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Preparation and optical properties of sputtered-deposition yttrium fluoride film
Nuclear Instruments and Methods in Physics Research B 307 (2013) 429–433
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Preparation and optical properties of sputtered-deposition yttrium fluoride film Lei Pei, Zhu Jiaqi ⇑, Zhu Yuankun, Han Jiecai Center for Composite Materials, Harbin Institute of Technology, P.O. Box 3010, Yikuang Street 2, Harbin 150080, P.R. China
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 2 February 2013 Accepted 5 February 2013 Available online 23 March 2013 Keywords: Magnetron sputtering Substrate temperature Yttrium fluoride Microstructure Optical properties
a b s t r a c t Yttrium fluoride films were grown on silicon wafers by magnetron sputtering at different substrate temperatures (ST). The composition, structure and optical properties have been investigated systematically. X-ray photoelectron spectroscopy (XPS) analysis shows that the films mainly contain Y and F elements. The deficiency of F element in yttrium fluoride films is generally inevitable by magnetron sputtering. Glancing incident X-ray diffraction (GIXRD) analysis demonstrates that the films have the orthorhombic structure with different growth orientations. Spectroscopic Ellipsometer (SE) analysis on films exhibits that the refractive index of yttrium fluoride film grown at 400 °C posses lower value than these of ones under other temperatures. The absorption of films gradually diminishes as the substrate temperature increases. It has been convinced that the YF3 films with optimal optical properties can be achieved by adjustment of substrate temperature for desirable optical design and applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Metal fluorides, compared with oxide counterparts, represent outstanding optical properties due to the more ionic nature of the M–F versus M–O bond, including high bandgap leading to increased transparency window, low polarisability resulting in lower refractive index [1], and low vibrational energy [2,3]. Thus, metal fluorides are considered as attractive and excellent materials in many fields of science and technology. Particularly, yttrium fluoride, as an important fluoride, has been extensively studied and employed in various applications involving phosphors [4], ionic conductors [5], scintillators [6], host crystal materials with interesting luminescent properties [7,8]. More importantly, Yttrium fluoride has been widely used as optical coatings in a wide range of applications, such as UV and IR lasers, high reflective mirrors, thin film interference filters [9–12], substitute for radioactive ThF4 in the IR range [13], antireflective coatings with other optical films in the range from the near-UV to IR [14]. These functions are available based on the nature of yttrium fluoride with low absorption and low refractive index (about 1.55 at 598 nm), excellent transmittance from the UV to IR ranges and desirable ability to match other materials for multilayer. In order to obtain optimized films for different purposes, various deposition methods have been performed. The most popular techniques belong to physical vapor deposition (PVD), such as electron beam deposition [12–15], thermal evaporation [16,17]. ⇑ Corresponding author. Tel./fax: +86 451 86417970. E-mail address: [email protected] (Z. Jiaqi). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.02.047
The evaporated films are generally porous with low packing density, ion assisted deposition has been resorted to get rather dense ones [18]. Meanwhile, yttrium fluoride films have been widely fabricated by chemical vapor deposition (CVD) for controllable synthesis [2,19,20]. In interference optical film applications, PVD technologies are dominant rather than CVD methods [9,11]. Among PVD technologies, magnetron sputtering is an important and useful method in both scientific and technological fields. However, fluoride films are difficult to be deposited by sputtering processes, deposition rates are low and film quality is generally lower than evaporated ones [21,22]. Sputtering process tends to cause fluorine deficient and oxygen contamination [23]. Dudney found that resputtering degraded both the optical properties and quality of CaF2 coatings [24]. Martin showed that the stoichiometry and optical properties of HfF4 films were sensitive to resputtering effects according to different locations [21]. To our knowledge, little work has been conducted to focus on the sputtered yttrium fluoride films and investigate how to grow the optimal thin films. Therefore, it is worthy of studying how the growth parameters (especially, ST) affect on the composition, structure and optical properties. Furthermore, studying on the sputtered-deposited yttrium fluoride is desirable for both the fundamental and technological applications. The aim of the present work is to discover if magnetron sputtering technology could deposit high quality yttrium fluoride with sputtered ceramic target, meanwhile, study how the important parameter (ST) adjusts the composition, structure and optical properties of yttrium fluorides films.
