Method and apparatus for reducing charge injection in control of MEMS electrostatic actuator array

• US 6,970,031 B1
• Filed: 05/28/2004
• Issued: 11/29/2005
• Est. Priority Date: 05/28/2004
• Status: Expired due to Fees

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Inventors: Martin, Eric T., Piehl, Art, Ghozeil, Adam
• Primary Examiner(s): Zweizig, Jeffrey
• Application Number:
  US10/855,359
• Publication Number:
  US 20050264340A1
• Time in Patent Office: 550 Days
• Field of Search: 327/108, 327/109, 327/110, 327/111, 327/112, 327/309, 327/320, 327/325, 327/379, 327/380, 327/381, 327/382, 327/384, 327/389
• US Class Current: 327/382
• CPC Class Codes: G09G 2300/0809 Several active elements per pixel in active matrix panels
• CPC Class Codes: G09G 2300/0814 used for selection purposes, e.g. logical AND for partial update
• CPC Class Codes: G09G 2300/0838 with level shifting
• CPC Class Codes: G09G 2300/088 using a non-linear two-terminal element
• CPC Class Codes: G09G 2320/0219 Reducing feedthrough effects in active matrix panels, i.e. voltage changes on the scan electrode influencing the pixel voltage due to capacitive coupling
• CPC Class Codes: G09G 3/3433 using light modulating elements actuated by an electric field and being other than liquid crystal devices and electrochromic devices using liquid crystal devices G09G3/36; using electrochromic devices G09G3/38
• CPC Class Codes: G09G 3/3466 based on interferometric effect

Abstract

A control circuit for a MEMS (Micro-Electro-Mechanical System) has a semiconductor switch which has a source, a drain and a gate, which is associated with a selected one of spatially arranged fixed and movable plates of a variable capacitor, and is arranged to selectively connect the selected one of the fixed and movable plates with a voltage source. A charge injection control circuit is associated with the semiconductor switch and attenuates current injection into the selected one of the fixed and movable plates of the capacitor.

Claims

1. A control circuit for a MEMS (Micro-Electro-Mechanical System) comprising:

a variable capacitor having a fixed plate and movable plate disposed in predetermined spatial relationship with respect to the fixed plate;

a semiconductor switch which has a source, a drain and a gate, which is associated with a selected one of the fixed and movable plates of the capacitor and which is arranged to selectively connect the selected one of the fixed and movable plates with a voltage source; and

a charge injection control circuit associated with the semiconductor switch which attenuates current injection into the selected one of the fixed and movable plates of the capacitor.

2. A control circuit as set forth in claim 1, wherein the charge injection control circuit comprises:

first and second semiconductor elements which are circuited with a gate of the semiconductor switch and which modify a gate signal which is applied to the gate of the semiconductor switch in a manner wherein at least one of;

a) a voltage variation time of the gate signal is set so that accumulated charge can predominantly drain from a channel of the semiconductor switch to the source when the semiconductor switch is closing, and

b) the voltage of the signal which is applied to the gate is limited to limit the degree to which the semiconductor switch enters into an inversion region and/or an accumulation region.

3. A control circuit as set forth in claim 2, wherein the first and second semiconductor elements are first and second MOSFET transistors wherein the drains are both connected to the gate of the semiconductor switch and wherein the sources are respectively connected to a source of reference voltage and ground respectively.

4. A control circuit as set forth in claim 3, wherein the first and second MOSFET transistors have gates which are respectively connected with sources of signals which respectively control the opening and closing of the semiconductor switch.

5. A control circuit as set forth in claim 2, wherein the first and second semiconductor elements are third and fourth MOSFET transistors wherein drains of the third and fourth MOSFET transistors are both connected to the gate of the semiconductor switch and which have sources which are respectively connected to a source of reference voltage and a source of a predetermined high voltage.

6. A control circuit as set forth in claim 5, wherein the third and fourth MOSFET transistors have gates which are respectively connected with sources of signals which respectively control the opening and closing of the semiconductor switch.

7. A control circuit as set forth in claim 6, wherein the first and second semiconductor elements are fifth and sixth MOSFET transistors wherein drains of the fifth and sixth MOSFET transistors are both connected to the gate of the semiconductor switch wherein the sources of the fifth and sixth MOSFET transistors are respectively connected sources of voltage which are limited to respectively limit the degree to which the semiconductor switch enters into an inversion region and/or an accumulation region.

8. A control circuit as set forth in claim 2, wherein the first and second semiconductor elements are first and second diodes which are connected in parallel between a source of reference voltage and the gate of the semiconductor switch which is connected with a voltage source which controls the opening and closing of the semiconductor switch via a resistor.

