Fusion-Bonded Linear CMUT Arrays Based on Low Resistivity Thermally Stable Titanium Silicide Electrodes

Capacitive Micromachined Ultrasound Transducers (CMUTs), fabricated using the LOCal Oxidation of Silicon (LOCOS) process and fusion bonding, offer precise control over cavity geometry and enable fabrication of large-scale arrays. However, conventional bottom electrode materials such as highly doped silicon (1-5 mΩ.cm) have limited conductivity, leading to signal attenuation in largerow-column addressed (RCA) arrays due to Resistor-Capacitor (RC) delay line effects. This work proposes to use the stable C54 phase titanium silicide as an alternative, offering low resistivity (13-20 μΩ.cm) and thermal stability up to 950°C, making it compatible with high-temperature bonding processes.A 15 MHz linear test array with half-wavelength pitch and 375 cells per element was fabricated to evaluate the feasibility of TiSi₂ electrodes in RCA configurations. The bottom electrodes were formed using a Self ALigned silICIDE (SALICIDE) process. The bonding surface exhibited low roughness (R_a < 0.3 nm), enabling fusion bonding. Resistivity measurements yielded 24.9±0.1 μΩ.cm before bonding and 32.5 ± 0.1 μΩ.cm after bonding. These findings suggest that TiSi₂ is a viable candidate for large-scale CMUT fabrication.