Method for making a with bulk acoustic wave filter

A method for making a bandpass filter comprising a first bulk acoustic wave resonator and a second bulk acoustic wave resonator, the resonant frequency of the second resonator being offset from the resonant frequency of the first resonator by a predetermined offset, the method comprising:

• providing a piezoelectric on insulator substrate, forming a donor substrate, comprising a piezoelectric layer at the top thereof,
• forming a lower electrode of the first resonator on a first predefined zone of the piezoelectric layer, and a lower electrode of the second resonator on a second predefined zone of the piezoelectric layer,
• assembling by bonding the donor substrate to a receiver substrate, so that the lower electrodes of the first and second resonators are disposed between the receiver substrate and the donor substrate,
• removing, in assembling the receiver substrate and the donor substrate, the donor substrate with a barrier on the piezoelectric layer,
• forming on the piezoelectric layer, an upper electrode of the first resonator at the first zone, and an upper electrode of the second resonator at the second zone,
• forming the lower electrodes being preceded by a step of forming a mass overload pattern at the second zone, forming a mass overload pattern of the lower electrode,
• and/or forming the upper electrodes being preceded by a step of forming a mass overload pattern in the second zone, forming a mass overload pattern of the upper electrode,
• a total thickness of the mass overload pattern or patterns being chosen to offset resonant frequency of the second resonator by the predetermined offset,
• each mass overload pattern being formed by lift-off of a sacrificial layer formed beforehand on the piezoelectric layer.