Method for fabricating a layered structure using wafer bonding
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 method for fabricating a layered structure, the method comprising:
(a) depositing a first polymer-based layer on a substrate assembly that functions as a bottom electrode;
(b) patterning the first polymer-based layer to be a cavity;
(c) depositing a sacrificial layer on a separate substrate;
(d) depositing a second polymer-based layer over the sacrificial layer;
(e) depositing a top electrode on the second polymer-based layer;
(f) depositing a third polymer-based layer on the top electrode such that the top electrode is between the second and third polymer-based layers;
(g) adhering the first and third polymer-based layers together such that the cavity is closed by the first and third polymer-based layers; and
(h) etching away the sacrificial layer such that the second polymer-based layer is released from the separate substrate.
2. The method of claim 1 , wherein the top electrode is embedded within the second and third polymer-based layers.
3. The method of claim 1 , further comprising cross-linking the first and third polymer-based layers prior to adhering the first and third polymer layers together.
4. The method of claim 1 , wherein patterning the first polymer-based layer and etching away the sacrificial layer are performed using organic and non-toxic solvents.
5. The method of claim 1 , wherein the first polymer-based layer is photosensitive, and wherein patterning the first polymer-based layer to be the cavity comprises:
(a) cross-linking a portion of the first polymer-based layer to remain following the etching by exposing the portion to ultraviolet radiation; and
(b) applying a photoresist developer to etch uncross-linked areas of the first polymer-based layer.
6. The method of claim 1 , wherein:
(a) relative thickness of the second polymer-based layer to the first polymer-based layer is selected such that the top electrode resonates at a frequency of at least 1 MHz; or
(b) the second polymer-based layer is at least five times thicker than the first polymer-based layer.
7. The method of claim 1 , wherein the fabrication of the layered structure is performed at a temperature of no more than 150° C.
8. The method of claim 1 , wherein the substrate assembly is flexible and bonded to a rigid carrier.
9. The method of claim 1 , wherein the top electrode comprises a conductive polymer.
10. The method of claim 1 wherein adhering the first and third polymer layers together comprises:
(a) treating surfaces of the first and third polymer layers to be adhered to each other with plasma;
(b) aligning the treated surfaces to each other; and
(c) pressing the treated surfaces together until a watertight seal is formed around the cavity.
11. The method of claim 1 , wherein the adhering is done in a bonding chamber at pressure of no more than 0.001 Torr.
12. The method of claim 1 , further comprising, after the adhering, trapping charge in the first polymer-based layer by:
(a) applying a voltage across the top electrode and the substrate assembly such that a portion of the first polymer-based layer contacting the top electrode is pulled into contact with the substrate assembly;
(b) maintaining the portion of the first polymer-based layer contacting the top electrode and the substrate assembly in contact for a period of time; and then
(c) ceasing applying the voltage.
13. The method of claim 1 , wherein the sacrificial layer comprises a polymer.
14. The method of claim 1 , wherein depositing the second polymer-based layer on the sacrificial layer comprises completely covering the sacrificial layer with the second polymer-based layer.
15. The method of claim 1 wherein the substrate assembly comprises a non-conductive substrate with a conductive bottom electrode on the substrate.
16. The method of claim 1 , wherein the substrate assembly comprises an optically-transparent conductive bottom electrode on an optically-transparent substrate.
17. The method 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.
18. The method of claim 1 , wherein the first, second, and third polymer-based layers comprise SU8 photoresist and the sacrificial layer comprises an OmniCoat™ composition.
19. The method of claim 1 , wherein the cavity has a height selected such that an operating voltage of the transducer is no more than 50 Volts.
20. The method of claim 1 , wherein depositing the sacrificial layer comprises evaporating a composition that comprises a solvent, and then depositing the composition as the sacrificial layer, wherein at least 70% and no more than 90% of the solvent is evaporated.