Low loss acoustic device

A method of fabricating an acoustic wave filter, the method comprising:

• forming a first layer that has a first acoustic impedance on top of a substrate such that the first layer covers a first region of the substrate;
• forming a second layer on top of the first layer, the second layer having a second acoustic impedance that is lower than the first acoustic impedance;
• forming a planar lateral bulk acoustic wave (LBAW) resonator on top of the second layer, and in parallel to a top surface of the substrate, the LBAW resonator comprising a piezoelectric layer, an electrode and a counter-electrode, the electrode comprising an input electrode and an output electrode, the input electrode and the output electrode being arranged on a first side of the piezoelectric layer and the counter electrode being arranged on a second side of the piezoelectric layer across the first side such that application of a radio frequency voltage between the input electrode and the counter-electrode creates acoustic resonance waves in the piezoelectric layer and causes generation of output signal voltage in the output electrode;
• and
• removing at least part of the first region of the substrate that is covered by the first layer to form a gap between the first layer and the substrate,
• wherein a combination of the first and the second layers reflects back to the acoustic wave resonator acoustic resonance waves that have frequencies greater than a cutoff frequency of a passband of the acoustic wave resonator.

A method of fabricating an acoustic wave filter, the method comprising:

• depositing an intermediary layer on top of a substrate;
• forming a first layer on top of the intermediary layer, the first layer having a first acoustic impedance;
• forming a second layer on top of the first layer, the second layer having a second acoustic impedance that is lower than the first acoustic impedance;
• forming a planar lateral bulk acoustic wave (LBAW) resonator on top of the second layer, and in parallel to a top layer of the substrate, the LBAW resonator comprising a piezoelectric layer, an electrode and a counter-electrode, the electrode comprising an input electrode and an output electrode, the input electrode and the output electrode being arranged on a first side of the piezoelectric layer and the counter electrode being arranged on a second side of the piezoelectric layer across the first side such that application of a radio frequency voltage between the input electrode and the counter-electrode creates acoustic resonance waves in the piezoelectric layer and causes generation of output signal voltage in the output electrode;
• and
• removing the intermediary layer to form a gap between the first layer and the substrate such that the gap blocks propagation of at least part of the acoustic resonance waves out of the acoustic wave resonator to the substrate,
• wherein a combination of the first layer and the second layer reflects back to the acoustic wave resonator acoustic resonance waves that have frequencies greater than a cutoff frequency of a passband of the acoustic wave resonator.