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Deposition of MgO films by pulsed mid-frequency magnetron sputtering
Applied Surface Science 200 (2002) 117–124
Deposition of MgO films by pulsed mid-frequency magnetron sputtering Y.H. Cheng*,1, H. Kupfer, F. Richter, Andreea Maria Paraian Institut fu¨r Physik, Technische Universita¨t Chemnitz, 09107 Chemnitz, Germany Received 26 April 2002; received in revised form 15 June 2002; accepted 15 June 2002
Abstract MgO films were deposited by pulsed mid-frequency magnetron sputtering from metallic targets in the mixture of Ar and O2 gas. The surface morphology, crystalline structure, and optical properties were characterized by using atomic force microscopy (AFM), X-ray diffraction (XRD), and spectroscopic ellipsometry, respectively. The secondary electron emission coefficients of MgO films were measured by using a self-made apparatus in He gas. A pronounced hysteresis phenomenon of target voltage, current, and deposition rate with increasing and decreasing O2 flow rate was observed. The structure of films deposited at a metallic mode changes from Mg phase to the mixed Mg and MgO phase, and the films have a very rough surface. All the films deposited at oxide mode have high transparency and smooth surface, and show (2 2 0) preferred orientation growth. The refractive index and extinction coefficient at a wavelength of 670 nm for MgO films deposited at oxide mode with a O2 flow rate of 3 sccm are 1.698 and 1:16 104 , respectively. The secondary emission coefficient at a E/p of 57.8 V/(cm Torr) for MgO films deposited at a O2 flow rate of 3 sccm is 0.16, which is higher than that of MgO films deposited by e-beam evaporation. # 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.C; 81.65.M; 79.20.H Keywords: MgO films; Pulsed mid-frequency magnetron sputtering; Secondary electron emission coefficient
1. Introduction The magnetron sputter method has been widely used in the industry for large area thin film deposition because of its very good coating uniformity and good film properties. For the production of conductive films (metallic, most of the nitride, and few oxide films), * Corresponding author. Tel.: þ49-371-531-3570; fax: þ49-371-531-3042. E-mail address: [email protected] (Y.H. Cheng). 1 On leave from School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 40074, PR China.
direct current (dc) magnetron sputtering is the best choice due to the high deposition rate and good uniformity. However, for the deposition of dielectric films (most of the oxide and few nitride films), dc magnetron sputtering has suffered from the serious drawback of arcing, which results from the charging up of the dielectric films formed on the target surface during sputtering [1]. Arcing may result in the degradation of the film properties and instabilities of the deposition process [2]. Although rf sputtering is an alternative method for sputtering of dielectric films, it is limited by low deposition rates, high equipment costs (power supply and matching network), and scaling problems [3,4].
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 1 8 - 9
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To solve the problem of arcing, a pulsed midfrequency magnetron sputtering technique has been developed to deposit insulating films [6–12]. In this case, the polarity of the power supplied to the target oscillates between negative and positive potential. During the positive period, positive electric charges accumulated on the covered target areas during the negative period are neutralized by electrons before breakthrough occurs. As pulsing frequency is higher than a critical frequency, arcing could be avoided [5]. Highly insulating films, such as SiO2 [6], Si3N4 [7], Ta2O5 [8], ZrO2 [9], Al2O3 [10,11], and TiO2 [12] have been deposited on large scale at rates exceeding 5 nm/s in a stable continuous process for more than 300 h by using pulsed mid-frequency magnetron sputtering [6]. Recent results have shown that the growing film is bombarded by a high flux of energetic ions during the deposition of films by pulsed magnetron sputtering. Szyszka et al. [13] studied the TwinMag deposition process and found that ion density at the substrate is 10 times higher than that in dc sputtering, and the ion energy distribution has a maximum at around 50 eV (5 eV for dc) and a high energetic tail up to values of 200 eV. Glocker [14] also observed that the ion densities in the ac plasmas are several times greater than the ion densities in similar dc plasmas, which was attributed to the target voltage spikes as the plasma re-ignites on each half-cycle, causing rapid electron acceleration in the presheath region. The intense bombardment by ions with high energy results in the formation of high quality films with a denser structure, improved hardness, and excellent optical properties [6]. MgO is a highly insulating ionic material with the rock-salt structure. It has high physical strength and stability, excellent optical transparency throughout the range 0.25–6.8 mm [15], very low sputtering yield, very large band gap (7.3 eV), and large secondary electron emission (g) coefficient. Due to these unique properties, MgO films have been widely used as a surface protective layer for dielectric materials in alternating current plasma display panel (PDP) [16]. Typically, MgO films were prepared by e-beam evaporation [17–19]. However, MgO films deposited by evaporation have a porous columnar structure, causing low transparency, high erosion rate resulted from ion bombardment in the glow discharge [16], and low secondary electron emission resulted from
the adsorption of hydrogen-bonded hydroxyl groups along the columnar boundary [20]. In order to improve the properties, rf magnetron sputtering using a MgO target [21], ion beam assisted deposition (IBAD) [22], and aerosol-assisted chemical vapor deposition (AACVD) [23] have been used to deposited MgO films. In this paper, we report, for the first time, the deposition of MgO films from Mg target in mixed Ar and O2 atmosphere by pulsed mid-frequency magnetron sputtering. The structure and surface morphology of MgO films were characterized by X-ray diffraction and atomic force microscopy (AFM), respectively. The optical properties of MgO films were studied by spectroscopic ellipsometry, and the secondary electron emission coefficient of the films was measured using a self-made apparatus.
2. Experimental details MgO films were deposited by a commercial PLS 500P magnetron sputtering system (Balzers Ltd.) in mixed Ar and O2 atmosphere. A high purity Mg (99.99%) plate with the diameter of 100 mm and thickness of 8 mm was used as target, which was located at the bottom of the chamber in a sputter-up configuration. Highly conductive (0.005 O cm) n-type (1 0 0) Si wafer was used as substrate. The substrate holder was electrical isolated from the ground chamber, and no bias was applied to the substrate. The distance between target and substrate was 80 mm. The chamber was pumped down to a base pressure of 5 105 Pa by a turbomolecular pump with a rotary backing pump. During the deposition, the flow rate of Ar gas was fixed at 50 sccm, the flow rate of O2 gas can be varied from 0 to 50 sccm, and the total pressure was kept at 0.4 Pa by adjusting the position of the valve connecting to a turbomolecular pump. No intentional substrate heating was used in this experiment. Before deposition, the Mg target was presputtered with 100% Ar gas for 5 min for the films deposited at metallic mode, or with the mixture of 50%Ar and 50% O2 gas for 15 min for the films deposited at oxide mode. The magnetron operates in a pulsed regime. The mid-frequency power supply used in this study was a Pinnacle plus magnetron driver (Advanced Energy Inc.), which worked in a constant power mode.
