Behavior of optical thin-film materials and coatingsunder proton and gamma irradiation

Ilaria Di Sarcina, Maria Luisa Grilli, Francesca Menchini, Angela Piegari,Salvatore Scaglione,* Anna Sytchkova, and Danilo Zola

Optical Coatings Laboratory, ENEA, Via Anguillarese 301, 00123 Roma, Italy

*Corresponding author: salvatore.scaglione@enea.it

Received 4 September 2013; accepted 15 November 2013;posted 27 November 2013 (Doc. ID 196800); published 15 January 2014

Optical materials and coatings are exposed to the flux of energetic particles when used in either spaceapplications or nuclear energy plants. The study of their behavior in such an environment is important toavoid failure of the optical components during their operation. The optical performance of several thin-film materials (HfO2, Ta2O5, Nb2O5, TiO2, SiO2) and coatings, under irradiation with high-dose gammarays (5.8 MGy) and exposure to low-energy (60 keV) protons, has been investigated. Some variations ofoptical properties have been detected in silicon oxide after irradiation, while the other materials arestable in such conditions. © 2014 Optical Society of AmericaOCIS codes: (310.1620) Interference coatings; (310.6860) Thin films, optical properties; (310.6870)

Thin films, other properties.http://dx.doi.org/10.1364/AO.53.00A314

1. Introduction

In several applications of coated optics, the thin-filmcoatings are exposed to the bombardment of ener-getic particles. Typical examples of application areasare space missions, high-energy physics, and nuclearfacilities, in which the presence of gamma rays, pro-tons, and neutrons could cause a variation of both op-tical materials’ properties and coating performanceduring their operation in such an environment.An induced change of the optical constants of thin-film materials (refractive index and extinction coef-ficient) will influence the spectral transmittance orreflectance of a multilayer coating containing suchmaterials and could deteriorate its performance.Moreover, the behavior of the optical components(filters, mirrors, etc.) could undergo some furthermodifications because of increased stress or scatter-ing phenomena.

In the literature, there are only a few data aboutthe effects of irradiation of optical coatings with

energetic particles [1–6], especially at high-leveldoses. In this work, several thin-film materials wereselected, and some typical optical coatings wereanalyzed to study their behavior under irradiationof gamma rays and protons.

The analyzed samples are described in Section 2,the irradiation facilities and the optical measure-ments in Section 3, and finally a discussion of resultsis reported in Section 4.

2. Irradiated Samples

The first set of samples consists of single-layer coat-ings made of materials (oxides) that are used forthe production of stable and resistant optical coatings(HfO2, Ta2O5, Nb2O5, TiO2, SiO2). The second set ofsamples includes antireflection coatings, mirrors, andfilters made of such materials. The list of the irradi-ated samples is reported in Table 1. The single layercoatings, as well as the fused silica substrates (Supra-sil 2, grade B), were exposed independently to gammarays and protons. The antireflection coating and thehigh-reflectance coating are components of a viewingsystem for a fusion plant and were dedicated to thestudy of gamma irradiation, while the narrow-band

1559-128X/14/04A314-07$15.00/0© 2014 Optical Society of America

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filters are part of optical instruments for space andwere irradiated with protons. As a consequence, thechoice of the irradiation parameters was determinedby the final application.

The coatings were deposited by ion-assistedelectron beam evaporation (Balzers, mod.BAK 640,equipped with an end-Hall ion source Common-wealth Scientific Corporation, mod. Mark I), withthe Ar (Xe) ion beam energy between 40 and 150 eV.The vacuum chamber was evacuated by a cryopumpat a back pressure of about 5 × 10−5 Pa. Depositionparameters such as oxygen partial pressure, deposi-tion rate, deposition temperature, and ion sourceparameters were optimized in order to obtain denseand high optical quality films. For all materials, asubstrate temperature of 250°C, an oxygen pressureof 1.2 × 10−2 Pa, a flux of 40 sccm, and a depositionrate of 0.2–0.3 nm/s were chosen. Only for hafniafilms the substrate temperature was 150°C. Ar gaswas used for TiO2 and Nb2O5 films ion-assisted dep-osition, while Xe gas was chosen for HfO2 and Ta2O5films. SiO2 films were fabricated with both Ar and Xeion assistance. The thicknesses of the single layerswere chosen to have several oscillations in the spec-tral curves of transmittance/reflectance and detectmore easily the variations of these spectra. The anti-reflection coating, the mirror, and the narrow-bandfilters were centered either in the ultraviolet or inthe near infrared (UV–NIR) to analyze a wide spec-tral range.

3. Irradiation Experiments

Typical conditions of irradiation from energeticparticles, both in the space environment and a fusionenergy plant were considered.

