Image sensor pixel cell with global shutter having narrow spacing between gates
A pixel cell includes a photodiode, a storage transistor, a transfer transistor and an output transistor disposed in a semiconductor substrate. The transfer transistor selectively transfers image charge accumulated in the photodiode from the photodiode to the storage transistor. The output transistor selectively transfers the image charge from the storage transistor to a readout node. A first isolation fence is disposed over the semiconductor substrate separating a transfer gate of the transfer transistor from a storage gate of the storage transistor. A second isolation fence is disposed over the semiconductor substrate separating the storage gate from an output gate of the output transistor. Thicknesses of the first and second isolation fences are substantially equal to spacing distances between the transfer gate and the storage gate, and between the storage gate and the output gate, respectively.
1. A pixel cell, comprising:
a photodiode disposed in a semiconductor substrate to accumulate image charge;
a single gate oxide layer having a thickness disposed over the semiconductor substrate;
a single polysilicon gate layer disposed over the single gate oxide layer, wherein the single polysilicon gate layer includes a storage gate, a transfer gate, and an output gate;
a storage transistor disposed in the semiconductor substrate to store the image charge, the storage transistor including the storage gate disposed over the single gate oxide, wherein the storage gate is separated from the semiconductor substrate by only the thickness of the single gate oxide layer;
a transfer transistor coupled between the photodiode and an input of the storage transistor to selectively transfer the image charge from the photodiode to the storage transistor, the transfer transistor including the transfer gate disposed over the single gate oxide layer, wherein the transfer gate is separated from the semiconductor substrate by only the thickness of the single gate oxide layer;
an output transistor coupled to an output of the storage transistor to selectively transfer the image charge from the storage transistor to a readout node, the output transistor including the output gate disposed over the single gate oxide, wherein the output gate is separated from the semiconductor substrate by only the thickness of the single gate oxide layer;
a first isolation fence disposed over the semiconductor substrate separating the transfer gate from the storage gate, wherein a thickness of the first isolation fence is substantially equal to a spacing distance between the transfer gate and the storage gate; and
a second isolation fence disposed over the semiconductor substrate separating the storage gate from the output gate, wherein a thickness of the second isolation fence is substantially equal to a spacing distance between the storage gate and the output gate.
2. The pixel cell of claim 1 wherein the first and second isolation fences have thicknesses that are substantially equal to a minimum line width of a semiconductor device fabrication process.
3. The pixel cell of claim 1 wherein the first and second isolation fences have thicknesses that are less than a minimum polysilicon-to-polysilicon spacing distance of a semiconductor device fabrication process.
4. The pixel cell of claim 1 wherein the first and second isolation fences have thicknesses that are less than or equal to 0.06 microns.
5. The pixel cell of claim 1 wherein the first and second isolation fences comprise nitride.
6. The pixel cell of claim 1 wherein the transfer gate, the storage gate and the output gate comprise polysilicon.
7. The pixel cell of claim 1 wherein the spacing distance between the transfer gate and the storage gate is sufficiently narrow such that the transfer gate and the storage gate substantially shield light from leaking between the transfer gate and the storage gate.
8. The pixel cell of claim 1 wherein the spacing distance between the storage gate and the output gate is sufficiently narrow such that the storage gate and the output gate substantially shield light from leaking between the transfer gate and the storage gate.
9. The pixel cell of claim 1 wherein the readout node comprises a floating diffusion disposed in the semiconductor substrate.
10. The pixel cell of claim 1 further comprising:
a reset transistor disposed in the semiconductor substrate and coupled to the readout node;
an amplifier transistor disposed in the semiconductor substrate having an amplifier gate coupled to the readout node; and
a row select transistor disposed in the semiconductor substrate coupled between a bitline and the amplifier transistor.
11. The pixel cell of claim 1 further comprising a shutter gate transistor disposed in the semiconductor substrate and coupled to the photodiode to selectively deplete the image charge from the photodiode.
12. An imaging system, comprising:
a pixel array of pixel cells, wherein each one of the pixel cells includes:
a photodiode disposed in a semiconductor substrate to accumulate image charge;
a single gate oxide layer having a thickness disposed over the semiconductor substrate;
a single polysilicon gate layer disposed over the single gate oxide layer, wherein the single polysilicon gate layer includes a storage gate, a transfer gate, and an output gate;
a storage transistor disposed in the semiconductor substrate to store the image charge, the storage transistor including the storage gate disposed over the single gate oxide layer, wherein storage gate is separated from the semiconductor substrate by only the thickness of the single gate oxide layer;
a transfer transistor coupled between the photodiode and an input of the storage transistor to selectively transfer the image charge from the photodiode to the storage transistor, the transfer transistor including the transfer gate disposed over the single gate oxide layer, wherein the transfer gate is separated from the semiconductor substrate by only the thickness of the single gate oxide layer;
an output transistor coupled to an output of the storage transistor to selectively transfer the image charge from the storage transistor to a readout node, the output transistor including the output gate disposed over the single gate oxide layer, wherein the output gate is separated from the semiconductor substrate by only the thickness of the single gate oxide layer;
a first isolation fence disposed over the semiconductor substrate separating the transfer gate from the storage gate, wherein a thickness of the first isolation fence is substantially equal to a spacing distance between the transfer gate and the storage gate; and
a second isolation fence disposed over the semiconductor substrate separating the storage gate from the output gate, wherein a thickness of the second isolation fence is substantially equal to a spacing distance between the storage gate and the output gate;
control circuitry coupled to the pixel array to control operation of the pixel array; and
readout circuitry coupled to the pixel array to readout image data from the plurality of pixels.
13. The imaging system of claim 12 wherein the control circuitry is coupled to selectively send a global shutter signal to the pixel array to simultaneously enable all of the pixel cells in pixel array to simultaneously transfer the image charge from each respective photodiode during a single acquisition window.
14. The imaging system of claim 12 wherein the first and second isolation fences have thicknesses that are substantially equal to a minimum line width of a semiconductor device fabrication process.
15. The imaging system of claim 12 wherein the first and second isolation fences have thicknesses that are less than a minimum polysilicon-to-polysilicon spacing distance of a semiconductor device fabrication process.
16. The imaging system of claim 12 wherein the first and second isolation fences have thicknesses of less than or equal to 0.06 microns.
17. The imaging system of claim 12 wherein the first and second isolation fences comprise nitride.
18. The imaging system of claim 12 wherein the transfer gate, the storage gate and the output gate comprise polysilicon.
19. The imaging system of claim 12 wherein the spacing distance between the transfer gate and the storage gate is sufficiently narrow such that the transfer gate and the storage gate substantially shield light from leaking between the transfer gate and the storage gate.
20. The imaging system of claim 12 wherein the spacing distance between the storage gate and the output gate is sufficiently narrow such that the storage gate and the output gate substantially shield light from leaking between the transfer gate and the storage gate.
21. The imaging system of claim 12 wherein the readout node comprises a floating diffusion disposed in the semiconductor substrate.
22. The imaging system of claim 12 wherein each one of the pixel cells further comprises:
a reset transistor disposed in the semiconductor substrate and coupled to the readout node;
an amplifier transistor disposed in the semiconductor substrate having an amplifier gate coupled to the readout node; and
a row select transistor disposed in the semiconductor substrate coupled between a bitline and the amplifier transistor.
23. The imaging system of claim 12 wherein each one of the pixel cells further comprises a shutter gate transistor disposed in the semiconductor substrate and coupled to the photodiode to selectively deplete the image charge from the photodiode.