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Deposition of MgF2 thin films for antireflection coating by using thermionic vacuum arc (TVA)
Optics Communications 285 (2012) 2373–2376
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Deposition of MgF2 thin films for antireflection coating by using thermionic vacuum arc (TVA) Şadan Korkmaz a,⁎, Saliha Elmas a, Naci Ekem a, Suat Pat a, M. Zafer Balbağ b a b
Department of Physics, Eskisehir Osmangazi University, Meselik, Eskisehir, Turkey Education Faculty, Eskisehir Osmangazi University, Meselik, Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 3 July 2011 Received in revised form 23 December 2011 Accepted 28 December 2011 Available online 11 January 2012 Keywords: Plasma TVA MgF2 thin film AR coating
a b s t r a c t In this study, optical and surface properties of MgF2 thin films produced by thermionic vacuum arc (TVA) technique have been investigated. By means of this technique the MgF2 thin film produced by condensing the plasma of anode material generated by using TVA under high vacuum conditions on the glass. The optical properties have been investigated by using Filmetrics F20 and UV/VIS spectrometer. For surface properties of produced thin films EDS, SEM and AFM have been used. Our analysis shows that MgF2 thin films produced by using TVA are proper single and multi layer anti-reflective (AR) coating and TVA technique brings very important advantages for ophthalmic glass coating and industrial applications' optical purposes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Antireflection coatings which reduce surface reflection have attracted much more interest in the application of optical and electro optical devices such as photovoltaic cells, solar collectors, and IR diodes [1,2]. AR coating on optical substrates have been carried out using various deposition techniques such as sol–gel, physical vapor deposition (PVD), chemical vapor deposition (CVD), etc. [3–13] and it has been shown that optical properties of thin films for AR coatings depend on deposition conditions and deposition techniques [14]. In this study we present another deposition technique namely TVA to prepare thin films for AR coatings. In that technique, thin films are produced under high vacuum condition and by the use of the bombardment of energetic ions. Recently, it has been shown that the quality of surfaces covered with different materials using this technique improved significantly [15–20]. In the present study, MgF2 single layer coating has been deposited on glass substrates using TVA. Magnesium fluoride has been chosen because it is the most widely used single-layer AR (SLAR) coating material due to its high transmittance and low refractive index. So MgF2 for AR coating has been deposited as thin film by TVA technique for the first time successfully.
TVA technique to deposit thin film is a different discharge plasma source which generates pure metal and non metal plasma [15–20]. TVA discharge occurring between cathode and anode under high or ultra vacuum conditions. The cathode in the Wehnelt cylinder contains heated tungsten filament which emits electron and the anode contains the material to be coated. The accelerated electrons with high dc voltage collide with the anode. The heated anode material due to electron bombardment starts to melt firstly, afterwards to evaporate and a steady state concentration of the evaporated atoms of anode materials is ensured between anode and cathode. With further measure of the applied high voltage a bright thermionic vacuum arc is established in the vapors at anode material in the vacuum chamber. Due to the suitable pressure of vacuum chamber the material generated plasma is deposited on substrate as thin films. TVA setup is schematically shown in Fig. 1. In our study MgF2 slug as anode material in tungsten crucible have been used. The working parameters of TVA as cathode filament current (Ifilament), current of generated plasma in interelectrodic space namely discharge current (Idischarge), applied voltage on anode (V) and vacuum vessel pressure (p) are presented in Table 1. MgF2 thin films were deposited on glass slides by using TVA technique which is one of the plasma assisted techniques. In the plasma assisted deposition technique, to characterize the plasma of coating material is important because the quality of thin films depends on plasma of coated material. For this purpose optical emission spectroscopy (OES) of MgF2 were obtained during the deposition in the range of 200–850 nm and OES of MgF2 are shown in Fig. 2. As seen in Fig. 2, intensities of peaks show the formation of Mg and Flor atoms in
⁎ Corresponding author. E-mail address: [email protected] (Ş. Korkmaz). 0030-4018/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.12.095
Ş. Korkmaz et al. / Optics Communications 285 (2012) 2373–2376
2374
Fig. 1. Schematic view of the electrodes arrangement for TVA discharges.
