Integrating porous silicon layer backing to capacitive micromachined ultrasonic transducers (CMUT)-based linear arrays for acoustic Lamb wave attenuation

Abstract

Lamb waves propagating in the substrate of linear arrays integrated on a silicon (Si) chip may degrade the elementary performances of the imaging device. In fact, these waves are radiated in the imaging medium. Their superimposition with the relevant ultrasonic signals alters the image performances (i.e., lateral and axial resolutions). In this article, we investigate the interest of using a thin layer of porous silicon (PS) as an absorbing material, aiming to reduce the total device dimensions compared to more traditional backing materials and facilitate device integration with on-chip electronics. The proposed method was applied to Capacitive Micromachined Ultrasonic Transducers. To this purpose, a PS layer with a thickness of 60 μm and a porosity of 50% was etched on the rear side of a 256-elements linear array. The electroacoustic response of the elements integrated on the Si substrate was compared to those on the Si/PS substrate, showing no deterioration of the acoustic characteristics (i.e., center frequency and bandwidth) after PS layer fabrication. To assess the PS silicon layer influence on Lamb wave attenuation, acoustic cross-talks were measured for each array element. The radio-frequency dataset was used to determine the dispersion curves of Lamb waves in the substrate. The comparison between the two substrates showed a significant attenuation value (superior to 30 dB) of Lamb waves induced by the PS layer.