PECVD Sinx Thin Films for Protecting Highly Reflective Silver Mirrors

Article PECVD SiNx Thin Films for Protecting Highly Reﬂective Silver Mirrors: Are They Better Than ALD AlOx Films?

Pavel Bulkin 1,* , Patrick Chapon 2, Dmitri Daineka 1, Guili Zhao 1 and Nataliya Kundikova 3,4

1 Laboratoire de Physique des Interfaces et des Couches Minces, Centre Nationale de la Recherche Scientiﬁque, Ecole Polytechnique, Institute Polytechnique de Paris, 91128 Palaiseau, France; [email protected] (D.D.); [email protected] (G.Z.) 2 HORIBA FRANCE S.A.S., Boulevard Thomas Gobert, Passage Jobin Yvon, 91120 Palaiseau, France; [email protected] 3 Optoinformatics Department, Physics Faculty, Institute of Natural Sciences and Mathematics, South Ural State University, 454080 Chelyabinsk, Russia; [email protected] 4 Nonlinear Optics Laboratory, Institute of Electrophysics, Ural Branch of the Russian Academy of Sciences, 620016 Yekaterinburg, Russia * Correspondence: [email protected]

Abstract: Protection of silver surface from corrosion is an important topic, as this metal is highly susceptible to damage by atomic oxygen, halogenated, acidic and sulfur-containing molecules. Protective coatings need to be efficient at relatively small thicknesses, transparent and must not affect the surface in any detrimental way, during the deposition or over its lifetime. We compare PECVD-deposited SiNx films to efficiency of ALD-deposited AlOx films as protectors of front surface silver mirrors against damage by oxygen plasma. Films of different thickness were deposited at

Citation: Bulkin, P.; Chapon, P.; room temperature and exposed to O2 ECR-plasma for various durations. Results were analyzed with Daineka, D.; Zhao, G.; Kundikova, N. optical and SEM microscopy, pulsed GD-OES, spectroscopic ellipsometry and spectrophotometry PECVD SiNx Thin Films for on reflection. Studies indicate that both films provide protection after certain minimal thickness. Protecting Highly Reflective Silver While this critical thickness seems to be smaller for SiNx films during short plasma exposures, longer Mirrors: Are They Better Than ALD plasma treatment reveals that the local defects in PECVD-deposited films (most likely due to erosion Coatings 11 AlOx Films? 2021, , 771. of some regions of the film and pinholes) steadily multiply with time of treatment and lead to slow https://doi.org/10.3390/coatings drop of reflectance of SiNx-protected mirrors, whereas we showed before that ALD-deposited AlOx 11070771 films reliably protect silver surface during long plasma exposures.

Academic Editors: Charafeddine Keywords: Jama and Georgios Skordaris PECVD; front surface silver mirrors; pulsed GD-OES; protective coatings

Received: 3 May 2021 Accepted: 22 June 2021 Published: 26 June 2021 1. Introduction From conservation of cultural artefacts to protection of reflective optics, the slowing Publisher’s Note: MDPI stays neutral down or altogether stopping corrosion of silver is an important technological problem. with regard to jurisdictional claims in Being the metal with highest reflectivity in visible and infrared wavelength ranges, lowest published maps and institutional affil- emissivity and polarization splitting and highest electrical conductivity [1], silver has many iations. uses, but it is not very stable in the environment as it tarnishes easily in contact with acidic vapors, halogens, sulfuric compounds, ozone etc. The usual approach for protection of silver surface is a deposition of thin transparent overcoat film, which isolates the silver from the atmosphere. Depending on the specific application area and expected environment for Copyright: © 2021 by the authors. use, the requirements for protective film will vary. Licensee MDPI, Basel, Switzerland. Front surface mirrors are important parts of telescope designs. For extending from This article is an open access article visible to infrared range in telescopes, either silver or aluminum can be used. Aluminum is distributed under the terms and far more resistant to aging and corrosion, but inferior to silver for all wavelengths, except conditions of the Creative Commons ultraviolet. To minimize the losses and maximize signal-to-noise ratio, silver would be the Attribution (CC BY) license (https:// preferred material, but until recently its use was severely hampered by its sensitivity to creativecommons.org/licenses/by/ oxidation. For use in space optics, especially on low Earth orbit (LEO), where the most 4.0/).

