Monolithically stacked image sensors
An imaging system may be formed from multiple stacked wafers. A first wafer may include backside illuminated photodiodes, floating diffusion regions, and charge transfer gate structures. The first wafer may be bonded to a second wafer that includes pixel trunk transistors such as reset transistors, source-follower transistors, row-select transistors and associated logic circuits. The pixel trunk transistors may be formed using bottom-gate thin-body transistors. The first and second wafers may share the same backend metallization layers. The second wafer may further be bonded to a third wafer that includes digital signal processing circuits. The digital signal processing circuits may also be implemented using bottom-gate thin-body transistors. Additional metallization layers may be formed over the third wafer. The first, second, and third wafers may be fabricated using the same or different technology nodes.
1. Imaging circuitry, comprising:
a first substrate layer that includes photodiodes and floating diffusion regions;
a second substrate layer that is bonded to the first substrate layer and that includes pixel transistors;
an interconnect stack formed on the second substrate layer, wherein the interconnect stack includes metal structures that are coupled to the floating diffusion regions in the first substrate layer and to the pixel transistors in the second substrate layer, and wherein the second substrate layer is interposed between the first substrate layer and the interconnect stack;
a third substrate layer that is bonded to the second substrate layer and that includes digital signal processing circuits; and
an additional interconnect stack that is formed on the third substrate layer and that includes metal structures that are coupled to the pixel transistors in the second substrate layer, wherein the pixel transistors in the second substrate layer are fabricated using a first technology node and wherein the digital signal processing circuits on the third substrate layer are fabricated using a second technology node that is different than the first technology node.
2. The imaging circuitry defined in claim 1 , wherein the first substrate layer further includes charge transfer gate structures interposed between the photodiodes and the floating diffusion regions.
3. The imaging circuitry defined in claim 1 , further comprising:
at least one metallization layer interposed between the first substrate layer and the second substrate layer.
4. The imaging circuitry defined in claim 1 , wherein the pixel transistors comprise bottom-gate thin-body transistors.
5. The imaging circuitry defined in claim 1 , wherein the first substrate layer has a first surface and a second surface, wherein the photodiodes are formed in the first surface of the first substrate layer, and wherein incoming light is received through the second surface of the first substrate layer.
6. The imaging circuitry defined in claim 1 , wherein the third substrate layer includes digital memory elements.
7. A method of fabricating an image sensor, comprising:
forming photodiodes in a first wafer;
forming pixel transistors in a second wafer;
bonding the second wafer to the first wafer;
after bonding the second wafer to the first wafer, forming metallization layers on the second wafer;
forming digital signal processing circuits on a third wafer; and
after forming the metallization layers on the second wafer, bonding the third wafer to the metallization layers.
8. The method defined in claim 7 , further comprising:
after bonding the third wafer to the metallization layers, forming additional metallization layers on the third wafer.
9. The method defined in claim 8 , wherein forming metallization layers on the second wafer comprises forming a first number of metallization layers, and wherein forming the additional metallization layers comprises forming a second number of metallization layers that is greater than the first number of metallization layers.
10. The method defined in claim 7 , further comprising:
forming at least one metallization layer in the first wafer.
11. The method defined in claim 7 , wherein the second and third wafers each include a substrate, the method further comprising:
thinning the substrate of the second wafer; and
thinning the substrate of the third wafer after thinning the substrate of the second wafer.
12. The method defined in claim 7 , further comprising:
forming a trench in the first wafer; and
lining the trench with metal to form a bond pad region.
13. A system, comprising:
a central processing unit;
memory;
a lens;
input-output circuitry; and
an imaging device, wherein the imaging device comprises:
a first substrate layer in which photodiodes are formed, wherein the first substrate layer also includes floating diffusion regions and charge transfer gate structures interposed between the photodiodes and the floating diffusion regions; and
a second substrate layer that is bonded to the first substrate layer and that includes pixel transistors coupled to the photodiodes, wherein the pixel transistors include at least one transistor selected from the group consisting of: a row select transistor, a source follower transistor, and a reset transistor, and wherein the second substrate layer also includes analog-to-digital converter circuits interspersed among the pixel transistors.
14. The system defined in claim 13 , wherein each floating diffusion region is shared among multiple photodiodes.
15. The system defined in claim 13 , further comprising:
logic circuits that are formed in the second substrate layer and that are formed directly overlapping with at least some of the photodiodes.
16. The system defined in claim 15 , wherein the pixel transistors are grouped into discrete islands, and wherein the logic circuits surround each of the islands.
17. The system defined in claim 15 , wherein the pixel transistors are arranged in discrete groups, and wherein the logic circuits surround each of the groups.
18. The system defined in claim 13 , wherein the imaging device further comprises:
a third substrate layer that is bonded to the second substrate layer and that includes memory circuits configured to store digital images captured using the photodiodes in the first substrate layer.
19. The system defined in claim 13 , wherein the pixel transistors are grouped into discrete islands.
20. The system defined in claim 13 , wherein the pixel transistors are arranged in discrete groups.