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L. Pei et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 429–433
2. Experimental details 2.1. Film preparation All yttrium fluoride thin films were deposited on p-type silicon (1 0 0) wafers by radio frequency magnetron sputtering using yttrium fluoride ceramic target (99.99% purity, 49 mm diameter and 3 mm thickness) in an ultrahigh vacuum chamber. Before deposition, the samples were cleaned by acetone and alcohol in an ultrasonic bath for 30 min, respectively. The distance between target and substrate remained 70 mm. High purity argon (99.995%) was used as background gas and kept at a constant flow rate (about 100 SCCM). The vacuum chamber was evacuated down to a base pressure of 104 Pa with turbo molecular and oil diffusion pumps. Pre-sputtering was performed for about 30 min with a shutter covering the substrate before film deposition. Under the conditions of 110 W and 1.0 Pa, the shutter was subsequently removed to commence film growth. During the film deposition, the deposition temperature was varied from 200 °C to 600 °C. Considering the low deposition rate, deposition times lasted for 3 h, 4 h and 4 h at 200 °C, 400 °C and 600 °C, respectively. In order to avoid the effects of other factors except the substrate temperature, all the samples were located at the same position on the holder. After deposition, the samples were kept in vacuum chamber until substrate temperature (Tsub) dropped to room temperature (RT) and then were taken out for analysis. 2.2. Film characterization The composition was examined by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA) using a monochromatized Al Ka X-ray source with a step size of 0.8 eV. All samples were placed in a high vacuum chamber about 106 Pa and etched by argon ion for two minutes to remove contaminations before scanning. The binding energy of the C1s level of the adventitious carbon was taken as 284.6 eV as a reference. The composition of YF3 target was also detected by XPS as the standard for comparisons. The crystalline structure of the films was identified by glancing incident X-ray diffraction (GIXRD, Philips X’ Pert-Pro) with Cu Ka source (40 kV, 30 mA). Incidence angle was 1.5° and the samples were scanned in a 2h range of 20–80° with a scan step size of 0.05. Spectroscopic Ellipsometer (SE) system was conducted to investigate the optical properties of films in the spectral range of 380–800 nm at an incidence angle of 70°. Film thickness was also established with ellipsometry. 3. Results and discussion 3.1. Film composition Fig. 1 shows the XPS survey spectra for the surface of the yttrium fluoride thin films at different substrate temperatures after argon ions bombardment for two minutes, the peaks corresponding to Y, F, O and C elements are observed in all the samples. No peaks from other elements appear in the scan. Y and F elements belong to the native yttrium fluoride film, which are expectable, the C1s and O1s peaks appear due to the contamination during the deposition process. The C1s peak appears very weak after bombardment. It is also clear that the yttrium fluoride films by sputtering suffered from oxygen contamination. In order to investigate the chemical bondings of films, we detected the typical XPS spectra of thin film grown at 400 °C and target as standard for comparisons. Fig. 2(a) shows the XPS spectra of F1s core level from yttrium fluoride target, F1s binding energy at 685.7 eV corresponds to the pure yttrium fluoride [9]. As for the
Fig. 1. The XPS survey spectra for the surface of the yttrium fluoride thin films after argon ions bombardment for two minutes at different substrate temperatures.
