Oliver Ahrens

Fabrication of gap-optimized CMUT

A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer. All previous approaches either require an additional metallic electrode or do not possess a structured sacrificial layer and, consequently, may make exact adjustment of the membrane dimensions difficult. The present set ups are especially suited for the fabrication of cMUT with gap heights ranging between 50 nm and 2 /spl mu/m between the electrodes. Large gaps are a prerequisite to enabling sufficient deflections of the membrane and, therewith, to generating high pressure gradients. On the other hand, small gap sizes are desirable for detecting weak ultrasonic sources. This paper focuses on the fabrication process of cMUT to realize electrode separation above 500 nm and, in addition, on the manufacturing of cMUT with gaps below 500 nm. The successful realization has been proven by some basic experimental investigations. Finally, the fundamental equations of a frequently chosen simulation model are documented, as a number of ambiguities exist in the common literature.

Capacitive micromachined ultrasonic transducers and their application

Todays ultrasound technology covers a wide range of measurements, diagnostic, and other applications, e.g. from non-destructive testing to medical ultrasonic imaging or from surveillance in process plants to distance measurement. Capacitive micromachined ultrasonic transducers (cMUT) receive increasing acceptance as an alternative to piezoelectric transducers for certain fields of applications due to a number of distinctive features. Some of these cMUT applications recently presented are reviewed. Besides, the present paper focuses on novel technological aspects regarding the improvement in performance of cMUT consisting of silicon membranes. Within this context a partial doping of silicon membranes is introduced to confine the effective electrode area to an indispensable extent and, therefore, to reduce the offset and parasitic capacitance. Though the objective of this paper emphasizes applications and technology of cMUT, some further attention is paid to a frequently applied electrical equivalent circuit.

A new class of capacitive micromachined ultrasonic transducers

A new set-up of capacitive micromachined ultrasonic transducers (cMUT) will be described that recently have been successfully realized. This new transducer design utilizes a structured silicon oxide sacrificial layer as well as a highly doped and, therefore, conductive membrane of polycrystalline silicon. So far, cMUT have been produced with resonance frequencies in the range from 1 to 4 MHz. These devices have been applied to a displacement measurement in air.