1. Introduction

Magnesium fluoride (MgF₂) is a low refractive index material commonly used for deep/vacuum ultraviolet (DUV/VUV) optical coating applications owing to its high transparency in these wavelength ranges. The preparation of high-quality optical coatings at DUV/VUV spectral range requires careful minimization of the optical losses (both absorption and scattering) and structural improvement of dense coatings via process optimization. In the past, plentiful of investigations have been performed to optimize the deposition parameters of processes including thermal evaporation, magnetron sputtering, ion-beam-assisted deposition, ion beam sputtering and autoclaved sol [1–5], etc. Those optimized parameters are mainly substrate temperature, deposition rate, ion source, working gas, and so on. Nevertheless, as DUV wavelength optical systems progress towards higher numerical aperture (NA) [6, 7], many optical components with large dimension and strongly curved (either concave or convex) surfaces are used in high-NA optical systems. For these optical systems to function properly, the optical components have to be coated with highly uniform coatings. Up to now, the preparation of uniform coatings on strongly curved surfaces is mainly focused on the thickness uniformity correction. Little attention has been paid to the optical and micro-structural property uniformities of the optical coatings. However, in real applications, the optical, micro-structural, and thickness uniformities all have influence on the performances of the optical coatings and have to be taken into consideration.

When preparing optical coatings on strongly curved surfaces with conventional coating machines having a planetary rotation system, the deposition angle is a function of the position on the curved surface. Therefore, to analyze the optical and micro-structural uniformities of optical coatings on strongly curved surfaces, the dependences of the optical and micro-structural properties of optical coatings on the deposition angle have to be investigated. Up to now, Zaczek et al. [8, 9] and Jaing et al. [8, 9] have reported some results on the dependences of the refractive index and residual stress of single-layer MgF₂ coatings prepared by thermal evaporation on the deposition angle. However, in literature there was no report on the microstructure-related properties of MgF₂ films for DUV/VUV applications prepared by oblique-angle deposition.

In the present work, molybdenum boat evaporation is employed to prepare single layer MgF₂ films in DUV/VUV spectral range under different deposition angles. The dependences of the optical characteristics (transmittance, reflectance, optical loss, refractive index, extinction coefficient, packing density and IR absorbance spectrum) and the microstructure-related properties (cross-section and surface morphologies, crystallization) of the thin films on the deposition angle are comprehensively studied by UV-VIS and FTIR spectrophotometers, scanning electron microscope (SEM), atomic force microscope (AFM), and x-ray diffraction (XRD). These results will provide useful information to the uniformity improvement of optical coatings deposited on optics components with large dimension and/or strongly curved surfaces.

2. Experiments

2.1 Film preparation

Single layer MgF₂ films were prepared by molybdenum boat evaporation at substrate temperature of 300 °C, with oblique-angle deposition technique. The vacuum chamber was pumped down to a base pressure of 3.5 × 10⁻⁴ Pa by a cryopump set. The nominal deposition rate and the physical thickness of the MgF₂ films controlled by a quartz monitor were set to 0.2 nm/s and 300nm for normal deposition. For oblique-angle deposition, when setting the controlled thickness the impact of the deposition angle on the physical thickness of the film was taken into account. Fused silica plates (size: Φ25.4 mm × 4 mm, with a root-mean-square (rms) surface roughness of 0.5nm, measured by an AFM), BK7 slices (size: 25 mm × 25 mm × 0.5 mm) and silicon wafers (size: Φ25.4 mm × 3 mm) were used as substrates. The distance between the thermal evaporation boat and the substrates was approximate 450 mm. The substrates were first cleaned manually with alcohol and acetone, and then treated with a commercial UV photo-cleaner for 40 min before deposition to remove hydro-carbon contaminations at the surfaces [10]. The substrate tilt angle (that is, the deposition angle) was measured as the direction of the incident evaporation flux with respect to the substrate normal. The deposition angle was set to 0°, 30°, 40°, 50°, 60°, and 70° without substrate rotation. For each deposition angle, the single layer MgF₂ film was deposited simultaneously on the fused silica, BK7, and silicon substrates.

