Resonator
A resonator comprising a beam formed from a first material having a first Young's modulus and a first temperature coefficient of the first Young's modulus, and a second material having a second Young's modulus and a second temperature coefficient of the second Young's modulus, a sign of the second temperature coefficient being opposite to a sign of the first temperature coefficient at least within operating conditions of the resonator, wherein the ratio of the cross sectional area of the first material to the cross sectional area of the second material varies along the length of the beam, the cross sectional areas being measured substantially perpendicularly to the beam.
1. A resonator comprising a beam formed from a first material having a first Young's modulus and a first temperature coefficient of the first Young's modulus, and a second material having a second Young's modulus and a second temperature coefficient of the second Young's modulus, a sign of the second temperature coefficient being opposite to a sign of the first temperature coefficient at least within operating conditions of the resonator, wherein the second material forms a layer on the first material, and wherein the ratio of the cross sectional area of the first material to the cross sectional area of the second material varies along the length of the beam, the cross sectional areas being measured substantially perpendicularly to the beam.
2. A resonator according to claim 1 comprising a MEMS resonator wherein the first material comprises silicon.
3. A resonator according to claim 1 wherein the second material comprises silicon dioxide.
4. A resonator according to claim 1 further comprising an anchor, the beam having a tip spaced apart from the anchor.
5. A resonator according to claim 4 wherein the thickness of the second material is greater at or near the anchor.
6. A resonator according to claim 1 wherein the thickness of the first material is substantially constant along the length of the beam, and the thickness of the second material varies along the length of the beam.
7. A resonator according to claim 1 comprising a plurality of slits formed in the beam and extending longitudinally along at least part of the length of the beam.
8. A resonator according to claim 1 wherein the beam comprises a top surface and an opposite bottom surface and two opposite side walls, and wherein the second material forms a skin around the top, bottom and side walls of the beam, the thickness of the second material over the side walls being greater than the thickness of the second material over the top and bottom surfaces of the beam.
9. A resonator according to claim 1 wherein the first material is doped, and the level of doping varies along the length of the beam.
10. A method of manufacturing a MEMS resonator comprising the steps of:
forming a beam from a first material;
coating the beam with a second material;
such that the ratio of the cross-sectional area of the first material to the cross-sectional area of the second material varies along the length of the beam.
11. A method according to claim 10 wherein the ratio of the cross-sectional area of the first material to the cross-sectional area of the second material is varied by varying the rate at which the second material is coated onto the first material.
12. A method according to claim 11 wherein the ratio of the cross-sectional area of the first material to the cross-sectional area of the second material is varied by forming the beam from a first material having a cross-sectional area that varies along the length of the beam.
13. A method of manufacturing a MEMS resonator according to any one of claim 10 comprising the steps of forming the beam from a silicon substrate and coating the silicon substrate with a silicon oxide layer.
14. A method according to claim 13 wherein prior to the step of coating the beam with silicon dioxide the following further step is carried out:
implanting the silicon substrate with an impurity such that the concentration of the impurity varies along the length of the beam.