film, the XPS spectra of F1s core level for the yttrium fluoride film grown at 400 °C are shown in Fig. 2(c). The F1s binding energy from thin film is located at 685.3 eV, corresponding to F–Y bond [9]. Compared with the F1s binding energy between target and thin film, the F1s from yttrium fluoride film at 400 °C has lower binding energy, indicating that the other elements (C or O) with low electronegativity in the films weaken the binding energy. Fig. 2(b) shows the XPS spectra of Y3d core level from target, and the XPS spectra of Y3d core level from thin film at 400 °C are given in Fig. 2(d). The XPS Peak Fit with the optimal proportion of Gaussian to Lorentzian lineshape was used to deconvolute the overlapping peaks. Duo to the spin–orbit coupling with the constant split energy of 2 eV [25], Y3d is naturally split in two states (Y3d5/2 and Y3d3/2). From Fig. 2(b), the Y3d scan spectra can be deconvoluted into three peaks (157.2, 159.2 and 161.3 eV) corresponding to yttrium atoms bound to little oxygen or other contamination and fluorine atoms. However, the Y3d signals from thin film were deconvoluted into four peaks, corresponding to yttrium atoms bound to fluorine atoms (located at 159.1 and 161.4 eV) [2,9,26] and to oxygen (located at 158.1 and 160.4 eV) [27,28], respectively. Due to the mixture of oxygen in the films, there exist Y–O and Y–F bonds in the film, causing the lower binding energy. Moreover, the Y3d spectra indicate that the yttrium in the film form the chemical bonds with anions, since a metallic Y3d peak at 156.0 eV is not observed [29]. The concentration of the elements in the films was determined by XPS quantitative analysis, quantification was performed using relative sensitivity factors for each element. The results are shown in Table 1. It is observed that besides the Y and F elements all the samples contained the O and C. The exact F/Y ratio in the films was obtained by XPS analysis. All the samples show fluorine deficiency (F/Y < 3) due to the sputtering process. At 400 °C, the F/Y ratio reaches the higher value (2.4) approaching to the stoichiometric YF3 and the oxygen content is just 17.37%. The F/Y ratio reaches the smaller value at 200 °C and 600 °C, while the films have oxygen content of 18.65% and 25.33%, respectively. It seems that oxygen could have an impact on the Y/F ratio, and less oxygen exists, larger Y/F ration which is close to stoichiometry obtains. However, ST has not apparent influence on the composition. 3.2. Film crystallinity Fig. 3 shows the GIXRD patterns of as-deposited yttrium fluoride films grown on Si (1 0 0) wafers at 200 °C, 400 °C and 600 °C, respectively. The XRD patterns of all these samples could be
L. Pei et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 429–433
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Fig. 2. The typical XPS spectra of (a) F1s, (b) Y3d core level from the target, the (c) F 1s and (d) Y3d core level for the film at 400 °C.
Table 1 Element concentration in the as-deposited YF3 films after Ar+ bombardment for 2 min. Substrate temperature (°C)
F (at. %)
Y (at. %)
O (at. %)
C (at. %)
F/Y
200 400 600
52.46 56.27 46.81
23.30 22.57 23.98
18.65 17.37 25.33
5.58 3.79 3.87
2.25 2.49 1.95
indexed to orthorhombic phase YF3 (b-YF3, Pnma (D16 2h )) [30], which is in agreement with the reported values (JCPDS card files No. 740911). It can be clearly seen that all the samples are polycrystalline and growth orientations strongly depend on the substrate temperature. At low temperature (200 °C), YF3 film shows the preferential orientation (1 1 1) plane, however, it turns to the strong (1 0 1) preferential plane at 400 °C, which is in agreement with Barrioz et al.’ report [10]. It could also be seen that the weak peak belongs to cubic Y2O3 (JCPDS card files No. 79-1716) appears in the samples. These indicate that ST in the range of 200–600 °C contributes to form the orthorhombic YF3 crystal structure with different preferential orientations. Meanwhile, at 200 and 600 °C, the (4 0 0) peak of cubic Y2O3 appears, indicating the Y2O3 exists, which is attribute to the contaminated oxygen atoms and the formation of the Y–O bonds confirmed with the XPS analysis. 3.3. Optical properties Optical properties of the yttrium fluoride films have been examined by SE which measures the relative amplitude change (Psi) and phase change (Delta) of the linearly polarized monochromatic incident light upon oblique reflection from the sample surface. The measured dispersion relation of yttrium fluoride films can be well fitted with Cauchy relation given by the polynomials in the visible range, defined as
nðkÞ ¼ aþ
b k2
þ
c k4
;
kðkÞ ¼ dþ
e k2
þ
f k4
:
Fig. 3. The GIXRD patterns of as-deposited yttrium fluoride films grown at 200 °C, 400 °C and 600 °C, respectively.
