Effects of substrate temperatures on the characterization of magnesium fluoride thin films in deep-ultraviolet region Jian Sun,1,2 Jianda Shao,1 Kui Yi,1 and Weili Zhang1,* 1
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China *Corresponding author: [email protected]
Received 30 September 2013; revised 3 January 2014; accepted 24 January 2014; posted 27 January 2014 (Doc. ID 198531); published 24 February 2014
As a low refractive index material widely used in coatings for deep-ultraviolet optical systems, magnesium fluoride (MgF2 ) films were prepared by electron beam evaporation at different substrate temperatures. The effects of the substrate temperatures on the optical properties in vacuum and in air, microstructures, and composition were investigated, as were the microstructures, their composition, and the relation between them. In vacuum, the substrate temperature directly affected the microstructures which dominated the packing density and inhomogeneity along the film thickness. When the films were exposed to air, the refractive index increased and a nonmonotonic change trend of the refractive index with substrate temperature was observed due to adsorbed water and magnesium oxide (MgO) formed in the film. While a moderate amount of MgO reduced absorption loss by decreasing vacancy defects, excessive MgO increased the absorption loss because of the high extinction coefficient of the oxide. © 2014 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (310.3840) Materials and process characterization; (310.4925) Other properties (stress, chemical, etc.). http://dx.doi.org/10.1364/AO.53.001298
1. Introduction
The development of powerful deep-ultraviolet (DUV) light sources, especially excimer lasers, frequencymultiplied solid-state lasers, and storage-free electron lasers, has promoted the optical application of DUV devices in many areas, particularly in the semiconductor industry [1]. To date, the shortest wavelength used in semiconductor lithography mass production is 193 nm. At this wavelength region, few materials are available for use as laser system coatings because of the high optical absorption; commonly used coating materials include oxides (Al2 O3 and SiO2 ) and wide bandgap fluorides (MgF2 , LaF3 , 1559-128X/14/071298-08$15.00/0 © 2014 Optical Society of America 1298
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AlF3 , NdF3 and GdF3 ) [2]. Among the fluorides, MgF2 is widely used as a low refractive index material because of its low optical loss and high laser-induced damage threshold. MgF2 films have been studied using various methods, including thermal evaporation, magnetron sputtering, and ion-beam-assisted deposition [3–9]. Among these methods, thermal evaporation allows the manufacture of films with low optical loss, similar to many other fluorides used in the DUV region [8,9]. However, MgF2 films produced by thermal evaporation usually have a columnar microstructure directly influenced by the substrate temperature. In addition, upon deposition of the fluoride, residual O replaces F to form oxide or oxyfluoride, which changes the optical properties of the films according to the properties of the newly formed element. When
the films are exposed to air, water is adsorbed and the optical properties are changed according to different deposition parameters. These phenomena are significant in investigations of the application of MgF2 films in the DUV region. Jacob and co-workers [6,7,10] investigated the microstructures of MgF2 films but did not discuss the relationships between the film’s microstructures and its optical properties, especially in the DUV wavelength region. Liu and co-workers [3,4] prepared MgF2 films using resistive heating boat deposition under different substrate temperatures but neither studied the difference between the optical properties of the films in air and those in vacuum nor evaluated the effects of film composition on its optical properties. In this study, MgF2 films were prepared by electron beam evaporation to investigate how substrate temperatures affect the optical properties of the resultant films in vacuum and in air. The microstructures and composition of MgF2, as well as the relation between these parameters, are also determined. The packing density and inhomogeneity of the films were dominated by the microstructures, which changed with different substrate temperatures. Significant increases in the refractive index were observed when the MgF2 films were exposed to air, resulting in a different nonmonotonic changing trend with substrate temperature in air compared to that in vacuum. The observed change was induced by adsorbed water and MgO formed in the films together. A moderate amount of MgO could reduce the absorption losses by decreasing vacancy defects. However, excessive MgO increased the absorption losses because of its high extinction coefficient. 2. Experiments A.
Film Preparation
MgF2 films were prepared by an electron beam evaporation machine with a flat planetary rotation system. The vacuum chamber was pumped down to a base pressure of less than 2.7 × 10−4 Pa by a cryopump prior to deposition. The films were deposited on fused quartz substrates (∅30 mm × 3 mm) and (111) oriented single crystal silicon substrates (10 mm × 10 mm, 1 mm thick) from room temperature to 300°C at a deposition rate of 0.3 nm∕s. Fused quartz substrates with identical spectra and similar surface root mean square (RMS) roughness (0.6– 0.7 nm) were selected to minimize the influence of the substrates on the film properties. The fused quartz substrates were ultrasonically cleaned to eliminate organic contamination prior to deposition. Film thickness and deposition rates were controlled by a quartz monitor and an optical monitor. The optical monitor glass was placed at the central position of the calotte near the quartz crystal monitor. The temperature was controlled by a radiation pyrometer and four pairs of ceramic heaters mounted uniformly at the bottom of the chamber to obtain
Table 1.
Deposition Parameters and Physical Thickness of MgF2 Films
Substrate temperature (°C) Thickness of films on optical monitor (nm) Thickness of films on quartz substrate (nm)
Room
150
200
250
300
429
417
413
407
404
308
299
292
289
286
uniform temperature distribution on the calotte of the chamber. The optical thickness of the films on the optical monitor glass was nine quarter waves at 250 nm. Because a couple of uniformity masks were fixed in the chamber to improve the film thickness distribution uniformity, the optical thickness of the films on the fused quartz substrates was about 6.4 quarter waves at 250 nm. Table 1 provides detailed information of the deposition parameters applied in this study. B. Film Characterization
Spectra measurements in vacuum and in air were performed on different substrates. Before venting after the deposition, near normal incidence reflectance of the films on the optical monitor glass (1 mm thick, double-sided polished) was measured using an OMS 5000 optical monitor system with a measurement error of 0.05% in the vacuum chamber with a pressure of 1 × 10−4 Pa. The optical monitor glass used the same fused quartz glass as the ∅30 mm fused quartz substrates and also had RMS roughness of 0.6–0.7 nm. Transmittance and reflectance of the films on the quartz substrates were investigated by a Perkin-Elmer Lambda 1050 UV/ VIS/NIR spectrophotometer with a measurement error within 0.08% in air atmosphere. The refractive indices, extinction coefficients, and thicknesses in vacuum and in air shown in Table 1 were simultaneously obtained from the transmittance and reflectance spectra by the modified envelope method using the film analysis software Essential Macleod [11,12]. The substrate absorption, which was determined from the bare substrate, was also taken into account. To calculate of the inhomogeneity of the optical constants, the layer was assumed to have a constant refractive index gradient across its thickness, and the extinction coefficient was assumed to be stable across the layer thickness. If n0 was the refractive index at the film–medium (vacuum or air) interface and ni was the refractive index at the film–substrate interface, then Δn ( ni − n0 ) represented the variation of the refractive index across the film thickness and n ( ni n0 ∕2) represented the average index. Then the Cauchy formula was used to fit the dispersion of the refractive index and extinction coefficient. The cross-sectional morphology of the films was analyzed by a Zeiss SURPA scanning electron microscope (SEM). The surface morphology and roughness of the MgF2 films on the quartz substrates were determined by a NanoScope III PSI atomic force 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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microscope (AFM). Infrared transmittance spectra of the films on the silicon substrates were examined by a Bruker Vertex 70 Fourier transform infrared spectrometer. The chemical composition of the films was characterized by a Thermo K-Alpha x-ray photoelectron spectrometer (XPS), and Ar with 1 keV was used to etch the films to obtain depth profiles. 3. Results and Discussion A.
