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Influence of the seed layer on structural and electro-acoustic properties of sputter-deposited AlN resonators
Thin Solid Films 517 (2009) 6588–6592
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Influence of the seed layer on structural and electro-acoustic properties of sputterdeposited AlN resonators T. Riekkinen a,⁎, A. Nurmela a, J. Molarius a, T. Pensala a, P. Kostamo b, M. Ylilammi a, S. van Dijken c a b c
VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland Micro and Nanosciences Laboratory, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland Department of Applied Physics, Helsinki University of Technology, P.O. Box 5100, FI-02015 TKK, Finland
a r t i c l e
i n f o
Article history: Received 22 December 2008 Received in revised form 14 April 2009 Accepted 16 April 2009 Available online 3 May 2009 Keywords: Aluminium nitride Sputtering Piezoelectric effect Crystallization Surface morphology Molybdenum Nickel Titanium
a b s t r a c t The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Radio frequency resonators are used in microradios for ubiquitous communication and other high-frequency applications. Key requirements include low power consumption, small size, low cost, high spectral purity, and sufficient stability, all of which can be met with thin film bulk acoustic wave (BAW) resonators. At present, AlN is predominantly used as piezoelectric material in BAW devices [1,2] as it exhibits good electro-acoustic coupling, it is very stable, non-toxic, environmentally friendly and compatible with microelectronic ICprocessing. To fulfill the device requirements for e.g. mobile phone applications, the AlN film must be of very high quality [1,3–5]. Material requirements include exact stoichiometry, hexagonal wurtzite crystallographic structure with a well-defined (002) orientation, and strict control over film stress and roughness. Key parameters that determine the response of a thin film BAW structure are the effective electro-acoustic coupling coefficient (K2) and quality factor (Q). K2 depends strongly on the quality of the AlN film, the electrode configuration, and the substrate. For a good resonator response, the electrodes should have low electrical resistivity, small acoustic losses, and high acoustic impedance. Moreover, as the bottom electrode serves as seed layer for AlN growth, it should also provide a high degree of crystalline texture and ⁎ Corresponding author. Tel.: +358 40 825 1496; fax: +358 9 20 722 7012. E-mail address: tommi.riekkinen@vtt.fi (T. Riekkinen). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.04.060
limited surface roughness. The most commonly used electrode materials are (111) oriented face centered cubic (fcc) metals such as Al, Pt and Ni [2,5,6–10], (110) oriented body centered (bcc) materials like Mo and W [1,2,6,8,9,11–16], and hexagonal metals with a (002) orientation including Ti and Ru [1,6,8,10]. Mo is a fairly good electrical conductor (ρ 5.2 µΩ cm) and it has been found to promote the growth of highly textured AlN films. In this paper, we report on the influence of various seed layers on the growth and electro-acoustic properties of AlN-based thin film BAW resonators with Mo electrodes. 2. Materials and methods The AlN films were deposited using pulsed-dc magnetron sputtering from a pure Al target in N2/Ar atmosphere. To limit the detrimental influence of residual gasses on AlN columnar film growth [17], the sputtering chamber was baked before each AlN deposition run. This resulted in a base pressure of less than 1 × 10− 8 mbar. Moreover, prior to any film growth the wafers and substrate holders were degassed by a heating/cooling cycle. The AlN films were grown at high power (1750 W) to minimize oxygen incorporation, and a substrate temperature of 200–400 °C. All other metal films were deposited in neighboring vacuum chambers using magnetron sputtering slightly at or above room temperature. The growth of the various seed layers, the Mo electrodes, and the AlN films was optimized using 100 mm Si(001) n-type substrates with a thermal or native oxide. For the thin film BAW devices the films were grown
T. Riekkinen et al. / Thin Solid Films 517 (2009) 6588–6592
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Table 1 Overview of the deposition parameters for the different seed layer materials, the Mo electrodes and the AlN films. Seeding
Parameter
Value
Electrode
Parameter
Value
Piezo
Parameter
Value
Ti
Power (RF) Ar flow Pressure Thickness
500 W 60 sccm 4.0 µbar 20 nm
Mo
Power (DC) Ar flow Pressure Thickness
1000 W 16 sccm 0.9 µbar variable nm
AlN
Ni
Power (RF) Ar flow Pressure Thickness Power (DC) Ar flow Pressure Thickness
200 W 30 sccm 2.0 µbar 10 nm 500 W 27 sccm 6.1 µbar 8 nm
Base pressure Power Ar flow N2 flow Pressure Temperature Bias Thickness
b1 × 10− 8 mbar 1750 W 30 sccm 30 sccm 2.0 µbar 400 °C, down to 200 °C after 20 s 60 W variable nm
TiW
onto an acoustic Bragg reflector consisting of two pairs of SiO2–W layers with a quarter wavelength thickness. On top of the AlN, a Mo thin film was deposited in situ, and patterned to form resonators. The crystallographic texture of the films was analyzed using X-ray diffraction (XRD). A θ–2θ analysis was performed by using a Panalytical X'pert pro MRD diffractometer with Cu Kα radiation. The surface morphology was inspected by various techniques including optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The instrumentation used for morphology analysis include LEO 1560 SEM operated at 5 kV and Digital Instruments DI3100 AFM in tapping mode using 300 kHz silicon tips. Surface roughness was determined with Nanoscope III software. Precise control over the layer thickness was obtained with a NanoSpec AFT 4150 reflectometer and Veeco DekTak 200 Si surface profilometer. The latter instrument was also used to determine the intrinsic film stress. The effective electroacoustic coupling coefficient and Q-values of the AlN films were extracted from wafer level S-parameter measurements with a Hewlett Packard 8720D vector network analyzer with Cascade Microtech ACP-40 Wafer probes at GHz frequency. 3. Results and discussion High-quality electrodes promote the nucleation of textured AlN films. To optimize the growth of Mo electrodes we used Ti, Ni, and TiW (10/90 wt.%) seed layers and varied several deposition parameters, in particular the seed layer thickness and magnetron sputtering power.