Coatings 2021, 11, 771. https://doi.org/10.3390/coatings11070771 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 771 2 of 10

abundant source of corrosion is atomic oxygen (AO) [2], the main properties required from protective coatings are resistance to oxidation, minimal influence on mirror reflectance over the whole range of interest and the preservation of silver minimal emissivity. Currently, a number of different materials are used for the thin film protective coatings, including Al2O3, TiO2, HfO2,Y2O3, Nb2O5, Ta2O5, SiO2, SiONx and Si3N4, as single films or in stacks [3–16]. Most of them are magnetron sputtered, but also ion-beam sputtered, evaporated with ion assistance and deposited with atomic layer deposition (ALD). To best of our knowledge, the protection with SiNx films grown by plasma-enhanced chemical vapor deposition (PECVD) has never been previously reported, while this deposition technique can provide high quality layers and is used in flat panel display industry on a scale of 10 m2 [17]. This article attempts to fill this gap by studying the behavior of mirrors with PECVD-deposited SiNx protective overcoats under exposure to high intensity oxygen plasma and comparing these results to our previous studies of the efficiency of ALD-deposited AlOx films [16]. While PECVD has more limitations than sputtering, CVD or ALD in terms of uniformity and conformality of the deposition, it has some advantages, such as easily adjustable refractive index of deposited films (using simple flow control) and relatively low deposition temperature, as well as the possibility to do surface pretreatment in situ by Ar or H2 plasma. PECVD is also a much faster deposition rate technique than ALD. In PECVD, gaseous precursors are used for depositing films and chemical reactions are driven by non-equilibrium plasma. Depending on the requirements, plasma can be generated either by radiofrequency electric fields in capacitive or inductive plasma sources, or by microwaves. In the latter case, the technology to use is the electron-cyclotron resonance (ECR) PECVD at low pressure (0.5–8.0 mTorr range) [18]. Such ECR-plasma produces large flux of low energy ions, which do not damage surfaces while helping to densify growing film. Sputtering, apparently, is a leading technology for both silver layer and protective coatings deposition, although it has only limited value in reactive sputtering of oxides over silver, since ionized oxygen will damage the silver layer. Sputtering produces dense films without columnar structure, but that are still susceptible to dust falling onto the wafer, resulting in pinholes, as the configuration with the mirror facing down is not always possible. ALD is very new to the field and is in an active testing period right now. The great advantage of ALD is the virtual absence of pinholes, due to the capability of this technology to fill very narrow spaces, effectively assuring the deposition under the particles of dust.

2. Materials and Methods

The preparation of silicon wafer and the conditions for RF sputtering and AlOx film deposition were described in detail elsewhere [16] and here we will recall them only briefly. After stripping native oxide from 4 inch silicon (100) wafers in 5% HF, the high-quality silver layers with a thickness of 200 nm were sputtered onto the silicon wafers at 250 ◦C. Total sputtering time was 8 min. Before and after the deposition of protective coatings, all mirrors were characterized by spectroscopic ellipsometry Uvisel 1 system (from HORIBA FRANCE, Palaiseau, France) and AFM microscope Dimension 5000 (from Veeco/Digital Instruments, USA) with cantilever ARROW-NCR (NanoWorld, Neuchâtel, Switzerland) in a tapping mode. Reflectance measurements were performed on a Perkin Elmer spectrophotometer Lambda 950 (Waltham, MA, USA) in 200 nm to 2600 nm wavelength range at an 8 degree angle of incidence. Protective SiNx layers were deposited in a custom-built 2.45 GHz electron cyclotron resonance (ECR) PECVD system at room temperature. The deposition system was described in detail earlier and here only exact deposition conditions will be given for brevity [19]. Pure silane and nitrogen were used as the precursor gases. Silane was injected near the substrate through the gas distribution ring, whereas nitrogen was injected via radial openings at the top of plasma chamber. The deposition power was kept at 1 kW and the pressure in the chamber was 2 mTorr, with SiH4 and N2 flows being 10 and 80 sccm, respectively. At those conditions, the deposition rate was 0.6 nm per second. Deposition time varied Coatings 2021, 11, x FOR PEER REVIEW 3 of 10

