Characterization of an Immersion Lithium Niobate Piezoelectric Micromachined Ultrasound Transducer using Laser Doppler Vibrometry and Hydrophone Measurements

This study introduces the first demonstration and quantitative characterization of a lithium niobate (LN) piezoelectric micromachined ultrasound transducer (PMUT) in immersion. Each PMUT diaphragm is a unimorph comprising an 800 nm-thick LN layer on top of a 2.0-µm-thick silicon dioxide layer. A lateral electric field is induced in the LN film using electrodes on the diaphragm surface. A range of diaphragm sizes and shapes are possible. This study focuses on experimental characterization of the transmit capabilities of an isolated rectangular diaphragm with dimensions 240 µm × 75 µm. In the first study, light from a laser Doppler vibrometer (LDV) was focused through mineral oil to measure the diaphragm’s vibration in response to a broadband chirp input, thereby obtaining the transmit frequency response. In a second study using tone burst waveforms, a hydrophone was used to measure the free-field pressure in oil generated by the transducer. Our characterization shows a transmit sensitivity equal to 5.5 kPa surface pressure per volt. We estimate an achievable maximum surface pressure equal to 400 kPa across the 0.5–5 MHz frequency range. Future work entails further optimizing the transducers for larger output pressure, targeting 1 MPa, and using these transducers in implantable biomedical phased arrays.