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Influence of sputtering power on crystal quality and electrical properties of Sc-doped AlN film prepared by DC magnetron sputtering
Applied Surface Science 287 (2013) 355–358
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of sputtering power on crystal quality and electrical properties of Sc-doped AlN film prepared by DC magnetron sputtering Jian-cang Yang ∗ , Xiang-qin Meng, Cheng-tao Yang, Yao Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China
a r t i c l e
i n f o
Article history: Received 24 July 2013 Received in revised form 14 September 2013 Accepted 26 September 2013 Available online 5 October 2013 Keywords: ScAlN thin film Magnetron sputtering Sputtering power Crystal quality Electrical properties
a b s t r a c t Scandium-doped aluminum nitride alloy (ScAlN) thin films were deposited on (1 0 0) silicon substrates by DC reactive magnetron sputtering method using a scandium aluminum alloy (Sc0.06 Al0.94 ) target. The influence of sputtering power on the crystalline structure, surface morphology and electrical properties of ScAlN thin films were investigated. The XRD patterns indicated all the films showed a single pronounced hexagonal (0 0 2) peak. According to the peak intensities in /2 scans and rocking curve FWHM measurements of the (0 0 2) peaks, the crystalline quality of ScAlN thin film first increased and then decreased, reaching the best crystalline state at a sputtering power of 130 W. The best surface morphology of ScAlN thin film was obtained at 130 W and the surface roughness reached a minimum of 2.612 nm. Then the piezoelectric response of ScAlN thin films was measured and the highest value, 8.9 pC/N, was achieved at the sample with the best crystal quality. The resistivity and dielectric constant change in the same rule as the crystal quality, first increasing to a maximum value of 3.35 × 1012 cm and 13.6, and then decreasing with the sputtering power increasing. In addition, when the sputtering power was 130 W, the highest breakdown field strength and lowest leakage current were obtained, with values 1.12 MV/cm and 3 × 10−8 A, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Currently, aluminum nitride (AlN) thin film has become a standard electronic material for most conventional MEMS devices [1], especially for high-temperature piezoelectric sensors [2,3] and thin film bulk-acoustic resonator (FBAR) duplex filters [4], because of its unique combination of remarkable properties. C-axis-oriented AlN thin film has a high acoustic velocity (>10 km/s), wide energy band gap (6.2 eV), high hardness (>20 GPa) [5], high thermal conductivity (3.9 W/cm K at RT), high-temperature stability (melting point of >2000 ◦ C), high electrical resistivity and low acoustic loss [6]. Above all, AlN thin film’s ability to be deposited directly on top of CMOS would eventually enable a vision in which MEMS and circuit co-exist on the same chip. However, when compared with PZT and ZnO piezoelectric thin film, AlN exhibits a lower electro-mechanical coupling coefficient (kt 2 ) and piezoelectric constant (d33 ), which, to some extent, limits AlN’s wide applications in sensors and filters for the reason that kt 2 is the determining factor for bandwidth in filters and d33 is closely associated with the piezoelectric response and sensibility of piezoelectric sensors. Recently, it was shown that Al substitution by Sc allows for an increase of the piezoelectric respond [7–10]. Experiments
∗ Corresponding author. Tel.: +86 28 83208048; fax: +86 28 83202139. E-mail address: [email protected] (J.-c. Yang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.155
demonstrated a 400% increase in piezoelectric modulus d33 for scandium aluminum nitride (Scx Al1−x N) alloys with x = 0.43 [3]. Then, first-principles calculations confirmed that the 400% increase of the piezoelectric constant was an intrinsic alloying effect, which led to the large elastic softening along the crystal parameter c, and raised significantly the intrinsic sensitivity to axial strain resulting in the highly increased piezoelectric constant [11]. Also, it was proved that the electro-mechanical coupling coefficient kt 2 improved from 7% to 10% by alloying AlN with up to 20 mol% ScN [12]. So far, it’s noted that studies of the ScAlN system were more focused on the change of piezoelectric respond and dielectric properties with varying Sc concentration, lacking of the basic research on the influence of sputtering parameters on film’s electrical properties. The sputtering power is a critical factor in the process of depositing ScAlN film by DC reactive magnetron sputtering, directly affecting film’s crystal quality and electrical properties. In this paper, the effect of sputtering power on the crystal structure, surface topography and electrical properties were investigated systemically. 2. Experimental ScAlN films were prepared on n-type (1 0 0) silicon substrates and Platinum (Pt) thin films simultaneously by DC reactive magnetron sputtering system with the sputtering power varying from
