Capacitive micromachined ultrasonic transducer (CMUT) with varying thickness membrane

• US 20070215964A1
• Filed: 02/16/2007
• Published: 09/20/2007
• Est. Priority Date: 02/28/2006
• 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, Ergun, Arif, Hansen, Sean, Yaralioglu, G.
• Granted Patent: US 7,615,834 B2
• Time in Patent Office: Days
• US Class Current: 257/414
• CPC Class Codes: B06B 1/0292 Electrostatic transducers, e.g. electret-type
• CPC Class Codes: B81B 2201/0257 Microphones or microspeakers
• CPC Class Codes: B81B 2203/0127 Diaphragms, i.e. structures separating two media that can control the passage from one medium to another; Membranes, i.e. diaphragms with filtering function
• CPC Class Codes: B81B 3/007 For controlling stiffness, e.g. ribs
• CPC Class Codes: B81B 3/0086 Electrical characteristics, e.g. reducing driving voltage, improving resistance to peak voltage
• CPC Class Codes: H01L 28/40 Capacitors

Abstract

Structure for capacitive micromachined ultrasonic transducer (CMUT) device or other vibrating membrane device having non-uniform membrane so that membrane mass and stiffness characteristics may be substantially independently adjusted. CMUT having trenched membrane and/or membrane with non-uniform thickness or density. Method for operating transducer or vibrating membrane device. Array of devices at least some of which have non-uniform membrane properties. CMUT comprising substrate, support for membrane, and membrane extending over support to create cavity, membrane having non-uniform membrane thickness resulting from at least one of: thickening on upper surface of the membrane outside of cavity, thickening on lower surface of membrane inside cavity, trench on upper surface of membrane, trench on lower surface of the membrane, and any combination of two or more of these. Method for fabricating CMUT or vibrating membrane device having non-uniform membrane. High mechanical sensitivity transducer for sensor, microphone, and/or transmitter.

Claims

1. A capacitive micro-machined ultrasonic transducer (CMUT) comprising:

a substrate;

a support having a base in contact with the substrate, and a wall extending away from the base to a top; and

a membrane having an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;

the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of;

(i) a thickening on the upper surface of the membrane outside of the cavity, (ii) a thickening on the lower surface of the membrane inside of the cavity, (iii) a trench on an upper surface of the membrane, (iv) a trench on a lower surface of the membrane, and (v) any combination of two or more of these.

2. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the substrate is formed of a silicon material;

the support is formed of an oxide material; and

the membrane is formed of a silicon material.

3. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the non-uniformity in the membrane thickness resulting from at least one of:

(i) a thickening on the upper surface of the membrane outside of the cavity, and (ii) a thickening on the lower surface of the membrane inside of the cavity.

4. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the non-uniformity in the membrane thickness resulting from:

a thickening on at least one of the upper surface and the lower surface of the membrane, and at least one trench on an upper or lower surface of the membrane.

5. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the thickness of the membrane is made non-uniform in a region over the gap or cavity formed between the membrane and the substrate.

6. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane thickness and thickness profile, the lateral membrane dimensions (or diameter or radius), and the membrane material properties determine the membrane stiffness and membrane mass and determine capacitance, collapse voltage, and operating frequency.

7. A capacitive micro-machined ultrasonic transducer as in claim 6, wherein both the membrane stiffness and the membrane mass increase as the lateral dimensions and thickness increase and the membrane stiffness and the membrane mass are modified to some extent independently of each other by providing the non-uniform membrane thickness to achieve an optimum transducer performance.

8. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane has a central portion and the central portion of the membrane is made thicker than portions outside the central portion so that the transducer generated more transducer output pressure in operation than a transducer of the same membrane lateral dimension not having a thickened central portion.

9. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane thickness, the membrane lateral dimensions (or diameter or radius), and the membrane material properties determine the stiffness and the mass of the membrane and therefore determine transducer device parameters.

10. A capacitive micro-machined ultrasonic transducer as in claim 9, wherein the device parameters include at least one of capacitance, collapse voltage, and operation frequency.

11. A capacitive micro-machined ultrasonic transducer as in claim 10, wherein membrane stiffness and the membrane mass increase as the lateral dimensions and thickness increase, and a non-uniform membrane thickness is used so that the capacitance, collapse voltage, and operation frequency parameters can be adjusted substantially individually, separately, and independently.

12. A capacitive micro-machined ultrasonic transducer as in claim 11, wherein this separate, individually, and independent adjustability enables setting at least two transducer parameters to be adjusted substantially independent of each other.

13. A capacitive micro-machined ultrasonic transducer as in claim 12, wherein the at least two transducer parameters that may be adjusted substantially independently of each other are the collapse voltage and the operation frequency.

14. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the thicker central membrane portion is characterized by one of (i) the membrane being thicker or having added mass above a nominal lateral plane of the membrane, (ii) the membrane being thicker or having added mass below a nominal lateral plane of the membrane, and (iii) the membrane being thicker or having added mass above and below a nominal lateral plane of the membrane.

15. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the non-uniform membrane thickness is structured to achieve at least one of:

(i) control the average membrane displacement, (ii) control maximize the average membrane displacement, and (iii) maximize the membrane displacement.

16. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane has a non-uniform thickness profile that is structured to provide a more uniform harmonic vibration amplitude at the central portion of the membrane and at the membrane peripheral portion close to the edge of the membrane adjacent the support wall.

17. A capacitive micro-machined ultrasonic transducer as in claim 1, the non-uniformity in the membrane thickness resulting from at least one of:

(i) a thickening on the upper surface of the membrane outside of the cavity, and (ii) a thickening on the lower surface of the membrane inside of the cavity;

the thickening resulting in an increased mass toward the center of the membrane and the increased mass resulting in an increased capacitance of the CMUT device.

18. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane has a membrane thickness profile or membrane shape that is modified from uniform thickness or shape to modify membrane displacement characteristics to more nearly equalize the membrane displacement proximate the center of the membrane and near the peripheral edge of the membrane.

19. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane includes at least one of a trench on an upper surface of the membrane, and a trench on a lower surface of the membrane, and operates as a sensing microphone device having increased mechanical sensitivity.

20. A capacitive micro-machined ultrasonic transducer as in claim 19, wherein the at least one of a trench is a trench on a lower surface of the membrane within a sealed cavity, and the transducer is adapted for use in liquid immersion environment as either a sensing microphone input device having increased mechanical sensitivity or as output device.

21. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein a constant thickness membrane includes at least one trenched portion that has a membrane thickness that is thinner than the membrane thickness in untrenched portions of the membrane.

22. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein the membrane thickness in untrenched portions of the membrane is substantially constant.

23. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein the membrane thickness in untrenched portions of the membrane is non-uniform and includes thickened portions.

24. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein at least one trenched portion comprises an annular trench having an inner annular radius and an outer annular radius.

25. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the width of the trench, its location with respect to the edge of the membrane, and its depth are determined by the desired sensitivity and resonant frequency of the membrane.

26. A capacitive micro-machined ultrasonic transducer as in claim 24, wherein at least one trench comprises a trench having a U-shaped cross-section, a rectangular-shaped cross-section, or a circular-shaped cross-section, or a v-shaped cross-section.

27. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein at least one trench comprises a plurality of trenches.

28. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein there are a plurality of supports and membranes formed on the substrate.

29. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein the trenches are formed on at least one of:

(i) the upper surfaces of the membrane outside of the cavity, (ii) the lower surfaces of the membrane within the cavity, and (iii) both the upper surfaces of the membrane and on the lower surfaces of the membrane within a cavity in the transducer device.

30. A capacitive micro-machined ultrasonic transducer as in claim 29, wherein for transducers having trenches on both upper and lower portions of the membrane, the trenches are formed at the same or substantially the same locations on the upper and lower portions of the membrane so that they overlie (or underlie) each other.

31. A capacitive micro-machined ultrasonic transducer as in claim 29, wherein for transducers having trenches on both upper and lower portions of the membrane, the trenches are formed at different locations on the upper and lower portions of the membrane so that they do not overlie (or underlie) each other.

32. A capacitive micro-machined ultrasonic transducer as in claim 29, wherein the trenches comprises annular trenches formed in a circular membrane, and the different locations include locations at different radii from the center of a circular membrane.

33. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein trenches are formed having a trench width and a trench depth, and the trench width and trench depth for any may be independently varied.

34. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein a plurality of trenches are formed in the membrane and each of the plurality of trenches are formed having a trench width and a trench depth, and the trench width and trench depth for any one particular trench may be varied independently of the trench width and trench depth for any other particular trench.

35. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the device includes at least one trench and at least one additional thickened membrane portion.

36. A capacitive micro-machined ultrasonic transducer as in claim 35, wherein the at least one additional thickened portion includes a first thickened portion on or adjacent to an upper surface of the membrane.

37. A capacitive micro-machined ultrasonic transducer as in claim 35, wherein the at least one additional thickened portion includes a second thickened portion on or adjacent to a lower surface of the membrane and disposed within the cavity.

38. A capacitive micro-machined ultrasonic transducer as in claim 35, wherein the at least one additional thickened portion includes a first thickened portion on or adjacent to an upper surface of the membrane, and a second thickened portion on or adjacent to a lower surface of the membrane and disposed within the cavity.

39. A capacitive micro-machined ultrasonic transducer as in claim 35, wherein the first thickened portion and the second thickened portion overlie or substantially overly each other on opposite surfaces of the membrane.

40. A capacitive micro-machined ultrasonic transducer as in claim 35, wherein the first thickened portion and the second thickened portion do not overlie or substantially overly each other on opposite surfaces of the membrane.

41. A capacitive micro-machined ultrasonic transducer as in claim 35, wherein the first thickened portion and the second thickened portion partially overly each other on opposite surfaces of the membrane.

42. A capacitive micro-machined ultrasonic transducer as in claim 41, wherein the overlying portions comprise a circular or annular overlying.