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The pulsing frequency can be varied from 1 to 350 kHz with a 50% maximum reverse bias duty cycle, which is defined as the reverse time divided by the total time. The reverse voltage is fixed at 10% of the negative driving voltage. The pulsing frequency, reverse bias duty cycle, and power used in this experiment were set to 200 kHz, 40%, and 400 W, respectively. The surface morphology of MgO films was observed by atomic force microscopy in the contacting mode (Autoprobe from Park Scientific Instrument). The phase and crystal structure of MgO films were identified by using X-ray diffractometry (Seifert, Germany). A Cu Ka X-ray source (wave˚ ) was used at 40 kV and 20 mA. The length of 1.54 A thickness of MgO films was measured with a surface profilometer (Taylor–Hobson, Form Talysurf Series). The deposition rate was derived from the film thickness and deposition time. To investigate the optical properties, spectroscopic ellipsometric measurements were carried out on MgO films in the wavelength range between 300 and 1200 nm at various incident angles in the range 65–758 with a 58 increment by a variable angle spectroscopic ellipsometer VASE (J.A. Wollam Co. Inc.). The secondary electron emission coefficient was measured by a method proposed by Auday et al. [24,25]. Briefly, two plane-parallel electrodes: Cu electrode and sample with the size of 20 mm 20 mm, were placed in a vacuum chamber, which was evacuated to 3 104 Pa by using a turbomolecular pump. The pressure in the chamber can be adjusted by a pressure controller in the range 0.5– 100 Torr. The sample and Cu electrode were connected to cathode and anode of the dc power supply, respectively. The distance between the two electrode was fixed at 3 mm. Helium was used as working gas. The electrical measurements were made with a digital source-meter (Keithley model 2410 series). By using the sweep mode of the power supply, the voltage between the two electrode was steadily increased from 0 to 500 V, and the current was recorded. It can be observed that the current remains almost zero below a certain voltage (Vb), then increases suddenly to a certain value (1 nA). The voltage Vb is defined as breakdown voltage. Then, the secondary electron emission (g) coefficient can be
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calculated from Townsend’s first ionization coefficient (a) by the equation: 1 eða=pÞpd 1 where p is the pressure inside the cell, d the distance between two electrodes. The value of a/p versus the reduced field E/p, which is defined as Vb/pd, can be obtained from [26].
g¼
3. Results and discussion The influence of increasing and decreasing O2 flow rate on the target voltage and current is shown in Fig. 1. A pronounced hysteresis can be seen from the figure. At a low O2 flow rate, the target works in a metallic mode, which corresponds to a high target voltage (240 V) and low target current (1.67 A). As the O2 flow rate is increased, the target is covered in part with MgO. The high secondary electron emission of MgO films on the target surface makes it possible to maintain a discharge at a reduced discharge voltage and increased current [27]. At a critical O2 flow rate of
Fig. 1. Influence of increasing and decreasing O2 flow rate on the target voltage and current.
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Fig. 2. Dependence of the deposition rate on the increasing and decreasing O2 flow rate.
5 sccm, the whole target surface is covered by the MgO films, which causes a sudden decrease and increase of target voltage and current to 111 V and 3.62 A, respectively. The target will work in a complete oxide mode. The further increase of O2 flow rate leads to an increase in the thickness of MgO layer on the target surface, which in turn, results in a slight increase and decrease in the target voltage and current, respectively [28]. The decrease of O2 flow rate from 50 sccm causes a reverse change in the target voltage and current. However, the critical O2 flow rate shifts to 2.5 sccm. The hysteresis can also be seen from the change of film deposition rate with O2 flow rate. Fig. 2 shows the influence of increasing and decreasing O2 flow rate on the film deposition rate. It can be seen that the deposition rate in the metallic mode is greatly affected by the O2 flow rate. The increase in the O2 flow rate from 0 to 5 sccm causes a significant decrease in the deposition rate from 380 to 182 nm/min. However, the increase of O2 flow rate from 5 to 5.5 sccm leads to a sudden decrease of the deposition rate to 6.4 nm/min. The further increase in the O2 flow rate to 50 sccm results in no significant change in the deposition rate. As the O2 flow rate is decreased from 50 to 2.5 sccm, the deposition rate remains almost constant. However, the decrease of O2 flow rate from 2.5 to 2 sccm leads to a sudden increase in the deposition rate to 367 nm/ min. The deposition rate is mainly influenced by the sputtering yield of target surface, ion flux density, ion bombarding energy, and species of impinging ions [26]. It is well known that the sputter yield of metallic target is much higher than that of oxide target. As
discussed earlier, the increase in the O2 flow rate results in an increase in the coverage area of MgO on the target surface, and therefore a decrease in the sputter rate of target, which corresponds to the great decrease in the deposition rate as O2 flow rate is increased from 0 to 5 sccm. As O2 flow rate is increased to above 5.5 sccm, most part of the target surface was covered by oxide layer, which contributes to the sudden drop in the deposition rate. The surface morphology of MgO films was observed by a atomic force microscopy. Fig. 3 shows the typical AFM images for films deposited at metallic mode with the O2 flow rate of (a) 0 sccm, and (b) 5 sccm, and at oxide mode with a O2 flow rate of (c) 2.5 sccm, and (d) 50 sccm. The surface morphology of the deposited films are greatly affected by the sputtering mode and O2 flow rate. Generally, the films deposited at metallic mode are very rough and have a very large grain size, while the films deposited at oxide mode have a very smooth surface with small grains. For the films deposited at metallic mode with a O2 flow rate of 0 sccm. Large grains with a size of 800 nm can be observed, the root mean square (RMS) surface roughness is 5.35 nm, and the films exhibit a metallic color. The increase of O2 flow rate to 5 sccm results in a great decrease in the grain size (100– 400 nm), but a great increase in the RMS surface roughness (29.6 nm). In this case, the films are black and porous, and have a very low strength and scratch resistance. For the films deposited at oxide mode with a O2 flow rate of 2.5 sccm, the surface is very smooth (with a RMS of 2.84 nm). Nanograins with a size of 50–200 nm can be observed. For the films deposited at a O2 flow rate of 50 sccm, the grains is very small (40–60 nm), which contributes to a much smaller RMS roughness (2.31 nm). Fig. 4 shows the X-ray diffraction patterns for MgO films deposited in the metallic mode with a O2 flow rate of (a) 0 sccm, and (b) 5.5 sccm, and in the oxide mode with a O2 flow rate of (c) 2.5 sccm, and (d) 50 sccm. It is interesting to note that only one strong and sharp peak at a 2y of 34.538 can be observed in the X-ray diffraction patterns for the films deposited at a O2 flow rate of 0 sccm. This peak corresponds to the (0 0 2) plane of hexagonal Mg (JCPDS 35-821), which indicates that the films are Mg metallic films. The appearance of single peak indicates that the Mg films exhibit a (0 0 2) preferred orientation growth.
