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DC magnetron sputtering of Si to form SiO2 in low-energy ion beam
ARTICLE IN PRESS
Vacuum 80 (2006) 693–697 www.elsevier.com/locate/vacuum
DC magnetron sputtering of Si to form SiO2 in low-energy ion beam Cheng-Chung Leea,, Der-Jun Jana,b a
Thin film Technology Center, Institute of Optical Sciences, National Central University, Chung-Li 320, Taiwan b Plasma Technology Promotion Center, Institute of Nuclear Energy Research, Logtan, Taoyuan 325, Taiwan
Abstract Silicon dioxide (SiO2) films were deposited by magnetron sputtering and ion-beam oxidation (IBO) in separate zones at ambient temperature. The optical and structural characteristics of the films were analyzed by spectrophotometry, Fourier transform infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscope. The oxygen ratio in the ion beam and the energy of ion bombardment during deposition has strong influence on the optical and physical properties of SiO2 films. The experimental results indicated that the IBO method could finely manipulate the structure and properties of the growing films. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ion beam; Magnetron sputtering; Optical properties
1. Introduction Thin films of silicon dioxide (SiO2) are by far the most common low refractive index layers used in optical coatings. The conventional deposition method to deposit high-quality SiO2 films is electron beam evaporation with ion assistance [1–3]. The ion-assisted deposition (IAD) produces dense, environmentally stable thin films. However, the high melting point of metal oxides results in high heat radiation on the substrate emitted by the electron beam gun [4]. This thermal load on the substrate is particularly serious when the multilayer depositions of functional coatings are required. It is a process problem for optical coatings on temperature-sensitive substrates, such as polymethyl methacrylate and polyethlene terephthalate. In addition, the IAD method is not easily scaled to coat a large area due to the size limitation of the ion source. Magnetron sputtering is a low-temperature deposition process and easy to scale up. However, reactive magnetron sputtering to deposit metal oxides causes arcing and plasma instability, especially for the SiO2 (an insulator with high dielectric constant). Radio frequency sputtering can overcome this problem, but the low deposition rate and Corresponding author. Tel.: +886 3 4263918; fax: +886 3 4252897.
E-mail address: [email protected] (C.-C. Lee). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.026
high equipment costs make it difficult to use in mass production. Moreover, magnetron sputtering is performed at relatively high pressures, so the film properties are not very reproducible and the films are formed with gaseous impurity. The film quality of magnetron sputtering must be improved. Several methods have been studied to improve magnetron sputtering in the depositions of metal-oxide films [5,6]. One of these methods is to deposit a thin metal film and then to oxidize this film with ion bombardment to form the metal oxide in separate zones [7–9] of the deposition chamber. The combining of DC magnetron sputtering and ion assistance produces high-quality oxide films at high deposition rates. Although much research has been devoted to deposit oxide film using magnetron sputtering with ion assistance, little work has been published on full characterization of metal oxides by this method. The present paper reports an ion-beam oxidation (IBO) technique applied during the deposition of a SiO2 film by DC magnetron sputtering at room temperature. The Si thin film was deposited and then oxidized to form SiO2 in separate zones. Argon gas was introduced near the target and oxygen gas was introduced into an End–Hall gridless ion source to bombard the substrate. The effects of ion energy on the optical properties, structure and morphology of the deposited films were investigated.
