- Email: [email protected]
Characterization and properties of r.f.-sputtered thin films of the alumina–titania system
Thin Solid Films 460 (2004) 327–334
Characterization and properties of r.f.-sputtered thin films of the alumina–titania system Dong-Hau Kuo*, Kuo-Hwa Tzeng Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien, Taiwan, ROC Received 20 August 2003; received in revised form 15 January 2004; accepted 12 February 2004 Available Online 12 April 2004
Abstract Thin films of the aluminum oxide (Al2O3)–titanium oxide (TiO2 ) system including Al2 O3 , TiO2 , and Al2O3 yTiO2 were prepared by radio-frequency (r.f.) magnetron sputtering using ceramic targets of Al2 O3 , TiO2 , and Al2O3 yTiO2 composites with different Al2O3 yTiO2 ratio. These films were studied at different substrate temperatures, r.f. powers, and annealing temperatures. Composition, microstructure, thermomechanical property of internal stress, and mechanical property of scratch adhesion, were evaluated. A thin film with a dielectric constant of 62 and a loss tangent of 0.012 was obtained at 500 8C from a 10y90 target. This thin film remained the high dielectric constant of TiO2 , but had an improvement in the dielectric loss tangent. Al2O3containing films had a higher resistivity and breakdown field, which was improved further by annealing. Optical properties, such as refractive index and optical transmittance, were also investigated. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Aluminum oxide; Titanium oxide; Mechanical property; Electrical properties and measurements; Optical properties
1. Introduction Aluminum oxide (alumina, Al2O3) as an insulator has several advantages for semiconductor device applications w1x. Alumina films are a better barrier to mobile ionic species w2x and have high chemical stability and radiation resistance w3x. It also has dielectric constant two folds higher than silica. Moreover, alumina films can be used for passivation of bipolar devices, as a diffusion mask, and as a buffer layer in silicon-oninsulator (SOI) devices for three-dimensional integrated circuits w4x. Titanium oxide (Titania, TiO2) thin films, with high refractive index, excellent transmittance in the visiblewavelength region, and high chemical stability, have been used in anti-reflection coating, sensors, and photocatalysis. Recently, TiO2 film has attracted attention for use in fabricating capacitors in dynamic random access memory (DRAM) and gate oxide in transistor, due to their higher dielectric constant up to 100 w5,6x. *Corresponding author. Tel.: q011-886-3-863-4208; fax: q011886-3-863-4200. E-mail address: [email protected] (D.-H. Kuo).
In this work, a systematic study of depositing thin films of alumina–titania (Al2O3 –TiO2 ) system by r.f. magnetron sputtering was proceeded by adjusting film properties through the Al2O3 y(Al2O3 qTiO2 ) ratio in sputtering targets. Thin films of the Al2O3 –TiO2 system included pure Al2O3 and TiO2 films, and thin films with different Al2O3 y(Al2O3qTiO2 ) ratios or the composite thin films. The effects of process parameters on deposition kinetics, composition, thermomechanical property of internal stress, mechanical property of scratch adhesion, dielectric properties of dielectric constant and loss tangent, electric properties of resistivity and breakdown field, and optical properties of refractive index and optical transmittance were the focuses of this study. 2. Experiment Thin films of the Al2O3 –TiO2 system were prepared in a magnetron sputtering unit equipped with a r.f. generator of 13.56 MHz. Seven kinds of 50-mm-diameter ceramic targets were involved: pure Al2O3, composites with Al2O3 yTiO2 volume ratios of 90y10, 70y
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.026
328
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
Fig. 1. Effects of target composition on growth rate for dielectric thin films of the Al2O3 –TiO2 system deposited at 200 and 500 8C under r.f. powers of 50, 100, and 150 W.
30, 55.7y44.3 (50y50 molar ratio), 30y70, 10y90, and pure TiO2. These targets were prepared from Al2O3, TiO2, or by mixing the powders of Al2O3 and TiO2 in proportion, followed by hot pressing at 1300 8C under argon atmosphere for 30 min. These hot-pressed ceramic targets were annealed at 1200 8C in air for 2 h. The target–substrate distance was kept at 40 mm. These films were deposited on bare Si wafer, bottom electrodeplated PtyTiySiO2 ySi substrates, and microscope glass slides at conditions of r.f. powers of 50–150 W, substrate temperatures of 200–500 8C, reaction pressure of 0.65 Pa, and argon flow rate of 20 sccm for 1 h. Pt upper electrodes of 150=150 mm were applied on top of electroded films by a shadow-mask method to form metalyinsulatorymetal (MIM) capacitors. Film thickness was measured by a Tencor P-1 Profiler. Film structure was analyzed by X-ray diffraction (XRD, Bruker D8, Germany) with a Cu Ka beam under an accelerated voltage of 40 kV and a current of 30 mA at a scan rate of 38ymin. Scanning electron microscopy (SEM, Hitachi S-3500H, Japan) equipped with energy dispersive spectroscopy (EDS) was used to observe the morphology of the coatings and to analyze the composition of the films. X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI 1600, USA) equipped with spherical capacitor analyzer and multi-channel detector was operated at 250 W to generate Mg Ka radiation at 1253.6 eV for analysis. The composition of the films was semi-quantitatively analyzed by measuring the peak areas and correcting them for the appropriate instrumental factors. A surface-cleaning step was conducted with a 3 kV ion beam for 3 min before measurements. Internal stress was estimated from the change in the curvature of the substrateyfilm system using a Stoney formula w7x. The curvature was obtained through the Profiler measurement. A scratch testing machine (Quad group: Romulus III, USA) was employed to evaluate adhesion between the film and the glass sub-
strate with a loading up to 60 N. The onset of peeling was monitored by optical microscopy to determine a critical load. Dielectric properties were measured with a precision impedance–capacitance–resistance (LCR) meter (Model 4284A, Agilent Technologies, USA) at a frequency of 100 kHz. Resistivity and breakdown field were obtained by using an electrometeryhigh-resistance meter (Model 6517a, Keithley Instruments, Inc. USA). An ellipsometer (Rudolph Instrument, Auto EL III-REV 304, USA) was applied to measure refractive indices at ˚ Optical transmittance was a wavelength of 6328 A. measured for films deposited on microscope glass slides by using a UV–visible recording spectrophotometer (Model UV-160A, Shimadzu, Japan) at wavelength of 300–800 nm. 3. Results 3.1. Growth behavior of thin films of the Al2O3 –TiO2 system Effects of temperature and r.f. power on the growth rate of thin films of the alumina–titania system are shown in Fig. 1. Growth rates (film thickness) were 0.75 (0.045)–1.2 (0.072), 2.9 (0.174)–4.4 (0.264), and 4.3 (0.258)–8.2 (0.492) nmymin (mm) for r.f. powers of 50, 100, and 150 W, respectively. Deposition temperature had little effects on growth rate. Growth rates increased with increasing r.f. powers and, to some extent, the TiO2 content in targets. 3.2. Characterization of thin films of the Al2O3 –TiO2 system The compositions of films deposited at 100 W and different temperatures were analyzed by EDS. A comparison between the film and target composition (atomic ratio) for thin films of the Al2O3 –TiO2 system deposited at different deposition temperatures by utilizing targets of different composition ratios (volume ratios) is demonstrated in Fig. 2. The results showed that film composition changed slightly with deposition temperature and the Al ratio in film was lower than that in target. These results were consistent with effects of temperature and sputtering yield, as demonstrated in Fig. 1. Compositional analysis of TiO2 films was performed by XPS. A plasma pre-cleaning step was performed for 3 min before XPS analysis. The OyTi ratio changed from 2.33, 2.01, to 1.87 as the pre-cleaning time increased from 10, 20, to 180 s for films deposited at 100 W and 500 8C to remove absorbed CO2. Thin films of the alumina–titania system formed in the temperature range of 200–500 8C were essentially amorphous, as determined by XRD. Anatase and rutile phases of pure TiO2 were observed at higher deposition