Optical Properties
1. Optical Properties in Vacuum Figure 1 shows the reflectance spectra of MgF2 films in vacuum prepared at various substrate temperatures on the optical monitor glass. For a low refractive index material, the reflectance reaches the maximum value when the optical thickness of the film is an integral number of half-wavelength thick. The maximum reflectance is lower than the bare substrate if the refractive index is negatively inhomogeneous, which means the refractive index decreases as the film thickness increases. The larger the difference between the reflectance of the film and the substrate is, the greater the extent of inhomogeneity becomes. Rectangular Area 1 in Fig. 1 shows that the maximum reflectance values of all the films were lower than that of the bare substrate, which indicated the refractive index was negatively inhomogeneous. The maximum reflectance decreased from room temperature to 150°C and then increased as the substrate temperature increased, indicating that 150°C was a type of turning point for the films. Figure 2 shows the variation in the refractive index of the MgF2 films across the films’ thickness Δn at 280 nm. All of the Δn values were positive. At room substrate temperature Δn was small and the inhomogeneity was not obvious. At 150°C the inhomogeneity became pronounced and then decreased with increasing substrate temperature. At 300°C, the inhomogeneity decreased and appeared similar to that of the films produced at room temperature. For MgF2, the reflectance reaches the minimum value when the film optical thickness is an odd number of quarter-wavelength thickness and the average refractive index decreases as the minimum
Fig. 1. Reflectance spectra of MgF2 films on the optical monitor glass in vacuum. 1300
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Fig. 2. Δn of MgF2 films on the optical monitor glass in vacuum (wavelength 280 nm).
Fig. 3. n of MgF2 films on the optical monitor glass in vacuum.
reflectance decreases. In Area 2 in Fig. 1, the minimum reflectance increased with the increasing substrate temperature. Figure 3 shows the average refractive index n of the MgF2 films prepared at different temperatures. The error margin of the measurement was small, thus the influence of the error margin was negligible on the calculation of the optical constants. What is more, compared to the difference of the spectra between different substrate temperatures, the error margin had very little impact on the changing trend of the optical constants. The average refractive index n increased gradually as the substrate temperature increased, agreeing with the minimum reflectance in Fig. 1. 2. Optical Properties in Air When fluoride films are exposed to air, they adsorb water and their optical properties vary. Figure 4 shows the transmittance and reflectance spectra of MgF2 films in air prepared at various substrate temperatures on the quartz substrates. At 150°C and 200°C, the films showed obvious negative inhomogeneity of the refractive index. The minimum transmittance and maximum reflectance were higher and lower, respectively, than those of the bare substrate. This result was similar to the performance of the MgF2 films in vacuum. However, as shown
Fig. 4. Transmittance and reflectance spectra of MgF2 films on quartz substrates in air.
Fig. 6. Extinction coefficient of MgF2 films on quartz substrates in air.
in the rectangular area, it is clearly that the trend of the minimum reflectance underwent a great change compared to that in vacuum. As the substrate temperature increased from room temperature to 150° C, the minimum reflectance decreased. When the substrate temperature exceeded 150°C, however, an opposite trend was observed that the minimum reflectance increased as the temperature increased. Figure 5 shows the average refractive index n of MgF2 films in air. The influence of the error margin of the measurements was negligible on the changing trend of the optical constants, as mentioned before. There was an increase of the refractive index of all the films in air compared with those in vacuum. Furthermore, the increase was more obvious at lower substrate temperatures. A nonmonotonic changing trend of the refractive index with increasing substrate temperature was obtained. The refractive index of the films in air first decreased from room temperature to 150°C and then increased with further increases in substrate temperature. This was in accordance with the minimum reflectance of the films shown in the rectangular area in Fig. 4. The Δn at 280 nm is also shown in Fig. 5. A similar changing trend in air to the results in vacuum was observed in that the Δn increased from room temperature to 150°C and then decreased with increasing
substrate temperature. However, the Δn values in air were lower than those in vacuum, indicating that inhomogeneity of the refractive index of MgF2 films in air occurred to a lesser extent than in vacuum. Figure 6 shows the extinction coefficient of MgF2 films prepared at different substrate temperatures. The extinction coefficient increased from 150°C to 300°C. In addition, the extinction coefficient of the films prepared at room temperature was slightly larger than that obtained at 200°C. Similar results were found in MgF2 films prepared by resistive heating boat deposition, which will be reported in a future study. The difference between the optical performances of the MgF2 films in vacuum and in air was thought to be related to the microstructures of the films, as well as variations in the components when the films were exposed to air. Thus, a study of the microstructure and composition of the MgF2 films was carried out.
Fig. 5. n and Δn at 280 nm of MgF2 films on quartz substrates in air.
