Methods of Forming Micromechanical Resonators Having High Density Trench Arrays Therein that Provide Passive Temperature Compensation

• US 20100319185A1
• Filed: 06/04/2010
• Published: 12/23/2010
• Est. Priority Date: 06/19/2009
• Status: Active Grant

• Current Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Original Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Inventors: Casinovi, Giorgio, Tabrizian, Roozbeh, Ayazi, Farrokh
• Granted Patent: US 8,381,378 B2
• Time in Patent Office: Days
• US Class Current: 29/594
• CPC Class Codes: H03H 2009/2442 Square resonators
• CPC Class Codes: H03H 3/0076 for obtaining desired frequency or temperature coefficients
• CPC Class Codes: H03H 9/02448 of temperature influence
• CPC Class Codes: Y10T 29/42 Piezoelectric device making
• CPC Class Codes: Y10T 29/49005 Acoustic transducer
• CPC Class Codes: Y10T 29/49155 Manufacturing circuit on or in base

Abstract

A method of forming a micromechanical resonator includes forming a resonator body anchored to a substrate by at least a first anchor. This resonator body may include a semiconductor or other first material having a negative temperature coefficient of elasticity (TCE). A two-dimensional array of spaced-apart trenches are provided in the resonator body. These trenches may be filled with an electrically insulating or other second material having a positive TCE. The array of trenches may extend uniformly across the resonator body, including regions in the body that have relatively high and low mechanical stress during resonance. This two-dimensional array (or network) of trenches can be modeled as a network of mass-spring systems with springs in parallel and/or in series with respect to a direction of a traveling acoustic wave within the resonator body during resonance.

Claims

1. A method of forming a micromechanical resonator operable in a bulk acoustic mode, comprising:

forming a resonator body anchored to a substrate by at least a first anchor, said resonator body comprising a first material having a negative temperature coefficient of elasticity (TCE) and an array of spaced-apart trenches filled with a second material having a positive TCE, said array of spaced-apart trenches having a sufficient density in the resonator body to meet the following relationship;

1≦

−

R_V(TCE_I/TCE_S)[(E_I/E_S)+(ρ

_I/ρ

_S)^1/2(E_I/E_S)<sup>−

1/2</sup>]≦

3,

where R_V=(V_I/V_S);

TCE_S, E_S, ρ

_S and V_S represent the temperature coefficient of elasticity, Young'"'"'s modulus, density and volume of the first material in the resonator body, respectively; and

TCE_I, E_I, ρ

_I and V_I ρ

represent the temperature coefficient of elasticity, Young'"'"'s modulus, density and total volume of the second material in the array of spaced-apart trenches, respectively.

2. The method of claim 1, wherein the array of spaced-apart trenches comprises an array of through-body holes that extend entirely through the resonator body.

3. The method of claim 2, wherein the spaced-apart trenches in the array are arranged in a checkerboard pattern.

4. The method of claim 1, wherein the spaced-apart trenches in the array are arranged in a checkerboard pattern.

5. The method of claim 1, wherein the first material comprises silicon, the second material comprises silicon dioxide and R_V has a value in a range from 0.14 to 0.42.

6. The method of claim 1, wherein the spaced-apart trenches in the array are arranged as a two-dimensional array of spaced-apart trenches having a first plurality of rows of spaced-apart trenches and a second plurality of columns of spaced-apart trenches.

7. The method of claim 6, wherein each of the first plurality of rows of spaced-apart trenches has an equivalent number of spaced-apart trenches therein.

8. The method of claim 7, wherein each of the second plurality of columns of spaced-apart trenches has an equivalent number of spaced-apart trenches therein.

9. The method of claim 1, further comprising:

forming a bottom electrode on the resonator body;

forming a piezoelectric layer on the bottom electrode; and

forming at least one top electrode on the piezoelectric layer.

10. The method of claim 9, wherein a combination of a number of spaced-apart trenches in the array and the second material is sufficient to fully temperature compensate the micromechanical resonator, which comprises the resonator body, bottom electrode, piezoelectric layer and at least one top electrode.

11. The method of claim 1, wherein the resonator body has upper and lower surfaces thereon;

and wherein a density of the filled trenches in the upper surface of the resonator body is at least five trenches/square for each square region on the upper surface having an area equal to t², where t is a thickness of the resonator body as measured between the upper and lower surfaces.

12. The method of claim 4, wherein the resonator body has upper and lower surfaces thereon;

and wherein a density of the filled trenches in the upper surface of the resonator body is at least five trenches/square for each square region on the upper surface having an area equal to t², where t is a thickness of the resonator body as measured between the upper and lower surfaces.

