Open Access
Add to BibSonomy
Abstract
The potential of titanium and copper seed layers to enhance the optical properties of aluminum films for ultra-violet (UV) applications is analyzed. The seed layers significantly influence the initial layer growth of aluminum films. For the titanium-seeded aluminum, a surface roughness of 0.34 nm was observed. UV spectral reflectance measurements showed an average higher reflectivity of 4.8% for wavelengths from 120 nm to 200 nm for the aluminum film grown on the titanium seed layer. Furthermore, the titanium-seeded aluminum coatings were stable at an elevated temperature of 225°C and showed no increase in surface roughness or pinholes.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The recently published study of metal mirrors for the Large Ultraviolet Optical Infrared (LUVOIR) observatory is based on protected aluminum mirrors to cover the operating spectral range between 90 and 200 nm [1].
Aluminum has the highest intrinsic reflectance of metals for a broad spectrum ranging from the far-ultraviolet (FUV, 90-200 nm) to the infrared and beyond. The actual reflectivity of aluminum mirrors, however, can be significantly lower. The main reason is the formation of a native aluminum oxide layer immediately after contact with air that reduces the FUV reflectance.
Protective top coatings are usually used to prevent oxidation of the aluminum [2]. Metal fluorides are commonly used as topcoat materials at FUV wavelengths because of their high transparency in this region. The first inventions were made by Hass and Tousey back in 1959 with evaporated Al films protected with MgF2 (absorption edge at ∼115 nm) which achieved a reflectance of 80% down to 121.6 nm [3]. Other fluorides used are e.g. LiF (absorption edge at ∼102 nm), and AlF3 (∼113 nm) [4–6] or combinations of several fluorides [7]. These protective layers are applied immediately after the deposition of aluminum in the same process in order to prevent any formation of a natural oxidation layer. This leads to a number of challenges.
The application of protective topcoats leads to a conflict in deposition temperature for aluminum and the metal fluorides. Aluminum exhibits a higher surface roughness when deposited at elevated substrate temperatures [8]. The strong wavelength-dependence of scattering (∼ 1/λ2, 1/λ4) becomes a notable reason for losses in reflectance towards short wavelengths [9,10], making a smooth and closed aluminum film desirable. Therefore, it is usually deposited at room temperature. In contrast, metal fluorides are known to grow in a columnar, porous structure when deposited at room temperature, resulting not only in higher roughness and light scattering [11] but also to a higher affinity to contaminations [12] and thus FUV absorption [13,14].
Ion-beam sputtering (IBS) and atomic-layer deposition (ALD) are known to lead to highly dense layers. They have been investigated in the literature as deposition methods for protective layers on evaporated aluminum films [15,16]. Both approaches showed a reduced reflectance due to a vacuum break to transfer the samples from one chamber to another. Most recent research was done on finding an optimized deposition temperature for e-beam evaporated metal fluorides. For elevated deposition temperatures, pinholes in the aluminum films and a loss in reflectance were reported [17,18].
The quoted research above focuses on the protective metal fluoride layer and its properties. In this publication, we focus on the aluminum layer itself as it sets new challenges for the status quo deposition process for high reflecting mirrors in the UV spectral region. The goal was to develop smooth and closed aluminum films, which are temperature stable. To achieve this goal, we adapted the idea of a seed layer on which the aluminum layer is deposited. Seed layers were already used for thin silver films for plasmonic sensors and lenses [19]. A problem with thin metal films is their tendency to dewet and form isolated islands during thin film growth [20,21]. To inhibit dewetting and improve the surface roughness of the Ag films, thin seed layers of e.g. copper or germanium [22] were applied to the layer system. We use copper as it showed good results in [20] for Au films. Titanium seed layers are studied as well because of the promising results in [23] for epitaxial grown aluminum films for piezoelectric devices.
The characterization of the samples includes the topography of the Al films, their surface roughness, and crystallographic orientation. Furthermore, reflectance measurements were carried out to investigate the influence in the UV region and corresponding stray light losses. As mentioned before, the highest reflectivities for aluminum-based mirrors are achieved with hot deposited protective layers. These elevated temperatures can cause recrystallization and increased roughness or holes in the Al layer. To investigate the influence of the seed layers on the temperature stability of the aluminum, the surface topography and roughness after exposure to elevated temperatures were investigated.