ARTICLE IN PRESS 694
C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
2. Method
4.2 Suboxide
3. Results and discussion Deposition rate is important in film thickness control, especially for throughput of the optical coatings, and is a strong function of the oxygen partial pressure in reactive magnetron sputtering [11]. Fig. 1 shows the deposition rate of the deposited films as a function of oxygen gas ratio in the ion source at constant working parameters and a constant sputtering power of 600 W. The deposition rate increased slowly with increasing oxygen gas ratio at the
4.0 Dep. Rate (A/s)
The SiO2 films were deposited in a cryo-pumped vacuum chamber by DC magnetron sputtering, followed by oxygen ion bombardment, which has been described elsewhere in detail [10]. In brief, the deposition system was a box coater equipped with a 6-in. Si target, an End–Hall gridless ion source and a plasma-bridge neutralizer. The deposition zone of Si films and the oxidation zone of ion bombardment were separated by baffles. Ar or Ar/O2 gas mixture was admitted 20 mm above the target through a gas ring and O2 was introduced directly into a discharge chamber of the ion source. The magnetron sputter source was supplied by a constant DC power of 600 W connected with a pulse generator Sparc-LE. The anode voltage Va of the ion source varied from 50 to 100 V, and the current Ia of the ion source varied from 0.5 to 4.7 A in constant process pressure. B270 glass and a silicon wafer were used as substrates. The target-to-substrate distance was fixed at 8 cm and the substrates were held at the ambient temperature through the process. The deposition was performed by placing substrates on a substrate holder that was rotated around the central axis of the chamber at 300 rpm. As the substrate passed by the magnetron sputter source, a thin Si layer was thus deposited, and then passed by the ion source where oxygen ions were directed toward the substrates to oxidize the Si layer. This process continued until the desired thickness was achieved. In conventional reactive magnetron sputtering, an Ar/O2 gas mixture was directly introduced into the chamber without bombarding the substrates with ions. An thin film metrology system (SCI’s FilmTek 2000) was used to measure the refractive indices, extinction coefficients and thickness of the films. Films deposited on a twoside-polished Si wafer were used to obtain the infrared absorption spectra by a Fourier transform infrared (FTIR) spectrometer (BRUKER, VECTOR 33). X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical bonding of the films. XPS spectra were obtained using a Thermo VG-Scientific product model sigma probe system with an Al anode. The binding energies of silicon oxide on the Si 2p line were investigated. The scanning range was 90–110 eV with an interval of 0.5 eV, and a pass energy of 50 eV and 50 scans. An atomic force microscope (AFM) of Digital Instruments Nanoscope III was used to measure the surface morphology of the deposited films.
Oxide
Va = 60 V Ia = 2.4 A Power = 600 W
3.8
3.6
3.4
3.2 65
70
75 80 Oxygen Ratio (%)
85
90
Fig. 1. Deposition rate of the deposited films as a function of oxygen gas ratio in ion beam.
beginning. The deposition rate reached to a maximum at the oxygen gas ratio of 82%, and then decreased as oxygen gas ratio further increased. There was a drastic decrease at 87%. The result revealed that the deposition zone of metal film and the reaction zone of ion bombardment were not completely isolated, although they were separated by baffles. Oxygen gas diffused from the reaction zone to the deposition zone and affected the state of the metal target. When the oxygen gas ratio was lower than 82%, the target was in the metallic mode. The silicon flux to the substrate was constant and the thin film thickness increased with the adding of oxygen atoms from ion bombardment. Between 82% and 87%, the target was in the transition mode and the deposition rate decreased slowly. With the oxygen gas ratio more than 87% the target surface was completely poisoned, i.e., in the oxide mode. A large number of secondary electrons were emitted from the topmost of the oxide on the surface of the target, and the sputtering yield was reduced, which resulted in the abrupt change in the deposition rate and the target voltage (not shown). The variation in the optical constants of the films over oxygen gas ratio is shown in Fig. 2. Apparently, full oxidation can only be achieved by oxygen ion bombardment up to an oxygen gas ratio of 82%. The optical constant remained nearly unchanged when more oxygen was added. For oxygen gas ratios lower than 82%, the oxygen ion flux from the ion source to the substrate was not enough to react with the thin metal film deposited in the deposition zone due to the very short bombardment period in high rotation speed. At oxygen gas ratios of more than 82%, the excess oxygen gas in the oxidation zone diffused to the deposition zone and reacted with the target surface to form an oxide layer which reduced the deposition rate. From Figs. 1 and 2, we can see that the process window of the oxygen gas in this study is broader than the conventional reactive magnetron sputtering [12].
ARTICLE IN PRESS C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
n
2.0 1.8 1.6 1.4
0.02
k Va = 60 V Ia = 2.4 A Power = 600 W @ 550 nm
65
70
0.01
0.00 75 80 Oxygen ratio
85
Si-O(R)
Si-O(B)
2.2
Absorbance ( a. u.)
0.03 Extinction coefficient
Refractive index (n)
Oxide
Oxygen deficiency
Si-O(S)
2.4 Suboxide
695
87%
82% 77%
90
66% 1400
Fig. 2. Optical constants of the films deposited by different oxygen gas ratio.
1200
1000 800 Wavenumber (cm-1)
600
400
Fig. 3. FTIR spectra of the films deposited at V a ¼ 60 V, I a ¼ 2:4 A of ion source and sputtering power of 600 W.
X5 Si-OH Absorbance (a. u.)
