Preparation and Characterization of Electroless Deposited Fe-Ni Alloy Films

Abstract

The higher reliability of high-density packaging for electronic devices is considered to be the issue to achieve the sustainable semiconductor manufacturing. Fe–Ni alloy thin films exhibit an excellent wettability and a low reaction rate with lead-free solders compared with Ni thin films [1]. The Fe–Ni alloy films are thus promising as a highly reliable metallize material for semiconductor packaging. An electroless Fe-Ni alloy plating process is of current interest because it is also possible to form films metallized on non-conductive material such as an insulating substrate. Pyrometallurgically produced Fe-Ni alloys with 55 to 70 wt% Fe contents exhibited low coefficients of thermal expansion (CTEs) and are termed “Invar alloys” [2]. Therefore, electroless deposited Fe-Ni alloy films can be also expected to have a low CTE comparable with those of semiconductor and insulating substrates used in power semiconductor devices. We have prepared Fe–Ni–Boron (B) alloy thin films by using the electroless plating method, and their thermal-expansion properties have been evaluated [3]. The CTE of electroless deposited Ni–5wt%B alloy film was approximately 13 ppm/K, while those of the electroless deposited Fe–Ni–B alloy films with an Fe content of 55 and 64 wt% in the Invar composition range exhibited lower CTEs, approximately 8 ppm/K, than that of a conventional electroless deposited Ni film [3]. So far, stress generations in the electroless deposited Fe–Ni–B alloy films associated with those microstructure have not been studied extensively despite their importance in high-density package reliability. In this study, we produced a variety of electroless deposited Fe–Ni–B alloy films with different amounts of Fe/Ni ratio and investigated the internal stress in the alloy films. The electroless Fe–Ni alloy plating bath compositions were as follows: FeSO4 ·7H2O 0 to 0.02 mol/L, NiSO4·6H2O 0.03 to 0.05 mol/L, Tri-potassium citrate monohydrate 0.1 mol/L, Potassium pyrophosphoric 0.005 mol/L, and dimethylamine borane 0.025 mol/L as the reducing agent [3]. The pH of the electroless Fe–Ni alloy plating bath was adjusted to pH 10.0 using a KOH solution. The bath temperature was kept at 70 °C in a water bath. Ni/Cr/Si wafers (with thicknesses of approximately 10 nm, 10 nm, and 525 μm, respectively) contacted with pure aluminum plate were used as a substrate. The electroless plating was carried out with adjusting a plating time to obtain a film thickness of about 500 nm. The composition of the electroless deposited Fe–Ni alloy films was determined by an EPMA. The thickness of the electroless deposited film was measured by the fluorescent X-ray-FP method. The internal stress (σ film) of the electroless deposited film was calculated using Stoney’s equation from the change in the film's curvature at room temperature. Fig.1 shows the alloy composition and the deposition rate of the electroless deposited Fe–Ni–B alloy film obtained from the plating bath, where the ratio of Fe2+/(Fe2++Ni2+) was varied from 0 to 0.4. An electroless deposited Ni–B alloy film that contained approximately 5 wt%B was obtained from the plating bath without Fe2+ addition. When Fe2+ was added to the bath, the Ni and B contents of the resulting film decreased gradually, and the Fe content increased to 62 wt%. When the Fe2+/(Fe2++Ni2+) ratio was 0.3 or more, the B content was less than 0.3 wt% and hardly detectable. When the Fe2+/(Fe2++Ni2+) concentration ratio was 0.3 or more, electroless deposited Fe–Ni–B alloy films in the Invar composition region were obtained. Fig.2 shows effect of Fe content on the internal stress value (σ film) in an electroless deposited Ni–B alloy film and electroless deposited Fe–Ni–B alloy films. The stresses (σ film) of electroless deposited Ni–Fe alloy films with an Fe content of 0 to 10 wt% were approximately 100 to 200 MPa (tensile). For Fe content of 20 to 40 wt%, the stresses (σ film) increased significantly to approximately 800 MPa (tensile). In contrast, with respect to the Fe–Ni–B alloy films with an Fe content of 55 to 64 wt% in the Invar composition range with lower CTEs, the stresses (σ film) decreased and exhibited relatively low values, approximately 500 MPa (tensile). It is believed that the change of the stress in the electroless deposited film with the various film composition corresponds to the microstructural conversion in the film. Reference [1] H. Zhou, J. Guo, and J. Shang, J. Electrochem. Soc., 160, D233, (2013). [2] C. É. Guillaume, CR Acad. Sci., 125, 235 (1897). [3] T. Yamamoto, T. Nagayama, and T. Nakamura, J. Electrochem. Soc. (in press). Figure 1