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
329
temperature of 500 8C. Amorphous TiO2 films deposited at 200 8C were crystallized to from co-existing anatase and rutile structures above annealing temperatures of 300 8C. However, Al2O3 and Al2O3 –TiO2 composite films remained amorphous upon annealing. SEM micrographs of thin films of the alumina–titania system obtained from a 30y70 target at deposition temperatures of 200 8C and 500 8C were observed, as shown in Fig. 3a,b, respectively. Some rough surfaces with granular microstructure were observed for films deposited at higher temperature of 500 8C. 3.3. Properties of the r.f.-sputtered thin films of the Al2O3 –TiO2 system
Fig. 2. Comparison between the film and target composition (atomic ratio) for thin films of the Al2O3 –TiO2 system deposited at different deposition temperatures by utilizing targets with different composition ratios (volume ratios).
Variation of the internal stress with target composition for thin films of the Al2O3 –TiO2 system was measured on films deposited at 200 8C (Fig. 4a) and 500 8C (Fig. 4b) under different r.f. powers. Compressive internal stresses were 4–8 GPa, 1–2.5 GPa, and 0.5–2 GPa for films deposited at 50 W, 100 W, and 150 W, respectively. Internal stresses were compressive and not a strong function of substrate temperature, but functions of r.f.
Fig. 3. SEM microstructure of thin films deposited at (a) 200 8C and (b) 500 8C by utilizing a 30 Al2O3y70 TiO2 target.
Fig. 4. Variation of compressive internal stress with target composition for dielectric thin films of the Al2 O3 –TiO2 system deposited at (a) 200 8C and (b) 500 8C under different r.f. powers.
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
330
Fig. 5. Variation of specific critical load (critical loadyfilm thickness) with target composition for dielectric thin films of the Al2O3 –TiO2 system deposited at (a) 200 8C and (b) 500 8C under different r.f. powers.
power and target composition. The films deposited at 50 W displayed a higher compressive internal stress and a larger variation of internal stress with composition. The critical adherence load of scratch tests increases with film thickness. After scaled with film thickness, the specific critical load has been defined w8x. Fig. 5 shows the variation of specific critical load (critical loadyfilm thickness) with target composition for thin films of the Al2O3 –TiO2 system deposited at 200 8C (Fig. 5a) and 500 8C (Fig. 5b) under different r.f. powers. Specific critical loads were 100–240 Nymm, 30–85 Nymm, and 15–57 Nymm for films deposited at 50 W, 100 W, and 150 W, respectively. Pure alumina films had a better adhesion as compared with pure titania films. The adhesion of the composite films depended on film composition that changed with target composition, as shown in Fig. 2. Dielectric properties of thin films of the Al2O3 –TiO2 system deposited at 100 W and 200–500 8C were measured, as shown in Table 1. Pure TiO2 films had high dielectric constants and loss tangents. Pure Al2O3 films had low dielectric constants and loss tangents, which changed with deposition temperature. Composite films had lower dielectric constants and loss tangents at
lower TiO2 contents. Excellent dielectric properties of a high dielectric constant of 62 and a low loss tangent of 0.012 were obtained for a thin film deposited at 500 8C by utilizing a 10y90 target. Table 2 lists the dielectric properties of the 200 8Cdeposited and its 200 8C-, 300 8C-, and 400 8C-annealed thin films of the Al2O3 –TiO2 system obtained under a r.f. power of 100 W. Dielectric properties of pure TiO2 films degraded upon annealing. Their dielectric constant decreased from 104 to 37 without an improvement in loss tangent after annealed at 500 8C. For pure Al2O3 and Al2O3-rich thin films, annealing did not have an apparent effect on dielectric properties. Thin films obtained from a 10y90 target, which displayed excellent dielectric properties at higher deposition temperatures, had degraded dielectric properties either, with dielectric constants changing from 23 to 15 after annealed at 400 8C. Resistivities of the 200 8C-deposited and its 300 8Cannealed thin films of the Al2O3 –TiO2 system obtained under r.f. power of 100 W were measured, as shown in Table 3. Al2O3 and composite films showed an apparent improvement in resistivity upon annealing, with a resistivity of 2.0–8.1=109 V-cm increasing to 2.2–
Table 1 Dielectric constant (k) and dielectric loss tangent (tand) of thin films of the Al2 O3 –TiO2 system deposited from targets with different composition ratios (volume ratios) under r.f. power of 100 W and different deposition temperatures of 200–500 8C Target composition (vol. ratio)
Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
Dielectric property 200 8C
300 8C
400 8C
500 8C
k
tand
k
tand
k
tand
k
tand
7.0 6.4 8.6 11 14 23 104
0.036 0.041 0.014 0.015 0.020 0.030 0.097
10 7.4 8.1 13 22 43 169
0.038 0.009 0.011 0.045 0.014 0.037 0.600
13 9.4 11 11 14 46 398
0.013 0.011 0.019 0.033 0.012 0.043 2.520
14 11 13 14 19 62 280
0.011 0.013 0.016 0.026 0.020 0.012 4.603
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
331
Table 2 Dielectric constant (k) and dielectric loss tangent (tand) of the 200 8C-deposited and 200 8C-, 300 8C-, and 400 8C-annealed thin films of the Al2O3 –TiO2 system obtained from targets with different composition ratios (volume ratios) under r.f. power of 100 W Target composition (vol. ratio)
Dielectric property 200 8Cdeposited
Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
200 8Cannealed
300 8Cannealed
400 8Cannealed
k
tand
k
tand
k
tand
k
tand
7.0 6.4 8.6 11 14 23 104
0.036 0.041 0.014 0.015 0.020 0.030 0.097
7.2 6.4 8.8 11 22 22 71
0.015 0.016 0.012 0.012 0.012 0.027 0.127
8.9 8.6 9.8 11 26 15 48
0.010 0.009 0.010 0.011 0.015 0.024 0.080
8.6 6.9 7.8 11 29 15 37
0.016 0.021 0.012 0.019 0.034 0.031 0.188
Table 3 Resistivity of the 200 8C-deposited and 300 8C-annealed thin films of the Al2O3 –TiO2 system obtained from targets with different composition ratios (volume ratios) under r.f. power of 100 W Target composition (vol. ratio) Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
Resistivity (109 V-cm) 200 8C-deposited
300 8C-annealed
4.4 4.4 4.6 8.1 5.0 2.0 0.0066
4.4=103 2.2=103 3.8=103 3.0=103 4.7=103 4.1=103 29
Table 4 Breakdown field of the 200 8C-deposited and 300 8C-annealed thin films of the Al2O3 –TiO2 system obtained from targets with different composition ratios (volume ratios) under r.f. power of 100 W Target composition (vol. ratio) Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
Breakdown field (106 Vycm) 200 8C-deposited
300 8C-annealed
5.9 5.9 6.2 6.1 5.0 4.3 0.8
4.7 7.4 5.7 5.0 3.8 4.4 1.0
4.7=1012 V-cm. However, pure TiO2 films showed a lower resistivity, which increased from 6.7=106 to 29=109 V-cm upon annealing.
