Stress bimorph MEMS switches and methods of making same

• US 20030058069A1
• Filed: 09/19/2002
• Published: 03/27/2003
• Est. Priority Date: 09/21/2001
• Status: Active Grant

• Current Assignee: HRL Laboratories LLC (The Boeing Co.), Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Schaffner, James H., Hsu, Tsung-Yuan, Wu, Ming, Schmitz, Adele E., Schwartz, Robert N., Tangonan, Gregory L., Loo, Robert Y.
• Granted Patent: US 7,053,737 B2
• Time in Patent Office: Days
• US Class Current: 335/78
• CPC Class Codes: H01G 5/18 due to change in inclination, e.g. by flexing, by spiral wrapping
• CPC Class Codes: H01G 5/40 Structural combinations of variable capacitors with other electric elements not covered by this subclass, the structure mainly consisting of a capacitor, e.g. RC combinations
• CPC Class Codes: H01H 1/20 Bridging contacts for circuit breakers H01H73/045
• CPC Class Codes: H01H 2059/0018 Special provisions for avoiding charge trapping, e.g. insulation layer between actuating electrodes being permanently polarised by charge trapping so that actuating or release voltage is altered
• CPC Class Codes: H01H 2059/0072 with stoppers or protrusions for maintaining a gap, reducing the contact area or for preventing stiction between the movable and the fixed electrode in the attracted position
• CPC Class Codes: H01H 2059/0081 with a tapered air-gap between fixed and movable electrodes
• CPC Class Codes: H01H 59/0009 making use of micromechanics

Abstract

A micro-electromechanical system (MEMS) switch formed on a substrate, the switch comprising a transmission line formed on the substrate, a substrate electrostatic plate formed on the substrate, and an actuating portion. The actuating portion comprises a cantilever anchor formed on the substrate and a cantilevered actuator arm extending from the cantilever anchor. Attraction of the actuator arm toward the substrate brings an electrical contact into engagement with the portions of the transmission line separated by a gap, thus bridging the transmission line gap and closing the circuit. In order to maximize electrical isolation between the transmission line and the electrical contact in an OFF-state while maintaining a low actuation voltage, the actuator arm is bent such that the minimum separation distance between the transmission line and the electrical contact is equal to or greater than the maximum separation distance between the substrate electrostatic plate and arm electrostatic plate.

Claims

1. An electromechanical switch comprising:

a substrate;

a first electrical contact formed on said substrate;

a substrate electrostatic plate formed on said substrate;

a cantilever actuator arm anchored to said substrate at a first end of said actuator arm;

a second electrical contact disposed at a second end of said actuator arm, said second electrical contact being in electrically contact with said first electrical contact when said switch is in a closed position;

said substrate electrostatic plate disposed beneath said actuator arm and between said first end and said second end of said actuator arm; and

an arm electrostatic plate formed on said actuator arm and positioned above said substrate electrostatic plate when said switch is in a closed position,

wherein when said switch is in an open position, said actuator arm curves away from said substrate.

2. The switch of claim 1, wherein said first electrical contact has a gap that is closed by the second electrical contact when the switch is in a closed position.

3. The switch of claim 1, wherein said actuator arm comprises polycrystalline silicon.

4. The switch of claim 1, wherein said actuator arm is fabricated such that said actuator arm curves away from said substrate due to nonuniform stresses in said actuator arm.

5. The switch of claim 1, wherein one or more mechanical stops are formed on said substrate, said one or more mechanical stops preventing said actuator arm from contacting said substrate electrostatic plate when said switch is in the closed position.

6. A method of switching electrical energy between an input and an output, the method comprising the steps of:

providing an electrostatically actuated cantilevered arm on a substrate;

applying a voltage to attract said electrostatically actuated cantilevered arm towards said substrate, said cantilevered arm having an electrical contact that electrically connects said input to said output when the voltage is applied; and

removing said voltage or applying a second voltage to cause said cantilevered arm to move away from said substrate, said electrical contact no longer electrically connecting said input to said output when the voltage is removed or the second voltage is applied,

wherein said cantilevered arm curves away from said substrate when the voltage is removed or the second voltage is applied.

7. The method of claim 6, wherein said cantilevered arm curves away from said substrate due to nonuniform stresses in said cantilevered arm.

8. The method of claim 6, wherein a substrate electrostatic plate is disposed on said substrate and an arm electrostatic plate is disposed on said cantilevered arm, said arm electrostatic plate positioned above said substrate electrostatic plate when said cantilevered arm is attracted towards said substrate.

9. The method of claim 8, wherein one or more mechanical stops are disposed proximate to said substrate electrostatic plate, said one or more mechanical stops preventing the cantilevered arm from contacting the substrate electrostatic plate when the cantilevered arm is attracted towards said substrate.

