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Piezoelectric properties of polycrystalline AlN thin films for MEMS application
Sensors and Actuators A 132 (2006) 658–663
Piezoelectric properties of polycrystalline AlN thin films for MEMS application K. Tonisch a,∗ , V. Cimalla a , Ch. Foerster a , H. Romanus a , O. Ambacher a , D. Dontsov b a
Institute for Micro- and Nanotechnologies, Technical University Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany b SIOS Meßtechnik GmbH, Am Vogelherd 46, 98693 Ilmenau, Germany Received 29 April 2005; received in revised form 27 February 2006; accepted 7 March 2006 Available online 18 April 2006
Abstract The piezoelectric coefficient d33eff of aluminium nitride thin films was measured using both, the piezoresponse force microscopy and an interferometric technique. Wurtzite AlN thin films were prepared on Si (1 1 1) substrates by reactive dc-sputtering and by metal organic chemical vapor deposition (MOCVD). Direct measurements of the inverse piezoelectric effect in the picometer range showed that the acceptable tolerance in the crystal orientation is much larger for MEMS applications than expected previously. The value of the effective piezoelectric coefficient d33 for the prepared AlN thin films remained as high as 5.1 pm/V even for lower degrees of texture. © 2006 Elsevier B.V. All rights reserved. Keywords: MEMS; Actuation; Sputtering; Aluminium nitride; Piezoresponse; Interferometry
1. Introduction Micromechanical resonators show significant promise for many sensor applications such as chemical and biological sensing, electrometry and scanning probe techniques. In these applications, a change in mass, temperature, charge, or any other applied force induces a small shift in the resonance frequency of the oscillator [1]. Typically, resonators require both an actuation and a detection of the resonance frequency. Such responses can be observed through a variety of physical detection methods including electronic and optical effects, i.e. changes in resistance (piezoresistivity), changes in capacitance, and changes in charge (piezoelectricity) [2]. The limiting feature for the application of thin films as actuation layers in any kind of micromechanical applications, is the strength of the (inverse) piezoelectric effect in vertical and in-plane direction, if a vertical electrical field is applied. The main advantage of this application compared to SAW or BAW devices is simply the fact, that in the latter ones acoustic losses can reduce the performance drastically even if the piezoelectric effect is high.
∗
Corresponding author. Tel.: +49 3677 693352. E-mail address: [email protected] (K. Tonisch).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.03.001
The exceptional properties of wide-bandgap III–V nitride semiconductors are promising for such applications. The piezoelectricity of wurtzite group III-nitrides is characterized by three independent piezoelectric coefficients d33 , d31 (=d32 ) and d15 (=d24 ). Among the nitrides AlN has the highest thermal conductivity at low temperatures, good mechanical strength, high resistivity and corrosion resistance, and the largest piezoelectric coefficients. Thus, AlN resonators are attractive building blocks for electromechanical devices in micro- and nanometer scale. The highest piezoelectric performance would be achieved by a defect-free, epitaxial film with the c-axis or [0 0 0 1] direction normal to the substrate. Unfortunately, epitaxial growth of AlN only occurs at high temperatures and on specific substrates which makes the epitaxial deposition incompatible for the integration in CMOS or other technologies sensitive to heat. Nevertheless it is possible, and has been shown by many researchers, to grow highly textured AlN thin films at temperatures as low as 500 ◦ C [3,4]. But unlike epitaxial thin films which most often only display point defects and threading dislocations, polycrystalline films are associated with grain boundaries. Additionally, the individual grains may exhibit a deviation from the ideal orientation, which is commonly referred to as tilt. Since the lack of epitaxial growth can also lead to a change in the orientation of the single grains, polycrystalline thin films may also happen to display no piezoelectricity at all, if the distribution of the
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¯ orientation is equal. The above mentioned [0 0 0 1] and [0 0 0 1] tilt can be directly determined via measuring the full width at half maximum (FWHM) of the X-ray diffraction rocking curve. Since both the tilt and the film thickness have an influence on the FWHM of the rocking curve, the variation of the film thickness did not exceed the factor 2 between the thinnest and the thickest layer. While epitaxial or near-epitaxial thin films have FWHMs of tenth of a degree or less, polycrystalline films can have anything up to 15◦ depending on the chosen growth conditions. In this work we will show by direct measurements of the inverse piezoelectric effect in the picometer range that the acceptable tolerance in the crystal orientation for actuator and resonator applications is much larger than expected.
