Fabrication and characterization of surface micromachined capacitive ultrasonic immersion transducers

Abstract

In this paper, several innovative steps used in fabricating surface micromachined capacitive ultrasonic immersion transducers are reported. The investigation is focused on major steps in the device fabrication processes necessary to optimize transducer performance. Such steps include membrane formation, vacuum sealing, and electrode metallization. Three transducer membrane structures are evaluated: a nitride membrane with an oxide sacrificial layer; a polysilicon membrane with an oxide sacrificial layer; and a nitride membrane with a polysilicon sacrificial layer. Three vacuum sealing mechanisms are compared, each of which requires a different degree of lithographic sophistication, uses a particular sealing mechanism, and results in a sealed cavity. Submicrometer via sealing requires sophisticated lithography but is amenable to LPCVD nitride, LTO, and other sealing procedures. Standard g-line lithography results in vies 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 material, is demonstrated. Finally, the impact of electrode metallization on the impedance, bandwidth, and efficiency of the transducers is discussed. The experiments in the paper are guided by theoretical analysis and computer simulations when applicable. The new process results in optimized devices which have a broad-band 50-/spl Omega/ real part impedance in the megahertz range. A transducer dynamic range in excess of 100 dB is achieved around 4.5 MHz. An untuned transducer exhibits more than 100% bandwidth when connected to electronics with 50-/spl Omega/ input impedance. In addition, beam pattern measurement shows the immersion devices behave like uniform piston transducers and are readily suitable for array applications. The fabrication techniques and results herein reported indicate that surface micromachined ultrasonic immersion transducers are an attractive alternative to piezoelectric transducers in immersion applications.