2.2 Film characterization

The transmittance ($T$) and reflectance ($R$) spectra of the single layer MgF₂ films deposited on the fused silica substrates were measured by a Perkin Elmer Lambda 1050 spectrophotometer with angles of incidence of 0° and 8°, respectively. From the measured transmittance and reflectance spectra, the optical loss ($L$) of thin films at 193nm was calculated using the following formula,

The cross-section morphologies of MgF₂ films were analyzed by a Hitachi S-4800 SEM. The surface morphologies were characterized with a Veeco AFM, operated in a tapping mode for imaging surface over scanning size of 5 μm square with 256 × 256 data points. The crystalline structures were assessed with a Philips X’Pert-MRD XRD. The start and end incident angles were 20° and 70°, respectively, with a step of 0.03°. The grain size ($D$, nm) and dislocation density ($\delta$, nm⁻²) of the MgF₂ films were calculated from the main intensity peak with the following equations [15]:

3. Results and discussion

3.1 Optical properties

Figure 1(a) shows the measured transmittance spectra of the single-layer MgF₂ films deposited on fused silica substrates with different deposition angles. It is obvious that the minimum transmittance of the MgF₂ films is slightly larger than that of the bare substrate in the wavelength range of 300nm to 800nm, indicating that the refractive index of the MgF₂ films is negatively inhomogeneous. That is, the refractive index of the MgF₂ film decreases as the layer thickness accumulates. On the other hand, the transmittance of the MgF₂ films at the low wavelength end is smaller than that of the bare substrate, due to the optical loss (absorption and scattering).

 

Fig. 1 Transmittance spectra of fused silica substrate and of the MgF₂ films prepared with different deposition angles. (a)The measured spectra; (b) the corresponding calculated spectra with an assumed film thickness of 300 nm.

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The determined optical loss, refractive index and extinction coefficient at 193nm, and physical thickness of the MgF₂ films from the measured transmittance and reflectance spectra are listed in Table 1 . The fitted result suggests that the refractive index of the MgF₂ film is a function of the layer thickness. As film thickness accumulates, the refractive index decreases. Clearly, the average refractive index and packing densities of the MgF₂ films decrease as the deposition angle increases, meanwhile the extinction coefficient of the samples increases. The peak shift shown in Fig. 1(a) is due to difference of optical thickness of the thin films. For convenience of comparing the transmittance spectra of the MgF₂ films prepared under different deposition angles, the transmittance spectra of MgF₂ films calculated by the determined optical constants at an assumed physical thickness of 300nm are shown in Fig. 1(b).

Table 1. Parameters of MgF₂ films obtained from optical and microstructure characterization at193nm

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3.2 Microstructures and crystallization

3.2.1 Cross-sectional morphology

Cross-sectional SEM photographs of the MgF₂ films deposited on BK7 substrates at different deposition angles are shown in Fig. 2 . From the thin film growth fundamentals [16, 17], in an oblique-angle physical vapor deposition (PVD) process the microstructure of the deposited thin film depends on geometric self-shadowing and surface diffusion. As a result, all prepared MgF₂ films present columnar microstructure with different oblique orientations.

 

Fig. 2 Cross-sectional SEM micrographs of the MgF₂ films prepared with different deposition angles: (a) 0°; (b) 30°; (c) 40°; (d) 50°; (e) 60°; (f) 70°.

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The orientation (columnar angle) of the columnar microstructure with respect to the substrate normal is measured from micrographs of cross-sectional morphology of the MgF₂ films. Traditionally, the relationship between the columnar angle ($\phi$) and the deposition angle ($\varphi$) is given by either the tangent rule [18], $\tan \phi =C\tan \varphi$ (with C a constant depending on the process parameters of the film deposition), or Tait’s rule [19], $\phi =\varphi -\mathrm{arc}\sin ((1-\cos \varphi )/2)$. Figure 3 shows the measured columnar angle of the MgF₂ film versus the deposition angle and the corresponding best fit with the tangent rule. The constant C is estimated to be 0.65. As can be seen, the measured columnar angles follow the Tangent rule quite well. The measurement result cannot be well fitted by Tait’s rule. On the other hand, the increased columnar angle leads to a discontinuous columnar structure with more voids, which in turn deteriorate the overall optical performance of the prepared films.