Coatings 2021, 11, 771 3 of 10 pressure in the chamber was 2 mTorr, with SiH4 and N2 flows being 10 and 80 sccm, respectively. At those conditions, the deposition rate was 0.6 nm per second. Deposition time varied from 3 to 30 s resulting in SiNx protective layers with thickness ranging from from1.5 nm 3 toto 3017 snm. resulting In comparison, in SiNx protective the deposition layers of with equivalent thickness film ranging thickness from by 1.5 ALD nm re- to 17quired nm. Ina time comparison, from 2.5 themin deposition to 25 min ofdue equivalent to the N2 film purges thickness required by ALDafter requiredeach precursor a time frominjection 2.5 minpulse. to 25For min silicon due nitride to the Ncharacterization2 purges required by afterspectroscopic each precursor ellipsometry, injection films pulse. of For60–70 silicon nm were nitride deposited characterization onto crystalline by spectroscopic silicon wafers, ellipsometry, with native films oxide of 60–70 removed nm were by depositedthe immersion onto in crystalline 5% HF solution silicon wafers,for 30 s. with native oxide removed by the immersion in 5% HFIn solutionour prior for study, 30 s. the protective films were deposited in a commercial ALD system R200In Advanced our prior study, (by Picosun the protective Oy, Espoo, films Finland) were deposited at 150 in °C a fromcommercial trimethilaluminum ALD system ◦ R200(TMA) Advanced and water, (by with Picosun a thickness Oy, Espoo, ranging Finland) from at 3 150nm toC 15 from nm trimethilaluminum [16]. In the current (TMA)study, andwe used water, the with same a thickness conditions ranging for the from silver 3 nmsputtering to 15 nm and [16 ].oxygen In the plasma current study,treatment we usedwith theonly same the protection conditions film for thebeing silver different. sputtering and oxygen plasma treatment with only the protectionSilicon film wafer beings with different. the mirrors were then quartered to do further tests. The same ECR Siliconplasma wafers system with was the used mirrors for oxygen were then plasma quartered exposures to do in further the following tests. The conditions: same ECR plasmaO2 flow system 40 sccm, was pressure used for 2 oxygenmTorr plasmaand MW exposures power 1 inkW. the Treatment following conditions:time was usually O2 flow 1 40min, sccm, but pressurefor several 2 mTorr tests the and time MW was power increased 1 kW. Treatment up to 40 min. time was usually 1 min, but for severalPulsed tests RF the GD time-OES was measurements increased up to of 40 the min. selected samples were done using GD Pro- filer-Pulsed2 system RF (from GD-OES HORIBA measurements FRANCE, Palaiseau, of the selected France) samples in order wereto establish done usingthe degree GD Profiler-2of silver layer system degradation (from HORIBA [20]. Some FRANCE, samples Palaiseau, were also France) examined in order with toSEM establish S4800 (Hi- the degreetachi High of silver-Tech, layer Japan) degradation and with [the20]. optical Some samplesmicroscope were BX40 also (from examined Olympus, with SEM Hamburg, S4800 (HitachiGermany) High-Tech, with 100× Tokyo, objective. Japan) and with the optical microscope BX40 (from Olympus, Hamburg, Germany) with 100× objective. 3. Results 3. Results 3.1. Material Properties 3.1.1. Silver The ellipsometric analysisanalysis ofof thethe depositeddeposited silversilver mirrorsmirrors andand thethe AFMAFM scansscans overover itsits surface showshow that that the the silver silver layer layer is ofis highof high quality. quality. The The direct direct inversion inversion of the of experimental the experi- ellipsometricmental ellipsometric data provides data provides the refractive the refractive index that index is better that than is better the refractive than the index refractive from Palik’sindex from database Palik’s of database optical properties, of optical propert as is shownies, as in is Figure shown1[ in21 Figure]. RMS 1 roughness[21]. RMS rough- of the silverness of layers the silver is also layers very small,is also belowvery small, 1 nm forbelow freshly 1 nm sputtered for freshly silver sputtered layers, silver as is evident layers, fromas is evident Figure2 .from Figure 2.

