oxygen
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 OCT 2017

6. 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

7. 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 OCT 2017

8. 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 OCT 2017

9. 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 OCT 2017

10. 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 OCT 2017

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

12. Mirror coatings with protected Aluminium optimized for Lyman-a
hot deposition at 300 °C
Al/MgF (Jan. 16)
Al/AlF3/MgF2 (Feb. 15)
Al/AlF (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

13. 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 OCT 2017
(Hennessy et al., JATIS 2(4), 2016)
(Balasubramanian et al., Proc. SPIE 9602, 2015)

14. 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 OCT 2017

15. - 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 OCT 2017

16. 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 OCT 2017
(Quijada et al., Proc. SPIE Vol , 2017)

17. 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 OCT 2017

18. additional material
POLLUX WS, Marseille OCT 2017