Novel multi-layer polymer-metal structures for use in ultrasonic transducer impedance matching and backing absorber applications

Abstract

This paper presents a novel design principle for designing multilayer polymer-metal structures which are well suited for front surface impedance conversion (matching) and for back surface acoustic absorption. It is shown that a polymer layer with an outer metal layer, when loaded by a low impedance propagation medium, acts as an efficient impedance converter. The resulting impedance seen at the inner polymer surface is increased and the structure provides the same performance as a traditional quarter-wavelength matching layer. Experimental evidence is also shown for a double-matching scheme for a lead zirconate titanate (PZT) transducer using an inner polymer-metal multilayer and an outer polymer quarterwavelength layer, resulting in a 55% bandwidth at 2.6 MHz with air backing. Also, it is theoretically shown that multiple layers of a lossy polymer adhesive-metal structure produce low propagation velocity and high absorption. Experimental proof of this ultrasonic multilayer backing absorber is provided. Design theories based on both a simplified mass and spring model and a rigorous one-dimensional wave model have been developed and show fair agreement.

Vacuum deposition of mass-spring matching layers for high-frequency ultrasound transducers

“…The first matching layer system investigated was a massspring matching layer combination applied to the planar singleelement lithium niobate transducer. This matching layer combination approximated a single quarter-wavelength matching layer with an acoustic impedance calculated by [2] (Z λ/4 = 4.25 MRayl). By substituting this impedance into Equations (1) and (2) and using the material properties for our chosen spring and mass materials (parylene and copper), we determined that the required parylene (spring) thickness is 3.8 μm and the required copper (mass) thickness was 1.4 μm.…”

“…Recently, a new acoustic matching layer architecture has been developed by Toda and Thompson [2] that uses thin (<<1/4 λ) alternating soft and dense layers of materials, analogous to a classical "mass-spring" resonator system. The spring layers consist of a low-stiffness, low density material such as a polymer and its stiffness is determined by the combination of the material Young's modulus and its thickness.…”

“…In this work, we present a similar matching layer scheme to that of [2] , but with a design specifically created for highfrequency transducers. Since high frequency transducers require mass-spring layers that are very thin in comparison to low frequency transducers, they can be deposited using using vacuum deposition equipment.…”

“…In this study we have fabricated several high-frequency singleelement lithium niobate transducers with vacuum deposited matching layer networks, and the results are in excellent agreement with the theoretical predictions II. METHODS Toda and Thompson [2], derived formulas for the resonance frequency as well as the mechanical input impedance at the driving spring end in a mass spring layer combination. We have combined these expressions to derive two simple formulas for calculating the spring and mass thicknesses ts and required for approximating any conventional quarter wavelength matching layer (…”

Research Papers. An Approach to Design Broadband Air Backed Piezoelectric Sensor

“…This impedance difference generates ultrasonic pulses of pulse width much larger than electric signals. For reducing the multiple internal reflections, front impedance matching layers between piezoelectric transducer and load medium are required (Beerman, 1981;Persson et al, 1985;Toda et al, 2010). Several theoretical models for calculating the optimum acoustic impedance for the front matching layers are reported in the literature (Shung et al, 2007;Kazys et al, 2006).…”