2.0 10

1.8 n Palik 9 k Palik 1.6 n exp 8 k exp 1.4 7

1.2 6

1.0 5

n 0.8 4

0.6 3

0.4 2

0.2 1

0.0 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 eV

Figure 1.1. Comparison betweenbetween thethe experimentallyexperimentally measured measured complex complex refractive refractive index index of of silver silver layers lay- anders and data data taken taken from from Palik’s Palik’s book book [21]. [21].

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CoatingsCoatings 20212021,,11 11,, 771x FOR PEER REVIEW 44 ofof 1010

Figure 2. AFM scan image and cross-sectional data extracted along the white line for the estimation of the roughness of a silver Figurelayer. 2. Figure 2. AFMAFM scanscan imageimage andand cross-sectionalcross-sectional datadata extractedextracted alongalong thethe whitewhite lineline forfor thethe estimationestimation ofof thethe roughnessroughness ofof aa silversilver layer.layer. 3.1.2. Silicon Nitride 3.1.2. Silicon Nitride Silicon3.1.2. Silicon nitride, Nitride deposited at an ambient temperature and with an excess of N2 flow, is a non-absorbingSilicon nitride, material deposited in a visible at an wavelength ambient temperature range and andthe uniformity with an excess of a oflayer N2 ﬂow, Silicon nitride, deposited at an ambient temperature and with an excess of N2 flow, depositedis a non-absorbing on a 100 mm wafer material is about in a visible5%. As wavelengthfar as absorption range in and the theinfrared uniformity part of of the a layer is a non-absorbing material in a visible wavelength range and the uniformity of a layer spectrumdeposited is concerned, on a 100 the mm presence wafer is of about Si–H, 5%. Si–N As and far N as– absorptionH bonds in inthe the material infrared should part of the spectrumdeposited ison concerned, a 100 mm thewafer presence is about of 5%. Si–H, As Si–N far as and absorption N–H bonds in the in theinfrared material part should of the not create problems, given the strength of oscillators and the small thickness of a layer. notspectrum create is problems, concerned, given the thepresence strength of Si of– oscillatorsH, Si–N and and N– theH bonds small thicknessin the material of a layer. should The complex refractive index of SiNx is shown in Figure 3 with the corresponding not createThe complex problems, refractive given the index strength of SiN of oscillatorsis shown and in Figure the small3 with thickness the corresponding of a layer. parameters of Tauc–Lorentz dispersion formulax given in Table 1 [22]. With a bandgap parametersThe complex of Tauc–Lorentz refractive index dispersion of SiN formulax is shown given in Figure in Table 31 with[ 22]. the With corresponding a bandgap above 4 eV, this silicon nitride cannot affect the reflectance values of the silver mirrors. aboveparam 4eters eV, thisof Tauc silicon–Lorentz nitride dispersion cannot affect formula the reﬂectance given in valuesTable 1 of [22]. the silverWith mirrors.a bandgap above 4 eV, this silicon nitride cannot affect the reflectance values of the silver mirrors. n k 1.95 0.20 n k 1.95 0.18 0.20

1.90 0.16 0.18

1.90 0.14 0.16

1.85 0.12 0.14

1.85 0.10 0.12

1.80 0.08 0.10

n 1.80 0.06 0.08

1.75 0.04 0.06

1.75 0.02 0.04

1.70 0.00 0.02 1 2 3 4 5 1.70 eV 0.00 1 2 3 4 5 Figure 3. Complex refractive index of PECVD-depositedeV silicon nitride as obtained from the ﬁt of Figureellipsometric 3. Complex refractive data. index of PECVD-deposited silicon nitride as obtained from the fit of ellipsometric data. TableFigure 1. 3.Parameters Complex refractive of Tauc–Lorentz index of dispersion PECVD-deposited of SiNx. silicon nitride as obtained from the fit of ellipsometric data.

Eg ε∞ AE0 C 4.088 2.709 239.05 3.387 0.956

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Table 1. Parameters of Tauc–Lorentz dispersion of SiNx.

