Micro-electro-mechanical transducer having a surface plate
A micro-electro-mechanical transducer (such as a cMUT) is disclosed. The transducer has a base, a spring layer placed over the base, and a mass layer connected to the spring layer through a spring-mass connector. The base includes a first electrode. The spring layer or the mass layer includes a second electrode. The base and the spring layer form a gap therebetween and are connected through a spring anchor. The mass layer provides a substantially independent spring mass contribution to the spring model without affecting the equivalent spring constant. The mass layer also functions as a surface plate interfacing with the medium to improve transducing performance. Fabrication methods to make the same are also disclosed.
1. A method for fabricating a micro-electro-mechanical device having a movable mechanical part to transform energy, the method comprising the steps of:
forming at least one spring anchor on either a front side of a substrate wafer or a bottom side of a spring layer;
forming at least one spring-mass connector of a desired height either on the spring layer or on a mass layer; and
joining the substrate wafer, the spring layer and the mass layer such that the spring layer and the substrate wafer are joined through the spring anchor to define a gap therebetween, the mass layer and the spring layer are joined through the spring-mass connector.
2. The method of claim 1 wherein the step of joining the substrate wafer, the spring layer and the mass layer comprises a step using wafer bonding technique.
3. The method of claim 1 wherein the spring-mass connector is horizontally distanced from the spring anchor by a sufficient length to define a cantilever anchored at the spring anchor with an exerting end at the spring-mass connector, the cantilever and the gap enabling a vertical displacement of the spring-mass connector to transport the mass layer substantially vertically with a piston-like motion.
4. The method of claim 1 wherein spring-mass connector is formed on a top side of the spring layer, and the mass layer is connected to the spring-mass connector from the top side of the spring layer opposing the substrate wafer.
5. The method of claim 1 wherein spring-mass connector is formed on the bottom side of the spring layer, and the mass layer is connected to the spring-mass connector from the bottom side of the spring layer.
6. The method of claim 5 wherein the mass layer is disposed in the gap defined by the spring layer and the substrate wafer.
7. The method of claim 1 , wherein the step of forming at least one spring anchor comprises:
depositing and patterning a material on the spring layer or the substrate wafer to form the at least one spring anchor.
8. The method of claim 1 , wherein the step of forming at least one spring anchor comprises:
growing and patterning an oxide on the spring layer or the substrate wafer to form the at least one spring anchor.
9. The method of claim 1 , wherein the step of forming at least one spring anchor comprises:
forming a recess on the spring layer or the substrate wafer; and
growing and patterning an oxide on the spring layer or the substrate wafer to form the at least one spring anchor in the recess, the spring anchor attaching to a bottom of the recess and extending beyond the spring layer or the substrate wafer.
10. The method of claim 1 , wherein the step of forming at least one spring-mass connector comprises:
depositing and patterning a material on the mass layer or the spring layer to form the at least one spring-mass connector.
11. The method of claim 1 , wherein the step of forming at least one spring-mass connector comprises:
patterning and micromachining the mass layer or the spring layer to form the at least one spring-mass connector on the mass layer.
12. The method of claim 1 , wherein the step of forming at least one spring-mass connector comprises:
forming a sacrificial layer on the mass layer or the spring layer, the sacrificial layer defining a negative pattern for the spring-mass connector;
depositing a material over the sacrificial layer and into the negative pattern for the spring-mass connector; and
removing the sacrificial layer to form the spring layer and the spring-mass connector.
13. The method of claim 1 further comprising forming or effectuating a transducing member on the spring layer.
14. The method of claim 13 wherein the transducing member is an electrode.
15. The method of claim 1 wherein the mass layer is disposed between the substrate and the spring layer, the method further comprising forming or effectuating a transducing member on the mass layer.