2. Experimental Aluminium nitride thin films of different structural quality have been prepared on highly n-doped (1 1 1) silicon wafers using both a low and a high temperature method: reactive magnetron sputtering [5] and metal organic chemical vapor deposition (MOCVD) [6]. The sputtering was performed in pure nitrogen atmosphere at 3 × 10−3 mbar. The temperature of the substrates varied from 350 to 500 ◦ C. Prior to the sputtering of the AlN films, the silicon substrate was cleaned by sputter-etching and afterwards transferred to the sputter chamber remaining under high vacuum conditions. This surface treatment was applied in order to remove the native oxide which is usually not homogeneously distributed and could therefore lead to a local polar axis reversal in the AlN thin film [7]. Additional samples of aluminium nitride thin films were grown (MOCVD). Thereby TMA and NH3 were used as precursors while hydrogen served as carrier gas. The growth temperature for different samples was varied from 1050 to 1190 ◦ C. The resulting layers were polycrystalline with different degrees of texture in the case of MOCVD and nanocrystalline in the case of sputtering. The layers thickness were between 130 and 250 nm. The morphological analysis with the atomic force microscope (Fig. 1) showed polycrystalline surfaces with a grain size of approximately 20–30 nm for the sputtered films and 100 nm for the MOCVD-grown films. An additional examination via transmis-
Fig. 2. Cross-section of a sputtered AlN film examined with transmission electron microscopy.
sion electron microscopy clearly revealed a columnar structure even for the sputtered thin films, which is shown in Fig. 2. X-ray diffraction has been performed on all samples and showed a pure (0 0 0 1) or c-axis orientation without appearance of any other orientation. In Fig. 3 some typical X-ray diffraction rocking curves of samples which were chosen for piezoelectric measurements are displayed ranging from 0.8◦ to 10.7◦ . For the piezoelectric measurements, titanium and gold were sputtered on the surface to act as an electrical contact, the substrate served as a back contact. 3. Results and discussion The piezoelectric properties of thin films are often measured indirectly by analysing the scattering parameters of a bulk acoustic wave transducer (BAW). The electromechanical coupling factor k2 is then derived by fitting the measured S21 spectra
Fig. 1. Surface morphology of a sputtered (left) and a MOCVD-grown AlN film (right) measured with atomic force microscopy with a length of 1 m in square.
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is then detected as the first harmonic component of the deflection signal. Thus, the effective piezoelectric coefficient can be calculated as the ratio between displacement t and applied voltage V: d33 =
S3 t = , E3 V
(1)
where S3 = t/t is the change of strain along the c-axis and E3 = V/t is the electric field along the c-axis. Most applications of piezolelectric thin films are based on structures, where the piezoelectric film is rigidly clamped to a substrate. The substrate then constrains the in-plane contraction and expansion of the film. Thus, only an effective value of d33eff instead of d33 is measured. Therefore most often a correction term involving the in-plane piezoelectric coefficient d31 and the elastic compliances S11 , S12 , S13 is used in order to calculate the unclamped value of d33 [12]: E d31 S13 d33 = d33eff − 2 . (2) E + SE S11 12
Fig. 3. Rocking curve of the AlN (0 0 2) reflex for layers prepared by MOCVD (dashed line) and of sputtered AlN films (solid lines) of different quality.
to theoretical computations [8]. We used two direct methods to measure the inverse piezoelectric effect in order to correlate the texture of the AlN thin films with the effective piezoelectric coefficient d33 . First, the expansion (and contraction) of the thin film, which is proportional to the applied external electrical field (both parallel to the c-axis) via the piezoelectric coefficient d33 , was measured using piezoresponse force microscopy. Second, a homodyne Michelson laser interferometer [9] was used to complement the AFM technique. While measuring the piezoelectric effects of thin films, it is important to bear in mind that the macroscopic piezoelectric response is the result of the contributions of all grains with their individual dipoles. The highest values are reached when all the dipoles are aligned along the same direction and have the same polarity, thus contributing with the same sign to the net piezoelectric response. Therefore a preferential c-axis orientation is crucial for a high piezoelectric response of thin films. Unlike Ruffner et al. [7], who examined sputtered AlN on ruthenium, we found no evidence for a distribution of different oriented dipoles. On the contrary, we observed a high piezoelectric response close to the reported values for bulk material even in case of clearly polycrystalline films. For studying the inverse piezoelectric effect, a modulation voltage is applied between the top electrode and the substrate, which causes the piezoelectric film to oscillate at the same frequency as the applied voltage. In piezoresponse force microscopy (PFM) this bias-induced deformation is detected by a common silicon AFM-tip which is brought into contact with the surface of the metallic top electrode [10,11]. When a modulation voltage is applied to the piezoelectric film, the vertical displacement of the tip follows accurately the piezoelectric motion of the sample surface. The piezoresponse of the sample
The value for S11 = 3.0, S12 = −0.9, and S13 = −0.6 in 10−12 m2 N−1 are taken from Wright [13], d31 is assumed to be −d33eff /2. PFM measurements were performed with a commercial AFM (ATOS Solver), a function generator (Agilent 33220 A) and a lock-in amplifier (Stanford Research Systems SR 830 DSP). For the PFM-measurements described here, frequencies of 1 kHz were employed, because this frequency is greater than most environmental noise frequencies and well below the resonant frequency of the AFM tip. Using this setup, even a vertical displacement as small as 10 pm could be clearly identified and quantified. A schematic of the experimental setup is shown in Fig. 4. Since the output in V of the lock-in amplifier has to be converted into a displacement of the tip in pm, the d33 coefficient of a LiNbO3 single crystal (bulk material) was measured as a reference. Fig. 5 shows the linear dependence of the displacement of the sample surface from the applied voltage measured at a sputtered aluminium nitride film. The average effective piezoelectric constant d33eff measured with PFM was calculated to 5.4 ± 0.1 pm/V for the sputtered samples.