356
J.-c. Yang et al. / Applied Surface Science 287 (2013) 355–358
90 W to 140 W. The silicon substrates were cleaned successively by acetone, absolute ethyl alcohol and deionized water to ensure clean surfaces for film growth. Platinum (Pt) thin films were prepared on half of the substrates as bottom electrode for electrical properties measurements. Because ScAl alloy targets are effective for keeping scandium concentration constant in ScAlN thin film [8], therefore in this series of experiment, a 99.99% pure scandium aluminum (ScAl) alloy target (110 mm in diameter, quality component proportion Sc:Al = 0.1:0.9) was applied. The sputtering chamber was evacuated to a pressure below 4.0 × 10−4 Pa, and then highpurity argon (99.999%) and nitrogen (99.999%) were introduced. Before deposition, the ScAl alloy target was cleaned under same deposition conditions for 2 min. For deposition, the substrate was rotated at 20 revolutions per minute (rpm) to ensure film uniformity. All ScAlN films were controlled at 2 m thick. For the electrical measurements, Pt dots (0.4 mm) were deposited onto ScAlN samples with Pt bottom electrode through a shadow-mask technique to realize the top electrodes. The crystal orientations of ScAlN films were investigated by X-ray Diffraction (XRD, Bede D1). The surface topographies of ScAlN films were observed with atomic force microscope (AFM, SPA-300HV). The electrical properties were respectively analyzed by piezoresponse force microscopy (PFM, SPA-300HV), a standard ferroelectric test system (Radiant Precision LC 2000), insulate voltage test apparatus and Impedance Analyzer (Agilent 4294A). 3. Results and discussion 3.1. Crystal orientation It is generally known that the piezoelectric response of piezoelectric thin film strongly depends on crystal orientation [13–17]. The crystal structures of ScAlN films were investigated by XRD to clear the influence of sputtering power. Fig. 1 shows the relation between the preferential orientation of the ScAlN films and the sputtering power. All the films show a single pronounced hexagonal (0 0 2) peak, indicating that the c-axis of the hexagonal ScAlN structure is normal to the substrate surface. From Fig. 1(a), it is clearly shown that the intensity of the (0 0 2) peak increases significantly with increasing sputtering power, reaching a maximum value at the sputtering power of 130 W. Above 130 W, the (0 0 2) peak intensity begins to decrease. On the other hand, the full width at half maximum (FWHM) of X-ray rocking curves gradually decreases, and then increases above 130 W, as shown in Fig. 1(b). It indicates that the crystal orientation is strongly influenced by the sputtering power. This may be attributed to the fact that at high sputtering power, the ejected metal atoms (Al, Sc) possess higher kinetic energy when they arrive on the substrate. Consequently, these Al, Sc atoms have sufficient kinetic energy to rearrange themselves to form close-packed (0 0 2) plane, resulting in a highly c-axis-oriented crystalline film structure. When the sputtering power increases to 140 W, the energy of metal atoms is so large that the large grains have formed, which leads to the degradation of crystal orientation. 3.2. Surface morphology When applying piezoelectric films to devices, the surface roughness of the piezoelectric film has a critical impact on the quality of the device [18–20]. Atomic force microscopy in tapping mode was employed to characterize the surface morphology of all layers. For each sample, three regions on were scanned and the RMS roughness value was the result of three values averaging. Fig. 2 shows the AFM patterns and the RMS roughness of ScAlN thin films prepared at various sputtering powers. It is noted that the films prepared
Fig. 1. (a) XRD patterns and (b) FWHM of X-ray rocking curves of ScAlN films prepared at various sputtering powers from 90 W to 140 W.