Fig. 6. Variation of refractive index with target composition for dielectric thin films of the Al2O3 –TiO2 system deposited at 200–500 8C with r.f. power of 100 W.
Breakdown fields (EB) of the 200 8C-deposited and its 300 8C-annealed thin films of the Al2O3 –TiO2 system were measured, as shown in Table 4. The EB values ranged from 4.3 to ;6 MVycm for the 200 8C-deposited Al2O3 and composite films. Annealing did not have an apparent effect on EB. However, 200 8C-deposited TiO2 films had a lower EB value of 0.8 MVycm. The EB value slightly increased to 1.0 MVycm after annealing at 300 8C for 15 min. It is concluded that breakdown field is insensitive to annealing in air for thin films of the Al2O3 –TiO2 system. Refractive indices (n) for thin films of the Al2O3 – TiO2 system deposited at 100 W and different temperatures were measured, as shown in Fig. 6. Refractive indices were 2.40–2.70 for TiO2 and 1.66–1.71 for Al2O3. The refractive index of aluminum oxide films is usually between 1.54 and 1.70 w9x, while it is ;1.76 for a-alumina and ;1.7 for g-alumina bulk samples. For composite films, the n values mostly increased with the increase in the TiO2 content of thin films.
332
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
temperature indicates the irrelevance of surface diffusion, desorption or reactions of adatoms on growth rate. From the compositional analysis of XPS, oxygendeficient TiO2 films were obtained due to the lower factor of energy transfer of oxygen atoms (gAr–Os0.81) under argon plasma without a supply of additional oxygen. 4.2. Properties of r.f.-sputtered films of the Al2O3 –TiO2 system
Fig. 7. Spectral transmittance characteristics of sputtered thin films of the Al2O3 –TiO2 system deposited from the (a) Al2O3, (b) 55.7Al2O3y44.3TiO2, (c) TiO2 targets under r.f. power of 100 W and deposition temperatures of 200 and 500 8C.
Fig. 7 shows the spectral transmittance characteristics of thin films of the Al2O3 –TiO2 system obtained from (a) Al2O3; (b) 55.7 Al2O3 y44.3 TiO2; and (c) TiO2 targets as a function of substrate temperature. All samples were highly transparent in the visible range. 4. Discussion 4.1. Growth and characterization of r.f.-sputtered films of the Al2O3 –TiO2 system The increased power increases the ionized argon to bombard more atoms off a target, therefore leads to a higher growth rate. The slightly increased growth rate with TiO2 content in targets is due to a little higher sputtering yield of Ti than Al for different targets. The factor of energy transfer of Ti (gAr–Tis0.99) under argon plasma is slightly higher that of the Al atoms (gAr–Als0.96) w10x. The independence of deposition
Internal stress is composed of two components: intrinsic and thermal. The coefficients of thermal expansion (a are 2.1–2.8, 6.5, and 2.6=10y6 Ky1 for TiO2 film w11x, Al2O3 film w12x and Si wafer w12x, respectively, the elastic modulus (E) of Si wafer is ;180 GPa, and the temperature difference (DT) between deposition and stress measurement is approximately 200–500 8C. Estimated from a formula of ssEDaDT, the thermal stress is tensile and up to ;0.35 GPa. Therefore, the contribution of intrinsic stress (subtracting the thermal stress from the internal stress) in films increases as the r.f. power decreases. It is thought that higher r.f. power can provide higher impinging energy to the depositing particles. Instead, this higher collision kinetic energy did not cause a higher intrinsic stress. Obviously, intrinsic stress was related to film thickness. A thicker film obtained at higher r.f. power (Fig. 1) had a lower compressive internal stress due to stress relaxation of the packing imperfection. It was observed that internal stress was decreased for thicker films in the study of metal-organic CVD alumina w8x, but an opposite result was obtained for the r.f. sputtered zirconia (ZrO2) films w13x. Similar to internal stress, the specific critical load is not a strong function of substrate temperature, but functions of r.f. power and target composition. There is a tendency of obtaining better adhesion under a higher compressive internal stress for thin films of the Al2O3 – TiO2 system. TiO2 thin films have a reported dielectric constant of 4–86 w14–17x. Nevertheless, pure TiO2 possesses a problem of a higher loss tangent in this study, even upon annealing due to orientation polarization contributing from oxygen vacancy and the electron-trapped titanium polaron from the Ti4q™Ti3q transition. Thin films obtained from a 10y90 target with a small amount of Al2O3 have improved dielectric properties at higher deposition temperatures. The thin film deposited at 500 8C from this target has demonstrated an excellent dielectric constant of 62 comparable with that of TiO2, but does not have the problem of a high loss tangent. The lower dielectric loss tangent for the films from this 10y90 target can be related to a high oxygen potential to lessen the reduction reaction of the Ti4q™Ti3q transition and the formation of oxygen vacancies.
