Method for fabricating a layered structure using wafer bonding

Patent metadata

Field	Value
Publication number	US-10509013-B2
Application number	US-201916276548-A
Country	US
Kind code	B2
Filing date	Feb 14, 2019
Priority date	Dec 19, 2017
Publication date	Dec 17, 2019
Grant date	Dec 17, 2019

Abstract

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.

First claim

The invention claimed is:

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.

Assignees

• Univ British Columbia

Inventors

• Gerardo Carlos D
• Rohling Robert
• Cretu Edmond

Classifications

• B06B1/0292 Primary

  Electrostatic transducers, e.g. electret-type · CPC title

• B32B37/02

  characterised by a sequence of laminating steps, e.g. by adding new layers at consecutive laminating stations · CPC title

• G01N29/0654

  Imaging · CPC title

• B32B38/10

  Removing layers, or parts of layers, mechanically or chemically · CPC title

• G01N29/2406 Primary

  Electrostatic or capacitive probes, e.g. electret or cMUT-probes · CPC title