at 110–130 W exhibit better surface morphology and lower RMS roughness. However, when the sputtering power reaches 140 W, large grains can be observed and the RMS roughness increases dramatically. The possible reason is that large grains have formed in ScAlN films due to the larger energy, which will lead to a bad surface morphology. 3.3. Electrical properties The piezoelectric response is a critical performance indicator for piezoelectric materials. To ensure an effective value, five piezoelectric response values were obtained for each sample, and the data shown in figure was the result of five values averaging. Fig. 3 shows the dependence of the piezoelectric response on sputtering power. The sputtering power obviously influences the piezoelectric response. The mean piezoelectric response first increases and then decreases with increasing the sputtering power. The maximum of piezoelectric response of 8.9 pC/N is obtained at the sputtering power of 130 W. We think the better piezoelectric response can be attributed to the better crystal quality of ScAlN film. Also, a piezoelectric response testing on pure AlN film with (0 0 2) preferred orientation was also performed with the same equipment, obtaining a value of 6.5 pC/N, which indicated an increase of piezoelectric response with Sc doping. Resistivity and dielectric constant are also important indicators of measuring piezoelectric film’s electrical performance. Fig. 4 shows the varying pattern of the ScAlN films’ resistivity and dielectric constant at the sputtering power. All the data in the figure are the result of five test values averaging. With the sputtering power increasing, the resistivity first increases and then decreases,
J.-c. Yang et al. / Applied Surface Science 287 (2013) 355–358
357
Fig. 4. Dependence of resistivity and dielectric constant on sputtering powers.
Fig. 5. Dependence of breakdown field strength and leakage current on sputtering powers. Fig. 2. (a) AFM images and (b) the RMS roughness of ScAlN film surfaces prepared at various sputtering powers.
reaching a maximum of 3.35 × 1012 cm at the sputtering power 130 W. Dielectric constant changes in the same rule and gains a peak of 13.6 at sputtering power 130 W. We think the possible reason along with the increase of sputtering power is the high quality of film crystallization, that is, uniform size and less defects, which leads to high resistivity and low leakage current [21]. Accordingly, the dielectric constant is high. When the sputtering power is too large, the leakage current of film increases because
of excessive crystallization, grain coarsening and the increasing number of grain boundary, which results in the decreasing of resistivity and dielectric constant. In addition, the determinations of the leakage current and the dielectric breakdown field strength are both important for dielectrics. Fig. 5 shows the varying pattern of the ScAlN films’ leakage current and dielectric breakdown field strength at the sputtering power. All the data in the figure are the result of five test values averaging. With the sputtering power increasing, the breakdown field strength first increases and then decreases, reaching a maximum of 1.12 MV/cm at sputtering power 130 W. The leakage current first decreases to a minimum of 3 × 10−8 A at the sputtering power 130 W and then increases. 4. Conclusions
Fig. 3. Dependence of piezoelectric response on sputtering powers.
