Fabrication of gap-optimized CMUT

Abstract

A recently introduced set up of capacitive micromachined ultrasonic transducers (cMUT) combines a conductive membrane above a structured sacrificial layer [1]. All previous approaches either require an additional metallic electrode [2], [3] or do not possess a structured sacrificial layer [4] 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 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.

MEMS/NEMS Techniques and Applications

“…One possibility is to use LPCVD silicon dioxide as the sacrificial layer, and LPCVD deposited and doped poly-silicon as the membrane material ( [14,71,39]). Liquid or vapor phase hydrofluoric (HF) acid (which has excellent selectivity between silicon dioxide and poly-silicon) can be used to remove the sacrificial silicon dioxide layer.…”

“…Because the poly-silicon membrane is conductive, an insulation layer is required between the membrane and the silicon back electrode. The high stress levels of the LPCVD polysilicon must either be reduced (as in [71]) or taken into account in the design. Either way, process control is not good because stress levels will vary greatly from wafer to wafer and from process to process.…”

Capacitive micromachined ultrasonic transducer technology for medical ultrasound imaging

“…The mechanical membrane impedance predicts the linear membrane dynamics, and is derived by solving the equation of motion. 8,9 The analysis of CMUT transducers is greatly simplified with the use of electrical equivalent circuit modelling introduced by Mason for electro-acoustic devices, 10 and improved to accurately include the effect of acoustical loading of the medium. 11 For operation into fluids, the transducer's membrane impedance is below that of fluids over a large frequency range, resulting in very broad bandwidth operation.…”

Acoustic Doppler velocity measurement system using capacitive micromachined ultrasound transducer array technology

“…[12][13][14][15][16][17] However, the majority of cMUT work has been directed toward biomedical ultrasound or submerged ultrasound. This paper presents the first demonstration of cMUT based in-air Doppler ultrasound.…”