Morphological and Functional Modifications of Optical Thin Films for

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Morphological and Functional Modifications of Optical Thin Films for Space Applications Irradiated with Low-Energy Helium Ions Maria Guglielmina Pelizzo, Alain Jody Corso, Enrico Tessarolo, Roman Boettger, René Hübner, Enrico Napolitani, Marco Bazzan, Marzio Rancan, Lidia Armelao, Werner Jark, Diane Eichert, and Alessandro Martucci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13085 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Applied Materials & Interfaces

Morphological and Functional Modifications of Optical Thin Films for Space Applications Irradiated with Low-Energy Helium Ions M.G. Pelizzo1,2,*, A.J. Corso1,+, E. Tessarolo1, R. Böttger3, R. Hübner3, E. Napolitani4, M. Bazzan4, M. Rancan5, L. Armelao5,6, W. Jark7, D. Eichert7, A. Martucci1,8 1

Consiglio Nazionale delle Ricerche, Istituto di Fotonica e Nanotecnologie, via Trasea 7, 35131 Padova

2

Dipartimento di Ingegneria dell’Informazione, Università degli Studi di Padova, via Gradenigo 6B, 35131 Padova

3

Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research Ion Beam Center, Bautzner Landstraße 400, 01328 Dresden, Germany

4

Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, via Marzolo 8, 35131 Padova

5

Consiglio Nazionale delle Ricerche, Istituto di Chimica della Materia Condensata e di Tecnologie per l’Energia, c/o Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131, Padova

6

Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova

7

Elettra - Sincrotrone Trieste, S.S. 14 Km 163.5 in Area Science Park, 34149 Trieste

8

Dipartimento di Ingegneria Industriale, Università degli Studi di Padova, via Marzolo 9, 35131 Padova

Corresponding authors: *[email protected] +

[email protected]

Keywords: Optical thin films, gold coatings, ions irradiation, helium ions, space weather Abstract Future space missions will operate in increasingly hostile environments, such as those in low-perihelion solar orbits and Jovian magnetosphere. This exploration involves the selection of optical materials and components resistant to the environmental agents. The conditions in space are reproduced on ground through the use of ion accelerators. The effects of He particles coming from the solar wind impinging on a gold thin film have been systematically investigated, considering absorbed doses compatible with the duration of the ESA Solar Orbiter mission. Structural and morphological changes have been proved to be dependent on the dose, but also on the irradiation flux. A predictive model of the variation of thin film reflectance has been developed for the case of lower flux irradiation. The results are discussed regarding reliability and limitations

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of laboratory testing. The outcomes are important to address the procedures for the space qualification tests of optical coatings.

1. Introduction Optically coated elements are mainly optimized for their specific characteristics, such as transparency or reflectivity in the desired spectral region. However, for space missions of the very next future, it will be fundamental to ensure the sustainability of these optical elements at the harsh conditions of the relevant space environment, preventing any critical optical degradation of their performances. In fact, optical performances of the components do strongly affect the data outcomes, and their degradation can lead to misinterpretation of the scientific data due to an uncontrolled change of the instrument response. In a more dramatic scenario, the failure of a component can affect the operational capacity of the whole instrument. Future space missions will operate in unexplored and hostile environments, facing new problems regarding the stability and validation of optical components, materials and coatings. The effect of ion irradiation is already considered as a potential cause of component degradation, together with other influences, such as temperature, ultraviolet irradiation and debris1,2. Most of the qualifications on electronic components and systems are carried out using MeV protons and electrons3,4,5,6, which are abundant, for example, in low-earthorbit as trapped in the Van Allen belts7. On the other hand, optical coatings and materials are significantly affected by keV ions, since these implant within the material, changing their optical and structural properties. Such particles are considered as one of the main causes of degradation in Sun-close environment, where the quiet solar wind constantly transports such ions8. In fact, solar wind is an outflow of completely ionized gas originating from the solar corona and expanding outwards into the interplanetary regions. It consists of protons, electrons, alpha particles and much less abundant heavy ions. There are different types of solar wind, depending e.g. on the velocity and duration of the propagation, energy of the constituents and solar region of provenience. The ions of the quiet solar wind carry the lowest kinetic energy, typically 1 keV for protons and 4 keV for the alpha particles, but are considered a long-term source of degradation of optical components since they flow constantly8. Additionally, they have been demonstrated to be present also in planetary atmosphere, including terrestrial orbits9.


