Pixel and imager device having high-k dielectrics in isolation structures
An imager device that has an isolation structure such that pinned photodiode characteristics are maintained without increasing doping levels. The invention provides an isolation structure to maintain pinned photodiode characteristics without increasing doping levels around the photodiode. By creating a substrate region surrounding the charge-collection region of the photodiode, the photodiode may be electrically isolated from the bulk substrate. This region fixes the depletion region so that it does not migrate toward the surface of the substrate or the STI region. By doing so, the region prevents charge from being depleted from the substrate and the accumulation region, reducing dark current.
1. A pixel cell comprising:
a photodiode in a substrate, the photodiode comprising a top region, a middle region, and a bottom region connected to the top region through an n-type intermediate region wherein the middle region is a p-type material and the top and bottom are n-type material; and
a first high-k dielectric material having excess positive charge over the top n-type region to provide electron accumulation in the top region and intermediate region.
2. The pixel cell of claim 1 , wherein a second high-k dielectric material is provided in a trench.
3. The pixel cell of claim 2 , wherein the trench is filled with the second high-k dielectric material which comprises silicon nitride.
4. The pixel cell of claim 1 , wherein the first high-k dielectric material comprises silicon nitride.
5. The pixel cell of claim 1 , wherein the first high-k dielectric material prevents a depletion region of the photodiode from reaching a top surface of the substrate.
6. A pixel cell comprising:
a photodiode comprising a p-type charge-collection region, top accumulation region, bottom accumulation region and side accumulation region, wherein each of the top accumulation region, bottom accumulation region, and side accumulation region are n-type and adjacent the p-type charge-collection region; and
an isolation structure comprising;
a trench containing a first high-k dielectric material having excess positive charge, the trench being adjacent the side n-type accumulation region, wherein the excess positive charge of the first high-k dielectric material causes electron accumulation in the side n-type accumulation region; and
a second high-k dielectric material having excess positive charge residing over the trench and the photodiode, wherein the excess positive charge of the second high-k dielectric material causes electron accumulation in the top n-type accumulation region.
7. The pixel cell of claim 6 , wherein the trench of the isolation structure has at least a bottom and sidewalls.
8. The pixel cell of claim 6 , wherein the trench has a depth between about 1000 Å and about 4000 Å.
9. The pixel cell of claim 6 , wherein the trench has a depth between about 1500 Å and about 3000 Å.
10. The pixel cell of claim 6 , wherein the trench has a width between about 500 Å and about 10000 Å.
11. The pixel cell of claim 6 , wherein the trench has a width between about 1000 Å and about 3000 Å.
12. The pixel cell of claim 6 wherein the isolation structure further comprises a silicon dioxide layer between the walls of the trench and the first high-k dielectric material.
13. The pixel cell of claim 6 wherein the first and second high-k dielectric materials are silicon nitride.
14. The pixel cell of claim 6 further comprising an insulating material between the trench wall and the first high-k dielectric material.
15. The pixel cell of claim 14 wherein the insulating material is selected from the group consisting of silicon dioxide, oxide-nitride, nitride-oxide, and oxide-nitride-oxide.
16. The pixel cell of claim 6 further comprising silicon dioxide over the top n-type accumulation region.
17. The pixel cell of claim 6 wherein the second high-k dielectric material resides exclusively over the trench and the photodiode.
18. The pixel cell of claim 6 further comprising a spacer insulating layer formed over the trench and the photodiode.