Temperature stable MEMS resonator

• US 10,541,666 B1
• Filed: 03/08/2018
• Issued: 01/21/2020
• Est. Priority Date: 12/21/2007
• Status: Active Grant

• Current Assignee: SiTime Corporation (Megachips Corporation)
• Original Assignee: SiTime Corporation (Megachips Corporation)
• Inventors: Hagelin, Paul M., Grosjean, Charles I.
• Primary Examiner(s): Gannon, Levi
• Application Number:
  US15/916,088
• Time in Patent Office: 684 Days
• Field of Search: 310311, 310315, 310341, 310346, 310370, 331 66, 331154, 331156, 331176, 333186-189, 333197, 333200, 333219, 3332191, 333234
• CPC Class Codes: H02N 1/00 Electrostatic generators or motors using a solid moving electrostatic charge carrier
• CPC Class Codes: H03B 5/30 with frequency-determining element being electromechanical resonator
• CPC Class Codes: H03B 5/32 being a piezoelectric resonator selection of piezoelectric material H10N30/00
• CPC Class Codes: H03H 2009/02251 Design
• CPC Class Codes: H03H 2009/02283 Vibrating means
• CPC Class Codes: H03H 2009/02291 Beams
• CPC Class Codes: H03H 2009/02299 Comb-like, i.e. the beam comprising a plurality of fingers or protrusions along its length
• CPC Class Codes: H03H 2009/02322 Material
• CPC Class Codes: H03H 2009/0233 comprising perforations
• CPC Class Codes: H03H 2009/02496 Horizontal, i.e. parallel to the substrate plane
• CPC Class Codes: H03H 2009/155 using MEMS techniques
• CPC Class Codes: H03H 3/0072 of microelectro-mechanical resonators or networks micromembranes or microbeams B81B2203/01; manufacture of microstructural devices in general B81C
• CPC Class Codes: H03H 3/0073 Integration with other electronic structures
• CPC Class Codes: H03H 3/0076 for obtaining desired frequency or temperature coefficients
• CPC Class Codes: H03H 9/02244 of microelectro-mechanical resonators
• CPC Class Codes: H03H 9/02433 Means for compensation or elimination of undesired effects
• CPC Class Codes: H03H 9/02448 of temperature influence
• CPC Class Codes: H03H 9/125 Driving means, e.g. electrodes, coils
• CPC Class Codes: H03H 9/21 Crystal tuning forks
• CPC Class Codes: H03H 9/2405 of microelectro-mechanical resonators
• CPC Class Codes: H03H 9/2468 : Tuning fork resonators
• CPC Class Codes: H03H 9/2484 : with two fork tines, e.g. Y-beam cantilever
• CPC Class Codes: H10N 30/01 : Manufacture or treatment
• CPC Class Codes: H10N 30/04 : Treatments to modify a piezoelectric or electrostrictive property, e.g. polarisation characteristics, vibration characteristics or mode tuning
• CPC Class Codes: Y10T 29/42 : Piezoelectric device making
• CPC Class Codes: Y10T 29/49002 : Electrical device making
• CPC Class Codes: Y10T 29/49005 : Acoustic transducer
• CPC Class Codes: Y10T 29/4902 : Electromagnet, transformer or inductor
• CPC Class Codes: Y10T 29/4908 : Acoustic transducer

Abstract

A resonant member of a MEMS resonator oscillates in a mechanical resonance mode that produces non-uniform regional stresses such that a first level of mechanical stress in a first region of the resonant member is higher than a second level of mechanical stress in a second region of the resonant member. A plurality of openings within a surface of the resonant member are disposed more densely within the first region than the second region and at least partly filled with a compensating material that reduces temperature dependence of the resonant frequency corresponding to the mechanical resonance mode.

Claims

1. A microelectromechanical device comprising:

a resonating element having (i) an exterior surface that is predominantly silicon throughout and (ii) first and second regions with different material properties, the first and second regions having respective first and second temperature coefficients of stiffness that are opposite in sign at least at one temperature; and

drive circuitry to generate a drive signal that renders the resonator element into mechanically resonant motion;

wherein one of the first region and the second region is fabricated via a process that removes a material from the other of the first region and the second region, to form a trench therein, and that forms the one of the first region and the second region by depositing a first material within the trench and then caps the first material within the trench with silicon.