The experimental data (W and D) are fitted by the model with Cauchy film on Si substrate as shown in Fig. 4. The Mean Squared Error (MSE) values and the film thickness deduced from fitting process are shown in Table 2. The fitting results of refractive index (n) for yttrium fluoride films are shown in Fig. 5. The dispersion curves of the refractive
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L. Pei et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 429–433
Fig. 4. The experimental data (W and D) are fitted by the model with Cauchy film on Si substrate.
index (n) against wavelength increase greatly towards shorter wavelengths corresponding to fundamental absorption of energy. It can be clearly seen that, compared with the refractive indices of yttrium fluoride deposited at 200 °C and 600 °C, the refractive indices of film deposited at 400 °C reach the lowest values, typically about 1.65 at 600 nm. The values of n are higher than the refractive indices of yttrium fluoride films by conventional
evaporation due to the nonstoichiometry (the F anion deficiency). However, the samples at 200 °C and 600 °C have higher refractive indices, approaching to 1.71 and 1.72 at 600 nm, respectively. These could be attributed to the greater F deficiency, namely, the F/Y ratio declines to 2.25 and 1.95, respectively. Moreover, the contaminated O in the films formed the yttrium oxide with high refractive index (1.9). Therefore, it can be concluded that controlling the oxygen content would be available to gain the low refractive indices. Another information deduced from the W and D is the k(k) dispersion curve. Fig. 6 shows the extinction coefficient of yttrium fluoride at different ST from 200 °C to 600 °C. It can be clearly seen that the k curves are very sensitive to the ST. At 200 °C, the k values are high, reaching about 0.025. As the substrate temperature increases to 400 °C, the k declines to 0.02. When temperature
Fig. 5. The refractive index (n) for yttrium fluoride films grown at the substrate temperature of 200 °C, 400 °C and 600 °C.
Fig. 6. The extinction coefficient of yttrium fluoride films grown at the substrate temperature of 200 °C, 400 °C and 600 °C.
Table 2 The MSE values and the film thickness deduced from Psi and Delta. Substrate temperature (°C)
200
400
600
MSE Thickness (nm)
8.850 195.06 ± 0.604
11.664 294.62 ± 1.344
10.700 256.64 ± 0.305
L. Pei et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 429–433
increases to 600 °C, the k values decrease to near zero, which is favorable to interference optical application. At low temperature, the films may exist more defects causing high extinctive coefficient, which would be greatly relieved as ST increases, thus the film at higher temperature shows the lower k values.
4. Conclusion Yttrium fluoride films were grown on silicon wafers at different substrate temperatures by radio frequency magnetron sputtering. XPS results show that the sputtered films are generally more or less F-deficient and contain contaminated C and O elements from chamber. The oxygen in the film forms the Y–O bonds of yttrium oxide. The film deposited at 400 °C has the larger F/Y ratio, about 2.4. GIXRD analysis demonstrates that all the films are polycrystalline and have the orthorhombic structure with small amount of cubic yttrium oxide phase. Film appears (1 1 1) preferential orientation at 200 °C and 600 °C, however, turns to (1 0 1) growth orientation at 400 °C. Spectroscopic Ellipsometer (SE) fitting results exhibit that the refractive index of yttrium fluoride film grown at 400 °C posses lower value than these of ones under other temperatures, about 1.65 at 600 nm. The higher refractive indices of films at other temperatures are mainly attributed to the mixture of cubic yttrium oxide. However, with the increase of substrate temperature, the extinction coefficient declines gradually from 0.025 to near zero owing to less defects. Taking into account the low refractive and extinction coefficient, yttrium fluoride film at 400 °C possesses the high quality. It has been proved that the YF3 films with dense and low refractive index and absorption can be achieved by adjustment of substrate temperature for desirable optical design and applications.
