Marc Lukacs

Single element high frequency (<50 MHz) PZT sol gel composite ultrasound transducers

A sol gel composite process has been used to produce lead zirconate titanate coatings in the thickness range of 3 to 100 /spl mu/m on aluminum substrates. The complex permittivity (/spl epsi//sub 33//sup S/), elastic stiffness (c/sub 33//sup D/), and the piezoelectric constant (h/sub 33/) of the coating and the complex elastic stiffness (c/sub 33//sup D/) of the substrate have been determined using impedance measurements and a commercially available software program [Piezoelectric Resonance Analysis Program PRAP 2.0, TASI Technical Software, Kingston, Ontario, Canada]. The complex components of the material parameters account for the losses within the film and the substrate. Sol gel composite films on aluminum have a dielectric constant of 220 with an imaginary component of 1% and an electromechanical coupling coefficient of up to 0.24 with an imaginary component of 3%. These films are applied to the fabrication of a high frequency transducers suitable for ultrasound biomicroscopy (UBM). By combining the sol gel composite material with existing transducer fabrication techniques, single-element focusing transducers have been produced that operate in the frequency range of 70 to 160 MHz. Devices have -6-dB bandwidths up to 52% and minimum insertion losses ranging from -47 to -58 dB. Real-time images of phantom materials and ex vivo biological samples are shown.

Thickness mode material constants of a supported piezoelectric film

New thick film ceramic processing techniques and microelectromechanical systems require material characterization of a piezoelectric film supported by a substrate. An analytical solution of the one-dimensional wave equation for multiple layered systems driven in the thickness mode is presented. The impedance across the piezoelectric layer is derived and expressed in terms of the material properties of the two materials. This includes the open-circuit elastic stiffness c33D, the clamped permittivity e33S and the h33 piezoelectric constant of the piezoelectric layer and the elastic stiffness csD of the substrate. The properties are expressed as complex variables in order to account for the losses within the materials. The material parameters of the solution are extracted from experimental results using a modified Levenberg–Marquardt technique. The capabilities of this nondestructive technique are demonstrated using experimental and simulated impedance spectra of lead zirconium titanate sol gel composite coa...

Laser micromachined high frequency ultrasonic arrays

Piezoelectric ceramics have been patterned by means of laser micromachining to create ultrasonic arrays resonating in the frequency range of 35-45 MHz. The Lumonics PM-844 excimer laser with a KrF gas mixture (248 nm) has been used to micromachine trenches with a width to depth aspect ratio of up to 1:5. By using a projection etch approach, the large aperture of the excimer laser is used to simultaneously ablate the features of a complex geometrical pattern. Pre-poled ceramic structures >20 /spl mu/m wide and >35 /spl mu/m thick have been cut and validated with SEM images. The dielectric, piezoelectric and acoustic properties have been evaluated using an impedance analyser and pulse-echo techniques.

Novel PZT films for ultrasound biomicroscopy

Lead zirconate titanate (PZT) ceramic films of thickness 5-200 /spl mu/m have been produced on various substrates using a modified sol gel process. A subset of these films (5-27 /spl mu/m) open the possibility of fabricating ceramic ultrasound transducers in the frequency range of 80-200 MHz. The technique involves mixing PZT powder conventional PZT sol gel to form a paint which is then coated onto a flat substrate. Initial characterization has included the dielectric and impedance response of the PZT film supported by a metal substrate. The films resonate within the desired frequency range, but the substrate is not conducive for ultrasonic transducers. A technique has been developed to mount the PZT onto conductive silver epoxy using 80 /spl mu/m aluminum foil as a sacrificial layer. The pulse echo response of this structure demonstrates the broadband response required for ultrasound imaging transducers.