Fig. 4. Schematic of the PFM experimental setup. A modulation voltage is applied to the conducting tip and to the top electrode, which causes the piezoelectric film to oscillate at the same frequency as the applied voltage.
K. Tonisch et al. / Sensors and Actuators A 132 (2006) 658–663
Fig. 5. Typical piezoelectric displacement vs. applied voltage estimated by piezoresponse force microscopy of a sputtered sample with a thickness of 130 nm and a FWHM (rocking curve) of 8.7◦ .
Additional measurements using a homodyne Michelson interferometer setup supported the above obtained results. Here an integrated spectrum ranging from 5 to 22 kHz was recorded. An extract of the spectra for: (a) a sputtered and (b) for a MOCVD-grown (low texture, FWHM 10.7◦ ) AlN film ranging from 17.5 to 19 kHz is provided in Fig. 6 and shows the vertical displacement of the sample surface due to the applied voltage of 14 V at a modulation frequency of 18.3 kHz. The calculation of the displacement versus applied voltage results in a effective d33 of 5.5 and 5.14 pm/V, respectively. Whereas the sputtered films all showed high piezoelectric values, in the case of the MOCVD-grown samples only the intentionally low textured sample had a d33eff as high as 5.14 pm/V (interferometry). For a sample with a FWHM of 2.5◦ (not shown in Fig. 1) it was remarkably reduced to 4.15 pm/V (measured with PFM) while it was not measurable at all for the thin film with the highest degree of texture (FWHM of 0.8◦ ). The reason was an increase of the vertical conductivity of the AlN films.
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Coming close to the growth mode of heteroepitaxy (by adjusting the growth conditions towards higher degrees of texture) resulted in an increase of the tensile strain in the AlN film. This led to an increased out-diffusion from the silicon substrate. Since silicon is a common dopant for group III-nitrides, this resulted in an increased conductivity and even to the formation of conducting path where the silicon “decorated” grain boundaries and threading dislocations [17,18]. In total, and especially for the sputtered films, the measured values were much higher than could have been expected for clamped and polycrystalline thin films. The corresponding unclamped values as calculated with the correction term in Eq. (2) are given in Table 1 and reach surprisingly high values up to 7.07 pm/V. Nevertheless these high values are consistent with several results that have been reported elsewhere [10,14–16]. An overview of values measured in this work and of those reported elsewhere (measured and calculated) are given in Table 1. All AlN films showed high piezoelectric responses and only a slight reduction of d33 due to lower degree of texture, which is consistent with a theoretical evaluation made by Mishin et al. [19] of the effect of an increased FWHM on the piezoelectric coefficient d33 . While the coupling factor k2 is reported to decrease dramatically for a FWHM higher than 4◦ [20], the piezoelectric coefficient d33 decreases only moderately from 5.5 pm/V down to 5.1 pm/V if the FWHM of the rocking curve reached 10◦ or higher. Both parameters, k2 (displayed as k for a better comparison) and d33eff (from both, PFM and interferometry) and their dependency from the degree of texture of the film, represented by the FWHM of the rocking curve, are displayed in Fig. 7. Even films that were only poorly textured showed high values of the effective piezoelectric constant d33eff . Therefore we assume that the degree of texture is less important for measuring d33eff , than it is for measuring the coupling factor k2 because it is a more direct approach to the piezoelectric performance of
Fig. 6. Typical piezoelectric displacement vs. frequency estimated by optical interferometry for: (a) sputtered AlN and (b) MOCVD-grown AlN with an applied modulation voltage of 14 V at 18.3 kHz.
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Table 1 Values of the piezoelectric coefficient d33 found in this work compared to literature values Sample (top/bottom)
Growth method
AFM d33 (pm/V)
Interferometry d33 (pm/V)
This work AlN/Si (1 1 1) AlN/Si (1 1 1) AlN/Si (1 1 1)
Sputtering MOCVDb MOCVDc
6.94 (5.4 ± 0.1)a 5.47 (4.15 ± 0.1)a
7.07 (5.5 ± 0.1)a
Sputtering Sputtering MBE
4.0 ± 1.8 [10] 6.0 ± 3.0 [10] 3.7 ± 1.9 [10]
Literature AlN/Si AlN/Si AlN/Si AlN/Pt (1 1 1) AlN/Si (1 0 0) AlN/Si (1 0 0) AlN AlN AlN a b c d
CVD MPACVDd Bulk Calculated Calculated
6.56 (5.1 ± 0.1)a
6.83 [16] 4.0 ± 0.1 [21] 3.2 ± 0.3 [21] 5.6 [13] 6.72 [15] 6.4 [22]
The corresponding values for the effective d33eff (which has been measured) are given in parantheses. Small FWHM. High FWHM. Microwave plasma assisted CVD.
