Progress in UCO's search for silver-based telescope mirror coatings

Phillips, Andrew C.; Miller, Joseph S.; Bolte, Michael; Dupraw, Brian; Radovan, Matthew V.; Cowley, David J.

doi:10.1117/12.925502

Cited by 8 publications

(6 citation statements)

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“…Ion bombardment of immediately postdeposited Ag films has been shown to enhance optical properties of protected Ag films. However, previous work with the same deposition chamber has shown decreased Ag film reflectivity when Ag deposition is ion-assisted throughout the entire 120 nm film thickness . Huang et al have also reported less desirable optical and electrical properties of Ag films deposited with ion assist when compared to evaporation without any ions.…”

Section: Results and Discussionmentioning

confidence: 97%

“…Long-lasting Ag mirrors must be protected (i.e., Ag-based protected mirror) by barrier overlayers of transparent dielectrics to prevent tarnish and corrosion. The University of California Observatories has undertaken a program to develop and/or identify high-performance coatings useful for astronomical optics (Phillips et al − ). A strong motivation for our research is to develop durable Ag-based protected mirrors that meet the requirements of the Thirty-Meter Telescope (TMT) project, which requires high reflectivity in the spectrum 0.34 < λ < 28 μm…”

Section: Introductionmentioning

confidence: 99%

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Silver Film Surface Modification by Ion Bombardment Decreases Surface Plasmon Resonance Absorption

Fryauf

León

Phillips

et al. 2017

ACS Appl. Mater. Interfaces

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Silver thin films covered with dielectric films serving as protective coatings are desired for telescope mirrors, but durable coatings have proved elusive. As part of an effort to develop long-lived protected-silver mirrors, silver thin films were deposited by electron beam evaporation using a physical vapor deposition system at the University of California Observatories Astronomical Coatings Lab. The silver films were later covered with a stack of dielectric films utilizing silicon nitride and titanium dioxide deposited by ion-assisted electron beam evaporation to fabricate protected mirrors. In-situ argon ion bombardment was introduced after silver deposition and prior to the deposition of dielectric films to assess its effects on the performance of the mirrors. We found that ion bombardment of the silver influenced surface morphology and reflectivity, and these effects correlated with time between silver deposition and ion bombardment. The overall reflectivity at wavelengths in the range of 350-800 nm was found to improve due to ion bombardment, which was qualitatively interpreted as a result of decreased surface plasmon resonance coupling. We suggest that the observed decrease in coupling is caused by silver grain boundary pinning due to ion bombardment suppressing silver surface diffusion, forming smoother silver-dielectric interfaces.

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Section: Results and Discussionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Silver Film Surface Modification by Ion Bombardment Decreases Surface Plasmon Resonance Absorption

Fryauf

León

Phillips

et al. 2017

ACS Appl. Mater. Interfaces

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“…With an established scalable deposition process of low-temperature AlO x , future work will utilize the scaling process parameters identified in Table to study scalability of different thin-film chemistry processes in the MSAS. While AlO x is generally a proficient corrosion barrier for many applications, extensive studies have shown that it is not an optimized stand-alone coating material to protect against environmental corrosion because of its eventual dissolution from atmospheric moisture. − Therefore, future work will explore low-temperature deposition processes with the MSAS of thin-film materials that have shown promising corrosion barrier properties on Ag in previous work when deposited by PVD techniques, such as TiO 2 and Si 3 N 4 . − The optimized process conditions and lessons in this work will be used to further optimize MSAS process performance for metal oxide thin films, specifically with the use of alternative oxidizing reactants that can be purged quicker than H 2 O, such as ozone, oxygen plasma, and wet Ar plasma. Detecting and eliminating vacuum leaks, minimizing gas flow-dependent growth patterns with improved flow diverting hardware, and precisely calibrating and controlling chamber and substrate temperatures are unique challenges of this new large-area ALD technique that will be addressed in future work exploring protected Ag mirror barrier coatings.…”

Section: Resultsmentioning

confidence: 99%

“…Even the Gemini coating comes with the cost of sacrificing deep blue and UV portions of the spectrum because of unwanted absorption primarily from the nickel–chromium nitride adhesion layer, , which is an unacceptable compromise for many astronomical programs. The Advanced Coatings Lab of the University of California Observatories (UCO) has been identifying and developing high-performance coatings useful for astronomical optics. − The Thirty-Meter Telescope (TMT) project has stringent requirements of broadband high reflectivity (0.34 < λ < 28 μm) and “lifetimes” of 5–10 years between recoating, which has supplied the motivation for this research and development of durable Ag mirror coatings.…”

