Jean-Francois Gelly

Dual frequency hybrid ultrasonic transducers - design and simulations

This paper studies a design concept of a dual frequency ultrasonic transducer, in which we use different technologies for each band. A hybrid structure consisting of a piezoelectric stack used for the lower frequency (LF) band and a Capacitive Micromachined Ultrasound Transducer (CMUT) array placed on the top surface used for the higher frequency (HF) band is demonstrated by Finite Element Method (FEM) simulations. Simulated results show that LF structure introduces ripples into the HF. Simulations also show that using an RTV lens on top of the device is sufficient to damp the vibrations from the CMUTs interfering to the LF band. An array to test the LF band of the design was fabricated and measured electrical impedances show a good correspondence to simulations. The work shows a potential of combining piezoelectric and MUTs technologies in ultrasound transducer design for use in medical applications.

Au-Sn Solid-Liquid Interdiffusion (SLID) bonding for piezoelectric ultrasonic transducers

This paper presents a bonding technique based on gold-tin (Au-Sn) intermetallics to bond an active layer to a backing material in a piezoelectric ultrasonic transducer stack. The bonding process was performed at 310°C. The temperature ramping rate was found to play an important role to the bond quality. Three different bonding temperature profiles P1, P2 and P3 corresponding to a ramping rate 120°C/min., 45°C/min. and 20°C/min., respectively, were investigated. The bonded samples were characterized by electrical impedance measurements in air and cross-sectional microscopy inspection. The most reproducible bondlines were found to be with the heating rate 120°C/min. The resulted bondlines show a layered structure of Au/Au-Sn intermetallic/Au with a corresponding average thickness of 5 μm/17 μm/5 μm, respectively. Based upon energy dispersive spectroscopy (EDS) analysis, the obtained Au-Sn intermetallic compounds are found in good agreement with those from literature. A 64-element one dimensional array formed by Au-Sn SLID method was fabricated and electrical impedance across elements was measured. The results show a good uniformity across elements in the array, and good accordance to Finite Element simulations from COMSOL. This shows the feasibility of using SLID bonding technology to assembly stacks of piezoelectric ultrasonic transducers.