References
Fig. 7. Comparison of the dependency of the coupling factor k in % [13] and of the effective piezoelectric coefficient d33eff in pm/V (this work) on the FWHM in degree.
a crystal or thin film. As long as the dipoles of all grains (or most of them) have the same orientation, the inverse piezoelectric effect of the thin film will be high and does not seem to be affected by other effects caused by the polycrystalline structure. Since the examination with TEM did not give any indication that the thin films might be porous, we consider the assumption d31 = −d33eff /2 to be valid even for poorly textured AlN films. Thus, polycrystalline AlN thin films sputtered on silicon substrates are well suited for the use as actuator layers in micromechanical systems such as resonator beam or active cantilever structures. Acknowledgements This work has been funded and supported by the Deutsche Forschungsgemeinschaft, SPP 1157: “Integrierte elektrokeramische Funktionsstrukturen”, contract no. AM 105/2-1
[1] J.W. Gardner, V.K. Varadan, O.O. Awadelkarim, Microsensors MEMS and Smart Devices, Wiley, 2001. [2] Proceedings of the International Conference on Solid State Sensors and Actuators, June 1997. Proceedings of the IEEE Workshop Micro Electro Mechanical Systems, Heidelberg, 1998. [3] T.T. Leung, C.W. Ong, Nearly amorphous to epitaxial growth of aluminium nitride films, Diamond Rel. Mater. 18 (5) (2005) A79. [4] T.P. Dr¨usedau, J. Bl¨asing:, Optical and structural properties of highly c-axis oriented aluminum nitride prepared by sputter-deposition in pure nitrogen, Thin Solid Films 377–378 (2000) 27–31. [5] 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. [6] O. Ambacher, Growth and application of Group III-nitrides, J. Phys. D: Appl. Phys. 31 (1998) 2653. [7] J.A. Ruffner, P.G. Clem, B.A. Tuttle, D. Dimos, D.M. Gonzales, Effect of substrate composition on the piezoelectric response of reactively sputtered AlN thin films, Thin Solid Films 354 (1999) 256. [8] L. Vergara, M. Clement, E. Iborra, A. Sanz-Hervas, J. Garcia Lopez, Y. Morilla, J. Sangrador, M.A. Respaldiza, Influence of oxygen and argon on the crystal quality and piezoelectric response of AlN sputtered thin films, Diamond Rel. Mater. 13 (2004) 839. [9] D. Dontsov, G. J¨ager, H. B¨uchner, U. Gerhardt, Laser interferometric techniques for vibration analysis, in: Proceedings of the Fifth Seminar on Quantitative Microscopy Nanoscale PtP-F-44, 2001, p. 166. [10] B.J. Rodriguez, D.J. Kim, A.I. Kingon, R.J. Nemanich, Measurement of the effective piezoelectric constant of nitride thin films and heterostructures using scanning force microscopy, Mater. Res. Soc. Symp. Proc. 693 (2002) I9.9. [11] B.J. Rodriguez, A. Gruvermann, A.I. Kingon, R.J. Nemanich, O. Ambacher, Piezoresponse force microscopy for polarity imaging of GaN, Appl. Phys. Lett. 80 (2002) 4166. [12] D. Royer, V. Kmetik, Measurement of piezoelectric constants using an optical heterodyne interferometer, Electron. Lett. 28 (1992) 1828–1830. [13] A.F. Wright, J. Appl. Phys. 82 (1997) 2833. [14] I.L. Guy, S. Muensit, E.M. Goldys, Extensional piezoelectric coefficients of gallium nitride and aluminium nitride, Appl. Phys. Lett. 75 (1999) 4133–4135. [15] T. Kamiya, Calculation of crystal structures, dielectric constants und piezoelectric properties of wurtzite-type crystals using ab-initio periodic HartreeFock method, Jpn. J. Appl. Phys. 35 (1996) 4421–4426.
K. Tonisch et al. / Sensors and Actuators A 132 (2006) 658–663 [16] F. Martin, P. Muralt, M.-A. Dubois, A. Pezous, J. Vac. Sci. Technol. A 22 (2) (2004) 361–365. [17] V. Lebedev, Thesis, FSU Jena, 2005. [18] V. Lebedev, F.M. Morales, H. Romanus, S. Krischok, G. Ecke, V. Cimalla, M. Himmerlich, T. Stauden, D. Cengher, O. Ambacher, The role of Si as surfactant and donor in molecular beam epitaxy of AlN, J. Appl. Phys. 98 (2005) 093508. [19] S. Mishin, D.R. Marx, B. Sylvia, V. Lughi, K.L. Turner, D.R. Clarke, Sputtered AlN thin films on Si and electrodes for MEMS resonators, in: Proceedings of the 2003 IEEE Ultrasonic Symposium, 2003, p. 2028. [20] R.S. Naik, J.J. Lutsky, R. Reif, C.G. Sodini, A. Becker, L. Fetter, H. Huggins, R. Miller, J. Pastalan, G. Rittenhouse, Y.-H. Wong, IEEE Trans. Ultrason., Ferroelectr. Freq. Contr. 47 (2000) 292. [21] K. Tsubouchi, K. Sugai, N. Mikoshiba, Proceedings of the IEEE Ultrasonic Symposium, New York, 1982, p. 340. [22] F. Bernardini, V. Fiorentini, First principles calculation of the piezoelectric tensor of III–V nitrides, APL 80 (22) (2002) 4145–4147.