B. Microstructures
Figures 7(a)–7(d) show SEM photographs of the cross-sectional morphology of the MgF2 films on quartz substrates. As described in Section 3.A.1, the optical properties of MgF2 films in vacuum
Fig. 7. SEM cross-sectional morphology of MgF2 films on the quartz substrates prepared at the substrate temperature of (a) room, (b) 150°C, (c) 200°C, and (d) 300°C. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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may be explained in terms of their microstructures. The classical structure zone model (SZM) described by Movchan and co-workers [13–16] was used to model the films as a function of T s ∕T m, where T s is the substrate temperature and T m is the melting temperature of the material. According to the model, four kinds of microstructures were associated with four temperature zones: zone I T s ∕T m ≦0.15, zone T (0.15 < T s ∕T m ≦0.3), zone II (0.3 < T s ∕T m ≦0.5), and zone III (T s ∕T m > 0.5). Furthermore, boundaries between the zones were diffused and transitions occurred gradually over wide ranges of T s ∕T m. The T s ∕T m ratio for the MgF2 films was 0.19 at room temperature, 0.27 at 150°C, 0.31 at 200°C, 0.34 at 250°C, and 0.37 at 300°C; these values lay in zone T, zone T, zone II, zone II, and zone II, respectively. (The melting temperature of MgF2 is T m 1266°C.) As 0.19 was in the initial temperature region of zone T, the cross-sectional morphology of the film deposited at room temperature shown in Fig. 7(a) exhibited obvious characteristic microstructures of zone I. In zone I, the film was composed of fibers of small diameter that increased with substrate temperature. The fibers were collected into bundles and grew out of the primary nuclei toward the top of the film, forming a rather homogeneous structure along the film thickness. Because of limited surface diffusion and atomic shadowing effects, the film was extensively porous and the packing density was low. These characteristics were in accordance with the optical properties in vacuum of the films deposited at room temperature. The inhomogeneity of the refractive index was weak and the average refractive index was low. Figure 8(a) shows the surface morphology of the films prepared at room temperature. The surface of the films sloped relative gently and the surface RMS roughness was 1.52 nm,
whereas the size and distribution of the granules were nonuniform. Figure 7(b) shows the cross-sectional morphology of the film deposited at 150°C, which was located in zone T. In zone T, surface diffusion was remarkable but grain boundary migration was negligible. The main features of the film structure were determined by competitive grain growth induced by different growth rates in various crystallographic directions. Near the substrate, the microstructure consists of randomly oriented small grains out of which V-shaped columns with the favored orientations grew, forming an inhomogeneous structure along the film thickness. In Fig. 7(b), the size of the V-shaped columnar structures increased as the film grew, leaving a discontinuous columnar structure with more voids, which caused the refractive index to decrease. Therefore, the film structure showed pronounced negative inhomogeneity, as discussed in Section 3.A.1. In addition, because of increased atomic mobility with increasing substrate temperature, the average packing density of the films in zone T was higher than that of the films in zone I and increased as the substrate temperature increased. Consequently, the refractive index in vacuum of the films prepared at 150°C was larger than that of the films prepared at room temperature. Figure 8(b) shows the surface morphology of the films prepared at 150°C. The faceted column tops led to a considerable surface roughness of 3.1 nm. The film prepared at 200°C lay near the initial temperature region of zone II. Thus the structure shown in Fig. 7(c) still exhibited characteristic of zone T. The refractive index was higher than that of the films prepared at 150°C and the negative inhomogeneity of the former films decreased but remained obvious. Open column boundaries were still obvious, as shown in Fig. 8(c), and the roughness was 2.2 nm. Figure 7(d) shows the cross-sectional morphology of the film deposited at 300°C, which placed it in zone II. In this zone, bulk diffusion effects began to take place and the structure was controlled by the minimization of the interface and surface energy. Grain boundary migration took place throughout the film growth process. The film was composed of columnar crystals with a homogeneous structure. The lateral size of the grains, as well as the packing density of the films, increased with increasing substrate temperature. As shown in Fig. 7(d), the column diameter of the films was homogeneous and obviously larger than that of the films prepared at lower temperature. Therefore, the film had the largest homogeneous refractive index in vacuum. Meanwhile, benefiting from more energy supplied to molecule migration and grain boundary diffusion, the surface was flat with the roughness of 0.8 nm, as shown in Fig. 8(d). C.
Fig. 8. Three-dimensional surface morphology by AFM of MgF2 films prepared at substrate temperatures of (a) room, (b) 50°C, (c) 200°C, and (d) 300°C. 1302
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Composition
1. Infrared Transmittance Figure 9 shows the infrared transmittance spectra of MgF2 films on the silicon substrates. An absorption
Fig. 9. Infrared transmittance spectra of MgF2 films on silicon substrates.
band centered at 3330 cm−1 appeared, which could be ascribed to stretching vibrations of O–H groups in water. When films are exposed to air, water adsorption is a common phenomenon. Water molecules fill into the pores of the films due to capillary action or chemical adsorption, which increases the refractive index and optical thickness. The packing density of the MgF2 films increased as the substrate temperature increased. Thus, the lower the substrate temperature, the larger the amount of absorbed water, as shown in Fig. 9, and the larger increase in refractive index when the films were exposed to air, as described in Section 3.A.. All the films exhibited negative inhomogeneous structures in vacuum, which meant more voids existed as the film thickness increased, especially for the films prepared at 150°C and 200°C. More water was adsorbed with the increasing film thickness and the enlargement of the refractive index induced by adsorbed water increased as the film thickness increased. Thus, adsorption of water induced lower MgF2 film negative inhomogeneity in air than in vacuum. The thickness of the films on the quartz substrates was lower than that of the films on the optical glass, which was another reason the Δn of the films on the optical glass was smaller than that of the films on the quartz substrates. Because the properties of the substrates were considered in the calculation of Δn, the influence of the difference of the optical glass and the fused quartz substrate materials was negligible. 2. XPS Results XPS was carried out to obtain detailed information on the composition of the MgF2 films. Figure 10 shows the XPS survey spectra of the MgF2 films prepared at 200°C at the surface and a depth of 50 nm. It can be seen that the films contained F, Mg, O, and trace C. The C content on the surface was higher than that at the 50 nm depth because the surface was easily polluted by organic contamination in air. Figures 11(a) and 11(b), respectively, show the F1s and Mg1s depth profiles of the films prepared at 200°C. The peak position and the peak intensity had reached stable values for both F1s and Mg1s
Fig. 10. XPS survey spectra of MgF2 films prepared at 200°C at the surface and at 50 nm depth.
at the depth of 10 nm. The phenomenon occurred similarly among all of the films prepared at different substrate temperatures. Figures 12(a)–12(d) show the high-resolution XPS spectra of O1s at 50 nm depth of MgF2 films prepared at different substrate temperatures. The O1s peak of all the films split into two peaks around 530.0 and 532.8 eV, indicating two chemical states. The ratio of oxygen content with high binding energy (scan A) to that with low binding energy (scan B) increased as the substrate temperature increased. We concluded that the oxygen with high binding energy came from
Fig. 11. High-resolution depth spectra of (a) F1s and (b) Mg1s of MgF2 films prepared at 200°C. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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Fig. 12. O content of MgF2 films prepared at substrate temperatures of (a) room, (b) 150°C, (c) 200°C, and (d) 300°C.
