Comparison of conventional and collapse-mode CMUT in 1-D array configuration

This paper presents a comparison between two types of capacitive micromachined transducers (CMUTs), one operating in the conventional mode and the other one in the collapse-mode. Both types of devices are designed to have the same membrane properties (silicon, 1-μm thick), the same peak frequency (10 MHz) and similar operating bias voltages (100 V - 130 V). In the finite element analysis (FEA) of transmit pressure, both types of devices have similar fractional bandwidth (FBW) more than 120%, whereas, the collapse-mode CMUT has higher output pressure (46.5 kPa/V) than that of the conventional CMUT (13.1 kPa/V). To validate the FEA results, these two devices are fabricated using LOCOS/wafer-bonding process. The fabricated devices have a phased array configuration with an element pitch of 75 μm. The output pressure and the FBW of the conventional CMUT is 110% and 12.7 kPa/V, respectively, which match well with FEA results. The collapse-mode CMUT has a FBW of 102.5% and output pressure of 26.4 kPa/V. With a large AC excitation and high DC bias, the conventional CMUT has an output pressure of 1.25 MPa_p-p with 56.4-V_p-p AC and 180-V DC. The collapse-mode CMUT has an output pressure of 2.37 MPa_p-p with 56.2-V_p-p AC, and 150-V DC.