Fabrication and characterization of thickness mode piezoelectric micromachined ultrasonic transducer (Tm-pMUT) based on PMN-PZT

Abstract

In this study, the lead magnesium niobate–lead zirconate titanate (PMN-PZT) single crystal based thickness mode piezoelectric micromachined ultrasonic transducer (Tm-pMUT) array was fabricated using microelectromechanical systems (MEMS) process. The PMN-PZT single crystal was grown by solid-state single crystal growth (SSCG) method. The inductively coupled plasma (ICP) etching was used to make a PMN-PZT based Tm-pMUT array. The ICP etching characteristics of PMN-PZT single crystal were investigated in a BCl3/Cl2/Ar gas mixture environment. The ICP etching parameters were RF power, chamber pressure, bias power, gas mixing ratio. The RF power was varied from 200 W to 1000 W. The bias power was controlled from 50 W to 450 W, and the gas flow rate was fixed as 20 sccm. The PMN-PZT surface was analyzed using X-ray photoelectron spectroscopy (XPS) to investigate the etching mechanism. The result showed that the BCl3/Cl2/Ar plasma increased etch selectivity ratio without decreasing etch rate of PMN-PZT single crystal and the physical etching was dominant. The sidewall angles was increased along the increase of the bias power. The optimized ICP etching conditions were BCl3/Cl2/Ar= 5/2/3 (10/4/6 sccm), 800 W RF power, 350 W bias power, 2 mTorr chamber pressure. The optimized ICP etching conditions gave a 107.6 nm/min etch rate and 4.22 etch selectivity ratio. Those optimized ICP conditions were used to fabricate the PMN-PZT based Tm-pMUT array. The PMN-PZT based Tm-pMUT array has 8 annular elements. The thickness of PMN-PZT single crystal was 300 m. The average resonance frequency and anti-resonance frequency were 2.66 (± 0.04) and 3.18 (± 0.03) MHz, respectively. The measured effective electromechanical coupling coefficient (keff2) was 30.05 %. These resonance frequency has high potential for ultrasound imaging applications. ⓒ 2016 DGIST