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Fig. 3. AFM image of MgO films deposited in metallic mode at O2 flow rate of (a) 0 sccm, and (b) 5 sccm, and in oxide mode at (c) 2.5 sccm, and (d) 50 sccm.
For the films deposited at a O2 flow rate of 5 sccm, the peak at a 2y of 34.538 can also be observed, indicating the existence of hexagonal metallic Mg phase in the films. However, a remarkable reduction in the intensity can be observed. In addition, one weak and broad peak at 2y of 42.88 can also be seen. The two peaks correspond to the (2 0 0) plane of cubic MgO (JCPDS 4-829), indicating the coexistence of cubic MgO face in the films. For the films deposited in oxide mode
with a O2 flow rate of 2.5 sccm, two weak peaks at 42.8 and 62.258 can be observed. These two peaks correspond to the (2 0 0) and (2 2 0) plane of cubic MgO phase, which indicates that the films are composed of single cubic MgO phase. The relative higher intensity of the peak at 62.258 indicates that MgO films show (2 2 0) preferred orientation growth. This is the same as that of MgO films deposited by rf magnetron sputtering, which also shows a strong
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The optical properties of MgO films were studied by spectroscopic ellipsometry. The obtained data were fitted by using a Cauchy mode. Within this model, the index of refraction (n) and extinction coefficient (k) are represented by a slowly varying function of wavelength (l), and an exponential absorption tail, respectively: nðlÞ ¼ An þ
Bn Cn þ l2 l4
kðlÞ ¼ Ak eBk ð12;400ð1=l1=Eg ÞÞ
Fig. 4. X-ray diffraction patterns of MgO films deposited in metallic mode at O2 flow rate of (a) 0 sccm, and (b) 5 sccm, and in oxide mode of (c) 2.5 sccm, and (d) 50 sccm.
(2 2 0) preferred orientation growth [29]. For the films deposited at 50 sccm, only one weak and broad peak at 62.258 can be observed. The weak feature indicates the small crystalline size, which fits well with the AFM results.
the six parameters in this dispersion model are An, Bn, Cn, the extinction coefficient amplitude (Ak), the exponent factor (Bk), and the absorption band gap (Eg). Due to the high transparency of MgO films in the UV–VIS range, the absorption band edge was chosen outside the measured spectral domain. Fig. 5 shows the typical measured and fitted spectra at the incident angle of 65, 70 and 758 for MgO films deposited at a O2 flow rate of 3 sccm. It is clear from the figure that the fitted spectra fit well with the measured spectra. The fitted refractive index and extinction coefficient as a function of the wavelength of incident light for MgO films deposited at a O2 flow rate of 3 sccm are shown in Fig. 6. As can be seen, the refractive index of MgO films decreases gradually from 1.76 to 1.69 as the wavelength is increased from
Fig. 5. Typical measured and fitted spectra at the incident angle of 65, 70 and 758 for MgO films deposited at a O2 flow rate of 3 sccm.
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Fig. 6. Typical refractive index and extinction coefficient as a function of the wavelength of incident light for MgO films deposited at a O2 flow rate of 3 sccm.
310 to 1200 nm. The refractive index at a wavelength of 670 nm for MgO films deposited at 3 sccm is 1.698, which is higher than that of MgO films (1.634) deposited by evaporation [30]. This indicates the higher density for the films deposited by pulsed mid-frequency magnetron sputtering. Meanwhile, it can also observed that the extinction coefficient of MgO films remains constant at a value smaller than the detection limit of the ellipsometer at a wavelength above 400 nm, and the decrease of wavelength to 310 nm results in a steep increase in the extinction coefficient to 0.008. This indicates the high transparency of the deposited MgO films in the visible wavelength range. Fig. 7 shows the typical secondary electron emission (g) coefficient versus E/p for MgO films deposited at a O2 flow rate of 3 sccm. It can be seen from the figure that g coefficient of MgO films increases
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gradually from 0.08 to 0.16 with increasing E/p from 28 to 57.8 V/(cm Torr). However, the further increase of E/p to 86 V/(cm Torr) results in a significant increase of g coefficient to 0.92. It is well known that potential emission and kinetic ejection are the two main mechanisms for secondary electron emission from a surface [31–33]. Potential emission occurs via Auger neutralization of the incident ion involving two valence electrons in the solid. In the present case, the discharge gas and cathodic material were the same, and only the discharge pressure and voltage were varied. The changes resulted from the potential emission should be negligible. However, due to the high gas pressure, some of the emitted secondary electrons are back-scattered without exciting or ionizing neutral gas atoms [34]. This part of secondary electrons will not contribute to the breakdown voltage, and therefore the measured g coefficient. The increase in the E/p will lead to an increase in the energy of the emitted secondary electrons, increasing the probability of ionization or excitation of the neutral atom, which in turn, leads to a slight increase of the measured g coefficient with increasing E/p. The large increase in the g coefficient at a E/p above 57.8 V/(cm Torr) may result from the kinetic ejection mechanism of the secondary electron emission, i.e. the direct emission of secondary electron by the bombardment of energetic ions. This is consistent with Moon et al. reports [18]. The measured g coefficient of MgO films in the flat region as shown in Fig. 7 is consistent with the value reported in the literature measured by different methods as summarized by Motoyama et al [35]. However, the measured g coefficient of MgO films at a E/p of 57.8 V/(cm Torr) is higher than that (0.13) of the MgO films deposited by evaporation at a E/p of 60 V/(cm Torr) in He gas measured by using the same method [25]. This may indicate that the MgO films deposited by pulsed mid-frequency magnetron sputtering have better secondary electron emission properties than that of the MgO films deposited by evaporation.
4. Conclusion
Fig. 7. Typical secondary electron emission (g) coefficient vs. E/p for MgO films deposited at a O2 flow rate 3 sccm.