thank the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20112302110036). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51072039 and 51222205). This work was also financially supported by Program for New Century Excellent Talents in University (NCET-10-0070). In addition, the authors
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[27] [28] [29] [30]
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Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Preparation and optical properties of sputtered-deposition yttrium fluoride film Lei Pei, Zhu Jiaqi ⇑, Zhu Yuankun, Han Jiecai Center for Composite Materials, Harbin Institute of Technology, P.O. Box 3010, Yikuang Street 2, Harbin 150080, P.R. China
a r t i c l e
i n f o
Article history: Received 10 September 2012 Received in revised form 2 February 2013 Accepted 5 February 2013 Available online 23 March 2013 Keywords: Magnetron sputtering Substrate temperature Yttrium fluoride Microstructure Optical properties
a b s t r a c t Yttrium fluoride films were grown on silicon wafers by magnetron sputtering at different substrate temperatures (ST). The composition, structure and optical properties have been investigated systematically. X-ray photoelectron spectroscopy (XPS) analysis shows that the films mainly contain Y and F elements. The deficiency of F element in yttrium fluoride films is generally inevitable by magnetron sputtering. Glancing incident X-ray diffraction (GIXRD) analysis demonstrates that the films have the orthorhombic structure with different growth orientations. Spectroscopic Ellipsometer (SE) analysis on films exhibits that the refractive index of yttrium fluoride film grown at 400 °C posses lower value than these of ones under other temperatures. The absorption of films gradually diminishes as the substrate temperature increases. It has been convinced that the YF3 films with optimal optical properties can be achieved by adjustment of substrate temperature for desirable optical design and applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Metal fluorides, compared with oxide counterparts, represent outstanding optical properties due to the more ionic nature of the M–F versus M–O bond, including high bandgap leading to increased transparency window, low polarisability resulting in lower refractive index [1], and low vibrational energy [2,3]. Thus, metal fluorides are considered as attractive and excellent materials in many fields of science and technology. Particularly, yttrium fluoride, as an important fluoride, has been extensively studied and employed in various applications involving phosphors [4], ionic conductors [5], scintillators [6], host crystal materials with interesting luminescent properties [7,8]. More importantly, Yttrium fluoride has been widely used as optical coatings in a wide range of applications, such as UV and IR lasers, high reflective mirrors, thin film interference filters [9–12], substitute for radioactive ThF4 in the IR range [13], antireflective coatings with other optical films in the range from the near-UV to IR [14]. These functions are available based on the nature of yttrium fluoride with low absorption and low refractive index (about 1.55 at 598 nm), excellent transmittance from the UV to IR ranges and desirable ability to match other materials for multilayer. In order to obtain optimized films for different purposes, various deposition methods have been performed. The most popular techniques belong to physical vapor deposition (PVD), such as electron beam deposition [12–15], thermal evaporation [16,17]. ⇑ Corresponding author. Tel./fax: +86 451 86417970. E-mail address: [email protected] (Z. Jiaqi). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.02.047
The evaporated films are generally porous with low packing density, ion assisted deposition has been resorted to get rather dense ones [18]. Meanwhile, yttrium fluoride films have been widely fabricated by chemical vapor deposition (CVD) for controllable synthesis [2,19,20]. In interference optical film applications, PVD technologies are dominant rather than CVD methods [9,11]. Among PVD technologies, magnetron sputtering is an important and useful method in both scientific and technological fields. However, fluoride films are difficult to be deposited by sputtering processes, deposition rates are low and film quality is generally lower than evaporated ones [21,22]. Sputtering process tends to cause fluorine deficient and oxygen contamination [23]. Dudney found that resputtering degraded both the optical properties and quality of CaF2 coatings [24]. Martin showed that the stoichiometry and optical properties of HfF4 films were sensitive to resputtering effects according to different locations [21]. To our knowledge, little work has been conducted to focus on the sputtered yttrium fluoride films and investigate how to grow the optimal thin films. Therefore, it is worthy of studying how the growth parameters (especially, ST) affect on the composition, structure and optical properties. Furthermore, studying on the sputtered-deposited yttrium fluoride is desirable for both the fundamental and technological applications. The aim of the present work is to discover if magnetron sputtering technology could deposit high quality yttrium fluoride with sputtered ceramic target, meanwhile, study how the important parameter (ST) adjusts the composition, structure and optical properties of yttrium fluorides films.