In space, the requirements about gamma irradia-tion range from a dose of 1 Gy/year to 10 kGy/year(10 Gy � 1 krad) depending on the mission environ-ment (Earth observation, planetary exploration,etc.). About protons, the presence of both low-energy(60 keV) and high-energy (30 MeV) protons has beentaken into account, with a fluence of the orderof 1010–1013 p∕cm2.

In the case of a fusion plant, a high total dose ofgamma rays, up to 5 MGy, is foreseen along withthe presence of neutrons. The irradiation of opticalcoatings with fast neutrons has also been performedat a fluence of 2.3 × 1017 n∕cm2, but the data are still

under evaluation and will be reported in a dedi-cated paper.

A. Irradiation Facilities

Several irradiation facilities were used for theexperiments with gamma rays and protons.

The exposure to gamma rays has been performedat ENEA, Casaccia Research Center, Roma—Italy,using the 60Co radiation source of the CALLIOPEplant [7]. A maximum total dose of 5.8 MGy wasreached at a rate of about 2.5 kGy/h (mean energy1.25 MeV); thus the irradiation duration was longerthan two months. However, the sample performanceat intermediate levels of total dose (starting fromabout 1.5 MGy) was also checked by adding similarsamples in the irradiation chamber, to be extractedduring the experiments.

Irradiation with high-energy protons (30 MeV) hasbeen carried out at INFN-LNS, Catania—Italy [8]while irradiation with low-energy protons (60 keV)has been performed at the Rudjer Boskovic InstituteZagreb—Croatia [9]. The total fluence was, respec-tively, 108 p∕cm2 and 1013 p∕cm2 because in spacethe flux of low-energy protons is usually higher thanthe flux of high-energy protons.

B. Optical Measurements

The spectral transmittance and/or reflectance of allsamples was measured, before and after irradiation,by a Perkin-Elmer Lambda 950 spectrophotometerin the wavelength range of 200–1100 nm, with aspectral resolution <0.05 nm and an accuracy ontransmittance measurements of 0.5%. A dedicatedsample holder was used to ensure a correct reposi-tioning of the samples. For measurements after irra-diation, that is important to avoid misinterpretationof the detected changes that could be caused by a pos-sible nonuniformity of the coatings. An example ofmeasurement repeatability is shown in Subsection 2.

1. Gamma RaysThe transmittance spectra of substrate and single-layer coatings, measured before and after irradiationwith gamma rays, are reported in Figs. 1 and 2.In each graph, the measurement after exposure tothe total dose of 1.79 MGy is compared withthe transmittance of nonirradiated samples. Thedegradation of the transmittance of the fused silica

Table 1. Irradiated Samples

No. Samplesa Materials (Thickness)

1 Substrate Fused silica SiO2 (1 mm)2 Single layers HfO2 (687 nm), Ta2O5 (820 nm), Nb2O5 (783 nm),

TiO2 (727 nm), SiO2 (944 nm)3 Antireflection coating: quartz/(0.25H) (0.4L)

(2.2H) (1.04L)/airL � SiO2, H � Ta2O5 (centered in the NIR wavelength

range: 750–950 nm)4 Dielectric mirror∶quartz∕�HL�10H∕air L � SiO2, H � TiO2, λ0 � 870 nm (NIR)5 Narrow-band filters: quartz∕�HL�7 HH �LH�7∕air L � SiO2, H � HfO2, λ0 � 316 nm (UV)

L � SiO2, H � TiO2, λ0 � 770 nm (NIR)aL and H indicate quarter-wave layers at the reference wavelength λ0.

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substrate in the ultraviolet, below 350 nm, can benoticed; Therefore the transmittance spectrum ofsome coatings (HfO2, Ta2O5) was normalized tothe substrate transmittance to eliminate the sub-strate influence from the graphs. The materials withhigh absorption below 350 nm (Nb2O5, TiO2) do notsuffer from this problem and their spectra arereported as measured (Fig 2). The behavior of all filmmaterials remains the same till the maximum exper-imented total dose of 5.8 MGy, while the substrateUV transmittance is worsening more and more.

A thin film of SiO2 was also deposited on fusedsilica, even though the evaluation of its behavior isnot easy, its transmittance being similar to thesubstrate transmittance. However, they can bedistinguished owing to the different properties offilm and bulk materials and degradation is noticedalso in SiO2 films. From all measurements, itcomes out that the optical behavior of the analyzed

thin-film materials is not affected by gamma rays,even at high dose levels, except for SiO2.

