Challenges with acoustic backing of CMUT arrays on silicon with integrated electronics

Capacitive micromachined ultrasonic transducers (CMUTs) have in the last decade shown promising qualities for medical imaging. But there are some acoustical challenges that have to be overcome before the performance of a CMUT array with integrated electronics is satisfactory. Acoustic backing is necessary to avoid surface acoustic waves (SAW) excited in the surface of the silicon substrate to affect the transmit pattern from the array. For the backing to be able to damp the SAW, the silicon wafer stack has to be thin enough for some of the acoustic energy to reach the backing. We will present simulations showing that the thickness of the silicon substrate and thicknesses and acoustic properties of the bonding material have to be considered, especially when designing high frequency transducers. Through simulations we compare the acoustic properties of 3D stacks bonded with three different bonding techniques; anisotropic conductive adhesives (ACA), Solid-Liquid Interdiffusion (SLID) bonding and direct fusion bonding. We look at a CMUT array with a center frequency of 30 MHz and three silicon wafers underneath. We find that fusion bonding is most beneficial if we want to prevent surface waves from damaging the array response, but SLID and ACA are also promising if bonding layer thicknesses can be reduced.