Biographies Katja Tonisch is currently a PhD student at the Technical University of Ilmenau, Germany. She recieved her Diploma (MS) in Electrical Engineering in 2005. Her research interests include the growth of group III-Nitrides with MOCVD and nitride-based MEMS. Volker Cimalla is currently working as assistant professor at the TU Ilmenau. He received his Diploma (MS) in Electrical Engineering and his PhD from the
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TU Ilmenau in 1993 and 1998, respectively. He spend several years as post doctorate at the North Carolina State University, USA and on Crete, Greece. His current research interest includes III-Nitrides and MEMS/NEMS technology. Christian Foerster is currently working as research engineer at Infineon, Villach (Austria). He received his Diploma (MS) in Electrical Engineering and his PhD from the TU Ilmenau in 2002 and 2005, respectively. Henry Romanus is currently a scientific staff member of the Center for Microand Nanotechnologies (ZMN) at Technical University Ilmenau. He received his Diploma (MS) and his PhD in Electrical Engineering and Materials Science from the TU Ilmenau in 1995 and 2004, respectively. His research interests include material research with electron microscopy (SEM, TEM) and X-ray diffraction, focussed ion beam technology and contact metallization. Oliver Ambacher is currently working as professor at the TU Ilmenau. He received his Diploma (MS) in Physics in 1989 from the LMU Munich and his PhD in 1993 from the Technical University of Munich. He worked several years as researcher at the Walter-Schottky-Institute Munich and at Cornell University, USA. Since 2002 he is head of Department of Nanotechnology and since 2004 he is head of the Center for Micro- and Nanotechnologies, TU Ilmenau. His research interests includes the growth and application of Group III-Nitrides. Denis Dontsov is currently working as research engineer at SIOS GmbH. He received his MS from the Polytechnical Institute Kiev in 1993 and his PhD from the TU Ilmenau in 2001. His research interests include the developement of piezoelectrical measurement techniques.
Piezoelectric properties of polycrystalline AlN thin films for MEMS application K. Tonisch a,∗ , V. Cimalla a , Ch. Foerster a , H. Romanus a , O. Ambacher a , D. Dontsov b a
Institute for Micro- and Nanotechnologies, Technical University Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany b SIOS Meßtechnik GmbH, Am Vogelherd 46, 98693 Ilmenau, Germany Received 29 April 2005; received in revised form 27 February 2006; accepted 7 March 2006 Available online 18 April 2006
Abstract The piezoelectric coefficient d33eff of aluminium nitride thin films was measured using both, the piezoresponse force microscopy and an interferometric technique. Wurtzite AlN thin films were prepared on Si (1 1 1) substrates by reactive dc-sputtering and by metal organic chemical vapor deposition (MOCVD). Direct measurements of the inverse piezoelectric effect in the picometer range showed that the acceptable tolerance in the crystal orientation is much larger for MEMS applications than expected previously. The value of the effective piezoelectric coefficient d33 for the prepared AlN thin films remained as high as 5.1 pm/V even for lower degrees of texture. © 2006 Elsevier B.V. All rights reserved. Keywords: MEMS; Actuation; Sputtering; Aluminium nitride; Piezoresponse; Interferometry
1. Introduction Micromechanical resonators show significant promise for many sensor applications such as chemical and biological sensing, electrometry and scanning probe techniques. In these applications, a change in mass, temperature, charge, or any other applied force induces a small shift in the resonance frequency of the oscillator [1]. Typically, resonators require both an actuation and a detection of the resonance frequency. Such responses can be observed through a variety of physical detection methods including electronic and optical effects, i.e. changes in resistance (piezoresistivity), changes in capacitance, and changes in charge (piezoelectricity) [2]. The limiting feature for the application of thin films as actuation layers in any kind of micromechanical applications, is the strength of the (inverse) piezoelectric effect in vertical and in-plane direction, if a vertical electrical field is applied. The main advantage of this application compared to SAW or BAW devices is simply the fact, that in the latter ones acoustic losses can reduce the performance drastically even if the piezoelectric effect is high.