10. The method of claim 6, wherein said electrical contact comprises two or more dimples projecting through said cantilevered arm, said two or more dimples being electrically connected together.

11. A micro-electromechanical switch formed on a substrate, said switch comprising:

a transmission line formed on said substrate, said transmission line having a transmission line gap forming an open circuit;

a substrate electrostatic plate formed on said substrate; and

an actuating portion, said actuating portion comprising;

a cantilever anchor formed on said substrate;

a cantilevered actuator arm extending from said cantilever anchor;

an electrical contact formed on said actuator arm and positioned facing said gap in said transmission line; and

an arm electrostatic plate formed on said actuator arm, said arm electrostatic plate having a first portion formed proximate said cantilever anchor and a second portion extending from said first portion along said actuator arm,

wherein when said switch is in an open position, said actuator arm has a bend such that a minimum separation distance between said transmission line and said electrical contact is equal to or greater than a maximum separation distance between said substrate electrostatic plate and said arm electrostatic plate, said arm electrostatic plate and a segment of said actuator arm on which said arm electrostatic plate is formed defining a structure electrostatically attractable toward said substrate electrostatic plate upon selective application of a voltage to said arm electrostatic plate.

12. The micro-electromechanical switch of claim 11, wherein the electrostatic attraction of said electrostatically attractable structure toward said substrate electrostatic plate causes said electrical contact on said actuator arm to bridge said transmission line gap.

13. The micro-electromechanical switch of claim 11, wherein said substrate comprises a semi-insulating substrate.

14. The micro-electromechanical switch of claim 13, wherein said semi-insulating substrate comprises gallium arsenide (GaAs).

15. The micro-electromechanical switch of claim 11, wherein said actuator arm comprises polycrystalline silicon.

16. The micro-electromechanical switch of claim 11 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said arm electrostatic plate formed on said actuator arm.

17. The micro-electromechanical switch of claim 11 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said actuator arm.

18. The micro-electromechanical switch of claim 11, wherein said substrate electrostatic plate and said transmission line comprise gold microstrips on said substrate.

19. The micro-electromechanical switch of claim 11, wherein said electrical contact comprises a metal selected from the group consisting of gold, platinum, and gold palladium.

20. A method for minimizing capacitive coupling between elements of a micro-electromechanical switch formed on a substrate, said method comprising:

forming a substrate electrostatic plate on said substrate;

forming a transmission line on said substrate, said transmission line having a gap forming an open circuit;

forming an actuating portion, said actuating portion comprising;

a cantilever anchor formed on said substrate; and

a cantilevered actuator arm extending from said cantilever anchor above said substrate, said actuator arm having an arm electrostatic plate formed thereon, said actuator arm having an electrical contact facing said transmission line, wherein when said switch is in an open position said actuator arm has a bend such that a minimum separation distance between said transmission line and said electrical contact is greater than or equal to a maximum separation distance between said substrate electrostatic plate and said arm electrostatic plate,

whereby capacitive coupling between said transmission line and said electrical contact is minimized by maximizing the distance between said transmission line and said electrical contact formed on said actuator arm.

21. The method of claim 20, wherein said substrate comprises a semi-insulating substrate.

22. The method of claim 21, wherein said semi-insulating substrate comprises gallium arsenide (GaAs).

23. The method of claim 20, wherein said actuator arm comprises polycrystalline silicon.

24. The method of claim 20 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said arm electrostatic plate formed on said actuator arm.

25. The method of claim 20 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said actuator arm.

26. The method of claim 20, wherein said substrate electrostatic plate and said transmission line comprise gold microstrips on said substrate.

27. The method of claim 20, wherein said electrical contact comprises a metal selected from the group consisting of gold, platinum, and gold palladium.

28. A method of fabricating an electromechanical switch on a substrate comprising the steps of:

a) applying electrically conductive material to said substrate to form a substrate electrostatic plate region, an input transmission line region, and an output transmission line region, said regions being electrically isolated from one another;

b) depositing one or more sacrificial layers over said regions;

c) forming an electrical contact region on said one or more sacrificial layers, said electrical contact region being applied above said input transmission line region and said output transmission line region and said electrical contact region comprising electrically conductive material;

d) forming an actuating arm on said one or more sacrificial layers, said actuating arm contacting said substrate at a first end and contacting said second layer of electrically conductive material at a second end;

e) forming an arm electrostatic plate region on said actuating arm, said arm electrostatic plate region being disposed above said substrate electrostatic pate region and comprising electrically conductive material; and

f) removing said one or more sacrificial layers,

wherein said steps of forming an actuating arm and/or forming an arm electrostatic plate region are performed to induce nonuniform stress in said actuating arm so that actuating arm will bow upwards when the one or more sacrificial layers are removed.

