Design and fabrication of SU-8 CMUT arrays through grayscale lithography

The authors report, for the first time, a simple one-step manufacturing process, without the use of any sacrificial layers, applied for the fabrication of SU-8 ultrasound transducers. Based on grayscale lithography, the method has a high degree of simplicity compared to traditional microfabrication flows. The fabrication of SU-8 CMUT arrays is a case study that validates the application potential of the developed process, its robustness, reproducibility and design predictability. To demonstrate the predictability of the fabrication process, a custom calibration of the SU-8 grayscale lithography has been performed, and the mechanical performance of the CMUT cells has been simulated through FEA tools based on calibration measurement. The dynamical properties of the manufactured SU-8 CMUTs have been experimentally assessed and compared with the simulation results. The CMUT cells have a fundamental resonant frequency at 1.75 MHz, suitable for biomedical imaging; their statistical characterization shows a good match with design simulations with small deviations, indicating acceptable reproducibility, predictability, and robustness. Both mechanical and electrical characterizations show that the proposed grayscale manufacturing technology led to functional CMUT arrays with robust performances. This low-cost, rapid micromanufacturing technique is suitable for any out-of-plane SU-8 based MEMS devices, from pressure sensors to inertial devices and ultrasonic transducer arrays.