Semiconductor device and bump formation process

1. A method of forming a semiconductor device, the method comprising:

forming a pad region on a semiconductor substrate;

connecting a solder bump to the pad region;

forming a metal cap layer on a middle sidewall surface of the solder bump, wherein the metal cap layer has a first melting temperature greater than a second melting temperature of the solder bump, wherein connecting the solder bump to the pad region comprises the solder bump having a bottom portion disposed outside lateral extents of the metal cap layer; and

exposing a top portion of the solder bump.

2. The method of claim 1 , further comprising reflowing the solder bump.

3. The method of claim 1 , wherein forming the metal cap layer comprises the metal cap layer comprising nickel.

4. The method of claim 1 , wherein forming the metal cap layer comprises the metal cap layer comprising palladium or gold.

5. The method of claim 1 , wherein forming the metal cap layer comprises the metal cap layer comprising copper.

6. The method of claim 1 , wherein connecting the solder bump to the pad region comprises the solder bump comprising a lead-free solder material.

7. A method comprising:

forming a conductive pillar over a first conductive pad of a first substrate;

forming a metal cap layer over a top surface and over upper sidewalls of the conductive pillar, wherein the metal cap layer has a higher melting temperature than the conductive pillar;

removing portions of the metal cap layer to expose the top surface of the conductive pillar;

reshaping the conductive pillar after removing portions of the metal cap layer; and

after reshaping the conductive pillar, attaching the top surface of the conductive pillar to a second conductive pad of a second substrate.

8. The method of claim 7 , wherein the conductive pillar is formed of a solder material.

9. The method of claim 7 , wherein the metal cap layer is formed of a material harder than a material of the conductive pillar.

10. The method of claim 7 , wherein the metal cap layer is formed of a material selected from the group consisting of copper, nickel, gold, silver, palladium, indium, and alloy thereof.

11. The method of claim 7 , wherein removing portions of the metal cap layer further comprises removing portions of the metal cap layer along sidewalls of the conductive pillar and proximate the top surface of the conductive pillar.

12. The method of claim 7 , wherein the conductive pillar has a first shape before reshaping the conductive pillar and has a second shape different from the first shape after reshaping the conductive pillar.

13. The method of claim 12 , wherein the first shape of the conductive pillar has straight sidewalls, and the second shape of the conductive pillar has curved sidewalls.

14. The method of claim 7 , wherein reshaping the conductive pillar comprises performing a first reflow process.

15. The method of claim 14 , wherein attaching the top surface of the conductive pillar to the second conductive pad comprises performing a second reflow process separate from the first reflow process.

16. The method of claim 7 , wherein lower sidewalls of the conductive pillar are free of the metal cap layer, wherein after reshaping the conductive pillar, a lower portion of the conductive pillar proximate the first conductive pad protrudes beyond exterior sidewalls of the metal cap layer.

17. A method comprising:

forming a solder bump over a pad region of a first substrate, wherein a lower portion of the solder bump is embedded in a mask layer and an upper portion of the solder bump is exposed by the mask layer;

forming a metal cap layer over an upper surface of the solder bump and over sidewalls of the upper portion of the solder bump while sidewalls of the lower portion of the solder bump are covered by the mask layer;

removing the mask layer to expose sidewalls of the lower portion of the solder bump;

removing the metal cap layer from the upper surface of the solder bump to expose the upper surface of the solder bump;

performing a first reflow process to reshape the solder bump; and

after the first reflow process, attaching the upper surface of the solder bump to a second pad region of a second substrate.

18. The method of claim 17 , wherein after the first reflow process, portions of the solder bump extend beyond lateral extents of the metal cap layer.

19. The method of claim 17 , wherein the solder bump has straight sidewalls before the first reflow process, and wherein the solder bump has curved sidewalls after the first reflow process.

20. The method of claim 17 , wherein the metal cap layer has a first shape before the first reflow process, and wherein the first reflow process reshapes the metal cap layer into a second shape different from the first shape.