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Methods for Characterizing Thin Film Reflective for Next-Generation Astronomical Instruments

James Wiley

Undergraduate Honors Thesis

University of Colorado Boulder, Department of

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 so thin that it protects the eLiF from degrading without ad- versely affecting the LUV reflectivity. These protected eLiF coated 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 Processes ...... 3 1.2.1 High Temperature Physical Vapor Deposition ...... 3 1.2.2 ...... 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 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 ...... 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 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. (from Quijada et. al. 2014) [1]

Figure 1.5: Reflectivies of hot deposition vs. cold deposition compared to theoretical Al. (from Quijada et. al. 2014) [1]

5 λn We aim for a 4 thickness of the LiF layer to maximize reflectivity at the target

wavelength due to constructive interference, where λn is the target wavelength for peak reflectivity of the instrument within the LiF layer. For an astronomical instrument op-

1100 timized for 1100 A,˚ we calculate that λn = , where n1100 = 1.64 is the index of n1100 of LiF at 1100 A,˚ giving λn = 671 A.˚ Our target thickness is then 167.7 A˚ in the total LiF layer.

1.2.2 Atomic Layer Deposition

Figure 1.6: ALD chamber at the Microdevices Laboratory at JPL (from Balasubramanian et. al. 2015) [7]

We send the eLiF coated mirrors to the Microdevices Laboratory (MDL) at NASA’s Jet Propulsion Laboratory (JPL) to overcoat them with a very small dielectric layer using atomic layer deposition (ALD) (figure 1.6) [3, 4]. ALD gives us a very small and

6 uniform layer of MgF2 or AlF3 to protect the eLiF. We have shown during this project that such protective capping layers decrease the degradation rate of the eLiF while not significantly affecting the lower bandpass cutoff [2]. This is an important development as previous efforts to cap the LiF with PVD layers all resulted in significant losses of bandpass and reflectivity [7]. Due to the nature of ALD depositing one monolayer at a time, it does not significantly affect surface roughness. The work is also naturally scalable to large area substrates. [3, 8] The ALD method grows thin films by introducing a sequence of precursor gases that chemically react in a self-limiting manner at the surface of the substrate [4]. The reaction steps are intermediated by purges of inert gas to remove any unreacted precursor [8]. This is what enables the prescription of monolayers at a time. Table 1.2 shows what gases are used for each type of fluoride coating, and the thickness of each layer deposited. [9]

◦ ◦ Material Co-reactant with Anhydrous HF Tsubstrate( C) A/Cycle˚ (100 C)

MgF2 bis(ethylcyclopentadienyl) magnesium 100-250 0.6 AlF3 trimethylaluminum 100-250 1.2 LiF lithium bis(trimethylsilyl)amide 100-250 0.2

Table 1.2: ALD processes for FUV fluoride materials. (From Hennessy et. al. 2017) [9]

Figure 1.7: Pre and post ALD Surface Microroughness of a 10 A˚ AlF3 deposition. the Pre RMS is 17.54 and post RMS is 16.79. (From Egan et. al. 2017) [10]

7 Figure 1.7 shows pre and post ALD surface microroughness of AlF3 samples as mea- sured by a Wyko NT2000 Profilometer in the JILA optics metrology laboratory. This data shows that the surface microroughness is not significantly changed during the deposition process.

ALD is not currently being used for depositing aluminum or the initial fluoride layer due to it still being in the early stages of development. Although the work is promising for the future, current work does not achieve as high reflectivities as PVD, as thus far the JPL team has been unable to prevent aluminum oxidation in the ALD chamber. Our hybrid ALD-PVD approach may be a solution for high reflectivity broadband coatings for near-term astronomical instruments without requiring onerous environmental procedures.

1.3 Degradation Measurements, Motivation for a New

Reflectivity Chamber

Protected eLiF coatings have been tested using an old vacuum chamber setup, known as the CU/LASP square tank[11], and degradation measurements have been made by keep- ing the eLiF coatings in a set of custom humidor chambers (figure 1.8a) and measuring reflectivity periodically.

The humidor chambers were held at 70% and 50% relative humidity and controlled

(b) Humidity Log (a) Humidity Controlled Chambers

Figure 1.8: Humidor chambers (a) and recorded humidity (b). (from Fleming et. al. 2018) [6]

8 Figure 1.9: The CU/LASP Square Tank assembly (from Fleming et. al. 2015) [2] via humidity gauges and a Raspberry Pi. The humidity was maintained within ±1% and logged every 30 seconds (figure 1.8b). The humidor chambers were stored in a clean room and samples were kept under a nitrogen purge when transporting to the square tank for testing.

