Relation of optical properties and femtosecond laser damage resistance for Al2O3/AlF3 and Al2O3/SiO2 composite coatings Mathias Mende,1 Istvan Balasa,1 Henrik Ehlers,1,* Detlev Ristau,1,2 Dam-be Douti,3 Laurent Gallais,3 and Mireille Commandré3 1 2 3

Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany

QUEST: Centre of Quantum Engineering and Space-Time Research, Welfengarten 1, D-30167 Hannover, Germany

Institut Fresnel, CNRS, Aix-Marseille Université, Ecole Centrale Marseille, Campus de St Jérôme, 13013 Marseille, France *Corresponding author: [email protected] Received 9 September 2013; revised 1 December 2013; accepted 9 December 2013; posted 9 December 2013 (Doc. ID 195700); published 24 January 2014

We report on the realization of aluminum oxyfluoride thin films and alumina/silica mixture coatings with different ratios by ion beam sputtering. The atomic compositions quantified by energy dispersive x-ray spectroscopy are correlated with the optical properties calculated from spectrophotometry and laser calorimetry measurements. Furthermore, the femtosecond laser damage resistance (τ  400 fs) of single layers is investigated in the infrared at 1030 nm and in the ultraviolet at 343 nm wavelengths. Experimental results on the wavelength scaling of the laser-induced damage threshold for oxyfluoride and oxide composite coatings are presented. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (320.0320) Ultrafast optics; (140.3330) Laser damage. http://dx.doi.org/10.1364/AO.53.00A383

1. Introduction

UV optics are required for industrial semiconductor lithography and micromaterial processing applying excimer or frequency converted solid state lasers as well as for various scientific projects, e.g., Laser Mégajoule or National Ignition Facility. This increases the demand for UV coatings with the lowest optical losses, enhanced mechanical and environmental stability, and the highest laser damage resistance and pushes the development of coating technology. In the last two decades, the optical and microstructural properties as well as the laserinduced damage threshold (LIDT) of fluoride UV coatings fabricated by different deposition techniques have been the subject of several publications [1–10]. Although the deposition of mixture coatings 1559-128X/14/04A383-09$15.00/0 © 2014 Optical Society of America

with adjustable refractive indices, optical band gap energies, and laser damage resistance has been demonstrated for many oxide material combinations [11–14], oxyfluoride or fluoride composite coatings are rarely investigated [15–19]. The reason for this lack of knowledge might be related to the more complex deposition process for fluoride thin films. Nevertheless, a combination of oxide and fluoride coating materials in mixture thin films will offer an extension to lower refractive indices, higher optical band gap energies, and potentially higher LIDT in the femtosecond regime compared to oxide mixtures. In the present study, two different process approaches are evaluated with regard to the realization of aluminum oxyfluoride mixture coatings with continuously adjustable optical properties by ion beam sputtering (IBS). The refractive indices, the optical band gap energies, and the femtosecond LIDT of Al2 O3 ∕AlF3 and Al2 O3 ∕SiO2 mixture thin films are compared. Furthermore, the influence of two 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A383

different substrate materials on the radiation resistance of the mixture coatings is analyzed. Finally, the wavelength scaling of the femtosecond LIDT for mixture thin films is discussed. 2. Experimental Setup

All thin films prepared for this study are coated in a cryo-pumped BALZERS BAK640 deposition plant equipped with a Veeco 16 cm RF ion source and a modified 66 cm focal point molybdenum grid set. Xenon with a purity of 99.999% is used as the sputter gas. Oxygen and an argon/fluorine mixture can optionally be added through separate mass flow controllers into the vacuum chamber. The deposition process is started at a background pressure below 8.0 × 10−6 mbar. The substrate temperature during the process is about 50°C. Employing a broadband optical monitoring system in combination with precoated test samples, a film thickness of two quarter-wave optical thickness at 1064 nm wavelength is precisely realized. A more detailed description of the coating plant and some aspects of the deposition process for mixture thin films are published in [20]. A.

