Acoustic wave resonator and method of operating the same to maintain resonance when subjected to temperature variations

• US 6,420,820 B1
• Filed: 08/31/2000
• Issued: 07/16/2002
• Est. Priority Date: 08/31/2000
• Status: Expired due to Term

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Agilent Technologies, Inc.
• Inventors: Larson, John Dwight III
• Primary Examiner(s): Ramirez, Nestor
• Assistant Examiner(s): Addison, Karen B
• Application Number:
  US09/652,455
• Time in Patent Office: 684 Days
• Field of Search: H02/K 4.108, 310/346, 310/349, 310/324, 310/363
• US Class Current: 310/346
• CPC Class Codes: H03H 3/04 for obtaining desired frequency or temperature coefficient
• CPC Class Codes: Y10T 29/42 Piezoelectric device making
• CPC Class Codes: Y10T 29/49005 Acoustic transducer
• CPC Class Codes: Y10T 29/49128 Assembling formed circuit to base

Abstract

An acoustic resonator includes a ferromagnetic compensator 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 the preferred embodiment, the compensator is formed of a nickel-iron alloy, with the most preferred embodiment being one in which the alloy is approximately 35% nickel and approximately 65% iron. 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.

Claims

1. An acoustic resonator comprising:

a substrate; and

a layer stack integrated to said substrate such that said layer stack includes a suspended region and a metallic flashing layer, said suspended region including;

a piezoelectric body and electrodes positioned to apply an electrical field to said piezoelectric body, said piezoelectric body and electrodes having a resonance and a negative temperature coefficient of frequency; and

a compensator acoustically coupled to said piezoelectric body and electrodes, said compensator being formed of a material having properties by which said compensator at least partially offsets temperature-induced effects on said resonance, where said temperature-induced effects are a function of said negative temperature coefficient of frequency, said metallic flashing layer being on a side of said compensator opposite to said electrodes and said piezoelectric body.

2. The acoustic resonator of claim 1 wherein said compensator is a ferromagnetic layer that is spaced apart from said piezoelectric body by one of said electrodes, said ferromagnetic layer being associated with a positive temperature coefficient of frequency.

3. The acoustic resonator of claim 1 wherein said layer stack includes a peripheral region that contacts said substrate to support said suspended region, said compensator being a layer of a nickel-iron alloy.

4. The acoustic resonator of claim 1 wherein said layer stack is a thin film bulk resonator (FBAR) stack.

5. The acoustic resonator of claim 1 wherein said compensator is formed of a material having a positive temperature coefficient of frequency and has a thickness such that a magnitude of temperature-induced effects on said resonance by presence of said compensator is similar to a magnitude of said temperature-induced effects on said resonance as a function of said negative temperature coefficient of frequency.

6. The acoustic resonator of claim 1 wherein said substrate is a silicon substrate and wherein said electrodes and compensator are metallic layers.

7. An acoustic resonator comprising:

a substrate;

an electrode-piezoelectric stack having a target resonant frequency and having a negative temperature coefficient of frequency; and

a metallic compensator layer having a ferromagnetic material and having a positive temperature coefficient of frequency, said metallic compensator layer being acoustically coupled to said electrode-piezoelectric stack for maintaining said target resonant frequency.

8. The acoustic resonator of claim 7 wherein said electrode-piezoelectric stack and said metallic compensator layer combine to define an FBAR.

9. The acoustic resonator of claim 8 wherein a major portion of said FBAR is suspended from contact with said substrate.

10. The acoustic resonator of claim 7 wherein said metallic compensator layer is formed of a nickel-iron alloy.

11. The acoustic resonator of claim 10 wherein said nickel-iron alloy is approximately 35 percent nickel and approximately 65 percent iron.

12. The acoustic resonator of claim 7 wherein said metallic compensator layer has a thickness selected to neutralize influences of temperature variations on resonance of said electrode-piezoelectric stack such that said target resonant frequency is substantially maintained.