As shown in figure 1.9, the square tank is a large vacuum chamber with 4 stages (two rotation and two translation) for testing reflectivities. It is hooked up to a light source section that inputs monochromatic, collimated, ultraviolet light for testing the FUV reflectivities of the optics. A micro-channel plate (MCP) was used as the reference detector. The degradation data shown in figure 1.10 was taken using this setup.

The data shows decreased degradation for eLiF mirrors with a small capping layer of

AlF3. The decreased degradation is consistent for both levels of humidity, and is below 10% when stored at 50% humidity. This shows the heightened environmental resistance of the protected eLiF, making it more viable for future space instruments. Also, the low degradation at 50% is promising for attaining our goal of > 80% reflectivities as most

9 Sample Degradation with RH Exposure 1.0

0.9

0.8

LiF + 20 Å AlF3 at 50% RH 0.7 LiF at 50% RH

LiF + 20 Å AlF3 at 70% RH LiF at 70% RH 0.6 Relative Reflectivity (102 − 125 nm) 0 20 40 60 80 100 120 140 Time (Days)

Figure 1.10: Degradation measurements made by storing optics in humidity controlled chambers and testing their reflectivities in the square tank over time. labs are held around 40% humidity to help prevent electrostatic discharge.

During these tests, due to the size of the square tank, testing times were increased due to slow pumping speeds to get to testable vacuum pressures of < 10−5 torr. A rough pump was used to decrease the pressure to a crossover pressure of < 10−1 torr and a cryopump was used to get the chamber to high vacuum. This, combined with the time it took to purge the chamber with nitrogen to atmospheric pressure to remove the samples, created vast amounts of time spent waiting between tests.

Moving the stages also increased time during tests. To go from incident to reflected positions, the user had to hold buttons to move the swing arm, the center rotation stage, the translation stage on the swing arm holding the detector, and the translation stage mounted on the center rotation stage holding the optics. It was critically important to have the light beam incident on the MCP at the same angle for all positions as MCP quantum efficiency is angle dependent. This meant about 3 minutes of time was spent

10 during each move between incident and reflected positions holding buttons on the stage controllers. The square tank also had the problem of high background noise. Testing times were increased due to having to wait for background levels to decrease between measurements. This was probably due to moving stages causing ions that were picked up by the MCP. This high background noise also decreased precision of measurements, causing longer test times and less reliable results. These problems with the square tank, long testing times and high background counts causing maximal user oversight and decreased measurement reliability, are what created the need for a new chamber. The development of new optics increases the need for routine reflectivity measurements to track degradation, and the square tank is not an efficient enough system to keep up with the amount of measurements necessary. A new chamber with faster testing times, higher measurement precision, and lower user oversight is necessary to continue supporting the development of these state of the art optics. This is what led to the design and fabrication of the automated vacuum ultraviolet reflectivity chamber.

11 Chapter 2

Exprerimental Setup

Note: Much of this chapter is adapted from my previous publication ”Semi-automated high-efficiency reflectivity chamber for vacuum UV measurements” [12]

Figure 2.1: A rendering of the experimental setup. (from Wiley et. al. 2017) [12]

12 2.1 Instrument Overview

An automated vacuum ultraviolet reflectivity chamber (figure 2.1) was designed for high cadence sample measurement from the extreme ultraviolet (EUV) to the NIR. Group experience with other reflectance chambers influenced the design of the experimental setup [11, 13]. The setup consists of a vacuum chamber connected to a light source section and pumping/vacuum assembly. The whole setup is supported by a mobile cart made of parts from 80/20.

Inside the chamber is an optics plate which has a diode mount, an optics mount, and a cable guide connected to it. The diode and optics mount are placed in a way such that each optic will reflect into the diode by rotating the plate. The optics plate is mounted on top of a vacuum-safe rotation stage. For the light source, a tip-tilt stage on a colli- mating off-axis parabolic mirror (OAP) moves the beam to perform detector peak-ups making the setup capable of more precise measurements. This OAP reflects monochro- matic ultraviolet light from a monochromator. The monochromator is controlled to select wavelengths by rotating a grating that reflects light incident on a slit. Light is input to the monochromator from a flow lamp that excites hydrogen- gas. The flow lamp is RF excited and uses wall power increasing the mobility and efficiency of the setup as a traditional arc lamp would require high voltage and a water cooler.

