Fabricating capacitive micromachined ultrasonic transducers with direct wafer-bonding and LOCOS technology

We present an improved fabrication method for capacitive micromachined ultrasonic transducers (CMUTs). Recently, a process was developed to fabricate CMUTs using direct wafer-bonding instead of the traditional sacrificial release method. This paper presents a method based on local oxidation of silicon (LOCOS) and direct wafer-bonding to improve the controllability of gap heights and the parasitic capacitance. Critical vertical dimensions are determined by a thermal oxidation process, which allows tight vertical tolerances (≪ 10 nm) with unmatched uniformity over the entire wafer. Using this process we successfully fabricated CMUTs with gap heights as small as 40 nm with a uniformity of ± 2 nm over the entire wafer.