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Only few experiments have been performed with low energy particles. Previous studies with protons having energies 1-100 keV range have demonstrated that implantation into materials for space application can cause refraction index changes10, delamination11, surface nanobubbles formation and blistering12. Blistering and layer detachment are also reported in the case of Extreme Ultraviolet Lithography Mo/Si mirrors irradiated with low-energy hydrogen ions13,14,15. Regarding He irradiation to simulate space conditions, the literature is rather limited16. Some information comes from studies related to other disciplines, such as fusion reaction and materials processing, even though the experimental parameters used, such as ions energy and doses, are not always representative of space conditions17-21. In the case of 12 keV He irradiation, subsurface nanobubble formation in metallic copper samples have been observed. The bubbles are confined within a region of about few tens of nanometers below the top surface22. In such experiments, ion and damage depth profiles show a consistent 20-30 nm offset compared to SRIM software simulations23. Irradiation with 15-35 keV He on Au (111) samples has been carried out at doses above 1×1017cm-2 24. It has been demonstrated that at lower energies the implantation leads to the formation of subsurface bubbles that quickly reach the surface releasing the helium and developing a sponge-like surface. At 35 keV, nanobubbles form in the deeper part of the film. They grow with the dose, such that for a total dose of 4.8×1017cm−2 formation of large blisters is observed. The above mentioned studies are generally limited to specific values of energy of some selected ions species thus having more the character of a validation of components already selected25. Moreover, those studies are generally performed by mimicking the irradiation process with the help of ion accelerators and assuming that by selecting the ion type, energy and the total equivalent ionizing dose one can realistically simulate the same kind of damage which will affect the optical component in space. This latter assumption however needs a more careful verification: in ground-based laboratories, for practical reasons, the dose which the sample is implanted with in space environments during some years of flight is delivered to the sample in some hours or few days. This means that there is a difference of some orders of magnitude between the fluxes experienced in real application and in the laboratory tests. Moreover, it should be stressed that the increase of the exposure doses may determine a significant increase of the potential damage to the components of the missions operating in sun-close environment26,27 and in planet orbits28,29. The impact of the dose on the damaging processes needs therefore to be clarified. 3 ACS Paragon Plus Environment


In this work we present the results of a systematic study of the effect of irradiation in a Sun-close environment on gold thin films. Such thin films have been selected since they are commonly used as coatings for space mirrors. The particles considered are He ions with an energy of 4 keV, as those carried out within the quiet solar wind8. The doses used were calculated considering a solar orbit mission, which is ESA Solar Orbiter. The same doses, in the range of 1015-1017 cm-2 were reached using different fluxes, in order to provide, for the first time, a complete investigation on the effect of the particle rate on the implantation and damage dynamics. The implantation profiles in the thin films were verified by Secondary Ions Mass Spectrometry (SIMS) and compared with theoretical simulations. Morphological and structural properties prior and after irradiation were investigated using various techniques, such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and cross-sectional Transmission Electron Microscopy (TEM). X-ray Photoemission Spectroscopy (XPS) was used to evaluate the chemical composition of the films top surface and to verify for potential contamination during irradiation sessions. Virgin samples were fully characterized using spectroscopic ellipsometry to retrieve their optical constants, and a spectrophotometer for the reflectance measurements. After irradiation, a change in reflectance was observed, depending both on flux and dose. For the first time the modification induced by the particles in the film was put linked to the change of their optical performances. In fact, in the case of lower doses, an Effective Medium Approximation (EMA) model was used to fit the reflectance curve of the irradiated samples. The input parameters of such model are the results from the previous investigations, such as SIMS implantation profiles and thin film optical constants determined by ellipsometric technique. At higher doses, a fatal damage of the samples was observed, due to He bubble formation and finally He release associated with dramatic modifications of the top surface; such experimental output must be carefully considered in future qualifications. The results are discussed regarding reliability and limitations of laboratory testing. The outcomes are important to address the procedures for the space qualification tests of optical coatings.

2. Materials and methods Gold coatings were irradiated with 4 keV He ions in two experimental sets. In the first set, the implantation parameters were selected in order to simulate the effect of the solar wind ions in the sun-close environment during a space mission, such as the ESA Solar Orbiter, in particular by considering four different doses, 4 ACS Paragon Plus Environment

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corresponding to those accumulated during 1, 2, 4 and 6 years of mission operation. In a second set of irradiation sessions, the dose values were further increased up to an order of magnitude. In order to calculate the equivalent doses, a model of the solar wind alpha particle distribution along the spacecraft orbit in the heliosphere needs to be assumed; in fact, to date the most reliable data on solar wind density and speed are available at the Earth distance (1AU) and are based on observations from previous satellites. To estimate the flux at other distances from the sun, an isotropic model was adopted, which states that the particle density decreases in proportion to the inverse square of the distance from the Sun.

