coatings 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 res- onance (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 sub- strate 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, re- spectively. 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.