Wafer-compatible fabrication and characteristics of nanocrystalline silicon thermally induced ultrasound emitters

Novel thermally induced ultrasound emitter without using any mechanical surface vibration systems has been developed with a high reliability. This emitter is based on a characteristic thermal property of nanocrystalline silicon (nc-Si) of which thermal conductivity and heat capacity per unit volume are extremely lowered in comparison to those of single-crystalline silicon (c-Si) due to complete carrier depletion associated with a strong quantum confinement effect. It is demonstrated here that a wafer-compatible electrochemical processing is available for the fabrication of this device by the use of appropriate masking and isolation techniques, and that more than 300 chips can be produced from a 4-inch wafer with a sufficiently high yield. The fundamental ultrasonic emission characteristics of the fabricated emitter are evaluated in terms of frequency response and the emission angle dispersion. The experimental results show that the fabricated device exhibits a flat frequency response with little distortion over a wide range as expected. Also the ultrasonic pressure is generated isotropically. This emitter is promising for applications to functional ultrasonic devices.