An improved ultrasonic transducer fabricated on a silicon base has a piezoelectric layer of polyvinylidene fluoride-trfluroethylene copolymer. The piezoelectric layer is sandwiched between two conductive electrodes, all of which are supported on a dielectric layer on top of the silicon base. At least one of the electrodes forms a Fresnel zone plate to focus the ultrasonic signals from the transducers. To improve the performance of the transducer, the silicon base behind the active area is removed, leaving the dielectric layer as a membrane to support the electrodes and the piezoelectric layer. The resulting void in the silicon base is filled with an acoustically matched backing, such as an epoxy, to enhance the wideband performance of the transducer. The transducer is especially suited for characterizing anatomical structures or features requiring very high resolution.

Ultrasonic transducer and method for using same

A 100-MHz ultrasonic linear transducer array made from a piezoelectric zinc oxide thin film on a sapphire substrate was developed and evaluated. Epitaxial, high-acoustic quality 10-/spl mu/m-thick ZnO film layers were produced by rf-magnetron sputter deposition onto a (111)-oriented gold film (with a chromium adhesion layer) that was vacuum-evaporated onto a (0001) sapphire surface. We found that, in well-oriented growth of gold, it is important to control the chromium sublayer thickness (less than 5 nm). An array was constructed by photolithography with an appropriate etch. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCl and HNO/sub 3/-based) that preferentially etched the c-plane of ZnO. Typical array elements were 90 /spl mu/m wide, 3.2 mm long, and 10 /spl mu/m thick, and the pitch of an array was typically 100 /spl mu/m. Our fine uniform array resulted in uniform ultrasonic response of individual elements throughout the array. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 /spl mu/m at the focal depth, agreeing well with theoretical predictions. Besides the use demonstrated with this present transducer, piezoelectric thin films should also lead to fabrication of various other kinds of ultrasonic transducers that can operate at high frequencies and should provide opportunities for miniaturizing transducers and making integrated ultrasonic devices. >

A 100-MHz ultrasonic transducer array using ZnO thin films

Acoustic Fresnel lenses have emerged in recent years as an alternative to the conventional spherical lenses for focusing sound waves in applications such as acoustic microscopy. Fresnel lenses offer the advantage of near-planar geometry and, therefore, ease of fabrication compared to spherical lenses. The Fresnel acoustic lenses reported so far, however, have the disadvantage of low efficiency; only about 40% of the input signal is directed towards the focus. The design and fabrication of lbinaryr acoustic Fresnel lenses that offer much higher efficiencies are described. These lenses, while being still nearly planar, have multiple phase levels to achieve phase shifts other than 0 and 180 degrees as used in conventional, two phase Fresnel lenses. Acoustic Fresnel lenses were fabricated at frequencies of about 1 MHz and 170 MHz. Measurements of the focusing efficiency and point spread function have been performed to characterize the operation of these lenses. Focusing efficiencies in excess of 80% have been achieved with these lenses. The measurements compare well to theoretical simulations

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High-efficiency Fresnel acoustic lenses

Scanning acoustic microscopy

An acoustic beam focused on a free liquid surface is used to eject discrete ink droplets of controlled diameter. The liquid surface, adjusted to be at the focal plane of a suitable focusing element, is excited with a burst of acoustic energy. Spherical lead zirconate titanate (PZT) shells, acoustic microscope lenses, spherical lenses etched in silicon, and Fresnel acoustic lenses are used successfully to eject droplets. Droplet diameter scales directly with the focal spot size, and inversely with the acoustic frequency. Droplet formation is experimentally demonstrated over the frequency range of 5 to 300 MHz, with corresponding droplet diameters from 300 to 5 mu m. This droplet ejection process is successfully utilized for a printing application by using ink as the liquid medium. Acoustic ink printing with a single lens and with an array of lenses is described. >

Acoustic ink printing

In the previous report, t he authors proposed a new planar-structure acoustic-microsc ope lens designed according to the theory of Fresnel zone plate, and demonstrated its good focusing capability. Since the l ens is composed of a number of concentric annular grooves of uniform depth on its radiation plane, it has an advantage over the conventional acoustic lens with a spherical concave in being fabricated easily using a photolithographi c technique. In our first model of the planar lens, which had an aperture angle of 47.3" and operated at 100 MHz, the grooves could be formed successfully using an ordinary chemical etching technique. However, the formation of finer grooves is inevitably limited by the undercutting effect so far as a wet-process is used. Therefore, some advanced photolithographi c technique s hould be employed for fabricating the lens with a wider aperture a ngle and a higher o peration frequency. Recently, the authors have introduced the ECR (electron cyclotron resonance) reactive ion etching technique for forming fine grooves. Using this dry-process, a planar lens designed for an operation frequency of 200 MHz and an aperture a ngle of 90' has been obtained. I n the present report, we describe the fabrication process and some results of imaging experiments by an acoustic microscope equipped with the p lanar lens.

http://ieeexplore.ieee.org/iel5/10286/32719/01535774.pdf

K. Yamada

Papers

Planar-Structure Focusing Lens for Operation at 200 MHz and Its Application to the Reflection-Mode Acoustic Microscope