Stacked photo sensor assembly with pixel level interconnect
Embodiments relate to a stacked photo sensor assembly where two substrates that are stacked vertically. The two substrates are connected via interconnects at a pixel level to provide a signal from a photodiode at a first substrate to circuitry on a second substrate. The circuitry on the second substrate performs operations that were conventionally performed on first substrate. By stacking the first and second substrates, the photo sensor assembly can be made more compact while increasing or at least retaining the photodiode fill factor of the photo sensor assembly.
1. A pixel of a photo sensor comprising:
a portion of a first substrate comprising:
a photodiode for detecting light, and
a diffusion well associated with the photodiode to store charge generated from the photodiode and to generate a signal based on the stored charge, an amount of the charge stored in the diffusion well dependent on an intensity of the light incident on the photodiode;
a portion of a second substrate having a circuit for processing the signal from the first substrate, the circuit formed on a surface of the second substrate facing away from the first substrate; and
a direct pixel-level interconnect that connects the diffusion well of the first substrate to the circuit of the second substrate to carry the signal from the diffusion well to the circuit.
2. The pixel of a photo sensor according to claim 1 , wherein the circuit is a first circuit; and
wherein the first substrate further comprises:
a first transistor to be turned on by a second circuit during a non-exposure time of the pixel to divert the charge away from the diffusion well; and
a second transistor to be turned on by the second circuit to transfer the charge stored in the photodiode to the diffusion well.
3. The pixel of a photo sensor according to claim 1 , wherein the first substrate is P doped, and the diffusion well is N doped.
4. The pixel of a photo sensor according to claim 1 , wherein the first substrate has a front surface formed with the photodiode and the diffusion well, the front surface facing the second substrate.
5. The pixel of a photo sensor according to claim 1 , wherein the second substrate has a front surface formed with the circuit and a rear surface opposite to the front surface, the front surface facing away from the first substrate and the rear surface facing the first substrate such that the direct pixel-level interconnect penetrates through the second substrate from the rear surface to reach the circuit formed on the front surface.
6. The pixel of a photo sensor according to claim 1 , wherein the signal is a voltage signal dependent on the amount of charge stored in the diffusion well and a capacitance of the diffusion well.
7. The pixel of a photo sensor according to claim 1 , wherein the processing comprises at least one of sampling and analog-to-digital conversion.
8. A method for operating a photo sensor, the method comprising:
detecting light by a photodiode in a first substrate;
storing charge generated by the photodiode by a diffusion well in the first substrate, an amount of the charge dependent on an intensity of the light incident on the photodiode;
generating a signal in the first substrate based on the charge stored in the diffusion well;
sending the signal from the first substrate to a circuit formed on a surface of a second substrate facing away from the first substrate via a direct pixel-level interconnect that connects the diffusion well of the first substrate to the circuit; and
processing the signal by the circuit in the second substrate.
9. The method according to claim 8 , further comprising:
turning on a first transistor during a non-exposure time to divert the charge away from the diffusion well; and
turning on a second transistor to transfer the charge stored in the photodiode to the diffusion well.
10. The method according to claim 8 , wherein the first substrate is P doped, and the diffusion well is N doped.
11. The method according to claim 8 , wherein the first substrate has a front surface formed with the photodiode and the diffusion well facing the second substrate.
12. The method according to claim 8 , wherein the second substrate has a front surface formed with the circuit and a rear surface at an opposite side of the second substrate, the rear surface facing the first substrate such that the direct pixel-level interconnect penetrates through the second substrate from the rear surface to reach the front surface.
13. The method according to claim 8 , wherein the signal is a voltage signal dependent on the amount of the charge transferred from the photodiode to the diffusion well.
14. The method according to claim 8 , wherein the processing comprises at least one of sampling and analog-to-digital conversion.
15. The pixel of a photo sensor according to claim 1 , further comprising an insulation layer sandwiched between the first substrate and the second substrate; and
wherein the direct pixel-level interconnect penetrates through the insulation layer.
16. The pixel of a photo sensor according to claim 2 , wherein the second circuit is formed in the second substrate.