Fig. 1. Dependence of the Mo surface roughness on the Ti seed layer thickness and sputtering power used during Ti growth. The thickness of the Mo film is 300 nm.
Variations in the growth conditions of the Mo electrodes were found to have only little effect and consequently were kept constant in the experiments. For all films, the sputtering pressure was kept low to induce compressive stress. This promotes a dense and smooth microstructure with uniform grain size [9,11,16]. The optimized deposition parameters for all films are summarized in Table 1. The AFM root mean square (RMS) surface roughness of x nm Ti/ 300 nm Mo bilayers as a function of seed layer thickness and sputtering power is shown in Fig. 1. A smooth surface is obtained for relatively thin seed layers and a medium sputtering power of 500 W. The nonmonotonic dependence of the surface roughness on sputtering power is most likely due to a trade-off between the energy of the deposited adatoms and the deposition rate, i.e., limited mobility of adatoms enhances the surface roughness at low sputtering power, whereas fast nucleation roughens the film morphology at high sputtering power. A qualitatively similar dependence of the Mo surface roughness on seed layer thickness and sputtering power is found for Mo electrodes grown onto thin Ni and TiW films. The optimal seed layer thickness and sputtering power for the three systems are 20 nm/500 W (Ti), 10 nm/ 200 W (Ni), and 8 nm/500 W (TiW). Under these conditions, the RMS surface roughness of the Mo electrode is 0.4, 0.6, and 1.8 nm for Ti, Ni, and TiW, respectively. SEM images of the Mo surface morphology are shown in Fig. 2. The Mo film consists of elongated grains whose size varies strongly with seed layer material. The largest grains are obtained with TiW, while small regular Mo grains grow on Ti films. The various seed layers are expected to provide different crystalline templates for Mo electrode growth. The preferred orientation of hexagonal close packed (hcp) Ti and fcc Ni is (002) and (111), respectively [6]. TiW, on the other hand, tends to be polycrystalline. Fig. 3 show θ–2θ XRD scans for the three optimized systems. Besides a small Ti(002) reflection, the seed layers are too thin to further contribute to the XRD data. The 300 nm thick Mo film is polycrystalline on top of all
Fig. 2. SEM images of the Mo surface morphology after deposition on thin TiW, Ni, and Ti seed layers.
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Fig. 3. θ–2θ XRD scans of 300 nm thick Mo films on different seed layers.
seed layers with a preferred (110) orientation. However, compared to Ni and TiW, the presence of other crystalline orientations is drastically reduced when the Mo film is grown onto Ti. The improved (110) orientation of the Mo film is also confirmed by the enhanced intensity and reduced width of the (110) reflection. The full width at half maximum (FWHM) of the Mo(110) peak is 0.30°, 0.33°, and 0.40° for Ti, Ni and TiW, respectively. The Mo peak positions are the same for all samples indicating similar defect densities and film strain. With a smooth surface morphology and (110)-oriented film texture the Tiseeded Mo electrodes provide the best template for high-quality AlN resonator films. The crystalline texture and morphology of AlN films depend strongly on the substrate temperature during magnetron sputtering [18]. In our experiments, we first grew the thin seed layer and Mo bottom electrode slightly above room temperature and then heated the substrate to 400 °C. At this deposition temperature, the AlN film forms densely packed columnar grains with a preferred (002) orientation. The morphology and crystallinity of the films can be controlled also by a variation of the substrate bias during film growth [11,19]. The substrate bias enhances adatom mobility and removes loose bonds and these effects result in denser void free films. In addition, the AlN film strain can be tailored by the selection of an appropriate substrate bias during deposition [11,19]. Despite the possibility to control the properties of AlN films by a variation of the deposition temperature and substrate bias, it is difficult to prevent the growth of some very large clusters. Large
Fig. 4. AFM images of the AlN surface morphology after deposition on 10 nm Ni/300 nm Mo (left) and 20 nm Ti/300 nm Mo (right). The thickness of the AlN film is 1300 nm.
Fig. 5. θ–2θ XRD scans of 1300 nm thick AlN films on top of a Bragg reflector and TiW-, Ni-, and Ti-seeded Mo electrodes.
variations in grain size and multicrystallinity reduce the effective electro-acoustic coupling coefficient and should be avoided in highquality thin film BAW resonators. In our experiments, we found that a reduction of the substrate temperature from an initial 400 °C to a final 200 °C during AlN film growth drastically limited the formation of large clusters while a high degree of (002) texture was maintained. During this process, the initial 20 s of deposition at 400 °C provides a highquality template for AlN growth and the subsequent deposition at lower temperature avoids the formation of large clusters without severely compromising the crystalline quality of the film. Fig. 4 shows the AlN surface morphology after deposition on a Ni/Mo bilayer at constant 400 °C temperature (left) and a Ti/Mo bilayer with the temperature ramp (right). The AlN grain size is substantially smaller when the Mo bottom electrode is seeded by a thin Ti film instead of a Ni layer. The AlN film qualitatively mimics the columnar grain morphology of the Mo film (see Fig. 2). When grown onto a Ti/Mo bilayer, the AlN film consists of densely packed small grains with a narrow size distribution. The RMS surface roughness is only 1.8 nm in this case, which is much smaller than the 8.8 nm RMS roughness of the AlN film on top of Ni/Mo. The thickness
Fig. 6. XRD rocking curves of the AlN (002) reflection, normalized to the peak values. The maximum intensities were 530, 4600 and 16,000 counts/s for AlN on TiW/Mo, Ni/ Mo and Ti/Mo, respectively.
