Electromagnetic interference shielding performance and structure of multilayered NiFe/Cu thin films: Effects of impedance and defects

The electromagnetic wave-shielding performance of metallic multilayer thin films composed of NiFe and Cu was analyzed by comparing the theoretically calculated and experimentally determined shielding efficiencies of various films. Previous studies have shown that the total conductivity of multilayer films does not directly correlate with their electromagnetic interference (EMI) shielding efficiency. Therefore, in this study, we analyzed the impact of the structure and electrical conductivity of each layer on the EMI shielding performance of the multilayer structures. The electrical conductivities of the single-layer Cu and NiFe thin films fabricated via electroplating were measured. The measured electrical conductivities of the three-layer (M3 and SM3) and five-layer films (M5 and SM5) were used to identify the multilayer film with the optimal shielding efficiency. In the gigahertz region, the maximum shielding effectiveness (SE) of M5 reached -82 dB; however, deviations from the theoretical SE values were noted at frequencies > ∼7 GHz. Experimental and theoretical data for M3 showed a similar trend to M5, with variations attributed to the presence of defects. These observations were further explored by comparing the maximum shielding efficiencies derived from the multilayer shielding model to the experimental results. Theoretical analyses suggest that metallic multilayers with an odd number of layers typically provide better EMI shielding than those with an even number of layers due to significant differences in electrical impedances between the outer layers and interlayers. However, deviations from theoretical predictions were noted and linked to interface defects in the thin films. Furthermore, the EMI shielding performance was assessed using vector network analysis with a GPC-7 fixture. To explore the relationships between various parameters, such as defects in the Cu layer, grain size, conductivity, and NiFe composition, and their impact on EMI shielding effectiveness, several experimental techniques were employed. These included focused ion beam spectroscopy, secondary electron microscopy, transmission electron microscopy, four-point probe analysis, and inductively coupled plasma-optical emission spectroscopy. These experiments helped clarify the effects of the detailed structural and compositional attributes of the metallic multilayer films on their shielding performance.