the adsorbed water and the oxygen with low binding energy came from MgO formed in the films. Figure 13 shows the contents of the two kinds of oxygen in the films prepared at various substrate temperatures. The content of adsorbed water and MgO both decreased as the substrate temperature increased. When MgF2 films were prepared at a lower substrate temperature, a more porous structure formed. Consequently, more water was absorbed and a greater increase of the refractive index occurred. However, the increase in refractive index brought about by adsorbed water was insufficient
Fig. 13. O content of MgF2 films prepared at various substrate temperatures. 1304
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to induce the nonmonotonic change trend of the refractive index with substrate temperature shown in Fig. 5, because the refractive index of water is lower than that of bulk MgF2 [17]. Given that the refractive index of MgO was much higher that of MgF2 [2], the MgO formed in the film promoted increases in the refractive index. The increased amount of MgO formed with an increasing amount of adsorbed water may be attributed to the catalytic action of water [18,19]. Consequently, the change trend of the refractive index in air compared to that in vacuum was induced by the combined effect of adsorbed water and MgO together. At room temperature, a large amount of water was adsorbed and MgO was formed, thus the refractive index of the films exposed to air was even higher than that of the films deposited at 200°C. The absorption behavior of the films shown in Fig. 6 was also thought to be related to its composition. While water shows absorption below the wavelength of 200 nm, its extinction coefficient was about 1 × 10−7 at 200 nm [17], much smaller than that of MgF2 films. Additionally, the extinction coefficient of the MgF2 films increased as the water decreased when the substrate temperature was higher than 150°C. Therefore, adsorbed water had little effect on the absorption loss of MgF2 films. In fluoride films, the F deficiency occurs inevitably and the vacancy defect has been certified as an important absorption source [20]. By filling O into F anion vacancies to reduce the color centers, the absorption
can be reduced. While the ratios of the F to Mg content were nearly identical in all of the films, MgO proportion decreased when the substrate temperature exceeded 150°C, resulting in increased absorption. The effect has also been observed in AlF3 films that Al2 O3 could reduce the absorption [21]. Nevertheless, the film prepared at room temperature had the largest content of MgO, but its extinction coefficient was higher than that of the films prepared at 200°C, as shown in Fig. 6. We thought it was because the extinction coefficient of MgO was much higher than that of MgF2 [2]. While some MgO reduced the number of F anion vacancies, excessive amounts would induce increased absorption because of its own high extinction coefficient. We concluded that there were two kinds of effects of MgO on the absorption of MgF2 films. On the one hand, the formation of MgO could reduce the film absorption by reducing the color centers. On the other hand, MgO could also aggravate absorption by its own high extinction coefficient. At low MgO contents, the former effect prevailed; by contrast, when MgO content exceeded a certain value, the latter effect dominated the absorption of MgF2 films. 4. Conclusion
MgF2 films were prepared by electron beam evaporation at different substrate temperatures. In vacuum, the microstructures of the films changed significantly with the increasing substrate temperature. The packing density increased as the substrate temperature increased, resulting in increasing average refractive index. The negative inhomogeneity of the refractive index increased from room temperature to 150°C and then decreased with further increase in substrate temperature. When the films were exposed to air, the refractive index increased and a different nonmonotonic change trend of the refractive index with substrate temperature in air compared to that in vacuum was observed. This phenomenon was thought to be induced by water adsorption and MgO formation in the films together. A moderate amount of MgO was conducive to reducing absorption losses by decreasing F anion vacancy defects, but excess MgO increased the absorption losses due to its high extinction coefficient. To obtain excellent MgF2 films, deposition parameters must be optimized to meet practical requirements. The authors would like to thank Prof. Zhengxiu Fan for helpful discussion. References 1. H. Blaschke, J. Kohlhaas, P. Kadkhoda, and D. Ristau, “DUV/ VUV spectrophotometry for high precision spectral characterization,” Proc. SPIE 4932, 536–543 (2003). 2. F. Rainer, W. H. Lowdermilk, D. Milam, C. K. Carniglia, T. T. Hart, and T. L. Lichtenstein, “Materials for optical coatings in the ultraviolet,” Appl. Opt. 24, 496–500 (1985).
3. M. C. Liu, C. C. Lee, M. Kaneko, K. Nakahira, and Y. Takano, “Microstructure of magnesium fluoride films deposited by boat evaporation at 193 nm,” Appl. Opt. 45, 7319–7324 (2006). 4. H. Yu, H. J. Qi, Y. Cui, Y. M. Shen, J. D. Shao, and Z. X. Fan, “Influence of substrate temperature on properties of MgF2 coatings,” Appl. Surf. Sci. 253, 6113–6117 (2007). 5. 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, 465–469 (2000). 6. D. Jacob, F. Peiro, E. Quesnel, and D. Ristau, “Microstructure and composition of MgF2 optical coatings grown on Si substrate by PVD and IBS processes,” Thin Solid Films 360, 133–138 (2000). 7. U. Kaiser, N. Kaiser, P. Weissbrodt, U. Mademann, E. Hacker, and H. Muller, “Structure of thin fluoride films deposited on amorphous substrates,” Thin Solid Films 217, 7–16 (1992). 8. Y. Taki, “Film structure and optical constants of magnetronsputtered fluoride films for deep ultraviolet lithography,” Vacuum 74, 431–435 (2004). 9. D. Ristau, S. Gunster, S. Bosch, A. Duparre, E. Masetti, J. Ferre-Borrull, G. Kiriakidis, F. Peiro, 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). 10. U. Kaiser, M. Adamik, G. Safran, P. B. Barna, S. Laux, and W. Richter, “Growth structure investigation of MgF2 and NdF3 films grown by molecular beam deposition on CaF2(111) substrates,” Thin Solid Films 280, 5–15 (1996). 11. B. Bovard, F. J. Vanmilligen, M. J. Messerly, S. G. Saxe, and H. A. Macleod, “Optical-constants derivation for an inhomogeneous thin-film from in situ transmission measurements,” Appl. Opt. 24, 1803–1807 (1985). 12. R. Swanepoel, “Determination of the thickness and opticalconstants of amorphous-silicon,” J. Phys. E 16, 1214–1222 (1983). 13. B. A. Movchan and A. V. Demchishin, “Investigation of the structure and properties of thick vacuum- deposited films of nickel, titanium, tungsten, alumina and zirconium dioxide,” Fiz. Met. Metalloved. 28, 653–660 (1969). 14. I. Petrov, P. B. Barna, L. Hultman, and J. E. Greene, “Microstructural evolution during film growth,” J. Vac. Sci. Technol. A 21, S117–S128 (2003). 15. P. B. Barna and M. Adamik, “Fundamental structure forming phenomena of polycrystalline films and the structure zone models,” Thin Solid Films 317, 27–33 (1998). 