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Making parabolic mirrors by electron-beam gun evaporation method with ion-assisted deposition
Applied Surface Science 169±170 (2001) 654±657
Making parabolic mirrors by electron-beam gun evaporation method with ion-assisted deposition Cheng-Chung Jainga,*, Cheng-Chung Leeb, Jin-Cherng Hsub, Chuen-Lin Tienb a
Precision Instrument Development Center, 20 R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu 300, Taiwan, ROC b Institute of Optical Sciences, National Central University, Chung-Li 320, Taiwan, ROC Received 29 July 1999; accepted 26 November 1999
Abstract A spherical surface was modi®ed to form a parabolic surface by the evaporation of Al2O3 ®lms and an ion-assisted deposition process using a designed mask. The stress of the evaporated ®lms was measured by a phase shifting Twyman± Green interferometer. Ion beam bombardment was proven to reduce the stress of Al2O3 ®lms. The optimum ion beam voltage and ion beam current density for reducing the ®lm stress were 300 V and 16 mA/cm2 for Al2O3 ®lms. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin ®lm; Ion beam bombardment; Stress; Phase shifting; Interference
1. Introduction A surface can be modi®ed by the additive processes of thin ®lm deposition in vacuum. However, the ®lm thickness between a surface and a modi®ed surface is so thick that the stress of the ®lm is large. Strong and Gaviola [1] used aluminium to modify spherics ®nding that the coating began to show a bloom when the thickness of the aluminium became greater than 2 mm. Schulz [2] used lithium ¯uoride to make aspherical refracting surfaces ®nding that the ®lms began to peel off when the thickness became greater than 5 mm. Dobrowolski and Weinstein [3] used zinc sul®de to make aspherical surfaces showing that the ®lms exhibited no peeling or cracking until the ®lm thickness reached 20 mm. Kurdock and Austin [4] suggested the *
Corresponding author. Tel.: 886-3-5779911, ext. 309; fax: 886-3-5780003. E-mail address: [email protected] (C.-C. Jaing).
use of a zinc sul®de/thorium ¯uoride mixture evaporation source to make aspherics showing that completely stress-free ®lms were dif®cult to achieve. Al ®lm with a thickness of 5.567 mm produced by electron-beam evaporation showed no peeling or cracking when a spherical surface was modi®ed to become a parabolic surface [5], but the re¯ectance of the Al ®lms was low due to the rough surface. The replacement of thick Al layers by dielectric ®lms during the manufacture of high re¯ectivity aspherical mirrors was investigated showing that the thick SiO2, ZrO2 and TiO2 ®lms cracked and peeled off, while Al2O3 ®lms did not [6]. Aluminium oxide was thus found to be a suitable material for modifying a surface. However, the stress of thick Al2O3 ®lms is still large and degenerates the modi®ed surface. In this paper, ion-assisted deposition (IAD) was applied to reduce the ®lm stress of Al2O3. A phase shifting Twyman±Green interferometer along with the phase reduction algorithm was set up to measure the Al2O3 ®lm stress. A parabolic surface was modi®ed
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 0 6 - 0
C.-C. Jaing et al. / Applied Surface Science 169±170 (2001) 654±657
655
from a spherical surface by the deposition of Al2O3 ®lms using the optimum ion source parameters. 2. Experiments The arrangement of the deposition system and the design of the mask has been reported previously [5]. The diameter of the spherical substrate was 100 mm and its curvature radius was 500 mm. Al2O3 and Al were evaporated using an electron-beam gun made by Temescal Company, USA. The evaporation distribution of Al2O3 was cosn y with n 2:69, as calculated by the ®lm thickness distribution on the ¯at substrates, using an electron beam voltage of 7 kV and currents of 220 to 270 mA to get a deposition rate of 1 nm/s. The maximum thickness of the Al2O3 ®lms required to make a parabolic surface from a spherical surface was 1.661 mm. A thin Al ®lm (thickness 0.11 mm) was overcoated to provide the re¯ecting surface. A Kaufman-type ion-beam source with a plasma bridge neutralizer (PBN) was used to assist the growth of the Al2O3 ®lms. The PBN provided neutralizing electrons into the ion beam to balance the positive charge of the ions. After deposition, aberrations in the parabolic mirrors were measured by using the null re¯ective method with a phase shifting function. BK-7 glass, polished on one side to a ¯atness of one wavelength and ground on the other side, was used to measure the stress of Al2O3 ®lms prepared at various ion beam voltages and ion beam current densities. The ion beam current density was measured by a Faraday cup at a position beneath the substrate before deposition. The glass was 25.4 mm in diameter and 2 mm in thickness. The deposition rate of Al2O3 ®lms was 1 nm/s and the thickness of the Al2O3 ®lms was 1.661 mm. 7 sccm of argon gas were fed into the ion source as the working gas and reactive oxygen gas was introduced to the chamber at a ®xed rate during the deposition process. The deposition pressure was therefore, maintained at 1:5 10ÿ4 Torr. The chamber temperature was 258C at the start of deposition but rose to 140±1508C at the end, due to radiant heating from the electron-beam gun and the ion source. The refractive index of the Al2O3 ®lms was measured by a variable angle spectroscopic ellipsometer made by J.A. Wollam Company. A non-contact surface pro®ler, a WYKO TOPO-3D, was used to
Fig. 1. Schematic drawing of a phase shifting Twyman±Green interferometer for measuring ®lm stress.
