Non- flexural parallel piston movement across CMUT with substrate-embedded springs

In this paper, we introduce a modified fabrication method for CMUTs with substrate-embedded springs (post-CMUTs or PCMUTs) to increase the fabrication yield. This modified fabrication process includes three additional steps from the previous process: vacuum cleaning prior to the wafer bonding step, thermal oxidation of the silicon piston top plate, and preservation of the 150-nm buried oxide (BOX) layer on top of the 250 nm Si layer of the bonded SOI wafer. These modifications increased the fabrication yield from 10% to 60%. For these newly fabricated PCMUT devices, we measured the electrical input impedance with a precision impedance analyzer, the plate displacement with a laser Doppler vibrometer (LDV), and the output pressure with a calibrated hydrophone. The measured electrical input impedance matches well with the FEA simulation results; the LDV measurement in air confirmed a non-flexural plate displacement; and the hydrophone measurement showed a peak-to-peak acoustic pressure of 27.6 kPa at a distance of 3.6 mm in immersion, corresponding to 1.05 MPa at the face of the transducer, for a particular test 2-D array element.