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L. Pei et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 429–433
2. Experimental details 2.1. Film preparation All yttrium fluoride thin films were deposited on p-type silicon (1 0 0) wafers by radio frequency magnetron sputtering using yttrium fluoride ceramic target (99.99% purity, 49 mm diameter and 3 mm thickness) in an ultrahigh vacuum chamber. Before deposition, the samples were cleaned by acetone and alcohol in an ultrasonic bath for 30 min, respectively. The distance between target and substrate remained 70 mm. High purity argon (99.995%) was used as background gas and kept at a constant flow rate (about 100 SCCM). The vacuum chamber was evacuated down to a base pressure of 104 Pa with turbo molecular and oil diffusion pumps. Pre-sputtering was performed for about 30 min with a shutter covering the substrate before film deposition. Under the conditions of 110 W and 1.0 Pa, the shutter was subsequently removed to commence film growth. During the film deposition, the deposition temperature was varied from 200 °C to 600 °C. Considering the low deposition rate, deposition times lasted for 3 h, 4 h and 4 h at 200 °C, 400 °C and 600 °C, respectively. In order to avoid the effects of other factors except the substrate temperature, all the samples were located at the same position on the holder. After deposition, the samples were kept in vacuum chamber until substrate temperature (Tsub) dropped to room temperature (RT) and then were taken out for analysis. 2.2. Film characterization The composition was examined by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA) using a monochromatized Al Ka X-ray source with a step size of 0.8 eV. All samples were placed in a high vacuum chamber about 106 Pa and etched by argon ion for two minutes to remove contaminations before scanning. The binding energy of the C1s level of the adventitious carbon was taken as 284.6 eV as a reference. The composition of YF3 target was also detected by XPS as the standard for comparisons. The crystalline structure of the films was identified by glancing incident X-ray diffraction (GIXRD, Philips X’ Pert-Pro) with Cu Ka source (40 kV, 30 mA). Incidence angle was 1.5° and the samples were scanned in a 2h range of 20–80° with a scan step size of 0.05. Spectroscopic Ellipsometer (SE) system was conducted to investigate the optical properties of films in the spectral range of 380–800 nm at an incidence angle of 70°. Film thickness was also established with ellipsometry. 3. Results and discussion 3.1. Film composition Fig. 1 shows the XPS survey spectra for the surface of the yttrium fluoride thin films at different substrate temperatures after argon ions bombardment for two minutes, the peaks corresponding to Y, F, O and C elements are observed in all the samples. No peaks from other elements appear in the scan. Y and F elements belong to the native yttrium fluoride film, which are expectable, the C1s and O1s peaks appear due to the contamination during the deposition process. The C1s peak appears very weak after bombardment. It is also clear that the yttrium fluoride films by sputtering suffered from oxygen contamination. In order to investigate the chemical bondings of films, we detected the typical XPS spectra of thin film grown at 400 °C and target as standard for comparisons. Fig. 2(a) shows the XPS spectra of F1s core level from yttrium fluoride target, F1s binding energy at 685.7 eV corresponds to the pure yttrium fluoride [9]. As for the
Fig. 1. The XPS survey spectra for the surface of the yttrium fluoride thin films after argon ions bombardment for two minutes at different substrate temperatures.
