## 4 Fabrication Process
Before the piezoelectric layer is deposited, the wafer undergoes a planarization process via chemical-mechanical polishing (CMP) to ensure a uniform top surface across all structures by thinning them down to 5 \(\upmu\)m. CMP process ensures devices modal vibration functionality at the targeted resonance. In addition, due to the CMP's high-precision process and fabrication tolerance, the employment of the CMP step ensures the reliability and repeatability of the measured devices. Similar to transistor foundry processes, this CMP planarization is also crucial for the subsequent deposition and patterning of the piezoelectric material, as shown in Figure 3b. The RF magnetron sputtering of the piezoelectric ZnO layer is critical for the fabrication of piezoelectric resonators, particularly due to the strong correlation between the degree of c-axis orientation and the value of the \(d_{31}\) transverse piezoelectric coefficient. Achieving a high transverse piezoelectric coefficient \(d_{31}\) is essential for optimizing the performance of ZnO resonators, necessitating the deposition of high-quality (002) c-axis-oriented ZnO thin films with densely packed columnar structures. The process begins with the RF sputtering of ZnO onto the resonator body structure, which includes a conductive bottom electrode. For piezoelectrically transduced contour-mode resonators and filters, where in-plane extensional motions are critical, the quality of the ZnO films is vital. These films must exhibit strong c-axis orientation to convert electrical signals into mechanical vibrations and realize a high electromechanical coupling coefficient. The substrate temperature during deposition plays a crucial role in the piezoelectric properties of the ZnO films. The optimal results were observed at a substrate temperature of 300 \({}^{\circ}\)C [26; 33; 40], under which the ZnO film demonstrated the highest intensity peak of c-axis orientation. This condition leads to better piezoelectric characteristics compared to films deposited at higher or lower temperatures, where the quality and dielectric properties degrade. Maintaining an optimal substrate temperature ensures that the 500 nm-thick ZnO films are deposited under conditions that favor the growth of piezoelectrically active phases with solid electromechanical coupling coefficients. The oxygen environment during sputtering is essential for ensuring an oxygen-rich ZnO film. It is crucial to maintain a balanced argon-oxygen (Ar:O\({}_{2}\)) gas flow ratio; our experiments have shown that a 1:1 ratio promotes the strongest (002) crystalline orientation. Deviations in this ratio can significantly affect the film's quality. An increase in oxygen ratio can lead to degradation, while a decrease can negatively impact the dielectric effect and increase the conductivity of the film. The precise control of the oxygen environment during the deposition process is vital for achieving the desired electrical and mechanical properties of the ZnO layer. Following deposition, the ZnO film undergoes an annealing process at 400 \({}^{\circ}\)C for one hour under a forming gas environment with Ar and O\({}_{2}\). This post-deposition treatment is vital for enhancing the piezoelectric crystal characteristics and improving the c-axis alignment. The annealing process helps to relieve internal stresses in the film and promotes the reorientation of piezoelectric crystal domains, which enhances the material's piezoelectric coefficient. The exploration of deposition conditions, from the sputtering environment to the post-deposition annealing, optimizes the ZnO's piezoelectric properties, thus ensuring that the final hybrid resonators meet the stringent demands of advanced RF applications. Each process parameter is tuned to optimize the resonator's response, proving essential for achieving high performance and reliability for advanced RF MEMS devices. After the sputter deposition of ZnO film under optimized process conditions, the next crucial step is the selective wet etching of the Al\({}_{2}\)O\({}_{3}\) sacrificial gap layer.