Hypersonic band gap in an AlN-TiN bilayer phononic crystal slab

Abstract

We show both theoretically and experimentally the existence of a complete band gap in a phononic crystal (PnC) constituted by periodical air holes drilled in a solid bi-layer slab. The composite slab is formed with an aluminum nitride (AlN) layer deposited on a titanium nitride (TiN) thin metallic film. Indeed, AlN slabs are of great interest in many technological applications, in particular, owing to the capabilities of AlN as a complementary metal-oxide-semiconductor compatible material for integration in piezoelectric radio frequency filters (thin-film bulk acoustic resonator technology). The TiN layer was chosen as a buffer to enable small lattice mismatch with AlN, thus resulting in highly c-axis oriented and low stresses at the interface. We calculate the band structure of the crystal by using a finite element method and discuss the frequency and width of the band gap as a function of the thicknesses of both layers and the shape of the holes, i.e., from cylindrical to conical. The introduction of the TiN slab contributes to slowly widen the band gap. The band gap narrows when the holes become conical and closes when the radius of one face in the cone exceeds by 15% the radius of the other face. Experimentally, the measurement of the elastic wave transmission through eight rows of the bi-layer PnC shows a band gap around 950 MHz with an attenuation higher than 20 dB. The experimental results are well reproduced by the simulations provided the conical shape of the air inclusions is taken into account.