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1. A method of fabricating a slanted surface-relief structure in a material layer, the method comprising:

injecting a first reactive gas into an reactive ion source generator, wherein the first reactive gas comprises a low-molecular-weight gas;

generating a plasma in the reactive ion source generator, the plasma including reactive ions;

extracting at least some of the reactive ions from the plasma to form a collimated reactive ion beam towards the material layer; and

injecting a second reactive gas onto the material layer,

wherein the collimated reactive ion beam and the second reactive gas etch the material layer both physically and chemically to form the slanted surface-relief structure.

2. The method of claim 1 , further comprising:

rotating the material layer based on a desired slant angle of the slanted surface-relief structure.

3. The method of claim 1 , wherein the first reactive gas comprises at least one of CF 4 , CHF 3 , CH 2 F 2 , CH 3 F, C 4 F 8 , C 4 F 6 , C 2 F 6 , C 2 F 8 , NF 3 , CLF 3 , N 2 O, N 2 , O 2 , SF 6 , C 12 , BCl 3 , HBr, Hz, Ar, He, or Ne.

4. The method of claim 1 , wherein the material layer includes a semiconductor substrate, a SiO 2 layer, a Si 3 N 4 material layer, a titanium oxide layer, an alumina layer, a SiC layer, a SiO x N y layer, an amorphous silicon layer, a spin on carbon (SOC) layer, an amorphous carbon layer (ACL), a diamond like carbon (DLC) layer, a TiO x layer, an AlO x layer, a TaO x layer, or a HFO x layer.

5. The method of claim 1 , wherein extracting at least some of the reactive ions from the plasma to form the collimated reactive ion beam comprises:

applying an extraction voltage on an extraction grid adjacent to the reactive ion source generator; and

applying an acceleration voltage on an acceleration grid to extract and accelerate at least some of the reactive ions,

wherein the extraction grid and the acceleration grid are aligned; and

wherein the acceleration voltage is lower than the extraction voltage.

6. The method of claim 1 , wherein the second reactive gas comprises at least one of CF 4 , CHF 3 , N 2 , O 2 , SF 6 , Cl 2 , BCl 3 , or HBr.

7. The method of claim 1 , wherein:

the slanted surface-relief structure comprises a slanted surface-relief optical grating.

8. The method of claim 7 , wherein:

the slanted surface-relief optical grating comprises a plurality of ridges; and

a leading edge of each ridge is parallel to a trailing edge of the ridge.

9. The method of claim 8 , wherein:

a slant angle of the leading edge and a slant angle of the trailing edge are greater than 30 degrees with respect to a surface normal of the material layer.

10. The method of claim 8 , wherein a difference between a length of the leading edge and a length of the trailing edge is less than 10% of the length of the trailing edge.

11. The method of claim 7 , wherein:

a depth of the slanted surface-relief optical grating is greater than 100 nm.

12. The method of claim 7 , wherein:

a duty cycle of the slanted surface-relief optical grating is greater than 60%.

13. The method of claim 1 , wherein:

the first reactive gas comprises H 2 ;

the reactive ions include H + ions;

the material layer includes a SiO m N n layer or a Si 3 N 4 layer; and

the H + ions react with the SiO m N n layer or the Si 3 N 4 layer to form a SiH x N y layer.

14. The method of claim 13 , wherein the collimated reactive ion beam and the second reactive gas react with the SiH x N y layer to generate SiF 4 , NH 3 , and HCN gases.

15. A chemically assisted reactive ion beam etching (CARIBE) system for fabricating a slanted surface-relief structure in a material layer, the CARIBE system comprising:

an reactive ion source generator configured to generate a plasma using a first reactive gas, the plasma including reactive ions, and the first reactive gas including a low-molecular-weight gas;

one or more aligned collimator grids configured to extract and accelerate at least some of the reactive ions in the plasma to form a collimated reactive ion beam towards the material layer; and

a gas ring configured to inject a second reactive gas onto the material layer,

wherein the collimated reactive ion beam and the second reactive gas etch the material layer both physically and chemically to form the slanted surface-relief structure in the material layer.

16. The CARIBE system of claim 15 , wherein:

the reactive ion source generator includes an internal cavity configured to enclose the plasma; and

a surface layer of the internal cavity includes an oxide material.

17. The CARIBE system of claim 16 , wherein the oxide material includes Y 2 O 3 or aluminum oxide.

18. The CARIBE system of claim 15 , further comprising:

a neutralizer configured to inject an electron beam into the collimated reactive ion beam to neutralize the collimated reactive ion beam.

19. The CARIBE system of claim 15 , wherein:

the reactive ion source generator comprises an inductively coupled plasma generator.

20. The CARIBE system of claim 15 , wherein:

the first reactive gas comprises at least one of CF 4 , CHF 3 , CH 2 F 2 , CH 3 F, C 4 F 8 , C 4 F 6 , C 2 F 6 , C 2 F 8 , NF 3 , CLF 3 , N 2 O, N 2 , O 2 , SF 6 , H 2 , Cl 2 , BCl 3 , HBr, Ar, He, or Ne; and

the second reactive gas comprises at least one of CF 4 , CHF 3 , N 2 , O 2 , SF 6 , Cl 2 , BCl 3 , or HBr.