Solidly mounted BAW filters for 8 GHz based on AlN thin films

This paper presents the fabrication and characterization of solidly mounted resonators (SMR) passband filters on silicon substrates based on piezoelectric AlN thin films working near 8GHz. Ladder filters with 3 to 14 resonators were investigated experimentally and theoretically. Simulation calculations of the transfer function yield a 3dB bandwidth of 3.5 to 4.5% and insertion losses of 1 to 3dB, neglecting losses in electrodes. The fabricated versions utilize Pt or Mo bottom and Al top electrodes. The frequency tuning of the shunt resonators was performed by an additional loading, i.e. by increasing the top electrode thickness. Molybdenum based SMR are theoretically better suited than platinum based SMR for frequencies above 5 GHz where the individual films constituting the resonator become very thin. Molybdenum electrodes offer a lower electrical resistivity than platinum and superior acoustic properties. The piezoelectric coefficient d₃₃ of AlN thin films deposited on Mo yields 3.2 pm/V compared with 4.0 pm/V for Pt. The highest measured coupling coefficient is thus achieved with platinum electrodes. Pt based resonators show coupling coefficients k_t² of 5.6% and quality factors of 414, the corresponding Mo SMR exhibit coupling coefficients of 3.9% and quality factors of 523. Fabricated SMR ladder filters with the pass-band at 8GHz exhibit -3.2dB, -5.5dB and -7.5dB insertion loss for 1, 2, respectively 3π-sections, and an out-of-band rejection of -13dB, -32dB and -33dB. The 3dB band width is 244MHz (3.1%), 224MHz (2.8%), respectively 194MHz (2.4%).