Rayleigh waves in nonpolar Al_0.7Sc_0.3N(11$2¯$0) films with enhanced electromechanical coupling and quality factor

The 5G mobile standard is key driver for the development of novel radio frequency (RF) filter components with increased bandwidths and high frequencies. Typically, RF filters are implemented using microacoustic technologies based on piezoelectric materials, enabling the fabrication of passive components at high integration levels. Well-known representatives for the design of such filters and oscillators are surface acoustic wave (SAW) and bulk acoustic wave (BAW) resonators. In recent years, sputtered wurtzite-type aluminum nitride (AlN) thin films were nearly a stand-alone material for BAW resonator structures due to its high elastic and piezoelectric properties as well as low-cost fabrication in combination with its CMOS compatibility.¹ However, the limited electromechanical coupling, which essentially defines the maximum bandwidth of passive filters components,² has driven the material to its physical limits.³ The ternary nitride Al_1−xSc_xN (AlScN), which is formed by the substitution of Al atoms with Sc atoms,⁴ opens a new window for the realization of broadband microacoustic technologies. This is primarily due to an increase in the piezoelectric response along the c-axis [0001] of the hexagonal crystal lattice [Fig. 1(a)]. Meanwhile, AlScN has become a state of the art piezoelectric material for BAW applications.^5–7 In addition, various concepts for AlScN based thin film SAW applications with enhanced electromechanical coupling are reported.^8–12 However, the incorporation of Sc into the AlN crystal has a negative effect on the elastic properties, leading to a lower acoustic velocity and, thus, reduced resonance frequencies. Moreover, there may be degradations of crystal quality, as increased phase segregation and misoriented grain growth are observed with rising Sc composition.¹³ This leads to an increase in acoustic wave attenuation losses (⁠$1/Q$ factor) as the Sc content of the film increases.^6,14

In the meantime, the range of $x=0.1−0.3$ in Al_1−xSc_xN has been shown to provide a favorable trade-off between increased piezoelectric properties and crystal quality and is under investigation within certain research activities.^11,15–17 We report on a special Al_1−xSc_xN orientation with composition x = 0.3 to decouple the trade-off between electromechanical coupling and phase velocity by taking advantage of properties of high sound velocity substrates.

The majority of microacoustic technologies use sputtered AlN and AlScN thin films with a fiber-texture. These films consist of columnar fibers of the piezoelectric material with a strong texture of the c-axis [0001] along growth direction, which means the c-plane (0001) is parallel to the wafer surface [Fig. 1(a)]. Nevertheless, it is possible to increase the crystallinity by suitable growth conditions. It is known that AlN as well as Al_1−xSc_xN can be grown via magnetron sputter epitaxy.¹⁸ Here, the columns of the film have a fixed epitaxial relationship with suitable substrates.¹⁹ A widely used substrate for epitaxial growth of wutzite-type materials is sapphire (Al₂O₃), which stands out as a cost-efficient high sound velocity substrate with low acoustic losses. Furthermore, epitaxial growth on Al₂O₃ may be used to realize crystal orientations of the film that differ from the convectional (0001) plane. For instance, the epitaxial growth of wurtzite-type crystals in the a-plane (11$2¯$0) orientation on the r-plane (1$1¯$02) oriented Al₂O₃ is well known^20,21 [Fig. 1(b)]. Prominent examples are GaN,²¹ ZnO,^22–24 and AlN²⁵ using various deposition methods, including molecular beam epitaxy, metal-organic chemical vapor deposition, and magnetron sputter epitaxy. Recently, the growth of nonpolar a-plane oriented Al_1−xSc_xN(11$2¯$0) films with x = 0.2 and 0.3 by magnetron sputter epitaxy has been demonstrated.^26,27 Simulations indicate the potential of such layered structures for microacoustic applications.²⁸ We report on the growth of Al_0.7Sc_0.3N(11$2¯$0) via magnetron sputter epitaxy and the characterization of SAW modes.

