Anti-resonant reflecting optical waveguide for imager light pipe
An anti-resonant reflecting optical waveguide structure for reducing optical crosstalk in an image sensor and method of forming the same. The method includes forming a trench within a plurality of material layers and over a photo-conversion device. The trench is vertically aligned with the photo-conversion device and is filled with materials of varying refractive indices to form an anti-resonant reflecting optical waveguide structure. The anti-resonant reflecting optical waveguide structure has a core and at least two cladding structures. The cladding structure in contact with the core has a refractive index that is higher than the refractive index of the core and the refractive index of the other cladding structure. The cladding structures act as Fabry-Perot cavities for light propagating in the transverse direction, such that light entering the anti-resonant reflecting optical waveguide structure remains confined to the core. This reduces the chance of photons impinging upon neighboring photo-conversion devices.
1. A pixel cell comprising:
a photo-conversion device formed in association with a substrate;
a plurality of material layers over said substrate; and
an anti-resonant reflecting optical waveguide structure formed over said photo-conversion device and within said plurality of material layers, said waveguide structure comprising:
a trench within at least a portion of said plurality of material layers;
at least three cladding structures formed along a sidewall of said trench, the at least three cladding structures being contiguous along at least the entire length of said trench sidewall; and
a core material formed over the at least three cladding structures and filling a remaining portion of said trench, wherein a refractive index of said core material is lower than a refractive index of a first cladding structure in contact with said core material, and wherein a refractive index of a second cladding structure in contact with said first cladding structure is lower than said refractive index of said first cladding structure.
2. The pixel cell of claim 1 , wherein said trench is substantially vertically aligned with said photo-conversion device.
3. The pixel cell of claim 1 wherein cross-sectional shapes of said trench and said photo-conversion device are approximately the same.
4. The pixel cell of claim 2 , wherein said trench has a circular cross-sectional shape.
5. The pixel cell of claim 1 , wherein the at least three cladding structures and said core material are optically transparent.
6. The pixel cell of claim 1 , wherein the at least three cladding structures comprise one or more of Si x N y , Si 1−x Ti x O 2 , and doped SiO 2 .
7. The pixel cell of claim 1 , wherein said core material comprises one or more of a polymer or SiO 2 .
8. The pixel cell of claim 1 , wherein said core material is SiO 2 and the at least three cladding structures alternate between Si x N y and SiO 2 .
9. A pixel cell comprising:
a photo-conversion device formed in association with a substrate;
a plurality of material layers over said substrate; and
an anti-resonant reflecting optical waveguide structure formed over said photo-conversion device and within said plurality of material layers, said waveguide structure comprising:
a trench within at least a portion of said plurality of material layers;
a plurality of cladding structures formed along a sidewall of said trench, said plurality of cladding structures being contiguous along at least the entire length of said trench sidewall; and
a core material formed over said plurality of cladding structures and filling a remaining portion of said trench, wherein a refractive index of said core material is lower than a refractive index of a first cladding structure in contact with said core material, and wherein a refractive index of a second cladding structure in contact with said first cladding structure is lower than said refractive index of said first cladding structure,
wherein a thickness of a cladding structure is approximated by equation (1):
d
i
=
λ
4
(
n
i
2
-
n
c
2
)
+
λ
2
4
d
c
2
(
1
)
wherein d i , is a thickness of an i th cladding structure, n i is a refractive index of said i th cladding structure, n c is said refractive index of said core material, λ is a light wavelength, and d c is a width of said core material filling said trench.
10. A method of forming a pixel cell, said method comprising:
forming a photo-conversion device in a substrate;
forming a plurality of material layers over said substrate; and
forming an anti-resonant reflecting optical waveguide structure over said photo-conversion device and within said plurality of material layers, the act of forming said waveguide structure comprising:
forming a trench within at least a portion of said plurality of material layers,
forming at least three cladding structures along a sidewall of said trench, the at least three cladding structures being contiguous along at least the entire length of said trench sidewall, and
forming a core material over the at least three cladding structures to fill a remaining portion of said trench, wherein a refractive index of said core material is lower than a refractive index of a first cladding structure in contact with said core material, and wherein a refractive index of said first cladding structure is higher than a refractive index of a second cladding structure in contact with said first cladding structure.
11. The method of claim 10 , wherein said trench is substantially vertically aligned with said photo-conversion device.
12. The method of claim 10 , wherein said trench has a circular cross-sectional shape.
13. The method of claim 10 , wherein the at least three cladding structures and said core material are optically transparent.
14. The method of claim 10 , wherein the at least three cladding structures comprise one or more of Si x N y , Si 1−x Ti x O 2 , and doped SiO 2 .
15. The method of claim 10 , wherein said core material comprises one or more of a polymer or SiO 2 .
16. The method of claim 10 , wherein the act of forming the at least three cladding structures comprises alternating between Si x N y and SiO 2 .