Micromachined ultrasonic transducers and method of fabrication

• US 20040085858A1
• Filed: 08/07/2003
• Published: 05/06/2004
• Est. Priority Date: 08/08/2002
• Status: Active Grant

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Huang, Yongli, Khuri-Yakub, Butrus T., Ergun, Arif S.
• Granted Patent: US 6,958,255 B2
• Time in Patent Office: Days
• US Class Current: 367/181
• CPC Class Codes: B06B 1/0292 Electrostatic transducers, e.g. electret-type
• CPC Class Codes: B06B 1/0688 with foil-type piezoelectric elements, e.g. PVDF
• CPC Class Codes: B81B 2201/0257 Microphones or microspeakers
• CPC Class Codes: B81C 1/00158 Diaphragms, membranes manufacture process for semi-permeable inorganic membranes B01D67/0039
• CPC Class Codes: B81C 2203/036 Fusion bonding
• CPC Class Codes: G01H 11/08 using piezoelectric devices
• CPC Class Codes: H01L 21/2007 Bonding of semiconductor wafers to insulating substrates or to semiconducting substrates using an intermediate insulating layer H01L21/2011 takes precedence; bonding of semiconductor wafers to semiconductor wafers for junction formation H01L21/187
• CPC Class Codes: H01L 21/764 Air gaps H01L21/76264 takes precedence

Abstract

There is described a micromachined ultrasonic transducers (MUTS) and a method of fabrication. The membranes of the transducers are fusion bonded to cavities to form cells. The membranes are formed on a wafer of sacrificial material. This permits handling for fusions bonding. The sacrificial material is then removed to leave the membrane. Membranes of silicon, silicon nitride, etc. can be formed on the sacrificial material. Also described are cMUTs, pMUTs and mMUTs.

Claims

1. The method of fabricating a microfabricated ultrasonic transducer comprising the steps of:

selecting a silicon carrier wafer;

selecting a silicon-on-insulator wafer having a thin silicon layer supported on an oxide layer;

thermally oxidizing either the silicon wafer or the silicon layer of the silicon-on-insulator wafer to form a silicon oxide layer of predetermined thickness;

applying a mask having openings of predetermined size and shape, to expose areas of the oxide layer;

etching away the oxide at the exposed openings to define oxide walls;

bonding the two wafers with the thin silicon layer facing the silicon carrier wafer and spaced therefrom by the oxide layer whereby cells are formed; and

removing the wafer and oxide layer of the silicon-on-oxide-on insulator wafer leaving the thin silicon membrane supported spaced from the silicon carrier wafer by the silicon oxide.

2. A method as in claim 1 in which the oxide layer is formed on the silicon carrier wafer.

3. The method of claim 1 in which the oxide is formed on the silicon layer of the silicon-on-oxide insulator.

4. The method as in claim 2 including the additional step of applying a thin oxide layer over the support oxide and carrier wafer prior to bonding.

5. The method of claim 3 including the steps of forming a thin oxide layer on the carrier wafer prior to bonding the wafers.

6. The method of claim 1 including the additional steps of selecting an additional silicon-on-insulator wafer having a thin silicon layer supported on an oxide layer;

masking an etching away portions of said thin silicon layer to leave islands of predetermined size and shape;

bonding said islands to the thin silicon of the silicon-on-insulator wafer;

removing the oxide and wafer to leave the selected wafer with a thin silicon layer having areas of different thicknesses whereby the resulting silicon membranes have areas of different thicknesses.

7. The method of claim 2 including the additional step of etching the carrier wafer between the oxide cavity walls to configure the bottom wall.

8. The method of claim 3 including the additional step of etching the carrier wafer between the oxide cavity walls to configure the bottom wall.

9. The method of forming an ultrasonic transducer the type comprising a membrane supported on a carrier wafer by patterened oxide supports of predetermined size and shape;

selecting a carrier wafer;

selecting a silicon-on-insulator wafer having a thin silicon layer supported by an oxide;

forming an oxide layer of predetermined thickness and by masking and etching removing the oxide layer from selected regions to provide regions of predetermined size and shape;

bonding the carrier and silicon of the silicon-on-insulator wafer; and

removing the support oxide and wafer leaving the silicon layer to form membranes which defines cavities of predetermined size and shape.

10. An ultrasonic transducer the type having cells with one wall defined by membrane comprising:

a silicon body having a surface forming one wall of the cavity;

a silicon oxide layer configured to define the side walls of said cavities; and

a membrane supported fused to and supported space from the silicon body by said silicon oxide walls to define with said silicon body and said cells.

11. An ultrasonic transducer as in claim 10 wherein said silicon membrane at each of said cells has regions of different thickness.

12. An ultrasonic transducer as in claim 10 wherein said silicon body is configured with regions of different height at each of said cells.

13. An ultrasonic transducer as in claim 10 wherein said silicon membrane at each of said cell has regions of different thicknesses and wherein said silicon body is configured with regions of different height at each of said cavities.

14. An ultrasonic transducer as in claim 10 wherein the single crystal membrane is formed by the silicon layer of a silicon-on-insulator wafer.

15. The method of fabricating a micromachined ultrasonic transducer which comprises the steps of:

micromachining a carrier wafer to form cavities of selected size and shape, said cavities having membrane support walls; and

fusion bonding a membrane of selected material having selected mechanical characteristics to said support walls whereby to form cells.

16. The method of claim 15 which includes the step of forming a piezoelectric transducer on the membrane of each of said cells.

17. The method of claim 15 which includes the step of forming a magnetic transducer on the membrane of each of said cells.

18. The method of claim 15 which includes the step of forming a conductor electrode on said membrane whereby it can be electrostatically vibrated.

19. The method of claim 15 in which the membrane is a layer supported on sacrificial material and the sacrificial material is removed after the fusion bonding.

20. The method of claim 15 in which the layer is a stressed region in a sacrificial material and the unstressed material is removed by thermal shock.

21. The method of fabricating ultrasonic transducers which comprises the steps of defining cavities by micromachining a support structure and fusion bonding under vacuum a membrane to the support structure to seal the cavities and form evacuated cells.

22. The method of claim 21 including the steps of forming piezoelectric transducer to vibrate the membrane.

23. The method of claim 21 including the steps of forming magnetic transducer to vibrate the membrane.

24. The method of claim 21 including the steps of forming electrostatic transducers to vibrate the membrane.