Tactile Threshold Reduction Through Angle-Steered Focused Ultrasound Stimulation Using Capacitive Micromachined Ultrasound Transducer (CMUT)

This work presents the design, fabrication, and characterization of a Capacitive Micromachined Ultrasonic Transducer (CMUT) for reducing tactile thresholds through angle-steered focused ultrasound stimulation. The device was fabricated using a double oxidation process to achieve a high pull-in voltage and stable operation. A 20-channel annular array was designed with field programmable gate array (FPGA)-based pulser control, enabling phase modulation to generate focused ultrasound at multiple depths. The designed pressure sensitivity was 10.4 kPa/V at 5 MHz, while the measured surface sensitivity ranged from 4 to 7 kPa/V due to fabrication variation. On-axis pressures were measured at focal depths from 2 to 15 mm, and the results were compared with simulations using effective aperture compensation to account for hydrophone acceptance angle. Psychophysical experiments presented that tactile thresholds were lower when the finger pulp surface was aligned with the beam axis than when perpendicular, indicating that beam orientation and contact area play a critical role in perception. These results provide key insights for optimizing ultrasound-based tactile interfaces.