A series of ScAlN films was prepared by DC reactive magnetron sputtering at different sputtering powers varying from 90 W to 140 W. The influence of sputtering power on the crystalline structure, surface topography and electrical properties were systematically studied. The XRD intensity of (0 0 2) oriented peak increases with increasing sputtering power, reaching a maximum value at a sputtering power of 130 W. Above 130 W, the (0 0 2) peak intensity dramatically decreases. According to the peak intensities in /2 scans and rocking curve FWHM measurements of the (0 0 2) peaks, the crystalline quality of ScAlN thin film first increases and then decreases, reaching the best crystalline state at a sputtering power of 130 W. The best surface morphology of ScAlN thin film was also obtained at 130 W and the surface roughness reaches a minimum of 2.612 nm. The piezoelectric response of ScAlN thin films
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J.-c. Yang et al. / Applied Surface Science 287 (2013) 355–358
were measured and the highest value, 8.9 pC/N, was achieved at the sample with the best crystal quality. The resistivity and dielectric constant change in the same rule as the crystal quality, first increasing to a maximum value of 3.35 × 1012 cm and 13.6, and then decreasing with the power increasing. In addition, when the sputtering power was 130 W, the highest breakdown field strength and lowest leakage current were obtained, with values 1.12 MV/cm and 3 × 10−8 A, respectively. Acknowledgement This article is supported by Project supported by the Fundamental Research Funds for the Central Universities of Ministry of Education of China. References [1] G. Piazza, V. Felmetsger, P. Muralt, Roy H. Olsson III, R. Ruby, Piezoelectric aluminum nitride thin films for microelectromechanical systems, MRS Bull. 37 (2012) 1051–1061. [2] C. Zuniga, M. Rinaldi, S.M. Khamis, A.T. Johnson, G. Piazza, Nanoenabled microelectromechanical sensor for volatile organic chemical detection, Appl. Phys. Lett. 94 (22) (2009) 223122. [3] M. Akiyama, T. Kamohara, A. Teshigahara, Y. Takeuchi, N. Kawahara, Enhancement of piezoelectric response in scandium aluminum nitride alloy thin films prepared by dual reactive cosputtering, Adv. Mater. 21 (2009) 593–596. [4] P. Muralt, Recent progress in materials issues for piezoelectric MEMS, J. Am. Ceram. Soc. 91 (5) (2008) 1385–1396. [5] P. Limsuwan, N. Udomkan, S. Meejoo, P. Winotai, Surface morphology of submicron crystals in aluminum nitride films grown by DC magnetron sputtering, Int. J. Mod. Phys. B 19 (2005) 2073–2083. [6] J. Hao, F. Bin, D. Shurong, Z. Changjian, Z. Jian, Y. Yi, et al., A model for rapid Tin Whisker growth on the surface of ErSn3 phase, J. Electron. Mater. 41 (2012) 184–189. [7] M. Akiyama, T. Kamohara, K. Kano, A. Teshigahara, N. Kawahara, Influence of oxygen concentration in sputtering gas on piezoelectric response of aluminum nitride thin films, Appl. Phys. Lett. 93 (2008) 021903.
[8] M. Akiyama, T. Tabaru, K. Nishikubo, A. Teshigahara, Preparation of scandium aluminum nitride thin films by using scandium aluminum alloy sputtering target and design of experiments, J. Ceram. Soc. Jpn. 118 (2010) 1166–1169. [9] R. Matloub, A. Artieda, C. Sandu, E. Milyutin, P. Muralt, Electromechanical properties of Al0.9Sc0.1N thin films evaluated at 2.5 GHz film bulk acoustic resonators, Appl. Phys. Lett. 99 (2011) 092903. [10] M. Moreira, J. Bjurstrom, I. Katardjev, V. Yantchev, Aluminum scandium nitride thin-film bulk acoustic resonators for wide band applications, Vacuum 86 (2011) 23–26. [11] F. Tasnádi, B. Alling, C. Höglund, G. Wingqvist, J. Birch, L. Hultman, et al., Origin of the anomalous piezoelectric response in wurtzite Scx Al1−x N alloys, Phys. Rev. Lett. 104 (2010) 137601. [12] G. Wingqvist, F. Tasnadi, A. Zukauskaite, J. Birch, H. Arwin, L. Hultman, Increased electromechanical coupling in w-Scx Al1−x N, Appl. Phys. Lett. 97 (2010) 112902. [13] V. Ranjan, L. Bellaiche, E.J. Walter, Strained hexagonal ScN: a material with unusual structural and optical properties, Phys. Rev. Lett. 