Section: Introductionmentioning

confidence: 99%

Scaling Atomic Layer Deposition to Astronomical Optic Sizes: Low-Temperature Aluminum Oxide in a Meter-Sized Chamber

Fryauf¹,

Phillips²,

Bolte³

et al. 2018

ACS Appl. Mater. Interfaces

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Atomic Layer Deposition (ALD) is very attractive for producing optical quality thin films, including transparent barrier films on metal-coated astronomical mirrors. To date, ALD of mirror coatings has been limited to relatively small-sized substrates. A new ALD tool has been designed, constructed, and tested to apply uniform protective coatings over a 0.9 m diameter substrate in a 1 m diameter scale deposition plane. The new tool, which we have named the meter scale ALD system (MSAS), employs a unique chamber design that isolates a large substrate surface to be coated by utilizing the substrate as a wall of the reaction chamber. The MSAS is mechanically designed to be rapidly reconfigurable for selective area coating of custom substrates with arbitrary shape, size, and permanent backside hardware attachments. The design, implementation, results, and future applications of this new tool are discussed for coating large-area optical substrates, specifically protective coatings for silver mirrors, and other future large astronomical optics. To demonstrate the potential of this new design, aluminum oxide was deposited by thermal ALD using trimethylaluminum and water at a low reaction temperature of 60 °C. Growth rate and uniformity, which are dependent on precursor pulse times and chamber purge times, show that the two half-reactions occur in a saturated regime, matching typical characteristics of ideal ALD behavior. Aluminum oxide deposition process parameters of the MSAS are compared with those of a conventional 100 mm wafer-scale ALD tool, and saturated ALD growth over the 0.9 m substrate is realized with a simple scaling factor applied to precursor pulse and purge times. This initial test shows that lateral thickness uniformity across a 0.9 m substrate is within 2.5% of the average film thickness, and simple steps to realize 1% uniformity have been identified for next growths. Results show promising application of transparent robust dielectric films as uniform coatings across large optical components scaled to meter-sized substrates.

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“…However, Xu et al have studied the adherence difference between Ag and MgF 2 , Al 2 O 3 , SiO 2 . They found SiO 2 cannot be used as a protective coating due to its weak adherence and humidity for the metallic Ag layer [ 23 , 24 ]. In Figure 4 , Layer 2 and 4 are designed as buffer layers on the silver layer for binding glass substrate and the enhancement stack.…”

Section: Discussionmentioning

confidence: 99%

Highly Reflective Silver-Enhanced Coating with High Adhesion and Sulfurization Resistance for Telescopes

et al. 2022

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Highly reflective metal coatings are essential for manufacturing reflecting telescope mirrors to achieve the highest reflectivity with broad spectral bandwidth. Among metallic materials, enhanced silver-based coatings can provide higher reflectivity in the 400–500 nm spectral range to better performance from visible to near IR. Moreover, over-coating a dielectric protective layer on the mirror’s front side attains additional hardness and oxidation stability. In this paper, we study a combination of thermal and electron beam evaporation as a technology to form protected enhanced high reflective Ag coatings. A newly designed multiplayer film can pass ASTM 5B adhesive performance testing and give sulfurization inhibition. The average specular reflectivity for the enhancement coating is about 98% in wavelengths across the spectral range from 400–1000 nm. This innovation has been demonstrated on a Newtonian type telescope, with storage in an ambiance humidity H = 60–85%, and temperature T = 10–35 °C, for more than six months without degradation in coating performance.

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Progress in UCO's search for silver-based telescope mirror coatings

Cited by 8 publications

References 0 publications

Silver Film Surface Modification by Ion Bombardment Decreases Surface Plasmon Resonance Absorption

Silver Film Surface Modification by Ion Bombardment Decreases Surface Plasmon Resonance Absorption

Scaling Atomic Layer Deposition to Astronomical Optic Sizes: Low-Temperature Aluminum Oxide in a Meter-Sized Chamber

Highly Reflective Silver-Enhanced Coating with High Adhesion and Sulfurization Resistance for Telescopes

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