16. M. Adamik, P. B. Barna, and I. Tomov, “Columnar structures in polycrystalline thin films developed by competitive growth,” Thin Solid Films 317, 64–68 (1998). 17. G. M. Hale and M. R. Querry, “Optical-constants of water in the 200 nm to 200 μm wavelength region,” Appl. Opt. 12, 555–563 (1973). 18. G. L. Tan, X. J. Wu, L. R. Wang, and Y. Q. Chen, “Investigation for oxygen sensor of LaF3 thin film,” Sens. Actuators B Chem. 34, 417–421 (1996). 19. S. Selvasekarapandian, M. Vijayakumar, T. Gnanasekaran, S. Fujihara, and S. Koji, “Characterization of oxygen impurities in thermally evaporated LaF3 thin films suitable for oxygen sensor,” Appl. Surf. Sci. 222, 125–130 (2004). 20. S. Niisaka, T. Saito, J. Saito, A. Tanaka, A. Matsumoto, M. Otani, R. Biro, C. Ouchi, M. Hasegawa, Y. Suzuki, and K. Sone, “Development of optical coatings for 157 nm lithography. I. Coating materials,” Appl. Opt. 41, 3242–3247 (2002). 21. J. Sun, X. Li, W. L. Zhang, K. Yi, and J. D. Shao, “Effects of substrate temperatures and deposition rates on properties of aluminum fluoride thin films in deep-ultraviolet region,” Appl. Opt. 51, 8481–8489 (2012).
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Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China *Corresponding author: [email protected]
Received 30 September 2013; revised 3 January 2014; accepted 24 January 2014; posted 27 January 2014 (Doc. ID 198531); published 24 February 2014
As a low refractive index material widely used in coatings for deep-ultraviolet optical systems, magnesium fluoride (MgF2 ) films were prepared by electron beam evaporation at different substrate temperatures. The effects of the substrate temperatures on the optical properties in vacuum and in air, microstructures, and composition were investigated, as were the microstructures, their composition, and the relation between them. In vacuum, the substrate temperature directly affected the microstructures which dominated the packing density and inhomogeneity along the film thickness. When the films were exposed to air, the refractive index increased and a nonmonotonic change trend of the refractive index with substrate temperature was observed due to adsorbed water and magnesium oxide (MgO) formed in the film. While a moderate amount of MgO reduced absorption loss by decreasing vacancy defects, excessive MgO increased the absorption loss because of the high extinction coefficient of the oxide. © 2014 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (310.3840) Materials and process characterization; (310.4925) Other properties (stress, chemical, etc.). http://dx.doi.org/10.1364/AO.53.001298
1. Introduction
The development of powerful deep-ultraviolet (DUV) light sources, especially excimer lasers, frequencymultiplied solid-state lasers, and storage-free electron lasers, has promoted the optical application of DUV devices in many areas, particularly in the semiconductor industry [1]. To date, the shortest wavelength used in semiconductor lithography mass production is 193 nm. At this wavelength region, few materials are available for use as laser system coatings because of the high optical absorption; commonly used coating materials include oxides (Al2 O3 and SiO2 ) and wide bandgap fluorides (MgF2 , LaF3 , 1559-128X/14/071298-08$15.00/0 © 2014 Optical Society of America 1298
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AlF3 , NdF3 and GdF3 ) [2]. Among the fluorides, MgF2 is widely used as a low refractive index material because of its low optical loss and high laser-induced damage threshold. MgF2 films have been studied using various methods, including thermal evaporation, magnetron sputtering, and ion-beam-assisted deposition [3–9]. Among these methods, thermal evaporation allows the manufacture of films with low optical loss, similar to many other fluorides used in the DUV region [8,9]. However, MgF2 films produced by thermal evaporation usually have a columnar microstructure directly influenced by the substrate temperature. In addition, upon deposition of the fluoride, residual O replaces F to form oxide or oxyfluoride, which changes the optical properties of the films according to the properties of the newly formed element. When
the films are exposed to air, water is adsorbed and the optical properties are changed according to different deposition parameters. These phenomena are significant in investigations of the application of MgF2 films in the DUV region. Jacob and co-workers [6,7,10] investigated the microstructures of MgF2 films but did not discuss the relationships between the film’s microstructures and its optical properties, especially in the DUV wavelength region. Liu and co-workers [3,4] prepared MgF2 films using resistive heating boat deposition under different substrate temperatures but neither studied the difference between the optical properties of the films in air and those in vacuum nor evaluated the effects of film composition on its optical properties. In this study, MgF2 films were prepared by electron beam evaporation to investigate how substrate temperatures affect the optical properties of the resultant films in vacuum and in air. The microstructures and composition of MgF2, as well as the relation between these parameters, are also determined. The packing density and inhomogeneity of the films were dominated by the microstructures, which changed with different substrate temperatures. Significant increases in the refractive index were observed when the MgF2 films were exposed to air, resulting in a different nonmonotonic changing trend with substrate temperature in air compared to that in vacuum. The observed change was induced by adsorbed water and MgO formed in the films together. A moderate amount of MgO could reduce the absorption losses by decreasing vacancy defects. However, excessive MgO increased the absorption losses because of its high extinction coefficient. 2. Experiments A.
Film Preparation
MgF2 films were prepared by an electron beam evaporation machine with a flat planetary rotation system. The vacuum chamber was pumped down to a base pressure of less than 2.7 × 10−4 Pa by a cryopump prior to deposition. The films were deposited on fused quartz substrates (∅30 mm × 3 mm) and (111) oriented single crystal silicon substrates (10 mm × 10 mm, 1 mm thick) from room temperature to 300°C at a deposition rate of 0.3 nm∕s. Fused quartz substrates with identical spectra and similar surface root mean square (RMS) roughness (0.6– 0.7 nm) were selected to minimize the influence of the substrates on the film properties. The fused quartz substrates were ultrasonically cleaned to eliminate organic contamination prior to deposition. Film thickness and deposition rates were controlled by a quartz monitor and an optical monitor. The optical monitor glass was placed at the central position of the calotte near the quartz crystal monitor. The temperature was controlled by a radiation pyrometer and four pairs of ceramic heaters mounted uniformly at the bottom of the chamber to obtain
Table 1.