Ion Beam Sputtering of AlF3 Coatings

Before describing the deposition approaches for oxyfluoride and oxide mixture coatings, the process optimization and the limitations for the realization of pure aluminum fluoride coatings are briefly discussed. In different optimization series, AlF3 single layers are sputtered applying a hot pressed AlF3 target specified with a purity of at least 99.9%. The following parameters and process steps have shown a significant influence on the optical properties of the AlF3 coatings: 1. the ion source parameters, especially the ion energy distribution, 2. the reactive gas environment and the chamber base pressure, and 3. the ex-situ post-treatment of coatings by UV irradiation. The fact that the listed parameters and process steps are dependent on each other to some extent makes the process optimization and the interpretation of the results even more difficult. The energy distribution of the xenon ions extracted in the Veeco 16 cm RF source is studied by replacing the AlF3 target with a retarding field analyzer (RFA) which is described in more detail in [21]. For different beam voltage set-point values, a collector current Icol is measured as a function of the retarding voltage Uret , which is ramped from zero up to 1200 V. The different ion energy distributions are determined as the derivative of the collector current with respect to the retarding voltage. Sections of the ion energy distributions showing the main peak are plotted in Fig. 1 for different beam voltage set-point values. The correlation of the ion energy distributions maxima with the optical properties of AlF3 single layers deposited with similar beam voltage set-point A384

APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014

Fig. 1. Ion energy distributions measured with a RFA in the target plane for the employed Veeco 16 cm RF ion source operated with different beam voltage set-point values.

values on fused silica substrates indicates the lowest extinction coefficients for ion energies around 800 eV. By adjusting the ion energy distributions maxima and keeping the reactive gas flows and the beam current constant, extinction coefficients at 193 nm between 2.0 × 10−3 and 5.5 × 10−2 are observed. The influence of the reactive gas environment on the optical properties of AlF3 thin films is investigated by adding defined gas flows of the argon/ fluorine mixture into the vacuum chamber while coating a single layer on fused silica substrates. Applying an LZH DUV/VUV spectrophotometer [22], the transmittance (T) and the reflectance (R) spectra of those single layers are measured under nitrogen atmosphere to suppress the absorption band of oxygen. In Fig. 2, the transmittance and the optical losses in terms of (100%-T-R) are plotted for AlF3 thin films deposited in different reactive gas environments. These results obviously show that the argon/ fluorine mixture is essential as a reactive gas to realize aluminum fluoride coatings with a stoichiometric composition and the lowest optical losses. A reduction of optical losses by ex-situ UV irradiation has been reported for ion-assisted fluoride coatings in [10]. The ex-situ UV treatment in this

Fig. 2. T and (100%-T-R) spectra for AlF3 thin films deposited under different reactive gas environments.

sample preparation or the measurement itself. For AlF3 films, it cannot be ruled out that the darker spots and the Moiré patterns are caused by electronbeam-induced crystallization inside the amorphous film as reported in [23]. Furthermore, the atomic composition of an AlF3 thin film is investigated by x-ray photoelectron spectroscopy (XPS) employing monochromatic MgK α x-rays (1253.6 eV). In Fig. 5, high-resolution XPS spectra of the Al2p and the Al2s [Fig. 5(a)], the F1s [Fig. 5(b)], and the O1s [Fig. 5(c)] peaks are shown. As a result, an oxygen fraction of the F  O content of about 20% is

Fig. 3. T, R, and (100%-T-R) spectra for an AlF3 thin film before and after the UV irradiation procedure.

study is realized by applying a xenon light source emitting at 172 nm in wavelength. In Fig. 3, T, R, and (100%-T-R) spectra of an AlF3 thin film deposited on a MgF2 substrate are compared before and after 12 hours of UV irradiation. The nitrogen absorption lines in the spectra are generated during the spectrophotometric measurement by the nitrogen purging gas. According to the ion-assisted fluoride coatings, clearly reduced optical losses are achieved by an ex-situ UV treatment for IBS AlF3 single layers. A possible explanation for a reduction of absorption by UV irradiation could be that the mobility of unbonded fluorine is increased and local substoichiometry with metallic behavior is healed. To avoid oxygen diffusion into the thin film which could induce oxide formation, the UV treatment is performed under nitrogen atmosphere. In an extensive characterization taking the chemical and structural properties into account, some limitations for the realization of pure aluminum fluoride coatings are identified. To obtain detailed information about the microstructure, an AlF3 coating is analyzed by transmission electron microscopy (TEM). In Fig. 4, a high-resolution bright-field image and a diffraction image are presented. Both images indicate numerous crystallites with diameters in the nanometer range embedded in a mainly amorphous matrix. Although TEM is an established technique to characterize the structural properties of amorphous thin films [5,17,20], the results can be affected by the

Fig. 4. TEM high-resolution bright-field image and diffraction image of an AlF3 thin film.

