Mixed valence sol-gels for high refractive index, transparent optical coatings

Abstract

Techniques disclosed herein relate to sol-gel materials that include at least one metal halide precursor and at least one alcohol. After being annealed, the metal in the metal halide is in at least two different oxidation states, both of which are stable and transparent to visible light. By producing a mixture of the same metal in multiple oxidation states that are all transparent to visible light, an amorphous state of the sol-gel material without any noticeable domain formation that would otherwise distribute stress locally and lead to voiding inside nano-gratings to be overcoated is obtained. Highly condensed states of the sol-gel have refractive index values in the 1.7-2.2 range, without sacrificing recessed-feature fill.

What is claimed is: 
     
         1 . A sol-gel material for overcoating surface-relief structures, the sol-gel material comprising:
 a metal halide precursor; and   at least one alcohol or glycol.   
     
     
         2 . The sol-gel material of  claim 1 , wherein the metal halide precursor comprises a source of tin(II) chloride or a source of tin(II) chloride dihydrate. 
     
     
         3 . The sol-gel material of  claim 1 , wherein the sol-gel material further comprises a solvent that is a source of oxide ligands during an annealing process. 
     
     
         4 . The sol-gel material of  claim 1 , wherein:
 after applying an annealing process to the sol-gel material, the sol-gel material comprises at least two different oxidation states of a metal in the metal halide precursor; and   the at least two different oxidation states of the metal are both transparent to visible light.   
     
     
         5 . The sol-gel material of  claim 4 , wherein after applying the annealing process to the sol-gel material, an oxychloride composition of the sol-gel material is non-stoichiometric. 
     
     
         6 . The sol-gel material of  claim 4 , wherein the metal comprises tin, and wherein the at least two different oxidation states of the metal include tin(II) and tin(IV). 
     
     
         7 . The sol-gel material of  claim 1 , wherein the sol-gel material is configured to fill recessed features on a substrate in a superconformal fashion without leading to voiding upon full thermal densification of the sol-gel material. 
     
     
         8 . The sol-gel material of  claim 1 , further comprising a stabilizer, an acid, a base, a peroxide, a surfactant, a cross-linker, a flexibilizer, a toughener additive, a solvent, or a combination thereof. 
     
     
         9 . The sol-gel material of  claim 8 , wherein the stabilizer comprises an ethanolamine, a diethanolamine, a triethanolamine, an aliphatic amine, a diamine, a triamine, a polyamine, or a combination thereof. 
     
     
         10 . The sol-gel material of  claim 8 , wherein the stabilizer comprises organic antioxidants, inorganic antioxidants, or a combination thereof. 
     
     
         11 . The sol-gel material of  claim 1 , wherein the sol-gel material further comprises a solvent comprising a propylene glycol methyl ether, a dipropylene glycol monomethyl ether, a propylene glycol methyl ether acetate, a tripropylene glycol monomethyl ether, a butyl lactate, a propylene carbonate, an isopropyl alcohol, a methanol, water, or a combination thereof. 
     
     
         12 . The sol-gel material of  claim 1 , wherein the sol-gel material comprises:
 10%-30% by weight of a tin salt; and   70%-90% by weight of a solvent mixture.   
     
     
         13 . The sol-gel material of  claim 12 , wherein the tin salt comprises:
 tin(II) chloride;   tin dichloride dihydrate;   anhydrous tin dichloride; or   any combination thereof.   
     
     
         14 . The sol-gel material of  claim 12 , wherein the solvent mixture comprises:
 27% by weight of propylene glycol methyl ether, 67% by weight of 1,3-dimethoxy-2-propanol, and 6% by weight of diethylene glycol;   27% by weight of propylene glycol methyl ether, 67% by weight id di(propylene glycol) methyl ether, and 6% by weight of diethylene glycol; or   100% by weight of 1,3-dimethoxy-2-propanol.   
     
     
         15 . An optical device for a display system comprising:
 a surface-relief structure including recessed features; and   a layer of a sol-gel material filling the recessed features of the surface-relief structure in a superconformal fashion without voiding,
 wherein the sol-gel material includes at least two different oxidation states of a metal, and
 wherein the at least two different oxidation states of the metal are transparent to visible light. 
 
   
     
     
         16 . The optical device of  claim 15 , wherein an absorption of the layer of the sol-gel material is lower than 0.1% per 100 nm for visible light and a refractive index of the layer of the sol-gel material is between 1.65 and 2.20. 
     
     
         17 . The optical device of  claim 15 , wherein the at least two different oxidation states of the metal include tin(II) and tin(IV). 
     
     
         18 . The optical device of  claim 17 , wherein a ratio of tin(II) to tin(IV) in the layer of the sol-gel material is between 1:5 to 4:1, and wherein the sol-gel material is in an amorphous state. 
     
     
         19 . The optical device of  claim 15 , wherein the surface-relief structure includes features characterized widths between 5 nm and 200 nm, and aspect ratios between 1:1.5 and 1:50. 
     
     
         20 . A method of producing a superconformal optical coating, the method comprising:
 depositing, on a surface-relief structure, a layer of a sol-gel material, wherein the sol-gel material comprises:
 a metal halide precursor; and 
 at least one alcohol or glycol; and 
   annealing the layer of the sol-gel material at temperatures lower than or equal to 300° C. for less than about 10 minutes to superconformally fill the surface-relief structure with the sol-gel material.