Adhesion and Interfacial Structure of Metal Film Electrolessly Deposited on Si Using Au Nanoparticles as Catalysts

Surface metallization of silicon (Si), that is, adhesive metal-film formation on Si is important for obtaining infallible electrical contacts in various devices, such as solar cells and power devices. Autocatalytic electroless deposition, which is a conventional method to metallize nonmetallic substrates, has several advantages including simplicity of process, uniformity of films, and the covering of complicated structures. So, the autocatalytic electroless deposition is expected to replace sputtering of back metal of power devices and screen printing of metal electrodes of solar cells. This deposition requires surface activation (catalyzation pretreatment) of nonmetallic substrates, generally using tin and palladium (1). It is difficult to obtain adhesive metal films on Si substrates with conventional catalyzation pretreatments. Heat treatments before and after the deposition improved the adhesion of electrolessly deposited metal films on Si (2, 3). We recently developed a new surface-activation process for the direct electroless deposition of adhesive metal films on silicon substrates that consists of two steps (4): Step 1) formation of metal nanoparticles by electroless displacement deposition; and Step 2) Metal-film formation on the Si surface by autocatalytic electroless deposition. In previous study, we found that gold nanoparticles have an excellent ability to bind metal films to silicon surfaces (4). In addition, the contact resistance between Si substrate and the nickel-phosphorus alloy (Ni-P) film formed by this process was approximately 0.01 Ωcm²(5). In this study, we investigated the interfacial structure of Ni-P film/Si substrate by a transmission electron microscope (TEM) analysis.

                           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.

ACKNOWLEDGEMENTS

The present work was partly supported by JSPS KAKENHI (23560875) and A-STEP from JST.

REFERENCES

1. M. Paunovic and M. Schlesinger, Fundamentals of Electrochemical Deposition, 2nd. ed., Wiley, NY (2006).
2. V. M. Dubin, J. Electrochem. Soc., 139, 1289 (1992).
3. S. Karmalkar and V. P. Kumar, J. Electrochem. Soc., 151, C554 (2004).
4. S. Yae, et al., ECS Trans., 53 (6), 99 (2013).
5. Y. Orita, et al., Abst. 225th. ECS Meeting, Symp. D2 (2014).