Gamma rays could create different types of defectsin the fused silica. In a paper by Karlitschek et al.[10], two absorption bands of irradiated bulk fusedsilica, peaked at 211� 3 nm and 267 nm withFWHM of 17 nm, are assigned to the so-calledE0-center. In Fig 3, the absorbance of the irradiatedfused silica, as simply deduced from the transmit-tance curve of Fig. 1(a), is shown. This curve canbe well reproduced by combining two Gaussiancurves (reported in the same figure) centered atthe wavelengths λ1 � 213 nm and λ2 � 258 nm,which are very similar to the absorption bandsreported in [10]. The E0-center is stable and is related

Fig. 1. Transmittance of (a) fused silica (suprasil) substrate,(b) Ta2O5, and (c) HfO2 thin films, before and after gamma irra-diation at 1.79 MGy.

Fig. 2. Transmittance of (a) Nb2O5 and (b) TiO2 thin films beforeand after gamma irradiation at 1.79 MGy.

Fig. 3. Measurement (blue curve) and fitting (red curve) of theabsorbance of the irradiated fused silica substrate.

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to Oxygen vacancy with an electron trapped in the 3porbital of Si–Si bond [11].

The behavior of the AR coating and of the high-reflectance coating, containing SiO2 layers, wasthen investigated. Their transmittance spectra are,respectively, reported in Figs. 4(a) and 4(b), beforeand after irradiation at 5.8 MGy.

An increased absorption in the ultraviolet can benoticed, together with a slight shift of the transmit-tance curve toward longer wavelengths. A wide spec-trum is reported, and not only the wavelength rangeof interest for the application, to show the effects ofthe gamma irradiation from UV to NIR.

Both coatings contain Ta2O5 and SiO2 layers. Thebehavior of tantalum oxide films is not affected bygamma irradiation up to 5.8 MGy; thus the observedchanges could be explained by the variation of theproperties of SiO2.

2. ProtonsAbout the effects of protons, it is expected that high-energy protons do not interact with the coating butonly with the substrate, because they come at restin a depth of about 4–5 mm, as explained in the nextsection. The experiment carried out with 30 MeVprotons on the NIR narrow-band filter of Table 1,containing SiO2 and TiO2 layers and centered at770 nm, confirmed this hypothesis; in fact no varia-tions were detected in its transmittance, as shown inFig. 5 at a fluence of 108 p∕cm2. Higher values of

fluence were also used for testing, but no changeswere detected in the whole measured spectrum.No variations were found in thin (1–2 mm) fusedsilica substrates, too.

The effect of low-energy protons on coating behav-ior is more interesting. In Fig. 6, the transmittancecurves of the fused silica substrate, measured beforeand after 60 keV proton irradiation at a fluence of1013 p∕cm2, are compared. After the irradiation,the transmittance decreases for wavelengths shorterthan 300 nm, whereas in the visible–near infraredspectral region the protons seem to have negligibleeffects on the substrate optical behavior.

Single layers of TiO2 and HfO2, which are the ma-terials used (together with SiO2) for the narrow-bandfilters, were also irradiated with low-energy (60 keV)protons. Their transmittance does not change afterthe irradiation, as shown in Figs. 7(a) and 7(b), eventhough they are deposited on fused silica.

In fact low-energy protons do not reach the sub-strate but come at rest into the thickness of thesecoatings, as will be shown in the next section; thusthe proton irradiation effects are due to the coatingmaterials only.

The UV narrow band filter of Table 1 is made of asequence of 29 layers of SiO2 and HfO2. The filter

Fig. 4. Transmittance of (a) the AR coating and (b) the high-reflectance coating before and after gamma irradiation at 5.8 MGy.

Fig. 5. Normalized transmittance of the NIR narrow-band filterbefore and after 30 MeV proton irradiation (enlarged wavelengthscale).

Fig. 6. Transmittance of the fused silica substrate before andafter 60 keV proton irradiation.

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transmittance before and after the proton irradiationis reported in Fig. 8, in which both a decrease and ashift of the peak transmittance toward longer wave-lengths can be noticed, for the irradiated sample. Onthe contrary, the peak of the NIR filter irradiatedwith such protons does not change; thus their influ-ence seems limited to short wavelengths. By analyz-ing Figs. 6 and 7, it can be observed that thefused silica substrate (SiO2) is the material thatsuffers the most damage; therefore the change inthe transmittance of the interference filter isprobably due to SiO2 layers.

4. Simulation of Proton Interaction with Coatings

In order to understand the effect of protons on sub-strate and coatings, the irradiation process was si-mulated by the stopping and range in ion matter(SRIM) code [12] based on the statistics of the MonteCarlo method. The number of 104 ions was consid-ered suitable in these calculations to obtain repre-sentative results.