90 (2003) 257602. [14] X.B. Wang, C. Song, D.M. Li, K.W. Geng, F. Zeng, F. Pan, The influence of different doping elements on microstructure, piezoelectric coefficient and resistivity of sputtering ZnO film, Appl. Surf. Sci. 253 (2006) 1639–1643. [15] F. Martin, P. Muralt, M.A. Dubois, A. Pezous, Thickness dependence of the properties of highly c-axis textured AlN thin films, J. Vac. Sci. Technol. A 22 (2004) 361–365. [16] A. Ababneh, M. Alsumady, H. Seidel, T. Manzaneque, J. Hernando-Garcia, J.L. Sanchez-Rojas, A. Bitter, U. Schmid, C-axis orientation and piezoelectric coefficients of AlN thin films sputter-deposited on titanium bottom electrodes, Appl. Surf. Sci. 259 (2012) 59–65. [17] A. Ababneh, U. Schmid, J. Hernando, J.L. Sanchez-Rojas, H. Seidel, The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films, Mater. Sci. Eng. B-Adv. 172 (2010) 253–258. [18] X.H. Xu, H.S. Wu, C.J. Zhang, Z.H. Jin, Morphological properties of AlN piezoelectric thin films deposited by DC reactive magnetron sputtering, Thin Solid Films 388 (2001) 62–67. [19] J.G. Rodriguez-Madrid, G.F. Iriarte, O.A. Williams, F. Calle, High precision pressure sensors based on SAW devices in the GHz range, Sensor Actuat. A 189 (2013) 364–369. [20] M. Clement, L. Vergara, J. Sangrador, E. Iborra, A. Sanz-Hervas., SAW characteristics of AlN films sputtered on silicon substrates, Ultrasonics 42 (2004) 403–407. [21] A. Bittner, A. Ababneh, H. Seidel, U. Schmid, Influence of the crystal orientation on the electrical properties of AlN thin films on LTCC substrates, Appl. Surf. Sci. 257 (2010) 1088–1091.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of sputtering power on crystal quality and electrical properties of Sc-doped AlN film prepared by DC magnetron sputtering Jian-cang Yang ∗ , Xiang-qin Meng, Cheng-tao Yang, Yao Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, China
a r t i c l e
i n f o
Article history: Received 24 July 2013 Received in revised form 14 September 2013 Accepted 26 September 2013 Available online 5 October 2013 Keywords: ScAlN thin film Magnetron sputtering Sputtering power Crystal quality Electrical properties
a b s t r a c t Scandium-doped aluminum nitride alloy (ScAlN) thin films were deposited on (1 0 0) silicon substrates by DC reactive magnetron sputtering method using a scandium aluminum alloy (Sc0.06 Al0.94 ) target. The influence of sputtering power on the crystalline structure, surface morphology and electrical properties of ScAlN thin films were investigated. The XRD patterns indicated all the films showed a single pronounced hexagonal (0 0 2) peak. According to the peak intensities in /2 scans and rocking curve FWHM measurements of the (0 0 2) peaks, the crystalline quality of ScAlN thin film first increased and then decreased, reaching the best crystalline state at a sputtering power of 130 W. The best surface morphology of ScAlN thin film was obtained at 130 W and the surface roughness reached a minimum of 2.612 nm. Then the piezoelectric response of ScAlN thin films was measured and the highest value, 8.9 pC/N, was achieved at the sample with the best crystal quality. The resistivity and dielectric constant change in the same rule as the crystal quality, first increasing to a maximum value of 3.35 × 1012 cm and 13.6, and then decreasing with the sputtering power increasing. In addition, when the sputtering power was 130 W, the highest breakdown field strength and lowest leakage current were obtained, with values 1.12 MV/cm and 3 × 10−8 A, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Currently, aluminum nitride (AlN) thin film has become a standard electronic material for most conventional MEMS devices [1], especially for high-temperature piezoelectric sensors [2,3] and thin film bulk-acoustic resonator (FBAR) duplex filters [4], because of its unique combination of remarkable properties. C-axis-oriented AlN thin film has a high acoustic velocity (>10 km/s), wide energy band gap (6.2 eV), high hardness (>20 GPa) [5], high thermal conductivity (3.9 W/cm K at RT), high-temperature