Membrane Grating for Beam Steering Device and Method of Fabricating Same

• US 20080247043A1
• Filed: 04/03/2008
• Published: 10/09/2008
• Est. Priority Date: 04/06/2007
• Status: Active Grant

• Current Assignee: Rosemount Aerospace, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: Rosemount Aerospace, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Nesnidal, Michael P.
• Granted Patent: US 7,989,250 B2
• Time in Patent Office: Days
• US Class Current: 359/576
• CPC Class Codes: G02B 26/0808 by means of one or more diffracting elements
• CPC Class Codes: G02B 27/0087 Phased arrays
• CPC Class Codes: G02B 27/4277 being separated by an air space
• CPC Class Codes: G02B 5/1857 using exposure or etching means, e.g. holography, photolithography, exposure to electron or ion beams
• CPC Class Codes: G02B 5/1871 Transmissive phase gratings

Abstract

A method of fabricating a membrane structure for a diffractive phased array assembly is provided. The method includes the steps of providing a wafer having a body and at least a membrane layer and a backside layer disposed on opposite faces of the body, forming a grating pattern on a surface of the membrane layer, and forming a window through the wafer to expose a back surface of the membrane, thereby allowing light to pass through the membrane.

Claims

1. A method of fabricating a membrane structure for a diffractive phased array assembly, comprising the steps of:

a) providing a wafer having a body and at least a membrane layer and a backside layer disposed on opposite faces of the body;

b) forming a grating pattern on a surface of the membrane layer; and

c) forming a window through the wafer to expose a back surface of the membrane, thereby allowing light to pass through the membrane.

2. A method of fabricating a membrane structure according to claim 1, wherein the step of forming a window through the backside layer and wafer body is performed through a photolithographic technique, and includes:

a) applying a photoresist layer on the face of the backside layer;

b) exposing the photoresist in a pattern to define the window;

c) removing the photoresist in regions where etching is to occur;

d) etching through at least the backside layer to partially form the window.

3. A method of fabricating a membrane structure according to claim 2, wherein the photoresist layer has a thickness of about 7 μ

m.

4. A method of fabricating a membrane structure according to claim 2, wherein the wafer body is first etched to a predetermined depth with a first etching technique, and subsequently etched with a second etching technique to complete the etch of the wafer body.

5. A method of fabricating a membrane structure according to claim 4, wherein the first etching technique is deep reactive ion etching, and the second etching technique is a chemical etch using tetramethylammonium hydroxide.

6. A method of fabricating a membrane structure according to claim 4, wherein the predetermined depth is about 100 μ

m.

7. A method of fabricating a membrane structure according to claim 2, wherein the thickness of the membrane layer is between about 10 and 75 μ

m.

8. A method of fabricating a membrane structure according to claim 1, wherein the wafer further includes an etch-stop layer underlying the membrane layer, and wherein the method further comprises the steps of:

a) etching through the wafer to partially form the window; and

b) etching through the etch-stop layer to the membrane layer to fully form the window.

9. The method of fabricating a membrane structure according to claim 8, further comprising the step of mounting the membrane structure in a protective fixture prior to the step of etching through the etch stop layer, in order to protect the membrane layer.

10. The method of fabricating a membrane structure according to claim 8, wherein the etch stop layer is removed with a buffered oxide etch.

11. A method of fabricating a membrane structure according to claim 1, wherein the materials of the wafer body, backside layer, the etch stop layer, and the membrane layer are selected so that when a window is formed through the wafer up to the membrane layer, the materials therein exhibit hoop stresses sufficient to provide tension to the membrane layer, to maintain the membrane layer substantially flat.

12. A method of fabricating a membrane structure according to claim 1, wherein the wafer body is formed of silicon, the backside layer is formed of silicon nitride, the etch stop layer is formed of silicon dioxide, and the membrane layer is formed of silicon.

13. A method of fabricating a membrane structure according to claim 1, further comprising the step of confirming that the flatness of the membrane structure is within allowable limits.

14. A method of fabricating a membrane structure according to claim 1, further comprising the step of baking the membrane structure for a length of time at a temperature and humidity sufficient to promote native oxide growth on the membrane.

15. A diffractive phased array assembly comprising first and second membrane structures, each membrane structure having a membrane layer with a patterned grating defined thereon, wherein the membrane layers of the two membrane structures are in operative contact with one another so as to form a gap therebetween.

16. A diffractive phased array assembly as recited in claim 15, wherein an index matching fluid is provided within the gap formed between the patterned surfaces of the first and second membrane structures.

17. A diffractive phased array assembly as recited in claim 16, wherein the index matching fluid has an ultra low viscosity.

18. A diffractive phased array assembly as recited in claim 15, wherein the first and second membrane structures are adapted for relative translational movement, substantially parallel to the membrane surface.

19. A diffractive phased array assembly as recited in claim 15, wherein each of the first and second membrane structures is provided with internal stresses to apply tension to the membrane layer to maintain the membrane layer flat.

20. A diffractive phased array assembly comprising:

a plate structure having a patterned grating defined thereon and a membrane structure having a patterned grating defined thereon, wherein the patterned grating of the plate and the patterned grating of the membrane are in operative contact with one another so as to form a gap therebetween.

21. A diffractive phased array assembly as recited in claim 20, wherein an index matching fluid is provided within the gap formed between the patterned surfaces of the plate structure and membrane structure.

22. A diffractive phased array assembly as recited in claim 20, wherein the plate structure and membrane structure are adapted for relative translational movement, substantially parallel to the respective gratings.