Aluminum Nitride Piezoelectric Micromachined Ultrasound Transducer Arrays for Non-Invasive Monitoring of Radial Artery Stiffness

Abstract

An aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array was proposed and fabricated for non-invasive radial artery stiffness monitoring, which could be employed in human vascular health monitoring applications. Using surface micromachining techniques, four hexagonal PMUT arrays were fabricated within a chip area of 3 × 3 mm². The mechanical displacement sensitivity and quality factor of a single PMUT were tested and found to be 24.47 nm/V at 5.94 MHz and 278 (in air), respectively. Underwater pulse-echo tests for the array demonstrated a -3 dB bandwidth of 0.76 MHz at 3.75 MHz and distance detection limit of approximately 25 mm. Using the PMUT array as an ultrasonic probe, the depth and diameter changes over cardiac cycles of the radial artery were measured to be approximately 3.8 mm and 0.23 mm, respectively. Combined with blood pressure calibration, the biomechanical parameters of the radial artery vessel were extracted using a one-dimensional vascular model. The cross-sectional distensibility, compliance, and stiffness index were determined to be 4.03 × 10^-3/mmHg, 1.87 × 10^-2 mm²/mmHg, and 5.25, respectively, consistent with the newest medical research. The continuous beat-to-beat blood pressure was also estimated using this model. This work demonstrated the potential of miniaturized PMUT devices for human vascular medical ultrasound applications.

Keywords: blood pressure monitoring; miniaturized ultrasonic device; piezoelectric micromachined ultrasound transducer array; pulse-echo method; radial artery stiffness.

Figure 1

(a) The three-dimensional schematic of our PMUT unit. (b) The cross-section of a single PMUT unit and the enlarged view of the cavity structure.

Figure 2

The relationship between the effective coupling coefficient and thickness of AlN film (red curve) and the relationship between the flexural rigidity and thickness of AlN film (blue curve).

Figure 3

(a) Two-dimensional model of a PMUT with a perfect matching layer (PML) in COMOSL. (b) Frequency response of absolute acoustic pressure on the axial direction with the distance of 2.8 mm for one PMUT unit in water medium.

Figure 4

(a) Three-dimensional schematic view of a hexagonal PMUT array. (b) Acoustic power direction comparison between the square array and the hexagonal array.

Figure 5

(a) The MBVD equivalent circuit model of the PMUT array. (b) The simulated frequency response of array impedance for both the no medium and water medium situations.

Figure 6

Schematic of PMUT fabrication process. (a) Deposition and patterning of sacrificial layer. (b) Deposition of elastic layer. (c) Etching of micro-holes. (d) Wet etching of sacrificial layer. (e) Sealing of the low-pressure cavity. (f) Thermal oxidation to form insulation layer. (g) Deposition of bottom electrodes and piezoelectric layer. (h) Deposition and patterning of top electrodes. (i) Deposition and patterning of gold layer on pads.

Figure 7

(a) SEM image of the fabricated PMUT chip with four arrays. (b) Enlarged view of a single PMUT unit. (c) Cross-sectional view of the PMUT structure via FIB cutting.

Figure 8

(a) Schematic of the pulse-echo detection signal transfer system. (b) Radial artery monitoring (corresponding to the dashed box in (a)). (c) Customized PCB for PMUT package and size comparison with a coin.

Figure 9

(a) Frequency response of underwater acoustic pulse-echo signals. (b) Pulse-echo signals with reflector at resonance frequency with three different detection distance.

Figure 10

(a) Pulse-echo signals of the single acquisition and data process in MATLAB. (b) The three-dimensional view of the receiving signal amplitude of continuous recording.

Figure 11

Dynamic depth change of two artery walls and diameter change.

Figure 12

Continuous monitoring of radial artery diameter and estimated continuous beat-to-beat blood pressure.