Understanding CMUTs with substrate-embedded springs

A capacitive micromachined ultrasonic transducer (CMUT) with substrate-embedded springs, as named post-CMUT or PCMUT, provides many benefits over a conventional CMUT having flexural plate movement. Since the PCMUT structure resembles an ideal piston transducer, the improvements in performance mainly stem from the higher average displacement of the top plate for a given gap height. In this work, comprehensive 3-D finite element analysis (FEA) models are developed to further our understanding of the PCMUT structure. The 3-D FEA models include 3 analysis types, i.e. static, modal, and harmonic analyses, to fully understand the static and dynamic behavior of the PCMUT structure. To reduce the long simulation runtime, simplified 3-D FEA models with quarter symmetry and ideal springs are used instead of full transducer element models. We show that modal analysis results of the 3-D FEA models are in good agreement with the experimental results obtained from the first-generation fabricated PCMUT devices. Two top plate types are considered for the PCMUT structure and hence included in our model: a uniform plate (type 1) and a non-uniform plate (type 2) having a thinner edge portion than its center. Based on the 3-D FEA models, comprehensive parametric simulations were performed for both plate types to understand the effect of each parameter on the static and dynamic behavior. The plate type 2 provided better overall performance than the plate type 1. For the simulated designs the average DC displacement achieved using the plate type 2 was over 50% larger than that for the plate type 1. The FEA simulations also revealed that careful attention must be paid to the engineering of the top plate to assure that unwanted higher plate modes do not interfere with the normal operation of the device within the desired frequency band.