Fabrication of CMUT Cells with Gold Center Mass for Higher Output Pressure

Abstract

For decades, high intensity focused ultrasound (HIFU) transducers have been developed for minimally invasive and non‐invasive therapies. Capacitive micromachined ultrasonic transducer (CMUT) technology is a promising candidate for HIFU therapy as it allows the fabrication of arbitrary array geometries and is inherently magnetic resonance (MR) compatible. In this study we investigate a way to improve the output pressure of a single CMUT cell by a modification to the basic CMUT cell structure: adding a gold mass over the center of the top CMUT plate. Using the direct wafer bonding fabrication process we realized linear 1D CMUT arrays. On top of the 0.86 μm thick silicon plate, a 200‐nm thick aluminum layer and a 10‐nm thick titanium adhesion layer were deposited. A lift‐off technique was used to deposit a gold mass on top of the adhesion layer, at the center of each cell. The 1‐μm thick gold layer was deposited in multiple steps with intervening cool‐down periods to ensure low thermal‐induced stress between the gold and the metalized CMUT plates. Electrical impedance measurements of the devices reveal improved performance due to the gold mass, and the average resonance frequency in air for the elements in the 1D array decreased from 7 MHz to 3.6 MHz with a standard deviation of 0.125 MHz and 0.157 MHz, respectively. A direct comparison of cells with and without the gold mass in terms of measured output pressure at the surface of a single cell demonstrated a 23% improvement. When biased with a DC voltage equal to 75% of the pull‐in voltage, the device with the gold mass delivered 1.875 MPa peak‐to‐peak surface pressure at a frequency of 2.6 MHz (single cell measurement). The results indicate that adding a center‐mass to regular CMUT cells improves device performance in terms of acoustic output pressure. In the future, we plan to investigate the acoustic crosstalk between cells and ways to mitigate it.