Investigation of nonlinear CMUT behavior

Abstract

Due to the electrostatic excitation principle, capac- itive micromachined ultrasound transducers (CMUT) exhibit a nonlinear behavior. This effect cannot be neglected in case of transmission where CMUTs are driven with high-level signals and are operated near the collapse point. Especially for medical ultrasonic diagnostics, nonlinear distortions of the transmitted sound pressure signals are highly undesirable for harmonic imaging. This paper presents a comprehensive investigation of the CMUT nonlinearities with the aim to optimize the input voltage signal for high transmit sound pressure amplitudes and for less harmonic distortion of the sound pressure signal. The investigations are mainly based on calculations using the 1D CMUT model which is presented in (1). In comparison to existing equivalent circuit models, which are linearized at the operating point, this 1D model has the advantage of being suitable also for fast nonlinear calculations. The model parameters are derived from FEM simulations of an arbitrary CMUT design. Therefore this approach guarantees a good description of a large CMUT arrays operated near the first order membrane mode and can be used for the calculation of pre-distorted input voltage signals to avoid harmonic distortions of the transmit sound pressure signals. Results compared to FEM simulations will be discussed.