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
333
The resistivity of pure TiO2 films is about one thousand time less than that of the Al2O3 yTiO2 composite film. The lower resistivity of TiO2 is related to the reduction tendency of Ti4q to Ti3q and the accompanying oxygen vacancies. The added Al2O3 during cosputtering with TiO2 increases the oxygen potential and results in a less-defective microstructure, which improves electrical properties of TiO2. Although annealing has an effect on the defective TiO2 structure, the addition of adequate amount of Al2O3 to TiO2 can have even more benefits not only on dielectric properties but also on electrical properties. As TiO2 films have attracted attentions for use in fabricating capacitors in DRAM and gate oxide in transistor and much work has been conducted on the growth of TiO2 thin films, we find that the addition of adequate amount of Al2O3 to TiO2 can be a better choice in applications. Refractive indices of TiO2 were 2.40–2.70 in this study, comparable with 2.0–2.67 prepared at different methods w18–20x. There are abnormal refractive indices occurred for composite films deposited at 400 and 500 8C from the 30y70 and 70y30 targets, which were 1.36– 1.45 below the values of individual components. The microstructure of these thin films wad examined by SEM. Fig. 3 shows the microstructure of the films deposited at (a) 200 8C; and (b) 500 8C from a 30y70 target. The films produced from the 30y70 and 70y30 targets have a tendency to form granular microstructure at higher temperatures of 400 and 500 8C. This rough and less-compact microstructure leads to abnormal refractive indices. For the transmittance characteristics in Fig. 7, no apparent interference patterns were observed for Al2O3 and composite films except for the TiO2 films. From the XRD analysis, TiO2 films can crystallize easier than Al2O3 and composite films. A decrease in the average transmission for TiO2 (Fig. 7c) can attribute to its partial reduction. There was a shift of the interference pattern in the transmission spectrum to a higher wavelength region for TiO2 deposited at higher temperature of 500 8C. This might be due to the combined effects of a different thickness and a differece index.
The compressive internal stresses were 4–8 GPa, 1– 2.5 GPa, and 0.5–2 GPa for films deposited at 50 W, 100 W, and 150 W, respectively. The thinner films deposited at 50 W displayed a higher internal stress due to the difficulty in stress relaxation of the packing imperfection. The specific critical loads were 100–240 Nymm, 30–85 Nymm, and 15–57 Nymm for films deposited at 50 W, 100 W, and 150 W, respectively. A film possessing a higher compressive internal stress had a better adhesion. Excellent dielectric properties of a dielectric constant of 62 and a loss tangent of 0.012 were obtained for a thin film deposited at 500 8C using a 10y90 target. The lower dielectric loss tangent for films from a 10y90 target can be related to a high oxygen potential to lessen the reduction reaction of the Ti4q™Ti3q transition and the formation of oxygen vacancies. The Al2O3 and Al2O3-containing composite films had resistivities of 2.0–8.1=109 V-cm, which increased to 2.2–4.7=1012 V-cm upon annealing. However, pure TiO2 films showed a lower resistivity, which increased from 6.7=106 to 29=109 V-cm upon annealing. The values of breakdown field ranged from 4.3 to ;6 MVycm for the 200 8C-deposited Al2O3 and composite films. The 200 8C-deposited TiO2 films had a lower value of 0.8 MVycm. Annealing did not have an obvious effect on breakdown field of thin films of the Al2O3 –TiO2 system. Refractive indices were 2.40–2.70 for TiO2 and 1.66– 1.71 for Al2O3. For composite thin films, refractive index increased with the increase in the TiO2 content. The Al2O3, TiO2, and composite films all displayed a good optical transmittance.
5. Conclusions
References
Amorphous thin films of the Al2O3 –TiO2 systems including Al2O3, TiO2, and composite thin films were deposited by r.f. magnetron sputtering. The deposition rate was not a function of substrate temperature, but changed with r.f. power. The growth rates were 0.75– 1.2 nmymin, 2.9–4.4 nmymin, and 4.3–8.2 nmymin for r.f. powers of 50 W, 100 W, and 150 W, respectively. The lower Aly(AlqTi) ratio in a film, as compared with the sputtered target, was explained by the effect of sputtering yield.
Acknowledgments The authors would like to acknowledge Prof. M.H. Hon of National Cheng Kung University for assisting in scratch tests, and Miss S.Y. Tsai of NSC Instrument center in National Tsing Hua University for helping in the XPS analysis. Funding for this study was provided by the National Science Council of the Republic of China under Grant no. NSC 90-2216-E-259-005.
w1x W.S. Rees Jr, CVD of Non-metals, VCH Publishers, Inc, New York, NY, USA, 1996, p. 282. w2x N.C. Tombs, H.A. Wegener, R. Newman, B.T. Kenny, A.J. Coppola, Proc. IEEE 55 (1967) 1168. w3x K.H. Zaininger, A.S. Waxman, IEEE Trans. Electron. Dev. 16 (1963) 333. w4x S. Hashimoto, J.L. Peng, W.M. Gibson, Appl. Phys. Lett. 47 (1985) 1071. w5x N. Rausch, E.P. Burte, J. Electrochem. Soc. 140 (1993) 145. w6x L. Messick, J. Appl. Phys. 47 (1976) 4949. w7x M. Ohring, The Materials Science of Thin Films, Academic Press, 1992, p. 552.
334
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
w8x D.H. Kuo, B.Y. Cheung, R.J. Wu, Thin Solid Films 398–399 (2001) 35. w9x J. Saraie, K. Ono, S. Takeuchi, J. Electrochem. Soc. 136 (1989) 3139. w10x H.F. Winters, P. Sigmund, J. Appl. Phys. 45 (1974) 4760. w11x C.R. Ottermann, K. Bange, Thin Solid Films 286 (1996) 32. w12x A.A.R. Elshabini-Riad, F.D. Barlow III, Thin Film Technology Handbook, McGraw-Hill International, 1997, in Table 3.1. w13x S. Ben Amur, B. Rogier, G. Baud, M. Jacquet, M. Mardin, Mater. Sci. Eng. B 57 (1998) 28.
