SCANDIUM DOPED ALUMINUM NITRIDE BASED PIEZOELECTRIC MICROMACHINED ULTRASOUND TRANSDUCERS

Abstract

This paper presents the design, fabrication and characterization of piezoelectric micromachined ultrasound transducers (PMUTs) based on scandium doped aluminum nitride (ScxAl1-xN) thin films (x = 20%). ScAlN thin film was prepared with a dual magnetron system and patterned by a reactive ion etching system utilizing chlorine-based chemistry with an etching rate of 160 nm/min. The film was characterized by X-ray diffraction (XRD) which indicated a crystalline structure expansion change compared to pure AlN and a well-aligned ScAlN film. ScAlN PMUTs were fabricated by a 2mask process based on cavity SOI wafers. ScAlN PMUTs with 50 m diameter had a large dynamic displacement sensitivity of 25 nm/V at 17 MHz in air, twice that of AlN PMUTs with the same dimensions. Electrical impedance measurements indicated that the ScAlN PMUTs had 20% greater electromechanical coupling coefficient (kt 2 ) compared to AlN PMUTs. The output pressure of a 7x7 ScAlN PMUT array was 0.7 kPa/V at ~1.7mm away from the array, which is ~3 times greater that of an 8x8 AlN PMUT array with the same element geometry and fill factor measured at the same distance.