In this paper, MgO films were deposited by pulsed mid-frequency magnetron sputtering. AFM and XRD were used to characterize the surface morphology and
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crystalline structure of the deposited films. Spectroscopic ellipsometry and a self-made apparatus were used to measure the optical properties and secondary electron emission coefficients of MgO films. A pronounced hysteresis phenomenon of target voltage, current, and deposition rate with increasing and decreasing O2 flow rate can be clearly observed. The surface morphology and crystalline structure are strongly dependent on the O2 flow rate. As the target operates at a metallic mode, the deposited films are metallic and have a very high surface roughness. The films deposited at oxide mode are MgO films with high transparence and small surface roughness. The refractive index and extinction coefficient at l ¼ 670 nm for MgO films deposited at oxide mode with a O2 flow rate of 3 sccm are 1.698 and 1:16 104 , respectively. The secondary emission coefficient, measured in He gas, of the MgO films deposited at a O2 flow rate of 3 sccm increases gradually from 0.08 to 0.16 with increasing E/p from 28 to 57.8 V/ (cm Torr), which is higher than that of the MgO films deposited by e-beam evaporation. Acknowledgements This work was supported by the German Bundeministerium Fu¨ r Bildurg und Forschung under grant no. 13N8053. The authors would like to thank Dr. H. Giegengack for the XRD measurement. References [1] P.C.L. Pfeil, L.B. Griffiths, Nature 183 (1959) 1481. [2] P.J. Kelly, R.D. Arnell, Vacuum 56 (2000) 159. [3] G. Brauer, J. Szczyrbowski, G. Teschner, J. Non-Cryst. Solids 218 (1997) 19. [4] R.K. Waits, Planar magnetron sputtering, in: J.L. Vossen, W. Kern (Ed.), Thin Film Processes, Vol. 1, Academic Press, Orlando, FL, 1978. [5] A. Belkind, A. Freilich, R. Scholl, J. Vac. Sci. Technol. A 17 (1999) 1934. [6] G. Brauer, M. Ruske, J. Szczyrbowski, G. Teschner, A. Zmelty, Vacuum 51 (4) (1998) 655. [7] M. Ruske, G. Brauer, J. Pistner, J. Szczyrbowski, A. Zmelty, Mater. Sci. Form 287/288 (1998) 247. [8] J.-Y. Kim, M.C. Nielsen, E.J. Rymaszewski, T.-M. Lu, J. Appl. Phys. 87 (3) (2000) 1448. [9] K. Goedicke, J.-S. Liebig, O. Zywitzki, H. Sahm, Thin Solid Films 377/378 (2000) 37.
[10] O. Zywitzki, G. Hoetzsch, F. Fietzke, K. Goedicke, Surf. Coat. Technol. 82 (1996) 169. [11] P.J. Kelly, P.S. Henderson, R.D. Arnell, G.A. Roche, D. Carter, J. Vac. Sci. Technol. A 18 (2000) 2890. [12] J. Szczyrbowski, G. Brauer, M. Ruske, G. Teschner, A. Zmelty, J. Non-Cryst. Solids 218 (1997) 262. [13] B. Szyszka, F. Simon, M. Siegesmund, S. Jager, in: Proceedings of the 1st International Conference on Coatings on Glass (ICCG), Saarbrucken, 27–31 October 1996. [14] David A. Glocker, J. Vac. Sci. Technol. A 11 (6) (1993) 2989 [15] E.D. Palik, Handbook of optical constants of Solids II, Academic Press/Harcourt, Brace, and Jovanovich, New York, USA, 1991. [16] T. Urade, T. Imemori, M. Osawa, N. Nakayama, I. Morita, IEEE Trans. Electron. Devices 23 (1976) 313. [17] O. Sahni, M.O. Aboelfotoh, Proc. Soc. Inform. Display 22 (1981) 219. [18] Kyoung Sup Moon, Jihwa Lee, Ki-Woong Whang, J. Appl. Phys. 86 (7) (1999) 4049. [19] Eun-Ha Choi, Hyun-Joo Oh, Young-Guon Kim, Jae-Jun Ko, Jae-Yong Lim, Jin-Goo Kim, Dae-Il Kim, Guang-Sup Cho, Seung-Oun Kang, Jpn. J. Appl. Phys. 37 (1998) 7015. [20] Jin-Man Jeoung, Jae-Yong Lim, Young-Guon Kim, Yoonho Seo, Guang-Sup Cho, Eun-Ha Choi, Jpn. J. Appl. Phys. 40 (2001) 1433. [21] Choongyong Son, Jinhui Cho, Jong-Wan Park, J. Vac. Sci. Technol. A 17 (5) (1999) 2619. [22] Soon Joon Rho, Soon Moon Jeong, Hong Koo Baik, Kie Moon Song, Thin Solid Films 355/356 (1999) 55. [23] Jong-Gul Yoon, Hun Kyoo Oh, Sung Jong Lee, Phys. Rev. B 60 (4) (1999) 2839. [24] G. Auday, Ph. Guillot, J. Galy, H. Brunet, J. Appl. Phys. 83 (11) (1998) 5917. [25] G. Auday, Ph. Guillot, J. Galy, J. Appl. Phys. 88 (8) (2000) 4871. [26] W.L. Morgan, J.-P. Boeuf, L.C. Pitchford, The Siglo Data base, CPAT and Kinema Software, 1982, http://www.csn.net/ siglo. [27] Y. Matsuda, K. Otomo, H. Fujiyama, Thin Solid Films 390 (2001) 59. [28] T. Lange, W. Njoroge, H. Weis, M. Beckers, M. Wuttig, Thin Solid Films 365 (2000) 82. [29] Chung-Hoo Park, Young-Kee Kim, Sung-Hyun Lee, WooGeun Lee, Youl-Moon Sung, Thin Solid Films 366 (2000) 88. [30] Jinhui Cho, Jong-Wan Park, J. Vac. Sci. Technol. A 18 (2) (2000) 329. [31] D.V. McCaughan, R.A. Kushner, V.T. Murphy, Phys. Rev. Lett. 30 (13) (1973) 614. [32] M.O. Aboelfoton, J.A. Lorenzen, J. Appl. Phys. 48 (11) (1977) 4754. [33] E.S. Parilis, L.M. Kishinevsky, Atomic Collisions on Solid Surfaces, North-Holland, Amsterdam, 1993. [34] S.J. Yoon, I. Lee, J.-W. Lee, B. Oh, Jpn. J. Appl. Phys. Part 1 40 (2001) 809. [35] Y. Motoyama, H. Matsuzaki, H. Murakami, IEEE Trans. Electron. Devices 48 (8) (2001) 1568.
