Semiconductor device with metal plug having rounded top surface and method for forming the same

A semiconductor device, comprising:

• a first metal plug and a first dielectric layer disposed over a semiconductor substrate;
• an etch stop layer disposed over the first dielectric layer, wherein the first metal plug has an upper portion protruding from the etch stop layer, the upper portion of the first metal plug has a convex top surface, and the etch stop layer adjoins a lower portion of the first metal plug;
• a second dielectric layer disposed over the etch stop layer, wherein a topmost point of the convex top surface is higher than a top surface of the second dielectric layer;
• and
• a second metal plug disposed over the first metal plug, wherein the second metal plug extends to contact a top surface of the etch stop layer.

A method for forming a semiconductor device, comprising:

• forming a first dielectric layer over a semiconductor substrate;
• forming an etch stop layer over the first dielectric layer;
• forming a second dielectric layer over the etch stop layer;
• forming a first metal plug penetrating through the second dielectric layer, the etch stop layer and the first dielectric layer, wherein the first metal plug protrudes from the second dielectric layer;
• performing an anisotropic etching process to partially remove the first metal plug such that the first metal plug has a convex top surface;
• forming a third dielectric layer covering the second dielectric layer and the convex top surface of the first metal plug;
• and
• forming a second metal plug over the first metal plug, wherein the second metal plug penetrates through the third dielectric layer and extends to contact the etch stop layer.