Eg ε∞ A E0 C Coatings 2021, 11, 771 5 of 10 4.088 2.709 239.05 3.387 0.956

3.2. Silver Mirrors 3.2. SilverThe reflectance Mirrors of the fabricated silver mirrors (Figure 4) is 96% at 400 nm and above 98% Thein the reflectance visible range of the from fabricated 460 nm, silver exceeding mirrors (Figure99.5% in4) isthe 96% infrared at 400 range. nm and It aboveis not affected98% in the by the visible SiN rangex layers from in the 460 measured nm, exceeding range. 99.5% in the infrared range. It is not affectedThe bynon the-protected SiNx layers silver in the layer measured oxidizes range. quickly (full thickness of 200 nm with less than The1 min non-protected at test conditions) silver layerand at oxidizes just 1.5 quicklynanometers (full thicknessalready slows of 200 the nm process with less notice- than 1ably. min The at test reflectivity conditions) of andthe mirror at just 1.5 with nanometers a 4.5-nanometer already-thick slows coating the process does noticeably. not show Theany degradationreflectivity of of the reflection mirror with after a 4.5-nanometer-thick1 min of exposure to coating O2 plasma. does notA 40 show min any long degradation exposure, hoof reflectionwever, leads after to 1about min of10% exposure reflectivity to O 2dropplasma. across A all 40 spectrums, min long exposure, as can be however, seen in Figure leads 4.to Examination about 10% reflectivity of the surface drop acrossof the allcoating spectrums, reveals as cantwo bedistinct seen in kinds Figure of4 .defects Examination on the surface,of the surface seen below of the coatingin optical reveals and electron two distinct microscopy kinds of images. defects on the surface, seen below in optical and electron microscopy images.

Figure 4. Reflectance of the mirrors: unprotected (SiNx 0 nm) and protected with 1.5 nm of SiNx Figure 4. Reflectance of the mirrors: unprotected (SiNx 0 nm) and protected with 1.5 nm of SiNx and and 4.5 nm of SiNx before (non-oxidized) and after (oxidized) plasma treatment. For the mirror with 4.5 nm of SiNx before (non-oxidized) and after (oxidized) plasma treatment. For the mirror with 4.5 4.5 nm SiNx protective layer, 1 min of O2 plasma exposure does not change the reflectivity (the curves nm SiNx protective layer, 1 min of O2 plasma exposure does not change the reflectivity (the curves for non-oxidized and oxidized SiNx 4.5 nm layer are indistinguishable). for non-oxidized and oxidized SiNx 4.5 nm layer are indistinguishable). The scanning electron microscopy images (Figure5) and the optical microscopy imagesThe (Figure scanning6) reveal electron the microscopy extensive damage images (Figure for 4.5 nm5) and layers, the withoptical the microscopy film exposed im- agesduring (Figure 40 min 6) toreveal oxygen the extensive plasma demonstrating damage for 4.5 a nm halo layers, around with the the holes film resembling exposed dur- an ingundercut 40 min etch to oxygen defect. Gradualplasma demonstrating brightening of a the halo images around most the likely holes can resembling be explained an un- by dercutthe slow etch decrease defect. of Gradual the thickness brightening of a dielectric of the images layer. Themost left likely image can shows be explained one of the by rare the slowfabrication decrease defects, of the but thickness otherwise of itsa dielectric surface looks layer. very The uniform. left imag Twoe shows other imagesone of the display rare fabricationgradual deterioration defects, but of otherwise the surface its quality. surface looks very uniform. Two other images display gradual deterioration of the surface quality. The defect types seen in the electron microscopy images of the O2 plasma-treated mirrors suggest that these defects are rather the result of the erosion of SiNx films, most likely the spots with a high amount of the Si–H and N–H bonds that are present in this material [23]. On the other hand, the optical microscopy images show large-scale defects that look like deep craters with no silver left at the bottom (Figure6, right panel). The origin of such craters is not yet well understood; however, the pinholes due to the particulate contamination are unlikely to be the reason, as all defects of this kind have a nearly perfect circular shape. Again, the left image of the non-treated with O2 plasma mirror is only displaying a small scratch, as otherwise it was impossible to find the focus of the microscope, due to the perfect mirror surface. Coatings 2021, 11, 771 6 of 10 Coatings 2021, 11, x FOR PEER REVIEW 6 of 10

(a) (b) (c)

Figure 5. SEM photographs of the mirrors with a 4.5 nm SiNx protection film: (a) non-treated with O2 plasma; (b) treated Figure 5. SEM photographs of the mirrors with a 4.5 nm SiNx protection ﬁlm: (a) non-treated with O2 plasma; (b) treated with O2 plasma for 1 min; (c) treated with O2 plasma for 40 min. with O2 plasma for 1 min; (c) treated with O2 plasma for 40 min.

