Liquid Density Sensing by a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) Based on Aluminum Nitride

In this abstract, an aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT) based liquid density sensor is presented. CMOS-compatible AlN is utilized as the piezoelectric material in the PMUT. The presented sensor operates by measuring the change in resonant frequency of the PMUT to calculate the liquid density. In experiment, the resonance of the PMUT sensor is measured by both an electrical impedance analyzer and a digital holographic microscope (DHM). The quality factor (Q) value of the PMUT sensor is measured to be 49.38 in water. A high sensitivity of 12.71kHz/g/cm^-3 was determined for this sensor by measuring the resonant frequencies immersed in different glycerol aqueous solutions. 12 PMUTs are measured and the maximum deviation of resonant frequency between different PMUTs is 9.85%. For the first time, the resolution of the PMUT density sensor is determined to be 5.9×10^-4g×cm^-3.

As an important physical parameter of liquids, the liquid density plays an important role in many applications. As a result, the development of liquid density sensors is critical. However, current liquid density sensors have drawbacks such as large size of conventional liquid density sensors, low quality factor of micro-cantilever beam structures, low sensitivity of quartz crystal tuning forks, high operating frequency requirement of surface-acoustic-wave love-mode resonators. As microfabricated piezotronics based sensors have advantages of small size, high quality factor and high sensitivity, an aluminum nitride based PMUT liquid density sensor is presented in this abstract to address the aforementioned drawbacks of traditional liquid density sensors.

The AlN based PMUT, which is microfabricated based on 1-μm-thick AlN deposited onto silicon-on-insulator (SOI) wafer, consists of an AlN piezoelectric layer sandwiched by two molybdenum metal electrodes, a silicon support layer, and a silicon substrate with a back-etched cavity. When the PMUT is immersed in liquid, the density of liquid causes the resonant frequency of the PMUT to shift. By measuring the resonant frequency shift, the density of liquid can be determined. In experiment, the resonance of the PMUT sensor is measured by both an electrical impedance analyzer and a digital holographic microscope (DHM). The resonant frequency measured by impedance analyzer agrees with that measured by the DHM. Measurement results shows that the signal to noise ratio of the electrical impedance measurement and DHM measurements are 14.77dB and 22.04dB respectively for 60% glycerol solution. The sensor sensitivities of PMUTs with different cavity diameters of 700, 1000, and 1300 μm are characterized, and it is found that a smaller PMUT cavity diameter results in higher sensitivity, yet lower SNR. A high sensitivity of 12.71kHz/g/cm^-3 was determined for this sensor by measuring the resonant frequencies immersed in different glycerol aqueous solutions, which is improved by 9.28 times compared with a tuning fork based liquid density sensor in prior studies. 12 PMUTs are measured and the maximum deviation of resonant frequency between different PMUTs is 9.85%. For the first time, the resolution of the PMUT density sensor is determined to be 5.9×10^-4 g×cm^-3. The presented piezotronics based PMUT liquid density sensor can be potentially applied to the density detection of physiological indicators, concentration or homogeneity in chemical industry, as well as automatic control.