Deposition of MgO films by pulsed mid-frequency magnetron sputtering Y.H. Cheng*,1, H. Kupfer, F. Richter, Andreea Maria Paraian Institut fu¨r Physik, Technische Universita¨t Chemnitz, 09107 Chemnitz, Germany Received 26 April 2002; received in revised form 15 June 2002; accepted 15 June 2002
Abstract MgO films were deposited by pulsed mid-frequency magnetron sputtering from metallic targets in the mixture of Ar and O2 gas. The surface morphology, crystalline structure, and optical properties were characterized by using atomic force microscopy (AFM), X-ray diffraction (XRD), and spectroscopic ellipsometry, respectively. The secondary electron emission coefficients of MgO films were measured by using a self-made apparatus in He gas. A pronounced hysteresis phenomenon of target voltage, current, and deposition rate with increasing and decreasing O2 flow rate was observed. The structure of films deposited at a metallic mode changes from Mg phase to the mixed Mg and MgO phase, and the films have a very rough surface. All the films deposited at oxide mode have high transparency and smooth surface, and show (2 2 0) preferred orientation growth. The refractive index and extinction coefficient at a wavelength of 670 nm for MgO films deposited at oxide mode with a O2 flow rate of 3 sccm are 1.698 and 1:16 104 , respectively. The secondary emission coefficient at a E/p of 57.8 V/(cm Torr) for MgO films deposited at a O2 flow rate of 3 sccm is 0.16, which is higher than that of MgO films deposited by e-beam evaporation. # 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.C; 81.65.M; 79.20.H Keywords: MgO films; Pulsed mid-frequency magnetron sputtering; Secondary electron emission coefficient
1. Introduction The magnetron sputter method has been widely used in the industry for large area thin film deposition because of its very good coating uniformity and good film properties. For the production of conductive films (metallic, most of the nitride, and few oxide films), * Corresponding author. Tel.: þ49-371-531-3570; fax: þ49-371-531-3042. E-mail address: [email protected] (Y.H. Cheng). 1 On leave from School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 40074, PR China.
direct current (dc) magnetron sputtering is the best choice due to the high deposition rate and good uniformity. However, for the deposition of dielectric films (most of the oxide and few nitride films), dc magnetron sputtering has suffered from the serious drawback of arcing, which results from the charging up of the dielectric films formed on the target surface during sputtering [1]. Arcing may result in the degradation of the film properties and instabilities of the deposition process [2]. Although rf sputtering is an alternative method for sputtering of dielectric films, it is limited by low deposition rates, high equipment costs (power supply and matching network), and scaling problems [3,4].
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 1 8 - 9
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To solve the problem of arcing, a pulsed midfrequency magnetron sputtering technique has been developed to deposit insulating films [6–12]. In this case, the polarity of the power supplied to the target oscillates between negative and positive potential. During the positive period, positive electric charges accumulated on the covered target areas during the negative period are neutralized by electrons before breakthrough occurs. As pulsing frequency is higher than a critical frequency, arcing could be avoided [5]. Highly insulating films, such as SiO2 [6], Si3N4 [7], Ta2O5 [8], ZrO2 [9], Al2O3 [10,11], and TiO2 [12] have been deposited on large scale at rates exceeding 5 nm/s in a stable continuous process for more than 300 h by using pulsed mid-frequency magnetron sputtering [6]. Recent results have shown that the growing film is bombarded by a high flux of energetic ions during the deposition of films by pulsed magnetron sputtering. Szyszka et al. [13] studied the TwinMag deposition process and found that ion density at the substrate is 10 times higher than that in dc sputtering, and the ion energy distribution has a maximum at around 50 eV (5 eV for dc) and a high energetic tail up to values of 200 eV. Glocker [14] also observed that the ion densities in the ac plasmas are several times greater than the ion densities in similar dc plasmas, which was attributed to the target voltage spikes as the plasma re-ignites on each half-cycle, causing rapid electron acceleration in the presheath region. The intense bombardment by ions with high energy results in the formation of high quality films with a denser structure, improved hardness, and excellent optical properties [6]. MgO is a highly insulating ionic material with the rock-salt structure. It has high physical strength and stability, excellent optical transparency throughout the range 0.25–6.8 mm [15], very low sputtering yield, very large band gap (7.3 eV), and large secondary electron emission (g) coefficient. Due to these unique properties, MgO films have been widely used as a surface protective layer for dielectric materials in alternating current plasma display panel (PDP) [16]. Typically, MgO films were prepared by e-beam evaporation [17–19]. However, MgO films deposited by evaporation have a porous columnar structure, causing low transparency, high erosion rate resulted from ion bombardment in the glow discharge [16], and low secondary electron emission resulted from
the adsorption of hydrogen-bonded hydroxyl groups along the columnar boundary [20]. In order to improve the properties, rf magnetron sputtering using a MgO target [21], ion beam assisted deposition (IBAD) [22], and aerosol-assisted chemical vapor deposition (AACVD) [23] have been used to deposited MgO films. In this paper, we report, for the first time, the deposition of MgO films from Mg target in mixed Ar and O2 atmosphere by pulsed mid-frequency magnetron sputtering. The structure and surface morphology of MgO films were characterized by X-ray diffraction and atomic force microscopy (AFM), respectively. The optical properties of MgO films were studied by spectroscopic ellipsometry, and the secondary electron emission coefficient of the films was measured using a self-made apparatus.
2. Experimental details MgO films were deposited by a commercial PLS 500P magnetron sputtering system (Balzers Ltd.) in mixed Ar and O2 atmosphere. A high purity Mg (99.99%) plate with the diameter of 100 mm and thickness of 8 mm was used as target, which was located at the bottom of the chamber in a sputter-up configuration. Highly conductive (0.005 O cm) n-type (1 0 0) Si wafer was used as substrate. The substrate holder was electrical isolated from the ground chamber, and no bias was applied to the substrate. The distance between target and substrate was 80 mm. The chamber was pumped down to a base pressure of 5 105 Pa by a turbomolecular pump with a rotary backing pump. During the deposition, the flow rate of Ar gas was fixed at 50 sccm, the flow rate of O2 gas can be varied from 0 to 50 sccm, and the total pressure was kept at 0.4 Pa by adjusting the position of the valve connecting to a turbomolecular pump. No intentional substrate heating was used in this experiment. Before deposition, the Mg target was presputtered with 100% Ar gas for 5 min for the films deposited at metallic mode, or with the mixture of 50%Ar and 50% O2 gas for 15 min for the films deposited at oxide mode. The magnetron operates in a pulsed regime. The mid-frequency power supply used in this study was a Pinnacle plus magnetron driver (Advanced Energy Inc.), which worked in a constant power mode.