Considering the trajectory of the spacecraft, a scale factor can be applied to calculate the flux as varying

with the position along the orbit. Starting with the knowledge of ion density (i.e. 0.41 He/cm3) and their

average velocity (i.e. 468 km/s) at 1AU8 as well as the mission trajectory, it is thus possible to calculate the total dose for a selected interval of mission operation time according to the following formula16: ∆ = = ∙ ∙

(1)

where is a distance-dependent scaling factor describing the change of the ion density with the distance from the Sun. The doses corresponding to 1, 2, 4 and 6 years mission are D1=2.6·1015 He/cm2, D2=5.2·1015 He/cm2, D3=1.0·1016 He/cm2 and D4=1.6·1016 He/cm2 respectively. In the second set of experiments, the total dose on the sample was increased up to D5=4.0·1016 He/cm2 and D6=4.0·1017 He/cm2. The samples were irradiated at the 40 kV ion implanter at the Ion Beam Center of the Helmholtz-Zentrum in Dresden-Rossendorf (Germany). The extraction potential used in this accelerator was −20 kV. The ions were then decelerated by setting a 16 kV potential so as to obtain 4 keV He ions in the experimental station. The beam of such ion implanter is focused (spot size 3-10 mm). The beam is raster scanned electrostatically using horizontal and vertical pairs of deflection plates. Raster frequencies are ~1kHz. Homogeneity is monitored comparing the measured ion beam currents in the four corner Faraday cups, which have to be equal. The experimental parameters of the implantation sessions are reported in Table 1. Various doses were reached by different fluxes varying the time of exposure. The experimental flux values were chosen taking into account various constraints, such as the facility capabilities, which define an inferior and superior limit, and the end station occupation time, which becomes unacceptable at high doses; 5 ACS Paragon Plus Environment


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therefore, a grid of significant dose/flux values compatible with such constraints was selected. The experiment was performed without any active control of the temperature the samples. During various irradiation sessions at LF, samples temperature was monitored using a thermocouple; a maximum value of 27°C was measured for such irradiation condition. At higher fluxes an increment of the temperature of few tens of degrees should be expected. Virgin samples fabricated in the same gold deposition run of the irradiated ones have been kept as references. During the implantation sessions, the chamber base vacuum was about 10-7 mbar. The samples were kept perpendicularly to the He ion flux. The ion current was integrated over time in order to control the total dose. 14 different implantation sessions were carried out.

Session #

Table.1 Irradiation parameters Sample labelling Dose L: low He/cm2 M: medium H: high D: dose F: flux

Flux He/cm2/s

1

LLDLF

D1=2.6·1015

F1=1.5·1011

2

LLDMF

D1=2.6·1015

F2=3.0·1012

3

LLDHF

D1=2.6·1015

F3=8.8·1012

4

LDLF

D2=5.2·1015

F1=1.5·1011

5

LDMF

D2=5.2·1015

F2=3.0·1012

6

LDHF

D2=5.2·1015

F3=8.81012

7

MDLF

D3=1.0·1016

F1=1.5·1011

8

MDMF

D3=1.0·1016

F2=3.0·1012

9

MDHF

D3=1.0·1016

F3=8.8·1012

10

HDLF

D4=1.6·1016

F1=1.5·1011

11

HDMF

D4=1.6·1016

F2=3.0·1012

12

HDHF

D4=1.6·1016

F3=8.8·1012

13

HHDHHF

D5=4.0·1016

F5=1.6·1013

14

HHHDHHF

D6=4.0·1017

F5=1.6·1013

For the first set of experiments, a gold film was deposited on a Si substrate coated by a 100 nm titanium adhesion layer (implantation sessions 1-12). In the second set of experiments (implantation sessions 13-14), gold was deposited on a Si substrate coated by few nm of chromium. The Si substrates are Czochralski6 ACS Paragon Plus Environment

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grown p-type polished wafers with a thickness of 650–700 µm. All metals were deposited by electron-beam evaporation starting from 99.99% pure pellets. The Ti film underneath the Au layer was deposited as adhesion layer. In the second experiment, Ti was replaced by a very thin (about few nm) Cr film used as adhesion layer. In both cases, He implantation occurs only within the gold films. The total film thicknesses were measured after deposition using a KLA Tencor P-16+ Profilometer. The gold layer thicknesses were retrieved by the cross-sectional TEM images, being 214 nm in the first set of samples and 217 nm in the second one. The thickness of the Au layers had been previously determined in order to guarantee that all He is implanted within the film itself, as verified by simulations of the Stopping Range of ions. Such simulations of collision dynamics were performed with TRIM/SRIM software23. In order to verify the correspondence between the delivered dose and the concentration of ions actually implanted in the sample, Secondary Ion Mass Spectrometry (SIMS) measurements were carried out on selected samples. The ion implantation profiles were recorded and compared with those obtained by simulations. Measurements were done by sputtering the samples with a 10 nA, 5.5 keV Cs+ beam, while collecting CsHe+, CsO+, CsSi+, CsCr+ secondary ions from a central area of the sputtering crater. Erosion time vs. depth calibration was done by measuring the crater depth after sputtering with the profilometer and assuming a constant erosion rate. To investigate the structural properties of the He-implanted films, cross-sectional bright-field Transmission Electron Microscopy (TEM) analysis was performed using an image Cs-corrected Titan 80-300 microscope (FEI) operated at an accelerating voltage of 300 kV. Classical TEM cross-sections of the He broad-beam irradiated Au films glued together in face-to-face geometry using G2 epoxy glue (Gatan), were prepared by sawing (Wire Saw WS 22, IBS GmbH), grinding (MetaServ 250, Bühler), polishing (Minimet 1000, Bühler), dimpling (Dimple Grinder 656, Gatan), and finally Ar+ ion milling (Precision Ion Polishing System PIPS 691, Gatan). X-ray diffraction experiments were performed using a Philips MRD X’Pert diffractometer, equipped with a Cu X-ray tube and a multilayer mirror to increase the beam collimation and spectral purity. The detector was a Xe proportional counter with a Soller slit having an angular acceptance of 0.04 rad and a parallel plate collimator mounted in front of it. Both the sample and the detector were mounted on two independent goniometers with nominal positioning resolution of 0.0001 degree and tested repeatability below 0.004 7 ACS Paragon Plus Environment


