Acoustic resonator including monolithic piezoelectric layer having opposite polarities
A method is provided for forming a piezoelectric layer during a corresponding deposition sequence. The method includes sputtering aluminum nitride onto a sputtering substrate inside a reaction chamber having a gas atmosphere, the gas atmosphere initially including nitrogen gas and an inert gas, causing growth of the piezoelectric layer with a polarity in a negative direction. The method further includes 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 onto the sputtering substrate during a remainder of the deposition sequence, such that the piezoelectric layer is monolithic. The predetermined amount of oxygen containing gas causes the polarity of the aluminum nitride piezoelectric layer to invert from the negative direction to a positive direction, opposite the negative direction.
1. 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 piezoelectric layer on the first electrode layer during a continuous 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 for a first period of time corresponding to the continuous deposition sequence, the gas atmosphere comprising nitrogen (N 2 ) 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, comprising oxygen (O 2 ) gas or ozone (O 3 ) gas, to the gas atmosphere over a predetermined second period of time, less than the first period of time, while continuing the sputtering of the aluminum nitride (AlN) onto the first electrode layer while adding the predetermined amount of oxygen gas and during a remainder of the continuous deposition sequence after the predetermined second period of time, 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, for the remainder of the continuous deposition sequence, and wherein the polarity in the negative direction is directed substantially toward the first electrode layer, and the polarity in the positive direction is directed substantially away from the first electrode layer; and
depositing a second electrode layer over the piezoelectric layer,
wherein the predetermined amount of oxygen gas added to the gas atmosphere is in a range from about 50 micromoles to about 5 millimoles, and wherein the predetermined second period of time is in a range from about one (1) second to about sixty (60) seconds.
2. The method of claim 1 , wherein there is no discernible interface in the piezoelectric layer where the polarity of the piezoelectric layer inverts from the negative direction to the positive direction.
3. The method of claim 1 , wherein the first electrode layer is formed of one of molybdenum (Mo) or tungsten (W).
4. The method of claim 1 , wherein adding the predetermined amount of oxygen gas causes the polarity of the piezoelectric layer to invert at half way through the continuous deposition sequence, such that a first half of the piezoelectric layer has the polarity in the negative direction and a second half of the piezoelectric layer has the polarity in the positive direction.
5. The method of claim 4 , wherein the piezoelectric layer, having the first half with the polarity in the negative direction and the second half with the polarity in the positive direction, causes the BAW resonator to resonate at a second harmonic of a resonance frequency of the BAW resonator, as opposed to a first harmonic of the resonance frequency.
6. The method of claim 5 , wherein by resonating at the second harmonic, the BAW resonator resonates at a frequency over twice the resonance frequency of a BAW resonator without a monolithic piezoelectric layer having different polarities, but having a piezoelectric layer with the same thickness.
7. The method of claim 1 , wherein the acoustic reflector comprises an air cavity formed in the resonator substrate.
8. The method of claim 1 , wherein the acoustic reflector comprises a distributed Bragg reflector (DBR).