The system is controlled by a computer using software written in LabVIEW. The soft- ware presents an intuitive graphical user interface which communicates with the detector readout on a nanovoltmeter, the rotation stage controller, the tip-tilt stage controller, the monochromator controller, the pumping system, and pressure gauges. The system performs automatic routines such as moving to incident and reflected positions, detec- tor peak-ups, data analysis and recording, and error handling. The LabVIEW interface gives extensive control to the user for uses such as non-reflectivity testing and mainte- nance while being optimized to reduce testing times and oversight when measuring eLiF coatings.

13 2.2 Vacuum Chamber

Figure 2.2: Solidworks rendering of the empty vacuum chamber with both lids removed. (From Wiley et. al. 2017) [12]

The vacuum chamber was fabricated by Nor-Cal Products out of AISI 304 steel with internal dimensions 27”x27”x17” (figure 2.2). It operates at pressures below 10−5 torr. There are two lids which are individually sealed; one lid is for installation of large features with the other, smaller lid hinged to the larger lid for ease when loading and removing samples. The chamber contains four NW-63 flanges, two NW-80 flanges, and one rectan- gular flange for feedthrus, light sources, detectors, and other hardware. The pump port is an NW-160 flange on the bottom of the chamber which is protected by a screen so that small objects do not fall into the pumping system. The chamber contains a welded-on bracket around its perimeter so it rests on the 80/20 frame with a bolt pattern to affix it.

A vacuum-safe Newport URM-150PP rotation stage is mounted in the center of the chamber. A light weighted 24” diameter optics plate is mounted to the top of the rotation

14 Figure 2.3: Benchtop setup at incident position. (from Wiley et. al. 2017) [12]

Figure 2.4: Renderings of reflected positions. (from Wiley et. al. 2017) [12] stage. The optics mount and the diode mount are placed on the optics plate such that the beam of light from the light source section will reflect off one of the three mirrors and in to the detector in three different positions while having the optics mount not block the light when taking incident measurements. The system allows three samples to be measured during a single testing session with only a single motorized stage, reducing complexity and background noise due to motion in a vacuum. Table 2.1 and figures 2.3 and 2.4 show the incident and reflected positions which are all at small enough angles to reduce splitting.

15 Position Rotation Stage Position Reflected Angle Detector Angle 1 0◦ — 9◦ 2 139◦ 10◦ 12◦ 3 150.5◦ 7.5◦ 5.5◦ 4 162.5◦ 4.5◦ 0.5◦

Table 2.1: Incident and reflected positions (from Wiley et. al. 2017) [12]

The detector cabling is connected to a cable guide made of delrin, which guides it to a feedthru on the NW-63 flange on the larger lid centered over the the optics plate. The guide rotates with the optics plate such that the cables do not interfere with the beam of light or get pulled or crimped. Power is supplied to the diode by a D-Sub9 connector and a BNC carries the diode readout. The rotation stage connector is a D-sub25 which connects to a feedthru on the NW-63 flange on the side of the chamber.

The optics mount (figure 2.5) was designed with 9◦ angles between each surface. This is for each optic to re- flect in to the diode at the three re- flected positions. The mount easily slides on and off its stand for ease when placing samples in the chamber. The optics mount can be placed in the humidor chambers, making transition- ing from humidor chamber to vacuum chamber more efficient.

The vacuum chamber was designed to reduce background noise as much as Figure 2.5: Rendering of the optics mount. possible. The vacuum-safe harnessing attached to the rotation stage and diode uses fully shielded cabling to reduce induced noise in signal carrying wires. Utilizing only a single stage and careful wire harnessing reduces ion generation and therefore electrical noise on signal lines. By attaching the

16 617MX vacuum preamplifier directly to the diode itself, the diode can resist noise on its low-level signal. This is done in contrast to an exterior amplifier where induced noise on the cable run to the amplifier would induce extra noise in the amplified signal.

The photodiode used for this setup is an Opto-Diode Corp AXUV100G, housed with the preamplifier in a mount made of PEEK. A photodiode was chosen as the detector for its sensitivity to a large bandpass, its linear responsivity, and its simple operational setup. Noise reduction efforts inside the chamber will enable high precision measurements with the photodiode detector.

