Variable-etch-depth gratings

Patent metadata

Field	Value
Publication number	US-10976483-B2
Application number	US-202016799532-A
Country	US
Kind code	B2
Filing date	Feb 24, 2020
Priority date	Feb 26, 2019
Publication date	Apr 13, 2021
Grant date	Apr 13, 2021

Abstract

Disclosed herein are techniques for fabricating straight or slanted variable-etch-depth gratings. A photoresist material for fabricating a variable-etch-depth grating in a substrate is sensitive to light with a wavelength shorter than 300 nm and has an etch rate comparable to the etch rate of the substrate. A depth of an exposed portion of a photoresist material layer including the photoresist material correlates with the exposure dose. After exposure using a gray-scale mask and development, the photoresist material layer has a non-uniform thickness. The photoresist material layer with the non-uniform thickness and the underlying substrate are etched using a straight etching or slanted etching process to form the straight or slanted variable-etch-depth grating in the substrate. The variable-etch-depth grating is characterized by a non-uniform depth profile corresponding to the non-uniform thickness of the photoresist material layer before etching.

First claim

What is claimed is:

1. A method comprising:
forming a patterned etch mask on a substrate;
depositing a photoresist material layer on the patterned etch mask, the photoresist material layer sensitive to ultra-violet (UV) light and having a linear response to UV light dose;
exposing the photoresist material layer to UV light through a variable transparency photomask;
developing the photoresist material layer exposed to the UV light to form a patterned photoresist layer having a non-uniform thickness;
etching the patterned photoresist layer and the substrate to obtain a grating having a non-uniform etch depth in the substrate, wherein the patterned photoresist layer is characterized by an etch rate comparable to an etch rate of the substrate; and
removing the patterned photoresist layer and the patterned etch mask from the substrate.

2. The method of claim 1 , further comprising at least one of:
depositing, before depositing the photoresist material layer, a first anti-reflective coating layer on the patterned etch mask, or
depositing, after depositing the photoresist material layer, a second anti-reflective coating layer on the photoresist material layer.

3. The method of claim 1 , wherein etching the patterned photoresist layer and the substrate comprises:
etching the patterned photoresist layer and the substrate at a slant angle greater than 10° with respect to a surface normal of the substrate.

4. The method of claim 3 , wherein the slant angle is between 30° and 70°.

5. The method of claim 1 , wherein the photoresist material layer is sensitive to light having a wavelength shorter than 300 nm, 250 nm, 193 nm, or 157 nm.

6. The method of claim 1 , wherein the etch rate of the patterned photoresist layer is between 0.5 and 5 times of the etch rate of the substrate in a same etching process.

7. The method of claim 1 , wherein a depth of an exposed portion of the photoresist material layer is a linear function of the UV light dose.

8. The method of claim 1 , wherein the photoresist material layer includes a positive-tone photoresist.

9. The method of claim 1 , wherein the photoresist material layer includes Poly(methyl methacrylate) (PMMA) sensitized with a photosensitive group.

10. The method of claim 9 , wherein the photosensitive group includes at least one of:
an acyloximino group;
methacrylonitrile;
terpolymer of methyl methacrylate;
oximino methacrylate;
benzoic acids;
N-acetylcarbazole; or
indenone.

11. The method of claim 1 , wherein the photoresist material layer includes at least one of:
poly(methyl methacrylate)-r-poly(tert-butyl methacrylate)-r-poly(methyl methacrylate) and a photo acid generator;
poly(methyl methacrylate)-r-poly(methacrylic acid);
poly(α-methylstyrene-co-methyl chloroacrylate) and an acid generator;
polycarbonate and a photo acid or base generator;
polylactide and a photo acid or base generator; or
polyphthalaldehyde and a photo acid generator.

12. The method of claim 1 , wherein the non-uniform etch depth in the substrate includes at least 8 different depth levels.

13. The method of claim 1 , wherein a maximum depth of the non-uniform etch depth in the substrate is greater than 100 nm.

14. The method of claim 1 , wherein etching the patterned photoresist layer and the substrate includes etching the patterned photoresist layer and the substrate using at least one of:
an oxygen source including O 2 , N 2 O, CO 2 , or CO;
a nitrogen source including N 2 , N 2 O, or NH 3 ; or
ions with an energy between 100-500 eV.

15. The method of claim 1 , wherein the variable transparency photomask includes a gray-scale photomask.

16. The method of claim 15 , wherein the gray-scale photomask includes at least 8 different transmissivity levels.

17. A slanted surface-relief structure for waveguide display, the slanted surface-relief structure comprising:
a substrate; and
a slanted surface-relief optical grating in the substrate, the slanted surface-relief optical grating having a flat top surface and including a plurality of grating ridges and a plurality of grating grooves, each of the plurality of grating grooves characterized by a corresponding depth,
wherein depths of the plurality of grating grooves include at least 8 different depth levels and vary in two directions across the slanted surface-relief optical grating according to a predetermined profile; and
wherein a slant angle of at least one grating ridge in the plurality of grating ridges is greater than 30° with respect to a surface normal of the substrate.

18. The slanted surface-relief structure of claim 17 , wherein the depths of the plurality of grating grooves range from 0 nm to greater than 100 nm.

19. The slanted surface-relief structure of claim 17 , wherein the corresponding depth of a grating groove in the plurality of grating grooves varies across the slanted surface-relief optical grating.

20. The slanted surface-relief structure of claim 17 , wherein the slanted surface-relief optical grating has different grating duty cycles at different regions of the slanted surface-relief optical grating.

21. The slanted surface-relief structure of claim 20 , wherein the slanted surface-relief optical grating is characterized by a duty cycle greater than 70%.

22. The slanted surface-relief structure of claim 20 , wherein the slanted surface-relief optical grating is characterized by a duty cycle less than 30%.

23. The slanted surface-relief structure of claim 17 , wherein the slanted surface-relief optical grating has different grating periods at different regions of the slanted surface-relief optical grating.

24. A photoresist material for fabricating a variable-etch-depth grating, wherein:
the photoresist material is sensitive to light with a wavelength shorter than 300 nm;
the photoresist material is characterized by an etch rate that is between 0.5 and 5 times of an etch rate of a substrate; and
the photoresist material is characterized by a linear response to UV light dose such that a depth of an exposed portion of the photoresist material is a linear function of the UV light dose.

25. The photoresist material of claim 24 , wherein the photoresist material includes at least one of:
Poly(methyl methacrylate) (PMMA) sensitized with a photosensitive group;
poly(methyl methacrylate)-r-poly(tert-butyl methacrylate)-r-poly(methyl methacrylate) and a photo acid generator;
poly(methyl methacrylate)-r-poly(methacrylic acid);
poly(α-methylstyrene-co-methyl chloroacrylate) and an acid generator;
polycarbonate and a photo acid or base generator;
polylactide and a photo acid or base generator; or
polyphthalaldehyde and a photo acid generator.

Assignees

• Facebook Tech Llc

Inventors

• Vora Ankit
• Mohanty Nihar Ranjan
• Lane Austin
• Colburn Matthew E
• Franke Elliott

Classifications

• B29D11/00769

  Producing diffraction gratings · CPC title

• G02B2027/0178

  Eyeglass type (eyeglass details G02C) · CPC title

• G02B27/0172

  characterised by optical features · CPC title

• G02B27/0081

  with means for altering, e.g. enlarging, the entrance or exit pupil · CPC title

• G03F7/2002

  with visible light or UV light, through an original having an opaque pattern on a transparent support, e.g. film printing, projection printing; by reflection of visible or UV light from an original such as a printed image · CPC title