stability (melting point of >2000 ◦ C), high electrical resistivity and low acoustic loss [6]. Above all, AlN thin film’s ability to be deposited directly on top of CMOS would eventually enable a vision in which MEMS and circuit co-exist on the same chip. However, when compared with PZT and ZnO piezoelectric thin film, AlN exhibits a lower electro-mechanical coupling coefficient (kt 2 ) and piezoelectric constant (d33 ), which, to some extent, limits AlN’s wide applications in sensors and filters for the reason that kt 2 is the determining factor for bandwidth in filters and d33 is closely associated with the piezoelectric response and sensibility of piezoelectric sensors. Recently, it was shown that Al substitution by Sc allows for an increase of the piezoelectric respond [7–10]. Experiments
∗ Corresponding author. Tel.: +86 28 83208048; fax: +86 28 83202139. E-mail address: [email protected] (J.-c. Yang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.155
demonstrated a 400% increase in piezoelectric modulus d33 for scandium aluminum nitride (Scx Al1−x N) alloys with x = 0.43 [3]. Then, first-principles calculations confirmed that the 400% increase of the piezoelectric constant was an intrinsic alloying effect, which led to the large elastic softening along the crystal parameter c, and raised significantly the intrinsic sensitivity to axial strain resulting in the highly increased piezoelectric constant [11]. Also, it was proved that the electro-mechanical coupling coefficient kt 2 improved from 7% to 10% by alloying AlN with up to 20 mol% ScN [12]. So far, it’s noted that studies of the ScAlN system were more focused on the change of piezoelectric respond and dielectric properties with varying Sc concentration, lacking of the basic research on the influence of sputtering parameters on film’s electrical properties. The sputtering power is a critical factor in the process of depositing ScAlN film by DC reactive magnetron sputtering, directly affecting film’s crystal quality and electrical properties. In this paper, the effect of sputtering power on the crystal structure, surface topography and electrical properties were investigated systemically. 2. Experimental ScAlN films were prepared on n-type (1 0 0) silicon substrates and Platinum (Pt) thin films simultaneously by DC reactive magnetron sputtering system with the sputtering power varying from
356
J.-c. Yang et al. / Applied Surface Science 287 (2013) 355–358
90 W to 140 W. The silicon substrates were cleaned successively by acetone, absolute ethyl alcohol and deionized water to ensure clean surfaces for film growth. Platinum (Pt) thin films were prepared on half of the substrates as bottom electrode for electrical properties measurements. Because ScAl alloy targets are effective for keeping scandium concentration constant in ScAlN thin film [8], therefore in this series of experiment, a 99.99% pure scandium aluminum (ScAl) alloy target (110 mm in diameter, quality component proportion Sc:Al = 0.1:0.9) was applied. The sputtering chamber was evacuated to a pressure below 4.0 × 10−4 Pa, and then highpurity argon (99.999%) and nitrogen (99.999%) were introduced. Before deposition, the ScAl alloy target was cleaned under same deposition conditions for 2 min. For deposition, the substrate was rotated at 20 revolutions per minute (rpm) to ensure film uniformity. All ScAlN films were controlled at 2 m thick. For the electrical measurements, Pt dots (0.4 mm) were deposited onto ScAlN samples with Pt bottom electrode through a shadow-mask technique to realize the top electrodes. The crystal orientations of ScAlN films were investigated by X-ray Diffraction (XRD, Bede D1). The surface topographies of ScAlN films were observed with atomic force microscope (AFM, SPA-300HV). The electrical properties were respectively analyzed by piezoresponse force microscopy (PFM, SPA-300HV), a standard ferroelectric test system (Radiant Precision LC 2000), insulate voltage test apparatus and Impedance Analyzer (Agilent 4294A). 