Cu Displacement Plating on Electroless Plated CoWB Layer on SiO2 and Its Adhesion Property

Abstract

Fabrication of TSV is one of key technology for 3D-ICs. For realizing conformal barrier layers against Cu diffusion with a low temperature process is important to achieve highly reliable TSVs. Electroless plated metals such as CoW-alloy and NiW-alloy are widely studied as a capping layer on Cu, and they are also considered to have a high potential as a barrier layer. We succeeded in conformal electroless plating of CoWB on SiO2, using DMAB as a reducing agent after forming self-assembled monolayer (SAM) of silane coupling agent on SiO2 [1-3], assisted with Pd nanoparticle catalyst. Electroless Cu plating on CoWB using reducing agent showed insufficient adhesion strength. The adhesion strength of CoWB decreased with an increase in film thickness [1], which might be due to an increase of film stress with increasing film stress. In this study, we tried to form electroless Cu seed layer by displacement plating without reducing agent, and investigated improvement of adhesion property. The Cu plating bath composed of sulfuric acid and copper sulfate , and its pH were varied between 1.5 to 3.5. Fig.1 shows cross-sectional SEM view after the Cu displacement plating in the case of pH 2.5 on CoWB film. Initially CoWB thickness was 70 nm (Fig.1-a), and 60 nm Cu layer was formed on the CoWB, while the thickness was decreased to 25 nm due to replacement reaction. Oxidation of Co atoms and reduction of Cu ions occurred concomitantly by displacement plating. The adhesion strength were evaluated by a stud-pull test. Table 1 shows the adhesion strengths of various films such as CoWB (70nm), displacement plated Cu/CoWB, electroless Cu plated with reducing agent on CoWB. The adhesion strength of the 70 nm CoWB film was 20MPa. That of displacement plated Cu/CoWB was 50 MPa. The adhesion strength of electroless plated Cu reducing agent was very poor. We were able to get enough adhesion strength with displacement plating. We are going to examine Cu displacement plating of various pH conditions. Furthermore, application of this process to TSV structure is in progress.