Single-element and linear-array transducer design for ultrasound biomicroscopy

Ceramic ultrasonic transducers with a frequency response <50MHz using lead zirconate titanate (PZT) layers in the 5-4Otm range have been difficult to achieve by bulk or thin film techniques. Advances in the production of small sized PZT ceramic powder allow the development ofthinner ceramic layers for this application. Sol gel composite thin film technology also provides a new technique for producing ceramic coatings of a thickness that successfully bridges the gap between traditional thin film and bulk techniques. So! gel composite PZT layers of 5-7Om have been coated on substrates (aluminum, platinized silicon, stainless steel, .. .) that can withstand the thermal processing of the ceramic. The thickness mode response of a thin piezoelectric layer supported by a thick substrate has been modeled from first principles using complex material constants. The LevenbergMarquart non linear regression technique has been used to extract the thickness mode elastic stiffness, dielectric constant and piezoelectric constant of the PZT, and the elastic stiffness of the substrate from the layered structure. This non-destructive technique allows for a reliable assessment of the quality of a coating prior to the fabrication of a transducer. The elastic stiffness of the substrate is not lossy enough for the required broadband response of an imaging transducer. However, aluminum can be preferentially etched, releasing the ceramic coating. Therefore it is possible to transfer the PZT film to a more suitable backing material. A processing sequence for single element PZT transducers in the frequency range of 50-200MHz has been developed. Characterization of transducers has been performed using pulse-echo techniques and by creating real time B-scan images of agar phantoms and biological tissue. Methods for patterning the PZT composite coatings are being developed with the intent to fabricate a linear array in the 40-60 MHz frequency range. Due to the very fine patterning and high concentration of cuts required for a high aspect ratio linear array, the limits of conventional etching techniques are surpassed. Laser micromachining using a frequency doubled Nd:YAG and a KrF excimer laser have the ability to pattern the array structure. In both cases, laser cuts <1Om wide have been achieved.

Single element and linear array PZT ultrasound biomicroscopy transducers

PZT coatings have been produced in the thickness range of 5-200 microns using a composite sol gel process (M. Lukacs et al., 1996). The application of this process in the fabrication of high frequency transducers suitable for ultrasound biomicroscopy (UBM) has been investigated. Curved single element transducers have been produced in a range of 70-165 MHz with -6dB bandwidths as high as 52% and minimum insertion losses ranging from -47 to -58 dB. Laser micromachining techniques for patterning linear array structures to operate at similar frequencies have been developed. Trenches <10 /spl mu/m wide with a 50% taper through 20 /spl mu/m of ceramic have been achieved with a pulsed frequency doubled Nd:YAG laser. Trenches 5 /spl mu/m wide with straight walls have been achieved using a pulsed KrF excimer laser.

Emerging technologies for ferroelectric films and coatings

Abstract Chemical solution deposition (CSD) allows the integration of ferroelectric and piezoelectric coatings with both silicon technology and in large scale macroscopic devices. Applications of coatings as actuators, in high frequency ultrasonic transducers and for piezoelectric transformers are discussed.

High frequency ultrasonics using PZT sol gel composites

Abstract Piezoelectric arrays suitable for high frequency ultrasound have been fabricated using PZT sol gel composite coatings in the thickness range of 5–100μm on aluminum and platinized alumina substrates. The coatings have been quantitatively characterized using impedance measurements to 100 MHz. The analysis provides a direct measure of the piezoelectric coupling coefficient of kt < 0.34. 16 element linear array structures suitable for ultrasound biomicroscopy at 30–40 MHz with elements having dimensions ∼40μm thick, 20–25μm wide and 15μm spacing have been patterned using excimer based laser micromachining. The dielectric and pulse echo responses have been measured.

Process Optimization for Piezoelectric Films and Coatings

The dielectric and piezoelectric response of sol gel based films and composite coatings is affected by the choice of process, gel composition, thermal cycle, processing atmosphere and poling conditions. Important general factors for thin films are the crystallization kinetics and the effects of residual carbon, while a new dielectric technique allows poling methods to be evaluated.