Beyond 5 GHz excitation of a ZnO-based high-overtone bulk acoustic resonator on SiC substrate

This work reports on the fabrication and characterization of an Au/ZnO/Pt-based high-overtone bulk acoustic resonator (HBAR) on SiC substrates. We evaluate its microwave characteristics comparing with Si substrates for micro-electromechanical applications. Dielectric magnetron sputtering and an electron beam evaporator are employed to develop highly c-axis-oriented ZnO films and metal electrodes. The crystal structure and surface morphology of post-growth layers are characterized using X-ray diffraction, atomic force microscopy, and scanning electron microscopy techniques. HBAR on SiC substrate results in multiple longitudinal bulk acoustic wave resonances up to 7 GHz, with the strongest excited resonances emerging at 5.25 GHz. The value of f.Q (Resonance frequency.Quality factor) parameter obtained using a novel Q approach method for HBAR on SiC substrate is 4.1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document}× 10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{13}$$\end{document}13 Hz, which to the best of our knowledge, is the highest among all reported values for specified ZnO-based devices.


Introduction
High-performance bulk acoustic wave (BAW) resonators have received a great deal of attention over the past few decades due to their potential use as radio frequency (RF) sources, sensors, filters, and actuators.
1, 2 Quartz crystal resonator (QCR), which typically operates in the several MHz to tens of MHz range, is a common type of BAW resonator.Another type of BAW resonator is the High Overtone Bulk Acoustic Resonator (HBAR), also referred to as a composite resonator is composed of a piezoelectric layer sandwiched between two metal electrodes on a low acoustic loss substrate.3, 4 With a simple yet robust structure, a compact size, and an impressively high-quality factor (Q), HBAR has the ability to demonstrate highly acute resonances (f) at GHz frequencies and above than that of QCR.6][7][8] Recently, there has been a noticeable growth in interest in the development of highly sensitive smart physical, chemical, and biological sensors based on acoustic resonators for noninvasive detection in real-time applications without utilizing any external reagents/chemicals.The working principle here is to integrate a biological/chemical element with the physical transducer of the acoustic device since it is sensitive to the atomic, ionic, or molecular chemical bond strength at the microwave frequency range.9, 10 Hence, HBAR can be widely employed to analyse a broad range of small volumes of fluidic materials, including human physiological fluids, and is suitable for Lab-on-a-Chip (LoC) systems.) value, however, it is not suited for HBAR applications due to its lower acoustic velocities, higher acoustic wave attenuation, and challenges in preparing thin films.1, 11 GaN films are substantially less prevalent due to their poor piezoelectric properties and low k 2 eff value. 17Although AlN films possess high acoustic velocity compared to ZnO films, it again suffers from a low k 2 eff value. 17Among the numerous piezoelectric materials described above, ZnO films with enhanced electro-acoustic characteristics have been found to be the most promising for the development of HBAR devices.Nevertheless, as reported in the literature, HBAR devices with ZnO piezoelectric layer have mostly been restricted to sapphire substrates with a f.Q product value of around 4.8 × 10 13 Hz using Lakin's Q method.Furthermore, SiC is frequently used in high-temperature and high-power electronic devices due to its high hardness, high thermal conductivity, chemical resistance, and so on.SiC substrates also play a vital role in the new genera-tion of hybrid quantum sensors and systems since they generate high stress at GHz frequencies than other substrates do. 20It is, therefore, imperative and pertinent to investigate the microwave resonant properties of ZnO-based HBAR on SiC substrates.
This work presents a novel device comprising a c-axis oriented ZnO piezoelectric film deposited on a Pt/Ti-coated SiC substrate to realize an efficient yet simple and scalable heterostructure for fabricating high-overtone bulk acoustic resonators.The detailed HBAR characteristics of the ZnO film on a SiC substrate is compared to those of a Si substrate over a broad frequency range.HBAR on SiC performs well with multiple longitudinal bulk acoustic wave resonances up to 7 GHz and a f.Q value up to 4.1 × 10 13 Hz, which is superior to any reported f.Q value among the specified ZnO-based HBAR on any other substrate.