13. A method of forming a micromechanical resonator operable in a bulk acoustic mode, comprising:

forming a resonator body having upper and lower surfaces thereon, said resonator body comprising a first material having a negative temperature coefficient of elasticity (TCE) and an array of spaced-apart trenches filled with a second material having a positive TCE, said array of spaced-apart trenches having a sufficient density in the resonator body to meet the following relationship;

0.5≦

−

R_V(TCE_I/TCE_S)[(E_I/E_S)+(ρ

_I/ρ

_S)^1/2(E_I/E_S)<sup>−

1/2</sup>],

where R_V=(V_I/V_S);

TCE_S, E_S, ρ

_S and V_S represent the temperature coefficient of elasticity, Young'"'"'s modulus, density and volume of the first material in the resonator body, respectively; and

TCE_I, E_I, ρ

_I and V_I represent the temperature coefficient of elasticity, Young'"'"'s modulus, density and total volume of the second material in the array of spaced-apart trenches, respectively; and

wherein a density of the filled trenches in the upper surface of the resonator body is at least five trenches/square for each square region on the upper surface having an area equal to t², where t is a thickness of the resonator body as measured between the upper and lower surfaces.

14. The method of claim 13, wherein the array of spaced-apart trenches comprises an array of through-body holes that extend entirely through the resonator body.

15. The method of claim 13, wherein the spaced-apart trenches in the array are arranged in a checkerboard pattern.

16. The method of claim 13, further comprising:

forming a bottom electrode on the resonator body;

forming a piezoelectric layer on the bottom electrode; and

forming at least one top electrode on the piezoelectric layer.

17. A method of forming a micromechanical resonator operable in a bulk acoustic mode, comprising:

forming a resonator body having upper and lower surfaces thereon, said resonator body comprising a first material having a negative temperature coefficient of elasticity (TCE) and a two-dimensional array of spaced-apart through-body holes filled with a second material having a positive TCE, said through-body holes having minimum and maximum lateral dimensions in a range from about 0.1 T to about 0.2 T, where T is a thickness of the resonator body and T is greater than about 10 microns; and

wherein a density of the filled through-body holes is at least five trenches/square for each square region on the upper surface having an area equal to t², where t is a thickness of the resonator body as measured between the upper and lower surfaces.

18. The method of claim 17, wherein said forming comprises:

etching a two-dimensional array of spaced-apart trenches in the resonator body;

depositing the electrically insulating material onto a surface of the resonator body and into the two-dimensional array of spaced-apart trenches;

dry etching the electrically insulating material to thereby remove portions of the electrically insulating material from the surface of the resonator body and from within the two-dimensional array of spaced-apart trenches; and

repeatedly performing said depositing and said dry etching in sequence at least once until the two-dimensional array of spaced-apart through-body holes is formed and completely filled with the electrically insulating material.

19. The method of claim 18, wherein said dry etching comprises blanket dry etching the electrically insulating material across the surface of the resonator body.

20. The method of claim 18, further comprising:

forming a bottom electrode on a layer of the electrically insulating material on the resonator body;

forming a piezoelectric layer on the bottom electrode; and

forming at least one top electrode on the piezoelectric layer.

21. The method of claim 18, wherein said depositing comprises depositing the electrically insulating material using a low pressure chemical vapor deposition technique.

22. The method of claim 18, wherein said etching comprises etching a two-dimensional array of spaced-apart trenches in the resonator body using an etching process that alternates repeatedly between an isotropic plasma etching of the resonator body and a deposition of a chemically inert passivation layer on the resonator body.

23. A method of forming a micromechanical resonator operable in a bulk acoustic mode, comprising:

forming a resonator body having upper and lower surfaces thereon, said resonator body comprising a first material having a negative temperature coefficient of elasticity (TCE) and an array of spaced-apart trenches filled with a second material having a positive TCE, said array of spaced-apart trenches having a sufficient density in the resonator body to meet the following relationship;

0.5≦

−

R_V(TCE_I/TCE_S)[(E_I/E_S)+(ρ

_I/ρ

_S)^1/2(E_I/E_S)<sup>−

1/2</sup>],

where R_V=(V_I/V_S);

TCE_S, E_S, ρ

_S and V_S represent the temperature coefficient of elasticity, Young'"'"'s modulus, density and volume of the first material in the resonator body, respectively; and

TCE_I, E_I, ρ

_I and V_I represent the temperature coefficient of elasticity, Young'"'"'s modulus, density and total volume of the second material in the array of spaced-apart trenches, respectively; and

wherein a density of the filled trenches in the upper surface of the resonator body is at least one trench/square for each square region on the upper surface having an area equal to t², where t, which is a thickness of the resonator body as measured between the upper and lower surfaces, is in a range from about 2 microns to about 10 microns.