Fig. 3. Transmittance spectra of produced MgF2 thin films.
Table 1 Working parameters from the TVA discharge. Pressure
Ifilament
Idischarge
(Torr)
(A)
(A)
Voltage (V)
5.27 × 10− 5
19
0.8
2500
plasma. To analyze to the quality of the produced MgF2 thin films, the optical and surface properties have been investigated. 3. Results and discussion 3.1. Optical characteristics of MgF2 thin films Optical characteristics of the AR coatings depend on film thickness of coated layers' transparency and refractive index. To determine thickness of MgF2 film was used and it was measured as 230 nm. To measure the transmittance of coating samples, Perkin-Elmer UV/vis Lambda 2S spectrometer was used. Transmittance spectra of produced MgF2 thin films are compared with the spectra of uncoated glass samples and are shown in Fig. 3. As seen in Fig. 3, the transmittance of the samples coated with MgF2 is higher than that of uncoated samples. The transparency of MgF2 thin film is 93% at 550 nm and it is in good agreement with the literature [6,8].
The reflectance of MgF2 thin films versus to the wavelength is measured by using Filmetrics F20 system and is shown in Fig. 4. As seen in Fig. 4 the reflectance of MgF2 is very low as we expected.
Fig. 2. Optical emission spectroscopy of MgF2 plasma.
Fig. 5. Refractive index of produced MgF2 thin films.
Fig. 4. Reflectance of produced MgF2 thin films.
Ş. Korkmaz et al. / Optics Communications 285 (2012) 2373–2376
2375
Fig. 6. SEM images of MgF2 thin film.
Table 2 EDS measurement of MgF2 coated substrate. Elt.
Line
Intensity (c/s)
Error 2-Sig
Atomic (%)
Conc
Units
O F Mg Si
Ka Ka Ka Ka
298.11 53.43 113.27 868.63
10.915 4.621 6.728 18.631
54.081 13.624 5.070 27.225 100.000
43.007 12.865 6.124 38.004 100.000
wt.% wt.% wt.% wt.% Total
The spectral dependence of the refractive index for MgF2 film is presented Fig. 5. Refractive index of MgF2 thin films is 1.379 at 550 nm. The change of refractive index with wavelength is in good agreement with literature [21].
3.2. Surface morphologies of produced MgF2 thin films The surface properties of MgF2 thin films have been investigated by using SEM, EDS and AFM. SEM images of MgF2 layer coated on the glass are presented in Fig. 6a with a magnification of 0.5 kx and
Fig. 7. 3D AFM images (4000 nm × 4000 nm) of produced MgF2 thin films.
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Ş. Korkmaz et al. / Optics Communications 285 (2012) 2373–2376
Fig. 8. Indent analysis of produced MgF2 thin films.
