Design, Simulation, Microfabrication and Characterization of a 16 MHz 1D-pMUT-Array

Abstract

Ultrasound transducers are being used in a variety of biomedical applications. Conventional ultrasound transducers used currently for medical imaging are quite bulky, these transducers also consume considerable amounts of power during their operation. They are used in pre and post operative scenarios for diagnostics and follow-up, respectively. There is a need to develop small footprint, low-power consuming ultrasound transducers that can be used intraoperatively. The modern development in microfabrication techniques make pMUT (piezoelectric Micromachined Ultrasound Transducers) as a potential candidate to replace the conventional ultrasound transducers. Imaging with pMUT Arrays is demonstrated at frequency ranges of 1 MHz - 12 MHz in earlier works. In this work we focus on design, simulation, microfabrication, and characterization of high-frequency (16 MHz) 1D pMUT-Array for high-resolution medical imaging application that require identification of fine structures such as nerves and blood vessels in and around a tumor/surgery site. We started with simulation study of a freely vibrating plate under electrical input loading condition and extended the same to understand the working of pMUT through simulations built in COMSOL Multiphysics platform. The aim of the simulations was to build a model that would allow for design of circular pMUT’s for different combinations of geometric parameters and material stack. The model allowed for varying material properties and dimensions to arrive at the specific values suitable for our application. 5% mismatch was noted between the analytical and simulation models for resonance frequencies and displacement at resonance. Imaging simulations were done using MATLAB Ultrasound Toolbox (MUST), to check the imaging abilities of the designed pMUT array and validates the design suitability for our specific clinical need even prior to microfabrication. Microfabrication steps were selected to ensure over all process compatibility. Aluminium Nitride (AlN) thin film was chosen as the piezoelectric material in the vibrating stack. We obtained a c-axis (<002>) oriented AlN thin-film through improvement in parameter control of RF Magnetron Sputtering process. At 350oC substrate temperature, 150 W RF power and 62% Nitrogen gas sputtering condition; a Full Width Half Maximum (FWHM) of 4.4 was achieved for the piezoelectric axis of AlN (36o of XRD peak). AlN thin film was further characterized using Piezo response Force Microscopy (PFM), effective piezoelectric co-efficient (d33) of 7.28 pm/V was obtained. To the best of our knowledge this is the highest value of d33 reported for an AlN thin film obtained through RF sputtering process. The fabricated pMUT’s were characterised in terms of electrical, mechanical, and acoustic properties. As per simulated design the 16 element 1D-array is housed in a chip size of 1.6mm2. The pMUT’s were interfaced into a photoacoustic setup wherein a graphite lead of 0.7mm diameter was the target to be imaged in an agar-based phantom. The graphite lead was illuminated using a laser at wavelength range: 660nm-2500nm and pulse width: 7ns along with a repetition rate of 30Hz. The laser was set to operate at a wavelength of 740 nm and the ultrasound created by the target was received using the pMUT. The pMUT’s captured the photoacoustic signals from a depth of 32mm. Photoacoustic data from 10mm, 20mm and 32mm were reconstructed to form photoacoustic images. A custom-built experimental setup was used to note the working of pMUT as transmitter: Hydrophone was aligned with the actuated pMUT to record the output pressure values from our device. We obtained output pressure values varying from 38 kPa to 15 kPa for distances of 1mm to 5mm, respectively. Two ultrasound imaging experiments were performed: 2 mm diameter copper wire and three 0.5 mm diameter copper wires embedded in an agar phantom. The imaging was carried out using the synthetic aperture scheme, wherein one pMUT element is transmitting the ultrasound wave and the other elements in the array are receiving the reflected echoes from the imaging medium to form one image frame. 16 image frames were captured in each experiment and averaged to form one effective output frame. Imaging experiments highlight the applicability of using this high-frequency pMUT-array as a multi-modal imaging device (photoacoustic + ultrasound). Imaging an object of size 0.5mm at a depth of 6mm from the transducer demonstrates the usefulness of high-frequency imaging in visualizing smaller structures. Further improvements in piezoelectric thin-film deposition coupled with efforts to reduce residual stresses in the material stack can result in superior imaging performance of the pMUT. Through-silicon-vias can allow for monolithically integrating pMUT-array with on-chip circuitry to enable future miniature high frequency medical imaging solutions. The AlN thin film deposited in this work has further scope for improvement, considering all these future scope and current limitations there are a few steps left before we can enable the surgeons to visualize blood vessels and nerves around a tumor allowing for preserving vasculature in the brain during surgery.