film, the XPS spectra of F1s core level for the yttrium fluoride film grown at 400 °C are shown in Fig. 2(c). The F1s binding energy from thin film is located at 685.3 eV, corresponding to F–Y bond [9]. Compared with the F1s binding energy between target and thin film, the F1s from yttrium fluoride film at 400 °C has lower binding energy, indicating that the other elements (C or O) with low electronegativity in the films weaken the binding energy. Fig. 2(b) shows the XPS spectra of Y3d core level from target, and the XPS spectra of Y3d core level from thin film at 400 °C are given in Fig. 2(d). The XPS Peak Fit with the optimal proportion of Gaussian to Lorentzian lineshape was used to deconvolute the overlapping peaks. Duo to the spin–orbit coupling with the constant split energy of 2 eV [25], Y3d is naturally split in two states (Y3d5/2 and Y3d3/2). From Fig. 2(b), the Y3d scan spectra can be deconvoluted into three peaks (157.2, 159.2 and 161.3 eV) corresponding to yttrium atoms bound to little oxygen or other contamination and fluorine atoms. However, the Y3d signals from thin film were deconvoluted into four peaks, corresponding to yttrium atoms bound to fluorine atoms (located at 159.1 and 161.4 eV) [2,9,26] and to oxygen (located at 158.1 and 160.4 eV) [27,28], respectively. Due to the mixture of oxygen in the films, there exist Y–O and Y–F bonds in the film, causing the lower binding energy. Moreover, the Y3d spectra indicate that the yttrium in the film form the chemical bonds with anions, since a metallic Y3d peak at 156.0 eV is not observed [29]. The concentration of the elements in the films was determined by XPS quantitative analysis, quantification was performed using relative sensitivity factors for each element. The results are shown in Table 1. It is observed that besides the Y and F elements all the samples contained the O and C. The exact F/Y ratio in the films was obtained by XPS analysis. All the samples show fluorine deficiency (F/Y < 3) due to the sputtering process. At 400 °C, the F/Y ratio reaches the higher value (2.4) approaching to the stoichiometric YF3 and the oxygen content is just 17.37%. The F/Y ratio reaches the smaller value at 200 °C and 600 °C, while the films have oxygen content of 18.65% and 25.33%, respectively. It seems that oxygen could have an impact on the Y/F ratio, and less oxygen exists, larger Y/F ration which is close to stoichiometry obtains. However, ST has not apparent influence on the composition. 3.2. Film crystallinity Fig. 3 shows the GIXRD patterns of as-deposited yttrium fluoride films grown on Si (1 0 0) wafers at 200 °C, 400 °C and 600 °C, respectively. The XRD patterns of all these samples could be
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Fig. 2. The typical XPS spectra of (a) F1s, (b) Y3d core level from the target, the (c) F 1s and (d) Y3d core level for the film at 400 °C.
Table 1 Element concentration in the as-deposited YF3 films after Ar+ bombardment for 2 min. Substrate temperature (°C)
F (at. %)
Y (at. %)
O (at. %)
C (at. %)
F/Y
200 400 600
52.46 56.27 46.81
23.30 22.57 23.98
18.65 17.37 25.33
5.58 3.79 3.87
2.25 2.49 1.95
indexed to orthorhombic phase YF3 (b-YF3, Pnma (D16 2h )) [30], which is in agreement with the reported values (JCPDS card files No. 740911). It can be clearly seen that all the samples are polycrystalline and growth orientations strongly depend on the substrate temperature. At low temperature (200 °C), YF3 film shows the preferential orientation (1 1 1) plane, however, it turns to the strong (1 0 1) preferential plane at 400 °C, which is in agreement with Barrioz et al.’ report [10]. It could also be seen that the weak peak belongs to cubic Y2O3 (JCPDS card files No. 79-1716) appears in the samples. These indicate that ST in the range of 200–600 °C contributes to form the orthorhombic YF3 crystal structure with different preferential orientations. Meanwhile, at 200 and 600 °C, the (4 0 0) peak of cubic Y2O3 appears, indicating the Y2O3 exists, which is attribute to the contaminated oxygen atoms and the formation of the Y–O bonds confirmed with the XPS analysis. 3.3. Optical properties Optical properties of the yttrium fluoride films have been examined by SE which measures the relative amplitude change (Psi) and phase change (Delta) of the linearly polarized monochromatic incident light upon oblique reflection from the sample surface. The measured dispersion relation of yttrium fluoride films can be well fitted with Cauchy relation given by the polynomials in the visible range, defined as
nðkÞ ¼ aþ
b k2
þ
c k4
;
kðkÞ ¼ dþ
e k2
þ
f k4
:
Fig. 3. The GIXRD patterns of as-deposited yttrium fluoride films grown at 200 °C, 400 °C and 600 °C, respectively.
