Capacitive MEMS device with programmable offset voltage control

• US 7,978,395 B2
• Filed: 05/27/2010
• Issued: 07/12/2011
• Est. Priority Date: 11/12/2007
• Status: Expired due to Fees

• Current Assignee: Snaptrack Incorporated (Qualcomm, Inc.)
• Original Assignee: Qualcomm MEMS Technologies Incorporated (Qualcomm, Inc.)
• Inventors: Felnhofer, Daniel, Gousev, Evgeni
• Primary Examiner(s): Choi; William C
• Application Number:
  US12/789,195
• Publication Number:
  US 20100238537A1
• Time in Patent Office: 411 Days
• Field of Search: 359/199.2, 359/200.6, 359/230, 359290-292, 359/302
• US Class Current: 359/290
• CPC Class Codes: B81B 2201/045 Optical switches
• CPC Class Codes: B81B 3/0086 Electrical characteristics, e.g. reducing driving voltage, improving resistance to peak voltage
• CPC Class Codes: G02B 26/001 based on interference in an adjustable optical cavity interference filters G02B5/28; devices or arrangements using multiple reflections in spectrometry or monochromators G01J3/26
• CPC Class Codes: G11C 23/00 Digital stores characterised by movement of mechanical parts to effect storage, e.g. using balls; Storage elements therefor

Abstract

A capacitive MEMS device is formed having a material between electrodes that traps and retains charges. The material can be realized in several configurations. It can be a multilayer dielectric stack with regions of different band gap energies or band energy levels. The dielectric materials can be trappy itself, i.e. when defects or trap sites are pre-fabricated in the material. Another configuration involves a thin layer of a conductive material with the energy level in the forbidden gap of the dielectric layer. The device may be programmed (i.e. offset and threshold voltages pre-set) by a method making advantageous use of charge storage in the material, wherein the interferometric modulator is pre-charged in such a way that the hysteresis curve shifts, and the actuation voltage threshold of the modulator is significantly lowered. During programming phase, charge transfer between the electrodes and the materials can be performed by applying voltage to the electrodes (i.e. applying electrical field across the material) or by UV-illumination and injection of electrical charges over the energy barrier. The interferometric modulator may then be retained in an actuated state with a significantly lower actuation voltage, thereby saving power.

Claims

1. A method of addressing a microelectromechanical (MEMS) element that comprises an electrode, a movable layer, and a material for trapping charge, the method comprising:

transferring charge to or from the material to configurably change an actuation threshold voltage of the element to a target level in response to actuation of the element.

2. The method of claim 1, wherein said MEMS element exhibits hysteresis, and wherein said transferred charge is sufficient to shift a center of said hysteresis to a desirable position.

3. The method of claim 1, wherein the target level is near zero volts.

4. The method of claim 1, further comprising actuating said MEMS element to move said movable layer toward said material.

5. The method of claim 1, wherein the charge is transferred by an electrostatic method.

6. The method of claim 1, wherein the charge is transferred by exciting electrons using ultraviolet illumination.

7. The method of claim 1, wherein the charge is transferred such that an initial offset voltage of the MEMS element is set to a target level.

8. The method of claim 1, wherein the charge is transferred such that an offset voltage of the MEMS element is set to a target level during operation.

9. A method of manufacturing a microelectromechanical (MEMS) device, the method comprising:

forming an electrode;

forming a movable layer extending over the electrode and forming a cavity therebetween; and

forming a material for trapping charge, the material being coupled to one or both the electrode and the movable layer,

wherein the movable layer is configured to move in response to an electrical potential between the electrode and at least a portion of the movable layer, the electrical potential being greater than an actuation threshold voltage, and wherein charge is transferred to or from the material to configurably change the actuation threshold voltage to a target level in response to actuation of the device.

10. The method of claim 9, wherein forming the movable layer comprises forming an interferometric cavity.

11. The method of claim 9, wherein the dielectric layer comprises process induced defects throughout the layer.

12. The method of claim 9, wherein the material is a multi-layer material comprising a first dielectric layer, a second dielectric layer, and an additional material layer, and wherein the additional material layer has a conduction band that is offset from the conduction band of the first and/or second dielectric layers.

13. The method of claim 12, wherein the additional material layer is situated between the first and second dielectric layers, the first and second dielectric layers having a thickness of about 50 to 10000 Angstroms and the additional material layer having a thickness of about 2 to 200 Angstroms.

14. The method of claim 9, wherein the material comprises a dielectric layer with introduced charge trapping sites.

15. The method of claim 14, wherein the charger trapping sites are introduced at a particular depth within the dielectric layer.

16. The method of claim 14, wherein the charger trapping sites are formed by implanted dopant atoms.

17. The method of claim 9, wherein the material is a multi-layer material comprising a first insulating layer, a second insulating layer, and a charge trapping material located between the first and second insulating layer, the charge trapping material forming a potential well for charge carriers.

18. The method of claim 17, wherein the charge trapping material has a Fermi energy level below the conduction band of the first and second insulating layers.

19. The method of claim 17, wherein the charge trapping material has a bottom of its conduction band lower than the first and second insulating layers.

20. The method of claim 17, wherein the charge trapping material has a top of its valence band higher than the first and second insulating layers.

21. The method of claim 9, wherein the material is a multi-layer material comprising a first material and at least one other material with an energy level below the conduction band of the first material.

22. The method of claim 9, wherein the material is a multi-layer material comprising a first material and at least one other material with an electron energy band gap that is less than the electron energy band gap of the first material.

23. The method of claim 22, wherein the first material comprises a dielectric or insulator material.

24. The method of claim 22, wherein the first material comprises at least one of a silicon nitride, a silicon dioxide, a metal oxide, or a silicate.

25. The method of claim 22, wherein the at least one other material comprises a metal, a semi-conductor, or a dielectric material.

26. The method of claim 9, wherein the material comprises regions of different electron energy band gaps.