Results and Discussion
Crystal structure and morphology of ZnO films.
A 650±20 nm thick ZnO piezoelectric film was grown on the Pt/Ti-coated SiC and Si substrate using the RF sputtering method.The structural properties of the synthesized ZnO film on the Pt/Ti coated oxidized Si and SiC substrates were investigated using X-ray diffraction (XRD), and the results are shown in Fig. 1.ZnO layers deposited on Pt/Ti/SiC exhibit a stronger (0002) orientation as compared to ZnO deposited on an oxidized Si substrate with a Pt/Ti coating.The (0002) rocking curve for ZnO on SiC is depicted in the inset of Fig. 1, with a full width at half maximum (FWHM) of 2.45°.This outcome is consistent with the cross-sectional scanning electron microscope (SEM) observation, as shown in Fig. 2 (a), where ZnO exhibits columnar microstructure that is normal to the SiC substrate surface.The surface morphology was measured using atomic force microscopy (AFM) in the tapping mode, and a representative result for ZnO/Pt/Ti/SiC is displayed in Fig. 2 (b).The ZnO films exhibit RMS surface roughness of 9.7±0.3nm and 4.9±0.2nm for ZnO/Pt/Ti/Si and ZnO/Pt/Ti/SiC, respectively.If the c-axis of ZnO film is perfectly oriented with respect to the normal of the substrate surface (no tilted angle), then the effective electromechanical coupling coefficient for longitudinal acoustic propagation ) is around 8.53% and for shear acoustic propagation (k S 2 ,eff ) is 0%.However, if the c-axis of the ZnO is tilted at an angle with the normal of the substrate, the shear mode acoustic wave takes precedence over the longitudinal mode acoustic wave. 23In the study, ZnO film is more strongly oriented along the (0002) direction on SiC substrate, as evidenced by the rocking curve and cross-sectional SEM micrograph in Fig. 1 and Fig. 2 compared to a Si substrate.Additionally, Si has relatively high longitudinal acoustic propagation losses (8.3 dB/cm @ 1 GHz), which are contrasted with its shear acoustic propagation losses (3.0 dB/cm @ 1 GHz) and a lower acoustic velocity than the other substrate materials. 21Hence, HBAR on a Si substrate only exhibits shear resonance.SiC, on the other hand, has a low acoustic loss (3.0 dB/cm @ 1 GHz) for both acoustic propagations.As a result, the longitudinal acoustic resonance predominates when ZnO-based HBAR is mounted to a SiC substrate compared to a Si substrate.
Fig. 3 (c) represents the measured impedance, or Z11 parameter, of the HBAR close to the strongest excited resonances on the SiC.The frequency range between each narrow resonance depends on the thickness (ts) of the substrate since the acoustic energy from the piezoelectric layer is coupled to it.This frequency spacing (fovertone) between narrow resonances is determined as fovertone = vs/2ts, where vs is the acoustic velocity of the substrate. 4The computed acoustic velocity from the equation is often a few percent lower than the real acoustic velocity because this expression is produced by ignoring the action of the piezoelectric layer on the substrate.The equation below describes the discrepancy between the calculated and actual acoustic velocities.
where the mass density and thickness of the substrate are represented by ρs and ls, respectively, and those of the piezoelectric film are represented by ρp and lp.The measured fovertone is around 12.9 and 17.8 MHz for the HBAR on the Si (thickness 250 ± 5 µm) and SiC (thickness 350 ± 5 µm), respectively.After the acoustic velocity has been rectified using the aforementioned equation, the corrected acoustic velocities of the Si and SiC substrates are esteemed to be 6490 and 12500 m/sec, respectively.The acoustic velocity for the Si substrate is measured to be greater than the shear acoustic velocity value, despite the fact that it is remarkably equivalent to the reported longitudinal acoustic velocity value for the SiC substrate.21, 25   This multitude of modes offers a special opportunity to use the HBAR as a biofluid sensor.To comprehend the detailed behaviour of device parameters, a modified Butterworth-Van Dyke (mBVD) model has been designed using Advance Design System (ADS, Keysight) software.Fig. 4(a) depicts a typical measured and mBVD fitted S11 parameter for ZnO-based HBAR on a SiC substrate at 5.25 GHz resonance.The mBVD model comprises circuit parameters such as motional resistance (Rm), motional capacitance (Cm), motional inductance (Lm), and static capacitance (C0), resistance (R0), which is shown as an equivalent circuit in Fig. 4(b).From the equivalent circuit, the fr is the resonance frequency where the series resonance occurs, fa is the anti-resonance frequency where the parallel resonance occurs and the effective electromechanical coupling coefficient (k 2 eff ) is given by the below equations.The electromechanical characteristics extracted from the mBVD model and the Q-factor of HBAR on Si and SiC substrates at the strongest excited resonances using the new Q approach are listed in Table 1.The product of resonance frequency and the measured quality factor (f.Q products) are then determined for the HBAR devices.We observed that the resonators for Si and SiC substrates, respectively, exhibit an f.Q product of 0.06 × 10 13 and 4.1 × 10