∗
Corresponding author. Tel.: +49 3677 693352. E-mail address: [email protected] (K. Tonisch).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.03.001
The exceptional properties of wide-bandgap III–V nitride semiconductors are promising for such applications. The piezoelectricity of wurtzite group III-nitrides is characterized by three independent piezoelectric coefficients d33 , d31 (=d32 ) and d15 (=d24 ). Among the nitrides AlN has the highest thermal conductivity at low temperatures, good mechanical strength, high resistivity and corrosion resistance, and the largest piezoelectric coefficients. Thus, AlN resonators are attractive building blocks for electromechanical devices in micro- and nanometer scale. The highest piezoelectric performance would be achieved by a defect-free, epitaxial film with the c-axis or [0 0 0 1] direction normal to the substrate. Unfortunately, epitaxial growth of AlN only occurs at high temperatures and on specific substrates which makes the epitaxial deposition incompatible for the integration in CMOS or other technologies sensitive to heat. Nevertheless it is possible, and has been shown by many researchers, to grow highly textured AlN thin films at temperatures as low as 500 ◦ C [3,4]. But unlike epitaxial thin films which most often only display point defects and threading dislocations, polycrystalline films are associated with grain boundaries. Additionally, the individual grains may exhibit a deviation from the ideal orientation, which is commonly referred to as tilt. Since the lack of epitaxial growth can also lead to a change in the orientation of the single grains, polycrystalline thin films may also happen to display no piezoelectricity at all, if the distribution of the
K. Tonisch et al. / Sensors and Actuators A 132 (2006) 658–663
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¯ orientation is equal. The above mentioned [0 0 0 1] and [0 0 0 1] tilt can be directly determined via measuring the full width at half maximum (FWHM) of the X-ray diffraction rocking curve. Since both the tilt and the film thickness have an influence on the FWHM of the rocking curve, the variation of the film thickness did not exceed the factor 2 between the thinnest and the thickest layer. While epitaxial or near-epitaxial thin films have FWHMs of tenth of a degree or less, polycrystalline films can have anything up to 15◦ depending on the chosen growth conditions. In this work we will show by direct measurements of the inverse piezoelectric effect in the picometer range that the acceptable tolerance in the crystal orientation for actuator and resonator applications is much larger than expected.
2. Experimental Aluminium nitride thin films of different structural quality have been prepared on highly n-doped (1 1 1) silicon wafers using both a low and a high temperature method: reactive magnetron sputtering [5] and metal organic chemical vapor deposition (MOCVD) [6]. The sputtering was performed in pure nitrogen atmosphere at 3 × 10−3 mbar. The temperature of the substrates varied from 350 to 500 ◦ C. Prior to the sputtering of the AlN films, the silicon substrate was cleaned by sputter-etching and afterwards transferred to the sputter chamber remaining under high vacuum conditions. This surface treatment was applied in order to remove the native oxide which is usually not homogeneously distributed and could therefore lead to a local polar axis reversal in the AlN thin film [7]. Additional samples of aluminium nitride thin films were grown (MOCVD). Thereby TMA and NH3 were used as precursors while hydrogen served as carrier gas. The growth temperature for different samples was varied from 1050 to 1190 ◦ C. The resulting layers were polycrystalline with different degrees of texture in the case of MOCVD and nanocrystalline in the case of sputtering. The layers thickness were between 130 and 250 nm. The morphological analysis with the atomic force microscope (Fig. 1) showed polycrystalline surfaces with a grain size of approximately 20–30 nm for the sputtered films and 100 nm for the MOCVD-grown films. An additional examination via transmis-
Fig. 2. Cross-section of a sputtered AlN film examined with transmission electron microscopy.
sion electron microscopy clearly revealed a columnar structure even for the sputtered thin films, which is shown in Fig. 2. X-ray diffraction has been performed on all samples and showed a pure (0 0 0 1) or c-axis orientation without appearance of any other orientation. In Fig. 3 some typical X-ray diffraction rocking curves of samples which were chosen for piezoelectric measurements are displayed ranging from 0.8◦ to 10.7◦ . For the piezoelectric measurements, titanium and gold were sputtered on the surface to act as an electrical contact, the substrate served as a back contact. 3. Results and discussion The piezoelectric properties of thin films are often measured indirectly by analysing the scattering parameters of a bulk acoustic wave transducer (BAW). The electromechanical coupling factor k2 is then derived by fitting the measured S21 spectra
Fig. 1. Surface morphology of a sputtered (left) and a MOCVD-grown AlN film (right) measured with atomic force microscopy with a length of 1 m in square.
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is then detected as the first harmonic component of the deflection signal. Thus, the effective piezoelectric coefficient can be calculated as the ratio between displacement t and applied voltage V: d33 =
S3 t = , E3 V
(1)
where S3 = t/t is the change of strain along the c-axis and E3 = V/t is the electric field along the c-axis. Most applications of piezolelectric thin films are based on structures, where the piezoelectric film is rigidly clamped to a substrate. The substrate then constrains the in-plane contraction and expansion of the film. Thus, only an effective value of d33eff instead of d33 is measured. Therefore most often a correction term involving the in-plane piezoelectric coefficient d31 and the elastic compliances S11 , S12 , S13 is used in order to calculate the unclamped value of d33 [12]: E d31 S13 d33 = d33eff − 2 . (2) E + SE S11 12
Fig. 3. Rocking curve of the AlN (0 0 2) reflex for layers prepared by MOCVD (dashed line) and of sputtered AlN films (solid lines) of different quality.