During the collisions, the protons are able to dis-place the atoms of the target from their equilibriumposition and SRIM is able to follow the trajectory ofthe atoms, calculating the number of atoms that be-come at rest in the interstitial position and in thesubstitutional configuration. The 30 MeV protonssimulation shows that these protons penetrate intothe coating with a very low number of interactions;in fact their ion range is about 4.5 mm. The numberof vacancies generated per proton during the travelinto the coating is very low, less than 10−7 per proton.

With 60 keV protons, the simulation of ions andrecoils collision cascade, for the fused silica sub-strate, gives the result reported in Fig. 9(a). Theion average range, Rion of the protons is 578.2 nmand the collision cascade is approximately effectiveup to 800 nm. The distribution of oxygen vacancieswith depth, i.e., the oxygen atoms in the interstitialconfiguration, is reported in Fig. 9(b).Fig. 7. Transmittance of (a) HfO2 and (b) TiO2 films before and

after 60 keV proton irradiation (several measurements are madeto check repeatability).

Fig. 8. Transmittance of the UV narrow-band filter, before andafter 60 keV proton irradiation.

Fig. 9. (a) Distribution of ions and recoils in a SRIM simulation,for 60 keV protons, into the fused silica substrate and (b) oxygenvacancies distribution in the fused silica substrate.

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A similar simulation, for the UV narrow-bandfilter, is shown in Figs. 10(a) and 10(b). As can beseen, the protons are stopped in the first 640 nm(14 layers) of the filter while the remaining 750 nmare not interested by the collision cascade (totalcoating thickness 1390 nm).

With the hypothesis that only the low refractive in-dex layers, SiO2, are responsible for the change in thetransmittance curves after proton irradiation, it canbe considered that vacancies are generated removingthe oxygen atoms from the equilibrium position. Thevacancy distribution depends on the depth, in boththe substrate and filter [Figs. 9(b) and 10(b)].

A mechanism similar to the case of gamma irradi-ation, in which the oxygen vacancies were supposedresponsible for the formation of E0-center, can be con-sidered also valid for the proton irradiation of fusedsilica material.

In order to evaluate the increase of the opticalabsorption at short wavelengths that is due to theoxygen vacancies, the low refractive index layerswere considered as a mixing of SiO2 and SiO, whichhas a higher refractive index and a higher extinctioncoefficient. The transmittance of the fused silica sub-strate has been calculated dividing the SRIM simu-lated region of Fig. 9(b) in 20 layers, with differentpercentages of SiO content. The result is shown in

Fig. 11, in which the calculated silica substratetransmittance, after the partial replacement ofSiO2 with SiO, is reported to be compared with Fig 6.

In the same way, the transmittance of the narrow-band filter has been calculated considering a percent-age SiO∕SiO2 in the first seven L layers of the filter,as deduced from Fig. 10(b). The transmittancecurves, calculated with this procedure, are in agree-ment with the experimental curves after irradiation(Figs. 6 and 8), confirming the hypothesis that eachoxygen atom removed from the equilibrium positiongenerates a SiO molecule. The increase in both re-fractive index and absorption coefficient that isdue to SiO induces a decrease of the transmittancevalue and a slight shift of the coating transmissionband, as noticed in the measurements. The conclu-sion that the SiO2 material is more sensitive tothe proton irradiation than the other analyzeddielectric materials is confirmed by the absence ofvariations in the transmittance curves of Fig. 7.

5. Conclusions

The behavior of several thin-film materials and opti-cal coatings, under irradiation of gamma rays andprotons, was studied. All materials, except forSiO2, were able to withstand high doses of bothgamma rays and low-energy protons. The penetra-tion depth of high-energy protons is typically greaterthan the coating thickness, different from low-energyprotons, which means a more effective interaction oflow-energy protons with the coatings.

Fig. 10. (a) Distribution of ions and recoils in a SRIM simulationfor 60 keV protons, into the UV narrow-band filter and (b) oxygenvacancies distribution in the SiO2 layers of the UV narrow-bandfilter.

Fig. 11. (a) SiO content in the substrate according to Fig. 9(b) and(b) calculated transmittance of the fused silica substrate withoutand with SiO content.

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A transmittance decrease in the ultraviolet of thefused silica substrate was detected both with high-dose gamma rays and low-energy protons. A similarvariation of properties is foreseen for SiO2 thin films,in which the oxygen vacancy formation could causean increase of the optical absorption.

Numerical calculations have been carried out bythe SRIM code, in the case of proton irradiation, toevaluate the observed changes of the transmittancecurves.

The authors are grateful to S. Baccaro and A.Cemmi for gamma irradiation, to M. Jaksic and M.Karlusic for irradiation tests at RBI facilities withlow-energy protons, to G. Cuttone and M. Lattuadafor irradiation at INFN-LNS facilities with high-energy protons.

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