<p>Metal thin films serve as effective absorbing layers across the microwave to infrared regions. In sputtering and evaporation methods, the initial collisions among growing nuclei—that occur at film thicknesses below approximately 10 nm—are used to achieve a lower electrical conductivity relative to bulk metals (e.g., 1.0 × 10⁶ S/m). However, at these electrical conductivity levels, transmittance and reflectance can vary by 15–20% in response to thickness variations of a few nanometers. Electroless Ni–P plated films can achieve a conductivity of approximately 0.1 × 10⁶ S/m at thicknesses near 100&#xa0;nm, which is over an order of magnitude lower than that of bulk amorphous nickel alloys. In this study, we investigated the origin of conductivity from a film’s structural perspective. We found that anisotropic growth was promoted in the thickness direction. Such a columnar growth mechanism was based on a co-deposition process involving hydrogen atoms. Investigation of the sub-terahertz optical properties of these electroless Ni–P plated films suggested an absorptance close to the theoretical limit at 46%, estimated from the measured transmittance and calculated reflectance. Additionally, the variation in transmittance and reflectance was only 5% of the total in response to a 10-nm thickness fluctuation. This indicates that the optical properties are more robust against thickness fluctuations of a few nanometers compared with conventional films. This characteristic is crucial for ensuring the reproducibility of the optical properties of metal thin films.</p>