2.3 Light Source

The three components of the light source section (figure 2.6) in the experimental setup are the collimator, monochromator, and lamp. These components are necessary for producing a monochromatic, UV, collimated beam of light. The light source section is under a differential vacuum with the lamp held at an operating pressure of about 25mtorr and the monochromator and collimator held at high vacuum. The lamp does not contain a window, but a pinhole serves as the barrier between the two vacuum sections. Light from the lamp reflects off the monochromator’s grating on to a slit at the monochromator exit plane. An OAP folds the beam in to the chamber.

The lamp used is a Resonance LTD. EUV-XL-L Flow Lamp for its mobility and ease of use. Because it is an RF excited lamp, it does not need high volts or liquid cooling like an arc lamp would. The RF flow lamp uses hydrogen-argon gas to test the eLiF samples, but can produce spectra from many different gas sources, enabling extreme vacuum UV to IR light sourcing. The pressure and flow rate in the lamp is controlled by a micrometer. There will be some gas leakage through the pinhole into the chamber, but previous experience with similar lamps shows this will have a negligible effect. The lamp uses a fan for cooling, and is connected to the setup with a 2.75” conflat flange that connects to an adapter to the monochromator.

The monochromator used is an Princeton Instruments VM-502, which contains an

17 Figure 2.6: Rendering of the light source section. (from Wiley et. al. 2017) [12]

aberration-corrected concave holographic grating in a kinematic mount which is controlled by a motor. Light input to the monochromator is reflected off the grating onto a slit to select the wavelength. The slit width which defines spectral purity is controlled by a micrometer. A secondary diverter mirror in the monochromator can be moved in front of the input light for performing dark measurements. The monochromator has a flange on its bottom which to the pumping system. Testable wavelengths of light are limited by the monochromator’s operational bandpass of 70-550 nm. Tests in the IR are possible by replacing the monochromator with a halogen source.

The collimator (figure 2.7) is housed in a vacuum box that attaches to the monochro- mator by a custom flange and to the vacuum chamber with an NW-63 flange. The collimator consists of an OAP mounted to a Newport 8821-L-UHV tip-tilt stage placed

18 Figure 2.7: Rendering of the inside of the collimator box. (from Wiley et. al. 2017) [12] on a Thorlabs PY005 five-axis stage. The five-axis stage is for initial alignment and collimation to center the tip-tilt range of motion. The tip-tilt stage will then be used for beam scanning and detector peak-ups. The stage is controlled by wires connected to a D-Sub9 feedthru on the top flange. Other flanges are placed around the collimating box for other feedthrus and hardware. Figure 2.8 shows the maximum distance scannable by the tip-tilt stage (±5◦), which is about 5” at the optics mount in the reflected position. Beam steering increases versatility and has many applications such as testing larger optics and detectors. The ability to steer the beam allows for more precision when measuring reflectivities and supports the use of a single stage inside the chamber.

19 Figure 2.8: Rendering of the cross-section view of the beam at ±5◦ vertically . (from Wiley et. al. 2017) [12]

2.4 Computer Control

A custom graphical user interface written in LabVIEW consisting of multiple virtual instruments (VI) corresponding to real instrument drivers controls the automation as- pect of the experimental setup. The program communicates with a Newport MM3000 motion controller which controls the rotation stage, a Keithley 2182A Nanovoltmeter for taking measurements from the detector, a Leybold 450ix TurboPump, a Newport 8742 Picomotor for controlling the collimator tip-tilt stage, a Princeton Instruments SD-3 monochromator controller for selecting wavelengths, and a LabJack U6 which reads from MKS 925 MicroPirani and 972B DualMag vacuum gauges for monitoring pressures. Ex- tra functionality is added to each VI for modularity and future incorporation into other programs. Most instruments communicate over USB, with the MM3000 connected via RS232 and the 2182A Nanovoltmeter via GPIB.

The MM3000 VI provides acceleration and velocity control for the rotation stage, but limits the user to remain within preset safety limits. The VI saves incident and reflected positions so the user can return to the exact location for measurement repeatability. The system automatically peaks-up the detector through the 8742 Picomotor VI by scanning

20 Figure 2.9: Control program written in LabVIEW. (from Wiley et. al. 2017) [12] the beam to account for hysteresis in the rotation stage. Because the rotation stage and the tip-tilt stage are the only two moving parts other than the monochromator’s grating, the VI controlling the MM3000 and the 8742 Picomotor Controller provide everything necessary for autonomously moving between incident and reflected positions.

