Image sensors with voltage-biased trench isolation structures
View Patent ↗An 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 semiconductor substrate. Buried light shields may be formed on the substrate to prevent regions between two adjacent photodiodes from being exposed to incoming light. In one embodiment, a shallow trench isolation (STI) structure may be formed between the photodiode and the storage diode, and a conductive layer formed from optically absorptive material may be constructed at the bottom of the STI structure. A via may be formed through the STI structure to help bias the conductive layer using a ground or negative voltage. In another embodiment, an isolation ring structure may be formed at the base of the buried light shields. The isolation ring structure may be formed from optically absorptive material and can optionally be biased using a ground or negative voltage.
1. An image sensor, comprising:
a substrate;
a first charge storage region formed in the substrate;
a second charge storage region formed in the substrate;
a shallow trench isolation (STI) structure that is interposed between the first and second charge storage regions in the substrate;
a conductive layer that is formed at the bottom of the STI structure; and
a conductive via that is formed through the STI structure to make direct contact with the conductive layer, wherein the conductive via is formed separately from the conductive layer and wherein the conductive layer is biased with a bias voltage to provide a potential barrier between the first charge storage region and the second charge storage region.
2. The image sensor defined in claim 1 , wherein the conductive via is configured to receive the bias voltage and wherein the bias voltage is a ground voltage.
3. The image sensor defined in claim 1 , wherein the conductive via is configured to receive the bias voltage and wherein the bias voltage is a negative voltage.
4. The image sensor defined in claim 1 , wherein the first charge storage region comprises a photodiode, the image sensor further comprising:
a buried light shielding structure that is formed at least partially over the second charge storage region and that is shorted to the conductive layer.
5. The image sensor defined in claim 4 , further comprising:
an isolation ring structure that is formed at the base of the buried light shielding structure and that is formed from optically absorptive material.
6. The image sensor defined in claim 1 , wherein the conductive layer is formed from optically absorptive material.
7. A system, comprising:
a central processing unit;
memory;
a lens;
input-output circuitry; and
an imaging device, wherein the imaging device comprises:
a substrate;
a photodiode formed in the substrate;
a storage diode formed in the substrate;
a shallow trench isolation (STI) structure formed in the substrate between the photodiode and the storage diode;
a conductive via that is formed through the STI structure;
a buried light shield that is formed at least partly over the storage diode, wherein the conductive via is coupled to the buried light shield; and
an optically absorptive layer that is formed within the STI structure and that is electrically coupled to the conductive via, wherein the conductive via is formed separately from the optically absorptive layer.
8. The system defined in claim 7 , wherein the optically absorptive layer is electrically biased to provide a potential barrier between the photodiode and the storage diode.
9. The system defined in claim 7 , wherein the imaging device further comprises:
an optical isolation ring structure that is formed on the surface of the substrate.
10. The system defined in claim 9 , wherein the optical isolation ring structure is formed from materials selected from the group consisting of: tungsten, tantalum, titanium, and nitride.
11. The system defined in claim 7 , wherein the STI structure has first and second opposing sides, wherein the optically absorptive layer is on the first side of the STI structure, wherein the buried light shield is on the second side of the STI structure, and wherein the conductive via extends from the first side to the second side to couple the buried light shield to the optically absorptive layer.