M.D. Stamate, Thin Solid Films 372 (2000) 246. N. Rausch, E.P. Burte, Engineering 19 (1992) 725. T. Fuyuki, H. Matsunami, Jpn. J. Appl. Phys. 25 (1986) 1288. W.D. Brown, W.W. Grannemann, Solid-State Electron. 21 (1978) 837. w18x K. Okimura, N. Maeda, A. Shibata, Thin Solid Films 281–282 (1996) 427. w19x K. Fukushima, I. Yamada, T. Takagi, J. Appl. Phys. 58 (1985) 4146. w20x J. Aarik, A. Aidla, A.A. Kiisler, T. Uustare, V. Sammelselg, Thin Solid Films 305 (1997) 270. w14x w15x w16x w17x
Characterization and properties of r.f.-sputtered thin films of the alumina–titania system Dong-Hau Kuo*, Kuo-Hwa Tzeng Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien, Taiwan, ROC Received 20 August 2003; received in revised form 15 January 2004; accepted 12 February 2004 Available Online 12 April 2004
Abstract Thin films of the aluminum oxide (Al2O3)–titanium oxide (TiO2 ) system including Al2 O3 , TiO2 , and Al2O3 yTiO2 were prepared by radio-frequency (r.f.) magnetron sputtering using ceramic targets of Al2 O3 , TiO2 , and Al2O3 yTiO2 composites with different Al2O3 yTiO2 ratio. These films were studied at different substrate temperatures, r.f. powers, and annealing temperatures. Composition, microstructure, thermomechanical property of internal stress, and mechanical property of scratch adhesion, were evaluated. A thin film with a dielectric constant of 62 and a loss tangent of 0.012 was obtained at 500 8C from a 10y90 target. This thin film remained the high dielectric constant of TiO2 , but had an improvement in the dielectric loss tangent. Al2O3containing films had a higher resistivity and breakdown field, which was improved further by annealing. Optical properties, such as refractive index and optical transmittance, were also investigated. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Aluminum oxide; Titanium oxide; Mechanical property; Electrical properties and measurements; Optical properties
1. Introduction Aluminum oxide (alumina, Al2O3) as an insulator has several advantages for semiconductor device applications w1x. Alumina films are a better barrier to mobile ionic species w2x and have high chemical stability and radiation resistance w3x. It also has dielectric constant two folds higher than silica. Moreover, alumina films can be used for passivation of bipolar devices, as a diffusion mask, and as a buffer layer in silicon-oninsulator (SOI) devices for three-dimensional integrated circuits w4x. Titanium oxide (Titania, TiO2) thin films, with high refractive index, excellent transmittance in the visiblewavelength region, and high chemical stability, have been used in anti-reflection coating, sensors, and photocatalysis. Recently, TiO2 film has attracted attention for use in fabricating capacitors in dynamic random access memory (DRAM) and gate oxide in transistor, due to their higher dielectric constant up to 100 w5,6x. *Corresponding author. Tel.: q011-886-3-863-4208; fax: q011886-3-863-4200. E-mail address: [email protected] (D.-H. Kuo).
In this work, a systematic study of depositing thin films of alumina–titania (Al2O3 –TiO2 ) system by r.f. magnetron sputtering was proceeded by adjusting film properties through the Al2O3 y(Al2O3 qTiO2 ) ratio in sputtering targets. Thin films of the Al2O3 –TiO2 system included pure Al2O3 and TiO2 films, and thin films with different Al2O3 y(Al2O3qTiO2 ) ratios or the composite thin films. The effects of process parameters on deposition kinetics, composition, thermomechanical property of internal stress, mechanical property of scratch adhesion, dielectric properties of dielectric constant and loss tangent, electric properties of resistivity and breakdown field, and optical properties of refractive index and optical transmittance were the focuses of this study. 2. Experiment Thin films of the Al2O3 –TiO2 system were prepared in a magnetron sputtering unit equipped with a r.f. generator of 13.56 MHz. Seven kinds of 50-mm-diameter ceramic targets were involved: pure Al2O3, composites with Al2O3 yTiO2 volume ratios of 90y10, 70y
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.026
328
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
Fig. 1. Effects of target composition on growth rate for dielectric thin films of the Al2O3 –TiO2 system deposited at 200 and 500 8C under r.f. powers of 50, 100, and 150 W.
30, 55.7y44.3 (50y50 molar ratio), 30y70, 10y90, and pure TiO2. These targets were prepared from Al2O3, TiO2, or by mixing the powders of Al2O3 and TiO2 in proportion, followed by hot pressing at 1300 8C under argon atmosphere for 30 min. These hot-pressed ceramic targets were annealed at 1200 8C in air for 2 h. The target–substrate distance was kept at 40 mm. These films were deposited on bare Si wafer, bottom electrodeplated PtyTiySiO2 ySi substrates, and microscope glass slides at conditions of r.f. powers of 50–150 W, substrate temperatures of 200–500 8C, reaction pressure of 0.65 Pa, and argon flow rate of 20 sccm for 1 h. Pt upper electrodes of 150=150 mm were applied on top of electroded films by a shadow-mask method to form metalyinsulatorymetal (MIM) capacitors. Film thickness was measured by a Tencor P-1 Profiler. Film structure was analyzed by X-ray diffraction (XRD, Bruker D8, Germany) with a Cu Ka beam under an accelerated voltage of 40 kV and a current of 30 mA at a scan rate of 38ymin. Scanning electron microscopy (SEM, Hitachi S-3500H, Japan) equipped with energy dispersive spectroscopy (EDS) was used to observe the morphology of the coatings and to analyze the composition of the films. X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI 1600, USA) equipped with spherical capacitor analyzer and multi-channel detector was operated at 250 W to generate Mg Ka radiation at 1253.6 eV for analysis. The composition of the films was semi-quantitatively analyzed by measuring the peak areas and correcting them for the appropriate instrumental factors. A surface-cleaning step was conducted with a 3 kV ion beam for 3 min before measurements. Internal stress was estimated from the change in the curvature of the substrateyfilm system using a Stoney formula w7x. The curvature was obtained through the Profiler measurement. A scratch testing machine (Quad group: Romulus III, USA) was employed to evaluate adhesion between the film and the glass sub-
strate with a loading up to 60 N. The onset of peeling was monitored by optical microscopy to determine a critical load. Dielectric properties were measured with a precision impedance–capacitance–resistance (LCR) meter (Model 4284A, Agilent Technologies, USA) at a frequency of 100 kHz. Resistivity and breakdown field were obtained by using an electrometeryhigh-resistance meter (Model 6517a, Keithley Instruments, Inc. USA). An ellipsometer (Rudolph Instrument, Auto EL III-REV 304, USA) was applied to measure refractive indices at ˚ Optical transmittance was a wavelength of 6328 A. measured for films deposited on microscope glass slides by using a UV–visible recording spectrophotometer (Model UV-160A, Shimadzu, Japan) at wavelength of 300–800 nm. 3. Results 3.1. Growth behavior of thin films of the Al2O3 –TiO2 system Effects of temperature and r.f. power on the growth rate of thin films of the alumina–titania system are shown in Fig. 1. Growth rates (film thickness) were 0.75 (0.045)–1.2 (0.072), 2.9 (0.174)–4.4 (0.264), and 4.3 (0.258)–8.2 (0.492) nmymin (mm) for r.f. powers of 50, 100, and 150 W, respectively. Deposition temperature had little effects on growth rate. Growth rates increased with increasing r.f. powers and, to some extent, the TiO2 content in targets. 3.2. Characterization of thin films of the Al2O3 –TiO2 system The compositions of films deposited at 100 W and different temperatures were analyzed by EDS. A comparison between the film and target composition (atomic ratio) for thin films of the Al2O3 –TiO2 system deposited at different deposition temperatures by utilizing targets of different composition ratios (volume ratios) is demonstrated in Fig. 2. The results showed that film composition changed slightly with deposition temperature and the Al ratio in film was lower than that in target. These results were consistent with effects of temperature and sputtering yield, as demonstrated in Fig. 1. Compositional analysis of TiO2 films was performed by XPS. A plasma pre-cleaning step was performed for 3 min before XPS analysis. The OyTi ratio changed from 2.33, 2.01, to 1.87 as the pre-cleaning time increased from 10, 20, to 180 s for films deposited at 100 W and 500 8C to remove absorbed CO2. Thin films of the alumina–titania system formed in the temperature range of 200–500 8C were essentially amorphous, as determined by XRD. Anatase and rutile phases of pure TiO2 were observed at higher deposition
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
329
temperature of 500 8C. Amorphous TiO2 films deposited at 200 8C were crystallized to from co-existing anatase and rutile structures above annealing temperatures of 300 8C. However, Al2O3 and Al2O3 –TiO2 composite films remained amorphous upon annealing. SEM micrographs of thin films of the alumina–titania system obtained from a 30y70 target at deposition temperatures of 200 8C and 500 8C were observed, as shown in Fig. 3a,b, respectively. Some rough surfaces with granular microstructure were observed for films deposited at higher temperature of 500 8C. 3.3. Properties of the r.f.-sputtered thin films of the Al2O3 –TiO2 system
Fig. 2. Comparison between the film and target composition (atomic ratio) for thin films of the Al2O3 –TiO2 system deposited at different deposition temperatures by utilizing targets with different composition ratios (volume ratios).