Fig. 5. High-resolution XPS spectra of (a) Al2p and Al2s, (b) Fls, and (c) O1s peaks measured for an AlF3 thin film. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A385

determined. Both the formation of nanocrystallites and the oxygen fraction are limitations for the realization of an ideal AlF3 coating. B. Deposition of Al2O3/AlF3 and Al2O3/SiO2 Mixture Thin Films

Two different approaches are evaluated for the deposition of the aluminum oxyfluoride mixture coatings. In the first approach, a hot pressed Al2 O3 target specified with a purity of at least 99.99% and the previously mentioned AlF3 target are used to deposit single layers in an argon/fluorine or an oxygen reactive atmosphere. In Fig. 6, the transmittance spectra of the four different thin films are compared. Applying the pure target materials in combination with an adjustable reactive atmosphere only half of the optical band gap and refractive index range between aluminum fluoride and oxide could be covered. The observed band gap shift of 0.4 eV for the Al2 O3 target material is slightly higher than the shift of 0.3 eV reported in [17] for oxyfluoride thin films deposited by plasma-ion-assisted electron beam evaporation. In the second approach, an Al2 O3 ∕AlF3 zone target with comparable specifications is applied to coat a set of nine aluminum oxyfluoride thin films on MgF2 and fused silica substrates, as well as on silicon wafers. By adjusting the zone target position and the reactive gas environment, the full optical band gap and refractive index range could be covered. The atomic compositions for the Al2 O3 ∕AlF3 sample set are determined by energy dispersive x-ray spectroscopy (EDX) applying a CamScan scanning electron microscope equipped with a Noran x-ray detector. The measurements are performed on the silicon wafers at an electron accelerating voltage of 15 kV and an x-ray take-off angle of 35° measured from the sample surface plane. The reproducibility of the atomic composition measured for a mixture thin film is within 2%. For the correlation of the composition with the optical properties, the O fraction of the sum of the

Fig. 6. Transmittance spectra of four different Al2 O3 ∕AlF3 oxyfluoride thin films deposited applying an AlF3 or an Al2 O3 target combined with either F2 ∕Ar or O2 reactive gas. A386

APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014

Fig. 7. Refractive indices as a function of wavelength from 200 to 1200 nm for the set of Al2 O3 ∕AlF3 mixture thin films including the SiO2 reference sample.

O and the F content in the thin film is used as a parameter (see Figs. 7 and 8). The Al2 O3 ∕SiO2 composite coatings are deposited applying a zone target consisting of an Al-target with a purity of 99.999% and a SiO target with a purity of 99.95%. A set of ten mixture thin films is coated on fused silica substrates and on molybdenum sheets in an oxygen reactive atmosphere. Additionally, a pure SiO2 thin film is deposited on MgF2 and on fused silica substrates as a reference layer for the Al2 O3 ∕ AlF3 sample set. The atomic compositions for the Al2 O3 ∕SiO2 sample set are also determined by EDX on the molybdenum sheets under the same conditions as describe above. For this sample set, the Al fraction of the sum of the Al and the Si content is used as a parameter for the correlation of the composition with the optical properties (see Figs. 9 and 10). C.

Optical Properties

For both mixture sample sets, the refractive indices and the extinction coefficients are determined from transmittance and reflectance measurements performed with a Perkin Elmer Lambda 900 and a LZH DUV/VUV spectrophotometer under 6° angle of incidence. The refractive indices are calculated

Fig. 8. Absorption coefficients as a function of photon energy for Al2 O3 ∕AlF3 mixture coatings of different compositions and the SiO2 reference thin film.