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The pulsing frequency can be varied from 1 to 350 kHz with a 50% maximum reverse bias duty cycle, which is defined as the reverse time divided by the total time. The reverse voltage is fixed at 10% of the negative driving voltage. The pulsing frequency, reverse bias duty cycle, and power used in this experiment were set to 200 kHz, 40%, and 400 W, respectively. The surface morphology of MgO films was observed by atomic force microscopy in the contacting mode (Autoprobe from Park Scientific Instrument). The phase and crystal structure of MgO films were identified by using X-ray diffractometry (Seifert, Germany). A Cu Ka X-ray source (wave˚ ) was used at 40 kV and 20 mA. The length of 1.54 A thickness of MgO films was measured with a surface profilometer (Taylor–Hobson, Form Talysurf Series). The deposition rate was derived from the film thickness and deposition time. To investigate the optical properties, spectroscopic ellipsometric measurements were carried out on MgO films in the wavelength range between 300 and 1200 nm at various incident angles in the range 65–758 with a 58 increment by a variable angle spectroscopic ellipsometer VASE (J.A. Wollam Co. Inc.). The secondary electron emission coefficient was measured by a method proposed by Auday et al. [24,25]. Briefly, two plane-parallel electrodes: Cu electrode and sample with the size of 20 mm 20 mm, were placed in a vacuum chamber, which was evacuated to 3 104 Pa by using a turbomolecular pump. The pressure in the chamber can be adjusted by a pressure controller in the range 0.5– 100 Torr. The sample and Cu electrode were connected to cathode and anode of the dc power supply, respectively. The distance between the two electrode was fixed at 3 mm. Helium was used as working gas. The electrical measurements were made with a digital source-meter (Keithley model 2410 series). By using the sweep mode of the power supply, the voltage between the two electrode was steadily increased from 0 to 500 V, and the current was recorded. It can be observed that the current remains almost zero below a certain voltage (Vb), then increases suddenly to a certain value (1 nA). The voltage Vb is defined as breakdown voltage. Then, the secondary electron emission (g) coefficient can be
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calculated from Townsend’s first ionization coefficient (a) by the equation: 1 eða=pÞpd 1 where p is the pressure inside the cell, d the distance between two electrodes. The value of a/p versus the reduced field E/p, which is defined as Vb/pd, can be obtained from [26].
g¼
3. Results and discussion The influence of increasing and decreasing O2 flow rate on the target voltage and current is shown in Fig. 1. A pronounced hysteresis can be seen from the figure. At a low O2 flow rate, the target works in a metallic mode, which corresponds to a high target voltage (240 V) and low target current (1.67 A). As the O2 flow rate is increased, the target is covered in part with MgO. The high secondary electron emission of MgO films on the target surface makes it possible to maintain a discharge at a reduced discharge voltage and increased current [27]. At a critical O2 flow rate of
Fig. 1. Influence of increasing and decreasing O2 flow rate on the target voltage and current.
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Fig. 2. Dependence of the deposition rate on the increasing and decreasing O2 flow rate.
5 sccm, the whole target surface is covered by the MgO films, which causes a sudden decrease and increase of target voltage and current to 111 V and 3.62 A, respectively. The target will work in a complete oxide mode. The further increase of O2 flow rate leads to an increase in the thickness of MgO layer on the target surface, which in turn, results in a slight increase and decrease in the target voltage and current, respectively [28]. The decrease of O2 flow rate from 50 sccm causes a reverse change in the target voltage and current. However, the critical O2 flow rate shifts to 2.5 sccm. The hysteresis can also be seen from the change of film deposition rate with O2 flow rate. Fig. 2 shows the influence of increasing and decreasing O2 flow rate on the film deposition rate. It can be seen that the deposition rate in the metallic mode is greatly affected by the O2 flow rate. The increase in the O2 flow rate from 0 to 5 sccm causes a significant decrease in the deposition rate from 380 to 182 nm/min. However, the increase of O2 flow rate from 5 to 5.5 sccm leads to a sudden decrease of the deposition rate to 6.4 nm/min. The further increase in the O2 flow rate to 50 sccm results in no significant change in the deposition rate. As the O2 flow rate is decreased from 50 to 2.5 sccm, the deposition rate remains almost constant. However, the decrease of O2 flow rate from 2.5 to 2 sccm leads to a sudden increase in the deposition rate to 367 nm/ min. The deposition rate is mainly influenced by the sputtering yield of target surface, ion flux density, ion bombarding energy, and species of impinging ions [26]. It is well known that the sputter yield of metallic target is much higher than that of oxide target. As
discussed earlier, the increase in the O2 flow rate results in an increase in the coverage area of MgO on the target surface, and therefore a decrease in the sputter rate of target, which corresponds to the great decrease in the deposition rate as O2 flow rate is increased from 0 to 5 sccm. As O2 flow rate is increased to above 5.5 sccm, most part of the target surface was covered by oxide layer, which contributes to the sudden drop in the deposition rate. The surface morphology of MgO films was observed by a atomic force microscopy. Fig. 3 shows the typical AFM images for films deposited at metallic mode with the O2 flow rate of (a) 0 sccm, and (b) 5 sccm, and at oxide mode with a O2 flow rate of (c) 2.5 sccm, and (d) 50 sccm. The surface morphology of the deposited films are greatly affected by the sputtering mode and O2 flow rate. Generally, the films deposited at metallic mode are very rough and have a very large grain size, while the films deposited at oxide mode have a very smooth surface with small grains. For the films deposited at metallic mode with a O2 flow rate of 0 sccm. Large grains with a size of 800 nm can be observed, the root mean square (RMS) surface roughness is 5.35 nm, and the films exhibit a metallic color. The increase of O2 flow rate to 5 sccm results in a great decrease in the grain size (100– 400 nm), but a great increase in the RMS surface roughness (29.6 nm). In this case, the films are black and porous, and have a very low strength and scratch resistance. For the films deposited at oxide mode with a O2 flow rate of 2.5 sccm, the surface is very smooth (with a RMS of 2.84 nm). Nanograins with a size of 50–200 nm can be observed. For the films deposited at a O2 flow rate of 50 sccm, the grains is very small (40–60 nm), which contributes to a much smaller RMS roughness (2.31 nm). Fig. 4 shows the X-ray diffraction patterns for MgO films deposited in the metallic mode with a O2 flow rate of (a) 0 sccm, and (b) 5.5 sccm, and in the oxide mode with a O2 flow rate of (c) 2.5 sccm, and (d) 50 sccm. It is interesting to note that only one strong and sharp peak at a 2y of 34.538 can be observed in the X-ray diffraction patterns for the films deposited at a O2 flow rate of 0 sccm. This peak corresponds to the (0 0 2) plane of hexagonal Mg (JCPDS 35-821), which indicates that the films are Mg metallic films. The appearance of single peak indicates that the Mg films exhibit a (0 0 2) preferred orientation growth.