T. Riekkinen et al. / Thin Solid Films 517 (2009) 6588–6592
of the AlN film was 1300 nm for both structures. All results in this paper were obtained with the above-mentioned two-temperature deposition method. XRD measurements of the AlN films grown onto Mo bottom electrodes with different seed layer materials are summarized in Figs. 5 and 6. The AlN grains are mainly (002) oriented. However, for Ni and in particular TiW seed layers, an additional AlN (105) reflection is also measured, which is most likely related to the formation of some large misoriented clusters. The AlN film on top of the Ti/Mo layer shows the highest intensity of the (002) AlN peak of all samples which indicates the highest degree of texturation along the c-axis. The FWHM of the (002) reflection of this film is only 0.21°, which compares well with the FWHM values of 0.24° and 0.31° for the AlN films on top of Ni/ Mo and TiW/Mo. Also, the (002)-oriented grains are much better aligned along the substrate normal when the AlN film is grown onto Ti/ Mo, as illustrated by the XRD rocking curves in Fig. 6. The FWHM of the rocking curves is 2.3° (Ti seed layer), 5.0° (Ni seed layer), and 12.3° (TiW seed layer). To characterize the influence of surface roughness, grain size, and crystalline quality we fabricated a series of thin film BAW resonators. For these structures, the seed layer/Mo/AlN multilayers were deposited onto a SiO2–W Bragg reflector and the entire structure was capped by a Mo top electrode. The full stack consisted of Si(001)/774 nm SiO2/ 654 nm W/711 nm SiO2/654 nm W/751 nm SiO2/x nm seed layer/ 300 nm Mo/1319 nm AlN/300 nm Mo. Only the top electrode was patterned in these test devices. The bottom electrode was capacitively contacted through the AlN film via large field metal areas surrounding the resonators. The electrical characterization of the resonators was performed by measuring the one-port scattering parameter S11 on wafer level. From the S11, the input impedance Zin was calculated using: Zin =
1 + S11 Z ; 1 − S11 0
ð1Þ
where Z0 is the line impedance (50 Ω). Series and parallel resonance frequencies (fs and fp, respectively) were determined from zero
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Fig. 8. Impedance magnitude (upper plot) and phase of 50 Ω resonators. The TiW/Mobased device (thin solid line) shows the lowest K2, as indicated by the small separation of fs and fp. The value of K2 is considerably larger for resonators with a Ni/Mo bottom electrode (dashed line) and the largest electromechanical coupling coefficient is obtained when the Mo electrode is seeded by a thin Ti layer (thick solid line).
crossings of the impedance phase ϕ. The effective electromechanical coupling coefficient was then calculated from [2]: 2
K =
π 2 fp − fs : 4 fs
ð2Þ
The quality factors of series and parallel resonances were calculated from using [2]: Qs;p =
j j
ω d/ 2 dω
ð3Þ
ωs ;ωp
where ω is the angular frequency. Finally, a figure of merit (FoM) indicating resonators performance for low-loss wide bandwidth RFapplications was calculated from [4]: 2 FoM = K max Qs ; Qp :
ð4Þ
The characterization was performed on i) devices with areas that closely match the 50 Ω line impedance and ii) large area resonators with a low impedance level (b10 Ω). The measured S11 scattering parameter of 50 Ω resonators with TiW/Mo, Ni/Mo, and Ti/Mo bottom electrodes are displayed in Fig. 7. The input impedances are shown in Fig. 8. To facilitate comparison, the
Table 2 Electro-acoustic parameters of 50 Ω and large area (b 10 Ω) resonator.
Fig. 7. S11 scattering parameter of 50 Ω resonators displayed on a Smith chart. The S11 values of the Ti/Mo-based resonator form the largest circle, which indicates a large FoM. The ripples are due to laterally standing plate wave resonances, which have not been suppressed in these resonators.
