Research on micro-sized acoustic bandgap structures.

Abstract

Phononic crystals (or acoustic crystals) are the acoustic wave analogue of photonic crystals. Here a periodic array of scattering inclusions located in a homogeneous host material forbids certain ranges of acoustic frequencies from existence within the crystal, t hus creating what are known as acoustic (or phononic) bandgaps. The vast majority of phononic crystal devices reported prior to this LDRD were constructed by hand assembling scattering inclusions in a lossy viscoelastic medium, predominantly air, water or epoxy, resulting in large structures limited to frequencies below 1 MHz. Under this LDRD, phononic crystals and devices were scaled to very (VHF: 30-300 MHz) and ultra (UHF: 300-3000 MHz) high frequencies utilizing finite difference time domain (FDTD) mo deling, microfabrication and micromachining technologies. This LDRD developed key breakthroughs in the areas of micro-phononic crystals including physical origins of phononic crystals, advanced FDTD modeling and design techniques, material co nsiderations, microfabrication processes, characterization methods and device structures. Micro-phononic crystal devices realized in low-loss solid materials were emphasized in this work due to their potential applications in radio frequency co mmunications and acoustic imaging for medical ultrasound and nondestructive testing. The results of the advanced modeling, fabrication and integrated transducer designs were t hat this LDRD produced the 1 st measured phononic crystals and phononic crystal devices (waveguides) operating in the VHF (67 MHz) and UHF (937 MHz) frequency bands and est ablished Sandia as a world leader in the area of micro-phononic crystals. 4 AcknowledgementsThe authors would like to acknowledge the memory of our late colleague and friend Jim Fleming. It was Jim's vision to pursue a research program in the area of acoustic bandgap crystals and his vision lives on today through the team he assembled. The authors would like that thank the MDL staff at Sandia National Laboratories including Jim Stevens for his work developing the AlN transducer film, Craig Nakakura for his work on AlN etching, Keith Yoneshige for his work supporting the tungsten chemical mechanical polish (CMP) and Todd Bauer and Rob Jarecki for their work on process and etch development.