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Optimized response of AlN stack for chipscale GHz ultrasonics

In this paper we present designs of an aluminum nitride (AlN) based transducer stack for ultrasonic transmit/receive applications integrated in silicon. By optimal design of the mechanical layer thickness and material properties, channel gain, center frequency and bandwidth can be controlled to allow for the use of lower gain and on-chip power electronics for integrated ultrasonic information processing. Certain materials in the stack were fixed due to the fabrication processing capability, however some of the passive layers, and the thicknesses of the layers could be controlled. Simulations were done to select the desired thicknesses of each layer and the resulting chip was fabricated and verified. Previous tape-outs from the fab had resulted in receive signal levels of 200 μV at 1.3 GHz, whereas the current stack had signal levels of 18 mV at 1.3 GHz.