fs (MHz)
fp (MHz)
K2 (%)
Qmax
FoM
50 Ω TiW/Mo/AlN Ni/Mo/AlN Ti/Mo/AlN
2093.90 1885.00 1943.70
2124.50 1928.40 1993.20
3.61 5.69 6.28
1076 664 1276
39 38 80
b10 Ω TiW/Mo/AlN Ni/Mo/AlN Ti/Mo/AlN
2094.10 1883.20 1905.20
2126.50 1929.80 1958.40
3.82 6.10 6.89
720 690 1000
27 42 69
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T. Riekkinen et al. / Thin Solid Films 517 (2009) 6588–6592
data are plotted against frequency normalized by fp of each resonator and re-scaled to match the fp of the Ti/Mo-based resonator. The Ti/ Mo-based resonator response shows the largest circle on the Smith chart, indicating the largest FoM. All resonators except those with TiW/Mo show strong ripples due to spurious plate wave resonances. In our devices, no additional measures like the ones presented in Ref. [20] were taken to suppress spurious resonances. The lack of ripples in the TiW/Mo-based devices is probably due to low K2 values. The impedance plots clearly show that K2 drastically improves when the TiW/Mo bottom electrode is replaced by Ni/Mo. A further improvement is obtained for Ti/Mo bottom electrodes as indicated by an increased separation of the parallel and series resonance frequencies (fp − fs). Whilst the electromechanical coupling coefficient of BAW resonators with Ti/Mo is already very good, further increases of K2 are still anticipated upon a detailed optimization of the Mo electrode thickness [21]. The electro-acoustic performance parameters of 50 Ω and sub10 Ω resonators are summarized in Table 2. The K2 and FoM values of the optimized Ti/Mo structure are 6.28% and 80, respectively, for the 50 Ω devices. For the large area devices, K2 is 6.89% but the Q-values are slightly lower producing the FoM of 69. The Q-values of the different resonator structures do not reveal any clear trends and the extracted values are somewhat prone to errors due to spurious ripples in the electro-acoustic response. Higher Q-values might be obtained by a re-design of the Bragg reflector [22]. 4. Conclusions We have optimized the structure, morphology, and electroacoustic properties of magnetron-sputtered Mo electrodes and AlN piezoelectric films. The smoothest films are obtained when the Mo bottom electrode is seeded by a 20 nm Ti layer. In this case, the Mo electrode and AlN film consist of small elongated grains with a (110) and (002) orientation, respectively. The electro-acoustic properties of BAW resonators with a Ti-seeded Mo/AlN/Mo structure are very
good, as indicated by a maximum electromechanical coupling constant of 6.89%. The performance of resonators with thin seed layers of Ni and TiW is clearly inferior to that of Ti-seeded BAW structures due to enhanced surface roughness, larger grains, and the growth of misoriented clusters. The optimized piezoelectric response of our Ti/Mo/AlN/Mo devices holds a great potential for RF resonator applications. References [1] Y. Satoh, T. Nishihara, T. Yokoyama, M. Ueda, T. Miyashita, Jpn. J. Appl. Phys. 44 (2005) 2883. [2] R. Lanz, C. Lambert, IEEE Ultrason. Symp. Proc. (2005) 210. [3] R. Lanz, L. Senn, L. Gabathuler, W. Huiskamp, R.C. Strijbos, F. Vanhelmont, IEEE Ultrason. Symp. Proc. (2007) 1429. [4] J.-W. Lobeek, A.B. Smolders, IEEE MTT-S Int. Microw. Symp. Dig. (2006) 386. [5] K.M. Lakin, IEEE Ultrason. Symp. Proc. (1999) 895. [6] G.F. Iriarte, J. Bjurström, J. Westlinder, F. Engelmark, I.V. Katardjiev, IEEE Ultrason. Ferroelectr. Freq. Control Proc. (2005) 1170. [7] R. Ohara, K. Sano, N. Yanase, T. Yasumoto, T. Kawakubo, K. Itaya, IEEE Ultrason. Symp. Proc. (2005) 206. [8] J.-B. Lee, J.-P. Jung, M.-H. Lee, J.-S. Park, Thin Solid Films 447 (2004) 610. [9] S.-H. Lee, J.-K. Lee, K.H. Yoon, J. Vac. Sci. Technol. A 21 (1) (2003) 1. [10] M.-A. Dubois, P. Muralt, J. Appl. Phys. 89 (2001) 6389. [11] F. Martin, P. Muralt, M.-A. Dubois, J. Vac. Sci. Technol. A 24 (4) (2006) 946. [12] T. Kamohara, M. Akiyama, N. Ueno, K. Nonaka, H. Tateyama, J. Cryst. Growth 275 (2005) 383. [13] M. Akiyama, N. Ueno, H. Tateyama, K. Nagao, T. Yamada, J. Mater. Sci. 40 (2005) 1159. [14] H. Matsumoto, K. Asai, N. Kobayashi, S. Nagashima, A. Isobe, N. Shibagaki, M. Hikita, Jpn. J. Appl. Phys. 43 (2004) 8219. [15] H.-C. Lee, J.-Y. Park, K.-H. Lee, J.-U. Bu, J. Vac. Sci. Technol., B 22 (3) (2004) 1127. [16] S.-H. Lee, S.C. Kang, S.C. Han, B.K. Ju, K.H. Yoon, J.-K. Lee, J. Korean Ceram. Soc. 40 (4) (2003) 393. [17] R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Electrochem. Soc. 146 (1999) 691. [18] J.P. Kar, G. Bose, S. Tuli, Curr. Appl. Phys. 6 (2006) 873. [19] A.K. Chu, C.H. Chao, F.Z. Lee, H.L. Huang, Thin Solid Films 429 (2003) 1. [20] J. Kaitila, M. Ylilammi, J. Ellä, R. Aigner, IEEE Ultrason. Symp. Proc. (2003) 84. [21] K.M. Lakin, J. Belsick, J.F. McDonald, K.T. McCarron, IEEE Ultrason. Symp. Proc. (2001) 827. [22] S. Marksteiner, J. Kaitila, G.G. Fattinger, R. Aigner, IEEE Ultrason. Symp. Proc. (2005) 329.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Influence of the seed layer on structural and electro-acoustic properties of sputterdeposited AlN resonators T. Riekkinen a,⁎, A. Nurmela a, J. Molarius a, T. Pensala a, P. Kostamo b, M. Ylilammi a, S. van Dijken c a b c
VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland Micro and Nanosciences Laboratory, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland Department of Applied Physics, Helsinki University of Technology, P.O. Box 5100, FI-02015 TKK, Finland
a r t i c l e
i n f o
Article history: Received 22 December 2008 Received in revised form 14 April 2009 Accepted 16 April 2009 Available online 3 May 2009 Keywords: Aluminium nitride Sputtering Piezoelectric effect Crystallization Surface morphology Molybdenum Nickel Titanium