The IR absorption spectra of the deposited films for various oxygen gas ratios were measured using a FTIR from 400 to 4000 cm1. In the high-frequency region, no significant absorption peak of Si–OH bond at about 3600 cm1 was detected indicating no water was absorbed during and after deposition of the films. Fig. 3 illustrates the IR absorption spectra for the films deposited with various oxygen gas ratios in the range from 400 to 1500 cm1 to show the three characteristics IR bands of the Si–O–Si group [13]: 450 cm1 for the Si–O rocking mode, 800 cm1 for the Si–O bending mode, and 1050 cm1 for the Si–O stretching mode. The position of the strongest absorption peak varied from 1012 to 1050 cm1 and the full-width at half-maximum (FWHM) decreased as the oxygen gas ratio increased indicating that Si–Si bonds were replaced by Si–O bonds gradually. After reaching 82%, the peak position remained at the same frequency of 1050 cm1 which is the characteristic of SiO2 grown at room temperature [14]. Additionally, there is also an absorption band in the region of 850–950 cm1 for low oxygen gas ratios. As the oxygen gas ratio increased, this band gradually diminished, and disappeared as the oxygen gas ratio reached 82%. Chau et al. [15] had pointed out that this 850–950 cm1 band could be assigned to oxygen deficiency which is normally observed in pyrolytic silicon dioxide films. Thus, these results indicated that the oxygen ion bombardment from the ion source has promoted a phase change of the deposited film from a silicon rich to a more stoichiometric structure. Fig. 4 is a plot of the IR stretching frequency as a function of the anode voltage of the ion source. The IR stretching frequency increased from 1049 to 1062 cm1 as the anode voltage increased from 50 to 100 V while the FWHM of four IR spectra remained essentially constant. At first sight, we assumed that the upward shift in frequency was due to a more stoichiometric structure of the films from the higher energy bombardment of the oxygen ions. However, the concentration of oxygen in the
100 V 80 V 60 V 50 V
4000
3500
1200 Wavenumber (cm-1)
600
Fig. 4. FTIR spectra of the films deposited at different anode voltage of ion source.
films determined by Rutherford backscattering maintained a constant value and the index of refraction decreased as the anode voltage increased. In addition, we noted that the intensity of the absorption band near 3650 cm1 due to hydrogen-bonded hydroxyl groups and absorbed water increased when the anode voltage increased, as shown in Fig. 4. The increase of the anode voltage seems to increase the porosity of the films. Lucovsky et al. [13] presented a microscopic model for the low-temperature growth of SiO2 films, where the increase in the index of refraction accompanying the decrease in the IR stretching frequency is correlated with the increase in the film density. Our results agree well with the findings of Lucovsky et al. Thus, we concluded that the higher energy of ion bombardment resulted in the increase of the vacancies and the interstitials at the growing surface by radiation damage because every
ARTICLE IN PRESS C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
round of Si deposition was a very thin layer. The defects led to a lower packing density that simultaneously brought the index of refraction down [16]. Fig. 5(a) contains the XPS spectra showing the progressive oxidation of magnetron-sputtered Si films under 60 eV ion bombardment as a function of the oxygen gas ratio in the ion beam. As the oxygen gas ratio increased, the Si 2p photoelectron signal of the deposited Si film without ion bombardment at 99.3 eV decreased while the signal at higher binding energy (103.6 eV) appeared and increased gradually due to the formation of stoichiometric SiO2. At the lower oxygen ratio, Si suboxides were formed, as seen in the XPS spectrum. But as the oxygen gas ratio increased to 82%, the suboxides disappeared completely and only stoichiometric SiO2 films were formed. Deconvoluting the low-ratio XPS spectrum with Gaussian line shapes, based on the data from Himpsel et al. [17], gives the oxidation states of the sputtered films after oxygen ion bombardment, as shown in Fig. 5(b). The suboxide states are denoted as Si1+, Si2+, and Si3+ state, respectively. It appeared that the non-stoichiometric silicon oxide, i.e., SiOx (xo2), was not a simple mixture of Si and SiO2, as predicted by the random-bonding model [18]. In the deposition zone, the amorphous Si films were deposited on which each atom was surrounded by four other Si atoms as nearest neighbors. In the reaction zone, the oxygen ions bombarded the films to react with the Si atoms to form Si–O bonds. As more oxygen was added to the ion
Si 2p
108 (a)
104 100 96 Binding energy (eV)
92
0 ) 66 77 tio (% 82 a 87 nr ge y Ox
Counts
Si 2p
SiO2
Si3+
Si2+ Si1+
0.8
0.6 Roughness (nm)
696
0.4
0.2
0
20
40 60 Voltage of anode (V)
80
100
Fig. 6. Surface roughness of the deposited films as a function of the anode voltage.
