SPAD photodiode

1. A SPAD-type photodiode comprising:

a semiconductor substrate of a first conductivity type having a front side and a back side;

a first semiconductor region of the second conductivity type extending in the substrate from the front side thereof and towards the back side thereof, the lateral surfaces of the first region being in contact with the substrate and the junction between the lateral surfaces of the first region and the substrate defining an avalanche area of the photodiode; and

a first semiconductor layer coating the back side of the substrate, the first region thoroughly crossing the substrate and stopping in the first layer, the first layer being doped with the second conductivity type.

2. The photodiode of claim 1 , wherein the first layer has a doping level lower than that of the first region.

3. The photodiode of claim 1 , wherein the first layer has substantially the same doping level as the first region, and wherein the junction between the first layer and the substrate defines an additional avalanche area of the photodiode.

4. The photodiode of any of claim 1 , further comprising a first metallization for biasing the substrate, located on the front side of the substrate, and a second metallization for biasing the first region, located on the front surface of the first region.

5. The photodiode of claim 1 , wherein the first region comprises a plurality of semiconductor bars or tubes of the second conductivity type extending in the substrate.

6. The photodiode of claim 5 , wherein the distance, in transverse cross-section, between two neighboring bars or tubes of the first region is shorter than or equal to 3μm.

7. The photodiode of claim 1 , wherein the first region extends in the substrate down to a depth in the range from 5 to 25 μm.

8. The photodiode of claim 1 , further comprising a second semiconductor layer coating the front side of the substrate, the first region thoroughly crossing the second layer, the second layer being doped with the first conductivity type and having a doping level lower than that of the substrate.

9. The photodiode of claim 1 , further comprising a local doped region of the second conductivity type, having a doping level lower than that of the first region, extending in the substrate from the upper surface thereof and laterally surrounding the first region.

10. The photodiode of claim 1 , wherein the substrate is made of silicon, the first region being formed by filling, with polysilicon having a conductivity type opposite to that of the substrate, a trench formed in the substrate.

11. The photodiode of claim 10 , wherein the first region is formed by partial filling, with polysilicon having a conductivity type opposite to that of the substrate, a trench formed in the substrate, the trench further comprising a layer of an opaque material coating the polysilicon, and an insulating material ensuring the filling of the trench.

12. The photodiode of claim 1 , wherein the substrate rests, on its back side, on an insulating layer, and wherein the first region thoroughly crosses the substrate and stops on the front side of the insulating layer.

13. The photodiode of claim 12 , wherein the first region is formed by partially filling, with polysilicon having a conductivity type opposite to that of the substrate, a trench formed in the substrate, the trench being then filled with an insulating material to obtain an insulating wall coupling the front surface of the insulating layer to the front side of the substrate.