Image sensors with backside trench structures
A backside illumination image sensor with an array of image sensor pixels is provided. Each pixel may include a photodiode, a storage diode, and associated circuitry formed in a front side of a semiconductor substrate. In accordance with an embodiment, a trench isolation structure may be formed directly over the storage diode but not over the photodiode from a back side of the substrate. The backside trench isolation structure may be filled with absorptive material and can optionally be biased to a ground or negative voltage level. A light shielding layer may also be formed over the backside trench isolation structure on the back side of the substrate. The light shielding layer may be formed from absorptive material or reflective material, and may also be biased to a ground or negative voltage level.
1. An image sensor, comprising:
a substrate having a front surface and a back surface;
a first charge storage region that is formed in the front surface of the substrate;
a second charge storage region that is formed in the front surface of the substrate; and
a trench isolation structure that is formed in the back surface of the substrate, wherein the trench isolation extends from the back surface to a first depth in the substrate, wherein the second charge storage region extends from a second depth in the substrate to the front surface, wherein the trench isolation structure overlaps with the second charge storage region without overlapping the first charge storage region, and wherein the second depth is further from the back surface than the first depth.
2. The image sensor defined in claim 1 , wherein the trench isolation structure is formed from a material that exhibits the same refractive index as the substrate.
3. The image sensor defined in claim 1 , wherein the trench isolation structure is formed from a material selected from the group consisting of: polysilicon, silicon nitride, silicon oxynitride, silicon carbide, titanium oxide, aluminum silicide, tungsten silicide, tantalum silicide, titanium silicide, vanadium silicide, chromium silicide, and cobalt silicide.
4. The image sensor defined in claim 1 , wherein the trench isolation structure is conductive.
5. The image sensor defined in claim 4 , wherein the trench isolation structure is configured to receive a selected one of a ground voltage and a negative voltage.
6. The image sensor defined in claim 5 , further comprising:
a light shielding layer that is formed on the trench isolation structure at the back surface of the substrate and that has an opening for the first charge storage region.
7. A method of forming an image sensor on a substrate having a front side and a back side, comprising:
forming a photodiode in the front side of the substrate;
forming a storage diode in the front side of the substrate;
forming a cavity in the back side of the substrate; and
filling the cavity to form a backside trench isolation structure, wherein the backside trench isolation structure extends from the back side to a first depth in the substrate, the storage diode extends from a second depth in the substrate to the front side, and the second depth is further from the back side than the first depth.
8. The method defined in claim 7 , further comprising:
polishing the back side of the substrate to thin the substrate before forming the cavity.
9. The method defined in claim 7 , wherein the cavity is filled with optically absorptive material.
10. The method defined in claim 7 , wherein the cavity is filled with optically absorptive and conductive material.
11. The method defined in claim 7 , further comprising:
forming a via to make electrical contact with the backside trench isolation structure.
12. The method defined in claim 7 , further comprising:
forming a light shielding layer that only overlaps with the backside trench isolation structure on the back side of the substrate.
13. The method defined in claim 7 , further comprising:
forming a color filter array over the back side of the substrate.
14. A system, comprising:
a central processing unit;
memory;
a lens;
input-output circuitry; and
an imaging device, wherein the imaging device comprises:
a substrate having a first refractive index associated with a first real part;
a photodiode formed in the substrate;
a storage diode formed in the substrate; and
a trench isolation structure that is formed in the substrate and that overlaps with the storage diode, wherein the trench isolation structure is filled with a material having a second refractive index associated with a second real part that is substantially equal to the first real part.
15. The system defined in claim 14 , wherein the substrate has a first surface and a second opposing surface, wherein the photodiode and the storage diode are formed in the first surface of the substrate, and wherein the trench isolation structure is formed in the second surface of the substrate directly over the storage diode.
16. The system defined in claim 15 , wherein the imaging device further comprises:
microlens structures formed over the second surface of the substrate.
17. The system defined in claim 15 , wherein the first refractive index is associated with a first imaginary part, and wherein the second refractive index is associated with a second imaginary part that is greater than the first imaginary part.
18. The system defined in claim 15 , wherein the imaging device further comprises:
a light shielding structure that is formed over the trench isolation structure on the second surface of the substrate.