Layered structure and method for fabricating same
Methods and techniques for fabricating layered structures, such as capacitive micromachined ultrasound transducers, as well as the structures themselves. The layered structure has a membrane that includes a polymer-based layer and a top electrode on the polymer-based layer. The membrane is suspended over a closed cavity and may be actuated by applying a voltage between the top electrode and a bottom electrode that may be positioned along or be a bottom of the closed cavity. The layered structure may be fabricated using a wafer bonding process.
1. A layered structure, comprising:
(a) a substrate assembly that functions as a bottom electrode;
(b) a first polymer-based layer suspended above a closed cavity, wherein the closed cavity is between the first polymer-based layer and the substrate assembly;
(c) a second polymer-based layer placed on the first polymer-based layer, wherein a portion of the second polymer-based layer that is on the first polymer-based layer is thicker than the first polymer-based layer; and
(d) a top electrode between the first and second polymer-based layers.
2. The structure of claim 1 , wherein the top electrode is embedded within the first and second polymer-based layers.
3. The structure of claim 1 , wherein the portion of the second polymer-based layer that is on the first polymer-based layer is at least five times thicker than the first polymer-based layer.
4. The structure of claim 1 , wherein the cavity has a height selected such that an operating voltage of the structure is no more than 50 Volts.
5. The structure of claim 1 , wherein the cavity has a height of no more than 0.3 μm.
6. The structure of claim 1 , wherein the cavity is watertight.
7. The structure of claim 1 , wherein the substrate assembly comprises a conductive substrate.
8. The structure of claim 1 , wherein the substrate assembly comprises a non-conductive substrate and a conductive bottom electrode on the substrate.
9. The structure of claim 1 , wherein the substrate assembly comprises an optically-transparent material.
10. The structure of claim 9 , wherein the substrate assembly further comprises an optically-transparent conductive bottom electrode on the substrate.
11. The structure of claim 1 , wherein the sacrificial layer is non-reactive when exposed to the second polymer-based layer and to a photoresist developer used during the patterning of the second polymer-based layer, and wherein the second polymer-based layer is non-reactive when exposed to an etchant used to etch away the sacrificial layer.
12. The structure of claim 11 , wherein the sacrificial layer is also non-reactive when exposed to the first polymer-based layer, and wherein the first polymer-based layer is non-reactive when exposed to the etchant used to etch away the sacrificial layer.
13. The structure of claim 1 , wherein the first and second polymer-based layers comprise SU8 photoresist.
14. The structure of claim 1 , wherein the sacrificial layer comprises an OmniCoat™ composition.
15. The structure of claim 1 , wherein the structure comprises a capacitive micromachined ultrasound transducer and the closed cavity is sealed.
16. The structure of claim 1 , wherein the portion of the second polymer-based layer that is on the first polymer-based layer is cross-linked.