9. A control circuit as set forth in claim 8, wherein the first and second diodes are configured to permit current flow in opposite directions.

10. A control circuit as set forth in claim 1, wherein the charge control circuit comprises a capacitance load which is circuited in parallel with the gate of the semiconductor switch.

11. A control circuit as set forth in claim 10, wherein the capacitance load is interposed between the gate of the semiconductor switch and a voltage control circuit which controls the application of a voltage signal to the gate of the semiconductor switch.

12. A control circuit as set forth in claim 11, wherein the voltage control circuit comprises a voltage level-shifter circuit.

13. A control circuit as set forth in claim 12, wherein the voltage level-shifter circuit is connected with a voltage source having a first voltage level and a source of a control signal which has a voltage lower than the first voltage and which determines the opening and closing of the semiconductor switch and wherein an output of the voltage level-shifter circuit is connected with the capacitance load.

14. A control circuit as set forth in claim 1, further comprising a capacitance load which is interposed between the charge injection circuit and a voltage level-shifter circuit which is configured to step up a first voltage of a control signal to a second higher voltage.

15. A display device comprising:

a plurality of variable capacitors each having a fixed plate and a movable plate disposed in predetermined spatial relationship with respect to the fixed plate;

a plurality of semiconductor switches each associated with a selected one of the fixed and movable plates of the capacitors and which is arranged to selectively connect the selected one of the fixed and movable plates with a voltage source; and

a plurality of charge injection control circuits each associated with a semiconductor switch for attenuating charge injection into the selected one of the fixed and movable plates of the respective capacitor when the semiconductor switch is closing.

16. A display as set forth in claim 15, wherein the movable plate is at least partially transparent and the fixed plate is reflective so that light can be subjected to interference or diffraction in accordance with the variable distance between the fixed and movable plates.

17. A display as set forth in claim 15, wherein the plurality of charge injection control circuits each comprise:

first and second semiconductor elements which are circuited with a gate of a semiconductor switch, and which modify a gate signal which is applied to the gate in a manner wherein at least one of;

a) a voltage variation time of the gate signal is set so that current can predominantly drain from a channel of the semiconductor switch to the source when the semiconductor switch is closing, and

b) the voltage of the signal which is applied to the gate has a voltage close to and in excess of a threshold voltage at which a conduction state of the semiconductor switch is changes.

18. A method of making a control circuit for a MEMS (Micro-Electro-Mechanical System) comprising:

forming a variable capacitor having a fixed plate and movable plate disposed in predetermined spatial relationship with respect to the fixed plate;

forming a semiconductor switch which has a source, a drain and a gate, which is associated with a selected one of the fixed and movable plates of the capacitor and which is arranged to selectively connect the selected one of the fixed and movable plates with a voltage source;

forming a circuit associated with the semiconductor switch for attenuating current injection into the selected one of the fixed and movable plates of the capacitor, said circuit comprising;

first and second semiconductor elements which are circuited with a gate of the semiconductor switches and which modify a gate signal which is applied to the gate in a manner wherein at least one of;

a) a voltage variation time of the gate signal is set so that current can predominantly drain from a channel of the semiconductor switch to the source when the semiconductor switch is closing, and

b) the voltage of the signal which is applied to the gate is limited to limit the degree to which the semiconductor switch enters into an inversion region and/or an accumulation region.

19. A control circuit for a MEMS (Micro-Electro-Mechanical System) comprising:

variable capacitor means having a fixed plate and movable plate disposed in predetermined spatial relationship with respect to the fixed plate, for operative association with and motivating an arrangement associated with the MEMS;

semiconductor switch means which has a source, a drain and a gate, which is associated with a selected one of the fixed and movable plates of the capacitor for selectively connecting the selected one of the fixed and movable plates with a voltage source and for inducing a change in distance between the fixed and movable plates;

circuit means associated with the semiconductor switch for attenuating current injection into the selected one of the fixed and movable plates of the capacitor.

20. A control circuit as set forth in claim 19, wherein the circuit means comprises:

first and second semiconductor elements which are circuited with a gate of the semiconductor switches and which modify a gate signal which is applied to the gate in a manner wherein at least one of;

a) a voltage variation time of the gate signal is set so that current can predominantly drain from a channel of the semiconductor switch to the source when the semiconductor switch is closing, and

b) the voltage of the signal which is applied to the gate is limited to limit the degree to which the semiconductor switch enters into an inversion region and/or an accumulation region.