Porous silicon as backing material for Capacitive Micromachined Ultrasonic Transducers

Crosstalks caused by Lamb waves propagating in the CMUT supporting substrate must be suppressed to avoid artifacts in the ultrasonic images. The present paper investigates the use of porous silicon as backing to preserve the device thickness and to consider 3D integration. In this way porous silicon layers were formed on the rear sides of CMUT'wafers. The electrical performances of the devices before and after porous silicon formation were estimated by electrical impedance measurements. They show that the transducer resonance is conserved even after making porous silicon. Moreover, the CMUT softening effect is preserved. The influence of the backing on the CMUT response in acoustic load condition was estimated by membrane displacement measurements. The experiments show that the use of a porous silicon backing significantly reduces the resonance of the substrate thickness mode. The impulse duration is 3.5 µs shorter when a porous silicon layer is formed on the rear side of CMUT substrate. These first results confirm that porous silicon is a promising candidate as backing material for ultrasonic transducers.