in Fig. 6b with a magnification of 2 kx. When the SEM images of MgF2 are seen that surface of produced thin film was very smooth. However, some fractions were seen in images. EDS analysis of MgF2 thin films has been realized by using JEOLJSM-5600-LV SEM and analysis results are presented in Table 2. Analysis results show that the main components of films are magnesium and fluoride. Its ratio is very close to MgF2 formation. The quality of the deposited MgF2 thin film has been obtained using AFM images. AFM images of the deposited MgF2 thin film are shown in Fig. 7. The roughness measurements were realized from different 40 lines of the coated surface. The average roughness of the deposited MgF2 thin film was measured as 0.79 nm. Quantum dots were seen in AFM images, whose dimensions are approximately 80–100 nm. Maximum height is approximately 30–35 nm. Heights of the quantum dots were 30–35 nm. These results show that produced MgF2 thin films by using TVA are pure, compact, smooth, homogeny and nanostructure. Also the roughness of deposited thin films is very low and it is in agreement with TVA literature [20]. To show the plastic deformation properties of produced MgF2 thin films, the indent analysis was realized up to 900 μN. The indent analysis of produced MgF2 thin films is shown in Fig. 8. According to this analysis' results, surface of the film exhibits plastic behavior, so adhesion forces between glass and film are not strong. This situation is desired for ophthalmic lens coating. 4. Conclusion We have investigated that the optical and surface properties of MgF2 thin films were produced by TVA technique. The coating of glass with MgF2 increased the transmittance and measured refractive index values of produced films are in good agreement with literature. Surface analysis shows that of MgF2 surfaces produced films by TVA are smooth, rough, homogeny and adhesive. Our analysis' results show that MgF2 thin films produced by TVA technique are proper for single and multi layer AR coating. The use of TVA technique to produce MgF2 thin films has some advantages such as very short coating
time and being able to control thickness of thin film. So we conclude that TVA technique is very suitable for industrial ophthalmic application. Acknowledgements This research activity has been supported by TUBITAK numbered with 108M608. AFM measurements are realized by ESOGU scientific research commission numbered with 200819045. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
H.A. Macleod, Adam Hilger Ltd, London, 2001, p. 9, E. D. A. Roy Chowdhuri, Dong-Un Jin, C.G. Takoudis, Thin Solid Films 457 (2004) 402. F. Perales, J.M. Herrero, D. Jaque, C. de las Heras, Optical Materials 29 (2007) 783. Q. Zhang, J. Wang, G. Wu, J. Shen, S. Buddhudu, Materials Chemistry and Physics 72 (2001) 56. H. Krüger, E. Kemnitz, A. Hertwing, U. Beck, Thin Solid Films 516 (2008) 4175. H. Yu, H. Qi, Y. Cui, Y. Shen, J. Shao, Z. Fan, Applied Surface Science 253 (2007) 6113. S. Fujihara, M. Tada, T. Kimura, Thin Solid Films 304 (1997) 252. M. Leskela, M. Ritala, Thin Solid Films 409 (2002) 138. M. Ritala, M. Leskela, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, Academic Press, San Diego, 2002, p. 103. T. Pilvi, E. Puukilainen, U. Kreissig, M. Leskela, M. Ritala, Chemistry of Materials 20 (2008) 5023. J.J. McNally, G.A. Al-Jumaily, J.R. McNeil, N. Sendow, Applied Optics 25 (1986) 1973. P.J. Martin, W.G. Sainty, R.P. Netterfeld, D.R. McKenzie, D.J.H. Cockayne, S.H. Sie, O.R. Wood, H.G. Craighead, Applied Optics 26 (1987) 1235. J.I. Larruquert, R.A.M. Krski-Kuha, Optics Communication 215 (2003) 93. E.T. Hutcheson, G. Hass, J.T. Cox, Applied Optics 11 (1972) 2245. H. Ehrich, J. Schuhmann, G. Musa, A. Popescu, I. Mustata, Thin Solid Films 333 (1998) 95. G. Musa, H. Ehrich, J. Schuhmann, IEEE Transactions on Plasma Science 25 (1997) 386. G. Musa, I. Mustata, V. Ciupina, R. Vladoiu, G. Prodan, E. Vasile, H. Ehrich, Diamond and Related Materials 13 (2004) 1398. G. Musa, I. Mustata, V. Ciupina, R. Vladoiu, G. Prodan, C.P. Lungu, H. Ehrich, Journal of Optoelectronics and Advanced Materials 7 (5) (2005) 2485. S. Pat, N. Ekem, T. Akan, Ö. Küsmüs, S. Demirkol, R. Vladoiu, C.P. Lungu, G. Musa, Journal of Optoelectronics and Advanced Materials 7 (5) (2005) 2495. S. Okur, M. Kalkanci, S. Pat, T. Akan, Z. Balbag, et al., Physica C: Superconductivity 466 (2007) 205. T. Wiktorczyk, M. Oles, Optical Materials 29 (2007) 1768.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Deposition of MgF2 thin films for antireflection coating by using thermionic vacuum arc (TVA) Şadan Korkmaz a,⁎, Saliha Elmas a, Naci Ekem a, Suat Pat a, M. Zafer Balbağ b a b
