Image sensor pixels with self-aligned lateral anti-blooming structures
Pixels for solid-state CMOS image sensor arrays may be provided that have a lateral blooming control structure incorporated in them. The lateral blooming control structure is built as a separate structure from the charge transfer gate and it is fabricated in a self-aligned manner, which is particularly suitable for incorporating into small size pixels. The blooming control structure can be used for backside or for front side illuminated image sensors. When the lateral blooming control structure is provided with a separate bias means, it may also be used for the complete or partial charge removal from the photodiode and thus used in pixels that are designed for global shutter operation.
1. An image sensor pixel in an image sensor having a silicon substrate and an interconnect stack formed on a surface of the silicon substrate, wherein the interconnect stack comprises a plurality of oxide layers, the image sensor pixel comprising:
a photodiode formed in the surface of the silicon substrate;
a transfer gate formed on the surface of the silicon substrate, wherein the transfer gate is configured to transfer charge from the photodiode to a floating diffusion;
a blooming control structure, wherein the blooming control structure is configured to remove overflow charge from the photodiode and wherein the blooming control structure is separate from the transfer gate; and
a conductive contact material in the interconnect stack that provides a bias voltage to the blooming control structure, wherein the conductive contact material is in contact with the surface of the silicon substrate and is at least partially surrounded by spacer structures that align the conductive contact material with the blooming control structure.
2. The image sensor pixel defined in claim 1 wherein the blooming control structure comprises:
a deep p+ type doped region formed in the silicon substrate below the conductive contact material;
an n+ type doped drain formed at the surface of the silicon substrate and in contact with the conductive contact material; and
a potential pinning p+ type doped region of the photodiode that at least partially surrounds the n+ type doped drain.
3. The image sensor pixel defined in claim 2 wherein the spacer structures are formed from nitride.
4. The image sensor pixel defined in claim 2 wherein the image sensor pixel is operable in global shutter mode and wherein the blooming control structure is configured to completely remove charge from the photodiode.
5. The image sensor pixel defined in claim 2 wherein the potential pinning p+ type doped region of PD is partially compensated by an n-type doped implant.
6. The image sensor pixel defined in claim 2 wherein the image sensor pixel is separated from other image sensor pixels in the image sensor by a plurality of p-type doped implants and wherein the deep p+ type doped region is interposed between the n+ type doped drain and the plurality of p-type doped implants.
7. The image sensor defined in claim 2 wherein the spacer structures have sidewall surfaces that align with edges of the n+ type doped drain of the blooming control structure.
8. A method for forming an anti-blooming structure in an image sensor, wherein the image sensor includes a silicon substrate, a photodiode formed in a surface of the silicon substrate, and a transfer gate formed on the surface of the silicon substrate, the method comprising:
depositing an oxide layer and a photoresist layer over the transfer gate and over the surface of the silicon substrate;
etching the photoresist layer and the oxide layer to form an opening in an anti-blooming region, wherein the anti-blooming region is laterally separated from the transfer gate;
depositing alignment structures in the opening; and
forming the anti-blooming structure in the silicon substrate under the opening, wherein the anti-blooming structure is configured to drain excess charge from the photodiode; and
using the alignment structures, forming a conductive contact material in the opening, wherein the conductive contact material provides a bias voltage to the anti-blooming structure.
9. The method defined in claim 8 wherein forming the anti-blooming structure comprises:
implanting a deep p+ type doped region into the silicon substrate; and
forming a surface p+ type doped region in the surface of the silicon substrate, wherein the deep p+ type doped region and the surface p+ type doped region are aligned with the opening.
10. The method defined in claim 9 wherein forming the anti-blooming structure further comprises:
using the alignment structures, implanting an n+ type doped drain in the surface of the silicon substrate, wherein the alignment structures have sidewall surfaces that align with edges of the n+ doped drain.
11. The method defined in claim 10 wherein depositing the alignment structures comprises depositing and etching a layer of nitride on the surface of the silicon substrate and in the opening.
12. The method defined in claim 10 further comprising:
after forming the anti-blooming structure in the silicon substrate, depositing additional oxide layers over the silicon substrate; and
etching a plurality of via holes in the additional oxide layers, wherein the alignment structures are resistant to oxide etching.
13. The method defined in claim 12 wherein forming the conductive contact material comprises forming the conductive contact material in one of the via holes to form an electrical connection with the n+ type doped drain, wherein the alignment structures automatically align the conductive contact material with the n+ type doped drain.
14. A system, comprising:
a central processing unit;
memory;
input-output circuitry; and
an imaging device, wherein the imaging device comprises an image pixel array having a silicon substrate and an interconnect stack formed on a surface of the silicon substrate, wherein the interconnect stack comprises a plurality of oxide layers, and wherein an image sensor pixel in the image pixel array comprises:
a photodiode formed in the surface of the silicon substrate;
a transfer gate formed on the silicon substrate, wherein the transfer gate is configured to transfer charge from the photodiode to a floating diffusion;
a blooming control structure, wherein the blooming control structure is configured to remove overflow charge from the photodiode and wherein the blooming control structure is separate from the transfer gate; and
a conductive contact material in the interconnect stack that provides a bias voltage to the blooming control structure, wherein the conductive contact material is in contact with the surface of the silicon substrate and is at least partially surrounded by spacer structures that align the conductive contact material with the blooming control structure.
15. The system defined in claim 14 wherein the blooming control structure comprises:
a deep p+ type doped region formed in the silicon substrate below the conductive contact material;
an n+ type doped drain formed at the surface of the silicon substrate and in contact with the conductive contact material; and
a potential pinning p+ type doped region of the photodiode that at least partially surrounds the n+ type doped drain.
16. The system defined in claim 15 wherein the spacer structures are formed from nitride.
17. The system defined in claim 15 wherein the image sensor pixel is operable in global shutter mode and wherein the blooming control structure is configured to completely remove charge from the photodiode.
18. The system defined in claim 15 wherein the potential pinning p+ type doped region of the photodiode is partially compensated by an n-type doped implant.
19. The system defined in claim 15 wherein the image sensor pixel is separated from other image sensor pixels in the image pixel array by a plurality of p-type doped implants and wherein the deep p+ type doped region is interposed between the n+ type doped drain and the plurality of p-type doped implants.
20. The system defined in claim 15 wherein the spacer structures have sidewall surfaces that align with edges of the n+ type doped drain of the blooming control structure.