29. The method of claim 28, wherein the one or more sacrificial layers comprise silicon dioxide.

30. The method of claim 28, wherein the actuating arm comprises polycrystalline silicon.

31. The method of claim 28, wherein said arm electrostatic plate region comprises aluminum.

32. The method of claim 28, wherein said electrical contact region, said input transmission line region, and said output transmission line region comprise gold.

33. A micro-electromechanical switch formed on a substrate comprising:

a substrate transmission line formed on said substrate, said substrate transmission line having a gap forming an open circuit;

a substrate electrostatic plate formed on said substrate; and

an actuating portion, said actuating portion comprising;

a cantilever anchor formed on said substrate;

a cantilevered actuator arm extending from said cantilever anchor, said actuator arm having an upper side and a lower side;

a conducting transmission line formed on said upper side of said actuator arm, said conducting transmission line having one or more conducting dimples projecting through said actuator arm and positioned facing said transmission line gap; and

an arm electrostatic plate formed on said actuator arm, said arm electrostatic plate having a first portion formed proximate said cantilever anchor and a second portion extending from said first portion along said actuator arm,

wherein when said switch is in an open position, said actuator arm has a bend such that a minimum separation distance between said substrate transmission line and said one or more conducting dimples is equal to or greater than a maximum separation distance between said substrate electrostatic plate and said arm electrostatic plate, said arm electrostatic plate and a segment of said actuator arm on which said arm electrostatic plate is formed defining a structure electrostatically attractable toward said substrate electrostatic plate upon selective application of a voltage to said arm electrostatic plate.

34. The micro-electromechanical switch of claim 33, wherein the electrostatic attraction of said electrostatically attractable structure toward said substrate electrostatic plate causes said one or more conducting dimples to contact said substrate transmission line to bridge said gap.

35. The micro-electromechanical switch of claim 33, wherein said substrate comprises a semi-insulating substrate.

36. The micro-electromechanical RF switch of claim 35, wherein said semi-insulating substrate comprises gallium arsenide (GaAs).

37. The micro-electromechanical switch of claim 33, wherein said actuator arm comprises polycrystalline silicon.

38. The micro-electromechanical switch of claim 33 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said arm electrostatic plate formed on said actuator arm.

39. The micro-electromechanical switch of claim 33 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said actuator arm.

40. The micro electromechanical switch of claim 33, wherein said substrate electrostatic plate and said transmission line comprise gold microstrips on said substrate.

41. The micro-electromechanical switch of claim 33, wherein said one or more conducting dimples comprise a metal selected from the group consisting of gold, platinum, and gold palladium.

43. A method of fabricating an electromechanical switch on a substrate comprising the steps of:

a) forming a first layer of electrically conductive material over a surface of said substrate, said first layer comprising an arm plate contact region, a substrate electrostatic plate region, an input transmission line region, and an output transmission line region, wherein each of said regions is electrically isolated from other said regions;

b) depositing a sacrificial layer over said first layer of electrically conductive material;

c) forming an arm structural layer, said arm structural layer providing a cantilever with a proximate end and a distal end, said distal end located above said input transmission line region and above said output transmission line region, said cantilever having a cantilever anchor and a cantilever arm, said cantilever anchor formed at said proximate end and located above said arm plate contact region, said cantilever arm projecting from said cantilever anchor to said distal end and located above said substrate electrostatic plate region;

d) removing a portion of said arm structural layer at said distal end to create a dimple area above said input transmission line region and said output transmission line region;

e) removing a portion of said arm structural layer at said cantilever anchor to expose said arm plate contact region;

f) depositing a layer of metal on said arm structural layer, said layer of metal filling said dimple area and said layer of metal providing an arm electrostatic plate electrically connected to said arm plate contact region, said arm electrostatic plate located generally above said substrate electrostatic plate region; and

g) removing said sacrificial layer,

wherein the step of forming the arm structural layer is performed to induce stress in the arm structural layer that varies from the stress of the layer of metal deposited on the arm structural layer so that the cantilever arm will bow upwards when the sacrificial layer is removed.

44. The method of claim 43, wherein said sacrificial layer comprises a layer of silicon dioxide.

45. The method of claim 43 wherein said arm structural layer comprises silicon nitride.

46. The method of claim 43, wherein said step of depositing a layer of metal on said arm support layer comprises the steps of:

sputter depositing a thin film of titanium; and

depositing a layer of gold.

47. The method of claim 43, wherein the step of removing the sacrificial layer is performed by wet etching the sacrificial layer.