degrees. In order to avoid the strong 004 diffraction peak from the Si substrate, we set a small offset of 0.2 degrees in the sample goniometer and used an automatic attenuator to cut the intensity in the 2θ range between 69 and 70 degrees. The samples were mounted on an Eulerian cradle which permits to select an arbitrary orientation for the sample. Such device was used to perform a series of symmetric ω - 2θ scans by varying each time the tilt angle ψ formed by the sample surface with the scattering plane, i.e. the plane formed by the directions of the incident and diffracted beams. For a randomly oriented collection of crystallites, the sample is isotropic and the same diffraction pattern is obtained for any direction ψ of the scattering vector. In case of preferred orientation instead, the patterns do change as a function of ψ. The surface morphology of the samples, both irradiated and non-irradiated, was characterized by using Atomic Force Microscope (AFM) and Scanning Electron Microscopy (SEM). The AFM (XE-70, Park System) was operated in non-contact mode (XE-70, Park System), in order to verify a possible change of the surface morphology due to the ion bombardment. Top-view scanning electron microscopy using secondary electrons was performed with a S-4800 microscope (Hitachi) operated at an accelerating voltage of 10 kV. The surface composition was analyzed by X-Ray Photoemission Spectroscopy (XPS) to verify a potential change of composition and the presence of contaminants after irradiation sessions; XPS analyses were performed in a Perkin-Elmer Φ 5600-ci spectrometer using Al Kα radiation (1486.6 eV). The sample analysis area was 800 µm in diameter. Survey scans were obtained in the 0−1350 eV range (187.8 eV pass energy, 0.8 eV step-1, 0.05 sec step-1). Detailed scans were recorded for the C1s, O1s, F1s and Au4f (23.5 eV pass energy, 0.1 eV step-1, 0.1 sec step-1). The standard deviation for the binding energy (BE) values is ± 0.2 eV. The XPS spectrometer was calibrated by assuming the binding energy of the Au 4f7/2 line at 83.9 eV with respect to the Fermi level. The BE shifts were corrected by assigning to the C1s peak associated with adventitious hydrocarbons a value of 284.8 eV30. Samples were mounted on a steel holder and introduced directly in the fast-entry lock system of the XPS analytical chamber. The data analysis involved Shirley-type background subtraction, non-linear least-squares curve fitting adopting Gaussian-Lorentzian peak shapes, and peak area determination by integration31. The atomic compositions were evaluated from peak areas using sensitivity factors supplied by Perkin-Elmer, taking into account the geometric configuration of the apparatus32. The experimental uncertainty on the reported atomic composition values does not exceed ± 5%.


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The reflectance of the samples was measured in the 300-800 nm wavelength range by using a Carry 5000 double grating spectrophotometer. The measurements were performed by employing a VW-scheme which allows to obtain the absolute reflectance with an incidence angle of 7° and an accuracy better than 2%. The optical constants of the gold virgin samples were determined using a VASE Spectroscopic Ellipsometer (J.A. Woollam). The measurements were carried out in air at three different incidence angles (55°, 65°, 75°) in the 300-1000 nm wavelength range, with a step size of 1 nm. The output of ellipsometric analysis is the ratio

between the total p-polarised to s-polarised complex Fresnel reflection coefficients of the sample.