Al_0.7Sc_0.3N(11$2¯$0) thin films with 1 µm thickness were grown on 100 mm r-plane Al₂O₃(1$1¯$02) substrates with off-cut $α=+4°$ toward m-plane [Fig. 1(c)] using an Evatec Clusterline reactive pulsed-DC magnetron co-sputtering module. The films were deposited at 450 °C under base pressure of $8.5×10−6 Pa$ and working pressure of $8.0×10−2 Pa$ in 100% N₂ plasma. The Sc concentration of x = 0.3 was maintained by adjusting the magnetron power ratio of Al (99.999%) and Sc (99.99%) sputter targets under combined magnetron power of 1000 W. The targets were pre-sputtered before deposition under Ar gas to rid the surface of nitrides and other surface contaminants. The substrates were cleaned in situ before deposition using inductively coupled plasma etching. The detailed growth parameters of the deposition can be found elsewhere.²⁷ The orientation of Al_0.7Sc_0.3N film was clarified to be a-plane (11$2¯$0) using $2θ−θ$ x-ray diffraction scan [Fig. 2(a)]. Al₂O₃ 1$1¯$02,2$2¯$04, 3$3¯$06, and AlScN 11$2¯$0 reflections corresponding to the substrate and the film were detected. X-ray diffraction pole figure measurements were performed to analyze the epitaxial orientation of the AlScN film with respect to the substrate in order to generate pole figures corresponding to the off-axis reflections: 1$1¯$01, 1$1¯$00, and 0002 [Fig. 2(b)]. The surface of the films is characterized by an elongated grain structure seen under atomic force microscopy [Fig. 2(c)]. This grain structure is understood to develop as a consequence of the anisotropic growth of the a-plane AlScN(11$2¯$0). The grains are aligned along the c-axis of the AlScN film which is also the projected c-axis of the Al₂O₃ substrate.

We analyzed the electro-acoustic properties of the grown Al_0.7Sc_0.3N(11$2¯$0) via fabrication and electrical characterization of one-port SAW resonators. The interdigital transducers were fabricated by standard lithography on the Al_0.7Sc_0.3N(11$2¯$0)/Al₂O₃(1$1¯$02) structures by evaporation of 100 nm Pt electrodes. The high acoustic impedance of Pt provides increased mechanical loading and, thus, contributes to the confinement of the acoustic fields in the layer. The number of fingers for all resonators was defined as N = 100 and the aperture was set as $A=20 λ$⁠. The scattering parameters of SAW structures were measured using a vector network analyzer, whereas the admittance Y was calculated. The fundamental wave modes were then analyzed, and the series resonance f_s, and parallel resonance f_p were extracted. The effective electromechanical coupling was calculated using the expression⁸ $keff2=(π2/4)(fs(fp−fs)/fp2)$ and the acoustic phase velocity was defined as $v=λ(fs+fp)/2$ with wavelength λ. The mechanical quality factors Q were extracted by modified Butterworth Van-Dyke electrical circuit fits of the experimental data. The measurements were then compared with FEM simulations. We applied 3D periodic cell modeling in order to take into account the effects of the aperture of the SAW structures.²⁹ For the material input, the elastic and piezoelectric constants of AlN²⁵ were interpolated with the trend obtained by the density functional theory (DFT) computations³⁰ for Al_1−xSc_xN with composition x = 0.3. The theoretical orientations of film and substrate²⁰ [Fig. 1(b)] were used in order to transform the material tensors in the corresponding orientation. The Euler angles for the film is (90°, −90°, 90°+$φ$⁠) and for the substrate (0°, 57.6°−α, 90°+$φ$⁠) with offcut $α=+4°$⁠. Here, the angle $φ$ indicates the wave propagation direction within the wafer plane with respect to the direction along the c-axis of the film [Fig. 1(b)].

