Direct Cu electrodeposition on electroless deposited NiWP barrier layer on SiO₂ substrate for all-wet metallization process

Electroless deposition of a NiWP barrier layer on a SiO₂ substrate was investigated for all-wet Cu interconnect fabrication. In this study, the entire fabrication process including substrate activation, barrier layer electroless deposition, and direct Cu electrodeposition wasmodified. The SiO₂ substratewas activated via Pd nanoparticles that were immobilized on the substrate by using a preformed selfassembled monolayer composed of 3-aminopropyl-triethoxysilane. Reduction of NiWP layer resistivity was achieved by applying ultrasound during the substrate activation process and by adding poly(ethylene glycol) to the electroless deposition bath. The Cu electrodeposition was performed directly on the NiWP layer after performing coulometric oxide reduction, thus improving the adhesion and nucleation density of Cu on the NiWP layer. The electrodeposition process was conducted in two steps: Cu nucleation and Cu thin film formation at a high overpotential followed by additional Cu film growth at a low overpotential. As a result, a uniform, smooth Cu film covered the NiWP layer. In addition, bottom-up Cu filling was accomplished on a 120 nm wide, patterned substrate with a 2.5 aspect ratio. Barrier layer performance was evaluated by using a Cu / NiWP / Pd / SiO₂ stacked specimen formed by applying the proposed procedure.