3. Results and discussion 3.1. Crystal orientation It is generally known that the piezoelectric response of piezoelectric thin film strongly depends on crystal orientation [13–17]. The crystal structures of ScAlN films were investigated by XRD to clear the influence of sputtering power. Fig. 1 shows the relation between the preferential orientation of the ScAlN films and the sputtering power. All the films show a single pronounced hexagonal (0 0 2) peak, indicating that the c-axis of the hexagonal ScAlN structure is normal to the substrate surface. From Fig. 1(a), it is clearly shown that the intensity of the (0 0 2) peak increases significantly with increasing sputtering power, reaching a maximum value at the sputtering power of 130 W. Above 130 W, the (0 0 2) peak intensity begins to decrease. On the other hand, the full width at half maximum (FWHM) of X-ray rocking curves gradually decreases, and then increases above 130 W, as shown in Fig. 1(b). It indicates that the crystal orientation is strongly influenced by the sputtering power. This may be attributed to the fact that at high sputtering power, the ejected metal atoms (Al, Sc) possess higher kinetic energy when they arrive on the substrate. Consequently, these Al, Sc atoms have sufficient kinetic energy to rearrange themselves to form close-packed (0 0 2) plane, resulting in a highly c-axis-oriented crystalline film structure. When the sputtering power increases to 140 W, the energy of metal atoms is so large that the large grains have formed, which leads to the degradation of crystal orientation. 3.2. Surface morphology When applying piezoelectric films to devices, the surface roughness of the piezoelectric film has a critical impact on the quality of the device [18–20]. Atomic force microscopy in tapping mode was employed to characterize the surface morphology of all layers. For each sample, three regions on were scanned and the RMS roughness value was the result of three values averaging. Fig. 2 shows the AFM patterns and the RMS roughness of ScAlN thin films prepared at various sputtering powers. It is noted that the films prepared
Fig. 1. (a) XRD patterns and (b) FWHM of X-ray rocking curves of ScAlN films prepared at various sputtering powers from 90 W to 140 W.
at 110–130 W exhibit better surface morphology and lower RMS roughness. However, when the sputtering power reaches 140 W, large grains can be observed and the RMS roughness increases dramatically. The possible reason is that large grains have formed in ScAlN films due to the larger energy, which will lead to a bad surface morphology. 3.3. Electrical properties The piezoelectric response is a critical performance indicator for piezoelectric materials. To ensure an effective value, five piezoelectric response values were obtained for each sample, and the data shown in figure was the result of five values averaging. Fig. 3 shows the dependence of the piezoelectric response on sputtering power. The sputtering power obviously influences the piezoelectric response. The mean piezoelectric response first increases and then decreases with increasing the sputtering power. The maximum of piezoelectric response of 8.9 pC/N is obtained at the sputtering power of 130 W. We think the better piezoelectric response can be attributed to the better crystal quality of ScAlN film. Also, a piezoelectric response testing on pure AlN film with (0 0 2) preferred orientation was also performed with the same equipment, obtaining a value of 6.5 pC/N, which indicated an increase of piezoelectric response with Sc doping. Resistivity and dielectric constant are also important indicators of measuring piezoelectric film’s electrical performance. Fig. 4 shows the varying pattern of the ScAlN films’ resistivity and dielectric constant at the sputtering power. All the data in the figure are the result of five test values averaging. With the sputtering power increasing, the resistivity first increases and then decreases,
J.-c. Yang et al. / Applied Surface Science 287 (2013) 355–358
357
Fig. 4. Dependence of resistivity and dielectric constant on sputtering powers.