Department of Physics, Eskisehir Osmangazi University, Meselik, Eskisehir, Turkey Education Faculty, Eskisehir Osmangazi University, Meselik, Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 3 July 2011 Received in revised form 23 December 2011 Accepted 28 December 2011 Available online 11 January 2012 Keywords: Plasma TVA MgF2 thin film AR coating
a b s t r a c t In this study, optical and surface properties of MgF2 thin films produced by thermionic vacuum arc (TVA) technique have been investigated. By means of this technique the MgF2 thin film produced by condensing the plasma of anode material generated by using TVA under high vacuum conditions on the glass. The optical properties have been investigated by using Filmetrics F20 and UV/VIS spectrometer. For surface properties of produced thin films EDS, SEM and AFM have been used. Our analysis shows that MgF2 thin films produced by using TVA are proper single and multi layer anti-reflective (AR) coating and TVA technique brings very important advantages for ophthalmic glass coating and industrial applications' optical purposes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Antireflection coatings which reduce surface reflection have attracted much more interest in the application of optical and electro optical devices such as photovoltaic cells, solar collectors, and IR diodes [1,2]. AR coating on optical substrates have been carried out using various deposition techniques such as sol–gel, physical vapor deposition (PVD), chemical vapor deposition (CVD), etc. [3–13] and it has been shown that optical properties of thin films for AR coatings depend on deposition conditions and deposition techniques [14]. In this study we present another deposition technique namely TVA to prepare thin films for AR coatings. In that technique, thin films are produced under high vacuum condition and by the use of the bombardment of energetic ions. Recently, it has been shown that the quality of surfaces covered with different materials using this technique improved significantly [15–20]. In the present study, MgF2 single layer coating has been deposited on glass substrates using TVA. Magnesium fluoride has been chosen because it is the most widely used single-layer AR (SLAR) coating material due to its high transmittance and low refractive index. So MgF2 for AR coating has been deposited as thin film by TVA technique for the first time successfully.
TVA technique to deposit thin film is a different discharge plasma source which generates pure metal and non metal plasma [15–20]. TVA discharge occurring between cathode and anode under high or ultra vacuum conditions. The cathode in the Wehnelt cylinder contains heated tungsten filament which emits electron and the anode contains the material to be coated. The accelerated electrons with high dc voltage collide with the anode. The heated anode material due to electron bombardment starts to melt firstly, afterwards to evaporate and a steady state concentration of the evaporated atoms of anode materials is ensured between anode and cathode. With further measure of the applied high voltage a bright thermionic vacuum arc is established in the vapors at anode material in the vacuum chamber. Due to the suitable pressure of vacuum chamber the material generated plasma is deposited on substrate as thin films. TVA setup is schematically shown in Fig. 1. In our study MgF2 slug as anode material in tungsten crucible have been used. The working parameters of TVA as cathode filament current (Ifilament), current of generated plasma in interelectrodic space namely discharge current (Idischarge), applied voltage on anode (V) and vacuum vessel pressure (p) are presented in Table 1. MgF2 thin films were deposited on glass slides by using TVA technique which is one of the plasma assisted techniques. In the plasma assisted deposition technique, to characterize the plasma of coating material is important because the quality of thin films depends on plasma of coated material. For this purpose optical emission spectroscopy (OES) of MgF2 were obtained during the deposition in the range of 200–850 nm and OES of MgF2 are shown in Fig. 2. As seen in Fig. 2, intensities of peaks show the formation of Mg and Flor atoms in
⁎ Corresponding author. E-mail address: [email protected] (Ş. Korkmaz). 0030-4018/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.12.095
Ş. Korkmaz et al. / Optics Communications 285 (2012) 2373–2376
2374
Fig. 1. Schematic view of the electrodes arrangement for TVA discharges.
