Stress and Microstructure of Electroless Deposited Fe-Ni-B Films
Authors: Yamamoto Takayo , Nagayama Tomio , Nakamura Toshihiro 
Published: December 2020

An electroless Fe–Ni alloy plating process have attracted the interest of researchers, because, these Fe–Ni films can be expected to have a low CTE (coefficients of thermal expansion) comparable to those of semiconductors and insulating substrates used in power semiconductor devices [1].

We have prepared electroless Fe–Ni–boron (B) alloy films, and their stress generation, thermal-stress, thermal-expansion behavior, and crystal structure were evaluated [2]. The results of our previous investigation suggested that microstructures, i.e., sub-microscopic textures of the films affected the stress generated by film formation. In addition, the CTE values and change of microstructure with phase separation upon heating affected the thermal stresses. However, the microstructures of the electroless Fe–Ni–B alloy films have not been sufficiently explored, although the literature contains several reports that address those of Ni–B films.

In this study, in addition to the previous work [2], transmission electron microscopy (TEM) characterization of the fine scale microstructure of Fe–Ni–B alloy films with Fe contents from 0 to 63 wt% were proposed. Furthermore the microstructure of the films before and after heating to 300 °C was associated with their stress.

For the plating baths and conditions as shown Table 1, the films with the alloy compositions in Table 2 were obtained. Ni/Cr/Si wafers contacted with pure aluminum plate were used as a substrate. The thickness of the film was approximately 500 nm.

The stress (σ
film) of the electroless film was calculated using Stoney’s equation from the change in the film's curvature during before and after heating to 300°C. The microstructure was evaluated from TEM observation of the cross-section of the electroless film.

Fig. 1 shows the effect of Fe content on σ
film at room temperature and after heating to 300 °C in an electroless Fe–Ni–B alloy films. The σ
film of the films with an Fe content from 0 wt% to 10 wt%, were approximately 100 MPa to 200 MPa (tensile). For Fe contents from 20 wt% to 40 wt%, the σ
film increased significantly to approximately 800 MPa (tensile). In contrast, with respect to the films with Fe contents of 55 wt% to 63 wt% in the Invar composition range having lower CTEs, the σ
film decreased and exhibited relatively low values of approximately 500 MPa (tensile).

After heating to 300 °C, due to a large contraction occurred in the the films with an Fe content from 0 wt% to 10 wt% originated from phase separation during heating, σ
film values exhibited excessive tensile stresses. In contrast, with respect to Fe content of 20 wt% or more, a slight contraction of the film was occured after heating to 300 °C, thus there was little change in σ
film values of the films with heating and cooling.

Fig. 2 shows the microstructures of the electroless Fe–Ni–B alloy films with an Fe content from 0 wt% to 63 wt%, before and after heating to 300 °C. In the Ni–B film without Fe, the grain size was too fine to be evaluated. This suggested that the film was an amorphous-like phase, which was conﬁrmed by the indistinct ring pattern of selected area electron diffraction. With respect to the Fe–Ni–B films with Fe contents from 20 wt% to 40 wt%, these films exhibited a very fine microstructure composed of granular grains, as the Fe content increased, the grain size increased from 10 nm to 20 nm. As for Fe content from 55 wt% to 63 wt%, which corresponds to the Invar composition, size of the granular grain increased from 40 nm to 50 nm. The microstructure of as-deposited films observed from TEM depended on the boron content in the films, as suggested by previous research.

Upon heating to 300 °C, the amorphous-like phase of the electroless Ni–5 wt% B alloy film transformed to a crystal phase composed of granular grains of approximately 100 nm. In contrast, the microstructure of the films with Fe contents of 20 wt% to 63 wt% in the as-deposited state and after heating to 300 °C was almost the same, exhibited a very fine microstructure composed of a few tens of nanometers-sized granular grains.

The microstructures, i.e., grain sizes affected the stress generated by film formation, this mechanism could be explained from the crystal coalescence theory. Furthermore, it was suggested that thermal stress in the film upon heating was associated with change of grain size and metallographic phase.

Reference

[1] H. Zhou, J. Guo, and J. Shang, J. Electrochem. Soc., 160, D233 (2013).

[2] T. Yamamoto, T. Nagayama, and T. Nakamura, ECS Transactions, 89(7), 53 (2019).

Figure 1