A Multiple Resonant Mode Film Bulk Acoustic Resonator Based on Silicon-on-Insulator Structures

Abstract

We report a multiple resonant mode film bulk acoustic resonator (FBAR) with different AlN film thicknesses of 605 nm, 640 nm and 680 nm. With the tilted c-axis orientation of the AlN piezoelectric film providing polarization vertical to the c-axis, acoustic wave resonant peaks have been observed for both the thickness shear modes (TSM 0th, TSM 1st, TSM 2nd) and the thickness extension modes (TEM 0th, TEM 1st). The corresponding parallel resonant frequencies are around 1.60 GHz, 2.41 GHz, 3.45 GHz, 2.75 GHz and 4.10 GHz, respectively. The latter two TEM modes also have good quality factors, and high equivalent electromechanical coupling coefficients Keff2 of 647, 3.13% and 113, 6.23%, respectively. By etching the 1.8 μm silicon sacrificial layer, the air gap FBAR devices have been fabricated in an easier and cleaner way resulting in a low insertion loss of −2.2 dB. The overall device structure of the top electrode/AlN film/bottom electrode on SiO2/silicon-on-insulator (SOI) substrate potentially enables CMOS compatibility. These multiple resonant mode FBAR devices will promote the integration of multi-band filters on a single chip. Improvements of the fabrication process, the influence of different AlN film thicknesses and theoretical analyses of the coexistence of multiple resonant modes are presented.