5F-5 Reducing Fluid Coupled Crosstalk Between Membranes in CMUT Arrays by Introducing a Lossy Top Layer

Capacitive micromachined ultrasound transducers (CMUTs) promise high transducer performance for several ultrasound applications. Likely the most promising opportunities will be found in applications which require arrays with a large numbers of individual elements for precise beam steering and focusing such as in ultrasound imaging. In off-axis beam steering neighbor elements operate at different phase. This leads to deformation gradients along the surface of the array which cause local tangential forces acting upon the adjacent medium. In the case of an adjacent lossless fluid this results in local high-Q resonances which have significant detrimental impact on the transducer array performance in off-axis operation. It is therefore of paramount importance to control and reduce the excitation of such resonances to an acceptable level through the design. The present paper presents one approach to this. Simulations show that by introducing a soft intermediate surface layer which is only a few per cent of its longitudinal wavelength in thickness, and which has high shear deformation losses - of the order of 0.4 in loss tangent, quite acceptable results may be obtained without adding more than 0.5-1 dB in transmit losses. Although not readily commercially available, it may be possible to develop adequate polymer materials for this purpose. It is shown also that such a layer may be mechanically protected by an additional stiff layer without significant degradation of the ultrasound transducer performance