Vertical transfer gate doping distribution for charge transfer from a photodiode
A pixel cell includes a photodiode disposed in a semiconductor material. A floating diffusion is disposed in the semiconductor material. A transfer gate includes a vertical transfer gate structure disposed in the semiconductor material between the photodiode and the floating diffusion. The transfer gate is coupled between the photodiode and the floating diffusion. A passivation layer is disposed in the semiconductor material and proximate to the vertical transfer gate. The passivation layer has a region with a non-uniformly distributed doping profile proximate to the vertical gate structure such that a first doping concentration of the region in the passivation layer proximate to the vertical gate structure along a first direction is less than a second doping concentration of the region in the passivation layer proximate to the vertical gate structure along a second direction.
1 . A pixel cell, comprising:
a photodiode disposed in a semiconductor material;
a floating diffusion disposed in the semiconductor material;
a transfer gate comprising a vertical transfer gate structure disposed in the semiconductor material between the photodiode and the floating diffusion, the transfer gate coupled between the photodiode and the floating diffusion; and
a passivation layer disposed in the semiconductor material and proximate to the vertical transfer gate, wherein the passivation layer has a region with a non-uniformly distributed doping profile proximate to the vertical gate structure such that a first doping concentration of the region in the passivation layer proximate to the vertical gate structure along a first direction is less than a second doping concentration of the region in the passivation layer proximate to the vertical gate structure along a second direction, and wherein a cross-section of the region with the non-uniformly distributed doping profile proximate to the vertical gate structure has an oval shaped profile.
2 . The pixel cell of claim 1 , wherein the first doping concentration of the region in the passivation layer proximate to the vertical gate structure is along the first direction that intersects with a charge transfer path between the photodiode and the floating diffusion, and wherein the second doping concentration of the region in the passivation layer proximate to the vertical gate structure is along the second direction that is parallel to the charge transfer path.
3 . The pixel cell of claim 1 , wherein the first direction is orthogonal to the second direction.
4 . The pixel cell of claim 1 , wherein a major axis of the oval shaped profile is along the second direction, and wherein a minor axis of the oval shaped profile is along the first direction.
5 . The pixel cell of claim 1 , wherein the first doping concentration in the passivation layer proximate to the vertical gate structure along the first direction is between 20% and 70% of the second doping concentration in the passivation layer proximate to the vertical gate structure along the second direction.
6 . The pixel cell of claim 1 , wherein the passivation layer is a doped region having an opposite conductivity type as the photodiode.
7 . The pixel cell of claim 6 , wherein the passivation layer is doped with boron.
8 . The pixel cell of claim 1 , wherein:
the photodiode is one of a plurality of photodiodes disposed in the semiconductor material;
the transfer gate is one of a plurality of transfer gates disposed in the semiconductor material,
each of the plurality of transfer gates is disposed in the semiconductor material between a respective one of the plurality of photodiodes and the floating diffusion;
the passivation layer is one of a plurality of passivation layers disposed in the semiconductor material; and
each of the plurality of passivation layers is proximate to the vertical transfer gate of a respective one of the plurality of transfer gates.
9 . A pixel cell, comprising:
a photodiode disposed in a semiconductor material;
a floating diffusion disposed in the semiconductor material;
a transfer gate comprising a vertical transfer gate structure disposed in the semiconductor material between the photodiode and the floating diffusion, the transfer gate coupled between the photodiode and the floating diffusion; and
a passivation layer disposed in the semiconductor material and proximate to the vertical transfer gate, wherein the passivation layer comprises doped material in the semiconductor material proximate to the vertical transfer gate and having a region with a non-uniformly distributed doping profile proximate to the vertical gate structure such that a first doping concentration of the region in the passivation layer proximate to the vertical gate structure along a first direction is less than a second doping concentration of the region in the passivation layer proximate to the vertical gate structure along a second direction, and wherein the doped material comprises:
a first doped region having the first doping concentration and surrounding the vertical transfer gate in the semiconductor material, wherein the first doped region has a first oval shaped profile with a major axis that intersects a charge transfer path between the photodiode and the floating diffusion; and
a second doped region having the second doping concentration and surrounding the vertical transfer gate in the semiconductor material, wherein the second doped region has a second oval shaped profile with a major axis that is parallel with charge transfer path, and wherein the second doping concentration is greater than the first doping concentration.
10 . The pixel cell of claim 9 , wherein the first doping concentration in the first doped region varies with respect to a distance from the charge transfer path.
11 . The pixel cell of claim 9 , wherein the first doping concentration in the passivation layer proximate to the vertical gate structure along the first direction is between 20% and 70% of the second doping concentration in the passivation layer proximate to the vertical gate structure along the second direction.
12 . The pixel cell of claim 9 , wherein the passivation layer is a doped region having an opposite conductivity type as the photodiode.
13 . The pixel cell of claim 9 , wherein the first direction is orthogonal to the second direction.
14 . The pixel cell of claim 9 , wherein:
the photodiode is one of a plurality of photodiodes disposed in the semiconductor material;
the transfer gate is one of a plurality of transfer gates disposed in the semiconductor material,
each of the plurality of transfer gates is disposed in the semiconductor material between a respective one of the plurality of photodiodes and the floating diffusion;
the passivation layer is one of a plurality of passivation layers disposed in the semiconductor material; and
each of the plurality of passivation layers is proximate to the vertical transfer gate of a respective one of the plurality of transfer gates.
15 . An imaging system, comprising:
a pixel array having a plurality of pixel cells, each of the plurality of pixel cells comprising:
a floating diffusion disposed in a semiconductor material,
four pixels, each pixel comprising:
a photodiode disposed in a semiconductor material; and
a transfer gate comprising a vertical transfer gate structure disposed in the semiconductor material between the photodiode and the floating diffusion,
wherein the four pixels are coupled to the floating diffusion, and wherein at least two of the pixels have a different first direction and a different second direction, and
a passivation layer disposed in the semiconductor material and proximate to the vertical transfer gate, wherein the passivation layer has a region with a non-uniformly distributed doping profile proximate to the vertical gate structure such that a first doping concentration of the region in the passivation layer proximate to the vertical gate structure along a first direction is less than a second doping concentration of the region in the passivation layer proximate to the vertical gate structure along a second direction; and
control circuitry coupled to the pixel array to control operation of the pixel array.
16 . The imaging system of claim 15 , wherein each passivation layer is a doped region having an opposite conductivity type as the photodiode.
17 . The imaging system of claim 15 , wherein each passivation layer is doped with boron.
18 . The imaging system of claim 15 , wherein the first doping concentration in the first direction that is less than the second doping concentration in the second direction causes charge to be transferred through the semiconductor material along a charge transfer path parallel to the second direction during a charge transfer time period.
19 . The imaging system of claim 15 , wherein the first doping concentration is between 20% and 70% of the second doping concentration.
20 . The imaging system of claim 15 , wherein the passivation layer comprises doped material in the semiconductor material proximate to the vertical transfer gate, and wherein the doped material comprises:
a first doped region having the first doping concentration and surrounding the vertical transfer gate in the semiconductor material, wherein the first doped region has a first oval shaped profile with a major axis that intersects a charge transfer path between the photodiode and the floating diffusion; and
a second doped region having the second doping concentration and surrounding the vertical transfer gate in the semiconductor material, wherein the second doped region has a second oval shaped profile with a major axis that is parallel with the charge transfer path, and wherein the second doping concentration is greater than the first doping concentration.
21 . The imaging system of claim 20 , wherein the first doping concentration of the first doped region varies with respect to a distance from the charge transfer path.