Fabrication and characterization of V-doped ZnO films implemented to lead-free piezoelectric micromachined devices

Piezoelectric micromachined ultrasonic transducers (p-MUTs) have been extensively utilized in medical imaging, range-finding, gesture recognition, and so on. However, the piezoelectric layer is dominated by the toxic Pb(Zr, Ti)O3, other materials possess inferior piezoelectric coefficients, and the traditional clamped diaphragm restricts the p-MUT response. In this work, lead-free ZnO films are doped by the vanadium nanostructures and are implemented to beam-island structure membranes, which are aimed to achieve non-toxic and high-performance p-MUTs. Firstly, the doping mechanism of ZnO is analyzed and the p-MUT structure is designed. Secondly, simulation based on the finite element method is conducted to evaluate the dynamic displacement of p-MUTs, after which prototypes are fabricated by the standard micromachined process. The effects of key fabrication parameters including O2 flow rates, sputtering targets, and annealing temperatures on V-doped ZnO films are investigated in detail. By using atomic force microscopy (AFM) and X-ray diffraction (XRD), the surface morphology and crystal structure of the films are analyzed respectively. Moreover, the piezoelectric properties are measured by piezo response force microscopy (PFM). The results indicate a piezoelectric coefficient as high as 194.5 p.m./V, which is superior to most doped ZnO films. Finally, an experimental testing system is established to examine the p-MUT performance. Compared with the clamped diaphragm, the beam-island structure can acquire better electromechanical coupling and achieve range-finding successfully. This work provides a fine application prospect for enhancing the performance of lead-free p-MUTs.