Integrated circuit structure having dies with connectors

1. An integrated circuit structure comprising:

a first die comprising a bump structure; and

a second die comprising a pad structure comprising at least one of a nickel (Ni) layer, a palladium (Pd) layer or a gold (Au) layer;

wherein the first die is attached to the second die by the bump structure being attached to the pad structure; and

wherein the bump structure comprises:

a metal pillar comprising copper;

a metal cap layer comprising nickel on the metal pillar, the metal cap layer having a first side facing away from the metal pillar;

a metal insertion layer comprising copper on the metal cap layer, the metal insertion layer completely covering the first side of the metal cap layer, the metal cap layer being disposed between the metal pillar and the metal insertion layer;

a solder layer on the metal insertion layer; and

an intermetallic compound (IMC) disposed between the solder layer and the metal insertion layer, the IMC comprising copper (Cu) and being substantially free of nickel (Ni).

2. The integrated circuit structure of claim 1 , wherein the pad structure comprises an electroless Ni layer, an electroless Pd layer and an immersion Au layer.

3. The integrated circuit structure of claim 1 , wherein the first die is an active die, and the second die is an interposer free of active devices.

4. The integrated circuit structure of claim 1 , wherein the second die comprises a through substrate via.

5. The integrated circuit structure of claim 1 , further comprising a workpiece electrically coupling the second die through a plurality of connectors.

6. The integrated circuit structure of claim 1 , wherein the second die comprises a metal layer comprising copper and underlying the pad structure.

7. The integrated circuit structure of claim 1 , wherein the first die comprises a post-passivation interconnect (PPI) layer underlying the bump structure.

8. The integrated circuit structure of claim 1 , wherein the solder layer is a lead-free solder layer.

9. An integrated circuit structure comprising:

a first die attached to a second die by a first connector,

wherein the first connector comprises:

a solder joint portion between a nickel layer and a pad structure;

a copper layer between the nickel layer and the solder joint portion; and

an intermetallic compound (IMC) comprising copper (Cu) between the solder joint portion and the copper layer, the IMC being substantially free of nickel (Ni), the copper layer completely separating the IMC from the nickel layer; and

wherein the pad structure comprises at least one of a nickel (Ni) layer, a palladium (Pd) layer or a gold (Au) layer.

10. The integrated circuit structure of claim 9 , wherein the solder joint portion comprises a lead-free solder material.

11. The integrated circuit structure of claim 9 , wherein the first connector comprises a copper pillar on a first substrate of the first die, and wherein the nickel layer is between the copper pillar and the copper layer.

12. The integrated circuit structure of claim 9 , wherein the second die comprises a metal layer comprising copper on a second substrate, and the pad structure is formed on the metal layer.

13. The integrated circuit structure of claim 9 , wherein the first die is an active die, and the second die is an interposer free of active devices.

14. The integrated circuit structure of claim 9 , wherein the second die comprises a through substrate via.

15. The integrated circuit structure of claim 9 , further comprising a workpiece electrically coupling the second die through a plurality of second connectors.

16. A method of forming an integrated circuit structure comprising:

forming on a first die a bump structure including:

forming on a surface of the first die a metal pillar comprising copper;

forming on the metal pillar a metal cap layer comprising nickel;

forming on the metal cap layer a metal insertion layer comprising copper; and

forming on the metal insertion layer a solder layer;

forming on a second die a pad structure by forming at least one of a nickel (Ni) layer, a palladium (Pd) layer and a gold (Au) layer;

forming an intermetallic compound (IMC) comprising copper (Cu) between the solder layer and the metal insertion layer, the IMC being substantially free of Ni; and

bonding the bump structure to the pad structure, at least a portion of the metal insertion layer contacting a center of a first surface of the metal cap layer facing the pad structure after the bonding, and the IMC being between the solder layer and the metal insertion layer after the bonding.

17. The method of claim 16 , wherein forming a pad structure comprises at least one of electroless plating Ni, electroless plating Pd, or an immersion Au process.

18. The integrated circuit structure of claim 1 , wherein the IMC has a thickness between about 3 μm and about 5 μm.

19. The integrated circuit structure of claim 9 , wherein the IMC has a thickness between about 3 μm and about 5 μm.

20. The method of claim 16 , wherein metal insertion layer has a thickness greater than about 1.5 μm.