. (a) (b) (c)

Figure 6. Optical micrographs of the silver mirrors protected with a 4.5 nm layer of SiNx film and taken with 100× objective: Figure(a) non- 6.treatedOptical with micrographs O2 plasma of (a the scratch silver is mirrors used for protected the focusing with a purpose 4.5 nm layer only); of ( SiNb) treatedx film and with taken O2 plasma with 100 for× 1objective: min; (c) (treateda) non-treated with O2 withplasma O2 forplasma 40 min. (a Black scratch scale is used bar in for top the right focusing corner purpose of each only);image (representsb) treated with10 microns. O2 plasma for 1 min; (c) treated with O2 plasma for 40 min. Black scale bar in top right corner of each image represents 10 microns. The defect types seen in the electron microscopy images of the O2 plasma-treated mirrors3.3. GD-OES suggest Studies that these defects are rather the result of the erosion of SiNx films, most likelyThe the protected spots with mirrors a high that amount are non-treated of the Si–H with and the N oxygen–H bonds plasma that showare present distinct in layer this materialstructure, [23]. as can be seen in Figure7 for a 12 nm layer of SiN x on top of 200 nm silver layer depositedOn the on other a silicon hand, wafer. the optical The silver microscopy emission images signal, show the strongestlarge-scale by defects far, was that scaled look likedown deep by craters a factor with of 50 no for silver the sake left at of the the bottom visibility (Figure of other 6, right elements. panel). The origin of such cratersFigure is not8 yetpresents well understood; the spectra however, of the non-treated the pinholes and due 1 min to the oxygen-treated particulate contami- silver nationmirrors are covered unlikely with to be a the 1.5 reason, nm SiN asx layer. all defects The of initial this kind stages have of thea nearly GD-OES perfect sputtering circular shape.clearly Again, demonstrate the left that image for of a non-treatedthe non-treated sample, with both O2 plasma Si and Nmirror signals is only start displaying before the asilver small signal, scratch, indicating as otherw theise full it was coverage impossible of the silverto find layer the focus by the of protective the microscope, coating. due After to the1 min perfect of oxygen mirror plasma surface. exposure, the protective layer is destroyed and the silver signal starts at the very beginning. Thus, GD-OES analysis allows verifying the state of the coating 3.3.after GD the-OES plasma Studies treatment. A strong oxygen signal within the silver layer unambiguously confirmsThe protected oxidation ofmirrors a mirror. that are non-treated with the oxygen plasma show distinct As for the mirrors with a 4.5 nm SiNx protective layer, the GDOES profiles presented layer structure, as can be seen in Figure 7 for a 12 nm layer of SiNx on top of 200 nm silver layerin Figure deposited9 demonstrate on a silicon that wafer. 1 min The of O silver2 exposure emission at firstsignal, glance the strongest does not by lead far, to was the change of a protective layer and, consequently, to the degradation in the reflectance of scaled down by a factor of 50 for the sake of the visibility of other elements. the mirror. However, the SEM image shown above (Figure5, central panel) demonstrates that the damage was already done and a small oxygen peak at the interface between the SiNx layer and the silver layer demonstrates that (Figure9, left panel). After 40 min of O 2 plasma exposure, a silver peak appears from the very beginning and oxygen has already penetrated a silver layer through the small and large defects, created in the SiNx protective layer, as seen in the right panel of Figure9.

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Figure 7. GD-OES profiles of the elements in a not-treated with O2 plasma SiNx/Ag mirror. Silver signal intensity is divided by 50. The silver layer is fully covered by SiNx:H; all interfaces are resolved.