2. Experimental procedure and analysis methods
This work shows the results of two sets of samples, each set consisting of 3 different types of samples and were prepared separately. The first sample type was the reference sample with about 75 nm aluminum (Al), which was deposited onto a Si-substrate without seed layer, acting as a reference sample. The other two types are Al films of the same thickness, deposited onto Si-substrates that were previously coated with a 3 nm thick seed layer of either copper (Cu/Al) or titanium (Ti/Al). The respective sample type, consisting of seed and aluminum layer, were deposited in the same coating run.
The samples were prepared in a Bühler Syrus Pro 1110 evaporation chamber equipped with a cryogenic and turbomolecular pump. The system is equipped with two electron guns and a plasma source. The deposition of the seed layer material and the aluminum was carried out from one electron source. The layer thickness and deposition rate were monitored using a quartz crystal. The evaporation material used was 99.98% pure aluminum, 99.5% titanium and 99.999% copper. As substrates, polished silicon (100) wafers with a size of 25 mm x 25 mm and a root mean square (RMS) surface roughness of σRMS = 0.14 nm were used.
For sample preparation, the chamber is heated to a temperature of 80°C and kept constant during the entire deposition run. After reaching the deposition temperature and a waiting period of 300 s, the substrates were cleaned in the chamber by plasma etching for 300 s and a BIAS voltage of 125 V. At a starting pressure < 8×10−5 Pa, the 3 nm thick seed layer was applied to the substrate: Titanium at a rate of 0.02 nm/s and copper at a rate of 0.2 nm/s. Subsequently, a 75 nm aluminum layer was deposited on the seeded substrates with a rate up to 20-25 nm/s. After coating, the aluminum surface was treated with an oxygen plasma for 180 s and 15 sccm oxygen flux to ensure standardized starting conditions of oxidation.
The first set of samples was prepared to investigate the surface morphology, the second set for the temperature influence on the aluminum films. For the investigation of temperature influence, after the aluminum deposition, the chamber was heated to a temperature of 225°C and kept constant for 180 s. The samples were then treated again with an oxygen plasma, as described before.
The surfaces of the samples were examined with a Carl Zeiss Σigma scanning electron microscope (SEM). For the detection of secondary electrons an InLens detector was used. The intersection method according to DIN ISO 643 [24] was used to determine the average grain size by SEM images.
Surface roughness analysis was performed using a Dimension Icon Atomic Force Microscope (AFM) from Bruker. The AFM was operated in tapping mode utilizing single-crystalline Si probes with a nominal tip radius of 7 nm.
Based on the roughness of the aluminum surfaces, the angle-resolved scattering was simulated using vector scattering theory [25] between 100 nm and 200 nm. These results were then integrated to yield the total scattering loss (TS), according to the ISO standard 13696 [26]. The total scattering loss can be directly used in the energy balance: 1 = R + T+A + TS, where R describes the reflectance, T the transmittance, and A the absorption.
The crystallinity of the prepared films was determined by X-ray diffraction (XRD) with a Bruker D8 diffractometer and Cu-Kα radiation (λ ≈ 0.154 nm).
The Physikalisch Technische Bundesanstalt (PTB), Berlin, carried out reflectivity measurements in the UV-range from 120-220 nm. The measurements were performed at a synchrotron (BESSY) using a VUV reflectometer, 34 days after the deposition.
3. Experimental results
3.1 Surface morphology of Al films deposited at 80°C
The influence of the different seed layer materials on the morphology of the aluminum layer was recorded as SEM images and is shown in Fig. 1. The single aluminum film (Al) in Fig. 1(a) shows a characteristic surface of a vapor-deposited aluminum layer. The individual grains of the layer are clearly visible with pronounced grain boundaries. This reveals that the aluminum layer grows initially in isolated islands, which indicates growth in the Volmer Weber mode. The characteristics of the Cu/Al seed sample are similar to the Al sample showing a smaller grain size (cf. Fig. 1(b)). The grain boundaries are still clearly visible. The isolated islands suggest a comparable Volmer Weber layer growth as for the single aluminum layer.
Fig. 1. SEM images of 75 nm thick aluminum layers grown; a) directly on the substrate; b) with copper seed layer; c) with titanium seed layer.