The VI controlling the 2182A Nanovoltmeter allows for the adjustment of a variety of parameters that affect the precision and speed at which measurements are gathered. An essential measurement parameter is integration time which is in units of power line cycles (PLC). A large source of noise comes from the use of 60Hz AC power, and integrating for at least 1 PLC can alleviate power line noise problems from a measurement. The Nanovoltmeter has a low-pass analog filter and a configurable digital averaging filter. The analog filter increases the noise rejection ratio at 60Hz, further attenuating the effects of AC power on the system. Digital filtering uses either a moving or repeating average filter that has a configurable stack size.

The measurements are taken from the device and stored in a buffer to enable high- speed, efficient communication and to allow the application of additional software filtering to the data. The data is time-stamped, taken from the buffer, and streamed to a file to allow for easy reduction. The software displays the photodiode measurements, with the capability to scale the data to a variety of formats appropriate for the dynamic range of

21 the measurements.

The SD-3 monochromator VI controls the grating in the monochromator. It can be used to scan through wavelengths and to find spectral lines using feedback from the Nanovoltmeter to peak-up spectral line intensity. The LabVIEW interface begins at a user defined wavelength, performs incident, reflected, and background measurements, and then switches wavelengths for the next round of measurements.

The software allows for control of the 450ix TurboPump and reads out stator fre- quency, motor current, temperature and intermediate circuit voltage. The pressure mon- itor VI reads the MKS pressure gauges from a LabJack U6 Analog and DIO module. The pressures are calculated from voltage differentials depending on the calibration set- tings for low vacuum or high vacuum. The software logs all pressures and handles errors appropriately.

The combination of all these VIs (figure 2.9) will be the base for complete automation of the experimental setup. Software will be written to take completely autonomous mea- surements with the press of a button. The goal of automation is to increase lab efficiency, increase precision and accuracy, and decrease variability due to human interaction.

2.5 Operational Walkthrough

To take reflectivity measurements, the user first purges the vacuum chamber to atmo- spheric pressure with N2 by opening a valve on the side of the chamber, and then places the sample mount on the mount holder in the chamber. The sample mount slides into place on a rail for repeatable mounting. The user then closes the lid, starts the IDP-15 rough pump, and opens gate valves to both the chamber and the lamp. When the pres- sure monitor VI reads out pressures in the chamber < 10−1 torr, the user then starts the turbo pump via the LabVIEW interface by enabling remote control and turning on the pump. To start the EUV-XL-L Flow Lamp, the user opens the hydrogen-argon supply and controls the flow of gas by using the micrometer on the lamp. The user brings the lamp to a lighting pressure of about 80 mtorr and then turns on the lamp and brings the

22 pressure down to an operating pressure of about 25 mtorr.

At pressures < 10−5 torr inside the chamber, the user moves to the incident position by using the MM3000 VI and scans to the desired starting wavelength (likely Lyman alpha) via the monochromator VI. Micrometers on the monochromator can be controlled to change intensity and spectral purity until the desired starting intensity is reached. Each set of measurements will begin at the same spectral line with the same starting incident intensity for consistency in an effort to reduce systematic error and for repeatability. The user then takes incident measurements and begins moving to the reflected position by inputing the position to the MM3000 VI. At each position, the Picomotor performs a peak-up routine to find the center of the diode. A dark measurement is taken before and after each incident or reflected measurement. After taking reflected measurements of each optic, the user returns to the incident position for another measurement to sandwich reflection data between incident measurements to check for lamp variability. If there is too much variability between before and after incident measurements for a given wavelength, the measurement is retaken.

After completing all measurements, the user slowly purges the chamber to atmospheric pressure with N2 to retrieve the samples. The user then pumps the chamber back to

−5 < 10 torr before purging it to a partial pressure of 170 torr with N2 to keep the chamber clean. When the chamber is unused for long periods of time, a long-term pressure monitoring program is set to run to monitor for pressure consistency.

