Methods for Characterizing Thin Film Reflective Mirror Coatings for Next-Generation Astronomical Instruments James Wiley Undergraduate Honors Thesis University of Colorado Boulder, Department of Physics Under the Supervision of Dr. Brian Fleming (APS) Honors Council Representative: Dr. John Cumalat (PHYS) Out-of-Department Representative: Dr. Ioana Fleming (CSCI) Defense Date: March 21, 2018 Abstract This thesis presents the design, theory of use, and calibration of a semi-automated vacuum ultraviolet reflectivity chamber. This chamber will be used to establish the ini- tial reflectivity and track the degradation of enhanced lithium fluoride (eLiF) protected aluminum mirror coatings. LiF provides the highest throughput of heritage dielectric coatings in the Lyman UV (912-1216 A).˚ Due to the hygroscopicity of LiF and the ab- sorption of UV light by water, we have developed a means of capping the eLiF with another dielectric layer so thin that it protects the eLiF from degrading without ad- versely affecting the LUV reflectivity. These protected eLiF coated mirrors are held in humidity controlled chambers and measured periodically to track degradation. Their performance and environmental resistance will be compared to other dielectric coatings to prove their space-worthiness. We designed and fabricated a reflectivity chamber to take measurements rapidly and with minimal oversight, increasing lab efficiency for high cadence and high accuracy vacuum UV reflectivity measurements. i Acknowledgements I would like to express my deepest gratitude to my advisor, Professor Brian Fleming, for his support throughout this honors thesis. Without his guidance, this work never would have been possible. For teaching me most of what I know about astronomical instrumentation, showing me how to conduct research, and helping me all the times I got stuck, I am truly appreciative. I would also like to thank the other members on my committee, Professor John Cumalat and Professor Ioana Fleming, for the support and learning opportunities they provided. I am grateful to Professor James Green for his insight and encouragement, and to ev- eryone else who helped during this project including Michael Kaiser, Arika Egan, Robert Kane, Nicholas Nell, Nicholas Renninger, and Nicholas Erickson. Last but not least, I would like to thank my family for supporting me through all my decisions, for always being there for me, and for helping me work towards making my dreams come true. ii Contents List of Figures . iv List of Tables . v 1 Introduction and Background 1 1.1 Introduction . 1 1.2 Coating Processes . 3 1.2.1 High Temperature Physical Vapor Deposition . 3 1.2.2 Atomic Layer Deposition . 6 1.3 Degradation Measurements, Motivation for a New Reflectivity Chamber . 8 2 Exprerimental Setup 12 2.1 Instrument Overview . 13 2.2 Vacuum Chamber . 14 2.3 Light Source . 17 2.4 Computer Control . 20 2.5 Operational Walkthrough . 22 2.6 Initial Calibration . 23 3 Future Work and Conclusion 26 3.1 Current State of Experimental Setup . 26 3.2 Future Work . 27 3.3 Conclusion . 30 Bibliography 32 iii List of Figures 1.1 LiF Reflectivity Curves . 2 1.2 16-Slide Tray . 3 1.3 PVD Chamber . 3 1.4 PVD Surface Microroughness . 5 1.5 Hot vs. Cold Reflectivities . 5 1.6 ALD Chamber . 6 1.7 Pre and Post ALD Surface Microroughness . 7 1.8 Humidor Chambers and Humidity Log . 8 1.9 CU/LASP Square Tank . 9 1.10 Degradation Measurements Made in the Square Tank . 10 2.1 Experimental Setup . 12 2.2 Vacuum Chamber . 14 2.3 Benchtop Setup . 15 2.4 Reflected Positions . 15 2.5 Optics Mount . 16 2.6 Light Source . 18 2.7 Inside of the Collimator Box . 19 2.8 Beam Steering . 20 2.9 LabVIEW Interface . 21 2.10 Residual Gas Analysis . 24 3.1 Current Experimental Setup . 26 3.2 Pinhole Mount . 27 3.3 SISTINE Raytrace . 29 iv List of Tables 1.1 Ambient vs. Hot PVD Microroughness . 5 1.2 ALD Processes . 7 2.1 Incident and Reflected Positions. 16 2.2 Data Taken with Laser . 24 v Chapter 1 Introduction and Background 1.1 Introduction Advances in coating processes for ultraviolet optimized optics have allowed for higher throughput in the Lyman UV bandpass (LUV, 912-1216 A)˚ [1, 2, 3]. Despite this band- pass being important for detecting and observing astrophysical processes such as spectral lines from carbon (C), oxygen (O), nitrogen (N), carbon monoxide (CO), hydrogen (H), molecular hydrogen (H2), and water (H2O) [4], it has been little explored due to the lim- itations of coatings on previous space observatories. The Hubble Space Telescope (HST) is coated with aluminum (Al) overcoated with magnesium fluoride (MgF2) which gives reflectivities over 80% from the far ultraviolet (FUV, 1150 - 2000 A)˚ to the near infrared (NIR), but has a steep bandpass cutoff at around 1150 A.