Fig. 3. Transmittance spectra of produced MgF2 thin films.
Table 1 Working parameters from the TVA discharge. Pressure
Ifilament
Idischarge
(Torr)
(A)
(A)
Voltage (V)
5.27 × 10− 5
19
0.8
2500
plasma. To analyze to the quality of the produced MgF2 thin films, the optical and surface properties have been investigated. 3. Results and discussion 3.1. Optical characteristics of MgF2 thin films Optical characteristics of the AR coatings depend on film thickness of coated layers' transparency and refractive index. To determine thickness of MgF2 film was used and it was measured as 230 nm. To measure the transmittance of coating samples, Perkin-Elmer UV/vis Lambda 2S spectrometer was used. Transmittance spectra of produced MgF2 thin films are compared with the spectra of uncoated glass samples and are shown in Fig. 3. As seen in Fig. 3, the transmittance of the samples coated with MgF2 is higher than that of uncoated samples. The transparency of MgF2 thin film is 93% at 550 nm and it is in good agreement with the literature [6,8].
The reflectance of MgF2 thin films versus to the wavelength is measured by using Filmetrics F20 system and is shown in Fig. 4. As seen in Fig. 4 the reflectance of MgF2 is very low as we expected.
Fig. 2. Optical emission spectroscopy of MgF2 plasma.
Fig. 5. Refractive index of produced MgF2 thin films.
Fig. 4. Reflectance of produced MgF2 thin films.
Ş. Korkmaz et al. / Optics Communications 285 (2012) 2373–2376
2375
Fig. 6. SEM images of MgF2 thin film.
Table 2 EDS measurement of MgF2 coated substrate. Elt.
Line
Intensity (c/s)
Error 2-Sig
Atomic (%)
Conc
Units
O F Mg Si
Ka Ka Ka Ka
298.11 53.43 113.27 868.63
10.915 4.621 6.728 18.631
54.081 13.624 5.070 27.225 100.000
43.007 12.865 6.124 38.004 100.000
wt.% wt.% wt.% wt.% Total
The spectral dependence of the refractive index for MgF2 film is presented Fig. 5. Refractive index of MgF2 thin films is 1.379 at 550 nm. The change of refractive index with wavelength is in good agreement with literature [21].
3.2. Surface morphologies of produced MgF2 thin films The surface properties of MgF2 thin films have been investigated by using SEM, EDS and AFM. SEM images of MgF2 layer coated on the glass are presented in Fig. 6a with a magnification of 0.5 kx and
Fig. 7. 3D AFM images (4000 nm × 4000 nm) of produced MgF2 thin films.
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Ş. Korkmaz et al. / Optics Communications 285 (2012) 2373–2376
Fig. 8. Indent analysis of produced MgF2 thin films.