Deposition Parameters and Physical Thickness of MgF2 Films
Substrate temperature (°C) Thickness of films on optical monitor (nm) Thickness of films on quartz substrate (nm)
Room
150
200
250
300
429
417
413
407
404
308
299
292
289
286
uniform temperature distribution on the calotte of the chamber. The optical thickness of the films on the optical monitor glass was nine quarter waves at 250 nm. Because a couple of uniformity masks were fixed in the chamber to improve the film thickness distribution uniformity, the optical thickness of the films on the fused quartz substrates was about 6.4 quarter waves at 250 nm. Table 1 provides detailed information of the deposition parameters applied in this study. B. Film Characterization
Spectra measurements in vacuum and in air were performed on different substrates. Before venting after the deposition, near normal incidence reflectance of the films on the optical monitor glass (1 mm thick, double-sided polished) was measured using an OMS 5000 optical monitor system with a measurement error of 0.05% in the vacuum chamber with a pressure of 1 × 10−4 Pa. The optical monitor glass used the same fused quartz glass as the ∅30 mm fused quartz substrates and also had RMS roughness of 0.6–0.7 nm. Transmittance and reflectance of the films on the quartz substrates were investigated by a Perkin-Elmer Lambda 1050 UV/ VIS/NIR spectrophotometer with a measurement error within 0.08% in air atmosphere. The refractive indices, extinction coefficients, and thicknesses in vacuum and in air shown in Table 1 were simultaneously obtained from the transmittance and reflectance spectra by the modified envelope method using the film analysis software Essential Macleod [11,12]. The substrate absorption, which was determined from the bare substrate, was also taken into account. To calculate of the inhomogeneity of the optical constants, the layer was assumed to have a constant refractive index gradient across its thickness, and the extinction coefficient was assumed to be stable across the layer thickness. If n0 was the refractive index at the film–medium (vacuum or air) interface and ni was the refractive index at the film–substrate interface, then Δn ( ni − n0 ) represented the variation of the refractive index across the film thickness and n ( ni n0 ∕2) represented the average index. Then the Cauchy formula was used to fit the dispersion of the refractive index and extinction coefficient. The cross-sectional morphology of the films was analyzed by a Zeiss SURPA scanning electron microscope (SEM). The surface morphology and roughness of the MgF2 films on the quartz substrates were determined by a NanoScope III PSI atomic force 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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microscope (AFM). Infrared transmittance spectra of the films on the silicon substrates were examined by a Bruker Vertex 70 Fourier transform infrared spectrometer. The chemical composition of the films was characterized by a Thermo K-Alpha x-ray photoelectron spectrometer (XPS), and Ar with 1 keV was used to etch the films to obtain depth profiles. 3. Results and Discussion A.
Optical Properties
1. Optical Properties in Vacuum Figure 1 shows the reflectance spectra of MgF2 films in vacuum prepared at various substrate temperatures on the optical monitor glass. For a low refractive index material, the reflectance reaches the maximum value when the optical thickness of the film is an integral number of half-wavelength thick. The maximum reflectance is lower than the bare substrate if the refractive index is negatively inhomogeneous, which means the refractive index decreases as the film thickness increases. The larger the difference between the reflectance of the film and the substrate is, the greater the extent of inhomogeneity becomes. Rectangular Area 1 in Fig. 1 shows that the maximum reflectance values of all the films were lower than that of the bare substrate, which indicated the refractive index was negatively inhomogeneous. The maximum reflectance decreased from room temperature to 150°C and then increased as the substrate temperature increased, indicating that 150°C was a type of turning point for the films. Figure 2 shows the variation in the refractive index of the MgF2 films across the films’ thickness Δn at 280 nm. All of the Δn values were positive. At room substrate temperature Δn was small and the inhomogeneity was not obvious. At 150°C the inhomogeneity became pronounced and then decreased with increasing substrate temperature. At 300°C, the inhomogeneity decreased and appeared similar to that of the films produced at room temperature. For MgF2, the reflectance reaches the minimum value when the film optical thickness is an odd number of quarter-wavelength thickness and the average refractive index decreases as the minimum
Fig. 1. Reflectance spectra of MgF2 films on the optical monitor glass in vacuum. 1300
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Fig. 2. Δn of MgF2 films on the optical monitor glass in vacuum (wavelength 280 nm).
Fig. 3. n of MgF2 films on the optical monitor glass in vacuum.
reflectance decreases. In Area 2 in Fig. 1, the minimum reflectance increased with the increasing substrate temperature. Figure 3 shows the average refractive index n of the MgF2 films prepared at different temperatures. The error margin of the measurement was small, thus the influence of the error margin was negligible on the calculation of the optical constants. What is more, compared to the difference of the spectra between different substrate temperatures, the error margin had very little impact on the changing trend of the optical constants. The average refractive index n increased gradually as the substrate temperature increased, agreeing with the minimum reflectance in Fig. 1. 2. Optical Properties in Air When fluoride films are exposed to air, they adsorb water and their optical properties vary. Figure 4 shows the transmittance and reflectance spectra of MgF2 films in air prepared at various substrate temperatures on the quartz substrates. At 150°C and 200°C, the films showed obvious negative inhomogeneity of the refractive index. The minimum transmittance and maximum reflectance were higher and lower, respectively, than those of the bare substrate. This result was similar to the performance of the MgF2 films in vacuum. However, as shown
Fig. 4. Transmittance and reflectance spectra of MgF2 films on quartz substrates in air.
