Methods of forming holographic gratings for optical systems
An electronic device may have a display system. The display system may include a waveguide, an input coupler, and a surface relief grating (SRG) structure. There are various ways to form a SRG structure. In one example, nanoimprinting may be performed on a low-index resin to form a mold for a subsequent deposition of high-index material. The high-index material conforms to the mold to form ridges for the SRG structure. A reusable mold may be formed with a plurality of ridges on a flexible substrate and coated by an anti-stick coating. The reusable mold may be coated with a high-index material that is attached to a glass substrate and cured to form a wafer-level SRG structure with high-index ridges on a glass substrate. The wafer-level SRG structure is subsequently diced. SRG structures may also be formed that include varying trough thicknesses, varying ridge thicknesses, and/or a sloped substrate.
1. A method of forming a surface relief grating comprising:
depositing a first material on a substrate;
nanoimprinting the first material to form a first plurality of ridges formed from the first material; and
depositing a second material over the first plurality of ridges, wherein the second material forms a second plurality of ridges for the surface relief grating, wherein the second plurality of ridges is formed between the first plurality of ridges, wherein the first plurality of ridges alternates with the second plurality of ridges, and wherein the second material has a higher index of refraction than the first material.
2. The method defined in claim 1 , further comprising:
removing some of the second material, wherein removing some of the second material results in the first and second pluralities of ridges having a single planar upper surface.
3. The method defined in claim 2 , further comprising:
applying an anti-reflection coating over the single planar upper surface.
4. The method defined in claim 2 , further comprising:
removing the first plurality of ridges, wherein removing the first plurality of ridges causes only the second plurality of ridges to be present on the substrate.
5. The method defined in claim 4 , wherein removing the first plurality of ridges comprises etching the first material.
6. The method defined in claim 1 , wherein each one of the second plurality of ridges has a width that is less than 1000 nanometers and a height that is less than 1000 nanometers.
7. The method defined in claim 1 , wherein the second material comprises titanium dioxide or a material with an index of refraction that is greater than 1.8.
8. The method defined in claim 2 , further comprising:
applying a coating over the single planar upper surface.
9. A method comprising:
depositing a first material on a first substrate while the first substrate is unwound from a first roller;
nanoimprinting the first material to define a first plurality of ridges;
depositing a second material on the first substrate and the first plurality of ridges while the first substrate is unwound, wherein the second material has a higher index of refraction than the first material;
applying a second substrate to the second material;
curing the second material while the second material is attached to the second substrate; and
removing the second substrate with the cured second material.
10. The method defined in claim 9 , wherein depositing the second material on the first substrate and the first plurality of ridges forms a second plurality of ridges between the first plurality of ridges.
11. The method defined in claim 10 , wherein applying the second substrate to the second material comprises applying the second substrate to the second plurality of ridges.
12. The method defined in claim 11 , further comprising:
after nanoimprinting the first material to define the first plurality of ridges, depositing an anti-stick coating over the first plurality of ridges, wherein the anti-stick coating conforms to the first plurality of ridges and the first substrate.
13. The method defined in claim 9 , wherein nanoimprinting the first material to define the first plurality of ridges comprises:
applying a nanoimprinting mold to the first material; and
exposing the first material to ultraviolet light while applying the nanoimprinting mold to the first material.
14. The method defined in claim 9 , wherein curing the second material comprises exposing the second material to ultraviolet light.
15. The method defined in claim 9 , wherein the first substrate comprises plastic and wherein the second substrate comprises glass.
16. The method defined in claim 9 , further comprising:
after removing the second substrate with the cured second material, dicing the second substrate with the cured second material to form a plurality of surface relief grating structures.
17. A method of forming a depth-modulated surface relief grating comprising:
forming a high-index layer over a substrate;
forming a patterned hard mask over the high-index layer;
forming a sloped layer over the patterned hard mask and the high-index layer; and
etching the sloped layer and the high-index layer through the hard mask.
18. A method of forming a depth-modulated surface relief grating comprising:
forming a high-index layer over a substrate;
forming a patterned hard mask over the high-index layer; and
etching the high-index layer through the patterned hard mask in a sloped manner to form troughs of varying depths in the high-index layer.
19. A method of forming a depth-modulated surface relief grating comprising:
forming a sloped high-index layer over a planar substrate;
forming a patterned hard mask over the sloped high-index layer; and
etching the sloped high-index layer through the patterned hard mask.
20. A method of forming a depth-modulated surface relief grating comprising:
forming a high-index layer having a uniform thickness over a substrate;
forming a sloped layer over the high-index layer;
patterning the sloped layer to have a first plurality of ridges; and
etching the first plurality of ridges and the high-index layer to produce a second plurality of ridges in the high-index layer having varying heights.
21. A method of forming a depth-modulated surface relief grating comprising:
forming a high-index layer having a uniform thickness over a substrate with a sloped upper surface;
forming a patterned hard mask over the high-index layer;
etching the high-index layer through the patterned hard mask to form a plurality of ridges in the high-index layer having uniform heights; and
removing portions of the plurality of ridges to cause the plurality of ridges to have non-uniform heights.