Two-stage lateral bulk acoustic wave filter

An acoustic wave filter device comprising:

• a first electrode comprising a first input electrode and a first output electrode;
• a second electrode comprising a second input electrode and a second output electrode, the second input electrode being electrically connected to the first output electrode;
• a piezoelectric layer beneath the first electrode and the second electrode;
• a counter electrode beneath the piezoelectric layer;
• an intermediary layer beneath the counter electrode, the intermediary layer comprising a first region and a second region, the first region having a first layer thickness and the second region having a second layer thickness different from the first layer thickness;
• and
• an acoustic reflector beneath the intermediary layer, the acoustic reflector comprising one or more layers,
• wherein the first electrode is located over the first region of the intermediary layer, and the second electrode is located over the second region of the intermediary layer, and
• wherein the filter device is configured such that application of a radio frequency voltage between the first input electrode and the counter electrode creates one or more acoustic thickness-extensional resonance modes in the piezoelectric layer between the first input and output electrodes and between the second input and output electrodes.