Formation of adhesive metal films on silicon wafers is important for electrodes of various devices. Autocatalytic electroless deposition has several advantages, e.g. simplicity of process, uniformity of deposited films, and covering on complicated structures. However, adhesion of metal films formed on silicon substrates using conventional pretreatments is not sufficient for the practical applications. We reported that adhesive metal films can be directly formed on silicon substrates by preliminarily depositing gold nanoparticles as catalysts without any other treatments.^1-3 When palladium nanoparticles are used as catalysts, heat-treatment is needed before and after plating to obtain adhesive films.⁴ We also reported that single crystalline gold nanoparticles are epitaxicially grown and gold-silicon alloy is formed at the gold-silicon interface, which improves the adhesion of films.³ In this study, we investigate the factors which affect the adhesion of metal films produced on silicon substrates by the present method.

The gold nanoparticles were deposited on p-Si (100) substrates by immersing in a 0.5 mM HAuCl₄ solution containing 0.15 M HF at 278 K. The Ni-P films were formed on the silicon substrates using a solution containing 0.1 M NiSO₄, 0.3 M NaPH₂O₂, 0.1 M (CH₂COONa)₂·6H₂O, and 0.1 M C₄H₆O₅ at 343 K. The solution pH was adjusted to 4.8 with NaOH. The adhesion of deposited films was examined by a tape test based on JIS H8504 corresponding to ISO 2819 after room temperature aging at atmospheric ambient.

Figure 1a shows the percentage of the area of the Ni-P films that remained on the silicon substrate after the tape test as a function of the film thickness. No peeling occurred for the films thinner than 0.8 µm immediately after the film deposition. As shown in Fig. 1b the starting film thickness of peering increased with aging time and reached the maximum after aging for 7 days. Since the starting film thickness of peering increased with the square root of aging time, we consider that the improvement in adhesion is due to the atomic diffusion of gold into silicon at room temperature. However, the starting film thickness of peering become constant after 7 days aging. This indicates that there are factors which affect the adhesion of metal films other than the formation of gold-silicon alloy.

Acknowledgement

This work was partly supported by JSPS KAKENHI Grant Number JP26289276.

References

1. S. Yae, M. Enomoto, H. Atsushiba, A. Hasegawa, N. Fukumuro, S. Sakamoto, and H. Matsuda., ECS Trans., 53(6), 99 (2013).
2. H. Atsushiba, Y. Orita, S. Sakamoto, N. Fukumuro, and S. Yae, ECS Trans., 61(10), 9 (2014).
3. N. Yamada, H. Atsushiba, S. Sakamoto, N. Fukumuro, and S. Yae, ECS Trans., 69(39), 59 (2015).
4. V. M. Dubin, J. Electrochem. Soc., 139, 1289 (1992).