Epitaxial ZnO piezoelectric layer on SiO2/Mo solidly mounted resonator fabricated using epitaxial Au sacrificial layer

In solidly mounted resonator (SMR) type of bulk acoustic wave resonator, it is difficult to fabricate single crystalline piezoelectric thin films in a bottom-up process due to the amorphous SiO2 in the low acoustic impedance layer of the acoustic Bragg reflector. In this study, single crystalline ZnO piezoelectric layer on amorphous SiO2/polycrystalline Mo acoustic Bragg reflector is fabricated using a wet etching process of the epitaxial Au sacrificial layer. Epitaxial growth of ZnO was confirmed by X-ray diffraction pole figure and transmission electron microscope electron diffraction pattern. Resonance frequency of 1.3 GHz of epitaxial ZnO SMR was observed using a network analyzer.

T he demand for bulk acoustic wave (BAW) filters has been increasing with the development of high frequency wireless communication bands.Two types of BAW filters: film bulk acoustic resonator (FBAR) and solidly mounted resonator (SMR) 1) are used in an RF front-end in the smartphone.FBAR has self-standing piezoelectric thin film structures without a supporting substrate.In contrast, SMR has an acoustic Bragg reflector consisting of a stack of different acoustic impedance as the supporting substrate.The power handling capability of SMR may be higher than FBAR because of the heat dissipation into the supporting substrate.
The power handling capability and Q factor are important parameters for BAW filters.In the transmitter, high RF power operation over 1 W is required.High Q resonator leads to sharp filter performance which can prevent interference between neighboring frequency bands.Unlike polycrystalline films with grain boundaries, single crystalline films are expected to possess lower dielectric and mechanical losses which may allow improvement in the power durability and Q factor.In general, single crystalline piezoelectric thin films can be obtained by cutting thin films from bulk single crystals using ion slicing and mechanical polishing, etc., or by epitaxial growth technique.[4][5][6][7][8][9][10][11][12][13] While sputtered AlN thin films can be mass-produced at 8 inches in a bottom-up process, bulk single crystals such as LiNbO 3 and LiTaO 3 are limited to 6 inches.On the other hand, by using an epitaxial growth technique, the fabrication of single crystalline AlN and ScAlN thin films over 8 inches might not even be difficult and can be relatively easily fabricated.
However, an epitaxial growth technique cannot be used to obtain a single crystalline piezoelectric thin film on an acoustic Bragg reflector in the usual bottom-up process, this is because amorphous SiO 2 is generally employed for a low acoustic impedance layer.In this study, we propose a method for fabricating the structure with an epitaxial piezoelectric layer on polycrystalline or an amorphous Bragg reflector using a wet etching process of an epitaxial sacrificial layer.The crystal orientation of the epitaxial thin film was evaluated by X-ray diffraction and electron diffraction.Impedance characteristics of SMR were measured by a network analyzer.
Figure 1 shows the fabrication process of the epitaxial piezoelectric thin film on an amorphous acoustic Bragg reflector.At first, epitaxial (111) Au thin films on epitaxial (111) Pt thin films were grown on a 0.65 mm-thick (0001) Al 2 O 3 single crystal substrate by DC and RF magnetron sputtering, as shown in Fig. 1(a).In order to obtain the Au epitaxial sacrificial layer with high crystallinity, an epitaxial Pt buffer layer was employed.Figures 2(a) and 2(b) show the 2θ-ω X-ray diffraction (XRD) patterns and the ω-scan rocking curves of the Au epitaxial sacrificial layer with and without an epitaxial Pt buffer layer on the Al 2 O 3 substrate, respectively.FWHM of the ω-scan rocking curves of (111) Au epitaxial layer clearly improved when an epitaxial (111) Pt buffer layer was inserted.The lattice mismatch between (0001) Al 2 O 3 and (111) Pt, (111) Au is estimated to be approximately 0.9% and 5%, respectively.This improvement in the crystallinity of the (111) Au layer is probably because the (111) Pt buffer layer possesses a smaller lattice mismatch with the (0001) Al 2 O 3 single crystal substrate compared with the (111) Au thin films.Next, the epitaxial (0001) ZnO piezoelectric layer was grown on the epitaxial (111) Au/(111) Pt/(0001) Al 2 O 3 substrate by using a standard planar RF magnetron sputtering.Although the lattice mismatch between (111) Au and (0001) ZnO is relatively large at 14%, epitaxial (0001) ZnO can be obtained on (111) Au since the (0001) ZnO with a close-packed structure is preferentially oriented.The growth conditions of the epitaxial thin films are shown in Table I.Afterwards, the polycrystalline Pt bottom electrode and the two pairs of SiO 2 /Mo acoustic Bragg reflector were sequentially deposited on the epitaxial ZnO piezoelectric layer by RF magnetron sputtering.Furthermore, a thin SiO 2 layer was fabricated on the bottom layer to prevent the Mo layer from damage due to the iodine solution during the wet etching of the Au sacrificial layer, as shown in Fig. 1(b).Next, the epitaxial Au sacrificial layer was etched using the iodine solution, as shown in Fig. 1(c).Since the reaction rate of Au with iodine is extremely high compared to other metals, a selective etching of the epitaxial Au sacrificial layer is possible.In this process, after the sample with the supporting Al 2 O 3 substrate was immersed in 0.5 mol l −1 iodine solution for 24 h, the epitaxial ZnO thin film with an acoustic Bragg reflector was released from the substrate.Finally, the Au top electrode pattern was fabricated on the epitaxial ZnO piezoelectric layer, as shown in Fig. 1(d).The epitaxial ZnO piezoelectric layer on SiO 2 /Mo acoustic Bragg reflector was then obtained.Figure 3 shows a cross sectional SEM image of SMR after etching of the epitaxial Au sacrificial layer.The epitaxial Au sacrificial layer was selectively etched without any damage on the epitaxial ZnO piezoelectric layer and the SiO 2 /Mo acoustic Bragg reflector.We can confirm that the epitaxial piezoelectric layer and the Bragg reflector layer have the desired thickness.
The crystal orientation of the epitaxial Pt, Au, and ZnO thin films was determined by X-ray diffraction (X'Pert PRO, PANalytical).FWHM values of the ω-scan rocking curves of the (111) Pt buffer layer and the (111) Au sacrificial layer were measured to be 0.25°and 0.27°, respectively.The Pt and Au layer exhibit good crystal orientation.025501-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd value of the ω-scan rocking curves of (0002) ZnO was measured to be 0.80°.Figure 5 shows the in-plane f-scan curve and the pole figure in the (10 11) plane of (0002) ZnO.FWHM values of the f-scan peak were determined to be 1.1°.ZnO piezoelectric layer exhibits good crystal orientation both out-of-plane and in-plane.We can confirm the epitaxial growth of the ZnO piezoelectric layer from six-fold symmetry around χ = 62°.Microstructure and local crystal orientation of the epitaxial ZnO piezoelectric layer were observed by transmission electron microscope (TEM) (JEM-2010, JEOL).Figure 6(a) shows a cross sectional TEM image of the ZnO piezoelectric layer on the SiO 2 /Mo acoustic Bragg reflector.(a)  The frequency characteristic of the epitaxial ZnO thin film SMR was measured by a network analyzer (E5071C, Keysight Technologies).This sample has a freestanding structure before transferring to the substrate.As shown in Fig. 7, a resonance peak was observed at 1.3 GHz in the real part of impedance and admittance.It was difficult to evaluate the effective electromechanical coupling coefficient k eff 2 and Q factor because a sharp resonance-antiresonance peak was not obtained.The ripples in the resonance peak may be caused by a laterally propagating acoustic wave.The reason for the weak resonance peak is due to the curved surface of the freestanding sample.Resonance characteristics should be improved by transferring the epitaxial ZnO piezoelectric layer/acoustic Bragg reflector structure to the other substrates using a thin-film bonding technique.
In conclusion, a method for fabricating a single crystalline piezoelectric layer on an amorphous acoustic Bragg reflector using a sputter-epitaxial growth technique and wet etching of the epitaxial sacrificial layer was demostrated.The epitaxial ZnO layer exhibits clear six symmetry in the pole figure and spot diffraction pattern in the TEM image.The thickness extensional mode resonance peak at 1.3 GHz was observed as expected.025501-4 © 2024 The Author(s).on behalf of The Society of Applied Physics by IOP Publishing Ltd