This ratio can be expressed in terms of the ellipsometric angles Ψ and Δ, according to: =

# $

= tan Ψ ( )*+

(2)

which is related to the sample structure and the complex dielectric functions of the materials33. Data were analysed with the WVASE32 software (J.A. Woollam). The gold layer thickness is above 200 nm and for this reason, the layer is considered optically thick (i.e. the thickness is larger than the skin depth) and was modelled as a semi-infinite film. The virgin sample was modelled as a homogenous layer described by a dielectric function dispersion based on a combination of a Drude oscillator and two Johs-Herzinger 34

generalized critical point oscillators

(named PSEMI-M1 in WVASE32 software) devoted to describe the

two critical points in the UV range. According to such model the film dielectric function can be written as 0

, + .,/ = 1

0 )1

0

+ 1

0 )1

4

3 53 − 1 6*4 + ∑/=E PSEMI= >, >= , @= , A= , BC= , BD= , @C= , @D= + ,F 1 53

(3)

where AH are the oscillator/pole amplitude coefficients, BJ are the damping coefficients, EJ are the oscillator/poles central energies, BC= and BD= represent the left and right sides widths of the PSEMI

oscillators, @C= and @D= indicate the relative magnitudes of the left and right control points and the ,F

parameter is the background dielectric constant35. To compute the complex refraction index from the fittings obtained it is possible to use the following relationship: KL = K + .M = N, + .,/

(4)

where n is the real part k the extinction coefficient. The quality of all fits was estimated by evaluating the mean square error function (MSE) MSE =

O χ/ /P)Q

YZ0

ψVWX )ψU U ∑P T YZ0 ^E /P)Q σψ,U

=S

/

[ +\


YZ0

∆VWX )∆U U YZ0 σ∆,U

/

]

(5)


where N is the number of experimental data and M is the number of fitting parameters. A fit was considered acceptable when the MSE value was below 2.

3. Results and discussion 3.1 Structural and morphological properties analysis

TRIM simulations with different He energies were performed to obtain the related implantation profiles (Fig.1). Such energies have been selected considering the He spectrum shape in solar wind36, which has the higher contribution in fluence at energies lower than 40 keV. For 4 keV energy, the peak of the implantation profile is at about 20 nm. The distribution profile changes with energy and, accordingly, different degradation effects can be expected. In the case of low-energy ions, the He particles cumulate in a small volume, so that it is reasonable to expect, that smaller doses are necessary to induce an appreciable modification in the thin film with respect to the case of higher energies.

Figure.1 Distribution of He ions implanted in gold (mass density of 19.3 g/cm3) as simulated by TRIM.

SIMS measurements are presented in Fig. 2a and b. The first figure shows the implantation profiles of all samples irradiated with dose D4, but at different fluxes. The implantation profiles are very similar in shape, 10 ACS Paragon Plus Environment

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demonstrating that flux does not have significant influence on the collisional dynamics and thus on the ion distribution inside the film. However, given the instrumental error affecting the measurements, we cannot conclude that the total number of implanted ions is the same in all three cases. This aspect must be critically evaluated during qualification tests and deserves further studies. With respect to simulation, the measured implantation profiles present a small peak shift and an overall enlargement, which is due to a lower density of the film with respect to the nominal value used in simulations and to the He diffusion dynamics37. In Fig. 2b, the profiles of samples irradiated at doses D5 and D6 are given; in particular, the sample irradiated with D6=4.0·1017 He/cm2 appears to be characterized by a strongly modified implantation profile, which is due to helium accumulation in bubbles as demonstrated in the following.



Figure.2 He implantation profiles in samples HDLF, HDMF and HDHF (a) and HHDHHF and HHHDHHF (b) as measured by Secondary Ion Mass Spectrometry. The theoretical curve computed by SRIM for an Au thin film with density of 19.3 g/cm3 is reported as dashed line in both figures.

The total film thicknesses of the virgin samples, as measured by the profilometer ((302 ± 5) nm for Au/Ti and (222 ± 5) nm for Au/Cr), were confirmed by cross-sectional TEM analysis on gold samples for both experimental sets (Fig. 3(a) and (b)); the gold layer thicknesses are 214 nm and 217 nm, respectively. Please, note that the Cr layer is not indicated in Fig. 3b due to its small thickness of only a few nanometers. The bright-field images in Fig.3 show a columnar growth structure of the gold films, which is compatible with the deposition parameters used38. Vertical Au grain sizes are comparable to the Au layer thicknesses, i.e. around 215 nm. Lateral Au grains sizes are (125 ± 45) nm in the case of Au/Ti and (100 ± 35) nm for Au/Cr. The standard deviation is given as error. For each sample, lateral grain size determination was done for at least 40 grains. Since sputtering processes/channeling depend on the crystallographic grain orientations due to different surface binding energies of surface atoms and due to different open channels39, XRD analysis was performed to investigate the texture of the Au films. In Fig.4, the result obtained for the virgin Au/Cr sample is plotted as a map of ω-2θ scans in dependence of the tilt angle ψ (sample Au/Ti shows similar results). The diffraction pattern obtained by summing the various ω-2θ scans for all the ψ values is also reported. It can be seen that the sample exhibits a strong texture: the strong peaks for ψ=0 at 2θ=38 and 2θ=82 degrees correspond to the 111 and 222 reflections of gold respectively and indicate clearly that this is the direction of preferred orientation (i.e. those planes are parallel to the sample surface). The other crystalline plane reflections are symmetric along ψ as expected for a fiber texture. After irradiation, samples do not show significant modification in crystallographic structure.