Fig. 9. Refractive indices as a function of wavelength from 200 to 1200 nm for the set of Al2 O3 ∕SiO2 mixture thin films.

by Sellmeiers dispersion formula (1) by applying the thin film design software SPEKTRUM [24–26] n2 λ  1 

3 X Aj λ 2 : λ2 − B j j1

(1)

The optical band gap energies are derived by two different approaches based on the absorption coefficient α, which is calculated from the extinction coefficient. The first method defines the optical band gap E04 as the photon energy where the absorption coefficient equals the value of 104 cm−1 [27]. In a second procedure, the optical band gap energy ETauc is determined by plotting αhν1∕2 as a function of the photon energy hν and extrapolating the linear curve progression to zero [28,29]. The refractive indices as a function of wavelength and the absorption coefficients as a function of the photon energy including the E04 level are plotted in Figs. 7 and 8 for the Al2 O3 ∕AlF3 sample set and in Figs. 9 and 10 for the Al2 O3 ∕SiO2 mixture thin films. The comparison of the absorption coefficients for the two pure SiO2 thin films plotted in Figs. 8 and 10 points out that the absorption coefficients and the deduced optical band gap energies for Al2 O3 ∕SiO2 samples with an

Fig. 10. Absorption coefficients as a function of photon energy for Al2 O3 ∕SiO2 mixture coatings of different compositions.

Al fraction of the Al and the Si content below 19% are affected by the absorption edge of the fused silica substrates. Furthermore, for Al2 O3 ∕AlF3 coatings with an oxygen fraction of the F  O content below 46% the optical losses could be reduced by an ex-situ UV treatment. In addition to the characterization by spectrophotometry, the Al2 O3 ∕AlF3 sample set is analyzed by laser calorimetry at 193 nm. The absorptance measurements using a LZH test system [30,31] are performed according to ISO 11551 [32]. The radiation source is an ArF excimer laser with a pulse duration of 12 ns. The repetition frequency during the tests was 1 kHz. The beam diameter in the sample plane was 4.9 mm. The general measurement error budget is 13% relative, whereas an uncertainty of 10% is attributed to the determination of the applied laser power. As the measured absorption at 193 nm can be dependent on the exposure dose as well as on the irradiation fluence, the measurement conditions were chosen carefully applying a fixed test procedure for all samples: at first, to consider any underlying dose dependence, each sample is measured at a fluence of 5 mJ∕cm2 for several times, to confirm, or if necessary, to reach the regime of static absorption with respect to the irradiation dose. After that, several measurements, each applying different irradiation fluence, are conducted. The fluence is varied by maintaining the beam diameter and tuning the laser pulse energy. For reasons of comparability to the results determined by spectrophotometry, the absorption at low irradiation fluence has to be chosen. Therefore, the absorption of interest is the extrapolated value at an irradiation fluence of H → 0 mJ∕cm2 . This is illustrated in Fig. 11 using the measurement results of the mixture thin film with 77% O fraction of the F  O content. The fluence dependence data reveal a linear relation with a slope of a few 100 ppm∕mJ∕cm2 for all samples, from

Fig. 11. Absorption measurements utilizing different irradiation fluence for an Al2 O3 ∕AlF3 mixture thin film (77% O fraction of the F  O content). The linear extrapolation to a fluence of 0 mJ∕cm2 allows us to compare the results with the data measured by spectrophotometry. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A387

on the fluence determination is estimated to be 4% (relative) considering the uncertainties in the energy and beam profile measurements. The samples are tested in the 1-on-1 mode with 20 sites tested for each fluence value. Damage was defined as any irreversible modification observed in the irradiated area with a Zeiss Axiotech microscope using a ×100 objective in Nomarski or dark-field mode. 3. Results and Discussion A. Laser-Induced Damage Threshold Data Processing

Fig. 12. Investigation of the absorption dependence on the laser irradiation dose for an Al2 O3 ∕AlF3 mixture coating (23% O fraction of the F  O content). After the ex-situ UV treatment using the xenon light source, only a negligible dose dependency can be observed.

which the absorption at fluence H → 0 mJ∕cm2 is easily extracted using a linear fit. Furthermore, the measurement results indicate a negligible dose dependence on the absorption. The qualitative characteristic for all samples is very similar to the example shown in Fig. 12. It can be assumed that optically as well as thermally induced structural changes in the coating (as well as in the substrate bulk material) have reached a state of saturation. D.