in Fig. 6b with a magnification of 2 kx. When the SEM images of MgF2 are seen that surface of produced thin film was very smooth. However, some fractions were seen in images. EDS analysis of MgF2 thin films has been realized by using JEOLJSM-5600-LV SEM and analysis results are presented in Table 2. Analysis results show that the main components of films are magnesium and fluoride. Its ratio is very close to MgF2 formation. The quality of the deposited MgF2 thin film has been obtained using AFM images. AFM images of the deposited MgF2 thin film are shown in Fig. 7. The roughness measurements were realized from different 40 lines of the coated surface. The average roughness of the deposited MgF2 thin film was measured as 0.79 nm. Quantum dots were seen in AFM images, whose dimensions are approximately 80–100 nm. Maximum height is approximately 30–35 nm. Heights of the quantum dots were 30–35 nm. These results show that produced MgF2 thin films by using TVA are pure, compact, smooth, homogeny and nanostructure. Also the roughness of deposited thin films is very low and it is in agreement with TVA literature [20]. To show the plastic deformation properties of produced MgF2 thin films, the indent analysis was realized up to 900 μN. The indent analysis of produced MgF2 thin films is shown in Fig. 8. According to this analysis' results, surface of the film exhibits plastic behavior, so adhesion forces between glass and film are not strong. This situation is desired for ophthalmic lens coating. 4. Conclusion We have investigated that the optical and surface properties of MgF2 thin films were produced by TVA technique. The coating of glass with MgF2 increased the transmittance and measured refractive index values of produced films are in good agreement with literature. Surface analysis shows that of MgF2 surfaces produced films by TVA are smooth, rough, homogeny and adhesive. Our analysis' results show that MgF2 thin films produced by TVA technique are proper for single and multi layer AR coating. The use of TVA technique to produce MgF2 thin films has some advantages such as very short coating
time and being able to control thickness of thin film. So we conclude that TVA technique is very suitable for industrial ophthalmic application. Acknowledgements This research activity has been supported by TUBITAK numbered with 108M608. AFM measurements are realized by ESOGU scientific research commission numbered with 200819045. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
H.A. Macleod, Adam Hilger Ltd, London, 2001, p. 9, E. D. A. Roy Chowdhuri, Dong-Un Jin, C.G. Takoudis, Thin Solid Films 457 (2004) 402. F. Perales, J.M. Herrero, D. Jaque, C. de las Heras, Optical Materials 29 (2007) 783. Q. Zhang, J. Wang, G. Wu, J. Shen, S. Buddhudu, Materials Chemistry and Physics 72 (2001) 56. H. Krüger, E. Kemnitz, A. Hertwing, U. Beck, Thin Solid Films 516 (2008) 4175. H. Yu, H. Qi, Y. Cui, Y. Shen, J. Shao, Z. Fan, Applied Surface Science 253 (2007) 6113. S. Fujihara, M. Tada, T. Kimura, Thin Solid Films 304 (1997) 252. M. Leskela, M. Ritala, Thin Solid Films 409 (2002) 138. M. Ritala, M. Leskela, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, Academic Press, San Diego, 2002, p. 103. T. Pilvi, E. Puukilainen, U. Kreissig, M. Leskela, M. Ritala, Chemistry of Materials 20 (2008) 5023. J.J. McNally, G.A. Al-Jumaily, J.R. McNeil, N. Sendow, Applied Optics 25 (1986) 1973. P.J. Martin, W.G. Sainty, R.P. Netterfeld, D.R. McKenzie, D.J.H. Cockayne, S.H. Sie, O.R. Wood, H.G. Craighead, Applied Optics 26 (1987) 1235. J.I. Larruquert, R.A.M. Krski-Kuha, Optics Communication 215 (2003) 93. E.T. Hutcheson, G. Hass, J.T. Cox, Applied Optics 11 (1972) 2245. H. Ehrich, J. Schuhmann, G. Musa, A. Popescu, I. Mustata, Thin Solid Films 333 (1998) 95. G. Musa, H. Ehrich, J. Schuhmann, IEEE Transactions on Plasma Science 25 (1997) 386. G. Musa, I. Mustata, V. Ciupina, R. Vladoiu, G. Prodan, E. Vasile, H. Ehrich, Diamond and Related Materials 13 (2004) 1398. G. Musa, I. Mustata, V. Ciupina, R. Vladoiu, G. Prodan, C.P. Lungu, H. Ehrich, Journal of Optoelectronics and Advanced Materials 7 (5) (2005) 2485. S. Pat, N. Ekem, T. Akan, Ö. Küsmüs, S. Demirkol, R. Vladoiu, C.P. Lungu, G. Musa, Journal of Optoelectronics and Advanced Materials 7 (5) (2005) 2495. S. Okur, M. Kalkanci, S. Pat, T. Akan, Z. Balbag, et al., Physica C: Superconductivity 466 (2007) 205. T. Wiktorczyk, M. Oles, Optical Materials 29 (2007) 1768.