Optimal acoustic impedance materials for polished substrate coating to suppress passband ripple in BAW resonators and filters

A method of reducing phase and amplitude ripples in a BAW resonator frequency response comprising:

• providing a substrate;
• fabricating a Bragg mirror having alternate layers of a high acoustic impedance material and a low acoustic impedance material on a first side of the substrate;
• fabricating a BAW resonator on the Bragg mirror;
• coating a second side of the substrate opposite the first side with a lossy material having an acoustic impedance in the range of 0.01× to 1.0× the acoustic impedance of the layers of high acoustic impedance material;
• the substrate of the BAW resonator having a thickness of 400 μm or less;
• the second side of the substrate being a polished surface with a peak-to-peak roughness of approximately 1000 A.