Bulk acoustic wave devices having electrodes with acoustic impedance gradients to improve coupling efficiency and related fabrication methods

A bulk acoustic wave (BAW) device, comprising:

• a piezoelectric layer;
• a first electrode layer on a first face of the piezoelectric layer;
• and
• a second electrode layer on a second face of the piezoelectric layer,
• wherein:
• the first electrode layer comprises:
• a first side comprising a first material adjacent to the first face of the piezoelectric layer;
• and
• a second side comprising a second material opposite to the first face of the piezoelectric layer;
• the first material comprises a first acoustic impedance;
• the second material comprises a second acoustic impedance different than the first acoustic impedance;
• the first electrode layer comprises a first layer and a second layer;
• the first layer is between the second layer and the piezoelectric layer;
• the first layer comprises:
• the first material on the first side adjacent to the first face of the piezoelectric layer;
• and
• the second material disposed on the first material;
• the second layer comprises:
• the first material disposed on the second material of the first layer;
• and
• the second material on the second side;
• an acoustic impedance of the first layer comprises at least one discontinuous change;
• and
• an acoustic impedance of the second layer changes continuously.

A bulk acoustic wave (BAW) device, comprising:

• a piezoelectric layer;
• a first electrode layer on a first face of the piezoelectric layer;
• and
• a second electrode layer on a second face of the piezoelectric layer,
• wherein:
• the first electrode layer comprises:
• a first side comprising a first material adjacent to the first face of the piezoelectric layer;
• and
• a second side comprising a second material opposite to the first face of the piezoelectric layer;
• the first material comprises a first acoustic impedance;
• the second material comprises a second acoustic impedance different than the first acoustic impedance;
• the first electrode layer comprises a first layer and a second layer;
• the first layer is between the second layer and the piezoelectric layer;
• the first layer comprises:
• the first material on the first side adjacent to the first face of the piezoelectric layer;
• and
• the second material disposed on the first material;
• the second layer comprises:
• the first material disposed on the second material of the first layer;
• and
• the second material on the second side;
• an acoustic impedance of the first layer changes continuously;
• and
• an acoustic impedance of the second layer comprises at least one discontinuous change.

A method of making a bulk acoustic wave (BAW) device, the method comprising:

• forming a piezoelectric layer;
• forming a second electrode layer on a second face of the piezoelectric layer;
• and
• forming a first electrode layer on a first face of the piezoelectric layer,
• wherein:
• the first electrode layer comprises:
• a first material on a first side of the first electrode layer adjacent to the first face of the piezoelectric layer;
• and
• a second material on a second side of the first electrode layer opposite to the first face of the piezoelectric layer;
• the first material comprises a first acoustic impedance;
• the second material comprises a second acoustic impedance different than the first acoustic impedance;
• and
• from the first side to the second side of the first electrode layer, an acoustic impedance of the first electrode layer changes continuously from the first acoustic impedance to the second acoustic impedance.