beam, the Si–O bond replaced the Si–Si bond gradually due to atomic collisions [19]. Thus, the full oxidation can only be achieved by an oxygen gas ratio up to a critical value which had enough oxygen ions to react with the Si atoms simultaneously. It seems that it is possible to manipulate the properties of the deposited films by varying the oxygen gas ratio. Fig. 6 is the variation of surface roughness of the films deposited by conventional magnetron sputtering and IBO. It is clear that the surface morphology of IBO was smoother than that of conventional magnetron sputtering. The sputtered atoms of conventional magnetron sputtering at low temperature had low kinetic energy and were immobile on the substrate. Therefore, they developed a columnar structure. The bombardment of oxygen ions not only reacted with the adatoms on the substrate to form SiO2 but also increased their mobility. The columnar growth was disturbed and the growth mode changed from island growth to layer growth [20,21]. However, the surface morphology of SiO2 films deposited by IBO critically depends on the ion energy under increased ion flux. The anode voltage of 100 V gives the lowest surface roughness. The increase of anode voltage appears to reduce the surface roughness of the deposited films. It might be that the surface adatom mobility increased with increased ion energy, caused by the excitation of surface phonons by the ion impact or else the formation of very shallow collision cascades [22]. 4. Conclusions
Si
106
(b)
104 102 100 Binding Energy (eV)
98
Fig. 5. (a) XPS spectra of the Si 2p of the films as a function of oxygen gas ratio in ion beam, (b) curve fitting of the Si 2p signal of SiOx deposited at oxygen gas ratio of 66%.
This study investigated the optical and structural properties of SiO2 films deposited by DC magnetron sputtering with low-energy ion-beam oxidation at ambient temperature. Because the separation between the deposition zone and the reaction zone was not complete, the oxygen gas ratio in ion beam affected the state of the target surface. By keeping the target surface in the transition
ARTICLE IN PRESS C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
mode, we obtained the stoichiometric SiO2 films at high deposition rate. The ion-beam oxidation method appears to have a broader process window than that of traditional reactive sputtering. The FTIR results showed that low-temperature stoichiometric SiO2 films were obtained by the ion-beam oxidation method when the oxygen gas ratio was over a critical value. However, high ion energy bombardment from the ion source resulted in porosity and water absorption of the films due to the radiation damage. The XPS results confirmed the presence of three intermediate oxidation states, attributed to Si1+, Si2+, and Si3+, when the oxygen gas ratio was insufficient in the ion beam. The transition from Si film to stoichiometric SiO2 was gradual because the Si–O bonds substituted for the Si–Si bonds by atomic collisions. It appeared that the ion-beam oxidation method achieved fine control of the physical and optical properties on the films by permitting a large adjustability in the energy of the incident ions and the chemical reactivity, which essentially influenced the structure and properties of the growing film. The present authors are currently conducting experiments on oxygen-deficient oxide deposition as a precoated binding layer for multiplayer coatings on flexible polymer substrates.
Acknowledgment We gratefully appreciate the financial support of the National Science Council of Taiwan under Contract numbers NSC 93-2215-E-008-030 and NSC 92-2622-E008-001.
