Demonstration of large complete phononic band gaps and waveguiding in high-frequency silicon phononic crystal slabs

Phononic crystals (PCs) are structures with periodic variations in their mechanical properties. PCs are especially of interest due to possibility of possessing frequency ranges in which propagation of elastic waves is completely prohibited; i.e., complete phononic band gaps (CPBGs). In this paper we first propose a PC slab structure created by a embedding a two dimensional array of void (air) inclusions in a solid slab with a finite thickness in the third dimension; using a plane wave expansion and a finite element code we show that wide CPBGs can be achieved by proper choice of geometrical parameters for the structure with void cylinders embedded in a thin silicon slab. Secondly, we report a CMOS-compatible fabrication procedure developed for fabrication and characterization of the proposed PC slabs operating at high frequencies (hundreds of megahertz to a few gigahertz). Using this fabrication procedure we fabricate and experimentally characterize the designed PC structures and show that strong attenuation (more than 30 dB) is observed in the transmission spectrum of elastic waves through eight layers of PC structure. The very good agreement between the frequency range of attenuation (119 MHz to 150 MHz) and the calculated CPBG provides an evidence of the validity and accuracy of our predictions of the existence of large CPBGs in the proposed structures. Using a PC structure with wide CPBG, a waveguide is fabricated by introducing a line defect in the PC structure. Characterization of the waveguide shows that high frequency (around 130 MHz) signals can be guided efficiently within the CPBG of the PC structure. These results show that the great capabilities of PCs can be utilized for realizing integrated micro/nano-mechanical devices with new and improved functionalities to be used in wireless communication and sensing applications.