Fig. 1 .
Fig. 1.Fabrication process of epitaxial ZnO piezoelectric layer on acoustic Bragg reflector based on two pairs of SiO 2 /Mo.(a) (b)

Fig. 2 .
Fig. 2. (a) XRD patterns and (b) ω-scan rocking curves of epitaxial Au layer with and without epitaxial Pt buffer layer.

Fig. 3 .
Fig. 3. Cross sectional SEM image of epitaxial ZnO piezoelectric layer on acoustic Bragg reflector based on 2 pairs of SiO 2 /Mo.

Figures 6 (
b) and 6(c) show the TEM micrograph and electron diffraction pattern of the ZnO layer on the selected area in Fig. 6(a).As shown in Fig. 6(b), few dislocations and defects indicate good crystal orientation of the ZnO layer.Moreover, the ZnO layer exhibits a spot electron diffraction pattern on the [1 ̅ 100] zone axis [Fig.6(c)].This indicates that the single crystalline ZnO piezoelectric layer was fabricated on the Bragg reflector consisting of polycrystalline or amorphous thin films.

Fig. 5 .
Fig. 5. (a) In plane f-scan curve and (b) the pole figure in the ( 1011) plane of (0001) ZnO piezoelectric layer on acoustic Bragg reflector after etching process.

Fig. 6 .
Fig. 6.(a) Cross sectional TEM image of epitaxial ZnO piezoelectric layer on SiO 2 /Mo acoustic Bragg reflector.Red circle shows the selected area for observing microstructure and local crystal orientation of ZnO layer, (b) TEM micrograph of ZnO layer on the selected area, (c) Electron diffraction pattern from ZnO layer on the selected area.

Fig. 7 .
Fig. 7. Frequency response of real part of (a) impedance and (b) admittance of epitaxial ZnO SMR.

Table I .
Growth conditions of epitaxial thin films by magnetron sputtering.