a b s t r a c t The growth of high-quality AlN films has been studied by reactive sputtering onto Mo electrodes with Ni, Ti, and TiW seed layers and subsequent integration into thin film bulk acoustic wave resonators. The crystalline structure and morphology of the Mo and c-axis oriented AlN films were found to vary strongly with seed layer material and thickness. The smoothest Mo electrodes were obtained on thin Ti films. Reactive sputtering of AlN on top of these optimized electrodes resulted in a dense columnar grain structure with a well-aligned (002) crystal orientation and good electro-acoustic properties, including an effective coupling coefficient of 6.89% and quality factor above 1000. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Radio frequency resonators are used in microradios for ubiquitous communication and other high-frequency applications. Key requirements include low power consumption, small size, low cost, high spectral purity, and sufficient stability, all of which can be met with thin film bulk acoustic wave (BAW) resonators. At present, AlN is predominantly used as piezoelectric material in BAW devices [1,2] as it exhibits good electro-acoustic coupling, it is very stable, non-toxic, environmentally friendly and compatible with microelectronic ICprocessing. To fulfill the device requirements for e.g. mobile phone applications, the AlN film must be of very high quality [1,3–5]. Material requirements include exact stoichiometry, hexagonal wurtzite crystallographic structure with a well-defined (002) orientation, and strict control over film stress and roughness. Key parameters that determine the response of a thin film BAW structure are the effective electro-acoustic coupling coefficient (K2) and quality factor (Q). K2 depends strongly on the quality of the AlN film, the electrode configuration, and the substrate. For a good resonator response, the electrodes should have low electrical resistivity, small acoustic losses, and high acoustic impedance. Moreover, as the bottom electrode serves as seed layer for AlN growth, it should also provide a high degree of crystalline texture and ⁎ Corresponding author. Tel.: +358 40 825 1496; fax: +358 9 20 722 7012. E-mail address: tommi.riekkinen@vtt.fi (T. Riekkinen). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.04.060
limited surface roughness. The most commonly used electrode materials are (111) oriented face centered cubic (fcc) metals such as Al, Pt and Ni [2,5,6–10], (110) oriented body centered (bcc) materials like Mo and W [1,2,6,8,9,11–16], and hexagonal metals with a (002) orientation including Ti and Ru [1,6,8,10]. Mo is a fairly good electrical conductor (ρ 5.2 µΩ cm) and it has been found to promote the growth of highly textured AlN films. In this paper, we report on the influence of various seed layers on the growth and electro-acoustic properties of AlN-based thin film BAW resonators with Mo electrodes. 2. Materials and methods The AlN films were deposited using pulsed-dc magnetron sputtering from a pure Al target in N2/Ar atmosphere. To limit the detrimental influence of residual gasses on AlN columnar film growth [17], the sputtering chamber was baked before each AlN deposition run. This resulted in a base pressure of less than 1 × 10− 8 mbar. Moreover, prior to any film growth the wafers and substrate holders were degassed by a heating/cooling cycle. The AlN films were grown at high power (1750 W) to minimize oxygen incorporation, and a substrate temperature of 200–400 °C. All other metal films were deposited in neighboring vacuum chambers using magnetron sputtering slightly at or above room temperature. The growth of the various seed layers, the Mo electrodes, and the AlN films was optimized using 100 mm Si(001) n-type substrates with a thermal or native oxide. For the thin film BAW devices the films were grown
T. Riekkinen et al. / Thin Solid Films 517 (2009) 6588–6592
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Table 1 Overview of the deposition parameters for the different seed layer materials, the Mo electrodes and the AlN films. Seeding
Parameter
Value
Electrode
Parameter
Value
Piezo
Parameter
Value
Ti
Power (RF) Ar flow Pressure Thickness
500 W 60 sccm 4.0 µbar 20 nm
Mo
Power (DC) Ar flow Pressure Thickness
1000 W 16 sccm 0.9 µbar variable nm
AlN
Ni
Power (RF) Ar flow Pressure Thickness Power (DC) Ar flow Pressure Thickness
200 W 30 sccm 2.0 µbar 10 nm 500 W 27 sccm 6.1 µbar 8 nm
Base pressure Power Ar flow N2 flow Pressure Temperature Bias Thickness
b1 × 10− 8 mbar 1750 W 30 sccm 30 sccm 2.0 µbar 400 °C, down to 200 °C after 20 s 60 W variable nm
TiW
onto an acoustic Bragg reflector consisting of two pairs of SiO2–W layers with a quarter wavelength thickness. On top of the AlN, a Mo thin film was deposited in situ, and patterned to form resonators. The crystallographic texture of the films was analyzed using X-ray diffraction (XRD). A θ–2θ analysis was performed by using a Panalytical X'pert pro MRD diffractometer with Cu Kα radiation. The surface morphology was inspected by various techniques including optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The instrumentation used for morphology analysis include LEO 1560 SEM operated at 5 kV and Digital Instruments DI3100 AFM in tapping mode using 300 kHz silicon tips. Surface roughness was determined with Nanoscope III software. Precise control over the layer thickness was obtained with a NanoSpec AFT 4150 reflectometer and Veeco DekTak 200 Si surface profilometer. The latter instrument was also used to determine the intrinsic film stress. The effective electroacoustic coupling coefficient and Q-values of the AlN films were extracted from wafer level S-parameter measurements with a Hewlett Packard 8720D vector network analyzer with Cascade Microtech ACP-40 Wafer probes at GHz frequency. 3. Results and discussion High-quality electrodes promote the nucleation of textured AlN films. To optimize the growth of Mo electrodes we used Ti, Ni, and TiW (10/90 wt.%) seed layers and varied several deposition parameters, in particular the seed layer thickness and magnetron sputtering power.