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Figure.3 Cross-sectional bright-field TEM micrographs of (a) Au/Ti and (b) Au/Cr reference samples



Figure.4 XRD patterns of Au/Cr depending on the tilt angle ψ and relative diffraction patterns; diffraction peaks related to Au are indicated.

After irradiation with the highest dose, sample HHHDHHF shows a modified top part of the Au film (Fig. 5a). In particular, He bubble formation occurs, which is best seen in the slightly defocused TEM micrograph in Fig. 5b. He bubbles are observed as deep as 100 nm below the Au surface which corresponds well with the implantation profile as measured by SIMS (Fig. 2b). More detailed investigations by high-resolution TEM including fast Fourier transform analysis indicate that the columnar grain structure and particularly the Au grain orientation are not influenced by the He bubble formation. As shown in Fig. 5c, the Au grain oriented in `011cd zone axis geometry with the [111] direction along to the sample surface normal does not

show an orientation variation in the top part of the Au film. Comparing top-view SEM images of the virgin samples (Fig. 6) with that of sample HHHDHHF (Fig. 7), the surface of the latter appears strongly modified. However, the sponge-like surface structure of HHHDHHF is actually compatible with the extensive bubble formation and consequently the He release, as shown by TEM.


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Figure.5 Cross-sectional TEM analysis of sample HHHDHHF: Irradiation at the highest dose leads to He bubble formation, as observed in the (a) overview and (b) slightly defocused bright-field images. (c) High-resolution TEM including Fourier transformation give no hint for modifications of the Au grain orientation by He irradiation.

Figure.6 Top-view SEM micrographs of (a) Au/Ti and (b) Au/Cr reference samples.



Figure.7 Top-view SEM micrograph of sample HHHDHHF.

In case of samples HDLF and HDHF, top-view SEM analysis (not shown here) does not show any significant surface modification for the most part of the samples with respect to the virgin state. Au grains appear to have the same size and the mean roughness values are preserved. Additionally, cross-sectional TEM analysis (not shown here) of sample HDLF confirms that the columnar structure observed in the virgin sample is preserved. Nevertheless, SEM analysis reveals very few disordered areas, present in both, HDLF and HDHF. The circular surface structures, as exemplarily shown in Fig.8a, have been also analyzed by AFM (Fig.8b). They are prominent with preliminary signs of detachment at the border. Even though the formation mechanism of such structures is still unclear, it might be possible that there is a relationship to the beginning of the degradation process observed for sample HHHDHHF.


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Figure.8 Top-view SEM micrograph of a disordered circular surface structure of samples HDLF (a) and AFM analysis (b).

3.2 Surface analysis Samples surface was analyzed by XPS to investigate the presence of contaminants or chemical modifications after irradiation sessions which could affect the optical performances of the films and, hence, data outcomes. Survey scans for the reference and virgin samples, HDLF and HDHF, reveal 17 ACS Paragon Plus Environment


similar spectra characterized by the photoemission peaks of Au along with carbon, oxygen and fluorine signals (Fig.9), confirming that samples were not contaminated during the implantation sessions.

Figure 9. Survey scans of reference, HDLF and HDHR samples

High resolution spectra for Au4f, C1s, O1s and F1s (Fig.10) were collected and used for quantitative analyses (Table 2). Here, we discuss in detail the results for sample HDLF, since the other samples have very similar spectra. Au4f7/2 has a binding energy of 84.0 eV ascribed to metallic gold. C1s has an asymmetric peak centered at 284.8 eV typical of adventitious carbon and it has been fitted with three components: the major one centered at 284.8 eV and two less intense contributions at 287.1 eV and 289.2 eV deriving from a C-C/C-H, C-O and O-C=O environments, respectively. O1s and F1s give symmetric photoemission peaks, well fitted with a single component, and centered at 532.5 eV and 689.5 eV, respectively. O1s binding energy value is compatible with oxygen atoms bound to carbon, while F1s has typical value of organic 18 ACS Paragon Plus Environment

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fluorine (C-F bonds). Carbon and oxygen clearly derive from adventitious contamination that generally comprises a variety of hydrocarbons species with small amounts of both singly and doubly bound oxygen atoms. Due to its nature, fluorine is probably originated from small contamination of vacuum oil accumulated during the different stages of samples preparation. Comparison of HDLF and HDHF surface composition with reference sample (Table 2) indicates the presence of a higher amount of adventitious carbon species in the irradiated samples that can be associated to storing and handling. This confirms that the chemical nature of outer layers of the optical films was not modified during the implantation sessions. Moreover, in both samples a very similar amount of contaminants has been revealed, even though the time of irradiation, and thus the time the samples lay in the vacuum chamber, was different by a factor of 58. We conclude that contaminants do not depend on flux. Moreover, the amount of carbon contaminant cannot explain the changes of the optical properties in the visible spectral range described in the following section.