˚ The Far Ultraviolet Spectro- scopic Explorer used Al overcoated with Lithium Fluoride (LiF) in two of its channels, extending the bandpass down to 1000 A˚ but failing to achieve reflectivities much higher than 60%. New physical vapor deposition techniques developed at Goddard Space Flight Center (GSFC) have allowed for enhanced reflectivities from LiF+Al (eLiF) optics in the LUV (figure 1.1). These eLiF coatings will extend the bandpass of future UV-sensitive space observatories [1, 2, 5] Although LiF has the lowest bandpass cutoff of heritage dielectric coatings and has been shown to have high reflectivities, it is hygroscopic and loses its reflectivity over time 1 1.0 0.8 0.6 0.4 Ref. Bare Al Reflectance Theory 180 Å LiF+Al Å 0.2 FUSE LiF+Al (160 ) 244 Å eLiF (Quijada 2014) 180 Å eLiF (Fleming 2015) 0.0 1000 1200 1400 1600 1800 Wavelength (Å) Figure 1.1: Reflectivity curves of Al, theoretical LiF+Al, measured FUSE witness sam- ples, and differing thicknesses of eLiF (from Fleming et. al. 2018). [6] with exposure to humidity [7]. Atomic layer deposition (ALD) of a very small layer of MgF2 or aluminum fluoride (AlF3) could decrease degradation while still allowing for high reflectivities without affecting the lower bandpass cutoff. The goal is to have reflectivities > 80% and a bandpass cutoff < 1000 A.˚ Qualifying the protected eLiF coatings for flight requires high cadence reflectivity testing to optimize the coating process and determine the durability over time with exposure to common environmental hazards. Measurements taken in vacuum in a clean- room environment require an onerous amount of person-time. It is for these reasons an automated vacuum ultraviolet reflectivity chamber was designed and built. This chamber will decrease user oversight and increase cadence for qualifying these state of the art coatings. 2 1.2 Coating Processes 1.2.1 High Temperature Physical Vapor Deposition Physical vapor deposition (PVD) is used as the coat- ing process for both the aluminum and the first pro- tective dielectric layer [1, 2]. PVD is characterized by a process where a material in a condensed phase is evaporated to a vapor phase and then condenses on the substrate, forming a thin film. PVD provides ex- tremely pure, thin, and high performance coatings, Figure 1.2: Rendering of the 16- and is currently superior to other deposition tech- slide tray used to coat samples in niques used for FUV optical coatings. the PVD chamber. A 16-slide tray (figure 1.2) containing 2x2 inch samples is hung upside down in the GSFC PVD chamber (figure 1.3). The chamber operates at pressures < 10−6 torr and contains a tungsten filament for heating and evaporating aluminum staples to produce a Figure 1.3: PVD chamber at GSFC. [1][2] 3 pure aluminum gas. A molybdenum crucible holds powdered LiF crystal at the chamber center. Molybdenum is used as it heats to the proper temperature for evaporating the LiF when an electrical current is run through it. Mechanical shutters are used to control deposition time for differing thicknesses of each layer. The evaporated gas is contained under the shutters and released when the shutter is moved. To coat the samples, a three step process was used[1]: 1. Coat the substrate with aluminum at room temperature to the planned thickness of 700 A.˚ This is because aluminum maintains higher reflectivities in the FUV when deposited at ambient temperatures. 2. Coat the aluminum with a small LiF layer of about 50 A˚ as soon as possible to prevent any degradation while in the coating chamber. This “flash coating" is used to prevent the aluminum from oxidizing while heating up the chamber in the next step, as even a small exposure to oxygen will cause rapid oxidation. 3. Heat the chamber to at least 220◦C and add the rest of the LiF coating to the intended total thickness of 170 A.˚ The high temperature slows the freeze-out time of the LiF molecules, enabling a higher material packing density and decreased surface roughness. Figure 1.4 and table 1.1 show the surface microroughness of two Al+MgF2 samples taken with a profilometer. The samples have a more even surface after a hot deposition than one at room temperature. Figure 1.5 shows the reflectivities of hot versus cold depositions of MgF2+Al. This shows why elevating the temperature in the PVD chamber is key to achieving high reflectivities. 4 Figure 1.4: Microroughness profiles of Al+MgF2 at ambient and elevated temperatures. (from Quijada et. al. 2014) [1] Ambient Hot PV(A)˚ Sq(A)˚ PV(A)˚ Sq(A)˚ top left 75.6 6.5 45.3 2.25 top right 101.2 5.20 40.2 2.33 center 128.0 4.02 51.0 3.30 bottom left 200.1 3.03 44.4 2.92 bottom right 100.0 3.28 50.8 3.85 average 120.97 4.33 46.3 2.93 Table 1.1: Measured peak to valley (PV) and RMS square (Sq) microroughness profiles of ambient and hot Al+MgF2 depositions.