Laser-Induced Damage Threshold Measurements

The test facility and the calibration procedures used for the LIDT measurements are described in detail in [14,33]. As the laser source, a commercial femtosecond diode-pumped ytterbium amplified laser (Amplitudes Systemes S-Pulse HP) operating at a wavelength of 1030 nm with a spatially Gaussian beam profile is applied. Additionally, a new line to generate third harmonic radiation at 343 nm, applying a nonlinear crystal, is implemented for the UV damage tests. The linear-polarized beam is focused at normal incidence on the front face of the coated sample by a plano-convex lens. The pulse duration at 1030 nm is obtained by autocorrelation traces of the laser beam, measured with an ASF Avesta system. The LIDT measurements at 1030 nm are performed at two slightly differing pulse durations of 500 fs for the Al2 O3 ∕SiO2 mixture thin films and 380 fs for the Al2 O3 ∕AlF3 composite coatings. Both LIDT test series at 343 nm are realized with a pulse duration of 380 fs for the first harmonic. The effective spot sizes, as defined in the corresponding ISO 21254:2011 international standard [34], are measured before each experiment to calibrate the fluence with a WinCam UV CCD camera and a magnification system. The LIDT measurements at 343 nm are performed at a beam diameter on the sample surface of about 20 μm at 1∕e, whereas the LIDT tests at 1030 nm are conducted with a beam diameter of about 50 μm at 1∕e. The error budget A388

APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014

The results are presented in terms of “internal” LIDT using Eq. (1), with Emax ∕ Einc being the ratio of the maximum of the standing-wave electric field distribution in the film to the incident electric field. This ratio is calculated numerically based on the determined refractive index and film thickness LIDTinternal  jEmax ∕Einc j2 LIDTmeasured :

(2)

Since the LIDT measurements at 1030 nm for the Al2 O3 ∕SiO2 mixture thin films are performed at a slightly different pulse duration, the results are rescaled for the comparison by a factor of 0.921. The applied scaling factor is determined from a temporal scaling law of the LIDT, obtained by Mero et al., based on measurements of different pure material single layers [35]. B. Relation between Optical Properties, Chemical Composition, and Laser-Induced Damage Threshold

The refractive indices of mixture thin films can be calculated from the refractive indices of the cosputtered pure materials and the film composition, using different effective medium approximation theories. In the present study, the Lorentz–Lorenz Eq. (3) is applied to calculate the refractive indices for both mixture sample sets [36,37] n2eff − 1 n2b − 1 n2a − 1  f  f : a b n2eff  2 n2b  2 n2a  2

(3)

In Fig. 13, the index of refraction at 343 and 1030 nm derived from transmittance and reflectance measurements is compared to the refractive index calculated from the atomic compositions. The two curves for the Al2 O3 ∕SiO2 sample set, plotted as a function of the Al fraction of the sum of the Al and the Si content, show an acceptable agreement for both wavelengths, whereas for the data of the Al2 O3 ∕AlF3 sample set, plotted as a function of the O fraction of the sum of the F and the O content, significant deviations are observed. Since the EDX measurement provides no information about the chemical bonding of the fluorine and the oxygen atoms, no explicit conclusion about the different types of molecules in the film can be obtained. For more precise calculations, the determination of the molecular composition for the aluminum oxyfluoride thin films requires further investigation.

Fig. 13. Refractive indices at 343 and 1030 nm wavelengths as a function of the Al fraction of the Al  Si content for the Al2 O3 ∕SiO2 composite coatings and as a function of the O fraction of the F  O content for the Al2 O3 ∕AlF3 mixture thin films. Results derived from transmittance and reflectance measurements by spectrophotometry (TuR) are compared to data calculated via Eq. (3) from the atomic compositions measured by EDX (LL).

Fig. 15. Extinction coefficients at 193 nm calculated from absorption (A) values measured by laser calorimetry and from T and R values determined by spectrophotometry for Al2 O3 ∕AlF3 composite coatings.

A continuous shift of the optical band gap energy as a function of the refractive index or the chemical composition has been demonstrated for various oxide mixture coatings [11,12,14]. The curve progression of the optical band gap energies derived by the E04 and the ETauc approach is analyzed as a function of the refractive index at 1030 and 343 nm. A linear relationship between the optical band gap energy and the index of refraction is observed for both wavelengths. Furthermore, the E04 band gap energies show an offset of about 5% compared to the ETauc values. In Fig. 14, the E04 and the ETauc optical band gap energies are presented as a function of the refractive index at 1030 nm for Al2 O3 ∕AlF3 sample set including the SiO2 reference layer. For correlation with the LIDT results, the E04 band gap energies are selected. To verify the extinction coefficients determined from the spectrophotometry measurements, the