Fig. 1. Dependence of the Mo surface roughness on the Ti seed layer thickness and sputtering power used during Ti growth. The thickness of the Mo film is 300 nm.
Variations in the growth conditions of the Mo electrodes were found to have only little effect and consequently were kept constant in the experiments. For all films, the sputtering pressure was kept low to induce compressive stress. This promotes a dense and smooth microstructure with uniform grain size [9,11,16]. The optimized deposition parameters for all films are summarized in Table 1. The AFM root mean square (RMS) surface roughness of x nm Ti/ 300 nm Mo bilayers as a function of seed layer thickness and sputtering power is shown in Fig. 1. A smooth surface is obtained for relatively thin seed layers and a medium sputtering power of 500 W. The nonmonotonic dependence of the surface roughness on sputtering power is most likely due to a trade-off between the energy of the deposited adatoms and the deposition rate, i.e., limited mobility of adatoms enhances the surface roughness at low sputtering power, whereas fast nucleation roughens the film morphology at high sputtering power. A qualitatively similar dependence of the Mo surface roughness on seed layer thickness and sputtering power is found for Mo electrodes grown onto thin Ni and TiW films. The optimal seed layer thickness and sputtering power for the three systems are 20 nm/500 W (Ti), 10 nm/ 200 W (Ni), and 8 nm/500 W (TiW). Under these conditions, the RMS surface roughness of the Mo electrode is 0.4, 0.6, and 1.8 nm for Ti, Ni, and TiW, respectively. SEM images of the Mo surface morphology are shown in Fig. 2. The Mo film consists of elongated grains whose size varies strongly with seed layer material. The largest grains are obtained with TiW, while small regular Mo grains grow on Ti films. The various seed layers are expected to provide different crystalline templates for Mo electrode growth. The preferred orientation of hexagonal close packed (hcp) Ti and fcc Ni is (002) and (111), respectively [6]. TiW, on the other hand, tends to be polycrystalline. Fig. 3 show θ–2θ XRD scans for the three optimized systems. Besides a small Ti(002) reflection, the seed layers are too thin to further contribute to the XRD data. The 300 nm thick Mo film is polycrystalline on top of all
Fig. 2. SEM images of the Mo surface morphology after deposition on thin TiW, Ni, and Ti seed layers.
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Fig. 3. θ–2θ XRD scans of 300 nm thick Mo films on different seed layers.
seed layers with a preferred (110) orientation. However, compared to Ni and TiW, the presence of other crystalline orientations is drastically reduced when the Mo film is grown onto Ti. The improved (110) orientation of the Mo film is also confirmed by the enhanced intensity and reduced width of the (110) reflection. The full width at half maximum (FWHM) of the Mo(110) peak is 0.30°, 0.33°, and 0.40° for Ti, Ni and TiW, respectively. The Mo peak positions are the same for all samples indicating similar defect densities and film strain. With a smooth surface morphology and (110)-oriented film texture the Tiseeded Mo electrodes provide the best template for high-quality AlN resonator films. The crystalline texture and morphology of AlN films depend strongly on the substrate temperature during magnetron sputtering [18]. In our experiments, we first grew the thin seed layer and Mo bottom electrode slightly above room temperature and then heated the substrate to 400 °C. At this deposition temperature, the AlN film forms densely packed columnar grains with a preferred (002) orientation. The morphology and crystallinity of the films can be controlled also by a variation of the substrate bias during film growth [11,19]. The substrate bias enhances adatom mobility and removes loose bonds and these effects result in denser void free films. In addition, the AlN film strain can be tailored by the selection of an appropriate substrate bias during deposition [11,19]. Despite the possibility to control the properties of AlN films by a variation of the deposition temperature and substrate bias, it is difficult to prevent the growth of some very large clusters. Large
Fig. 4. AFM images of the AlN surface morphology after deposition on 10 nm Ni/300 nm Mo (left) and 20 nm Ti/300 nm Mo (right). The thickness of the AlN film is 1300 nm.
