Image sensor and pixel having an anti-reflective coating over the photodiode
A pixel for use in CMOS or CCD image sensors is disclosed. The pixel includes a light sensitive element, such as a photodiode, formed in a semiconductor substrate. An anti-reflective coating is formed over the photodiode to reduce reflection of incident light. The reduced reflection results in greater “signal” reaching the photodiode.
1. A pixel comprising:
a light sensing element formed in a semiconductor substrate having a first index of refraction;
an anti-reflective coating formed atop said light sensing element; and
a planarized layer having a second index of refraction and formed directly atop said anti-reflective coating, wherein the anti-reflective coating is transparent to visible light and has an index of refraction between the first and second indexes of refraction, wherein said anti-reflective coating has a thickness approximately calculated as follows:
T=λ/ 4 N
where T is the thickness of the anti-reflective coating, λ is a wavelength for an incident light, and N is the index of refraction of the anti-reflective coating.
2. The pixel of claim 1 wherein said light sensing element is selected from the group of photodiode, pinned photodiode, partially pinned photodiode, or photogate.
3. The pixel of claim 1 further including:
a transfer transistor formed between said light sensing element and a floating node and selectively operative to transfer a signal from said light sensing element to said floating node; and
an amplification transistor controlled by said floating node.
4. The pixel of claim 1 further including:
a reset transistor formed between said light sensing element and a node and selectively operative to reset said node to a reference voltage; and
an amplification transistor controlled by said node.
5. The pixel of claim 1 wherein said light sensing element is a photodiode.
6. The pixel of claim 1 wherein said light sensing element includes a photodiode having a P+ pinning layer on the surface of said semiconductor substrate.
7. The pixel of claim 1 further including a buffer layer between said light sensing element and said anti-reflective coating.
8. The pixel of claim 7 wherein said buffer layer is a thin oxide layer.
9. The pixel of claim 7 wherein said thin oxide layer is silicon dioxide with a thickness of between 20 to 100 angstroms.
10. The pixel of claim 1 wherein the anti-reflective coating has a thickness of between 200–1500 angstroms thick.
11. The pixel of claim 10 wherein the anti-reflective coating has a thickness of between 300–1000 angstroms thick.
12. The pixel of claim 1 incorporated into a CMOS image sensor.
13. The pixel of claim 1 incorporated into a CCD image sensor.
14. The pixel of claim 1 wherein said anti-reflective coating is a multilayered stack.
15. The pixel of claim 14 wherein said multilayered stack includes at least two layers.
16. The pixel of claim 15 wherein said multilayered stack is SiO x N y /Si 3 N 4 .
17. The pixel of claim 15 wherein said multilayered stack is SiO X N Y /Si 3 N 4 /SiO W N Z .
18. The pixel of claim 1 wherein the anti-reflective coating is an index graded material.
19. The pixel of claim 18 wherein the index graded material is formed by the deposition of SiO X N Y wherein the flow of oxygen or nitrogen is varied during the deposition.
20. The pixel of claim 1 wherein said anti-reflective coating is silicon nitride.
21. A pixel comprising:
a photodiode formed in a semiconductor substrate having a first index of refraction;
a buffer layer formed atop of said photodiode;
an anti-reflective coating formed atop said photodiode; and
a planarized layer having a second index of refraction and formed directly atop said anti-reflective coating, wherein the anti-reflective coating is transparent to visible light and has an index of refraction between the first and second indexes of refraction, wherein said anti-reflective coating has a thickness approximately calculated as follows:
T=λ/ 4 N
where T is the thickness of the anti-reflective coating, λ is a wavelength for an incident light, and N is the index of refraction of the anti-reflective coating.
22. The pixel of claim 21 wherein said photodiode is a pinned photodiode.
23. The pixel of claim 21 further including:
a transfer transistor formed between said photodiode and a floating node and selectively operative to transfer a signal from said photodiode to said floating node; and
an amplification transistor controlled by said floating node.
24. The pixel of claim 21 further including:
a reset transistor formed between said photodiode and a node and selectively operative to reset said node to a reference voltage; and
an amplification transistor controlled by said node.
25. The pixel of claim 21 wherein said buffer layer is silicon oxide.
26. The pixel of claim 21 wherein said anti-reflective coating is silicon nitride.
27. The pixel of claim 21 wherein anti-reflective coating has a thickness of between 200–1500 angstroms thick.
28. The pixel of claim 21 incorporated into a CMOS image sensor.
29. The pixel of claim 21 incorporated into a CCD image sensor.
30. The pixel of claim 21 wherein said anti-reflective coating is a multilayered stack.
31. The pixel of claim 30 wherein said multilayered stack includes at least two layers.
32. The pixel of claim 31 wherein said multilayered stack is SiO x N y /Si 3 N 4 .
33. The pixel of claim 31 wherein said multilayered stack is SiO x N y /Si 3 N 4 /SiO w N z .
34. The pixel of claim 21 wherein the anti-reflective coating is an index graded material.
35. The pixel of claim 34 wherein the index graded material is formed by the deposition of SiO X N Y wherein the flow of oxygen or nitrogen is varied during the deposition.
36. A CMOS image sensor comprising:
a plurality of active pixels arranged in rows and columns, at least one of said active pixels comprising:
a light sensing element formed in a semiconductor substrate having a first index of refraction;
an anti-reflective coating formed above said light sensing element; and
a planarized layer having a second index of refraction and formed directly atop said anti-reflective coating, wherein the anti-reflective coating is transparent to visible light and has an index of refraction between the first and second indexes of refraction;
a processing circuit for receiving the output of said active pixels; and
an I/O circuit for outputting the output of said active pixels off of said CMOS image sensor, wherein said anti-reflective coating has a thickness approximately calculated as follows:
T=λ/ 4 N
where T is the thickness of the anti-reflective coating, λ is a wavelength for an incident light, and N is the index of refraction of the anti-reflective coating.
37. The image sensor of claim 36 further wherein said at least one active pixel further includes:
a transfer transistor formed between said light sensing element and a floating node and selectively operative to transfer a signal from said light sensing element to said floating node; and
an amplification transistor controlled by said floating node.
38. The image sensor of claim 37 further including:
a reset transistor formed between said light sensing element and a node and selectively operative to reset said node to a reference voltage; and
an amplification transistor controlled by said node.
39. The image sensor of claim 36 wherein said light sensing element is a photodiode.
40. The image sensor of claim 36 further including a buffer layer between said light sensing element and said anti-reflective coating.
41. The image sensor of claim 36 wherein said buffer layer is a thin oxide layer.
42. The image sensor of claim 41 wherein said thin oxide layer is silicon dioxide with a thickness of between 20 to 100 angstroms.
43. The pixel of claim 36 wherein anti-reflective coating has a thickness of between 200–1500 angstroms thick.
44. The pixel of claim 36 wherein said anti-reflective coating is a multilayered stack.
45. The pixel of claim 44 wherein said multilayered stack includes at least two layers.
46. The pixel of claim 45 wherein said multilayered stack is SiO x N y /Si 3 N 4 .
47. The pixel of claim 45 wherein said multilayered stack is SiO x N y /Si 3 N 4 /SiO w N z .
48. The pixel of claim 36 wherein the anti-reflective coating is an index graded material.
49. The pixel of claim 48 wherein the index graded material is formed by the deposition of SiO x N y wherein the flow of oxygen or nitrogen is varied during the deposition.