Performing die-to-wafer stacking by filling gaps between dies

1. A method of forming an integrated circuit structure, the method comprising:

providing a bottom wafer comprising a plurality of bottom semiconductor chips;

providing a plurality of top dies, each bonded to one of the plurality of bottom semiconductor chips;

forming an enclosure ring on, and close to an outer parameter of, the bottom wafer;

applying a protecting material filling spacings between the plurality of top dies, wherein a top surface of the protecting material, top surfaces of the top dies, and a top surface of the enclosure ring are leveled;

forming a planar dielectric layer on the plurality of top dies and the protecting material; and

forming a conductive feature in the planar dielectric layer, wherein the conductive feature is electrically connected to at least one of the plurality of top dies and the plurality of bottom semiconductor chips.

2. The method of claim 1 further comprising:

forming an addition dielectric layer over the dielectric layer; and

forming a dual damascene conductive structure in the addition dielectric layer and connected to the conductive feature.

3. The method of claim 1 , wherein the step of forming the enclosure ring comprises:

applying a polymer ring; and

pre-baking the polymer ring before the step of applying the protecting material.

4. The method of claim 1 , wherein the step of forming the enclosure ring comprises laminating a dry film as the enclosure ring.

5. The method of claim 1 further comprising, after the steps of forming the enclosure ring and applying the protecting material, performing a planarization to level the top surface of the protecting material, the top surfaces of the top dies, and the top surface of the enclosure ring.

6. The method of claim 5 , wherein after the planarization, a through-silicon via (TSV) in the top die is exposed through a top surface of one of the plurality of the top dies, wherein the conductive feature is connected to the TSV.

7. The method of claim 5 further comprising, after the step of planarization, forming a TSV in one of the plurality of top dies, wherein the conductive feature is connected to the TSV.

8. The method of claim 5 further comprising, after the step of planarization, forming a contact plug penetrating the protecting material, wherein the contact plug connects the conductive feature and one of the plurality of bottom semiconductor chips.

9. The method of claim 5 further comprising, before the step of performing the planarization, baking the protecting material and the enclosure ring.

10. The method of claim 1 , wherein the step of applying the protecting material comprises stencil printing.

11. The method of claim 1 , wherein the step of applying the protecting material comprises screen printing using a mask having openings corresponding to positions of the spacings between the top dies.

12. The method of claim 1 , wherein the conductive feature has a top surface leveled with a top surface of the planar dielectric layer.

13. A method of forming an integrated circuit structure, the method comprising:

providing a bottom wafer comprising a plurality of bottom semiconductor chips;

bonding a plurality of top dies, each onto one of the plurality of bottom semiconductor chips;

forming an enclosure ring on, and close to an outer parameter of, the bottom wafer;

applying a protecting material filling spacings between the plurality of top dies, wherein the protecting material is enclosed by the enclosure ring;

solidifying the protecting material;

polishing to planarize the protecting material, the plurality of top dies, and the enclosure ring;

forming a planar dielectric layer on the plurality of top dies, the protecting material, and the enclosure ring; and

forming a copper line in the planar dielectric layer, wherein the copper line is electrically connected to at least one of the plurality of top dies and the plurality of bottom semiconductor chips.

14. The method of claim 13 , wherein the plurality of top dies and the plurality of bottom semiconductor chips are bonded face-to-face.

15. The method of claim 13 , wherein after the polishing, through-silicon vias (TSV) in the plurality of top dies are exposed, and wherein the copper line is connected to one of the TSVs.

16. The method of claim 13 further comprising, after the polishing, forming a TSV in one of the plurality of top dies, wherein the copper line is connected to the TSV.

17. The method of claim 13 further comprising, after the polishing, forming a contact plug penetrating the protecting material, wherein the contact plug connects the copper line and one of the plurality of bottom semiconductor chips.

18. The method of claim 13 , wherein the copper line has a top surface leveled with a top surface of the planar dielectric layer.

19. The method of claim 1 wherein after the step of applying a protecting material, a top surface of the protecting material is below top surfaces of the top dies.

20. The method of claim 1 further comprising removing excess protecting material from top surfaces of the top dies after applying the protecting materials.