Figure 10. High resolution spectra and fitting for Au4f, C1s O1s and F1s in sample HDLF.



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Table 2: XPS quantitative analysis of the samples.

Sample

C (at. %)

O (at. %)

F (at. %)

Au (at. %)

Reference

46.8

11.5

5.1

36.6

HDLF

71.0

15.9

3.2

9.9

HDHF

64.9

16.9

0.9

17.3

3.3 Optical analysis

The dielectric function of the virgin samples was retrieved by fitting the measured ellipsometric curves Ψ

and Δ; the results are reported in Fig.11. This preliminary work was necessary because the optical constants of gold thin films available in literature40-44, are different from those of our samples, as demonstrated by the fact that it was not possible to satisfactorily fit the reflectance measurement obtained by the spectrophotometer (Fig.12).

Figure. 11 Dielectric function retrieved by spectroscopic ellipsometry.


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Figure. 12: Reflectance measurement of virgin gold samples and related fitting obtained using the dielectric function shown in Fig.12. For comparison, various curve obtained with data in literature are also reported.

The reflectance drop observed in the visible range (300 - 800 nm) after He irradiation was modelled by adopting the effective medium approximation (EMA). The effects produced by the vacancies and inclusions can be suitably modelled considering that they induced a change of the complex dielectric function of the material medium which depends on the fractional volume e : f

e = f3g

where

i1 is

(6)

h

the density of the vacancies and inclusions and

j

is the density of gold atoms in the film. In

particular, if the e is sufficiently small (i.e. e k 0.2), the changes in the optical constants can be modelled by using the Maxwell-Garnett (MG) formula45 which allows the calculation of the effective dielectric function ,e of the intermixed system corresponding to the implanted film volume. According to the MG

formula, the effective dielectric function, ,e , depends on the matrix medium, , j , the dielectric function of the inclusions, ,i1 , and the volume fraction e of the inclusions with respect to the matrix medium:

,e = , j

/)no ph 66/no p3g /6no ph 6)no p3g


(7)


The fractional volume e changes with the depth of the film proportionally to the He implantation profile measured by SIMS (Fig. 2a); correspondently, the irradiated region of the film is characterized by a complex dielectric function which varies continually in depth. The reflectance of such films can be computed by using the standard transfer matrix formalism by discretizing into layers the irradiated area and assuming each one as characterized by its local average fractional volume e (Fig. 13).

Figure. 13: Sketch of the irradiated gold films area discretized in layers with the superimposition of the fractional volume distributions adopted for the reflectance computation.

The reflectance measurements of the irradiated samples were fitted with the model just described by using a fraction volume distribution function qe , where B is the normalized depth profile and qe the maximum fraction volume of the distribution, which is used as a free parameter in the optimization. The first 150 nm of the films were discretised into 50 layers. The dielectric function for the materials is that derived by ellipsometric measurements (Fig. 11), while the one associated to vacancies and inclusions was assumed to be ,i1 equal to 1. This model was applied to fit the set of the reflectance curves of sample irradiated with different doses all with flux F1 (Fig. 14(a)). As an example, Figure 14(b) reports the reflectance curves measured for samples LDLF and HDLF together with their best fits. For each curve, the fit associated reduced r / is always lower than 1. 22 ACS Paragon Plus Environment

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Figure.14: (a) Reflectance of the samples irradiated at different doses with the lower flux F1. (b) Measurements (dots) and related fitting of samples LDLF and HDLF. (c) Maximum fractional volume qe derived by the fitting procedure associated to the curve of (a). The error bars reported for each point indicate the 95% confidence interval.

As it can be observed from Fig.14(b), the model has a good agreement with the measurements almost in the whole spectral range considered. Small discrepancies between the experimental points and the theoretical 23 ACS Paragon Plus Environment


