Temperature Stable MEMS Resonator

• US 20090158566A1
• Filed: 12/21/2007
• Published: 06/25/2009
• Est. Priority Date: 12/21/2007
• Status: Active Grant

• Current Assignee: SiTime Corporation (Megachips Corporation)
• Original Assignee: SiTime Corporation (Megachips Corporation)
• Inventors: Grosjean, Charles, Hagelin, Paul Merritt
• Granted Patent: US 8,234,774 B2
• Time in Patent Office: Days
• US Class Current: 29/25.350
• 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

One embodiment of the present invention sets forth a method for decreasing a temperature coefficient of frequency (TCF) of a MEMS resonator. The method comprises lithographically defining slots in the MEMS resonator beams and filling the slots with oxide. By growing oxide within the slots, the amount of oxide growth on the outside surfaces of the MEMS resonator may be reduced. Furthermore, by situating the slots in the areas of large flexural stresses, the contribution of the embedded oxide to the overall TCF of the MEMS resonator is increased, and the total amount of oxide needed to decrease the overall TCF of the MEMS resonator to a particular target value is reduced. As a result, the TCF of the MEMS resonator may be reduced in a manner that is more effective relative to prior art approaches.

Claims

1. A method for fabricating a microelectromechanical system (MEMS) resonator having a reduced magnitude of thermal coefficient of frequency (TCF), the method comprising:

defining one or more slots within the MEMS resonator;

fabricating the one or more slots; and

filling the one or more slots with a compensating material,

wherein a temperature coefficient of Young'"'"'s Modulus (TCE) of the compensating material has a sign opposite to a TCE of a material comprising the MEMS resonator.

2. The method of claim 1, wherein the one or more slots are defined in a region of the MEMS resonator subject to larger stresses relative to other regions of the MEMS resonator.

3. The method of claim 2, wherein the MEMS resonator includes a MEMS resonating element resonating via an extensional and/or flexural mode.

4. The method of claim 3, wherein a cross section of the MEMS resonating element that is perpendicular to the extensional and/or flexural mode comprises approximately 40% of the compensating material.

5. The method of claim 2, wherein the MEMS resonator includes a MEMS resonating element, the stresses decrease from an outside sidewall of the MEMS resonating element to the center of the MEMS resonating element, and a region proximate to the outside sidewall of the MEMS resonating element is the region subject to the relatively larger stresses.

6. The method of claim 2, wherein an outside sidewall of the MEMS resonating element has a serrated surface.

7. The method of claim 6, wherein the serrated surface comprises teeth.

8. The method of claim 7, wherein the MEMS resonator includes a MEMS resonating element configured to oscillate in a tuning fork fashion during operation, and a region proximate to the base of the teeth is the region subject to the largest flexural stresses.

9. The method of claim 6, wherein the serrated surface comprises an irregular profile.

10. The method of claim 2, wherein the MEMS resonator includes a MEMS resonating element configured to oscillate in an extensional fashion during operation, and a region proximate to where the MEMS resonating element connects to a support element in the MEMS resonator is the region subject to the relatively larger stresses.

11. The method of claim 1, wherein the one or more slots are filled completely with the compensating material.

12. The method of claim 1, wherein the one or more slots are partially filled with the compensating material to produce a gap within each of the one or more slots.

13. The method of claim 12, further comprising the step of filling the gap in each of the one or more slots with a low-stress cap layer.

14. The method of claim 13, wherein the low-stress cap layer comprises silicon.

15. The method of claim 1, further comprising the step of lining the one or more slots with a liner material resistant to an etch process, prior to the step of filling the one or more slots with the compensating material.

16. The method of claim 15, wherein the liner material comprises silicon.

17. The method of claim 15, further comprising the step of removing excess of the compensating material from the surfaces of the MEMS resonator such that the compensating material remains substantially only within the one or more slots.

18. The method of claim 17, further comprising the step of capping the one or more slots with a conductive capping material resistant to the etch process.

19. The method of claim 18, wherein the conductive capping material comprises silicon.

20. The method of claim 18, wherein the one or more slots are filled completely with the compensating material.

21. The method of claim 18, wherein the one or more slots are partially filled with the compensating material to produce a gap within each of the one or more slots.

22. The method of claim 21, further comprising the step of filling the gap in each of the one or more slots with a low-stress cap layer prior to the step of capping the one or more slots with the conductive capping material.

23. The method of claim 22, wherein the low-stress cap layer comprises silicon.

24. The method of claim 1, wherein the one or more slots are defined lithographically.

25. The method of claim 1, wherein the compensating material comprises oxide.

26. The method of claim 25, wherein the one or more slots are filled with oxide through the processes of oxide growth, oxide deposition, or a combination of oxide growth and oxide deposition.