to theoretical computations [8]. We used two direct methods to measure the inverse piezoelectric effect in order to correlate the texture of the AlN thin films with the effective piezoelectric coefficient d33 . First, the expansion (and contraction) of the thin film, which is proportional to the applied external electrical field (both parallel to the c-axis) via the piezoelectric coefficient d33 , was measured using piezoresponse force microscopy. Second, a homodyne Michelson laser interferometer [9] was used to complement the AFM technique. While measuring the piezoelectric effects of thin films, it is important to bear in mind that the macroscopic piezoelectric response is the result of the contributions of all grains with their individual dipoles. The highest values are reached when all the dipoles are aligned along the same direction and have the same polarity, thus contributing with the same sign to the net piezoelectric response. Therefore a preferential c-axis orientation is crucial for a high piezoelectric response of thin films. Unlike Ruffner et al. [7], who examined sputtered AlN on ruthenium, we found no evidence for a distribution of different oriented dipoles. On the contrary, we observed a high piezoelectric response close to the reported values for bulk material even in case of clearly polycrystalline films. For studying the inverse piezoelectric effect, a modulation voltage is applied between the top electrode and the substrate, which causes the piezoelectric film to oscillate at the same frequency as the applied voltage. In piezoresponse force microscopy (PFM) this bias-induced deformation is detected by a common silicon AFM-tip which is brought into contact with the surface of the metallic top electrode [10,11]. When a modulation voltage is applied to the piezoelectric film, the vertical displacement of the tip follows accurately the piezoelectric motion of the sample surface. The piezoresponse of the sample
The value for S11 = 3.0, S12 = −0.9, and S13 = −0.6 in 10−12 m2 N−1 are taken from Wright [13], d31 is assumed to be −d33eff /2. PFM measurements were performed with a commercial AFM (ATOS Solver), a function generator (Agilent 33220 A) and a lock-in amplifier (Stanford Research Systems SR 830 DSP). For the PFM-measurements described here, frequencies of 1 kHz were employed, because this frequency is greater than most environmental noise frequencies and well below the resonant frequency of the AFM tip. Using this setup, even a vertical displacement as small as 10 pm could be clearly identified and quantified. A schematic of the experimental setup is shown in Fig. 4. Since the output in V of the lock-in amplifier has to be converted into a displacement of the tip in pm, the d33 coefficient of a LiNbO3 single crystal (bulk material) was measured as a reference. Fig. 5 shows the linear dependence of the displacement of the sample surface from the applied voltage measured at a sputtered aluminium nitride film. The average effective piezoelectric constant d33eff measured with PFM was calculated to 5.4 ± 0.1 pm/V for the sputtered samples.
Fig. 4. Schematic of the PFM experimental setup. A modulation voltage is applied to the conducting tip and to the top electrode, which causes the piezoelectric film to oscillate at the same frequency as the applied voltage.
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Fig. 5. Typical piezoelectric displacement vs. applied voltage estimated by piezoresponse force microscopy of a sputtered sample with a thickness of 130 nm and a FWHM (rocking curve) of 8.7◦ .
Additional measurements using a homodyne Michelson interferometer setup supported the above obtained results. Here an integrated spectrum ranging from 5 to 22 kHz was recorded. An extract of the spectra for: (a) a sputtered and (b) for a MOCVD-grown (low texture, FWHM 10.7◦ ) AlN film ranging from 17.5 to 19 kHz is provided in Fig. 6 and shows the vertical displacement of the sample surface due to the applied voltage of 14 V at a modulation frequency of 18.3 kHz. The calculation of the displacement versus applied voltage results in a effective d33 of 5.5 and 5.14 pm/V, respectively. Whereas the sputtered films all showed high piezoelectric values, in the case of the MOCVD-grown samples only the intentionally low textured sample had a d33eff as high as 5.14 pm/V (interferometry). For a sample with a FWHM of 2.5◦ (not shown in Fig. 1) it was remarkably reduced to 4.15 pm/V (measured with PFM) while it was not measurable at all for the thin film with the highest degree of texture (FWHM of 0.8◦ ). The reason was an increase of the vertical conductivity of the AlN films.