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References [1] Sainty WG, Netterfield RP, Martin PJ. Appl Opt 1984;23:1116–9. [2] Niederwald H, Laux S, Kennedy M, Schallenberg U, Duparre A, Mertin M, et al. Appl Opt 1999;38:3610–3. [3] Schulz U, Jakobs S, Kaiser N. In: Reid I, editor. Developments in optical component coatings. Proc SPIE 1996;2776:169–74. [4] Schulz U, Schallenberg UB, Kaiser N. Appl Opt 2002;41:3107–10. [5] Safi I. Surf Coat Technol 2000;127:203–18. [6] Musil J, Baroch P, Vlcek J, Nam KH, Han JG. Thin Solid Films 2005;475:208–18. [7] Chiba S, Motok A, Fujikura K, Hata T. Vacuum 2004;74:449–54. [8] Song Y, Sakurai T, Maruta K, Matusita A, Matsumoto S, Saisho S, et al. Vacuum 2000;59:755–63. [9] Lehan JP, Sargent RB, Klinger RE. J Vac Sci Technol A 1992;10:3401–6. [10] Lee CC, Jan DJ. Thin Solid Films 2005;486:130–5. [11] Wu WF, Chiou BS. Appl Surf Sci 1996;99:237–43. [12] Takamura K, Abe Y, Sasaki K. Vacuum 2004;74:397–401. [13] Lucovsky G, Manitini MJ, Srivastava JK, Irene EA. J Vac Sci Technol B 1987;5:530–7. [14] Rochet F, Dufour G, Roulet H, Pelloie B, Perriere J, Fogarassy E, et al. Phys Rev B 1988;37:6468–77. [15] Chau TT, Mejia SR, Kao KC. Mater Res Soc Symp Proc 1991;223:69–74. [16] Tang Q, Kikuchi K, Ogura S, Macleod A. J Vac Sci Technol A 1999;17:1–6. [17] Himpsel FJ, McFeely FR, Taleb-Ibrahimi A, Yarmoff JA. Phys Rev B 1988;38:6084–95. [18] Philipp HR. J Non-Cryst Solids 1972;8–10:627–32. [19] Herbots N, Ye P, Wang X, Vancauwenberghe O. In: Rabalais JW, editor. Low energy ion-surface interaction. Chichester: Wiley; 1994. p. 424. [20] Ensinger W. Nucl Instrum Meth B 1997;127/128:796–808. [21] Koponen IT. Nucl Instrum Meth B 2000;171:314–24. [22] Kay E, Rossnagel SM. In: Cuomo JJ, Rossnagel SM, Kaufman HR, editors. Handbook of ion beam processing technology. Ridge: Noyes; 1989. p. 180.
Vacuum 80 (2006) 693–697 www.elsevier.com/locate/vacuum
DC magnetron sputtering of Si to form SiO2 in low-energy ion beam Cheng-Chung Leea,, Der-Jun Jana,b a
Thin film Technology Center, Institute of Optical Sciences, National Central University, Chung-Li 320, Taiwan b Plasma Technology Promotion Center, Institute of Nuclear Energy Research, Logtan, Taoyuan 325, Taiwan
Abstract Silicon dioxide (SiO2) films were deposited by magnetron sputtering and ion-beam oxidation (IBO) in separate zones at ambient temperature. The optical and structural characteristics of the films were analyzed by spectrophotometry, Fourier transform infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscope. The oxygen ratio in the ion beam and the energy of ion bombardment during deposition has strong influence on the optical and physical properties of SiO2 films. The experimental results indicated that the IBO method could finely manipulate the structure and properties of the growing films. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ion beam; Magnetron sputtering; Optical properties
1. Introduction Thin films of silicon dioxide (SiO2) are by far the most common low refractive index layers used in optical coatings. The conventional deposition method to deposit high-quality SiO2 films is electron beam evaporation with ion assistance [1–3]. The ion-assisted deposition (IAD) produces dense, environmentally stable thin films. However, the high melting point of metal oxides results in high heat radiation on the substrate emitted by the electron beam gun [4]. This thermal load on the substrate is particularly serious when the multilayer depositions of functional coatings are required. It is a process problem for optical coatings on temperature-sensitive substrates, such as polymethyl methacrylate and polyethlene terephthalate. In addition, the IAD method is not easily scaled to coat a large area due to the size limitation of the ion source. Magnetron sputtering is a low-temperature deposition process and easy to scale up. However, reactive magnetron sputtering to deposit metal oxides causes arcing and plasma instability, especially for the SiO2 (an insulator with high dielectric constant). Radio frequency sputtering can overcome this problem, but the low deposition rate and Corresponding author. Tel.: +886 3 4263918; fax: +886 3 4252897.
E-mail address: [email protected] (C.-C. Lee). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.026
high equipment costs make it difficult to use in mass production. Moreover, magnetron sputtering is performed at relatively high pressures, so the film properties are not very reproducible and the films are formed with gaseous impurity. The film quality of magnetron sputtering must be improved. Several methods have been studied to improve magnetron sputtering in the depositions of metal-oxide films [5,6]. One of these methods is to deposit a thin metal film and then to oxidize this film with ion bombardment to form the metal oxide in separate zones [7–9] of the deposition chamber. The combining of DC magnetron sputtering and ion assistance produces high-quality oxide films at high deposition rates. Although much research has been devoted to deposit oxide film using magnetron sputtering with ion assistance, little work has been published on full characterization of metal oxides by this method. The present paper reports an ion-beam oxidation (IBO) technique applied during the deposition of a SiO2 film by DC magnetron sputtering at room temperature. The Si thin film was deposited and then oxidized to form SiO2 in separate zones. Argon gas was introduced near the target and oxygen gas was introduced into an End–Hall gridless ion source to bombard the substrate. The effects of ion energy on the optical properties, structure and morphology of the deposited films were investigated.