Figure 8 presents the spectra of the non-treated and 1 min oxygen-treated silver mirrors covered with a 1.5 nm SiNx layer. The initial stages of the GD-OES sputtering clearly demonstrate that for a non-treated sample, both Si and N signals start before the silver signal, indicating the full coverage of the silver layer by the protective coating. After 1 min of oxygen plasma exposure, the protective layer is destroyed and the silver signal starts at the very beginning. Thus, GD-OES analysis allows verifying the state of the coating Figure 7. GD-OES profiles of the elements in a not-treated with O2 plasma SiNx/Ag mirror. Silver Figureafter the 7. GD-OES plasma proﬁlestreatment. of the A elementsstrong oxygen in a not-treated signal within with O the2 plasma silver SiN layxer/Ag unambiguously mirror. Silver signal intensity is divided by 50. The silver layer is fully covered by SiNx:H; all interfaces are re- signalsolved.confirms intensity oxidation is divided of bya mirror. 50. The silver layer is fully covered by SiNx:H; all interfaces are resolved.

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mirror. However, the SEM image shown above (Figure 5, central panel) demonstrates that the damage was already done and a small oxygen peak at the interface between the SiNx

layer and the silver layer demonstrates that (Figure 9, left panel). After 40 min of O2 (a) (b) plasma exposure, a silver peak appears from the very beginning and oxygen has already

FigureFigure 8. 8.GD-OES GD-OES proﬁles profiles of of elements: elements:penetrated (a ()a non-treated) non a -silvertreated inlayer in O 2O plasma2through plasma SiN SiN thex/Agx /Agsmallmirror mirror and with withlarge an an defects, 1.5 1.5 nm nm SiN createdSiNx xprotection protection in the layer;SiN layer;x protective (b) treated with O2 plasma for one minute. (b) treated with O2 plasma for onelayer, minute. as seen in the right panel of Figure 9. (a) (b)

Figure 8. GD-OES profiles of elements:As (fora) non the-treated mirrors in O2 plasma with SiN a x4.5/Ag mirrornm SiN withx an protective 1.5 nm SiNx protection layer, thelayer; GDOES profiles presented (b) treated with O2 plasma for one minute. in Figure 9 demonstrate that 1 min of O2 exposure at first glance does not lead to the

changeAs for of the a mirrors protective with a 4.5layer nm SiNand,x protective consequently, layer, the GDOESto the profilesdegradation presented in the reflectance of the in Figure 9 demonstrate that 1 min of O2 exposure at first glance does not lead to the change of a protective layer and, consequently, to the degradation in the reflectance of the

(a) (b)

FigureFigure 9. GD-OES 9. GD proﬁles-OES profiles of the elementsof the elements in a SiN inx /Aga SiN mirrorx/Ag mirror with awith 4.5 nm a 4.5 SiN nmx protection SiNx protection treated treated with O with2 plasma O2 plasma for for (a) 1 min(a) 1 and min (b and) 40 ( min.b) 40 min.

4. Discussion We have undertaken the study of the PECVD silicon nitride for the protective layers on front surface silver mirrors and compared their efficiency with our previous work on AlOx protective layers. We kept the testing and analysis methodology the same as in a prior study [16]. PECVD layers of SiNx provide the effective protection of a silver mirror during 1 min of exposure to oxygen plasma at smaller layer thickness compared to the AlOx layer (4.5 nm for SiNx versus 15 nm for AlOx). However, the types of defects created in the nitride layer even after 1 min of oxygen plasma treatment and the gradual erosion of the protective film and underlying silver during longer oxygen plasma treatment, with the appearance of a micrometric-scale defects, does not allow us to consider PECVD silicon nitride as a viable material for such applications at the moment. Comparison data are presented in Table 2.

Table 2. Comparative data for AlOx ALD and SiNx PECVD layers tested with O2 plasma exposure

Temperature during Range of Thicknesses for Range of Thicknesses for No Protection Tech- Range of Thick- the Deposition of Apparent Damage from 1 Damage from the Long-Term nology nesses Protection Coating min Plasma Exposure Exposure PECVD 1.5–17 nm Room temperature Below 4.5 nm All damaged ALD 3–15 nm 150 °C Below 15 nm 15 nm and above

The degradation of a silver layer under the silicon nitride protection during the exposure to atomic and ionized oxygen proceeds very differently compared to the degradation process of AlOx-protected silver. If AlOx layers deposited by ALD completely stop the oxidation and withstand considerably longer exposure times after reaching the layer thickness of 15 nm, the erosion of SiNx protection layer proceeds along two different lines, in specific points at nanoscale, most likely along the hydrogen-rich columnar areas in SiNx, and on a scale of tens of micrometers, the origin of which needs to be further investigated, but at first glance looks like blistering at the interfaces.

