Effects of Release Hole Size on Microscale Phononic Crystals

We have experimentally investigated the effects of release hole size on acoustic transmission through micromachined phononic band gap crystals. The results confirm previous theoretical studies and determine the range of release hole sizes for which the band gap is minimally compromised. The experiments were performed on micro fabricated 2D phononic crystal plates comprised of high acoustic impedance tungsten rods arranged in a square lattice inside a low acoustic impedance silicon dioxide host medium. The lattice constant and diameter of the W rods in this experiment were 45 μm and 28.8 μm corresponding to a phononic band-gap centered at 67 MHz. Phononic crystals were characterized with 15, 12.5, 10, and 7.5 μm diameter release holes, required to undercut the crystal and suspend it from the substrate, placed in the center of the W rods. As the air hole diameter decreases, the band gap frequency width increases. However, as the diameter of the release holes approach 5 μm, the fabrication yield significantly decreases. From these experiments an optimum release hole diameter of 7.5 μm was found that maximizes phononic band gap performance and manufacturability.