Fig. 5. θ–2θ XRD scans of 1300 nm thick AlN films on top of a Bragg reflector and TiW-, Ni-, and Ti-seeded Mo electrodes.
variations in grain size and multicrystallinity reduce the effective electro-acoustic coupling coefficient and should be avoided in highquality thin film BAW resonators. In our experiments, we found that a reduction of the substrate temperature from an initial 400 °C to a final 200 °C during AlN film growth drastically limited the formation of large clusters while a high degree of (002) texture was maintained. During this process, the initial 20 s of deposition at 400 °C provides a highquality template for AlN growth and the subsequent deposition at lower temperature avoids the formation of large clusters without severely compromising the crystalline quality of the film. Fig. 4 shows the AlN surface morphology after deposition on a Ni/Mo bilayer at constant 400 °C temperature (left) and a Ti/Mo bilayer with the temperature ramp (right). The AlN grain size is substantially smaller when the Mo bottom electrode is seeded by a thin Ti film instead of a Ni layer. The AlN film qualitatively mimics the columnar grain morphology of the Mo film (see Fig. 2). When grown onto a Ti/Mo bilayer, the AlN film consists of densely packed small grains with a narrow size distribution. The RMS surface roughness is only 1.8 nm in this case, which is much smaller than the 8.8 nm RMS roughness of the AlN film on top of Ni/Mo. The thickness
Fig. 6. XRD rocking curves of the AlN (002) reflection, normalized to the peak values. The maximum intensities were 530, 4600 and 16,000 counts/s for AlN on TiW/Mo, Ni/ Mo and Ti/Mo, respectively.
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of the AlN film was 1300 nm for both structures. All results in this paper were obtained with the above-mentioned two-temperature deposition method. XRD measurements of the AlN films grown onto Mo bottom electrodes with different seed layer materials are summarized in Figs. 5 and 6. The AlN grains are mainly (002) oriented. However, for Ni and in particular TiW seed layers, an additional AlN (105) reflection is also measured, which is most likely related to the formation of some large misoriented clusters. The AlN film on top of the Ti/Mo layer shows the highest intensity of the (002) AlN peak of all samples which indicates the highest degree of texturation along the c-axis. The FWHM of the (002) reflection of this film is only 0.21°, which compares well with the FWHM values of 0.24° and 0.31° for the AlN films on top of Ni/ Mo and TiW/Mo. Also, the (002)-oriented grains are much better aligned along the substrate normal when the AlN film is grown onto Ti/ Mo, as illustrated by the XRD rocking curves in Fig. 6. The FWHM of the rocking curves is 2.3° (Ti seed layer), 5.0° (Ni seed layer), and 12.3° (TiW seed layer). To characterize the influence of surface roughness, grain size, and crystalline quality we fabricated a series of thin film BAW resonators. For these structures, the seed layer/Mo/AlN multilayers were deposited onto a SiO2–W Bragg reflector and the entire structure was capped by a Mo top electrode. The full stack consisted of Si(001)/774 nm SiO2/ 654 nm W/711 nm SiO2/654 nm W/751 nm SiO2/x nm seed layer/ 300 nm Mo/1319 nm AlN/300 nm Mo. Only the top electrode was patterned in these test devices. The bottom electrode was capacitively contacted through the AlN film via large field metal areas surrounding the resonators. The electrical characterization of the resonators was performed by measuring the one-port scattering parameter S11 on wafer level. From the S11, the input impedance Zin was calculated using: Zin =
1 + S11 Z ; 1 − S11 0
ð1Þ
where Z0 is the line impedance (50 Ω). Series and parallel resonance frequencies (fs and fp, respectively) were determined from zero
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Fig. 8. Impedance magnitude (upper plot) and phase of 50 Ω resonators. The TiW/Mobased device (thin solid line) shows the lowest K2, as indicated by the small separation of fs and fp. The value of K2 is considerably larger for resonators with a Ni/Mo bottom electrode (dashed line) and the largest electromechanical coupling coefficient is obtained when the Mo electrode is seeded by a thin Ti layer (thick solid line).
crossings of the impedance phase ϕ. The effective electromechanical coupling coefficient was then calculated from [2]: 2
K =
π 2 fp − fs : 4 fs
ð2Þ
The quality factors of series and parallel resonances were calculated from using [2]: Qs;p =
j j
ω d/ 2 dω
ð3Þ
ωs ;ωp
where ω is the angular frequency. Finally, a figure of merit (FoM) indicating resonators performance for low-loss wide bandwidth RFapplications was calculated from [4]: 2 FoM = K max Qs ; Qp :
ð4Þ
The characterization was performed on i) devices with areas that closely match the 50 Ω line impedance and ii) large area resonators with a low impedance level (b10 Ω). The measured S11 scattering parameter of 50 Ω resonators with TiW/Mo, Ni/Mo, and Ti/Mo bottom electrodes are displayed in Fig. 7. The input impedances are shown in Fig. 8. To facilitate comparison, the
Table 2 Electro-acoustic parameters of 50 Ω and large area (b 10 Ω) resonator.
Fig. 7. S11 scattering parameter of 50 Ω resonators displayed on a Smith chart. The S11 values of the Ti/Mo-based resonator form the largest circle, which indicates a large FoM. The ripples are due to laterally standing plate wave resonances, which have not been suppressed in these resonators.