Variation of the internal stress with target composition for thin films of the Al2O3 –TiO2 system was measured on films deposited at 200 8C (Fig. 4a) and 500 8C (Fig. 4b) under different r.f. powers. Compressive internal stresses were 4–8 GPa, 1–2.5 GPa, and 0.5–2 GPa for films deposited at 50 W, 100 W, and 150 W, respectively. Internal stresses were compressive and not a strong function of substrate temperature, but functions of r.f.
Fig. 3. SEM microstructure of thin films deposited at (a) 200 8C and (b) 500 8C by utilizing a 30 Al2O3y70 TiO2 target.
Fig. 4. Variation of compressive internal stress with target composition for dielectric thin films of the Al2 O3 –TiO2 system deposited at (a) 200 8C and (b) 500 8C under different r.f. powers.
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
330
Fig. 5. Variation of specific critical load (critical loadyfilm thickness) with target composition for dielectric thin films of the Al2O3 –TiO2 system deposited at (a) 200 8C and (b) 500 8C under different r.f. powers.
power and target composition. The films deposited at 50 W displayed a higher compressive internal stress and a larger variation of internal stress with composition. The critical adherence load of scratch tests increases with film thickness. After scaled with film thickness, the specific critical load has been defined w8x. Fig. 5 shows the variation of specific critical load (critical loadyfilm thickness) with target composition for thin films of the Al2O3 –TiO2 system deposited at 200 8C (Fig. 5a) and 500 8C (Fig. 5b) under different r.f. powers. Specific critical loads were 100–240 Nymm, 30–85 Nymm, and 15–57 Nymm for films deposited at 50 W, 100 W, and 150 W, respectively. Pure alumina films had a better adhesion as compared with pure titania films. The adhesion of the composite films depended on film composition that changed with target composition, as shown in Fig. 2. Dielectric properties of thin films of the Al2O3 –TiO2 system deposited at 100 W and 200–500 8C were measured, as shown in Table 1. Pure TiO2 films had high dielectric constants and loss tangents. Pure Al2O3 films had low dielectric constants and loss tangents, which changed with deposition temperature. Composite films had lower dielectric constants and loss tangents at
lower TiO2 contents. Excellent dielectric properties of a high dielectric constant of 62 and a low loss tangent of 0.012 were obtained for a thin film deposited at 500 8C by utilizing a 10y90 target. Table 2 lists the dielectric properties of the 200 8Cdeposited and its 200 8C-, 300 8C-, and 400 8C-annealed thin films of the Al2O3 –TiO2 system obtained under a r.f. power of 100 W. Dielectric properties of pure TiO2 films degraded upon annealing. Their dielectric constant decreased from 104 to 37 without an improvement in loss tangent after annealed at 500 8C. For pure Al2O3 and Al2O3-rich thin films, annealing did not have an apparent effect on dielectric properties. Thin films obtained from a 10y90 target, which displayed excellent dielectric properties at higher deposition temperatures, had degraded dielectric properties either, with dielectric constants changing from 23 to 15 after annealed at 400 8C. Resistivities of the 200 8C-deposited and its 300 8Cannealed thin films of the Al2O3 –TiO2 system obtained under r.f. power of 100 W were measured, as shown in Table 3. Al2O3 and composite films showed an apparent improvement in resistivity upon annealing, with a resistivity of 2.0–8.1=109 V-cm increasing to 2.2–
Table 1 Dielectric constant (k) and dielectric loss tangent (tand) of thin films of the Al2 O3 –TiO2 system deposited from targets with different composition ratios (volume ratios) under r.f. power of 100 W and different deposition temperatures of 200–500 8C Target composition (vol. ratio)
Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
Dielectric property 200 8C
300 8C
400 8C
500 8C
k
tand
k
tand
k
tand
k
tand
7.0 6.4 8.6 11 14 23 104
0.036 0.041 0.014 0.015 0.020 0.030 0.097
10 7.4 8.1 13 22 43 169
0.038 0.009 0.011 0.045 0.014 0.037 0.600
13 9.4 11 11 14 46 398
0.013 0.011 0.019 0.033 0.012 0.043 2.520
14 11 13 14 19 62 280
0.011 0.013 0.016 0.026 0.020 0.012 4.603
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
331
Table 2 Dielectric constant (k) and dielectric loss tangent (tand) of the 200 8C-deposited and 200 8C-, 300 8C-, and 400 8C-annealed thin films of the Al2O3 –TiO2 system obtained from targets with different composition ratios (volume ratios) under r.f. power of 100 W Target composition (vol. ratio)
Dielectric property 200 8Cdeposited
Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
200 8Cannealed
300 8Cannealed
400 8Cannealed
k
tand
k
tand
k
tand
k
tand
7.0 6.4 8.6 11 14 23 104
0.036 0.041 0.014 0.015 0.020 0.030 0.097
7.2 6.4 8.8 11 22 22 71
0.015 0.016 0.012 0.012 0.012 0.027 0.127
8.9 8.6 9.8 11 26 15 48
0.010 0.009 0.010 0.011 0.015 0.024 0.080
8.6 6.9 7.8 11 29 15 37
0.016 0.021 0.012 0.019 0.034 0.031 0.188
Table 3 Resistivity of the 200 8C-deposited and 300 8C-annealed thin films of the Al2O3 –TiO2 system obtained from targets with different composition ratios (volume ratios) under r.f. power of 100 W Target composition (vol. ratio) Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
Resistivity (109 V-cm) 200 8C-deposited
300 8C-annealed
4.4 4.4 4.6 8.1 5.0 2.0 0.0066
4.4=103 2.2=103 3.8=103 3.0=103 4.7=103 4.1=103 29
Table 4 Breakdown field of the 200 8C-deposited and 300 8C-annealed thin films of the Al2O3 –TiO2 system obtained from targets with different composition ratios (volume ratios) under r.f. power of 100 W Target composition (vol. ratio) Al2O3 90 Al2O3 –10 TiO2 70 Al2O3 –30 TiO2 55.6 Al2O3 –44.4 TiO2 30 Al2O3 –70 TiO2 10 Al2O3 –90 TiO2 TiO2
Breakdown field (106 Vycm) 200 8C-deposited
300 8C-annealed
5.9 5.9 6.2 6.1 5.0 4.3 0.8
4.7 7.4 5.7 5.0 3.8 4.4 1.0
4.7=1012 V-cm. However, pure TiO2 films showed a lower resistivity, which increased from 6.7=106 to 29=109 V-cm upon annealing.