The experimental data (W and D) are fitted by the model with Cauchy film on Si substrate as shown in Fig. 4. The Mean Squared Error (MSE) values and the film thickness deduced from fitting process are shown in Table 2. The fitting results of refractive index (n) for yttrium fluoride films are shown in Fig. 5. The dispersion curves of the refractive
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Fig. 4. The experimental data (W and D) are fitted by the model with Cauchy film on Si substrate.
index (n) against wavelength increase greatly towards shorter wavelengths corresponding to fundamental absorption of energy. It can be clearly seen that, compared with the refractive indices of yttrium fluoride deposited at 200 °C and 600 °C, the refractive indices of film deposited at 400 °C reach the lowest values, typically about 1.65 at 600 nm. The values of n are higher than the refractive indices of yttrium fluoride films by conventional
evaporation due to the nonstoichiometry (the F anion deficiency). However, the samples at 200 °C and 600 °C have higher refractive indices, approaching to 1.71 and 1.72 at 600 nm, respectively. These could be attributed to the greater F deficiency, namely, the F/Y ratio declines to 2.25 and 1.95, respectively. Moreover, the contaminated O in the films formed the yttrium oxide with high refractive index (1.9). Therefore, it can be concluded that controlling the oxygen content would be available to gain the low refractive indices. Another information deduced from the W and D is the k(k) dispersion curve. Fig. 6 shows the extinction coefficient of yttrium fluoride at different ST from 200 °C to 600 °C. It can be clearly seen that the k curves are very sensitive to the ST. At 200 °C, the k values are high, reaching about 0.025. As the substrate temperature increases to 400 °C, the k declines to 0.02. When temperature
Fig. 5. The refractive index (n) for yttrium fluoride films grown at the substrate temperature of 200 °C, 400 °C and 600 °C.
Fig. 6. The extinction coefficient of yttrium fluoride films grown at the substrate temperature of 200 °C, 400 °C and 600 °C.
Table 2 The MSE values and the film thickness deduced from Psi and Delta. Substrate temperature (°C)
200
400
600
MSE Thickness (nm)
8.850 195.06 ± 0.604
11.664 294.62 ± 1.344
10.700 256.64 ± 0.305
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increases to 600 °C, the k values decrease to near zero, which is favorable to interference optical application. At low temperature, the films may exist more defects causing high extinctive coefficient, which would be greatly relieved as ST increases, thus the film at higher temperature shows the lower k values.
4. Conclusion Yttrium fluoride films were grown on silicon wafers at different substrate temperatures by radio frequency magnetron sputtering. XPS results show that the sputtered films are generally more or less F-deficient and contain contaminated C and O elements from chamber. The oxygen in the film forms the Y–O bonds of yttrium oxide. The film deposited at 400 °C has the larger F/Y ratio, about 2.4. GIXRD analysis demonstrates that all the films are polycrystalline and have the orthorhombic structure with small amount of cubic yttrium oxide phase. Film appears (1 1 1) preferential orientation at 200 °C and 600 °C, however, turns to (1 0 1) growth orientation at 400 °C. Spectroscopic Ellipsometer (SE) fitting results exhibit that the refractive index of yttrium fluoride film grown at 400 °C posses lower value than these of ones under other temperatures, about 1.65 at 600 nm. The higher refractive indices of films at other temperatures are mainly attributed to the mixture of cubic yttrium oxide. However, with the increase of substrate temperature, the extinction coefficient declines gradually from 0.025 to near zero owing to less defects. Taking into account the low refractive and extinction coefficient, yttrium fluoride film at 400 °C possesses the high quality. It has been proved that the YF3 films with dense and low refractive index and absorption can be achieved by adjustment of substrate temperature for desirable optical design and applications.
thank the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20112302110036). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51072039 and 51222205). This work was also financially supported by Program for New Century Excellent Talents in University (NCET-10-0070). In addition, the authors
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[27] [28] [29] [30]
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