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Coming close to the growth mode of heteroepitaxy (by adjusting the growth conditions towards higher degrees of texture) resulted in an increase of the tensile strain in the AlN film. This led to an increased out-diffusion from the silicon substrate. Since silicon is a common dopant for group III-nitrides, this resulted in an increased conductivity and even to the formation of conducting path where the silicon “decorated” grain boundaries and threading dislocations [17,18]. In total, and especially for the sputtered films, the measured values were much higher than could have been expected for clamped and polycrystalline thin films. The corresponding unclamped values as calculated with the correction term in Eq. (2) are given in Table 1 and reach surprisingly high values up to 7.07 pm/V. Nevertheless these high values are consistent with several results that have been reported elsewhere [10,14–16]. An overview of values measured in this work and of those reported elsewhere (measured and calculated) are given in Table 1. All AlN films showed high piezoelectric responses and only a slight reduction of d33 due to lower degree of texture, which is consistent with a theoretical evaluation made by Mishin et al. [19] of the effect of an increased FWHM on the piezoelectric coefficient d33 . While the coupling factor k2 is reported to decrease dramatically for a FWHM higher than 4◦ [20], the piezoelectric coefficient d33 decreases only moderately from 5.5 pm/V down to 5.1 pm/V if the FWHM of the rocking curve reached 10◦ or higher. Both parameters, k2 (displayed as k for a better comparison) and d33eff (from both, PFM and interferometry) and their dependency from the degree of texture of the film, represented by the FWHM of the rocking curve, are displayed in Fig. 7. Even films that were only poorly textured showed high values of the effective piezoelectric constant d33eff . Therefore we assume that the degree of texture is less important for measuring d33eff , than it is for measuring the coupling factor k2 because it is a more direct approach to the piezoelectric performance of
Fig. 6. Typical piezoelectric displacement vs. frequency estimated by optical interferometry for: (a) sputtered AlN and (b) MOCVD-grown AlN with an applied modulation voltage of 14 V at 18.3 kHz.
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Table 1 Values of the piezoelectric coefficient d33 found in this work compared to literature values Sample (top/bottom)
Growth method
AFM d33 (pm/V)
Interferometry d33 (pm/V)
This work AlN/Si (1 1 1) AlN/Si (1 1 1) AlN/Si (1 1 1)
Sputtering MOCVDb MOCVDc
6.94 (5.4 ± 0.1)a 5.47 (4.15 ± 0.1)a
7.07 (5.5 ± 0.1)a
Sputtering Sputtering MBE
4.0 ± 1.8 [10] 6.0 ± 3.0 [10] 3.7 ± 1.9 [10]
Literature AlN/Si AlN/Si AlN/Si AlN/Pt (1 1 1) AlN/Si (1 0 0) AlN/Si (1 0 0) AlN AlN AlN a b c d
CVD MPACVDd Bulk Calculated Calculated
6.56 (5.1 ± 0.1)a
6.83 [16] 4.0 ± 0.1 [21] 3.2 ± 0.3 [21] 5.6 [13] 6.72 [15] 6.4 [22]
The corresponding values for the effective d33eff (which has been measured) are given in parantheses. Small FWHM. High FWHM. Microwave plasma assisted CVD.
References
Fig. 7. Comparison of the dependency of the coupling factor k in % [13] and of the effective piezoelectric coefficient d33eff in pm/V (this work) on the FWHM in degree.
a crystal or thin film. As long as the dipoles of all grains (or most of them) have the same orientation, the inverse piezoelectric effect of the thin film will be high and does not seem to be affected by other effects caused by the polycrystalline structure. Since the examination with TEM did not give any indication that the thin films might be porous, we consider the assumption d31 = −d33eff /2 to be valid even for poorly textured AlN films. Thus, polycrystalline AlN thin films sputtered on silicon substrates are well suited for the use as actuator layers in micromechanical systems such as resonator beam or active cantilever structures. Acknowledgements This work has been funded and supported by the Deutsche Forschungsgemeinschaft, SPP 1157: “Integrierte elektrokeramische Funktionsstrukturen”, contract no. AM 105/2-1
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Biographies Katja Tonisch is currently a PhD student at the Technical University of Ilmenau, Germany. She recieved her Diploma (MS) in Electrical Engineering in 2005. Her research interests include the growth of group III-Nitrides with MOCVD and nitride-based MEMS. Volker Cimalla is currently working as assistant professor at the TU Ilmenau. He received his Diploma (MS) in Electrical Engineering and his PhD from the
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TU Ilmenau in 1993 and 1998, respectively. He spend several years as post doctorate at the North Carolina State University, USA and on Crete, Greece. His current research interest includes III-Nitrides and MEMS/NEMS technology. Christian Foerster is currently working as research engineer at Infineon, Villach (Austria). He received his Diploma (MS) in Electrical Engineering and his PhD from the TU Ilmenau in 2002 and 2005, respectively. Henry Romanus is currently a scientific staff member of the Center for Microand Nanotechnologies (ZMN) at Technical University Ilmenau. He received his Diploma (MS) and his PhD in Electrical Engineering and Materials Science from the TU Ilmenau in 1995 and 2004, respectively. His research interests include material research with electron microscopy (SEM, TEM) and X-ray diffraction, focussed ion beam technology and contact metallization. Oliver Ambacher is currently working as professor at the TU Ilmenau. He received his Diploma (MS) in Physics in 1989 from the LMU Munich and his PhD in 1993 from the Technical University of Munich. He worked several years as researcher at the Walter-Schottky-Institute Munich and at Cornell University, USA. Since 2002 he is head of Department of Nanotechnology and since 2004 he is head of the Center for Micro- and Nanotechnologies, TU Ilmenau. His research interests includes the growth and application of Group III-Nitrides. Denis Dontsov is currently working as research engineer at SIOS GmbH. He received his MS from the Polytechnical Institute Kiev in 1993 and his PhD from the TU Ilmenau in 2001. His research interests include the developement of piezoelectrical measurement techniques.