SPAD image sensor and associated fabricating method

1. A method of fabricating a single photon avalanche diode (SPAD) image sensor, the method comprising:

receiving a substrate having a front surface and a back surface, wherein the substrate has a first layer doped with dopants of a first conductivity type extending from the front surface to the back surface of the substrate;

forming two isolations extending from the front surface of the substrate toward the back surface of the substrate;

performing an ion implantation upon the substrate with dopants of a second conductivity type opposite to the first conductivity type to form a second layer along the front surface of the substrate to immediately abut the front surface of the substrate and immediately abut at least one of the isolations;

performing an ion implantation upon the front surface of the substrate with dopants of the second conductivity type opposite to the first conductivity type to form a sensing node within the second layer; and

performing an ion implantation upon the back surface of the substrate with dopants of the first conductivity type to form a common node.

2. The method of claim 1 , further comprising:

forming a grid structure including metal grid lines on the back surface of the substrate.

3. The method of claim 1 , wherein performing the ion implantation upon the back surface of the substrate with dopants of the first conductivity type to form the common node comprises:

forming the common node having a thickness less than about 0.5 μm.

4. The method of claim 1 , wherein the formation of the two isolations extending from the front surface of the substrate toward the back surface of the substrate comprises:

forming the two isolations extending from the front surface of the substrate toward the back surface of the substrate without exceeding the back surface of the substrate.

5. The method of claim 4 , wherein performing the ion implantation upon the substrate with dopants of the second conductivity type opposite to the first conductivity type to form the second layer along the front surface of the substrate to immediately abut the front surface of the substrate and immediately abut at least one of the isolations comprises:

performing the ion implantation upon the substrate with dopants of the second conductivity type opposite to the first conductivity type to form the second layer along the front surface of the substrate to immediately abut the front surface of the substrate and immediately abut both of the isolations.

6. The method of claim 5 , wherein a ratio of the dopant concentration of the sensing node to the dopant concentration of the second layer is in a range from about 10 to about 1000.

7. The method of claim 4 , further comprising:

performing an ion implantation upon the front surface of the substrate with dopants of the first conductivity type to form a third layer in the first layer and immediately abuts the second layer.

8. The method of claim 7 , wherein a dopant concentration of the third layer is greater than a dopant concentration of the first layer.

9. The method of claim 1 , further comprising:

forming an interconnect structure over the front surface of the substrate.

10. The method of claim 9 , further comprising:

bonding a chip to the interconnect structure.

11. The method of claim 10 , wherein performing the ion implantation upon the back surface of the substrate with dopants of the first conductivity type to form the common node comprises:

after the bonding, performing the ion implantation upon the back surface of the substrate with dopants of the first conductivity type to form the common node.

12. The method of claim 1 , further comprising:

forming a transparent conductive layer on the back surface of the substrate.

13. A method of fabricating a single photon avalanche diode (SPAD) image sensor, the method comprising:

receiving a substrate having a front surface and a back surface, wherein the substrate has a first layer doped with dopants of a first conductivity type extending from the front surface to the back surface of the substrate;

forming at least two trench isolations extending from the front surface of the substrate toward the back surface of the substrate;

performing an ion implantation upon the front surface of the substrate with dopants of a second conductivity type opposite to the first conductivity type to form a second layer to fully fills gaps between the at least two trench isolations; and

performing an ion implantation upon the front surface of the substrate with dopants of the second conductivity type to form a sensing node within the second layer.

14. The method of claim 13 , wherein performing the ion implantation upon the front surface of the substrate with dopants of the second conductivity type opposite to the first conductivity type to form the second layer comprises:

performing the ion implantation upon the front surface of the substrate with dopants of the second conductivity type opposite to the first conductivity type to form the second layer having a dopant concentration of about 1e17/cm 3 to about 1e19/cm 3 .

15. The method of claim 14 , wherein performing the ion implantation upon the front surface of the substrate with dopants of the second conductivity type to form the sensing node within the second layer comprises:

performing the ion implantation upon the front surface of the substrate with dopants of the second conductivity type to form the sensing node within the second layer abutting the front surface of the substrate.

16. The method of claim 13 , further comprising:

performing an ion implantation upon the back surface of the substrate with dopants of the first conductivity type to form a common node within the substrate and abutting the back surface of the substrate.

17. A method of fabricating a single photon avalanche diode (SPAD) image sensor, the method comprising:

receiving a substrate having a front surface and a back surface, wherein the substrate has a first layer doped with dopants of a first conductivity type extending from the front surface to the back surface of the substrate;

forming trench isolations extending from the front surface of the substrate toward the back surface of the substrate;

performing an ion implantation upon the front surface of the substrate with dopants of a second conductivity type opposite to the first conductivity type to form a second layer to fully fills gaps between the trench isolations;

performing an ion implantation upon the front surface of the substrate with dopants of the second conductivity type to form a sensing node within the second layer; and

performing an ion implantation upon the front surface of the substrate with dopants of the first conductivity type to form a third layer in the first layer and immediately abuts the second layer.

18. The method of claim 17 , further comprising:

performing an ion implantation upon the back surface of the substrate with dopants of the first conductivity type to form a common node within the substrate and abutting the back surface of the substrate.

19. The method of claim 18 , further comprising:

forming a grid structure on the back surface of the substrate.

20. The method of claim 18 , further comprising:

forming a transparent conductive layer on the back surface of the substrate.