In contrast, the sample with a seed layer of titanium (Fig. 1(c)) shows a different morphology of the aluminum surface and the grain boundaries are less distinct as compared to both other samples. Calculations of the grain size by the intersection method were performed and the results are shown in Table 1.
Table 1. Results of the calculations of the average grain size by intersection method
The single aluminum film has an average grain size of (112 ± 6) nm, while the copper seeded aluminum shows a grain size of (56 ± 3) nm. In contrast, the sample with titanium seed layer exhibits a larger average grain size of (128 ± 6) nm.
Further investigations included AFM measurements in different scan areas of 1 × 1 µm2 and 10 × 10 µm2. The topography images and calculated surface roughness are shown in Fig. 2. The Al sample grown on the unseeded silicon substrate has an rms-surface roughness of σRMS = 1.43 nm for the 1 × 1 µm2 scan area and σRMS = 1.32 nm for the larger area, shown in Figs. 2(a) and (d). The difference of these roughness values can easily be explained by the different spatial frequency ranges covered.
Fig. 2. AFM 3D-images for 1 × 1 µm2 scan area (a – c) and 2D images of a 10 × 10 µm2 scan area (d – f). Uniform color scales shown on the right side.
A substantial increase in surface roughness was also observed for the Cu/Al sample with σRMS = 1.99 nm for the 1 × 1 µm2 scan area and σRMS = 1.61 nm for the 10 × 10 µm2 scan area as shown in Figs. 2(b) and (e). The smaller scan area in Fig. 2(b) also shows the finer grain size compared to other samples. A significantly lower surface roughness of σRMS = 0.38 nm in the small scan area and σRMS = 0.39 nm in the larger scan area, was measured for the Ti/Al sample shown in Figs. 2(c) and (f).
The morphological differences between the three samples could be an indication of different layer growth modes and different crystalline structures. In order to investigate the crystallographic orientation of the aluminum layers XRD measurements were performed and the results are shown in Fig. 3.
Fig. 3. Diffractograms of XRD measurements for all three samples. Al sample is shown in the top section in black, Cu/Al sample in the middle section in red, and Ti/Al sample in the bottom section in blue. The different intensities of the Al (111) peak at 2θ = 38.5° show significant differences in crystallinity.
The measurements were carried out in a 2θ range from 20° to 80°. For all samples, the Al (111) peak at 2θ = 38.5° was measured with different intensities. The Si (400) peak from the substrate occurs at the same angle (69.1°2θ) and with the same order of intensity for all samples. This allows us to compare the independent measurements.
The first section shows the measurement for the single Al film. In the middle section, the Cu/Al sample shows less intensity by an order of magnitude compared to the Al sample. The Ti/Al sample in the third section shows a more distinct Al (111) peak with an intensity higher than an order of magnitude compared to the Al sample.
The results of the reflectance measurements performed at PTB and the results of the predicted total scatter are shown in Figs. 4(a) and 4(b), respectively.
Fig. 4. Reflectance curves for all three samples from 220 nm to 120 nm for aluminum (black), Cu/Al (red) and Ti/Al (blue) shown in section a). Section b) shows the simulated roughness induced total scattering loss of each sample with the same color code. Measurements were performed 34 days after deposition and storage under atmospheric conditions.
The reflectance of the single Al layer shown as a black curve resembles the characteristic profile for unprotected aluminum in the UV range with an oxidized surface. In contrast to the Al sample, the Cu/Al film (red curve) shows an average 6.8% (absolute) lower reflectance from 140 nm to 200 nm while for lower wavelengths the reflectance converges towards the reflection of the unseeded Al film. The Ti/Al film (blue curve) shows a significantly higher reflectance of approximatley 4.8% for wavelengths shorter than 200 nm.
The roughness-induced total scattering losses of all three samples shown in Fig. 4(b) reveal, the Al sample to exhibit the highest scattering of all for the calculated wavelengths with a maximum of TS = 2.5% (absolute) at 100 nm. The Cu/Al sample has lower scatter losses as the aluminum sample but with an increase for wavelengths shorter than 140 nm up to a maximum of ca. 1% at 100 nm. The Ti/Al sample shows the lowest scatter losses with a maximum below 0.5% towards the shortest simulated wavelengths.