Fig. 6. Extinction coefficient of MgF2 films on quartz substrates in air.
in the rectangular area, it is clearly that the trend of the minimum reflectance underwent a great change compared to that in vacuum. As the substrate temperature increased from room temperature to 150° C, the minimum reflectance decreased. When the substrate temperature exceeded 150°C, however, an opposite trend was observed that the minimum reflectance increased as the temperature increased. Figure 5 shows the average refractive index n of MgF2 films in air. The influence of the error margin of the measurements was negligible on the changing trend of the optical constants, as mentioned before. There was an increase of the refractive index of all the films in air compared with those in vacuum. Furthermore, the increase was more obvious at lower substrate temperatures. A nonmonotonic changing trend of the refractive index with increasing substrate temperature was obtained. The refractive index of the films in air first decreased from room temperature to 150°C and then increased with further increases in substrate temperature. This was in accordance with the minimum reflectance of the films shown in the rectangular area in Fig. 4. The Δn at 280 nm is also shown in Fig. 5. A similar changing trend in air to the results in vacuum was observed in that the Δn increased from room temperature to 150°C and then decreased with increasing
substrate temperature. However, the Δn values in air were lower than those in vacuum, indicating that inhomogeneity of the refractive index of MgF2 films in air occurred to a lesser extent than in vacuum. Figure 6 shows the extinction coefficient of MgF2 films prepared at different substrate temperatures. The extinction coefficient increased from 150°C to 300°C. In addition, the extinction coefficient of the films prepared at room temperature was slightly larger than that obtained at 200°C. Similar results were found in MgF2 films prepared by resistive heating boat deposition, which will be reported in a future study. The difference between the optical performances of the MgF2 films in vacuum and in air was thought to be related to the microstructures of the films, as well as variations in the components when the films were exposed to air. Thus, a study of the microstructure and composition of the MgF2 films was carried out.
Fig. 5. n and Δn at 280 nm of MgF2 films on quartz substrates in air.
B. Microstructures
Figures 7(a)–7(d) show SEM photographs of the cross-sectional morphology of the MgF2 films on quartz substrates. As described in Section 3.A.1, the optical properties of MgF2 films in vacuum
Fig. 7. SEM cross-sectional morphology of MgF2 films on the quartz substrates prepared at the substrate temperature of (a) room, (b) 150°C, (c) 200°C, and (d) 300°C. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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may be explained in terms of their microstructures. The classical structure zone model (SZM) described by Movchan and co-workers [13–16] was used to model the films as a function of T s ∕T m, where T s is the substrate temperature and T m is the melting temperature of the material. According to the model, four kinds of microstructures were associated with four temperature zones: zone I T s ∕T m ≦0.15, zone T (0.15 < T s ∕T m ≦0.3), zone II (0.3 < T s ∕T m ≦0.5), and zone III (T s ∕T m > 0.5). Furthermore, boundaries between the zones were diffused and transitions occurred gradually over wide ranges of T s ∕T m. The T s ∕T m ratio for the MgF2 films was 0.19 at room temperature, 0.27 at 150°C, 0.31 at 200°C, 0.34 at 250°C, and 0.37 at 300°C; these values lay in zone T, zone T, zone II, zone II, and zone II, respectively. (The melting temperature of MgF2 is T m 1266°C.) As 0.19 was in the initial temperature region of zone T, the cross-sectional morphology of the film deposited at room temperature shown in Fig. 7(a) exhibited obvious characteristic microstructures of zone I. In zone I, the film was composed of fibers of small diameter that increased with substrate temperature. The fibers were collected into bundles and grew out of the primary nuclei toward the top of the film, forming a rather homogeneous structure along the film thickness. Because of limited surface diffusion and atomic shadowing effects, the film was extensively porous and the packing density was low. These characteristics were in accordance with the optical properties in vacuum of the films deposited at room temperature. The inhomogeneity of the refractive index was weak and the average refractive index was low. Figure 8(a) shows the surface morphology of the films prepared at room temperature. The surface of the films sloped relative gently and the surface RMS roughness was 1.52 nm,
whereas the size and distribution of the granules were nonuniform. Figure 7(b) shows the cross-sectional morphology of the film deposited at 150°C, which was located in zone T. In zone T, surface diffusion was remarkable but grain boundary migration was negligible. The main features of the film structure were determined by competitive grain growth induced by different growth rates in various crystallographic directions. Near the substrate, the microstructure consists of randomly oriented small grains out of which V-shaped columns with the favored orientations grew, forming an inhomogeneous structure along the film thickness. In Fig. 7(b), the size of the V-shaped columnar structures increased as the film grew, leaving a discontinuous columnar structure with more voids, which caused the refractive index to decrease. Therefore, the film structure showed pronounced negative inhomogeneity, as discussed in Section 3.A.1. In addition, because of increased atomic mobility with increasing substrate temperature, the average packing density of the films in zone T was higher than that of the films in zone I and increased as the substrate temperature increased. Consequently, the refractive index in vacuum of the films prepared at 150°C was larger than that of the films prepared at room temperature. Figure 8(b) shows the surface morphology of the films prepared at 150°C. The faceted column tops led to a considerable surface roughness of 3.1 nm. The film prepared at 200°C lay near the initial temperature region of zone II. Thus the structure shown in Fig. 7(c) still exhibited characteristic of zone T. The refractive index was higher than that of the films prepared at 150°C and the negative inhomogeneity of the former films decreased but remained obvious. Open column boundaries were still obvious, as shown in Fig. 8(c), and the roughness was 2.2 nm. Figure 7(d) shows the cross-sectional morphology of the film deposited at 300°C, which placed it in zone II. In this zone, bulk diffusion effects began to take place and the structure was controlled by the minimization of the interface and surface energy. Grain boundary migration took place throughout the film growth process. The film was composed of columnar crystals with a homogeneous structure. The lateral size of the grains, as well as the packing density of the films, increased with increasing substrate temperature. As shown in Fig. 7(d), the column diameter of the films was homogeneous and obviously larger than that of the films prepared at lower temperature. Therefore, the film had the largest homogeneous refractive index in vacuum. Meanwhile, benefiting from more energy supplied to molecule migration and grain boundary diffusion, the surface was flat with the roughness of 0.8 nm, as shown in Fig. 8(d). C.
