Finite element method and normal mode modeling of capacitive micromachined SAW and Lamb wave transducers

Surface wave and Lamb wave devices without piezoelectricity are the latest breakthrough applications of the capacitive micromachined ultrasonic transducers (CMUTs). CMUTs were introduced for airborne and immersion applications. However, experiments showed that those devices couple energy not only to the medium but also to the substrate they are built on. By placing the CMUTs on a substrate in an interdigitated configuration, it is possible to couple energy to Lamb wave or Rayleigh wave modes with very high efficiency without a need for any piezoelectric material. In this study, we calculate the acoustic field distribution in a silicon substrate as well as the acoustic impedance of the CMUT membrane, which includes the power coupled to the substrate. We apply the normal mode theory to find the distribution of the acoustic power among different Lamb wave modes. For low frequency (1 MHz) devices, we find that the lowest order antisymmetric (A₀) mode Lamb wave is the dominant mode in the substrate, and 95% of the power propagates through this mode. For high frequency devices (100 MHz), interdigital CMUTs excite Rayleigh waves with efficiencies comparable to piezoelectric surface acoustic wave (SAW) devices