3.2 Topography after heating to 225°C
In order to investigate the influence of the seed layer on the temperature stability of the aluminum layer, we performed AFM measurements (scan areas of 1 × 1 µm2 and 10 × 10 µm2). The results depicted in Fig. 5.
Fig. 5. AFM 3D-images for 1 × 1 µm2 scan area (a – c) and 2D images of a 10 × 10 µm2 scan area (d – f). Uniform color scales shown on the right side. Samples were heated up to 225°C for 180 s after the deposition of the aluminum. The samples were treated with oxygen plasma at the end of the deposition run.
The single aluminum film shown in Figs. 5(a) and (d) on the unseeded silicon substrate has higher rms-surface roughness of σRMS = 1.73 nm for the 1 × 1 µm2 scan area and σRMS = 1.70 nm for the larger scan area, compared to the sample in Figs. 2(a)–(d), which was not heated to 225°C. For the Cu/Al sample a smoother surface observed with σRMS = 1.55 nm for the 1 × 1 µm2 scan area and σRMS = 1.04 nm for the 10 × 10 µm2 scan area as shown in Figs. 5(b)–(e). The Ti/Al sample has for both scan areas the same surface roughness of σRMS = 0.34 nm shown in Figs. 5(c) and (f). These samples were also measured by XRD to investigate the influence of elevated temperature on the crystallinity of the aluminum layer. The results are shown in Fig. 6.
Fig. 6. Comparison of intensity profiles gained from XRD measurements for samples exposed to a maximum temperature of 80°C (dashed lines) and up to 225°C (solid lines). The aluminum sample is shown in black, Cu/Al sample in red and Ti/Al in blue.
Since no other aluminum peaks were detected, we focused on the Al (111) Bragg peak at an angle of 38.5° and the angle region from 35° to 42°. The only two other peaks detected were at 36.9° for the Ti/Al samples and could be assigned to W-Lα radiation (λ ≈ 0.148 nm) from the x-ray source. The dashed lines show the intensity profiles for the samples from Fig. 3 and the solid lines the profiles for the samples, which were exposed to a temperature of 225° after deposition. The single Al samples are displayed as black profiles in the graph and show similar intensities in the same order of magnitude but with a less intense peak for the sample exposed to 225°C. Besides, the profile is slightly shifted to larger angles from 2θ80°=38.48° to 2θ225°=38.54°. A shift for the peak position could also be measured for the Cu/Al samples (red curves) from 2θ80°=38.49° towards 2θ225°=38.54° and increased intensity. The Ti/Al sample shows also a shift for the Al (111) peak from 2θ80°=38.54° to 2θ225°=38.6° with just a slightly higher peak intensity for the sample exposed to 225°C.
4. Discussion
The morphology differed drastically between the single Al film, the Cu/Al, and the Ti/Al film. The silicon wafer, with a native oxide layer, as a substrate for the aluminum has different properties concerning wetting behavior and chemical bonds than the silicon wafers with seed layers. Therefore, we propose two effects to explain the role of the seeding layer on the morphology of the aluminum layer.
The first one refers to the wetting behavior and is correlated with the surface free energy (SE) of the surface on which the aluminum (1.160 J m-2) layer is deposited. The surface free energies of titanium (1.989 J m-2) and copper (1.790 J m-2) [27] have significantly higher values than the native SiO2 layer on the silicon substrate (0.259 J m-2) [28]. The deposition of aluminum on a substrate with a higher SE leads to a positive difference between the SE of the substrate and the SE of the aluminum (Ti-Al = 0.829 J m-2 and Cu-Al = 0.630 J m-2). This results in a better wetting behavior for these two surfaces compared to the unseeded substrate were the difference becomes negative (SiO2-Al = -0.901 J m-2) [29]. This further leads to decreased diffusion of the aluminum on the seeded substrates and more nucleation sites [30] compared to the unseeded Si-wafer as substrate.
The second effect is the differences in the dissociation energy between the two seeding materials and aluminum. In the case of Cu and Al, the Cu-Al bonds have a slightly lower dissociation energy of 227 kJ mol-1 than the Al-Al bonds 264 kJ mol-1 [30]. This means that the deposited Al bonds preferentially to other Al atoms than to Cu. For the Ti-Al bonds, the dissociation energy of 263 kJ mol-1 [31] is close to the Al-Al bonds. This increases the probability that the deposited Al will bond to the titanium.