fs (MHz)
fp (MHz)
K2 (%)
Qmax
FoM
50 Ω TiW/Mo/AlN Ni/Mo/AlN Ti/Mo/AlN
2093.90 1885.00 1943.70
2124.50 1928.40 1993.20
3.61 5.69 6.28
1076 664 1276
39 38 80
b10 Ω TiW/Mo/AlN Ni/Mo/AlN Ti/Mo/AlN
2094.10 1883.20 1905.20
2126.50 1929.80 1958.40
3.82 6.10 6.89
720 690 1000
27 42 69
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data are plotted against frequency normalized by fp of each resonator and re-scaled to match the fp of the Ti/Mo-based resonator. The Ti/ Mo-based resonator response shows the largest circle on the Smith chart, indicating the largest FoM. All resonators except those with TiW/Mo show strong ripples due to spurious plate wave resonances. In our devices, no additional measures like the ones presented in Ref. [20] were taken to suppress spurious resonances. The lack of ripples in the TiW/Mo-based devices is probably due to low K2 values. The impedance plots clearly show that K2 drastically improves when the TiW/Mo bottom electrode is replaced by Ni/Mo. A further improvement is obtained for Ti/Mo bottom electrodes as indicated by an increased separation of the parallel and series resonance frequencies (fp − fs). Whilst the electromechanical coupling coefficient of BAW resonators with Ti/Mo is already very good, further increases of K2 are still anticipated upon a detailed optimization of the Mo electrode thickness [21]. The electro-acoustic performance parameters of 50 Ω and sub10 Ω resonators are summarized in Table 2. The K2 and FoM values of the optimized Ti/Mo structure are 6.28% and 80, respectively, for the 50 Ω devices. For the large area devices, K2 is 6.89% but the Q-values are slightly lower producing the FoM of 69. The Q-values of the different resonator structures do not reveal any clear trends and the extracted values are somewhat prone to errors due to spurious ripples in the electro-acoustic response. Higher Q-values might be obtained by a re-design of the Bragg reflector [22]. 4. Conclusions We have optimized the structure, morphology, and electroacoustic properties of magnetron-sputtered Mo electrodes and AlN piezoelectric films. The smoothest films are obtained when the Mo bottom electrode is seeded by a 20 nm Ti layer. In this case, the Mo electrode and AlN film consist of small elongated grains with a (110) and (002) orientation, respectively. The electro-acoustic properties of BAW resonators with a Ti-seeded Mo/AlN/Mo structure are very
good, as indicated by a maximum electromechanical coupling constant of 6.89%. The performance of resonators with thin seed layers of Ni and TiW is clearly inferior to that of Ti-seeded BAW structures due to enhanced surface roughness, larger grains, and the growth of misoriented clusters. The optimized piezoelectric response of our Ti/Mo/AlN/Mo devices holds a great potential for RF resonator applications. References [1] Y. Satoh, T. Nishihara, T. Yokoyama, M. Ueda, T. Miyashita, Jpn. J. Appl. Phys. 44 (2005) 2883. [2] R. Lanz, C. Lambert, IEEE Ultrason. Symp. Proc. (2005) 210. [3] R. Lanz, L. Senn, L. Gabathuler, W. Huiskamp, R.C. Strijbos, F. Vanhelmont, IEEE Ultrason. Symp. Proc. (2007) 1429. [4] J.-W. Lobeek, A.B. Smolders, IEEE MTT-S Int. Microw. Symp. Dig. (2006) 386. [5] K.M. Lakin, IEEE Ultrason. Symp. Proc. (1999) 895. [6] G.F. Iriarte, J. Bjurström, J. Westlinder, F. Engelmark, I.V. Katardjiev, IEEE Ultrason. Ferroelectr. Freq. Control Proc. (2005) 1170. [7] R. Ohara, K. Sano, N. Yanase, T. Yasumoto, T. Kawakubo, K. Itaya, IEEE Ultrason. Symp. Proc. (2005) 206. [8] J.-B. Lee, J.-P. Jung, M.-H. Lee, J.-S. Park, Thin Solid Films 447 (2004) 610. [9] S.-H. Lee, J.-K. Lee, K.H. Yoon, J. Vac. Sci. Technol. A 21 (1) (2003) 1. [10] M.-A. Dubois, P. Muralt, J. Appl. Phys. 89 (2001) 6389. [11] F. Martin, P. Muralt, M.-A. Dubois, J. Vac. Sci. Technol. A 24 (4) (2006) 946. [12] T. Kamohara, M. Akiyama, N. Ueno, K. Nonaka, H. Tateyama, J. Cryst. Growth 275 (2005) 383. [13] M. Akiyama, N. Ueno, H. Tateyama, K. Nagao, T. Yamada, J. Mater. Sci. 40 (2005) 1159. [14] H. Matsumoto, K. Asai, N. Kobayashi, S. Nagashima, A. Isobe, N. Shibagaki, M. Hikita, Jpn. J. Appl. Phys. 43 (2004) 8219. [15] H.-C. Lee, J.-Y. Park, K.-H. Lee, J.-U. Bu, J. Vac. Sci. Technol., B 22 (3) (2004) 1127. [16] S.-H. Lee, S.C. Kang, S.C. Han, B.K. Ju, K.H. Yoon, J.-K. Lee, J. Korean Ceram. Soc. 40 (4) (2003) 393. [17] R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Electrochem. Soc. 146 (1999) 691. [18] J.P. Kar, G. Bose, S. Tuli, Curr. Appl. Phys. 6 (2006) 873. [19] A.K. Chu, C.H. Chao, F.Z. Lee, H.L. Huang, Thin Solid Films 429 (2003) 1. [20] J. Kaitila, M. Ylilammi, J. Ellä, R. Aigner, IEEE Ultrason. Symp. Proc. (2003) 84. [21] K.M. Lakin, J. Belsick, J.F. McDonald, K.T. McCarron, IEEE Ultrason. Symp. Proc. (2001) 827. [22] S. Marksteiner, J. Kaitila, G.G. Fattinger, R. Aigner, IEEE Ultrason. Symp. Proc. (2005) 329.