Fig. 5. Dependence of breakdown field strength and leakage current on sputtering powers. Fig. 2. (a) AFM images and (b) the RMS roughness of ScAlN film surfaces prepared at various sputtering powers.
reaching a maximum of 3.35 × 1012 cm at the sputtering power 130 W. Dielectric constant changes in the same rule and gains a peak of 13.6 at sputtering power 130 W. We think the possible reason along with the increase of sputtering power is the high quality of film crystallization, that is, uniform size and less defects, which leads to high resistivity and low leakage current [21]. Accordingly, the dielectric constant is high. When the sputtering power is too large, the leakage current of film increases because
of excessive crystallization, grain coarsening and the increasing number of grain boundary, which results in the decreasing of resistivity and dielectric constant. In addition, the determinations of the leakage current and the dielectric breakdown field strength are both important for dielectrics. Fig. 5 shows the varying pattern of the ScAlN films’ leakage current and dielectric breakdown field strength at the sputtering power. All the data in the figure are the result of five test values averaging. With the sputtering power increasing, the breakdown field strength first increases and then decreases, reaching a maximum of 1.12 MV/cm at sputtering power 130 W. The leakage current first decreases to a minimum of 3 × 10−8 A at the sputtering power 130 W and then increases. 4. Conclusions
Fig. 3. Dependence of piezoelectric response on sputtering powers.
A series of ScAlN films was prepared by DC reactive magnetron sputtering at different sputtering powers varying from 90 W to 140 W. The influence of sputtering power on the crystalline structure, surface topography and electrical properties were systematically studied. The XRD intensity of (0 0 2) oriented peak increases with increasing sputtering power, reaching a maximum value at a sputtering power of 130 W. Above 130 W, the (0 0 2) peak intensity dramatically decreases. According to the peak intensities in /2 scans and rocking curve FWHM measurements of the (0 0 2) peaks, the crystalline quality of ScAlN thin film first increases and then decreases, reaching the best crystalline state at a sputtering power of 130 W. The best surface morphology of ScAlN thin film was also obtained at 130 W and the surface roughness reaches a minimum of 2.612 nm. The piezoelectric response of ScAlN thin films
358
J.-c. Yang et al. / Applied Surface Science 287 (2013) 355–358
were measured and the highest value, 8.9 pC/N, was achieved at the sample with the best crystal quality. The resistivity and dielectric constant change in the same rule as the crystal quality, first increasing to a maximum value of 3.35 × 1012 cm and 13.6, and then decreasing with the power increasing. In addition, when the sputtering power was 130 W, the highest breakdown field strength and lowest leakage current were obtained, with values 1.12 MV/cm and 3 × 10−8 A, respectively. Acknowledgement This article is supported by Project supported by the Fundamental Research Funds for the Central Universities of Ministry of Education of China. References [1] G. Piazza, V. Felmetsger, P. Muralt, Roy H. Olsson III, R. Ruby, Piezoelectric aluminum nitride thin films for microelectromechanical systems, MRS Bull. 37 (2012) 1051–1061. [2] C. Zuniga, M. Rinaldi, S.M. Khamis, A.T. Johnson, G. Piazza, Nanoenabled microelectromechanical sensor for volatile organic chemical detection, Appl. Phys. Lett. 94 (22) (2009) 223122. [3] M. Akiyama, T. Kamohara, A. Teshigahara, Y. Takeuchi, N. Kawahara, Enhancement of piezoelectric response in scandium aluminum nitride alloy thin films prepared by dual reactive cosputtering, Adv. Mater. 21 (2009) 593–596. [4] P. Muralt, Recent progress in materials issues for piezoelectric MEMS, J. Am. Ceram. Soc. 91 (5) (2008) 1385–1396. [5] P. Limsuwan, N. Udomkan, S. Meejoo, P. Winotai, Surface morphology of submicron crystals in aluminum nitride films grown by DC magnetron sputtering, Int. J. Mod. Phys. B 19 (2005) 2073–2083. [6] J. Hao, F. Bin, D. Shurong, Z. Changjian, Z. Jian, Y. Yi, et al., A model for rapid Tin Whisker growth on the surface of ErSn3 phase, J. Electron. Mater. 41 (2012) 184–189. [7] M. Akiyama, T. Kamohara, K. Kano, A. Teshigahara, N. Kawahara, Influence of oxygen concentration in sputtering gas on piezoelectric response of aluminum nitride thin films, Appl. Phys. Lett. 93 (2008) 021903.
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