For the aluminum layer deposited directly on the Si-wafer, the above-mentioned effects have no influence. The well known Volmer Weber mode can be observed, as seen in Fig. 2(a). The island-like growth leads to an average grain size of (112 ± 6) nm and surface roughness of σRMS = 1.43 nm.
The seeded aluminum layers show different morphologies compared to the simple aluminum layer. The higher surface free energy of copper and titanium leads to a reduced diffusion of the Al atoms on the seed layer surface and more nucleation sites. The difference in dissociation energies for the Cu/Al and Ti/Al samples influences the initial layer growth. For the Cu/al sample, the aluminum bonds predominant to other Al atoms, due to the lower Cu-Al dissociation energy, resulting in island-like growth. This leads to a lower average grain size of 56 ± 3 nm and an increased surface roughness of σRMS = 1.99 nm, compared to unseeded aluminum. The titanium-seeded aluminum layer shows a different morphology as shown in Fig. 2(c). The comparable dissociation energies of Ti and Al lead to a formation of a closed initial layer and resulting in more layer-like growth. For increasing layer thickness of the Al, the Ti-Al bonds which are isolated at the Ti/Al interface, have less effect and a change from layer-by-layer to island-like growth takes place resulting in Stranski-Krastanov growth mode. For the Ti/Al sample this leads to a decreased surface roughness σRMS = 0.38 nm and an increased average grain size of 128 ± 6 nm compared to unseeded aluminum.
The increased peak intensity of the Al (111) orientation is attributed to the different growth modes of the titanium-seeded aluminum. The reduced interface energy between the titanium and aluminum leads to a uniform orientation of the Al (111) planes parallel to the titanium surface and an increased initial orientation of the aluminum is formed and maintained during layer growth [23,32].
We further suppose, that the differences in crystallographic orientation also explains the different results in the reflectance measurements. Gartland et al. [33] demonstrated that the Al (111) plane has a lower sticking probability of oxygen compared to the other planes. As a result, the change in the work function due to oxidation is lower for the Al (111) plane, attributed to a thinner aluminum oxide layer [34]. Further influences on the reflectance are attributed to the different grain sizes and hence resulting amount of grain boundaries of the aluminum film. The larger grain size of the Ti/Al sample leads to fewer grain boundaries and decreased plasma resonance in the grain boundaries [35,36].
These assumptions are supported by the simulation of the scattering losses shown in Fig. 4(b). The average losses through scattering for the Ti/Al sample in the spectral range from 200 nm down to 100 nm are 0.7%, while the reflectivity of the Ti/Al sample is increased by an average of 4.8% compared to the Al sample.
The second set of samples, which was exposed to a temperature of 225°C, showed differences in the respective surface roughness. The Al sample showed an increased surface roughness of σRMS = 1.73 nm and a change in the Al (111) peak intensity, assuming recrystallization of the aluminum layer [8]. The same assumption is made for the Cu/Al sample with a decrease in surface roughness to σRMS = 1.55 nm and an increase in peak intensity. In contrast to the other samples, the titanium-seeded aluminum layer showed no significant crystallization changes and the surface roughness only slightly decreased to. Further were no pinholes detected for both AFM scan areas, so that we suppose no recrystallization occurred for this sample.
5. Conclusion
A significantly smoother and temperature stable aluminum surface was achieved with a titanium seed layer. That layer has a positive influence on the wetting behavior and the initial layer growth of the aluminum. A change in the growth mode from island to more layer-like is assumed. The change in initial layer growth also leads to larger Al grains, a smoother aluminum surface, and a distinct Al (111) crystallographic orientation. The improved reflectivity for wavelengths down to 120 nm is caused, as we suppose, by the reduced sticking probability of oxygen on the Al (111) plane resulting in a thinner absorbing oxide layer. The smoother surface of the Ti/Al sample further reduces stray light losses in the FUV. The first tests of the temperature stability of the titanium-seeded aluminum at 225°C showed no recrystallization or pinholes and keep a smoother surface as unseeded aluminum.
The addition of a seed layer to improve the optical properties of aluminum mirrors is a promising technique to address the conflict between the optimal deposition temperatures of aluminum and protective fluoride layers. Therefore, further tests to the temperature stability of the Ti/Al layer system with fluoride protective layers are already started.
