Classification of Palladium Nucleation Processes Based on Nucleus Size Distribution

Abstract

Catalytic Pd nucleation is a crucial preparatory step for the activation of electroless ͑EL͒ Cu deposition. We have observed that the time evolution of catalytic Pd nucleation can be classified into stages according to the Pd nuclei density and the nucleus size distribution. We have tentatively labeled these stages as growth, secondary nucleation, and ripening. The stage identification is assisted with the observation of distribution range and semi-interquartile range. In this study we found that the smoothest EL Cu film can be achieved with Pd activation corresponding to the beginning of the ripening stage. Activation process is required to overcome the energy barrier in electroless ͑EL͒ Cu process. 1 Bindra and Roldan determined that Pd and Pt are the best catalyst in alkaline media. 2 Various activation methods have been proposed to pretreat the specimen surface for EL Cu deposition, such as excimer lamp and laser-assisted processes. 3-7 However, Dubin et al. have proposed a nonvacuum, cost-effective, contact-displacement method for the purpose of Cu bulk-fill. 1 Be-sides the advantages mentioned, this method provides good selectiv-ity and good adhesion to the substrate. EL Cu deposition is initiated on the randomly distributed catalyst particles on the substrate, and the initial Cu grain structure is largely determined by the surface morphology. Patterson et al. reported that Cu morphology and grain structure are dependent on the Pd activa-tion process and the control of the subsequent Cu plating. 8,9 Naka-hara and Okinaka have also found that the properties of the seed layer determine the EL Cu morphology. 10 The nature and distribu-tion of Pd nuclei formed during the activation process have been proven to be very important to the EL deposition behavior and de-posit quality. 11 However, detailed investigations of the effect of Pd nuclei on the resultant EL film have yet to be carried out. Previous investigations showed that the roughness of EL Cu film was closely related to the Pd nuclei density and size. Pd nucleation process was categorized into three stages based on the change in Pd nucleus size and nuclei density. The stages are growth, secondary nucleation, and ripening. We report our analysis on Pd nucleation stages utilizing Pd nuclei size distributions. This corresponds well with previous analy-ses of nucleation stages based on nucleus size and nuclei density changes.