extinction coefficients at 193 nm are calculated on the basis of laser calorimetry measurements for the Al2 O3 ∕AlF3 mixture thin films. The results derived from the two different measuring techniques are plotted in Fig. 15 as a function of the O fraction of the sum of the F and the O content. Although the scattered light is not included in the extinction coefficients obtained via laser calorimetry, very good accordance between both datasets is observed. The laser damage threshold at 1030 nm in the femtosecond pulse regime is reported to correlate with intrinsic thin film material properties, e.g., the refractive index or the optical band gap energy [13,14]. For this reason, the internal damage fluences at 1030 nm of both mixture sample sets are compared as a function of the refractive index at 1030 nm in Fig. 16. Except for the sample with the highest fluorine content, the LIDT of the Al2 O3 ∕AlF3 mixture coatings is continuously increasing for decreasing refractive indices. The Al2 O3 ∕SiO2 sample set shows no significant evolution of the LIDT as a function of the

Fig. 14. Comparison of E04 and ETauc optical band gap energies as a function of the refractive index at 1030 nm for Al2 O3 ∕AlF3 mixture coatings of different compositions.

Fig. 16. Comparison of the LIDT at 1030 nm as a function of the refractive index at 1030 nm for Al2 O3 ∕AlF3 and Al2 O3 ∕SiO2 mixture thin films. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A389

above 4.3 J∕cm2 is measured for an oxyfluoride coating with a refractive index at 1030 nm of 1.411 and an E04 optical band gap energy of 8.0 eV. 4. Conclusions

Fig. 17. LIDTs measured at 343 and 1030 nm wavelengths as a function of the optical band gap energy E04 for Al2 O3 ∕AlF3 composite coatings.

refractive index. The distinct deviation of about 1.2 J∕cm2 measured for the two pure SiO2 thin films might be explained by the different optical band gap energies of the applied substrate materials. By comparing the results at 343 nm for the two SiO2 coatings on fused silica and MgF2 substrates, no difference in the LIDT is observed. This discrepancy is not understand so far and will need further investigation on the thin film properties as well as on the measurement conditions. To obtain information about the wavelength scaling of the LIDT, the results at 343 and 1030 nm are compared in Fig. 17 for the Al2 O3 ∕ AlF3 composite coatings and in Fig. 18 for the Al2 O3 ∕SiO2 mixture thin films. At both wavelengths, a linear dependency of the femtosecond LIDT with the optical band gap energy E04 is observed for the two different material combinations. For the Al2 O3 ∕AlF3 sample set a clear offset by a factor of 1.9 is determined, whereas the results for the Al2 O3 ∕SiO2 mixtures exhibit a wider scope for different interpretations. Compared to various pure and mixture materials [13,14] a very high internal LIDT at 1030 nm

Fig. 18. LIDTs measured at 343 and 1030 nm wavelengths as a function of the optical band gap energy E04 for Al2 O3 ∕SiO2 composite coatings. A390

APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014

The deposition of Al2 O3 ∕AlF3 and Al2 O3 ∕SiO2 mixture thin films with continuously adjustable refractive indices and the absorption coefficients is demonstrated by IBS applying a zone target setup. Although for the pure AlF3 coatings no oxygen is inserted during the deposition process, an oxygen fraction of about 16%–20% of the sum of the oxygen and fluorine content is quantified by two different measurement techniques. The optical losses of Al2 O3 ∕ AlF3 composite coatings with an oxygen fraction of the oxygen and fluorine content below 46% could be reduced by an ex-situ UV treatment. The calculation of the refractive indices by applying the Lorentz–Lorenz equation provides reasonable results only for the Al2 O3 ∕SiO2 mixture coatings. A comparison of the E04 and the ETauc optical band gaps for the Al2 O3 ∕AlF3 composite coatings revealed an offset of about 5%. By the comparison of the LIDT measurements at 343 and 1030 nm for the Al2 O3 ∕ AlF3 sample set, a linear dependency with the optical band gap energy E04 and a wavelength scaling factor of 1.9 is observed. Finally, a slightly higher LIDT than for the SiO2 reference coating is measured for an oxyfluoride thin films with 23% O fraction of the F  O content at both wavelengths. The authors thank the German Federal Ministry of Economics and Technology (BMWi) for the financial support of the research project “TAILOR” under Contract No. 16IN0667 within the framework of the “InnoNet” program. Also, funding by the German Research Foundation (DFG) within the Cluster of Excellence 201, “Centre of Quantum Engineering and Space Time Research,” QUEST, is gratefully acknowledged. References 1. J. Kolbe, H. Kessler, T. Hofmann, F. Meyer, H. Schink, and D. Ristau, “Optical properties and damage thresholds of dielectric UV/VUV coatings deposited by conventional evaporation, IAD, and IBS,” Proc. SPIE 1624, 221–235 (1992). 2. J. Dijon, E. Quesnel, B. Rolland, P. Garrec, C. Pelle, and J. Hue, “High-damage-threshold fluoride UV mirrors made by ion-beam sputtering,” Proc. SPIE 3244, 406–415 (1998). 3. M. Kennedy, D. Ristau, and H. S. Niederwald, “Ion beamassisted deposition of MgF2 and YbF3 films,” Thin Solid Films 333, 191–195 (1998). 4. J. Ferré-Borrull, A. Duparré, and E. Quesnel, “Roughness and light scattering of ion-beam-sputtered fluoride coatings for 193 nm,” Appl. Opt. 39, 5854–5864 (2000). 5. D. Ristau, S. Günster, S. Bosch, A. Duparré, E. Masetti, J. Ferré-Borrull, G. Kiriakidis, F. Peiró, E. Quesnel, and A. Tikhonravov, “Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation,” Appl. Opt. 41, 3196–3204 (2002). 6. K. Iwahori, M. Furuta, Y. Taki, T. Yamamura, and A. Tanaka, “Optical properties of fluoride thin films deposited by RF magnetron sputtering,” Appl. Opt. 45, 4598–4602 (2006). 7. F. Sarto, E. Nichelatti, D. Flori, M. Vadrucci, A. Santoni, S. Pietrantoni, S. Guenster, D. Ristau, A. Gatto, M. Trovò, M.

8.

9. 10.

11. 12. 13.

14. 15.

16. 17.

18. 19. 20.

Danailov, and B. Diviacco, “Vacuum-ultraviolet optical properties of ion beam assisted fluoride coatings for free electron laser applications,” Thin Solid Films 515, 3858–3866 (2007). H. Blaschke, N. Beermann, H. Ehlers, D. Ristau, M. Bischoff, D. Gäbler, N. Kaiser, A. Matern, and D. Wulff-Molder, “Investigation in the degradation of CaF2 outcouplers in excimer lasers operating at 193 nm,” Proc. SPIE 7132, 71321A (2008). C. J. Stolz, H. Blaschke, L. Jensen, H. Mädebach, and D. Ristau, “Excimer mirror thin film laser damage competition,” Proc. SPIE 8190, 819007 (2011). M. Bischoff, O. Stenzel, K. Friedrich, S. Wilbrandt, D. Gäbler, S. Mewes, and N. Kaiser, “Plasma-assisted deposition of metal fluoride coatings and modeling the extinction coefficient of as-deposited single layers,” Appl. Opt. 50, C232–C238 (2011). H. Demiryont, “Optical properties of SiO2–TiO2 composite films,” Appl. Opt. 24, 2547–2650 (1985). M. Cevro, “Ion-beam sputtering of (Ta2O5)x–(SiO2)1-x composite thin films,” Thin Solid Films 258, 91–103 (1995). B. Mangote, L. Gallais, M. Commandré, M. Mende, L. Jensen, H. Ehlers, M. Jupé, D. Ristau, A. Melninkaitis, J. Mirauskas, V. Sirutkaitis, S. Kičas, T. Tolenis, and R. Drazdys, “Femtosecond laser damage resistance of oxide and mixture oxide optical coatings,” Opt. Lett. 37, 1478–1480 (2012). M. Mende, S. Schrameyer, H. Ehlers, D. Ristau, and L. Gallais, “Laser damage resistance of ion-beam sputtered Sc2O3/SiO2 mixture optical coatings,” Appl. Opt. 52, 1368–1376 (2013). G. L. Harding, “Production and properties of high rate sputtered low index transparent dielectric materials based on aluminium-oxy-fluorine,” Sol. Energy Mater. 12, 169–186 (1985). R. Lewin, R. P. Howson, and C. A. Bishop, “Optical coatings for large area interference filters,” Vacuum 37, 257–260 (1987). O. Stenzel, D. Gäbler, S. Wilbrandt, N. Kaiser, H. Steffen, and A. Ohl, “Plasma ion assisted deposition of aluminium oxide and aluminium oxifluoride layers for applications in the ultraviolet spectral range,” Opt. Mater. 33, 1681–1687 (2011). X. Li, J. Sun, W. Zhang, Y. Hou, K. He, and K. Yi, “Effect of oxygen incorporation on the properties of AlF3 films,” Advan. Mater. Res. 631–632, 121–126 (2013). C. E. Anderson and J. P. Rousseau, “Transparent substrate provided with a thin-film coating,” U.S. patent 5,952,084 (September 14, 1999). M. Mende, L. O. Jensen, H. Ehlers, W. Riggers, H. Blaschke, and D. Ristau, “Laser-induced damage of pure and mixture material high reflectors for 355 nm and 1064 nm wavelength,” Proc. SPIE 8168, 816821 (2011).