 

Fig. 3 The measured dependence of tangent of columnar angle$\phi$on tangent of deposition angle $\varphi$ and the corresponding best fit with the Tangent rule [18].

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3.2.2 Surface morphology

For optical coatings for DUV/VUV applications, one of the important parameters is the scattering loss, which is determined by the surface roughness of the coated component. Figure 4 shows the AFM surface morphologies of the MgF₂ films deposited on fused silica substrates. From these pictures the surface roughness of MgF₂ films are determined and the root-mean-square (rms) roughness values are listed in Table 1. The measurement results show that the rms roughness of MgF₂ films increases as the deposition angle increases. This is due to the self-shadowing effect of material evaporation, which gives rise to a self-organized columnar growth [17]. A larger deposition angle results in more void growth, which leads to increase of the rms roughness. Consequently, the scattering loss of MgF₂ film increases as well with the increasing deposition angle.

 

Fig. 4 AFM surface morphologies of MgF₂ films prepared at different deposition angles. (a) 0°, (b) 30°, (c) 40°, (d) 50°,(e) 60°, and (f) 70°.

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3.2.3 Crystalline structure

The polycrystalline structures of the MgF₂ films deposited on BK7 substrates were revealed by XRD patterns, as presented in Fig. 5 . The positions of XRD peaks of the MgF₂ films, which are observed at $2\theta$ = 27.3°, 40.3°, 43.8°, 53.5°,and 56.3°, corresponding to (110), (111), (210), (211), and (220), respectively, are the same for films deposited at all different angles. The main peak appears at (110). Moreover, the intensities of the peaks at (110), (210) and (220) decrease as the deposition angle increases, meanwhile the FWHM of the main peak increases with the increasing deposition angle.

 

Fig. 5 X-ray diffraction patterns of MgF₂ films prepared with different deposition angles.

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Grain sizes and dislocation densities of the MgF₂ films are calculated by Eqs. (2) and (3) with the data obtained from the measured XRD patterns, and are also presented in Table 1. The calculated dislocation density of the MgF₂ film at deposition angle of 70° is nine times of that of the sample without tilting growth. The dependence of the determined dislocation density on the deposition angle is consistent with that of the optical loss obtained from the spectrophotometric measurements.

3.3 IR absorption spectra

Figure 6(a) shows the measured IR transmittance spectra of MgF₂ films fabricated on silicon wafers at different deposition angles in the range from 2500 cm⁻¹ to 4500 cm⁻¹. In the spectra between 2857cm⁻¹ and 2960cm⁻¹, no absorption peaks of C-H bonds are observed, indicating that the prepared films are not contaminated by hydrocarbons. On the other hand, the IR absorption spectra from 3000 cm⁻¹ to 3800cm⁻¹, which are related to incorporation of water in the thin films, have been extracted and plotted in Fig. 6(b). Obviously, as the deposition angle increases, the water content in the prepared single-layer MgF₂ film increases. This is expected as the film prepared at a larger deposition angle is more porous. The result agrees well with the determined packing density of the MgF₂ film from the measured transmittance and reflectance spectra, as presented in Table 1.

 

Fig. 6 IR spectra of MgF₂ films prepared at different deposition angles.

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4. Conclusions

Single-layer MgF₂ films were obliquely deposited by molybdenum boat evaporation on fused silica, BK7, and silicon substrates. Spectrophotometers, SEM, AFM, and XRD were adopted to investigate the dependences of the optical and structural properties, such as optical loss, refractive index, extinction coefficient, packing density, surface roughness, grain size, dislocation density and columnar angles of the MgF₂ films, on the deposition angle. These results would be of great importance to the preparation of highly uniform optical coatings in DUV/VUV spectral range on strongly curved substrates with large sizes.