Fabrication of Ni Microbumps With Small Feature Size on Au Using Electroless Ni Plating With Noncontact Induction

Abstract

This paper reports a method of fabrication for small-feature-sized nickel (Ni) microbumps on gold (Au) using a newly developed technique called electroless Ni plating with noncontact induction (ENPNI). This technique, differing from conventional electroless Ni plating with contact induction (ENPCI), which directly connects an active metal with an inactive metal to induce Ni electrochemical reaction, is characterized by separation of the active metal and the target metal. The mechanism of ENPNI is interpreted by employing the electric-double layer theory, and some phenomena are explained by the proposed mechanism. Ni microbumps with a diameter of 3–6 $\mu \text{m}$ and a height of 3–4 $\mu \text{m}$ have been successfully fabricated using ENPNI. The resistance of the Ni microbumps is measured, and yield and uniformity are evaluated. By breaking the restriction of contact, ENPNI has the advantages of no need for pretreatment and contact induction, allowing fabrication of microbumps with small feature sizes for applications in which direct contact of an active induction metal is impossible.