Direct Copper Electroplating on Polyimide Film By Using Ni As Barrier and Conductive Interlayer

Ni interlayer is formed on the polyimide(PI) film by sequential steps: PI ring-opening by KOH_(aq), ions exchange between K⁺ and Ni²⁺, and the reduction of Ni²⁺. Dimethylamine borane (DMAB) with a specific additive is used as the reducing agent. The specific additive plays an important role that it not only shortens the Ni²⁺ reduction incubation time, but also makes the Ni film uniform and conductive. We also compared reducing agent DMAB with reducing agent NaBH₄. The reducibility of NaBH₄ is higher than that of DMAB, but NaBH₄ produced a dark and non-conductive Ni film because it contained a small amount of boron. FTIR and AFM were used to measure the functional group and surface roughness after PI ring-opening, and surface morphology after each steps were observed by SEM. Pinholes were formed on the Cu/Ni/PI when KOH strongly reacted with PI, and these caused adhesion problem between the deposited metal and the PI film. In this process, adhesion issues are summarized in the following two points: one is PI shrink and cracks formation in the ions exchange step because the pH difference between the first two steps is too big , the other is the moisture content in the ring-opened PI would affect the subsequent steps. As a result, with the specific additive in the reducing agent, copper can directly electrodeposition on the Ni/PI, and the adhesion between the deposited metal and PI film is depended on the condition of the ring-opening and the ions exchange steps.