Stability Evaluation of Non-Agglomerated Pd Nanoparticle Catalysts for Electroless Deposition

Conventional physical vapor depositions (PVD) of diffusion barrier and seed layers have been facing its limits of application for high aspect ratio of through Si via (TSV). Electroless plating (ELP) is an alternative candidate for TSV metallization because of their promising potential, e.g. conformal deposition property, cost reduction possible and low temperature.

In order to deposit ELP film on dielectric layer, catalyst process is the most important. In general, Sn-Pd process has been used for catalyst process of ELP. In addition, palladium nanoparticles (Pd-NPs) showed high density of adsorption, which assisted achievement of thin and conformal ELP formation in high aspect ratio TSV [1, 2].

To deposit electroless plating, high density of adsorption of Pd-NPs is required. Furthermore, in actual manufacturing process, long term stability that assumed repeated use is strongly desired.

In this study, we have developed the Pd-NPs to shorten the processing time in TSV and disperse in water for a long period. We found that the addition of compound A(COM-A ) or B enable to significantly improve the stability without losing the adsorption property of Pd-NPs.

Fig.1 shows the time transient of the diffusion coefficient of Pd-NPs with and without COM-A in solution which were stored at 25 °C. Diffusion coefficient of the particles were measured by dynamic light scattering (DLS).

In the Pd-NPs solution without COM-A, the diffusion coefficient of the particles gradually decreased. However, the diffusion coefficient of particles maintained a constant value in the Pd-NPs solution with COM-A.

According to the Stokes-Einstein equation, if the temperature and viscosity of a solution is constant, the decreasing of the diffusion coefficient indicates that the particles aggregate each other. Thus, the substantial catalyst particle number can be decreased for the case without COM-A.

Fig.2 shows SEM images of Pd-NPs adsorbed on blanket SiO₂. The Pd-NPs solutions were stored 40 days at 25 °C before use. In the case of Pd-NPs solution with COM-A, there is no agglomeration of the particles on the sample. On the other hand, particles agglomeration were occurred in case without COM-A.

In addition, we also found that the addition of COM-A is possible to dispersed again agglomerated Pd-NPs. These results suggest that the Pd-NPs solution with COM-A is applicable to actual manufacturing process.

[1] F. Inoue, S. Shingubara, et al., ECS Transactions, 25 (38) 31-36 (2010)

[2] F. Inoue, S. Shingubara, et al., Electrochimica Acta 82 (2012) 372-277