Electroplating Low Coercivity Nanocrystalline Fe-Ni Magnetic Cores for High Performance On-Chip Microinductor

Due to outstanding magnetic properties, low cost, and high efficiency, electroplated Fe-Ni films were the most common magnetic core materials for inductors. In order to meet the requirement of a high-frequency on-chip microinductor in advanced 3-D electronic packaging, the properties of Fe-Ni films electroplated by a modified Watt type electrolyte with different concentrations of FeSO ₄ $\cdot 7\text{H}$ ₂ O were investigated. Key magnetic properties, including saturated magnetic induction ( $B_{s}$ ), coercivity ( $H_{c}$ ), and resistivity ( $\rho$ ) were measured, which were 0.98–1.62 T, 0.6–5.6 Oe, and 17.2– $33.4 ~\mu \Omega \cdot $ cm, respectively. Both $B_{s}$ and $\rho $ increased monotonically when the content of Fe within the films increased, which was similar to the bulk state Fe-Ni alloys. The $H_{c}$ first decreased from 5.6 to 0.6 Oe, then fluctuated at the range of 0.6–2.2 Oe when the Fe content increased. As revealed in transmission electron microscopy (TEM) observation, the Fe-Ni films with face-centered cubic (fcc) structure are consist of nanograins with an average grain size of 5–10 nm, resulting in an ultralow $H_{c}$ . The electroplated nanocrystalline Fe-Ni film (Fe 45 wt%) was further used as magnetic core materials due to its high $B_{s}$ and $\rho $ value. In addition, it was proven that the as-fabricated Fe-Ni microinductor possessed a high inductance of 75 nH and a quality factor of 5 at 30 MHz, respectively. In summary, through composition and microstructure optimization of Fe-Ni films, the quality and working frequency of the microinductor were both improved.