measure the surface roughness of the Al2O3 ®lms. Samples made with the ion-assisted deposition process were amorphous when measured by a Siments D5000 X-ray diffraction system. A phase shifting Twyman±Green interferometer, combing the phase reduction algorithm, was set up to measure the substrate deformation, which was bent by the Al2O3 coatings, as shown in Fig. 1. Phase shifting was performed by moving the reference plate to ®ve equally spaced positions of l/8 with a computer controlled piezoelectric transducer translation device. The phase shifting algorithm followed Hariharan algorithm requiring ®ve digitized intensity interferograms with a constant phase difference of 908 [7]. The phase in the interferogram was calculated by using the following equation; 2 I2 ÿ I4 (1) F x; y tanÿ1 2I3 ÿ I5 ÿ I1 Where F(x, y) is a phase function and I1 to I5 represent the digitized intensity at the pixels of the ®ve interferograms. A contour map of the substrate was obtained by subtracting the tilt terms from the phase terms after the Zernike polynomials were ®tted to the phase distribution data [8]. The Al2O3 ®lm stress was therefore, calculated by using the following equation [9±10]; s
Dh Es ds2 3r 2 1 ÿ ndf
(2)
656
C.-C. Jaing et al. / Applied Surface Science 169±170 (2001) 654±657
where s is the stress in the thin ®lm, Dh is the difference in the deformation at a radius r, 10 mm from the center of the substrate before and after ®lm deposition. ds, 2 mm, is the thickness of the substrate and a df, of 1.661 mm, is the ®lm thickness. Es, 8:1 1011 dyne/cm2, and n, 0.208, are the Young's modulus and the Poisson's ratio for the BK-7 substrate, respectively [11]. 3. Results and discussions Without ion-assistance, but with substrate heating to 2758C during deposition, the stress of Al2O3 ®lms is 0.339 GPa and the refractive index is 1.656. There were two circular fringes to be seen in the interferogram of a BK-7 glass before deposition, and there were six circular fringes in the interferogram of the same glass after deposition. Because the curvature of the deformation difference was concave, the stress was tensile. The beam voltage of the ion source was varied with a ®xed ion beam current density of 12 mA/cm2, and the stress and the refractive indices of the Al2O3 ®lms are shown in Fig. 2. All the stress is tensile in Fig. 2. The r.m.s. value of the surface roughness of Al2O3 ®lms remained 0.296 nm over the ion beam voltage, from 250 V to 350 V, but at 400 V it changed to 0.302 nm. The depositing atoms have their optimum mobility when the ion beam voltage is 300 V, with the result that the ®lm should exhibit less of a self-shadowing effect, a reduction in the columnar structure and an increase in the packing density
Fig. 2. The stress and refractive index (dashed line) for 1.661 mm thick IAD Al2O3 ®lms as functions of the ion beam voltage at a ®xed ion beam current density of 12 mA/cm2.
Fig. 3. The stress and refractive index (dashed line) for 1.661 mm thick IAD Al2O3 ®lms as functions of the ion beam current density at a ®xed ion beam voltage of 300 V.