Fig. 8. Three-dimensional surface morphology by AFM of MgF2 films prepared at substrate temperatures of (a) room, (b) 50°C, (c) 200°C, and (d) 300°C. 1302
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Composition
1. Infrared Transmittance Figure 9 shows the infrared transmittance spectra of MgF2 films on the silicon substrates. An absorption
Fig. 9. Infrared transmittance spectra of MgF2 films on silicon substrates.
band centered at 3330 cm−1 appeared, which could be ascribed to stretching vibrations of O–H groups in water. When films are exposed to air, water adsorption is a common phenomenon. Water molecules fill into the pores of the films due to capillary action or chemical adsorption, which increases the refractive index and optical thickness. The packing density of the MgF2 films increased as the substrate temperature increased. Thus, the lower the substrate temperature, the larger the amount of absorbed water, as shown in Fig. 9, and the larger increase in refractive index when the films were exposed to air, as described in Section 3.A.. All the films exhibited negative inhomogeneous structures in vacuum, which meant more voids existed as the film thickness increased, especially for the films prepared at 150°C and 200°C. More water was adsorbed with the increasing film thickness and the enlargement of the refractive index induced by adsorbed water increased as the film thickness increased. Thus, adsorption of water induced lower MgF2 film negative inhomogeneity in air than in vacuum. The thickness of the films on the quartz substrates was lower than that of the films on the optical glass, which was another reason the Δn of the films on the optical glass was smaller than that of the films on the quartz substrates. Because the properties of the substrates were considered in the calculation of Δn, the influence of the difference of the optical glass and the fused quartz substrate materials was negligible. 2. XPS Results XPS was carried out to obtain detailed information on the composition of the MgF2 films. Figure 10 shows the XPS survey spectra of the MgF2 films prepared at 200°C at the surface and a depth of 50 nm. It can be seen that the films contained F, Mg, O, and trace C. The C content on the surface was higher than that at the 50 nm depth because the surface was easily polluted by organic contamination in air. Figures 11(a) and 11(b), respectively, show the F1s and Mg1s depth profiles of the films prepared at 200°C. The peak position and the peak intensity had reached stable values for both F1s and Mg1s
Fig. 10. XPS survey spectra of MgF2 films prepared at 200°C at the surface and at 50 nm depth.
at the depth of 10 nm. The phenomenon occurred similarly among all of the films prepared at different substrate temperatures. Figures 12(a)–12(d) show the high-resolution XPS spectra of O1s at 50 nm depth of MgF2 films prepared at different substrate temperatures. The O1s peak of all the films split into two peaks around 530.0 and 532.8 eV, indicating two chemical states. The ratio of oxygen content with high binding energy (scan A) to that with low binding energy (scan B) increased as the substrate temperature increased. We concluded that the oxygen with high binding energy came from
Fig. 11. High-resolution depth spectra of (a) F1s and (b) Mg1s of MgF2 films prepared at 200°C. 1 March 2014 / Vol. 53, No. 7 / APPLIED OPTICS
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Fig. 12. O content of MgF2 films prepared at substrate temperatures of (a) room, (b) 150°C, (c) 200°C, and (d) 300°C.
the adsorbed water and the oxygen with low binding energy came from MgO formed in the films. Figure 13 shows the contents of the two kinds of oxygen in the films prepared at various substrate temperatures. The content of adsorbed water and MgO both decreased as the substrate temperature increased. When MgF2 films were prepared at a lower substrate temperature, a more porous structure formed. Consequently, more water was absorbed and a greater increase of the refractive index occurred. However, the increase in refractive index brought about by adsorbed water was insufficient
Fig. 13. O content of MgF2 films prepared at various substrate temperatures. 1304
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to induce the nonmonotonic change trend of the refractive index with substrate temperature shown in Fig. 5, because the refractive index of water is lower than that of bulk MgF2 [17]. Given that the refractive index of MgO was much higher that of MgF2 [2], the MgO formed in the film promoted increases in the refractive index. The increased amount of MgO formed with an increasing amount of adsorbed water may be attributed to the catalytic action of water [18,19]. Consequently, the change trend of the refractive index in air compared to that in vacuum was induced by the combined effect of adsorbed water and MgO together. At room temperature, a large amount of water was adsorbed and MgO was formed, thus the refractive index of the films exposed to air was even higher than that of the films deposited at 200°C. The absorption behavior of the films shown in Fig. 6 was also thought to be related to its composition. While water shows absorption below the wavelength of 200 nm, its extinction coefficient was about 1 × 10−7 at 200 nm [17], much smaller than that of MgF2 films. Additionally, the extinction coefficient of the MgF2 films increased as the water decreased when the substrate temperature was higher than 150°C. Therefore, adsorbed water had little effect on the absorption loss of MgF2 films. In fluoride films, the F deficiency occurs inevitably and the vacancy defect has been certified as an important absorption source [20]. By filling O into F anion vacancies to reduce the color centers, the absorption
can be reduced. While the ratios of the F to Mg content were nearly identical in all of the films, MgO proportion decreased when the substrate temperature exceeded 150°C, resulting in increased absorption. The effect has also been observed in AlF3 films that Al2 O3 could reduce the absorption [21]. Nevertheless, the film prepared at room temperature had the largest content of MgO, but its extinction coefficient was higher than that of the films prepared at 200°C, as shown in Fig. 6. We thought it was because the extinction coefficient of MgO was much higher than that of MgF2 [2]. While some MgO reduced the number of F anion vacancies, excessive amounts would induce increased absorption because of its own high extinction coefficient. We concluded that there were two kinds of effects of MgO on the absorption of MgF2 films. On the one hand, the formation of MgO could reduce the film absorption by reducing the color centers. On the other hand, MgO could also aggravate absorption by its own high extinction coefficient. At low MgO contents, the former effect prevailed; by contrast, when MgO content exceeded a certain value, the latter effect dominated the absorption of MgF2 films. 4. Conclusion
MgF2 films were prepared by electron beam evaporation at different substrate temperatures. In vacuum, the microstructures of the films changed significantly with the increasing substrate temperature. The packing density increased as the substrate temperature increased, resulting in increasing average refractive index. The negative inhomogeneity of the refractive index increased from room temperature to 150°C and then decreased with further increase in substrate temperature. When the films were exposed to air, the refractive index increased and a different nonmonotonic change trend of the refractive index with substrate temperature in air compared to that in vacuum was observed. This phenomenon was thought to be induced by water adsorption and MgO formation in the films together. A moderate amount of MgO was conducive to reducing absorption losses by decreasing F anion vacancy defects, but excess MgO increased the absorption losses due to its high extinction coefficient. To obtain excellent MgF2 films, deposition parameters must be optimized to meet practical requirements. The authors would like to thank Prof. Zhengxiu Fan for helpful discussion. References 1. H. Blaschke, J. Kohlhaas, P. Kadkhoda, and D. Ristau, “DUV/ VUV spectrophotometry for high precision spectral characterization,” Proc. SPIE 4932, 536–543 (2003). 2. F. Rainer, W. H. Lowdermilk, D. Milam, C. K. Carniglia, T. T. Hart, and T. L. Lichtenstein, “Materials for optical coatings in the ultraviolet,” Appl. Opt. 24, 496–500 (1985).
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