Image sensor comprising a plurality of SPAD photodiodes
An image sensor including a plurality of avalanche photodiodes formed inside and on top of a semiconductor substrate of a first conductivity type having a front side and a back side, wherein: trenches vertically extend in the substrate from its front side to its back side, the trenches having, in top view, the shape of a continuous grid laterally delimiting a plurality of substrate islands, each island defining a pixel including a single individually-controllable avalanche photodiode, and including a doped area of collection of an avalanche signal of the pixel photodiode the lateral walls of the trenches are coated with a first semiconductor layer having a conductivity type opposite to that of the collection area, and a conductive region extends in the trenches, the conductive region being in contact with the surface of the first semiconductor layer opposite to the substrate.
1. An image sensor comprising a plurality of avalanche photodiodes with a vertical PN junction formed inside and on top of a semiconductor substrate of a first conductivity type having a front side and a back side, wherein:
trenches vertically extend in the substrate from its front side to its back side, the trenches having, in top view, the shape of a continuous grid laterally delimiting a plurality of substrate islands, each island defining a pixel comprising a single individually-controllable avalanche photodiode with a vertical PN junction, and comprising a doped area of collection of an avalanche signal of the pixel photodiode;
the lateral walls of the trenches are coated with a first semiconductor layer having a conductivity type opposite to that of the collection area; and
a conductive region extends in the trenches, said conductive region being in contact with the surface of the first semiconductor layer opposite to the substrate,
the sensor further comprising, in a central portion of each substrate island, a central region having a conductivity type opposite to that of the substrate forming the collection area of the island, vertically extending in the substrate from its front side to its back side, the junction between the lateral surfaces of the central region having a conductivity type opposite to that of the substrate and the substrate defining an avalanche area of an avalanche photodiode of the sensor.
2. The sensor according to claim 1 , wherein the first semiconductor layer is of the first conductivity type.
3. The sensor according to claim 1 , further comprising, in an upper portion of each substrate island, a ring-shaped region having a conductivity opposite to that of the substrate and a doping level lower than that of the central region having a conductivity opposite to that of the substrate, laterally extending from the lateral walls of the central region having a conductivity opposite to that of the substrate.
4. The sensor according to claim 1 , wherein the collection areas of the different pixels are electrically insulated from one another.
5. The sensor according to claim 1 , wherein the conductive region is opaque to the radiation to be detected.
6. The method according to claim 1 , wherein the conductive region is made of metal.
7. The sensor according to claim 6 , wherein the conductive region comprises copper or tungsten.
8. The sensor according to claim 1 , wherein the first semiconductor layer is made of doped polysilicon.
9. The sensor according to claim 1 , wherein the substrate rests, on its back side, on a third semiconductor layer having a conductivity type opposite to that of the substrate, the trenches emerging into or onto the third layer or crossing the third layer.
10. The sensor according to claim 9 , wherein the doping level of the third layer is lower than that of the first layer.