Disclosures
The authors declare no conflicts of interest related to this article.
References
1. M. R. Bolcar, L. Feinberg, K. France, B. J. Rauscher, D. Redding, and D. Schininovich, “Initial technology assessment for the large-aperture uv-optical-infrafed (luvoir) mission concept study,” Proc. SPIE 9904, 99040J (2016). [CrossRef]
2. E. Spiller, “Reflective multilayer coatings for the far-UV region,” Appl. Opt. 15(10), 2333–2338 (1976). [CrossRef]
3. G. Hass and R. Tousey, “Reflecting coatings for the extreme ultraviolet,” J. Opt. Soc. Am. 49(6), 593–602 (1959). [CrossRef]
4. M. Zukic, D. G. Torr, J. F. Spann, and M. R. Torr, “Vacuum ultraviolet thin films. 1: optical constants of BaF2, CaF2, MgF2, Al2O3, HfO2 and SiO2,” Appl. Opt. 29(28), 4284–4292 (1990). [CrossRef]
5. V. Dauer, “Optical constants of lithium fluoride thin films in the far ultraviolet,” J. Opt. Soc. Am. 17(2), 300–303 (2000). [CrossRef]
6. C. Lee, M. Liu, M. Kaneko, K. Nakahira, and Y. Takano, “Characterization of AlF3 thin films at 193 nm by thermal evaporation,” Appl. Opt. 44(34), 7333–7338 (2005). [CrossRef]
7. S. Wilbrandt, O. Stenzel, H. Nakamura, D. Wulff-Molder, A. Duparré, and N. Kaiser, “Protected and enhanced aluminum mirrors for the VUV,” Appl. Opt. 53(4), A125–A130 (2014). [CrossRef]
8. Y. I. Dymshits, V. A. Korobitsyn, and A. A. Metel’nikov, “Effect of heating on the reflectivity of Aluminum coatings in the vacuum ultraviolet,” Sov. J. Opt. Technol. 46, 42 (1979). [CrossRef]
9. S. Schröder, A. von Finck, and A. Duparré, “Standardization of light scattering measurements,” Adv. Opt. Technol. 4(5-6), 361–375 (2015). [CrossRef]
10. M. Trost and S. Schröder, “Roughness and Scatter in Optical Coatings,” in Optical Characterization of Thin Solid Films, O. Stenzel and M. Ohlídal, eds., (Springer, 2018).
11. S. Schröder, H. Uhlig, A. Duparré, and N. Kaiser, “Nanostructure and optical properties of fluoride films for high-quality DUV/VUV optical components,” Proc. SPIE 5963, 59630R (2005). [CrossRef]
12. H. K. Pulker and E. Jung, “Correlation between film structure and sorption behaviour of vapour deposited ZnS, cryolite and MgF2 films,” Thin Solid Films 9(1), 57–66 (1972). [CrossRef]
13. L. Dumas, E. Quesnel, J.-Y. Robic, and Y. Pauleau, “Characterization of magnesium fluoride thin films deposited by direct electron beam evaporation,” J. Vac. Sci. Technol., A 18(2), 465–469 (2000). [CrossRef]
14. M. A. Quijada, S. Rice, and E. Mentzell, “Enhanced MgF2 and LiF over-coated Al mirrors for FUV space astronomy,” Proc. SPIE 8450, 84502H (2012). [CrossRef]
15. J. I. Larruquert and R. Keski-Kuha, “Far ultraviolet optical properties of MgF2 films deposited by ion-beam sputtering and their application as protective coatings for Al,” Opt. Commun. 215(1-3), 93–99 (2003). [CrossRef]
16. J. Hennessy, K. Balasubramanian, C. S. Moore, A. D. Jewell, S. Nikzad, K. France, and M. Quijada, “Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition,” J. Astron. Telesc. Instrum. Syst. 2(4), 041206 (2016). [CrossRef]
17. Luis L. Rodríguez De Marcos, J. I. Larruquert, J. A. Méndez, N. Gutiérrez-Luna, L. Espinosa-Yáñez, C. Honrado-Benítez, J. Chavero-Royán, and B. Perea-Abarca, “Optimization of MgF2-deposition temperature for far UV Al mirrors,” Opt. Express 26(7), 9363–9372 (2018). [CrossRef]
