Cavity-less film bulk acoustic resonator (FBAR) devices

A film bulk acoustic resonator (FBAR) device, comprising:

• a substrate;
• an acoustic Bragg reflector over the substrate, the acoustic Bragg reflector comprising a first Bragg layer and a second Bragg layer juxtaposed with the first Bragg layer, the first Bragg layer comprising a first material having an acoustic impedance less than five Mrayl, the second Bragg layer comprising a second material having an acoustic impedance greater than 50 Mrayl;
• a piezoelectric element over the acoustic Bragg reflector;
• and a remote-side electrode over the piezoelectric element, wherein:
• the FBAR device has a band-pass characteristic having a center frequency;
• at least one of the Bragg layers has a nominal thickness equal to one quarter of the wavelength in the material of the respective Bragg layer of an acoustic signal equal in frequency to the center frequency;
• and the metal Bragg layer is thinner than the nominal thickness.

A film bulk acoustic resonator (FBAR) device, comprising:

• a substrate;
• an acoustic Bragg reflector over the substrate, the acoustic Bragg reflector comprising a first Bragg layer and a second Bragg layer juxtaposed with the first Bragg layer, the first Bragg layer comprising a first material having a first acoustic impedance, the second Bragg layer comprising a second material having a second acoustic impedance, the second acoustic impedance and the first acoustic impedance having a ratio greater than ten;
• a piezoelectric element over the second Bragg layer;
• and a remote-side electrode over the piezoelectric element, wherein:
• the FBAR device has a band-pass characteristic having a center frequency;
• at least one of the Bragg layers has a nominal thickness equal to one quarter of the wavelength in the material of the respective Bragg layer of an acoustic signal equal in frequency to the center frequency;
• and the metal Bragg layer is thinner than the nominal thickness.