ARTICLE IN PRESS 694
C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
2. Method
4.2 Suboxide
3. Results and discussion Deposition rate is important in film thickness control, especially for throughput of the optical coatings, and is a strong function of the oxygen partial pressure in reactive magnetron sputtering [11]. Fig. 1 shows the deposition rate of the deposited films as a function of oxygen gas ratio in the ion source at constant working parameters and a constant sputtering power of 600 W. The deposition rate increased slowly with increasing oxygen gas ratio at the
4.0 Dep. Rate (A/s)
The SiO2 films were deposited in a cryo-pumped vacuum chamber by DC magnetron sputtering, followed by oxygen ion bombardment, which has been described elsewhere in detail [10]. In brief, the deposition system was a box coater equipped with a 6-in. Si target, an End–Hall gridless ion source and a plasma-bridge neutralizer. The deposition zone of Si films and the oxidation zone of ion bombardment were separated by baffles. Ar or Ar/O2 gas mixture was admitted 20 mm above the target through a gas ring and O2 was introduced directly into a discharge chamber of the ion source. The magnetron sputter source was supplied by a constant DC power of 600 W connected with a pulse generator Sparc-LE. The anode voltage Va of the ion source varied from 50 to 100 V, and the current Ia of the ion source varied from 0.5 to 4.7 A in constant process pressure. B270 glass and a silicon wafer were used as substrates. The target-to-substrate distance was fixed at 8 cm and the substrates were held at the ambient temperature through the process. The deposition was performed by placing substrates on a substrate holder that was rotated around the central axis of the chamber at 300 rpm. As the substrate passed by the magnetron sputter source, a thin Si layer was thus deposited, and then passed by the ion source where oxygen ions were directed toward the substrates to oxidize the Si layer. This process continued until the desired thickness was achieved. In conventional reactive magnetron sputtering, an Ar/O2 gas mixture was directly introduced into the chamber without bombarding the substrates with ions. An thin film metrology system (SCI’s FilmTek 2000) was used to measure the refractive indices, extinction coefficients and thickness of the films. Films deposited on a twoside-polished Si wafer were used to obtain the infrared absorption spectra by a Fourier transform infrared (FTIR) spectrometer (BRUKER, VECTOR 33). X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical bonding of the films. XPS spectra were obtained using a Thermo VG-Scientific product model sigma probe system with an Al anode. The binding energies of silicon oxide on the Si 2p line were investigated. The scanning range was 90–110 eV with an interval of 0.5 eV, and a pass energy of 50 eV and 50 scans. An atomic force microscope (AFM) of Digital Instruments Nanoscope III was used to measure the surface morphology of the deposited films.
Oxide
Va = 60 V Ia = 2.4 A Power = 600 W
3.8
3.6
3.4
3.2 65
70
75 80 Oxygen Ratio (%)
85
90
Fig. 1. Deposition rate of the deposited films as a function of oxygen gas ratio in ion beam.
beginning. The deposition rate reached to a maximum at the oxygen gas ratio of 82%, and then decreased as oxygen gas ratio further increased. There was a drastic decrease at 87%. The result revealed that the deposition zone of metal film and the reaction zone of ion bombardment were not completely isolated, although they were separated by baffles. Oxygen gas diffused from the reaction zone to the deposition zone and affected the state of the metal target. When the oxygen gas ratio was lower than 82%, the target was in the metallic mode. The silicon flux to the substrate was constant and the thin film thickness increased with the adding of oxygen atoms from ion bombardment. Between 82% and 87%, the target was in the transition mode and the deposition rate decreased slowly. With the oxygen gas ratio more than 87% the target surface was completely poisoned, i.e., in the oxide mode. A large number of secondary electrons were emitted from the topmost of the oxide on the surface of the target, and the sputtering yield was reduced, which resulted in the abrupt change in the deposition rate and the target voltage (not shown). The variation in the optical constants of the films over oxygen gas ratio is shown in Fig. 2. Apparently, full oxidation can only be achieved by oxygen ion bombardment up to an oxygen gas ratio of 82%. The optical constant remained nearly unchanged when more oxygen was added. For oxygen gas ratios lower than 82%, the oxygen ion flux from the ion source to the substrate was not enough to react with the thin metal film deposited in the deposition zone due to the very short bombardment period in high rotation speed. At oxygen gas ratios of more than 82%, the excess oxygen gas in the oxidation zone diffused to the deposition zone and reacted with the target surface to form an oxide layer which reduced the deposition rate. From Figs. 1 and 2, we can see that the process window of the oxygen gas in this study is broader than the conventional reactive magnetron sputtering [12].
