Effect of mechanical loading on the tuning of acoustic resonances in Ba (x) Sr1-x TiO3 thin films

The effect of mechanical loading on the tuning performance of a tunable Thin Film Bulk Acoustic Wave Resonator (TFBAR) based on a Ba0.3Sr0.7TiO3 (BST) thin film has been investigated experimentally and theoretically. A membrane-type TFBAR was fabricated by means of micromachining. The mechanical load on the device was increased stepwise by evaporating SiO2 on the backside of the membrane. The device was electrically characterized after each evaporation step and the results were compared to those obtained from modeling. The device with the smallest mechanical load exhibited a tuning of -aEuro parts per thousand 2.4% and -aEuro parts per thousand 0.6% for the resonance and antiresonance frequencies at a dc electric field of 615 kV/cm, respectively. With increasing mechanical load a decrease in the tuning performance was observed. This decrease was rather weak if the thickness of the mechanical load was smaller or comparable to the thickness of the active BST film. If the thickness of the mechanical load was larger than the thickness of the active BST layer, a significant reduction in the tuning performance was observed. The weaker tuning of the antiresonance frequency was due to a reduced tuning of the sound velocity of the BST layer with increasing dc bias. The resonance frequency showed a reduced tuning due to a decrease in the effective electromechanical coupling factor of the device with increasing mechanical load. With the help of the modeling we could de-embed the intrinsic tuning performance of a single, non-loaded BST thin film. We show that the tuning performance of the device with the smallest mechanical load we fabricated is close to the intrinsic tuning characteristics of the BST layer.