2. The microelectromechanical device of claim 1 wherein the first region comprises a silicon region having a negative temperature coefficient of stiffness at the at least one temperature and the second region has a positive temperature coefficient of stiffness at the at least one temperature.

3. The microelectromechanical device of claim 2 wherein the second region comprises silicon.

4. The microelectromechanical device of claim 2 wherein the second region comprises a compensating material.

5. The microelectromechanical device of claim 4 wherein the compensating material comprises silicon atoms in combination with atoms other than silicon.

6. The microelectromechanical device of claim 5 wherein the atoms other than silicon comprise oxygen atoms.

7. The microelectromechanical device of claim 2 wherein the first region constitutes a predominant percentage of the resonating element along at least one dimension.

8. The microelectromechanical device of claim 2 wherein the resonating element has a thickness in a dimension perpendicular to a direction of the mechanically resonant motion, and wherein the first region constitutes a predominant percentage of the thickness.

9. The microelectromechanical device of claim 1 wherein the second region is stacked on the first region in a thickness dimension of the resonating element.

10. The microelectromechanical device of claim 1 wherein the second region is disposed laterally with respect to the first region along an axis perpendicular to a thickness dimension of the resonating element.

11. The microelectromechanical device of claim 1 wherein the resonating element comprises at least one of a plate, a ring or a beam capable of mechanically resonant motion in an extensional mode.

12. The microelectromechanical device of claim 1 wherein the resonating element comprises at least one beam capable of mechanically resonant motion in a bending mode.

13. The microelectromechanical device of claim 1 wherein the resonating element comprises a beam, wherein the trench is formed at a position along a length dimension of the beam that will experience the greatest flexural stress during operation of the resonating element, wherein the trench is formed in a manner that extends parallel to the length dimension, and wherein the first material is formed within the trench prior to the capping of the trench with silicon.

14. A method of fabricating a microelectromechanical device, the method comprising:

forming, in a first semiconductor die, a resonating element having (i) an exterior surface that is predominantly silicon throughout and (ii) first and second regions with different material properties, the first and second regions having respective first and second temperature coefficients of stiffness that are opposite in sign at least at one temperature;

forming, in a second semiconductor die, drive circuitry to generate a drive signal that, when applied to the resonating element, will drive the resonator element into mechanically resonant motion; and

electrically coupling the first and second semiconductor dies to enable electrical coupling of the drive signal from the drive circuitry to the resonating element;

wherein forming the resonating element having first and second regions with respective first and second temperature coefficients of stiffness that are opposite in sign at least one temperature comprises forming a trench in the exterior surface of the resonating element, disposing a first material in the trench and then capping the trench with silicon such that the first material is disposed beneath the exterior surface of the resonating element.

15. The method of claim 14 further comprising forming, in the first semiconductor die, a drive electrode adjacent the resonating element, and wherein electrically coupling the first and second semiconductor dies to enable electrical coupling of the drive signal from the drive circuitry to the resonating element comprises electrically coupling the first and second semiconductor dies to enable conduction of the drive signal from the drive circuitry to the drive electrode and electrostatic coupling of the drive signal from the drive electrode to the resonating element.

16. The method of claim 14 wherein the first material comprises silicon atoms in combination with atoms other than silicon.

17. The method of claim 14 wherein forming the resonating element constituted at least in part by first and second regions with different material properties comprising forming the first region from silicon and forming the second region at least in part from a combination of silicon atoms and atoms other than silicon, the first region having a negative temperature coefficient of stiffness at the at least one temperature and the second region having a positive temperature coefficient of stiffness at the at least one temperature.

18. The method of claim 17 wherein forming the second region at least in part from the combination of silicon atoms and atoms other than silicon comprises forming the second region at least in part by a combination of silicon atoms and oxygen atoms.

19. The method of claim 14 wherein forming the resonating element comprises forming at least one of (i) a plate, a ring or a beam capable of mechanically resonant motion in an extensional mode, or (ii) a beam capable of mechanically resonant motion in a bending mode.

20. The method of claim 14 wherein the resonating element comprises a beam, and wherein forming the resonating element comprises identifying a position along a length dimension of the beam that will experience the greatest flexural stress during operation of the resonating element, forming the trench at the position in a manner that extends parallel to the length dimension, and forming the first material within the trench prior to capping the trench with silicon.