Near infrared optical interference filters with improved transmission

Abstract

An interference filter includes a layers stack comprising a plurality of layers of at least: layers of amorphous hydrogenated silicon with added nitrogen (a-Si:H,N) and layers of one or more dielectric materials, such as SiO 2 , SiO x , SiO x N y , a dielectric material with a higher refractive index in the range 1.9 to 2.7 inclusive, or so forth. The interference filter is designed to have a passband center wavelength in the range 750-1000 nm inclusive. Added nitrogen in the a-Si:H,N layers provides improved transmission in the passband without a large decrease in refractive index observed in a-Si:H with comparable transmission. Layers of a dielectric material with a higher refractive index in the range 1.9 to 2.7 inclusive provide a smaller angle shift compared with a similar interference filter using SiO 2 as the low index layers.

1 . An interference filter comprising:
 a layers stack comprising plurality of layers of at least:
 layers of amorphous hydrogenated silicon with added nitrogen (a-Si:H,N) and 
 layers of one or more dielectric materials having a refractive index lower than the refractive index of the a-Si:H,N. 
   
     
     
         2 . The interference filter of  claim 1  wherein the one or more dielectric materials include SiO 2 . 
     
     
         3 . The interference filter of  claim 1  wherein the one or more dielectric materials include a silicon suboxide (SiO x ). 
     
     
         4 . The interference filter of  claim 1  wherein the one or more dielectric layers include a silicon oxynitride (SiO x N y ). 
     
     
         5 . The interference filter of  claim 1  wherein the layers of one or more dielectric materials include layers of a dielectric material having a refractive index in the range 1.9 to 2.7 inclusive. 
     
     
         6 . The interference filter of  claim 5  wherein the layers of a dielectric material having a refractive index in the range 1.9 to 2.7 inclusive include one or more layers comprising Si 3 N 4 , SiO x N y  with y large enough to provide a refractive index of 1.9 or higher, Ta 2 O 5 , Nb 2 O 5 , or TiO 2 . 
     
     
         7 . The interference filter of  claim 6  wherein the layers of one or more dielectric materials further include SiO 2  layers. 
     
     
         8 . The interference filter of  claim 1  wherein the layers stack is configured to have a passband center wavelength in the range 800-1100 nm inclusive. 
     
     
         9 . The interference filter of  claim 1  wherein the layers stack is configured to have a passband center wavelength in the range 750-1100 nm inclusive. 
     
     
         10 . The interference filter of  claim 1  wherein the a-Si:H,N has an atomic concentration between 1% and 4% hydrogen and between 2% and 6% nitrogen. 
     
     
         11 . The interference filter of  claim 1  further comprising:
 a transparent substrate supporting the layers stack. 
 
     
     
         12 . The interference filter of  claim 11  wherein the transparent substrate comprises a glass substrate. 
     
     
         13 . The interference filter of  claim 11  wherein the layers stack includes a first layers stack on one side of transparent substrate and a second layers stack on the opposite side of the transparent substrate. 
     
     
         14 . The interference filter of  claim 13  wherein the first layers defines a low pass filter with a low pass cutoff wavelength, the second layers stack defines a high pass filter with a high pass cutoff wavelength, and the interference filter has a passband defined between the high pass cutoff wavelength and the low pass cutoff wavelength. 
     
     
         15 . An interference filter comprising:
 a layers stack comprising alternating a-Si:H,N and silicon based dielectric layers;   wherein the interference filter has at least one passband with center wavelength in the range 750-1100 nm inclusive.   
     
     
         16 . The interference filter of  claim 15  having at least one passband with center wavelength in the range of 800-1100 nm inclusive. 
     
     
         17 . The interference filter of  claim 15  wherein the passband center wavelength is 850 nm. 
     
     
         18 . The interference filter of  claim 15  wherein the silicon based dielectric layers comprise silicon oxide (SiO x ) layers. 
     
     
         19 . The interference filter of  claim 18  wherein the silicon oxide (SiO x ) layers comprise stoichiometric SiO 2  layers. 
     
     
         20 . The interference filter of  claim 15  wherein the silicon based dielectric layers comprise silicon oxidenitride (SiO x N y ) layers. 
     
     
         21 . The interference filter of  claim 15  wherein the a-Si:H,N has an atomic concentration between 4% and 8% hydrogen and between 2% and 12% nitrogen. 
     
     
         22 . The interference filter of  claim 15  further comprising:
 a transparent substrate supporting the layers stack. 
 
     
     
         23 . The interference filter of  claim 22  wherein the transparent substrate comprises a glass substrate. 
     
     
         24 . A method of manufacturing an interference filter comprising alternating a-Si:H,N and SiO x  layers, the method comprising:
 sputtering silicon from a silicon target onto a filter substrate; and   during the sputtering, alternating between (i) a process gas including hydrogen and nitrogen in order to deposit a-Si:H,N and (ii) a process gas including oxygen in order to deposit SiO x .   
     
     
         25 . The method of  claim 24  wherein the sputtering comprises:
 applying a negative bias to the silicon target; and 
 including an inert gas component in both (i) the process gas including hydrogen and nitrogen and (ii) the process gas including oxygen. 
 
     
     
         26 . The method of  claim 25  wherein the inert gas is argon. 
     
     
         27 . The method of  claim 24  wherein the sputtering and the alternating are configured to manufacture the interference filter having a passband center wavelength in the range 800-1000 nm inclusive. 
     
     
         28 . The method of  claim 24  wherein the sputtering and the alternating are configured to manufacture the interference filter having a passband center wavelength in the range 750-1000 nm inclusive. 
     
     
         29 . An interference filter comprising:
 a layers stack comprising plurality of layers of at least:
 layers of amorphous hydrogenated silicon and 
 layers of one or more dielectric materials having a refractive index lower than the refractive index of the amorphous hydrogenated silicon including layers of a dielectric material having a refractive index in the range 1.9 to 2.7 inclusive. 
   
     
     
         30 . The interference filter of  claim 29  wherein the layers of a dielectric material having a refractive index in the range 1.9 to 2.7 inclusive include one or more layers comprising Si 3 N 4 , SiO x N y  with y large enough to provide a refractive index of 1.9 or higher, Ta 2 O 5 , Nb 2 O 5 , or TiO 2 . 
     
     
         31 . The interference filter of  claim 29  wherein the layers of one or more dielectric materials further include SiO 2  layers. 
     
     
         32 . The interference filter of  claim 31  wherein the layers stack includes at least one SiO 2  layer immediately adjacent a layer of a dielectric material having a refractive index in the range 1.9 to 2.7 inclusive with no intervening layer of amorphous hydrogenated silicon. 
     
     
         33 . The interference filter of  claim 29  wherein the amorphous hydrogenated silicon includes nitrogen. 
     
     
         34 . The interference filter of  claim 33  wherein the amorphous hydrogenated silicon including nitrogen has an atomic concentration between 1% and 4% hydrogen and between 2% and 6% nitrogen.