Adhesion and Interfacial Structure of Metal Film Electrolessly Deposited on Si Using Au Nanoparticles as Catalysts
At the first step of the two-step process, gold nanoparticles, 5-25 nm in diameter and ca. 1011 cm-2in particles density, were deposited on a p-Si substrate by immersing the substrate in metal salt solution containing HF. At the second step, a Ni-P film was electrolessly formed on the Si substrate by using phosphinate as a reducing agent. The adhesion of deposited metal films on Si substrates was examined by a tape test based on Japanese Industrial Standard JIS H8504 corresponding to ISO 2819. We observed the interfacial structure by a TEM (JEOL JEM-2100). An X-ray analysis was performed using a FE-TEM (HITACHI HF-2000, 2-nm nanoprobe).
Figure 1 shows the percentage of the area of the electrolessly deposited Ni-P films that remained on the silicon substrates after the tape test as a function of the thickness of deposited Ni-P films. No peeling occurred for Ni-P films thinner than 1.3 μm after aging of samples at atmospheric ambient for a day. The thickness of starting point of peering increased to 1.7 μm after aging for 7 days. This shows that the adhesion of the films improves by aging under mild conditions.
Figure 2 shows the typical interfacial structures of metal-nanoparticle/Si. The Ag particles were in contact barely with Si (Fig. 2a). The Au nanoparticles formed mixing structure with Si (Fig. 2b and c). The formation of amorphous layer between Ni-P film and Si substrate was observed (Fig. 2c). These images indicate that a silicon and gold metallic alloy was formed at the interface between silicon and gold even at room temperature. This alloy formation is expected to improve the adhesion of metal film on silicon. Thus, gold nanoparticles work not only as catalysts to initiate autocatalytic electroless metal deposition but also as binding-points and as electric-contacts between the deposited metal film and the silicon surface.
The present work was partly supported by JSPS KAKENHI (23560875) and A-STEP from JST.
- M. Paunovic and M. Schlesinger, Fundamentals of Electrochemical Deposition, 2nd. ed., Wiley, NY (2006).
- V. M. Dubin, J. Electrochem. Soc., 139, 1289 (1992).
- S. Karmalkar and V. P. Kumar, J. Electrochem. Soc., 151, C554 (2004).
- S. Yae, et al., ECS Trans., 53 (6), 99 (2013).
- Y. Orita, et al., Abst. 225th. ECS Meeting, Symp. D2 (2014).