[12±16]. The stress may be at its minimum and the refractive index may be at its maximum. The deposition rate was apparently lower for a beam voltage of 400 V than for a beam voltage of 300 V, with the same electron-beam gun power. The condensed atoms could be resputtered colliding with the incoming atoms. The incoming atoms thus lost their mobility, resulting in a rougher surface, higher stress and a smaller refractive index. In another experiment, the ion beam current density was varied with a ®xed ion beam voltage of 300 V. As shown in Fig. 3, the stress decreased and the refractive index increased when the ion beam current density increased, with a minimum stress value of 0.022 GPa and a maximum refractive index value of 1.695, when the current density equals 16 mA/cm2. This is due to an increase in the surface mobility of the depositing atoms, as the ratio of the number of ions to the number of condensing atoms increased. There was less than a fringe added in the interferogram for BK-7 glass after a 16 mA/cm2 deposition. However, the decrease in the stress is not linearly correlated with the increase in the refractive index. This might be due to the change of strain in the ®lm not being so obvious when the ion current density was small. All the stress is tensile in Fig. 3. For a low ion beam current density, i.e. 4 mA/ cm2, the r.m.s. value of the surface roughness was 0.3 nm, which improved to 0.296 nm as the current density was increased to 8 mA/cm2 or higher. The surface mobility of the depositing atoms could be too small to get a smooth surface during the 4 mA/ cm2 deposition. It is obvious that aluminium oxide has
C.-C. Jaing et al. / Applied Surface Science 169±170 (2001) 654±657
much stress reduction and a higher refractive index, for ®lms with IAD rather than for ®lms without IAD. The reason is thought to be an increase in the surface mobility of the depositing atoms. A parabolic mirror with an r.m.s. 0.128l aberration was made by a thin-®lm coating technique, with a specially designed mask, starting from a spherical substrate with an aberration of r.m.s. 0.0725l, on which an Al2O3 ®lm was deposited by an electronbeam gun using IAD, with an ion beam voltage of 300 V and an ion beam current density of 16 mA/cm2. This result may be improved in future researches by better control of the electron beam during the Al2O3 deposition process.
contracts NSC 88-2215-E-097-001 and NSC 88-2215E-008-015. References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusions We have shown that ion beam bombardment not only decreases the stress but also increases the refractive index of Al2O3 ®lms. An optimum ion beam voltage and ion beam current, 300 V and 16 mA/cm2, was used to assist in the making of a parabolic mirror from a spherical substrate.
[9] [10] [11] [12] [13] [14]
Acknowledgements This research was sponsored by the National Science Council, Taiwan, Republic of China, under
657
[15] [16]
J. Strong, E. Gaviola, J. Opt. Soc. Am. 26 (1936) 153. L.G. Schulz, J. Opt. Soc. Am. 38 (1948) 432. J.A. Dobrowolski, W. Weinstein, Nature 175 (1955) 646. J.R. Kurdock, R.R. Austin, Phys. Thin Films 10 (1978) 261. C.C. Lee, D.S. Wan, C.C. Jaing, C.W. Chu, Appl. Optics 32 (1993) 5535. C.C. Jaing, C.C. Lee, Opt. Quant. Elect. 28 (1996) 1583. P. Hariharan, B.F. Oreb, T. Eiju, Appl. Opt. 26 (1987) 2504. D. Malacara, S.L. Devore, in: D. Malacara (Ed.), Optical Shop Testing, Chapter 13, Wiley, New York, 1992 p. 455. D.S. Campbell, in: L.I. Maissel, R. Glang (Eds.), Handbook of Thin Film Technology, Chapter 12, McGRAW-HILL, New York, 1970, pp. 12±15. R. Glang, R.A. Holmwood, R.L. Rosenfeld, Rev. Sci. Instrum. 36 (1965) 7. S. Musikant, Optical Materials, Paoil, Pennsylvania, 1985, p. 92. P.J. Martin, H.A. Macleod, R.P. Netter®eld, C.G. Pacey, W.G. Sainty, Appl. Opt. 22 (1983) 178. J.R. McNeil, G.A. Al-Jumaily, K.C. Jungling, A.C. Barron, Appl. Opt. 24 (1985) 486. F.L. Williams, D.W. Reicher, C.B. Juang, J.R. McNeil, J. Vac. Sci. Technol. 7 (1989) 2286. H.K. Pulker, M. Lardox, R. Buhl, E. Moll, J. Vac. Sci. Technol. 4 (1986) 2922. B.G. Bovard, in: F.R. Flory (Ed.), Thin Films for Optical Systems, Chap. 5, Marcel Dekker, New York, 1995, p. 117.
