Method for fabricating image sensor

1. A method for fabricating an image sensor, the method comprising:

doping a bottom portion of a semiconductor substrate with a first dopant to form a light-sensitive element in the bottom portion of the semiconductor substrate;

etching a top portion of the semiconductor substrate to form a post structure on the light-sensitive element;

forming a gate structure on at least one sidewall of the post structure, wherein the gate structure exposes a first part of the bottom portion of the semiconductor substrate;

after forming the gate structure, etching the exposed first part of the bottom portion of the semiconductor substrate, such that a top surface of the exposed first part of the bottom portion of the semiconductor substrate is lower than a top surface of a second part of the bottom portion of the semiconductor substrate below the gate structure;

after etching the exposed first part of the bottom portion of the semiconductor substrate, doping the exposed first part of the bottom portion of the semiconductor substrate with a second dopant to form a pinning layer on the light-sensitive element, wherein the second dopant has a conductivity type opposite a conductivity type of the first dopant, and a top surface of the pinning layer is lower than the top surface of the second part of the bottom portion of the semiconductor substrate below the gate structure; and

forming a contact on the post structure.

2. The method of claim 1 , wherein a top surface of the post structure is higher than a top surface of the gate structure.

3. The method of claim 1 , wherein after doping the exposed first part of the bottom portion of the semiconductor substrate with the second dopant, the second part of the bottom portion of the semiconductor substrate below the gate structure is free of the second dopant.

4. The method of claim 1 , wherein the pinning layer is laterally offset from the post structure.

5. The method of claim 1 , further comprises:

doping the second part of the bottom portion of the semiconductor substrate with a third dopant to form a doped region over the light-sensitive element, wherein the third dopant has a same conductivity type as the conductivity type of the first dopant, and the post structure is over the doped region.

6. The method of claim 1 , wherein forming the gate structure comprises:

forming a gate dielectric layer over the semiconductor substrate;

depositing a gate conductive layer over the gate dielectric layer, wherein the gate conductive layer and the gate dielectric layer covers the first part of the bottom portion of the semiconductor substrate; and

etching the gate conductive layer and the gate dielectric layer to form the gate structure.

7. The method of claim 1 , wherein a doping concentration of the second dopant in the pinning layer is greater than a doping concentration of the first dopant in the light-sensitive element.

8. A method for fabricating an image sensor, the method comprising:

doping a semiconductor substrate to form a light-sensitive element in a bottom portion of the semiconductor substrate;

etching a top portion of the semiconductor substrate to form a post structure on the light-sensitive element;

forming a gate structure having a first portion on a sidewall of the post structure and a second portion extending along a top surface of the bottom portion of the semiconductor substrate;

forming a pinning layer in the semiconductor substrate and on the light-sensitive element, wherein the pinning layer is in contact with a bottom surface of the second portion of the gate structure, and has a conductivity type opposite a conductivity type of the light-sensitive element; and

forming a gate contact on the second portion of the gate structure.

9. The method of claim 8 , wherein the first and second portions of the gate structure are over the light-sensitive element from a top view.

10. The method of claim 8 , wherein a top surface of the first portion of the gate structure is higher than a top surface of the second portion of the gate structure.

11. The method of claim 8 , further comprising:

doping a part of the top portion of the semiconductor substrate to form an ohmic contact layer prior to etching the top portion of the semiconductor substrate.

12. The method of claim 11 , further comprising:

forming a source contact on the post structure, wherein the source contact is in contact with the ohmic contact layer.

13. The method of claim 8 , wherein forming the gate structure comprises:

forming a gate dielectric layer on the sidewall of the post structure and the top surface of the bottom portion of the semiconductor substrate;

depositing a gate conductive layer over the gate dielectric layer; and

etching the gate conductive layer and the gate dielectric layer to form the gate structure.

14. The method of claim 8 , wherein the gate structure exposes a part of the bottom portion of the semiconductor substrate, and forming the pinning layer comprises:

doping the exposed part of the bottom portion of the semiconductor substrate to form the pinning layer on the light-sensitive element.

15. A method for fabricating an image sensor, the method comprising:

forming a light-sensitive element in a semiconductor substrate;

forming an ohmic contact node above the light-sensitive element;

etching the semiconductor substrate to form a post structure between the light-sensitive element and the ohmic contact node;

depositing a gate conductive layer over the post structure and the ohmic contact node;

etching the gate conductive layer into a gate conductor around the post structure, wherein a top surface of the gate conductor is below a bottom surface of the ohmic contact node; and

after etching the gate conductive layer into the gate conductor around the post structure, forming a first doped region in the post structure between the top surface of the gate conductor and the bottom surface of the ohmic contact node, wherein the first doped region surrounds a portion of the post structure, and the first doped region has a conductivity type the same as that of the light-sensitive element.

16. The method of claim 15 , wherein etching the gate conductive layer into the gate conductor is performed such that a bottom surface of the gate conductor is higher than a top surface of the light-sensitive element.

17. The method of claim 15 , further comprising:

forming a source contact over the ohmic contact node; and

forming a gate contact over the gate conductor.

18. The method of claim 15 , further comprising:

forming a second doped region in the semiconductor substrate over the light-sensitive element prior to forming the ohmic contact node, wherein the second doped region comprises a same conductivity type as that of the light-sensitive element, wherein etching the semiconductor substrate to form the post structure is performed such that the second doped region is etched to form a protrusion portion below the post structure.

19. The method of claim 15 , further comprising:

forming a gate dielectric layer, wherein a bottom surface of the gate dielectric layer is higher than a top surface of the semiconductor substrate; and

forming a pinning region in the semiconductor substrate, wherein the pinning region is in contact with the gate dielectric layer.