ARTICLE IN PRESS C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
n
2.0 1.8 1.6 1.4
0.02
k Va = 60 V Ia = 2.4 A Power = 600 W @ 550 nm
65
70
0.01
0.00 75 80 Oxygen ratio
85
Si-O(R)
Si-O(B)
2.2
Absorbance ( a. u.)
0.03 Extinction coefficient
Refractive index (n)
Oxide
Oxygen deficiency
Si-O(S)
2.4 Suboxide
695
87%
82% 77%
90
66% 1400
Fig. 2. Optical constants of the films deposited by different oxygen gas ratio.
1200
1000 800 Wavenumber (cm-1)
600
400
Fig. 3. FTIR spectra of the films deposited at V a ¼ 60 V, I a ¼ 2:4 A of ion source and sputtering power of 600 W.
X5 Si-OH Absorbance (a. u.)
The IR absorption spectra of the deposited films for various oxygen gas ratios were measured using a FTIR from 400 to 4000 cm1. In the high-frequency region, no significant absorption peak of Si–OH bond at about 3600 cm1 was detected indicating no water was absorbed during and after deposition of the films. Fig. 3 illustrates the IR absorption spectra for the films deposited with various oxygen gas ratios in the range from 400 to 1500 cm1 to show the three characteristics IR bands of the Si–O–Si group [13]: 450 cm1 for the Si–O rocking mode, 800 cm1 for the Si–O bending mode, and 1050 cm1 for the Si–O stretching mode. The position of the strongest absorption peak varied from 1012 to 1050 cm1 and the full-width at half-maximum (FWHM) decreased as the oxygen gas ratio increased indicating that Si–Si bonds were replaced by Si–O bonds gradually. After reaching 82%, the peak position remained at the same frequency of 1050 cm1 which is the characteristic of SiO2 grown at room temperature [14]. Additionally, there is also an absorption band in the region of 850–950 cm1 for low oxygen gas ratios. As the oxygen gas ratio increased, this band gradually diminished, and disappeared as the oxygen gas ratio reached 82%. Chau et al. [15] had pointed out that this 850–950 cm1 band could be assigned to oxygen deficiency which is normally observed in pyrolytic silicon dioxide films. Thus, these results indicated that the oxygen ion bombardment from the ion source has promoted a phase change of the deposited film from a silicon rich to a more stoichiometric structure. Fig. 4 is a plot of the IR stretching frequency as a function of the anode voltage of the ion source. The IR stretching frequency increased from 1049 to 1062 cm1 as the anode voltage increased from 50 to 100 V while the FWHM of four IR spectra remained essentially constant. At first sight, we assumed that the upward shift in frequency was due to a more stoichiometric structure of the films from the higher energy bombardment of the oxygen ions. However, the concentration of oxygen in the
100 V 80 V 60 V 50 V
4000
3500
1200 Wavenumber (cm-1)
600
Fig. 4. FTIR spectra of the films deposited at different anode voltage of ion source.
films determined by Rutherford backscattering maintained a constant value and the index of refraction decreased as the anode voltage increased. In addition, we noted that the intensity of the absorption band near 3650 cm1 due to hydrogen-bonded hydroxyl groups and absorbed water increased when the anode voltage increased, as shown in Fig. 4. The increase of the anode voltage seems to increase the porosity of the films. Lucovsky et al. [13] presented a microscopic model for the low-temperature growth of SiO2 films, where the increase in the index of refraction accompanying the decrease in the IR stretching frequency is correlated with the increase in the film density. Our results agree well with the findings of Lucovsky et al. Thus, we concluded that the higher energy of ion bombardment resulted in the increase of the vacancies and the interstitials at the growing surface by radiation damage because every
ARTICLE IN PRESS C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
round of Si deposition was a very thin layer. The defects led to a lower packing density that simultaneously brought the index of refraction down [16]. Fig. 5(a) contains the XPS spectra showing the progressive oxidation of magnetron-sputtered Si films under 60 eV ion bombardment as a function of the oxygen gas ratio in the ion beam. As the oxygen gas ratio increased, the Si 2p photoelectron signal of the deposited Si film without ion bombardment at 99.3 eV decreased while the signal at higher binding energy (103.6 eV) appeared and increased gradually due to the formation of stoichiometric SiO2. At the lower oxygen ratio, Si suboxides were formed, as seen in the XPS spectrum. But as the oxygen gas ratio increased to 82%, the suboxides disappeared completely and only stoichiometric SiO2 films were formed. Deconvoluting the low-ratio XPS spectrum with Gaussian line shapes, based on the data from Himpsel et al. [17], gives the oxidation states of the sputtered films after oxygen ion bombardment, as shown in Fig. 5(b). The suboxide states are denoted as Si1+, Si2+, and Si3+ state, respectively. It appeared that the non-stoichiometric silicon oxide, i.e., SiOx (xo2), was not a simple mixture of Si and SiO2, as predicted by the random-bonding model [18]. In the deposition zone, the amorphous Si films were deposited on which each atom was surrounded by four other Si atoms as nearest neighbors. In the reaction zone, the oxygen ions bombarded the films to react with the Si atoms to form Si–O bonds. As more oxygen was added to the ion
Si 2p
108 (a)
104 100 96 Binding energy (eV)
92
0 ) 66 77 tio (% 82 a 87 nr ge y Ox
Counts
Si 2p
SiO2
Si3+
Si2+ Si1+
0.8
0.6 Roughness (nm)
696
0.4
0.2
0
20
40 60 Voltage of anode (V)
80
100
Fig. 6. Surface roughness of the deposited films as a function of the anode voltage.