Conclusion
The acoustic properties at microwave frequencies have been investigated for ZnO-based HBARs on Si and SiC substrates.Highly c-axis oriented ZnO film is observed on Pt/Ti coated SiC substrate using the RF sputtering method.The novel c-axis oriented ZnO-HBAR on SiC substrate reveals multiple longitudinal bulk acoustic wave resonances and can be employed up to 7 GHz as a reliable wide-band piezoelectric transducer.The value of f.Q product for HBAR on SiC substrate, determined using a unique Q approach method is 4.1 × 10 13 Hz, which is superior to any reported f.Q value among the specified ZnO-based HBAR on any other substrate.The findings will be valuable in the manufacture of both low-phase noise microwave oscillators and highly sensitive acoustic sensors.For the fabrication of HBAR, we have chosen a double-side polished, oxidized Si (100) and semi-insulating 4H-SiC (0001) wafer to achieve a metal insulator metal (MIM) capacitor configuration using an electron beam evaporator and dielectric magnetron sputtering equipment.For the bottom electrode of the HBAR, initially, 15 nm/100 nm thick Ti/Pt (adhesive/conducting) layers were deposited on cleaned wafers at room temperature using the electron beam evaporator technique.Then, a 650±20 nm thick ZnO film was grown at 300 °C in an Ar:O2 (1:1) gas atmosphere using an RF sputtering method.Wet etching of ZnO is accomplished using a photo-resist mask in order to shape the Pt bottom electrode.Finally, using an electron beam evaporator, a stacked layer of Cr/Au with a thickness of 15/100 nm was deposited.This was followed by a lift-off photolithography procedure for the top electrode and active area patterning with 300 µm diameters.Fig. 5 displays the final manufactured devices along with their material stack and microscopic picture.RF measurements on the developed HBAR were carried out using an Agilent vector network analyser and a Ground-Signal-Ground probe station having an on-wafer pitch of 100 um.On a typical standard substrate, calibration was carried out using the short, open, and load procedures.HBAR devices are measured as one-port devices in the frequency range of 0.5 GHz to 10 GHz, with the top electrode functioning as the signal port and the bottom electrode functioning as the ground plane.

Figure 4 .
Figure 4. (a) The measured and mBVD fitted S11 parameter of ZnO-based HBAR resonator on SiC substrate at 5.25 GHz resonance, (b) The equivalent circuit diagram of mBVD model and (c) The measured quality factor of the ZnO-HBAR device on SiC substrate at various resonances using the new Qapproach based on S11 parameter.

27 Table 1 .
The quality factor of the HBAR device on SiC substrate at the various excited resonances is shown in Fig. 4(c), which is measured using the new Q approach based on S11 parameter proposed by Feld et.al. and it is related as follows; 26, The measured and mBVD fitted S11 parameters of ZnO-based HBAR resonator and their f.Q values on Si and SiC substrate.

Figure 5 .
Figure 5. Device configurations and material stacks for HBARs fabricated on Si and SiC substrates (a) cross-sectional view of HBAR on Si, (b) cross-sectional view of HBAR on SiC, (c) top view of HBAR device, and (d) optical microscope image of the fabricated device.
13Hz.Pang et.al., Baumgartel et.al. and Zhang et.al. has reported that , to the best of our knowledge stands out as the best among them.Using the novel Q approach developed by Feld et.al., the f.Q product of ZnO-based HBAR on Diamond is reported by Gosavi et.al. as 0.3 × 10 13 , which is substantially lower than this finding 20 .