The measured and computed admittance Y for SAW resonators with wavelength λ = 2.2 µm for the directions parallel (⁠$φ=0°$⁠) and perpendicular (⁠$φ=90°$⁠) to the c-axis [0001] of the Al_0.7Sc_0.3N(11$2¯$0) film is given in Fig. 3. It indicates a strong anisotropy of the SAW wave characteristics. Taking into account the error tolerances of the metal thickness, the phase velocity v, electromechanical coupling $keff2$⁠, as well as spurious modes are well represented by the FEM model. According to displacement fields from our simulations (Fig. 4), the fundamental wave mode for the direction with $φ=0°$ is a Rayleigh wave with moderate electromechanical coupling (⁠$keff2 ≈$ 2.2%) and low phase velocity (v = 3472 m/s). This wave mode exhibits sagittal polarization; therefore; the displacement fields perpendicular to propagation direction are zero. Since the displacement fields at resonance are almost entirely located within the piezoelectric film [Fig. 4(b)], the velocity of this mode is almost exclusively determined by the elastic properties of Al_0.7Sc_0.3N(11$2¯$0). The wave mode with propagation perpendicular to the c-axis [0001] direction (⁠$φ=90°$⁠) has a predominant shear horizontal (SH) polarization with low electromechanical coupling (⁠$keff2 ≈$ 0.3%). We note that this mode depends almost only on the untransformed film constants e₁₅ and c₄₄.

The trade-off between increased electromechanical coupling and reduced velocity due to the Sc incorporation may be bypassed by choosing the appropriate normalized film thickness $h/λ$ for Al_0.7Sc_0.3N(11$2¯$0) films. The dispersion branches of phase velocity v and effective electromechanical coupling $keff2$ for the fundamental SAW with propagation along the c-axis [0001] (⁠$φ=0°$⁠) were determined and compared with the fundamental wave mode in Al_0.7Sc_0.3N(0001)/Al₂O₃(0001)³¹ [Fig. 5(a)]. It is interesting to note that the experimental data follows the trend of our FEM results, whereas the maximum in the effective electromechanical coupling for Al_0.7Sc_0.3N(11$2¯$0) film is found at lower normalized thicknesses (⁠$h/λ=0.2$⁠), which supports a significantly larger phase velocity compared to conventional Al_0.7Sc_0.3N(0001). In this range, the elastic properties and, therefore, the high phase velocities, as well as the low acoustic losses of Al₂O₃ become significant for the wave characteristics. The frequency response of a SAW resonator at $h/λ=0.2$ (λ = 5 µm) is shown in Fig. 5(b), indicating a large phase velocity (⁠$v=4825 ms−1$⁠) and increased electromechanical coupling (⁠$keff2$ = 3.7%) as well as a large quality factor (Q > 1000).

In this work, we have demonstrated the growth of Al_0.7Sc_0.3N(11$2¯$0) films on Al₂O₃(1$1¯$02) using magnetron sputtering as the deposition method. Fabrication and measurements of one-port SAW resonators on the films revealed strong anisotropic effects of the electroacoustic behavior within the wafer plane. Our experimental results were well represented by FEM modeling by using DFT constants³⁰ and epitaxial relationships from the literature.²⁰ For wave propagation along the c-axis of Al_0.7Sc_0.3N(11$2¯$0), we have identified a sagittal polarized Rayleigh mode with a maximum of the electromechanical coupling for normalized thickness around $h/λ=0.2$⁠. The associated influence of the substrate increases both the phase velocity (⁠$v=4825 ms−1$⁠) and quality factor (Q > 1000) combined with large effective electromechanical coupling (⁠$keff2$ = 3.7%).

This work was partially supported by the Gips-Schüle-Stiftung and the Carl-Zeiss-Stiftung (project “SCHARF”). The authors would like to thank the Department of Technology and the Department of Epitaxy at the Fraunhofer Institute for Applied Solid State Physics IAF for their help in the fabrication of SAW structures. The authors thank M. Prescher for expert assistance for the XRD analysis.