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Fig. 3. AFM image of MgO films deposited in metallic mode at O2 flow rate of (a) 0 sccm, and (b) 5 sccm, and in oxide mode at (c) 2.5 sccm, and (d) 50 sccm.
For the films deposited at a O2 flow rate of 5 sccm, the peak at a 2y of 34.538 can also be observed, indicating the existence of hexagonal metallic Mg phase in the films. However, a remarkable reduction in the intensity can be observed. In addition, one weak and broad peak at 2y of 42.88 can also be seen. The two peaks correspond to the (2 0 0) plane of cubic MgO (JCPDS 4-829), indicating the coexistence of cubic MgO face in the films. For the films deposited in oxide mode
with a O2 flow rate of 2.5 sccm, two weak peaks at 42.8 and 62.258 can be observed. These two peaks correspond to the (2 0 0) and (2 2 0) plane of cubic MgO phase, which indicates that the films are composed of single cubic MgO phase. The relative higher intensity of the peak at 62.258 indicates that MgO films show (2 2 0) preferred orientation growth. This is the same as that of MgO films deposited by rf magnetron sputtering, which also shows a strong
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The optical properties of MgO films were studied by spectroscopic ellipsometry. The obtained data were fitted by using a Cauchy mode. Within this model, the index of refraction (n) and extinction coefficient (k) are represented by a slowly varying function of wavelength (l), and an exponential absorption tail, respectively: nðlÞ ¼ An þ
Bn Cn þ l2 l4
kðlÞ ¼ Ak eBk ð12;400ð1=l1=Eg ÞÞ
Fig. 4. X-ray diffraction patterns of MgO films deposited in metallic mode at O2 flow rate of (a) 0 sccm, and (b) 5 sccm, and in oxide mode of (c) 2.5 sccm, and (d) 50 sccm.
(2 2 0) preferred orientation growth [29]. For the films deposited at 50 sccm, only one weak and broad peak at 62.258 can be observed. The weak feature indicates the small crystalline size, which fits well with the AFM results.
the six parameters in this dispersion model are An, Bn, Cn, the extinction coefficient amplitude (Ak), the exponent factor (Bk), and the absorption band gap (Eg). Due to the high transparency of MgO films in the UV–VIS range, the absorption band edge was chosen outside the measured spectral domain. Fig. 5 shows the typical measured and fitted spectra at the incident angle of 65, 70 and 758 for MgO films deposited at a O2 flow rate of 3 sccm. It is clear from the figure that the fitted spectra fit well with the measured spectra. The fitted refractive index and extinction coefficient as a function of the wavelength of incident light for MgO films deposited at a O2 flow rate of 3 sccm are shown in Fig. 6. As can be seen, the refractive index of MgO films decreases gradually from 1.76 to 1.69 as the wavelength is increased from
Fig. 5. Typical measured and fitted spectra at the incident angle of 65, 70 and 758 for MgO films deposited at a O2 flow rate of 3 sccm.
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Fig. 6. Typical refractive index and extinction coefficient as a function of the wavelength of incident light for MgO films deposited at a O2 flow rate of 3 sccm.
310 to 1200 nm. The refractive index at a wavelength of 670 nm for MgO films deposited at 3 sccm is 1.698, which is higher than that of MgO films (1.634) deposited by evaporation [30]. This indicates the higher density for the films deposited by pulsed mid-frequency magnetron sputtering. Meanwhile, it can also observed that the extinction coefficient of MgO films remains constant at a value smaller than the detection limit of the ellipsometer at a wavelength above 400 nm, and the decrease of wavelength to 310 nm results in a steep increase in the extinction coefficient to 0.008. This indicates the high transparency of the deposited MgO films in the visible wavelength range. Fig. 7 shows the typical secondary electron emission (g) coefficient versus E/p for MgO films deposited at a O2 flow rate of 3 sccm. It can be seen from the figure that g coefficient of MgO films increases
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gradually from 0.08 to 0.16 with increasing E/p from 28 to 57.8 V/(cm Torr). However, the further increase of E/p to 86 V/(cm Torr) results in a significant increase of g coefficient to 0.92. It is well known that potential emission and kinetic ejection are the two main mechanisms for secondary electron emission from a surface [31–33]. Potential emission occurs via Auger neutralization of the incident ion involving two valence electrons in the solid. In the present case, the discharge gas and cathodic material were the same, and only the discharge pressure and voltage were varied. The changes resulted from the potential emission should be negligible. However, due to the high gas pressure, some of the emitted secondary electrons are back-scattered without exciting or ionizing neutral gas atoms [34]. This part of secondary electrons will not contribute to the breakdown voltage, and therefore the measured g coefficient. The increase in the E/p will lead to an increase in the energy of the emitted secondary electrons, increasing the probability of ionization or excitation of the neutral atom, which in turn, leads to a slight increase of the measured g coefficient with increasing E/p. The large increase in the g coefficient at a E/p above 57.8 V/(cm Torr) may result from the kinetic ejection mechanism of the secondary electron emission, i.e. the direct emission of secondary electron by the bombardment of energetic ions. This is consistent with Moon et al. reports [18]. The measured g coefficient of MgO films in the flat region as shown in Fig. 7 is consistent with the value reported in the literature measured by different methods as summarized by Motoyama et al [35]. However, the measured g coefficient of MgO films at a E/p of 57.8 V/(cm Torr) is higher than that (0.13) of the MgO films deposited by evaporation at a E/p of 60 V/(cm Torr) in He gas measured by using the same method [25]. This may indicate that the MgO films deposited by pulsed mid-frequency magnetron sputtering have better secondary electron emission properties than that of the MgO films deposited by evaporation.
4. Conclusion
Fig. 7. Typical secondary electron emission (g) coefficient vs. E/p for MgO films deposited at a O2 flow rate 3 sccm.