2.6 Initial Calibration

Figure 2.10 shows the residual gas analysis (RGA) of the chamber. This data was taken with an SRS RGA 200. The main molecules identified by the RGA are hydrogen (H, 1

amu; H2, 2 amu), nitrogen (N, 14 amu; N2, 28 amu), oxygen (O 16 amu, O2 32 amu),

water (OH, 17 amu; H2O, 18 amu), carbon monoxide (28 amu), and carbon dioxide (CO2, 44 amu) There were very low traces of heavy molecules and hydrocarbons, showing that the chamber is environmentally safe for testing sensitive optics.

23 Figure 2.10: RGA of the chamber.

Initial reflectivity measurements were made using a HeNe laser light source. This was due to the fact that a laser is easy to see, collimated, and monochromatic which simulates the type of tests that will be run in the chamber. The samples used were pristine AlF3+Al. Table 2.2 shows reflectivity data taken using the laser. Each reflectivity is calculated as:

I − d R = r . (2.1) I − d

where Ir is reflected intensity, I is the incident intensity, and d is the dark signal.

Light (mV) Dark (mV) Reflectivity Incident 21.4 ± 0.6 7.668 ± 0.002 — Sample 1 19.2 ± 0.5 7.716 ± 0.003 (84 ± 5)% Sample 2 20.7 ± 0.9 7.743 ± 0.002 (94 ± 8)% Sample 3 20.0 ± 0.5 7.7505 ± 0.0006 (89 ± 5)%

Table 2.2: Data taken at 633nm using a HeNe laser. The samples tested were AlF3+Al.

24 Each measurement was taken using an integration time of one PLC, and sampling time of 10 seconds. The uncertainty in the measurements is the standard deviation from the Nanovoltmeter readings. The reflectivities are close to what I would expect for the optics tested (∼ 90%). The high uncertainty in the light measurements was due to high variations in the laser intensity. This data is a proof of concept for the experimental setup, as it has not been fully optimized or calibrated yet. A problem this data exposes is the high background inside the chamber. The back- ground of 7.7 mV corresponds to 400 million photons a second being detected by the photodiode, which will make testing low-intensity ultraviolet beams difficult. This back- ground could be due to a number of things, from stray capacitance in the harnessing to light leaking in the chamber from feedthrus. More work needs to be done to identify any issues in the diode cabling and to reduce noise inside the chamber.

25 Chapter 3

Future Work and Conclusion

3.1 Current State of Experimental Setup

Figure 3.1: Current State of the Experimental Setup.

26 Figure 3.1 shows the current state of the experimental setup. The monochromator is the only part missing due to the kinematic motor breaking and currently be- ing fixed by Princeton Instru- ments. Due to not being able to test light from the monochroma- tor, a pinhole mount was designed SOLIDWORKS Educational Product. For Instructional Use Only and fabricated to simulate the Figure 3.2: Pinhole mount to simulate the monochro- mator exit slit. monochromator’s exit slit. This pinhole was designed to be at the same focal distance from the OAP as the monochromator’s slit and separated from the collimator box by a vacuum viewport. Optical light produced from a fiber source was input to the pinhole and used to configure the collimator.

Figure 3.2 shows the pinhole mount that was designed to simulate the monochroma- tor’s exit slit. A 30 micron pinhole is set 3” away from the OAP. A fiber is fed through the hole on the other side of the mount to keep it steady while outputting light through the pinhole. The fiber mount is attached to the outside of a 2.75” conflat window to take measurements at vacuum. This was the setup used when taking measurements as shown in table 2.2 from section 2.6 using a laser source instead of the optical fiber.

This setup provides collimated light that could be measured both at incident and reflected positions, and should make realigning the chamber and collimator much easier after installing the monochromator.

3.2 Future Work

Future work still necessary on the experimental setup is the installation and calibration of the monochromator. The motor which controls the kinematic mount for the grating is

27 currently being fixed by Princeton Instruments, and is the last part necessary for taking ultraviolet measurements. Once installed, it may be necessary to slightly reconfigure the collimator to produce a consistent beam of light. Calibration in the UV will then be necessary, and hopefully the work done calibrating the chamber at optical wavelengths will translate well to the ultraviolet.

Noise reduction efforts are very important, as background noise during optical testing was extremely high. This can be achieved by decreasing wire lengths inside the chamber and insulating the diode better. Some noise may be coming from the feedthrus on the chamber, so testing and possibly replacing them may be necessary. The detector is housed in a mount fabricated from PEEK, which has shown high initial background counts but reduced noise when left in vacuum for an extended amount of time on other detectors. There could also be a stray capacitance issue, or an issue with the detector cabling.