21. N. Beermann, H. Ehlers, and D. Ristau, “Ion source characterization based on an array of retarding field analyzers,” Proc. SPIE 5963, 59630H (2005). 22. H. Blaschke, J. Kohlhaas, P. Kadkhoda, and D. Ristau, “DUV/ VUV spectrophotometry for high precision spectral characterization,” Proc. SPIE 4932, 536–543 (2003).. 23. G. S. Chen, C. B. Boothroyd, and C. J. Humphreys, “Electronbeam induced crystallization transition in self-developing amorphous AlF3 resists,” Appl. Phys. Lett. 69, 170–172 (1996). 24. B. Tatian, “Fitting refractive-index data with the Sellmeier dispersion formula,” Appl. Opt. 23, 4477–4485 (1984). 25. D. Poelman and P. F. Smet, “Methods for the determination of the optical constants of thin films from single transmission measurements: a critical review,” J. Phys. D 36, 1850–1857 (2003). 26. M. Dieckmann, “SPEKTRUM, Thin Film Design Software” (Laser Zentrum Hannover e.V., 1990–2013). 27. E. C. Freeman and W. Paul, “Optical constants of RF sputtered hydrogenated amorphous Si,” Phys. Rev. B 20, 716–728 (1979). 28. J. Tauc, R. Grigorovici, and A. Vancu, “Optical properties and electronic structure of amorphous germanium,” Phys. Status Solidi B 15, 627–637 (1966). 29. G. D. Cody, T. Tiedje, B. Abeles, B. Brooks, and Y. Goldstein, “Disorder and the optical-absorption edge of hydrogenated amorphous silicon,” Phys. Rev. Lett. 47, 1480–1483 (1981). 30. H. Blaschke, M. Jupe, and D. Ristau, “Absorptance measurements for the DUV spectral range by laser calorimetry,” Proc. SPIE 4932, 467–474 (2003). 31. I. Balasa, H. Blaschke, L. Jensen, and D. Ristau, “Impact of SiO2 and CaF2 surface composition on the absolute absorption at 193 nm,” Proc. SPIE 8190, 81901T (2011). 32. International Organization for Standardization, “Test method for absorptance of optical laser components,” ISO 11551:2003. 33. B. Mangote, L. Gallais, M. Zerrad, F. Lemarchand, L. H. Gao, M. Commandré, and M. Lequime, “A high accuracy femto-/ picosecond laser damage test facility dedicated to the study of optical thin films,” Rev. Sci. Instrum. 83, 013109 (2012). 34. International Organization for Standardization, “Test methods for laser-induced damage threshold,” ISO 21254:2011. 35. M. Mero, J. Liu, W. Rudolph, D. Ristau, and K. Starke, “Scaling laws of femtosecond laser pulse induced breakdown in oxide films,” Phys. Rev. B 71, 115109 (2005). 36. H. A. Lorentz, “Über die Beziehung zwischen der Fortpflanzungsgeschwindigkeit des Lichtes und der Körperdichte,” Ann. Phys. 245, 641–665 (1880). 37. L. Lorenz, “Über die Refractionsconstante,” Ann. Phys. 247, 70–103 (1880).

1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A391

SiO2 composite coatings.

We report on the realization of aluminum oxyfluoride thin films and alumina/silica mixture coatings with different ratios by ion beam sputtering. The ...
2MB Sizes 2 Downloads 0 Views