In this paper, MgO films were deposited by pulsed mid-frequency magnetron sputtering. AFM and XRD were used to characterize the surface morphology and
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crystalline structure of the deposited films. Spectroscopic ellipsometry and a self-made apparatus were used to measure the optical properties and secondary electron emission coefficients of MgO films. A pronounced hysteresis phenomenon of target voltage, current, and deposition rate with increasing and decreasing O2 flow rate can be clearly observed. The surface morphology and crystalline structure are strongly dependent on the O2 flow rate. As the target operates at a metallic mode, the deposited films are metallic and have a very high surface roughness. The films deposited at oxide mode are MgO films with high transparence and small surface roughness. The refractive index and extinction coefficient at l ¼ 670 nm for MgO films deposited at oxide mode with a O2 flow rate of 3 sccm are 1.698 and 1:16 104 , respectively. The secondary emission coefficient, measured in He gas, of the MgO films deposited at a O2 flow rate of 3 sccm increases gradually from 0.08 to 0.16 with increasing E/p from 28 to 57.8 V/ (cm Torr), which is higher than that of the MgO films deposited by e-beam evaporation. Acknowledgements This work was supported by the German Bundeministerium Fu¨ r Bildurg und Forschung under grant no. 13N8053. The authors would like to thank Dr. H. Giegengack for the XRD measurement. References [1] P.C.L. Pfeil, L.B. Griffiths, Nature 183 (1959) 1481. [2] P.J. Kelly, R.D. Arnell, Vacuum 56 (2000) 159. [3] G. Brauer, J. Szczyrbowski, G. Teschner, J. Non-Cryst. Solids 218 (1997) 19. [4] R.K. Waits, Planar magnetron sputtering, in: J.L. Vossen, W. Kern (Ed.), Thin Film Processes, Vol. 1, Academic Press, Orlando, FL, 1978. [5] A. Belkind, A. Freilich, R. Scholl, J. Vac. Sci. Technol. A 17 (1999) 1934. [6] G. Brauer, M. Ruske, J. Szczyrbowski, G. Teschner, A. Zmelty, Vacuum 51 (4) (1998) 655. [7] M. Ruske, G. Brauer, J. Pistner, J. Szczyrbowski, A. Zmelty, Mater. Sci. Form 287/288 (1998) 247. [8] J.-Y. Kim, M.C. Nielsen, E.J. Rymaszewski, T.-M. Lu, J. Appl. Phys. 87 (3) (2000) 1448. [9] K. Goedicke, J.-S. Liebig, O. Zywitzki, H. Sahm, Thin Solid Films 377/378 (2000) 37.
[10] O. Zywitzki, G. Hoetzsch, F. Fietzke, K. Goedicke, Surf. Coat. Technol. 82 (1996) 169. [11] P.J. Kelly, P.S. Henderson, R.D. Arnell, G.A. Roche, D. Carter, J. Vac. Sci. Technol. A 18 (2000) 2890. [12] J. Szczyrbowski, G. Brauer, M. Ruske, G. Teschner, A. Zmelty, J. Non-Cryst. Solids 218 (1997) 262. [13] B. Szyszka, F. Simon, M. Siegesmund, S. Jager, in: Proceedings of the 1st International Conference on Coatings on Glass (ICCG), Saarbrucken, 27–31 October 1996. [14] David A. Glocker, J. Vac. Sci. Technol. A 11 (6) (1993) 2989 [15] E.D. Palik, Handbook of optical constants of Solids II, Academic Press/Harcourt, Brace, and Jovanovich, New York, USA, 1991. [16] T. Urade, T. Imemori, M. Osawa, N. Nakayama, I. Morita, IEEE Trans. Electron. Devices 23 (1976) 313. [17] O. Sahni, M.O. Aboelfotoh, Proc. Soc. Inform. Display 22 (1981) 219. [18] Kyoung Sup Moon, Jihwa Lee, Ki-Woong Whang, J. Appl. Phys. 86 (7) (1999) 4049. [19] Eun-Ha Choi, Hyun-Joo Oh, Young-Guon Kim, Jae-Jun Ko, Jae-Yong Lim, Jin-Goo Kim, Dae-Il Kim, Guang-Sup Cho, Seung-Oun Kang, Jpn. J. Appl. Phys. 37 (1998) 7015. [20] Jin-Man Jeoung, Jae-Yong Lim, Young-Guon Kim, Yoonho Seo, Guang-Sup Cho, Eun-Ha Choi, Jpn. J. Appl. Phys. 40 (2001) 1433. [21] Choongyong Son, Jinhui Cho, Jong-Wan Park, J. Vac. Sci. Technol. A 17 (5) (1999) 2619. [22] Soon Joon Rho, Soon Moon Jeong, Hong Koo Baik, Kie Moon Song, Thin Solid Films 355/356 (1999) 55. [23] Jong-Gul Yoon, Hun Kyoo Oh, Sung Jong Lee, Phys. Rev. B 60 (4) (1999) 2839. [24] G. Auday, Ph. Guillot, J. Galy, H. Brunet, J. Appl. Phys. 83 (11) (1998) 5917. [25] G. Auday, Ph. Guillot, J. Galy, J. Appl. Phys. 88 (8) (2000) 4871. [26] W.L. Morgan, J.-P. Boeuf, L.C. Pitchford, The Siglo Data base, CPAT and Kinema Software, 1982, http://www.csn.net/ siglo. [27] Y. Matsuda, K. Otomo, H. Fujiyama, Thin Solid Films 390 (2001) 59. [28] T. Lange, W. Njoroge, H. Weis, M. Beckers, M. Wuttig, Thin Solid Films 365 (2000) 82. [29] Chung-Hoo Park, Young-Kee Kim, Sung-Hyun Lee, WooGeun Lee, Youl-Moon Sung, Thin Solid Films 366 (2000) 88. [30] Jinhui Cho, Jong-Wan Park, J. Vac. Sci. Technol. A 18 (2) (2000) 329. [31] D.V. McCaughan, R.A. Kushner, V.T. Murphy, Phys. Rev. Lett. 30 (13) (1973) 614. [32] M.O. Aboelfoton, J.A. Lorenzen, J. Appl. Phys. 48 (11) (1977) 4754. [33] E.S. Parilis, L.M. Kishinevsky, Atomic Collisions on Solid Surfaces, North-Holland, Amsterdam, 1993. [34] S.J. Yoon, I. Lee, J.-W. Lee, B. Oh, Jpn. J. Appl. Phys. Part 1 40 (2001) 809. [35] Y. Motoyama, H. Matsuzaki, H. Murakami, IEEE Trans. Electron. Devices 48 (8) (2001) 1568.