18. F. Wang, S. Li, Z. Zhang, Z. Wang, H. Zhou, and T. Huo, “Effect of MgF2 deposition temperature on Al mirrors in vacuum ultraviolet,” Proc. SPIE 11064, 110640O (2019). [CrossRef]
19. V. J. Logeeswaran, P. Nobuhiko, M. Saif Islam, W. Wu, P. Chaturvedi, N. X. Fang, S. Y. Wang, and R. Stanley Williams, “Ultrasmooth Silver Thin Films Deposited with a Germanium Nucleation Layer,” Nano Lett. 9(1), 178–182 (2009). [CrossRef]
20. C. M. Müller and R. Spolenak, “Microstructure evolution during dewetting in thin Au films,” Acta Mater. 58(18), 6035–6045 (2010). [CrossRef]
21. S. Kundu, S. Hazra, S. Banerjee, M. K. Sanyal, S. K. Mandal, S. Chaudhuri, and A. K. Pal, “Morphology of thin silver film grown by dc sputtering on Si (001),” J. Phys. D: Appl. Phys. 31(23), L73–L77 (1998). [CrossRef]
22. H. R. Colenso, E. Z. Rafealov, M. Maddah, N. O. V. Plank, W. Chen, G. I. N. Waterhouse, J. Hao, G. J. Gouws, and C. P. Moore, “Comparison of seed layers for smooth, low loss silver films used in ultraviolet-visible plasmonic imaging devices,” Thin Solid Films 656, 68–74 (2018). [CrossRef]
23. O. Nakagawara, M. Saeki, M. Watanabe, K. Inoue, T. Hagi, T. Makino, and S. Arai, “Epitaxially grown aluminum films with titanium intermediate layer on θ rotated Y– X LiNbO 3 piezoelectric single crystal substrates,” J. Cryst. Growth 249(3-4), 497–501 (2003). [CrossRef]
24. DIN EN ISO 643, Steel - Micrographic determination of the apparent grain size (ISO/DIS 643:2017).
25. P. Bousquet, F. Flory, and P. Roche, “Scattering from multilayer thin films: theory and experiment,” J. Opt. Soc. Am. 71(9), 1115–1123 (1981). [CrossRef]
26. DIN EN ISO 13696, Optics and optical instruments - Test methods for radiation scattered by optical components (ISO 13696:2002).
27. L. Vitos, A. V. Ruban, H. L. Skriver, and J. Kollar, “The surface energy of metals,” Surf. Sci. 411(1-2), 186–202 (1998). [CrossRef]
28. S. Brunauer, D. L. Kantro, and C. H. Weise, “The surface energies of amorphous silica and hydrous amorphous silica,” Can. J. Chem. 34(10), 1483–1496 (1956). [CrossRef]
29. D. Bonn, J. Eggers, J. Indekeu, J. Meunier, and E. Rolley, “Wetting and Spreading,” Rev. Mod. Phys. 81(2), 739–805 (2009). [CrossRef]
30. J. N. Israelachvili, “Intermolecular and Surface Forces: Revised 3rd Edition”, (Academic, 2011).
31. Y. R. Luo, “Comprehensive Handbook of Chemical Bond Energies,” (CRC, 2007).
32. S. Roberts and P. J. Dobson, “The microstructure of aluminium thin films on amorphous SiO2,” Thin Solid Films 135(1), 137–148 (1986). [CrossRef]
33. P. O. Gartland, “Adsorption of oxygen on clean single crystal faces of aluminium,” Surf. Sci. 62(1), 183–196 (1977). [CrossRef]
34. P. Hofmann, W. Wyrobisch, and A. M. Bradshaw, “The interaction of oxygen with aluminium single crystal surfaces: mainly Δϕ aspects,” Surf. Sci. 80, 344–351 (1979). [CrossRef]
35. O. Hunderi, “Influence of Grain Boundaries and Lattice Defects on the Optical Properties of Some Metals,” Phys. Rev. B 7(8), 3419–3429 (1973). [CrossRef]
36. Y. Jiang, S. Pillai, and M. A. Green, “Grain boundary effects on the optical constants and Drude relaxation times of silver films,” J. Appl. Phys. 120(23), 233109 (2016). [CrossRef]
