Method of making an acoustic wave resonator
A method of fabricating an acoustic resonator includes forming a ferromagnetic compensator, such as one comprised of a nickel-iron alloy, which at least partially offsets temperature-induced effects introduced by an electrode-piezoelectric stack. The compensator has a positive temperature coefficient of frequency, while the stack has a negative temperature coefficient of frequency. By properly selecting the thickness of the compensator, temperature-induced effects on resonance may be neutralized. Alternatively, the thickness can be selected to provide a target positive or negative composite temperature coefficient of frequency. In order to prevent undue electromagnetic losses in the ferromagnetic compensator, a metallic flashing layer may be added to at least partially enclose the compensator.
1. A method of fabricating an acoustic resonator comprising the steps of:
providing a substrate; and
forming an electrode-piezoelectric stack on said substrate such that a portion of said electrode-piezoelectric stack is suspended from contact with said substrate by a cavity having a boundary defined by said electrode-piezoelectric stack, said electrode-piezoelectric stack having a negative temperature coefficient of frequency, and
(a) forming a compensator layer, comprised of a ferromagnetic material, in direct contact with said electrode-piezoelectric stack, including selecting a material for said compensator layer having a positive temperature coefficient of frequency.
2. The method of claim 1 wherein said step (a) that includes selecting said material includes selecting a nickel-iron alloy.
3. The method of claim 1 wherein said step (a) includes depositing said material as approximately 35 percent nickel and approximately 65 percent iron.
4. The method of claim 1 wherein said step (a) includes selecting a layer thickness to substantially match a magnitude of temperature-induced effects on resonance by operation of said electrode-piezoelectric stack with a magnitude of temperature-induced effects on said resonance as a consequence of said compensator layer.
5. The method of claim 1 wherein said step of forming said compensator layer further includes (b) forming a metallic flashing layer on a side of said compensator layer opposite to said electrode-piezoelectric stack.
6. The method of claim 5 further comprising using fabrication alignment techniques in said steps (a) and (b) to prevent spurious mode generation resulting from partial coverage of said electrode-piezoelectric stack by said compensator layer.