Fabrication and characterization of surface CMUT with a bossed membrane

This paper describes the fabrication and characterization of surface-micromachined capacitive ultrasonic transducers with a bossed membrane. The boss was formed using 3μm thick deposited tetraethoxysilane (TEOS) oxide on top of a suspended polysilicon membrane. This same oxide layer was also used to seal the release holes along the peripheral of the polysilicon membrane. No extra mask or processing step in addition to that used for fabricating planar-membrane capacitive micromachined ultrasonic transducer (CMUT) is needed for the addition of a boss on the polysilicon membrane. It was found from device characterization that a bossed device shifted the center frequency to a higher value, improved the fractional bandwidth in transmission mode, increased the receiver sensitivity, and augmented the electromechanical coupling efficiency compared to their planar-membrane counterpart with the same membrane diameter.