Fig. 6. Variation of refractive index with target composition for dielectric thin films of the Al2O3 –TiO2 system deposited at 200–500 8C with r.f. power of 100 W.
Breakdown fields (EB) of the 200 8C-deposited and its 300 8C-annealed thin films of the Al2O3 –TiO2 system were measured, as shown in Table 4. The EB values ranged from 4.3 to ;6 MVycm for the 200 8C-deposited Al2O3 and composite films. Annealing did not have an apparent effect on EB. However, 200 8C-deposited TiO2 films had a lower EB value of 0.8 MVycm. The EB value slightly increased to 1.0 MVycm after annealing at 300 8C for 15 min. It is concluded that breakdown field is insensitive to annealing in air for thin films of the Al2O3 –TiO2 system. Refractive indices (n) for thin films of the Al2O3 – TiO2 system deposited at 100 W and different temperatures were measured, as shown in Fig. 6. Refractive indices were 2.40–2.70 for TiO2 and 1.66–1.71 for Al2O3. The refractive index of aluminum oxide films is usually between 1.54 and 1.70 w9x, while it is ;1.76 for a-alumina and ;1.7 for g-alumina bulk samples. For composite films, the n values mostly increased with the increase in the TiO2 content of thin films.
332
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
temperature indicates the irrelevance of surface diffusion, desorption or reactions of adatoms on growth rate. From the compositional analysis of XPS, oxygendeficient TiO2 films were obtained due to the lower factor of energy transfer of oxygen atoms (gAr–Os0.81) under argon plasma without a supply of additional oxygen. 4.2. Properties of r.f.-sputtered films of the Al2O3 –TiO2 system
Fig. 7. Spectral transmittance characteristics of sputtered thin films of the Al2O3 –TiO2 system deposited from the (a) Al2O3, (b) 55.7Al2O3y44.3TiO2, (c) TiO2 targets under r.f. power of 100 W and deposition temperatures of 200 and 500 8C.
Fig. 7 shows the spectral transmittance characteristics of thin films of the Al2O3 –TiO2 system obtained from (a) Al2O3; (b) 55.7 Al2O3 y44.3 TiO2; and (c) TiO2 targets as a function of substrate temperature. All samples were highly transparent in the visible range. 4. Discussion 4.1. Growth and characterization of r.f.-sputtered films of the Al2O3 –TiO2 system The increased power increases the ionized argon to bombard more atoms off a target, therefore leads to a higher growth rate. The slightly increased growth rate with TiO2 content in targets is due to a little higher sputtering yield of Ti than Al for different targets. The factor of energy transfer of Ti (gAr–Tis0.99) under argon plasma is slightly higher that of the Al atoms (gAr–Als0.96) w10x. The independence of deposition
Internal stress is composed of two components: intrinsic and thermal. The coefficients of thermal expansion (a are 2.1–2.8, 6.5, and 2.6=10y6 Ky1 for TiO2 film w11x, Al2O3 film w12x and Si wafer w12x, respectively, the elastic modulus (E) of Si wafer is ;180 GPa, and the temperature difference (DT) between deposition and stress measurement is approximately 200–500 8C. Estimated from a formula of ssEDaDT, the thermal stress is tensile and up to ;0.35 GPa. Therefore, the contribution of intrinsic stress (subtracting the thermal stress from the internal stress) in films increases as the r.f. power decreases. It is thought that higher r.f. power can provide higher impinging energy to the depositing particles. Instead, this higher collision kinetic energy did not cause a higher intrinsic stress. Obviously, intrinsic stress was related to film thickness. A thicker film obtained at higher r.f. power (Fig. 1) had a lower compressive internal stress due to stress relaxation of the packing imperfection. It was observed that internal stress was decreased for thicker films in the study of metal-organic CVD alumina w8x, but an opposite result was obtained for the r.f. sputtered zirconia (ZrO2) films w13x. Similar to internal stress, the specific critical load is not a strong function of substrate temperature, but functions of r.f. power and target composition. There is a tendency of obtaining better adhesion under a higher compressive internal stress for thin films of the Al2O3 – TiO2 system. TiO2 thin films have a reported dielectric constant of 4–86 w14–17x. Nevertheless, pure TiO2 possesses a problem of a higher loss tangent in this study, even upon annealing due to orientation polarization contributing from oxygen vacancy and the electron-trapped titanium polaron from the Ti4q™Ti3q transition. Thin films obtained from a 10y90 target with a small amount of Al2O3 have improved dielectric properties at higher deposition temperatures. The thin film deposited at 500 8C from this target has demonstrated an excellent dielectric constant of 62 comparable with that of TiO2, but does not have the problem of a high loss tangent. The lower dielectric loss tangent for the films from this 10y90 target can be related to a high oxygen potential to lessen the reduction reaction of the Ti4q™Ti3q transition and the formation of oxygen vacancies.
