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

Abstract

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 FeSO₄<inline-formula> <tex-math notation="LaTeX">$\cdot 7\text{H}$ </tex-math></inline-formula>₂O were investigated. Key magnetic properties, including saturated magnetic induction (<inline-formula> <tex-math notation="LaTeX">$B_{s}$ </tex-math></inline-formula>), coercivity (<inline-formula> <tex-math notation="LaTeX">$H_{c}$ </tex-math></inline-formula>), and resistivity (<inline-formula> <tex-math notation="LaTeX">$\rho$ </tex-math></inline-formula>) were measured, which were 0.98&#x2013;1.62 T, 0.6&#x2013;5.6 Oe, and 17.2&#x2013;<inline-formula> <tex-math notation="LaTeX">$33.4 ~\mu \Omega \cdot $ </tex-math></inline-formula>cm, respectively. Both <inline-formula> <tex-math notation="LaTeX">$B_{s}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\rho $ </tex-math></inline-formula> increased monotonically when the content of Fe within the films increased, which was similar to the bulk state Fe-Ni alloys. The <inline-formula> <tex-math notation="LaTeX">$H_{c}$ </tex-math></inline-formula> first decreased from 5.6 to 0.6 Oe, then fluctuated at the range of 0.6&#x2013;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&#x2013;10 nm, resulting in an ultralow <inline-formula> <tex-math notation="LaTeX">$H_{c}$ </tex-math></inline-formula>. The electroplated nanocrystalline Fe-Ni film (Fe 45 wt&#x0025;) was further used as magnetic core materials due to its high <inline-formula> <tex-math notation="LaTeX">$B_{s}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$\rho $ </tex-math></inline-formula> 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.