Making parabolic mirrors by electron-beam gun evaporation method with ion-assisted deposition Cheng-Chung Jainga,*, Cheng-Chung Leeb, Jin-Cherng Hsub, Chuen-Lin Tienb a
Precision Instrument Development Center, 20 R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu 300, Taiwan, ROC b Institute of Optical Sciences, National Central University, Chung-Li 320, Taiwan, ROC Received 29 July 1999; accepted 26 November 1999
Abstract A spherical surface was modi®ed to form a parabolic surface by the evaporation of Al2O3 ®lms and an ion-assisted deposition process using a designed mask. The stress of the evaporated ®lms was measured by a phase shifting Twyman± Green interferometer. Ion beam bombardment was proven to reduce the stress of Al2O3 ®lms. The optimum ion beam voltage and ion beam current density for reducing the ®lm stress were 300 V and 16 mA/cm2 for Al2O3 ®lms. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin ®lm; Ion beam bombardment; Stress; Phase shifting; Interference
1. Introduction A surface can be modi®ed by the additive processes of thin ®lm deposition in vacuum. However, the ®lm thickness between a surface and a modi®ed surface is so thick that the stress of the ®lm is large. Strong and Gaviola [1] used aluminium to modify spherics ®nding that the coating began to show a bloom when the thickness of the aluminium became greater than 2 mm. Schulz [2] used lithium ¯uoride to make aspherical refracting surfaces ®nding that the ®lms began to peel off when the thickness became greater than 5 mm. Dobrowolski and Weinstein [3] used zinc sul®de to make aspherical surfaces showing that the ®lms exhibited no peeling or cracking until the ®lm thickness reached 20 mm. Kurdock and Austin [4] suggested the *
Corresponding author. Tel.: 886-3-5779911, ext. 309; fax: 886-3-5780003. E-mail address: [email protected] (C.-C. Jaing).
use of a zinc sul®de/thorium ¯uoride mixture evaporation source to make aspherics showing that completely stress-free ®lms were dif®cult to achieve. Al ®lm with a thickness of 5.567 mm produced by electron-beam evaporation showed no peeling or cracking when a spherical surface was modi®ed to become a parabolic surface [5], but the re¯ectance of the Al ®lms was low due to the rough surface. The replacement of thick Al layers by dielectric ®lms during the manufacture of high re¯ectivity aspherical mirrors was investigated showing that the thick SiO2, ZrO2 and TiO2 ®lms cracked and peeled off, while Al2O3 ®lms did not [6]. Aluminium oxide was thus found to be a suitable material for modifying a surface. However, the stress of thick Al2O3 ®lms is still large and degenerates the modi®ed surface. In this paper, ion-assisted deposition (IAD) was applied to reduce the ®lm stress of Al2O3. A phase shifting Twyman±Green interferometer along with the phase reduction algorithm was set up to measure the Al2O3 ®lm stress. A parabolic surface was modi®ed
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 0 6 - 0
C.-C. Jaing et al. / Applied Surface Science 169±170 (2001) 654±657
655
from a spherical surface by the deposition of Al2O3 ®lms using the optimum ion source parameters. 2. Experiments The arrangement of the deposition system and the design of the mask has been reported previously [5]. The diameter of the spherical substrate was 100 mm and its curvature radius was 500 mm. Al2O3 and Al were evaporated using an electron-beam gun made by Temescal Company, USA. The evaporation distribution of Al2O3 was cosn y with n 2:69, as calculated by the ®lm thickness distribution on the ¯at substrates, using an electron beam voltage of 7 kV and currents of 220 to 270 mA to get a deposition rate of 1 nm/s. The maximum thickness of the Al2O3 ®lms required to make a parabolic surface from a spherical surface was 1.661 mm. A thin Al ®lm (thickness 0.11 mm) was overcoated to provide the re¯ecting surface. A Kaufman-type ion-beam source with a plasma bridge neutralizer (PBN) was used to assist the growth of the Al2O3 ®lms. The PBN provided neutralizing electrons into the ion beam to balance the positive charge of the ions. After deposition, aberrations in the parabolic mirrors were measured by using the null re¯ective method with a phase shifting function. BK-7 glass, polished on one side to a ¯atness of one wavelength and ground on the other side, was used to measure the stress of Al2O3 ®lms prepared at various ion beam voltages and ion beam current densities. The ion beam current density was measured by a Faraday cup at a position beneath the substrate before deposition. The glass was 25.4 mm in diameter and 2 mm in thickness. The deposition rate of Al2O3 ®lms was 1 nm/s and the thickness of the Al2O3 ®lms was 1.661 mm. 7 sccm of argon gas were fed into the ion source as the working gas and reactive oxygen gas was introduced to the chamber at a ®xed rate during the deposition process. The deposition pressure was therefore, maintained at 1:5 10ÿ4 Torr. The chamber temperature was 258C at the start of deposition but rose to 140±1508C at the end, due to radiant heating from the electron-beam gun and the ion source. The refractive index of the Al2O3 ®lms was measured by a variable angle spectroscopic ellipsometer made by J.A. Wollam Company. A non-contact surface pro®ler, a WYKO TOPO-3D, was used to
Fig. 1. Schematic drawing of a phase shifting Twyman±Green interferometer for measuring ®lm stress.
