Silicon spatial light modulator

• US 5,170,283 A
• Filed: 07/24/1991
• Issued: 12/08/1992
• Est. Priority Date: 07/24/1991
• Status: Expired due to Term

• Current Assignee: Northrop Corporation
• Original Assignee: Northrop Corporation
• Inventors: Ionescu, Adrian C., Hu, King L., Burns, Brent E., O'Brien, Benedict B.
• Primary Examiner(s): LaRoche, Eugene R.
• Assistant Examiner(s): Lester, Evelyn A.
• Application Number:
  US07/735,392
• Time in Patent Office: 503 Days
• Field of Search: 359/230, 359/212, 359/224, 359/317, 359/318, 359/291-292, 359/295, 359/279, 358/233, 358/206, 358/234, 346/160
• US Class Current: 359/291
• CPC Class Codes: G02B 26/0825 the reflecting element being a flexible sheet or membrane, e.g. for varying the focus flexible mirrors for cosmetic use A45D42/24
• CPC Class Codes: G02B 26/0833 the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD G02B26/0825 takes precedence; micromechanical devices in general B81B
• CPC Class Codes: G02B 26/0841 the reflecting element being moved or deformed by electrostatic means

Abstract

A spatial light modulator including a silicon backplate having an insulating layer and a standoff grid of insulating material deposited on the backplate to define an array of cells. An electrode is deposited in each cell on said backplate. A thin membrane of doped silicon is mounted to the standoff grid and over said array of cells and electrodes. Mirrors are laid on the membrane to create an array of reflective pixels over the array of cells. When an electrode is selectively charged the portion of the membrane overlying that cell is deflected by electrostatic attraction between the membrane and the electrode. Taken overall, a pattern is assumed by the array of pixels which corresponds to the state of the electrical signals placed on the electrodes. A vent hole formed in each cell from the electrode side of the backplate to the opposite side thereof for venting gas from the cell when said diaphragm is deflected, and grooves extend across the face of the backplate and over the cell to intersect the vent hole to assist in venting gas out of the cell and through the hole. The vent hole is preferably of a size and constructed for critical damping of the movement of the mass of the deflected diaphragm at said cell to control the movement caused by the electrostatic deflection. Flexures of reduced cross-section are formed in the diaphragm along a closed line extending around the inside perimeter of the stand off grid, so that said diaphragm moves more nearly in piston mode within the flexure.

Claims

1. A spatial light modulator comprising:

a silicon backplate having an insulating layer therein,

a standoff grid of insulating material deposited on said backplate to define an array of cells covering an area thereof and forming a second layer thereon,

a plurality of electrodes, each electrode being deposited in one of said cells and on said backplate,

a membrane of doped silicon mounted to said standoff grid and over said array of cells and electrodes,

means forming a mirror surface on said membrane on the side facing away from said backplate.

2. The spatial light modulator as in claim 1 further including:

means for selectively charging and discharging each electrode,

whereby each charged electrode causes the individual portions of said membrane overlying each corresponding cell to be deflected by electrostatic attraction between the membrane and the electrode in accordance with a pattern corresponding to the state of the electrical signals placed on said electrodes.

3. The spatial light modulator as in claim 1 further including:

means forming a vent hole in each cell from the electrode side of the backplate to the opposite side thereof for venting gas from said cell when said membrane is deflected.

4. The spatial light modulator as in claim 3 further including:

groove means extending across the face of said backplate and over said cell to intersect said vent hole to assist in venting gas out of said cell and through said hole.

5. The spatial light modulator as in claim 3 wherein said vent hole is of a size and constructed for critical damping of the movement of the mass of said membrane at said cell.

6. The spatial light modulator as in claim 4 wherein said vent hole and said groove are collectively of a size for providing critical damping of the movement of the mass of said membrane.

7. The spatial light modulator as in claim 1 further including:

a plurality of electrical conductors connected to each of said electrodes respectively.

8. The spatial light modulator as in claim 4 further including:

a plurality of electrical conductors connected to each of said electrodes, respectively, through the respective vent hole for each element.

9. The spatial light modulator as in claim 1 wherein said membrane comprises:

a doped thin coating of silicon epitaxially grown on a silicon wafer from which substantially the entire original wafer has been etched away.

10. The spatial light modulator as in claim 1 further in which:

said electrodes are made of platinum silicide.

11. The spatial light modulator as in claim 1 further in which:

said electrodes are made of a layer of an elemental metal or layers of elemental metals.

12. The spatial light modulator as in claim 1 further in which:

said stand off grid is made of silicon dioxide (SiO₂) deposited on said silicon backplate.

13. The spatial light modulator as in claim 1 wherein:

said membrane is made about 3 microns in thickness, and

said stand off grid is made about 4 microns in height.

14. The spatial light modulator as in claim 1 further in which:

said backplate and said membrane are made of single crystal silicon material.

15. The spatial light modulator as in claim 1 further including:

a thin oxide layer covering said electrode to prevent sticking of said membrane to said electrode.

16. The spatial light modulator as in claim 1 further in which:

said membrane is provided with means forming flexures of reduced cross-section formed therein along a closed line extending around the inside perimeter of said stand off grid, so that said membrane moves more nearly in piston mode within said flexure means.

17. A spatial light modulator comprising:

a backplate made of doped single crystal high temperature material,

a diaphragm made of doped single crystal material having the same coefficient of thermal expansion as said material of said backplate,

a standoff grid formed on one side of said backplate,

said grid including a plurality of raised ridges formed to project up from one side of the backplate for defining the periphery of a plurality of area elements and for spacing said diaphragm from the backplate,

means bonding said diaphragm on said grid,

a plurality of electrodes disposed on said backplate and within each of said plurality of area elements respectively,

said diaphragm having a thickness small enough to be readily flexed by electrostatic forces applied between said electrodes and said diaphragm,

means in said diaphragm for forming flexures therein around the inside of each said area periphery so that, when flexed, the region of said diaphragm within the flexures preferentially moves in piston mode,

means forming a plurality of vent holes in said backplate with at least one vent hole for each area element, each of said vent holes having a size for critically damping the movement of said diaphragm as caused by said electrode,

electrical connector means for connecting to each of said electrodes through respective vent hole to the opposite side of said backplate.