5F-4 Acoustic Crosstalk Reduction Method for CMUT Arrays

Abstract

This paper reports on the finite element analysis (FEA) of crosstalk in capacitive micromachined ultrasonic transducer (CMUT) arrays. Finite element calculations using a commercial package (LS-DYNA) were performed for an immersed 1D CMUT array operating in the conventional and collapsed modes. LS-DYNA was used to model the crosstalk in CMUT arrays under specific voltage bias and excitation conditions, and such a modeling is well worth the effort for special-purpose CMUT arrays for ultrasound applications such as medical imaging and high intensity focused ultrasound (HIFU) treatment. Compared to the existing finite element analysis (FEA) in literature, our FEA is distinguished by having all 5 main features together: First, the explicit, time domain solver of LS-DYNA enables the modeling of the actual CMUT array in detail, i.e. all 41 array elements are modeled. Second, user-defined subroutines provide an efficient electrostatic-structural coupling method. Third, the robust contact capability offers the CMUT modeling in collapsed operation. Fourth, a fast method to bias the CMUT array in conventional and collapsed modes is implemented. Fifth, the FEA results are verified with interferometer measurements. Our finite element calculations show that the main crosstalk mechanism is the dispersive guided modes propagating in the fluid-solid interface. Conventional operation has a crosstalk level of -23 dB and the guided modes are not present above the cut-off frequency of 4 MHz. Most importantly, the crosstalk wave has a center frequency of 2.3 MHz with a narrow bandwidth although the transmitter element has a center frequency of 5.8 MHz with more than 130% fractional bandwidth. Crosstalk level is improved to -39 dB in collapsed operation, and the cut-off frequency becomes 10 MHz because of the contact between the membrane and the substrate. The Lamb wave modes have a crosstalk level around -43 dB in both operation modes. These finite element results show excellent agreement with the interferometer measurements of the fabricated CMUT array. Using our verified FEA, we implemented a powerful method for the first time to reduce the crosstalk by impeding the propagation of the guided interface waves. This method is based on the acoustic band gap resulting from the periodic CMUT membranes on the fluid-solid interface. The crosstalk was effectively reduced by 10 dB down to -33 dB in the conventional operation without loss of acoustic pressure of the transmitter element. This method can be easily introduced into the fabrication of 1D and 2D CMUT arrays to achieve superior crosstalk