measure the surface roughness of the Al2O3 ®lms. Samples made with the ion-assisted deposition process were amorphous when measured by a Siments D5000 X-ray diffraction system. A phase shifting Twyman±Green interferometer, combing the phase reduction algorithm, was set up to measure the substrate deformation, which was bent by the Al2O3 coatings, as shown in Fig. 1. Phase shifting was performed by moving the reference plate to ®ve equally spaced positions of l/8 with a computer controlled piezoelectric transducer translation device. The phase shifting algorithm followed Hariharan algorithm requiring ®ve digitized intensity interferograms with a constant phase difference of 908 [7]. The phase in the interferogram was calculated by using the following equation; 2 I2 ÿ I4 (1) F x; y tanÿ1 2I3 ÿ I5 ÿ I1 Where F(x, y) is a phase function and I1 to I5 represent the digitized intensity at the pixels of the ®ve interferograms. A contour map of the substrate was obtained by subtracting the tilt terms from the phase terms after the Zernike polynomials were ®tted to the phase distribution data [8]. The Al2O3 ®lm stress was therefore, calculated by using the following equation [9±10]; s
Dh Es ds2 3r 2 1 ÿ ndf
(2)
656
C.-C. Jaing et al. / Applied Surface Science 169±170 (2001) 654±657
where s is the stress in the thin ®lm, Dh is the difference in the deformation at a radius r, 10 mm from the center of the substrate before and after ®lm deposition. ds, 2 mm, is the thickness of the substrate and a df, of 1.661 mm, is the ®lm thickness. Es, 8:1 1011 dyne/cm2, and n, 0.208, are the Young's modulus and the Poisson's ratio for the BK-7 substrate, respectively [11]. 3. Results and discussions Without ion-assistance, but with substrate heating to 2758C during deposition, the stress of Al2O3 ®lms is 0.339 GPa and the refractive index is 1.656. There were two circular fringes to be seen in the interferogram of a BK-7 glass before deposition, and there were six circular fringes in the interferogram of the same glass after deposition. Because the curvature of the deformation difference was concave, the stress was tensile. The beam voltage of the ion source was varied with a ®xed ion beam current density of 12 mA/cm2, and the stress and the refractive indices of the Al2O3 ®lms are shown in Fig. 2. All the stress is tensile in Fig. 2. The r.m.s. value of the surface roughness of Al2O3 ®lms remained 0.296 nm over the ion beam voltage, from 250 V to 350 V, but at 400 V it changed to 0.302 nm. The depositing atoms have their optimum mobility when the ion beam voltage is 300 V, with the result that the ®lm should exhibit less of a self-shadowing effect, a reduction in the columnar structure and an increase in the packing density
Fig. 2. The stress and refractive index (dashed line) for 1.661 mm thick IAD Al2O3 ®lms as functions of the ion beam voltage at a ®xed ion beam current density of 12 mA/cm2.
Fig. 3. The stress and refractive index (dashed line) for 1.661 mm thick IAD Al2O3 ®lms as functions of the ion beam current density at a ®xed ion beam voltage of 300 V.
