Acoustic resonator including monolithic piezoelectric layer having opposite polarities

A method of forming a bulk acoustic wave (BAW) resonator, comprising:

• depositing a first electrode layer over a resonator substrate and an acoustic reflector;
• depositing a monolithic piezoelectric layer on the first electrode layer during a deposition sequence, wherein depositing the piezoelectric layer comprises:
• sputtering aluminum nitride (AlN) on the first electrode layer inside a reaction chamber having a gas atmosphere, the gas atmosphere initially comprising nitrogen (N2) gas and argon (Ar) gas, causing growth of the piezoelectric layer with a polarity in a negative direction;
• and
• adding a predetermined amount of oxygen gas to the gas atmosphere over a predetermined period of time, while continuing the sputtering of the aluminum nitride (AlN) onto the first electrode layer during a remainder of the deposition sequence, such that the piezoelectric layer is monolithic, wherein the predetermined amount of oxygen gas causes the polarity of the piezoelectric layer to invert from the negative direction to a positive direction, opposite the negative direction;
• and
• depositing a second electrode layer over the monolithic piezoelectric layer.

A method of forming a piezoelectric layer of aluminum nitride (AlN), doped with a rare earth element, during a corresponding deposition sequence, the method comprising:

• sputtering aluminum nitride (AlN) and the rare earth element onto a metal sputtering substrate inside a reaction chamber having a gas atmosphere, the gas atmosphere initially comprising nitrogen (N2) gas and argon (Ar) gas, causing growth of the piezoelectric layer with a negative polarity;
• and
• adding a predetermined amount of oxygen containing gas to the gas atmosphere over a predetermined period of time, while continuing the sputtering of the aluminum nitride (AlN) and the rare earth element onto the metal sputtering substrate during a remainder of the deposition sequence, such that the piezoelectric layer is monolithic,
• wherein the predetermined amount of oxygen containing gas causes the negative polarity of the piezoelectric layer to invert to a positive polarity, opposite the negative polarity, at half way through the deposition sequence, such that a first half of the piezoelectric layer has the negative polarity and a second half of the monolithic piezoelectric layer has the positive polarity.