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4. Discussion We have undertaken the study of the PECVD silicon nitride for the protective layers on front surface silver mirrors and compared their efﬁciency with our previous work on AlOx protective layers. We kept the testing and analysis methodology the same as in a prior study [16]. PECVD layers of SiNx provide the effective protection of a silver mirror during 1 min of exposure to oxygen plasma at smaller layer thickness compared to the AlOx layer (4.5 nm for SiNx versus 15 nm for AlOx). However, the types of defects created in the nitride layer even after 1 min of oxygen plasma treatment and the gradual erosion of the protective ﬁlm and underlying silver during longer oxygen plasma treatment, with the appearance of a micrometric-scale defects, does not allow us to consider PECVD silicon nitride as a viable material for such applications at the moment. Comparison data are presented in Table2.

Table 2. Comparative data for AlOx ALD and SiNx PECVD layers tested with O2 plasma exposure.

Temperature during Range of Thicknesses for Range of Thicknesses for No Protection Range of the Deposition of Apparent Damage from 1 min Damage from the Technology Thicknesses Protection Coating Plasma Exposure Long-Term Exposure PECVD 1.5–17 nm Room temperature Below 4.5 nm All damaged ALD 3–15 nm 150 ◦C Below 15 nm 15 nm and above

The degradation of a silver layer under the silicon nitride protection during the exposure to atomic and ionized oxygen proceeds very differently compared to the degradation process of AlOx-protected silver. If AlOx layers deposited by ALD completely stop the oxidation and withstand considerably longer exposure times after reaching the layer thickness of 15 nm, the erosion of SiNx protection layer proceeds along two different lines, in speciﬁc points at nanoscale, most likely along the hydrogen-rich columnar areas in SiNx, and on a scale of tens of micrometers, the origin of which needs to be further investigated, but at ﬁrst glance looks like blistering at the interfaces. Further investigation into the origin of both small-scale and large-scale defects will be required in order to evaluate the potential of the PECVD-deposited silicon nitride to serve as a protection layer for a silver front surface mirror.

5. Conclusions We fabricated and tested the protected front surface silver mirrors for their sensitivity to the atomic oxygen environment with a protection coating made of PECVD-deposited silicon nitride. We also compared the findings to our prior study on ALD-deposited AlOx- protected silver mirrors. Apparently, for a short-term exposure to atomic oxygen, the SiNx films show reasonable protection at very small (below 5 nm) thickness. However, even a short-term exposure to oxygen produces the small defects in the coating that later lead to slow degradation of the silver mirror reflection, thus proving that this erosion process, while slow, shows that the PECVD-deposited SiNx films are vulnerable to the aggressive environment. The thin layers of silicon nitride deposited in HD-PECVD system at low substrate temperature can provide reasonable short-term protection of a silver surface against corrosion by active oxygen. These layers also do not noticeably affect the reflection properties of a mirror in all its useful range. The strong points of using SiNx PECVD-deposited layers for the protection of silver mirrors are the existence of the industrially-proven large area deposition technology, the ability to fine-tune the refractive index of the material and the compatibility of PECVD technique with the sensitive silver surface. However, it is too early to suggest that it can be viable addition to the list of PVD and ALD deposited films already used in the protected mirrors technology. Clearly, more research is needed to particularly determine the origins of the defects that develop in a protective film during O2 plasma exposure. Coatings 2021, 11, 771 9 of 10

Author Contributions: Conceptualization, P.B. and N.K.; methodology, P.B. and P.C.; validation, P.B.; formal analysis, P.B. and P.C.; investigation, P.B., P.C., G.Z. and D.D.; resources, P.B. and P.C.; data curation, P.B.; writing—original draft preparation, P.B.; writing—review and editing, P.B. and P.C.; visualization, P.B.; supervision, P.B.; project administration, P.B.; funding acquisition, P.B. All authors have read and agreed to the published version of the manuscript. Funding: This activity was partially funded through the Total-LPICM ANR Industrial Chair “PIS- TOL” (Contract ANR-17-CHIN-0002-01). Institutional Review Board Statement: Not Applicable. Informed Consent Statement: Not Applicable. Data Availability Statement: Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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