[12±16]. The stress may be at its minimum and the refractive index may be at its maximum. The deposition rate was apparently lower for a beam voltage of 400 V than for a beam voltage of 300 V, with the same electron-beam gun power. The condensed atoms could be resputtered colliding with the incoming atoms. The incoming atoms thus lost their mobility, resulting in a rougher surface, higher stress and a smaller refractive index. In another experiment, the ion beam current density was varied with a ®xed ion beam voltage of 300 V. As shown in Fig. 3, the stress decreased and the refractive index increased when the ion beam current density increased, with a minimum stress value of 0.022 GPa and a maximum refractive index value of 1.695, when the current density equals 16 mA/cm2. This is due to an increase in the surface mobility of the depositing atoms, as the ratio of the number of ions to the number of condensing atoms increased. There was less than a fringe added in the interferogram for BK-7 glass after a 16 mA/cm2 deposition. However, the decrease in the stress is not linearly correlated with the increase in the refractive index. This might be due to the change of strain in the ®lm not being so obvious when the ion current density was small. All the stress is tensile in Fig. 3. For a low ion beam current density, i.e. 4 mA/ cm2, the r.m.s. value of the surface roughness was 0.3 nm, which improved to 0.296 nm as the current density was increased to 8 mA/cm2 or higher. The surface mobility of the depositing atoms could be too small to get a smooth surface during the 4 mA/ cm2 deposition. It is obvious that aluminium oxide has
C.-C. Jaing et al. / Applied Surface Science 169±170 (2001) 654±657
much stress reduction and a higher refractive index, for ®lms with IAD rather than for ®lms without IAD. The reason is thought to be an increase in the surface mobility of the depositing atoms. A parabolic mirror with an r.m.s. 0.128l aberration was made by a thin-®lm coating technique, with a specially designed mask, starting from a spherical substrate with an aberration of r.m.s. 0.0725l, on which an Al2O3 ®lm was deposited by an electronbeam gun using IAD, with an ion beam voltage of 300 V and an ion beam current density of 16 mA/cm2. This result may be improved in future researches by better control of the electron beam during the Al2O3 deposition process.
contracts NSC 88-2215-E-097-001 and NSC 88-2215E-008-015. References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusions We have shown that ion beam bombardment not only decreases the stress but also increases the refractive index of Al2O3 ®lms. An optimum ion beam voltage and ion beam current, 300 V and 16 mA/cm2, was used to assist in the making of a parabolic mirror from a spherical substrate.
[9] [10] [11] [12] [13] [14]
Acknowledgements This research was sponsored by the National Science Council, Taiwan, Republic of China, under
657
[15] [16]
J. Strong, E. Gaviola, J. Opt. Soc. Am. 26 (1936) 153. L.G. Schulz, J. Opt. Soc. Am. 38 (1948) 432. J.A. Dobrowolski, W. Weinstein, Nature 175 (1955) 646. J.R. Kurdock, R.R. Austin, Phys. Thin Films 10 (1978) 261. C.C. Lee, D.S. Wan, C.C. Jaing, C.W. Chu, Appl. Optics 32 (1993) 5535. C.C. Jaing, C.C. Lee, Opt. Quant. Elect. 28 (1996) 1583. P. Hariharan, B.F. Oreb, T. Eiju, Appl. Opt. 26 (1987) 2504. D. Malacara, S.L. Devore, in: D. Malacara (Ed.), Optical Shop Testing, Chapter 13, Wiley, New York, 1992 p. 455. D.S. Campbell, in: L.I. Maissel, R. Glang (Eds.), Handbook of Thin Film Technology, Chapter 12, McGRAW-HILL, New York, 1970, pp. 12±15. R. Glang, R.A. Holmwood, R.L. Rosenfeld, Rev. Sci. Instrum. 36 (1965) 7. S. Musikant, Optical Materials, Paoil, Pennsylvania, 1985, p. 92. P.J. Martin, H.A. Macleod, R.P. Netter®eld, C.G. Pacey, W.G. Sainty, Appl. Opt. 22 (1983) 178. J.R. McNeil, G.A. Al-Jumaily, K.C. Jungling, A.C. Barron, Appl. Opt. 24 (1985) 486. F.L. Williams, D.W. Reicher, C.B. Juang, J.R. McNeil, J. Vac. Sci. Technol. 7 (1989) 2286. H.K. Pulker, M. Lardox, R. Buhl, E. Moll, J. Vac. Sci. Technol. 4 (1986) 2922. B.G. Bovard, in: F.R. Flory (Ed.), Thin Films for Optical Systems, Chap. 5, Marcel Dekker, New York, 1995, p. 117.