48. A micro-electromechanical switch formed on a substrate comprising:

a substrate transmission line formed on said substrate, said substrate transmission line having a gap forming an open circuit;

a substrate electrostatic plate formed on said substrate; and

an actuating portion, said actuating portion comprising;

a cantilever anchor formed on said substrate;

a cantilevered actuator arm having a proximate end and a distal end, said cantilevered actuator arm attached to the cantilever anchor at the proximate end of the cantilevered actuator arm

a cantilever insulating layer attached at the distal end of the cantilevered actuator arm;

an electrical contact formed on said cantilever insulating layer and positioned facing said transmission line gap; and

an arm electrostatic plate formed on said actuator arm, said arm electrostatic plate having a first portion formed proximate said cantilever anchor and a second portion extending from said first portion along said actuator arm,

wherein when said switch is an open position, said actuator arm has a bend such that a minimum separation distance between said transmission line and said electrical contact is equal to or greater than a maximum separation distance between said substrate electrostatic plate and said arm electrostatic plate, said arm electrostatic plate and a segment of said actuator arm on which said arm electrostatic plate is formed defining a structure electrostatically attractable toward said substrate electrostatic plate upon selective application of a voltage to said arm electrostatic plate.

49. The micro-electromechanical switch of claim 48, said switch further comprising one or more mechanical stops formed on said substrate, said mechanical stops disposed adjacent said substrate electrostatic plate and having a height greater than a height of the substrate electrostatic plate

50. The micro electromechanical switch of claim 48, wherein the electrostatic attraction of said electrostatically attractable structure toward said substrate electrostatic plate causes said electrical contact on said actuator arm to bridge said transmission line gap.

51. The micro electromechanical switch of claim 48, wherein said substrate comprises a silicon substrate with a layer of silicon nitride.

52. The micro electromechanical switch of claim 48, wherein said actuator arm comprises polycrystalline silicon.

53. The micro electromechanical switch of claim 48 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said arm electrostatic plate formed on said actuator arm.

54. The micro-electromechanical switch of claim 48 wherein said bend in said actuator arm is produced by inducing a nonuniform level of residual stress in said actuator arm.

55. The micro-electromechanical switch of claim 48, wherein said substrate electrostatic plate comprises polysilicon.

56. The micro-electromechanical switch of claim 48, wherein said electrical contact comprises a metal selected from the group consisting of gold, platinum, and gold palladium.

57. The micro-electromechanical switch of claim 48, wherein said arm electrostatic plate comprises a metal selected from the group consisting of gold, platinum, and gold palladium.

58. The micro electromechanical switch of claim 48, wherein said cantilever insulating layer comprises silicon nitride.

59. A method of fabricating an electromechanical switch on a substrate comprising the steps of:

a) depositing a first layer of electrically conductive material over a surface of said substrate;

b) depositing a sacrificial cantilever support layer over said first layer of electrically conductive material;

c) forming a arm structural layer of electrically conductive material over said sacrificial cantilever support layer, said arm structural layer having a proximate end and a distal end, said arm structural layer having a cantilever anchor disposed at said proximate end, said cantilever anchor formed on said surface of said substrate, and said arm structural layer having a cantilever arm projecting from said cantilever anchor;

d) depositing a first metal layer on said cantilever arm, said first metal layer located generally above said first layer of electrically conductive material;

e) depositing a second metal layer on or above said substrate to form a transmission line with a gap in the middle, said second metal layer located adjacent said distal end;

g) depositing a conductor sacrificial layer on top of said second metal layer;

h) depositing a third metal layer on top of said conductor sacrificial layer, said third metal layer positioned above said second metal layer and extending across said gap;

i) forming an insulating layer above said third metal layer and located adjacent said distal end, said insulating layer attaching to said distal end and to said third metal layer; and

j) removing said sacrificial layers,

wherein the step of forming the arm structural layer is performed to induce stress in the arm structural layer that varies from the stress of the first layer of metal so that the cantilever arm will bow upwards when said sacrificial layers are removed.

60. The method of claim 59 further comprising the step of:

forming a mechanical stop layer over said surface of said substrate, said mechanical stop layer disposed proximate to said first layer of electrically conductive material and having a height greater than a height of said first layer of electrically conductive material,

wherein the step of forming a mechanical stop layer being performed prior to the step of depositing a sacrificial cantilever support layer.

61. The method of claim 59, wherein said first layer of electrically conductive material comprises polysilicon.

62. The method of claim 60, wherein said step of forming a mechanical stop layer comprises the steps of:

depositing a mechanical stop sacrificial layer on top of said first layer of electrically conductive material;

etching said mechanical stop sacrificial layer to form areas for the mechanical stops; and

filling the areas for the mechanical stops with polysilicon to form said mechanical stops.

63. The method of claim 59, wherein said arm structural layer comprises polysilicon.

64. The method of claim 59, wherein said insulating layer comprises silicon nitride.