CMUTs on glass with ITO bottom electrodes for improved transparency

In this work, we fabricated capacitive micromachined ultrasonic transducers (CMUTs) on a glass substrate with indium tin oxide (ITO) bottom electrodes for improved transparency. A 2-µm vibrating silicon plate was formed by anodic bonding. The fabrication process requires three masks. The fabricated devices show approximately 300% improvement of optical transmission in the visible to NIR wavelength range (400 nm - 1000 nm) compared to the devices with chromium/gold (Cr/Au) bottom electrodes. The measured static surface profile confirmed that the fabricated devices are vacuum-sealed. The electrical input impedance measurement shows the device has a resonant frequency of 4.75 MHz at 30-V DC voltage. The series resistance of the device is ~1 k?, which is mainly due to the ITO bottom electrode connections. Using a full bottom electrode or using parallel connections to the pads could reduce the resistance. The main hurdle for the transparency at shorter wavelength range is the 2-µm silicon plate. The transfer-matrix model shows the transparency could be improved to -80% across the measured spectrum, if silicon is replaced with a more transparent plate material such as ITO or silicon nitride.