Duty cycle range increase for waveguide combiners

Abstract

Techniques for fabricating a slanted structure are disclosed. In one embodiment, a method of fabricating a slanted surface-relief structure in a material layer includes forming a thin hard mask on top of an intermediate mask layer, etching the intermediate mask layer at a slant angle using the thin hard mask to form a slanted intermediate mask, and etching the material layer at the slant angle using the slanted intermediate mask to form the slanted surface-relief structure in the material layer. The intermediate mask layer is characterized by an etch rate greater than an etch rate of the material layer.

What is claimed is: 
     
         1 . A method of fabricating a slanted surface-relief structure in a material layer, the method comprising:
 forming a thin hard mask on top of an intermediate mask layer;   etching the intermediate mask layer at a slant angle using the thin hard mask to form a slanted intermediate mask, wherein the intermediate mask layer is characterized by an etch rate greater than an etch rate of the material layer; and   etching the material layer at the slant angle using the slanted intermediate mask to form the slanted surface-relief structure in the material layer.   
     
     
         2 . The method of  claim 1 , wherein a duty cycle of the slanted surface-relief structure is less than 30%. 
     
     
         3 . The method of  claim 1 , wherein:
 the slanted surface-relief structure includes a slanted surface-relief optical grating characterized by a duty cycle varying across areas of the slanted surface-relief optical grating; and   a minimum of the duty cycle of the slanted surface-relief optical grating is less than 30%.   
     
     
         4 . The method of  claim 1 , wherein the material layer comprises 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 the intermediate mask layer includes an organic material. 
     
     
         6 . The method of  claim 1 , wherein the thin hard mask includes a metal or a metallic compound. 
     
     
         7 . The method of  claim 1 , wherein a thickness of the thin hard mask is less than 20 nm. 
     
     
         8 . The method of  claim 1 , wherein an etch selectivity between the intermediate mask layer and the thin hard mask is greater than 500:1. 
     
     
         9 . The method of  claim 1 , wherein the etch rate of the intermediate mask layer is greater than three times of the etch rate of the material layer. 
     
     
         10 . The method of  claim 1 , further comprising:
 coating the intermediate mask layer on top of the material layer;   forming a thin hard mask layer on top of the intermediate mask layer; and   etching the thin hard mask layer to form the thin hard mask.   
     
     
         11 . The method of  claim 1 , wherein the slant angle is greater than 30° with respect to a surface normal of the material layer. 
     
     
         12 . The method of  claim 1 , wherein a depth of the slanted surface-relief structure is greater than 100 nm. 
     
     
         13 . The method of  claim 1 , wherein:
 a maximum duty cycle of the slanted surface-relief structure is greater than 75%.   
     
     
         14 . The method of  claim 1 , wherein etching parameters for etching the intermediate mask layer are different from etching parameters for etching the material layer. 
     
     
         15 . A method of fabricating a slanted surface-relief structure in a material layer, the method comprising:
 forming a thin hard mask on top of a first intermediate mask layer;   etching the first intermediate mask layer at a slant angle using the thin hard mask to form a first slanted intermediate mask, wherein the first intermediate mask layer is characterized by a first etch rate greater than an etch rate of the material layer;   etching a second intermediate mask layer below the first intermediate mask layer at the slant angle using the first slanted intermediate mask to form a second slanted intermediate mask, wherein the second intermediate mask layer is characterized by a second etch rate greater than the first etch rate of the first intermediate mask layer; and   etching the material layer at the slant angle using the second slanted intermediate mask to form the slanted surface-relief structure in the material layer.   
     
     
         16 . The method of  claim 15 , wherein a minimum duty cycle of the slanted surface-relief structure is less than 30%. 
     
     
         17 . The method of  claim 15 , wherein the slant angle is greater than 45° with respect to a surface normal of the material layer. 
     
     
         18 . The method of  claim 15 , wherein a thickness of the thin hard mask is less than 20 nm. 
     
     
         19 . The method of  claim 15 , wherein further comprising:
 coating the second intermediate mask layer on top of the material layer;   coating the first intermediate mask layer on top of the second intermediate mask layer;   forming a thin hard mask layer on top of the first intermediate mask layer; and   etching the thin hard mask layer to form the thin hard mask.