Pierre Maréchal

Electromechanical Properties of Piezoelectric Integrated Structures on Porous Substrates

The fabrication of piezoelectric transducers for high resolution medical imaging applications requires a backing material to damp the piezoelectric resonance, resulting in a shorter time response, i.e. improved resolution, but lower sensitivity. Thus, the choice of such a substrate must be made according to its acoustical properties, namely the ratio of acoustic impedance of the backing material and the piezoelectric layer. Moreover, this backing material must have a relatively high attenuation, and provide good surface properties such as a low roughness and diffusion potential. Finally, the substrate material must have a good mechanical behavior at high temperature (around 900°C for the sintering of the piezoelectric thick film). In this context a review of available materials is first given, and the corresponding list is found to be quite limited. Several PZT/PGO piezoelectric thick films deposited by screen-printing have been fabricated on dense alumina [1] and have delivered good electromechanical performance, but the substrate attenuation was too low to be used as a backing for high frequency transducers. In this paper, new porous Al 2 O 3 substrates have been fabricated and used for the fabrication of a batch of piezoelectric films. The effects of a barrier layer and bottom electrode (conductive material type and thickness) are studied and related to the electromechanical performance of the piezoelectric thick film and then to the electro-acoustic properties of the ultrasonic transducer integrating these structures. Finally, these results are compared with previous studies [2] using porous PZT substrate. These comparisons are performed in terms of fabrication facility, electromechanical constants of the thick films (in thickness mode) and electro-acoustic responses of ultrasonic transducers with center frequencies over 20 MHz.

High frequency transducer based on integrated piezoelectric thick films for medical imaging

A screen-printed PZT thick film with a final thickness around 40 µm is deposited on a porous PZT substrate to tend toward an integrated structure for ultrasonic transducer applications. This process allows to decrease the number of fabrication steps of high frequency single-element transducers. The porous PZT material has high acoustical impedance and attenuation, satisfying conditions as a backing for medical imaging. The piezoelectric thick film delivers high electro- mechanical performance comparable to those of standard bulk ceramic (thickness coupling factor near 50 %). Based on these structures, high frequency transducers with a center frequency around 25 MHz are fabricated and characterized. As a result, a good sensitivity and axial resolution are obtained in comparison with similar transducer integrating a PT disk as active material. The two types of transducers are integrated in high frequency imaging system and comparative skin images are shown.

P3P-7 Modeling of Lens Focused Piezoelectric Transducer for Medical Imaging

In previous work on acoustical lens effect, a model based on the annular decomposition of an axisymmetric focused piezoelectric transducer was described and found to be in agreement with FEM and experimental results. The design of a focused transducer must take into account both radiation and transduction properties. These complex models such as FEM, FDTD and PSTD methods take into account radial modes, but their influence is not significant compared to the thickness mode when the radius to thickness ratio of the piezoelectric source is high enough. This assumption was verified since the mean error was evaluated lower than 2% for the chosen lens materials and geometries. Therefore, an efficient transfer function to model the lens was proposed. This annular decomposition allows to determine the input acoustical impedance of the lens and it is shown that the lens can be considered as a semi-infinite propagation medium. For an experimental comparison, a transducer structure based on a 1-3 piezocomposite is chosen for its high thickness coupling factor kt = 64% and low acoustical impedance Z = 14 MRa. This piezoelectric source is damped by an epoxy backing and matched to the propagation medium with two quarter wavelength matching layers. Based on this structure, different types of focusing techniques are investigated: geometrical, concave lens with high velocity material and convex lens with low velocity material. Experimental results obtained on the manufactured transducers are finally compared to simulated ones and the performance of the different focusing strategies is discussed

Pseudospectral time-domain method to calculate radiation pattern of lens-focused transducers