beam, the Si–O bond replaced the Si–Si bond gradually due to atomic collisions [19]. Thus, the full oxidation can only be achieved by an oxygen gas ratio up to a critical value which had enough oxygen ions to react with the Si atoms simultaneously. It seems that it is possible to manipulate the properties of the deposited films by varying the oxygen gas ratio. Fig. 6 is the variation of surface roughness of the films deposited by conventional magnetron sputtering and IBO. It is clear that the surface morphology of IBO was smoother than that of conventional magnetron sputtering. The sputtered atoms of conventional magnetron sputtering at low temperature had low kinetic energy and were immobile on the substrate. Therefore, they developed a columnar structure. The bombardment of oxygen ions not only reacted with the adatoms on the substrate to form SiO2 but also increased their mobility. The columnar growth was disturbed and the growth mode changed from island growth to layer growth [20,21]. However, the surface morphology of SiO2 films deposited by IBO critically depends on the ion energy under increased ion flux. The anode voltage of 100 V gives the lowest surface roughness. The increase of anode voltage appears to reduce the surface roughness of the deposited films. It might be that the surface adatom mobility increased with increased ion energy, caused by the excitation of surface phonons by the ion impact or else the formation of very shallow collision cascades [22]. 4. Conclusions
Si
106
(b)
104 102 100 Binding Energy (eV)
98
Fig. 5. (a) XPS spectra of the Si 2p of the films as a function of oxygen gas ratio in ion beam, (b) curve fitting of the Si 2p signal of SiOx deposited at oxygen gas ratio of 66%.
This study investigated the optical and structural properties of SiO2 films deposited by DC magnetron sputtering with low-energy ion-beam oxidation at ambient temperature. Because the separation between the deposition zone and the reaction zone was not complete, the oxygen gas ratio in ion beam affected the state of the target surface. By keeping the target surface in the transition
ARTICLE IN PRESS C.-C. Lee, D.-J. Jan / Vacuum 80 (2006) 693–697
mode, we obtained the stoichiometric SiO2 films at high deposition rate. The ion-beam oxidation method appears to have a broader process window than that of traditional reactive sputtering. The FTIR results showed that low-temperature stoichiometric SiO2 films were obtained by the ion-beam oxidation method when the oxygen gas ratio was over a critical value. However, high ion energy bombardment from the ion source resulted in porosity and water absorption of the films due to the radiation damage. The XPS results confirmed the presence of three intermediate oxidation states, attributed to Si1+, Si2+, and Si3+, when the oxygen gas ratio was insufficient in the ion beam. The transition from Si film to stoichiometric SiO2 was gradual because the Si–O bonds substituted for the Si–Si bonds by atomic collisions. It appeared that the ion-beam oxidation method achieved fine control of the physical and optical properties on the films by permitting a large adjustability in the energy of the incident ions and the chemical reactivity, which essentially influenced the structure and properties of the growing film. The present authors are currently conducting experiments on oxygen-deficient oxide deposition as a precoated binding layer for multiplayer coatings on flexible polymer substrates.
Acknowledgment We gratefully appreciate the financial support of the National Science Council of Taiwan under Contract numbers NSC 93-2215-E-008-030 and NSC 92-2622-E008-001.
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