Method and apparatus for forming bump structure used for flip-chip mounting, the bump structure and the flip-chip

A method for forming a bump structure wherein a first bump is formed on a second bump, the first bump being made of a conductive material, a lower surface of which is fixed to an electrode of a substrate, and which is substantially spherical and has a projected portion at a center portion on an upper surface thereof, and the second bump being made of a conductive material, which covers said projected portion on said first bump and which has a diameter smaller than an outer diameter of said first bump, the method comprising the steps of:

• placing an opening of a mask which opening is located at a position corresponding to that of said first bump on said substrate, said opening having a diameter larger than the outer diameter of said first bump;
• applying a pressure to said second conductive material in a liquid form filled under said mask which is horizontally placed, thereby forming at said opening a protuberance of said second conductivity material;
• and
• causing said projected portion of the first bump to be sank in said protuberance, raising said substrate for said first bump to move away from said protuberance, and causing said second conductive material to adhere to a tip end of said first bump, thereby forming said second bump on said first bump.