Gray-tone lithography for precise control of grating etch depth

1. A method comprising:

receiving a substrate including a surface-relief grating formed thereon;

depositing an overcoat layer on the surface-relief grating to fill grating grooves of the surface-relief grating and cover top surfaces of grating ridges of the surface-relief grating, the overcoat layer having an uneven top surface;

depositing a gray-tone photoresist layer on the overcoat layer, the gray-tone photoresist layer characterized by an uneven top surface;

exposing, through a gray-scale photomask, the gray-tone photoresist layer to a light beam having a non-uniform intensity;

developing the gray-tone photoresist layer to remove exposed portions of the gray-tone photoresist layer; and

fully etching the gray-tone photoresist layer and partially etching the overcoat layer to form an even top surface at the overcoat layer.

2. The method of claim 1 , wherein the surface-relief grating is characterized by at least one of:

a variable etch depth;

a variable grating period;

a variable duty cycle;

a variable thickness; or

an uneven top surface.

3. The method of claim 1 , wherein the gray-scale photomask is characterized by a transmissivity profile corresponding to a height profile of the uneven top surface of the gray-tone photoresist layer.

4. The method of claim 1 , wherein an etch rate of the gray-tone photoresist layer is between 0.5 and 5 times of an etch rate of the overcoat layer during the etching.

5. The method of claim 1 , wherein the gray-tone photoresist layer is characterized by a non-binary response to exposure dosage such that a depth of an exposed portion of the gray-tone photoresist layer is a function of the exposure dosage.

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

7. The method of claim 1 , wherein the gray-tone photoresist layer is characterized by an even top surface after the developing.

8. The method of claim 1 , further comprising forming an anti-reflection coating layer or an angular selective transmission layer on the overcoat layer.

9. The method of claim 1 , wherein etching the gray-tone photoresist layer and the overcoat layer includes etching the gray-tone photoresist layer and the overcoat layer 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 and 500 eV.

10. A method comprising:

depositing a grating material layer on a substrate;

forming a patterned hard mask on a first area of the grating material layer but not on a second area of the grating material layer;

depositing a photoresist material layer on the patterned hard mask and the second area of the grating material layer, the photoresist material layer sensitive to exposure light and having a non-binary response to exposure dosage;

exposing the photoresist material layer on the patterned hard mask to a light beam for a period of time, the light beam characterized by a non-uniform light intensity;

developing the photoresist material layer to remove portions of the photoresist material layer exposed to the light beam;

etching the photoresist material layer and the grating material layer to form a grating with a variable depth in the grating material layer, wherein a portion of the photoresist material layer remains on the second area of the grating material layer;

removing the patterned hard mask on the first area of the grating material layer; and

etching, after removing the patterned hard mask, the first area of the grating material layer and the portion of the photoresist material layer on the second area of the grating material layer without using any etch mask.

11. The method of claim 10 , wherein an etch rate of the photoresist material layer is between 0.5 and 5 times of an etch rate of the grating material layer.

12. The method of claim 10 , wherein the light beam includes an ultraviolet light beam.

13. The method of claim 10 , further comprising:

exposing the photoresist material layer on the second area of the grating material layer to a light pattern corresponding to an alignment mark,

wherein etching the first area of the grating material layer and the photoresist material layer on the second area of the grating material layer forms the alignment mark in the second area of the grating material layer.

14. The method of claim 10 , further comprising, after developing the photoresist material layer, curing the photoresist material layer using heat or ultraviolet light.

15. The method of claim 10 , wherein etching the photoresist material layer and the grating material layer includes etching the photoresist material layer and the grating material layer 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 and 500 eV.

16. A method comprising:

depositing a gray-tone photoresist layer on a material layer to be etched by an etching system, wherein the etching system is characterized by an uneven etch rate in an etch area;

exposing, through a gray-scale photomask, the gray-tone photoresist layer to a light beam having a non-uniform intensity, wherein a transmissivity of the gray-scale photomask is determined based on the uneven etch rate of the etching system;

developing the gray-tone photoresist layer to remove exposed portions of the gray-tone photoresist layer, wherein remaining portions of the gray-tone photoresist layer have an uneven top surface; and

etching the remaining portions of the gray-tone photoresist layer and the material layer to be etched for a period of time to remove materials of the material layer uniformly across the material layer.

17. The method of claim 16 , wherein an etch rate of the gray-tone photoresist layer is between 0.5 and 5 times of an etch rate of the material layer to be etched.

18. The method of claim 16 , wherein the light beam includes an ultraviolet beam.

19. The method of claim 16 , further comprising, after developing the gray-tone photoresist layer, curing the gray-tone photoresist layer.

20. The method of claim 16 , wherein the transmissivity of the gray-scale photomask is complementary to the uneven etch rate of the etching system.