NEAR INFRARED OPTICAL INTERFERENCE FILTERS WITH IMPROVED TRANSMISSION
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.