Role of Glycine in the Electroless Fe-Ni-B Alloy Process for Power Semiconductor Devices
Authors: Yamamoto Takayo , Nagayama Tomio 
Published: December 2024

Pyrometallurgically produced Invar Fe-Ni alloys possessing an Fe content of 55–70 mass% exhibit a low coefficient of thermal expansion (CTE). Therefore, one can expect electroless deposited Invar Fe-Ni alloy films to also exhibit a low CTE comparable to those of semiconductor chips and insulating substrates used in semiconductor devices. However, a process for producing an Invar Fe-Ni alloy film by electroless plating has not been established and the thermal expansion properties of the obtained film have not been sufficiently investigated.
We prepared electroless Fe-Ni-B alloy thin films using a citrate-pyrophosphate bath as a complexing agent and dimethylamine borane (DMAB) as a reducing agent, whereupon the thermal stress behavior of the thin films was evaluated [1]. In a previously reported plating process [1], the deposition rate of the Invar alloy plated film was 0.6 µm·h−1, which is too slow for practical applications. This deterioration in reaction rate of the electroless plating is caused by the high concentration of Fe2+, known as a stabilizer [2], in the plating bath. It is therefore necessary to improve the process to obtain the ~5–10 μm thickness required for high-density semiconductor packaging.
In general, an increase in bath temperature and pH value improves the deposition rate, but this process limits the optimization of these operation conditions owing to the promotion of Fe2+ oxidation and the absence of stable complexes. Therefore, control of the deposition rate was attempted in this study by selecting a complexing agent in the plating bath. Here, we chose glycine [2], which is known to form a complex with Ni2+ in an electroless Ni plating bath and improve the deposition rate of electroless Ni. The glycine was added to the electroless Fe-Ni-B alloy plating bath, and the effect on the deposition of the Fe-Ni-B alloy film was investigated.
Table 1 shows the electroless plating bath and plating conditions. The total metal-ion concentration was 50 mmol·L−1. A pure copper plate was used for the substrate, and a contact plating method was used where an aluminum plate was brought into contact with the substrate to initiate the deposition reaction. A 1 h plating time was used in each case.
Fig. 1 shows Fe content and deposition rate of the electroless Fe-Ni-B alloy film obtained from the plating bath without or with addition of 1 to 100 mmol·L−
1 glycine, where ratios of Fe2+ / (Fe2+ + Ni2+) were 0.3 and 0.8. In both ratios of Fe2+ / (Fe2+ + Ni2+), Fe content was significantly reduced by adding of glycine of 10 mmol·L-1 or more. The deposition rate increased when glycine was added to the plating bath compared to that in the bath without glycine. When 100 mmol·L-1 of glycine was added, the deposition rate decreased compared to that when 10 mmol·L-1 was added.
Using the stability constants of the complexes formed in the bath, it is estimated that when the glycine concentration is up to 10 mmol·L-1, it predominantly coordinates with Ni2+, whereas upon the addition of 100 mmol·L-1, it also forms complexes with Fe2+. As previously mentioned, the electroless plating process utilizing Ni2+ complex ions with glycine as a ligand exhibits a faster rate compared to deposition reactions involving other stable complex ions [2]. It is assumed that the Ni-glycine complex ion species with a high reduction rate becomes dominant in the plating bath containing 10 mmol·L-1 of glycine, so that the reduction rate of the Fe and Ni alloy is accelerated accompanied by acceleration of Ni2+ reduction and reached the maximum value.
It turns out that by using glycine as a second complexing agent and optimizing its addition amount in an electroless Fe-Ni-B alloy plating bath, complex ions with a high reduction rate are formed, resulting in the improving of the deposition rate.

Acknowledgement

This work was supported by JSPS KAKENHI Grant Number JP24K08121.

References

[1] T. Yamamoto, T. Nagayama, T. Nakamura, J. Electrochem. Soc., 166(1), D3238-3245 (2019).
[2] I. Baskaran, T. S. Narayanan, A. Stephen,Mater. chem. phys., 99(1), 117-126 (2006).

Figure 1