43. A capacitive micro-machined ultrasonic transducer as in claim 21, wherein the membrane is substantially circular, and the first and second thickened portions are substantially circular, and the first thickened portion has a first radius measured from the center of the membrane, the second thickened portion has a second radius measured from the center of the membrane, and the circular membrane has a third radius measured from the center of the membrane.

44. A capacitive micro-machined ultrasonic transducer as in claim 43, wherein the first radius is equal to the second radius, and the first and second radius are less than the third radius.

45. A capacitive micro-machined ultrasonic transducer as in claim 43, wherein the first radius is greater than the second radius, and the first and second radius are less than the third radius.

46. A capacitive micro-machined ultrasonic transducer as in claim 43, wherein the first radius is less than the second radius, and the first and second radius are less than the third radius.

47. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane thickness profile adapted so that the membrane spring constant or membrane stiffness is defined substantially independently of the membrane mass.

48. A capacitive micro-machined ultrasonic transducer as in claim 47, wherein the independent specification of membrane, stiffness and membrane mass provides a transducer device having an average membrane displacement and corresponding output pressure and a greater transducer device sensitivity as compared to conventional transducer devices.

49. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the membrane is a substantially circular membrane, and comprises a plurality of substantially concentric annular or circular trenches disposed near the outer circular edge of the membrane;

and the substantially concentric annular or circular trenches disposed near the outer edge of the membrane are disposed within an annular regions that has a radius of between about 50% and 100% of the radius of the membrane.

50. A capacitive micro-machined ultrasonic transducer as in claim 1, wherein the provision at least one trench reduces the stiffness of the membrane while keeping the total membrane mass substantially the same as a membrane without a trench;

and the total mass of the trenched membrane is within substantially 10% of the mass of the untrenched membrane.

51. A capacitive micro-machined ultrasonic transducer device comprising:

a membrane supported by support structures or posts, the membrane having a nominal uniform thickness extending between the support structures or posts except for first and second trenches wherein some membrane material is absent from or has been removed from the upper surface of the membrane outside of a cavity formed between the membrane and the substrate; and

at least one of;

(i) a first thickened portion formed on or integral with the membrane on the inner surface of the membrane within the transducer cavity and having a first lateral dimension and a first thickness, and (ii) a second thickened portion formed on or integral with the membrane on the outer surface of the membrane outside of the transducer cavity and having a second lateral dimension and a second thickness.

52. A capacitive micro-machined ultrasonic transducer device as in claim 51, wherein the first lateral dimension is greater than the second lateral dimension and the first thickness is less than the second thickness.

53. A capacitive micro-machined ultrasonic transducer device as in claim 51, wherein both the first and second thickened portions are defined between the two trenches on either side.

54. A capacitive micro-machined ultrasonic transducer device as in claim 51, wherein the relative lateral and thickness dimensions of the first and second thickened portions in the structure may be modified to provide the desired membrane static and/or dynamic transducer characteristics.

55. A capacitive micro-machined ultrasonic transducer device as in claim 51, wherein the first thickened portion lateral dimension may be any of less-than, greater-than, or equal to the second thickened portion lateral dimension;

and, that the first thickened portion thickness dimension may be any of less-than, greater-than, or equal to the second thickened portion thickness dimension.

56. A capacitive micro-machined ultrasonic transducer device as in claim 51, wherein the at least one of the first and second thickened portions comprise a metal layer deposited by an electro-deposition process.

57. A capacitive micro-machined ultrasonic transducer (CMUT) array comprising:

a plurality of CMUT elements or cells disposed as an array;

each CMUT element or cell formed on a substrate and each CMUT element or cell further comprising;

a support having a base in contact with the substrate, and a wall extending away from the base to a top; and

a membrane having an upper surface and a lower surface and extending in a lateral dimension parallel to the support and between the support wall to create a cavity defined by the substrate, the wall, and the membrane lower surface and a defining a gap between the substrate and the membrane;

the membrane having a non-uniform membrane thickness extending in a direction orthogonal to the membrane surface, the non-uniformity in the membrane thickness resulting from at least one of;

(i) a thickening on the upper surface of the membrane outside of the cavity, (ii) a thickening on the lower surface of the membrane inside of the cavity, (iii) a trench on an upper surface of the membrane, (iv) a trench on a lower surface of the membrane, and (v) any combination of two or more of these.

58. A method for designing a capacitive micro-machined ultrasonic transducer (CMUT) comprising:

specifying a CMUT membrane mass characteristic to achieve a first performance metric;

specifying a CMUT membrane stiffness characteristic to achieve a second performance metric, the specification of the membrane mass and thickness being independent of each other; and

determining a CMUT membrane structure having a non-uniform membrane thickness using the specified membrane mass and membrane stiffness characteristics.

59. A method as in claim 58, further comprising including at least one trench in at least one of the steps of specifying the membrane mass characteristic and the membrane stiffness characteristic, and determining the CMUT membrane structure having a non-uniform membrane thickness using the specified membrane mass and membrane stiffness characteristics and a trench.