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
333
The resistivity of pure TiO2 films is about one thousand time less than that of the Al2O3 yTiO2 composite film. The lower resistivity of TiO2 is related to the reduction tendency of Ti4q to Ti3q and the accompanying oxygen vacancies. The added Al2O3 during cosputtering with TiO2 increases the oxygen potential and results in a less-defective microstructure, which improves electrical properties of TiO2. Although annealing has an effect on the defective TiO2 structure, the addition of adequate amount of Al2O3 to TiO2 can have even more benefits not only on dielectric properties but also on electrical properties. As TiO2 films have attracted attentions for use in fabricating capacitors in DRAM and gate oxide in transistor and much work has been conducted on the growth of TiO2 thin films, we find that the addition of adequate amount of Al2O3 to TiO2 can be a better choice in applications. Refractive indices of TiO2 were 2.40–2.70 in this study, comparable with 2.0–2.67 prepared at different methods w18–20x. There are abnormal refractive indices occurred for composite films deposited at 400 and 500 8C from the 30y70 and 70y30 targets, which were 1.36– 1.45 below the values of individual components. The microstructure of these thin films wad examined by SEM. Fig. 3 shows the microstructure of the films deposited at (a) 200 8C; and (b) 500 8C from a 30y70 target. The films produced from the 30y70 and 70y30 targets have a tendency to form granular microstructure at higher temperatures of 400 and 500 8C. This rough and less-compact microstructure leads to abnormal refractive indices. For the transmittance characteristics in Fig. 7, no apparent interference patterns were observed for Al2O3 and composite films except for the TiO2 films. From the XRD analysis, TiO2 films can crystallize easier than Al2O3 and composite films. A decrease in the average transmission for TiO2 (Fig. 7c) can attribute to its partial reduction. There was a shift of the interference pattern in the transmission spectrum to a higher wavelength region for TiO2 deposited at higher temperature of 500 8C. This might be due to the combined effects of a different thickness and a differece index.
The compressive internal stresses were 4–8 GPa, 1– 2.5 GPa, and 0.5–2 GPa for films deposited at 50 W, 100 W, and 150 W, respectively. The thinner films deposited at 50 W displayed a higher internal stress due to the difficulty in stress relaxation of the packing imperfection. The specific critical loads were 100–240 Nymm, 30–85 Nymm, and 15–57 Nymm for films deposited at 50 W, 100 W, and 150 W, respectively. A film possessing a higher compressive internal stress had a better adhesion. Excellent dielectric properties of a dielectric constant of 62 and a loss tangent of 0.012 were obtained for a thin film deposited at 500 8C using a 10y90 target. The lower dielectric loss tangent for films from a 10y90 target can be related to a high oxygen potential to lessen the reduction reaction of the Ti4q™Ti3q transition and the formation of oxygen vacancies. The Al2O3 and Al2O3-containing composite films had resistivities of 2.0–8.1=109 V-cm, which increased to 2.2–4.7=1012 V-cm upon annealing. However, pure TiO2 films showed a lower resistivity, which increased from 6.7=106 to 29=109 V-cm upon annealing. The values of breakdown field ranged from 4.3 to ;6 MVycm for the 200 8C-deposited Al2O3 and composite films. The 200 8C-deposited TiO2 films had a lower value of 0.8 MVycm. Annealing did not have an obvious effect on breakdown field of thin films of the Al2O3 –TiO2 system. Refractive indices were 2.40–2.70 for TiO2 and 1.66– 1.71 for Al2O3. For composite thin films, refractive index increased with the increase in the TiO2 content. The Al2O3, TiO2, and composite films all displayed a good optical transmittance.
5. Conclusions
References
Amorphous thin films of the Al2O3 –TiO2 systems including Al2O3, TiO2, and composite thin films were deposited by r.f. magnetron sputtering. The deposition rate was not a function of substrate temperature, but changed with r.f. power. The growth rates were 0.75– 1.2 nmymin, 2.9–4.4 nmymin, and 4.3–8.2 nmymin for r.f. powers of 50 W, 100 W, and 150 W, respectively. The lower Aly(AlqTi) ratio in a film, as compared with the sputtered target, was explained by the effect of sputtering yield.
Acknowledgments The authors would like to acknowledge Prof. M.H. Hon of National Cheng Kung University for assisting in scratch tests, and Miss S.Y. Tsai of NSC Instrument center in National Tsing Hua University for helping in the XPS analysis. Funding for this study was provided by the National Science Council of the Republic of China under Grant no. NSC 90-2216-E-259-005.
w1x W.S. Rees Jr, CVD of Non-metals, VCH Publishers, Inc, New York, NY, USA, 1996, p. 282. w2x N.C. Tombs, H.A. Wegener, R. Newman, B.T. Kenny, A.J. Coppola, Proc. IEEE 55 (1967) 1168. w3x K.H. Zaininger, A.S. Waxman, IEEE Trans. Electron. Dev. 16 (1963) 333. w4x S. Hashimoto, J.L. Peng, W.M. Gibson, Appl. Phys. Lett. 47 (1985) 1071. w5x N. Rausch, E.P. Burte, J. Electrochem. Soc. 140 (1993) 145. w6x L. Messick, J. Appl. Phys. 47 (1976) 4949. w7x M. Ohring, The Materials Science of Thin Films, Academic Press, 1992, p. 552.
334
D.-H. Kuo, K.-H. Tzeng / Thin Solid Films 460 (2004) 327–334
w8x D.H. Kuo, B.Y. Cheung, R.J. Wu, Thin Solid Films 398–399 (2001) 35. w9x J. Saraie, K. Ono, S. Takeuchi, J. Electrochem. Soc. 136 (1989) 3139. w10x H.F. Winters, P. Sigmund, J. Appl. Phys. 45 (1974) 4760. w11x C.R. Ottermann, K. Bange, Thin Solid Films 286 (1996) 32. w12x A.A.R. Elshabini-Riad, F.D. Barlow III, Thin Film Technology Handbook, McGraw-Hill International, 1997, in Table 3.1. w13x S. Ben Amur, B. Rogier, G. Baud, M. Jacquet, M. Mardin, Mater. Sci. Eng. B 57 (1998) 28.
M.D. Stamate, Thin Solid Films 372 (2000) 246. N. Rausch, E.P. Burte, Engineering 19 (1992) 725. T. Fuyuki, H. Matsunami, Jpn. J. Appl. Phys. 25 (1986) 1288. W.D. Brown, W.W. Grannemann, Solid-State Electron. 21 (1978) 837. w18x K. Okimura, N. Maeda, A. Shibata, Thin Solid Films 281–282 (1996) 427. w19x K. Fukushima, I. Yamada, T. Takagi, J. Appl. Phys. 58 (1985) 4146. w20x J. Aarik, A. Aidla, A.A. Kiisler, T. Uustare, V. Sammelselg, Thin Solid Films 305 (1997) 270. w14x w15x w16x w17x