The main future goal of the setup is complete automation. Future work on the software aspect of the experimental setup is necessary to create a completely autonomous experiment. The automated program should be built off of currently existing software, while providing more error handling due to decreased user input. The final goal would be to take reflectivity measurements at the push of a button. Once the chamber is fully autonomous, it will drastically improve testing times, decrease user oversight, and allow for more consistent and accurate measurements.

More effort could be placed in increasing the mobility and efficiency of the setup, such as installing a drawer with clean parts for easy maintenance of the chamber. Cable handling could be improved as currently all power cables are plugged in to a power strip on the floor and most other cables are randomly placed through the setup. A power strip could be installed along one of the rods of 80/20 and all cables can be shortened.

The chamber can also be used for more than reflectivity testing of mirror coatings. Some future work is necessary to configure the chamber to test gratings, detectors, and non-flat optics. This will require new hardware implementations, and new software to be written. Because of the modularity of the current programs written in LabVIEW, new software would only have to call upon current control programs and will hopefully

28 be easy to implement. The chamber is also configurable to take infrared measurements, although this will require changing the light source.

We currently are in possession of a set of eLiF, AlF3 and MgF2 protected eLiF, and

AlF3+Al mirror samples that are in safe storage in an N2 purge environment. Once the chamber is fully operational, the samples will be characterized and then aged in a set of custom humidor chambers and then remeasured periodically for over one year to track degradation with exposure. These samples will then be compared to show which material is the most spaceworthy. If the chamber proves to be as accurate and efficient as intended, then it may be used to characterize optics from other laboratories as well.

A proposed sounding rocket mission, The Suborbital Imaging Spectrograph for Tran- sition region Irradiance from Nearby Exoplanet host stars (SISTINE, PI KevinFrance, figure 3.3) [5] will serve as the flight testbed for eLiF coatings [5]. It will have a 0.5m eLiF coated primary mirror and grating, and may have eLiF variant coatings for the secondary and fold mirrors. More future work needs to be done for eLiF to be used for future astronomical observatories such as scaling the coating process to larger mirrors.

Although there is a lot of future work that could be done, the state of the chamber once the monochromator is installed should be a significant improvement over the square tank.

Figure 3.3: A raytrace of the SISTINE optical assembly (from. Fleming 2015)[2]

29 3.3 Conclusion

Enhanced LiF+Al coatings developed by GSFC have the potential to increase through- put in the LUV for future UV-sensitive space observatories. This automated reflectivity chamber will provide enhanced efficiency and accuracy compared to existing FUV reflec- tivity chambers currently being used for testing these coatings. The components were chosen for their ease of use, flexibility, and reliability. The chamber will operate with very low background noise, and will allow for a wide variety of tests and future modifications. Utilizing custom designed hardware and a specially designed vacuum chamber, a sim- ple optical setup, and computer control, this experimental setup will decrease oversight and testing times while still producing high quality measurements relative to previous characterization chambers.

The automation of the reflectivity chamber will further increase efficiency and allow faster coating characterization during the development of state of the art UV-optimized coatings. The preeminent future goal for the experimental setup is complete automation. It is currently controlled by a graphical user interface written in LabVIEW and provides some autonomous functionality to the user. Automation consists of building upon the foundational controls currently offered by the software and incorporating manual proce- dures recorded during calibration into the software. With proper error handling, these procedures can be run from the existing LabVIEW framework with minimal user input. This automation will further increase efficiency and allow faster development cycles while developing state of the art optical coatings.

Some future work is still necessary to get the chamber to a highly autonomous state. More work is necessary to test gratings and detectors. Currently, the only component missing to perform ultraviolet reflectivity tests is the monochromator. Once it is received, calibration and preliminary tests will commence. These tests will compare the eLiF, AlF3 and MgF2 protected eLiF, and AlF3 coated mirrors. A planned sounding rocket at CU Boulder, the Suborbital Imaging Spectrograph for Transition Region Irradiance from Nearby Exoplanet host stars (SISTINE), will test the eLiF coated mirrors in flight [5]. This sounding rocket and other missions are in

30 support of space observatories such as LUVOIR and HabEx, and will show the space worthiness of protected eLiF coatings. The automated vacuum ultraviolet reflectivity chamber will make characterizing these coatings easier, and increase support for future flagship missions.

31 Bibliography

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