Surface micromachined capacitive ultrasonic immersion transducers

Major steps used in fabricating surface micromachined capacitive ultrasonic immersion transducers are investigated. Such steps include membrane formation and cavity sealing under vacuum. Three transducer membrane structures are evaluated: a nitride membrane with an LTO sacrificial layer; a polysilicon membrane with an LTO sacrificial layer; and a nitride membrane with a polysilicon sacrificial layer. The major differences in the three processes are the conductivity, dielectric constant and residual stress of the membrane. Three vacuum sealing mechanisms are compared, each of which requires a different degree of lithographic sophistication, and results in a sealed cavity. Submicron via sealing requires sophisticated lithography, but is amenable to LPCVD nitride, LTO and other sealing procedures. Standard g-line lithography results in vias which seal only with high sticking coefficient species, such as LTO. A novel etch channel structure, which results in lateral sealing and requires neither sophisticated lithography nor a particular sealing is demonstrated. The experiments in the paper are guided by theoretical analysis and computer simulations when applicable. The optimized process based on a nitride membrane with a polysilicon sacrificial layer results in devices which have a broad band 50 Ω real part impedance in the megahertz range. In addition, a transducer dynamic range in excess of 100 dB is achieved with an untuned bandwidth of 50%. The fabrication techniques and results herein reported indicate that surface micromachined capacitive ultrasonic transducers are an alternative to piezoelectric transducers in immersion applications