curves occur at the wavelengths around 470 nm and 320 nm, where the effects of the inter-band transitions of gold are present, which the MG approximation cannot describe. qe obtained by fitting the curves reported in Fig. 14(c) reports the value of the maximum fractional volume Fig. 14(a) as function of the dose (the error bars indicate the 95% confidence interval). The retrieved values of the fractional volume show a linear relation with the dose, with the x-intercept not placed at zero as expected. This can be explained considering the presence of other effects, such as the thermal one as mentioned in section 3.1, which are not directly taken into account in the model. The model developed well applies to samples irradiated with F1, but not at higher fluxes F2 and F3. In fact, given a dose, the modification observed in the case of higher fluxes is lower, as for example for sample irradiated with D4 (Fig.15). In the case of F2 irradiated samples at different doses, the only non-zero qe is that related to sample HDMF. The model has thus been proved for fluxes of 1.5⋅1011 He/cm2/s, and doses in the range of 2.5⋅1015 – 1.6⋅1016 He/cm2. Its validity can be probably extended to lower fluxes values and higher doses, but its applicability beyond the verified range must be proven. Special caution should be given in the case of higher fluxes. For the sake of completeness, the graph of the reflectance of the sample HHHDHHF is shown in Fig.16, which demonstrates a very large drop in performances, as it was expected by considering both the results from TEM and SIMS analysis.

Figure. 15. Reflectance for samples irradiated at dose D4 with different fluxes.


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Figure 16. Reflectance for samples irradiated at dose D6.

4. Conclusions For the first time He ions irradiation on gold samples at different doses and fluxes was carried out systematically. Within this study, we demonstrated that upon irradiation at doses above 1017 He/cm2 a substantial degradation of surface morphology and optical performances is present. Such changes are induced by the formation of bubbles and subsequent He release in the top part of the layer, which corresponds to the implanted area, as demonstrated by SIMS. In future qualification planning, this experimental outcome must be taken into account. At dose levels below 2⋅1016 He/cm2, a slight change on the optical performances is observed. The effect of different fluxes was considered at all different doses. In particular, a SIMS investigation was systematically carried out for the first time. As a novelty, SIMS analysis seems to indicate as the implant profile remains substantially unvaried by changing the fluxes, although data are not conclusive regarding the concentration of He actually implanted. Optical modifications are more evident in samples irradiated at lower fluxes. This fact could be dependent on physical processes induced by the high particles rates, including a potential thermal effect, which contrasts the drop in reflectance associated to vacancy and inclusions formation only. Since these tests are accelerated with respect to what really happening in space, we conclude that qualifications should be performed at low fluxes, as in our case LF=1.5·1011 He/cm2/s, compatibly with the time of occupancy of the facility. This constraint on the flux is stated clearly for the first time and should be taken into account in future qualification procedures. Such flux limits the temperatures undergone by the thin 25 ACS Paragon Plus Environment


films during the tests, as discussed in section 2. The range of temperature faced by the component during the laboratory irradiation session is in fact narrower than in space, as optical systems are usually provided with an active control to maintain the operating temperature within the limits established by the opto-mechanical tolerances; typical excursion values are of a few tens of degrees. Therefore, the qualifications of components must be performed having temperatures within the operating limits of the instrument, as it is the case of LF. Higher temperature, eventually induced in the films by higher fluxes, could cause annealing effects in some exceptional cases when unprotected components are directly exposed to the environment; this is for example the case of the external occulter mirror of the ESA Solar Orbiter METIS instrument46, for which a deeper study involving a temperature dependence dynamic would be needed. The modification of the morphological and structural properties of the film induced by the particles simulating the solar wind has been correlated for the first time to the change of their optical performances through the use of a proper model. In the case of lower flux F1 of 1.5·1011 He/cm2/s, a model based on Maxwell-Garnett approximation has been developed to fit the experimental reflectance data and to retrieve the fractional volume values as function of the dose. Within this study, it has been proven that the dependence of the fractional volume is linear with the dose. Thus, the model can be used to predict the performances of a gold film at any different dose. The model could not be applied in the case of higher fluxes, since the qe approaches zero in all samples except to HDMF. The present study will be extended to different materials, including oxides. In this last case the variation of the dielectric function is expected to be much greater than for gold after the irradiation or, more in general, than expected for metal coatings.

Acknowledgments

The authors thank the technical and administrative staff of the Forschungszentrum Dresden Rossendorf for their support during ion implantation experiment. The authors are grateful to Dr. Omar Sqalli Houssini and Dominic Doyle from ESA for the fruitful discussions. This work has been performed with the financial support of the Italian Space Agency (ASI-INAF I/013/12/0 Solar Orbiter) and European Space Agency (ESA


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Contract No. 4000122836/18/NL/PS/gp). The authors thanks the Italian Space Agency, Unità Tecnologie ed Ingegneria, for its support.

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46. Antonucci, E., Fineschi, S., Naletto, G., Romoli, M., Spadaro, D., Nicolini, G., Nicolosi, P., Abbo, L., Andretta, V., Bemporad, A. and Auchère, F., 2012, September. Multi Element Telescope for Imaging and Spectroscopy (METIS) Coronagraph for the Solar Orbiter Mission. In Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray (Vol. 8443, p. 844309). International Society for Optics and Photonics. Graphic abstract


Morphological and Functional Modifications of Optical Thin Films for

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