The purpose of this study is to present a new pseudo- spectral time-domain (PSTD) algorithm to model wave propagation in axisymmetric multi-layer propagation media. The approach consists in meshing the medium and in solving local differential equations at each node. This paper describes first the model. Then, the validation is performed for a classical geometrically focused wave front configuration. The results are shown to be in good agreement with an analytical diffraction model. In the next section, a 10.5 MHz center frequency lens- focused transducer is modeled. The electro-acoustical response is calculated by coupling a KLM-based model of the transducer to the PSTD propagation code. Results are very similar to those obtained with other methods involving a KLM model extended to axisymmetric geometry coupled to an analytical propagation code using an axisymmetric formulation of the Rayleigh integral (1). Finally, the variation of the focal distance as a function of the Poisson ratio in the lenss material is shown and discussed.

Time-frequency analysis for surface roughness characterization using backscatter ultrasound

The determination of a surface roughness with translation velocity was investigated with an ultrasonic backscattering technique based on the Doppler effect. The difficulty of the Fourier analysis to detect with accuracy the changes of roughness led us to implement a time-frequency analysis. We demonstrate that the spectrum of the demodulated signal is related to the spatial spectrum of the surface roughness, and the velocity of the surface. An accurate detection of roughness can be performed thanks to the spectrogram representation.

P1J-5 Performance Comparison of Screen-Printed Piezoelectric Structures on Porous PZT and Alumina Substrates

In previous work, screen-printing technology was used to elaborate an integrated piezoelectric structure on a porous substrate [1]. The substrate was chosen to withstand the film sintering temperature which was lowered at 800degC thanks to the addition of PGO to a PZT composition. The acoustical impedance of this substrate is very close to that of the deposited piezoelectric thick film, making it an adequate backing to deliver a short pulse-echo response, but the resonance frequency is lowered. Intermediate functional layers were necessary between the substrate and the piezoelectric layer. The transducer based on a porous PZT structure including a dense barrier layer and a gold rear electrode is used as a reference device. Here, another set of materials is considered as possible candidate to fulfill both functions of substrate and backing: porous alumina associated with a dense alumina barrier layer and platinum rear electrode. Since the thicknesses of these layers is not negligible compared to the wavelength of the first thickness mode of the piezoelectric layer, they have a relatively strong influence on the electro- acoustic response. The input acoustic impedance of this stack must be controlled very precisely in order to make it an adequate backing The damping of the backing is optimized to deliver a relatively short pulse-echo response, without excessive lowering of the resonance frequency. The piezoelectric thick film on alumina substrate has similar properties to the one on porous PZT. A mean thickness around 30 micrometers and a thickness coupling factor around 40% were obtained in both cases. As a result of the damping, the structure resonates at 40 MHz, the anti-resonance of the piezoelectric thick film alone being around 65 MHz. Pulse-echo measurements allows comparison of the performance of the two devices in terms of sensitivity, axial resolution and bandwidth. The results are compared and discussed, showing that the sensitivity/bandwidth trade-offs of the two transducers are significantly different.

5C-4 Optimization of an Integrated Structure Including a Screen-Printed Piezoelectric Thick Film for High Frequency Transducers

Based on previous work, screen-printing technology was used to elaborate an improved integrated piezoelectric structure relatively to a reference sample [Marechal, et. al., 2000], An active PZT layer doped with PGO to lower sintering temperature at 800degC was printed on a porous unpoled PZT substrate which was chosen to withstand such a temperature. Moreover, the acoustical impedance of this substrate is close to that of the deposited piezoelectric thick film, making it an adequate backing for a wideband device. Thus, a short pulse-echo response is obtained, but the resonance frequency is lowered. Additional layers such as a back electrode and a dense barrier layer were interposed between the substrate and the piezoelectric film. These passive and active layers have a significant influence on the performance of the high frequency transducer. Therefore, their thickness and acoustic properties must be controlled precisely for an optimization of the structure. The center frequency and active source radius of the transducer were increased in comparison to the reference sample, resulting in an improved radiation pattern. In vivo human skin images were performed with an imaging system for two of the prototype broadband high frequency transducer