Thin metal films such as copper are essential to the design and fabrication of high-frequency circuits, such as microstrip and coplanar waveguide (CPW) on almost-as-thin dielectric substrates in the formation of RF/microwave and even millimeter-wave printed-circuit boards (PCBs). While many studies have been performed on the analysis of the properties of dielectric materials in those PCBs, such as dielectric constant and dissipation factor, much remains to be known about the metal conductors.
In quest of that knowledge, researchers from a number of locations—including the School of Mechanical Engineering of China’s Tianjin University and the School of Electrical and Computer Engineering of Oklahoma State University—pooled their resources into the investigation of different types of thin metal films. The researchers used terahertz spectroscopy to study two key material parameters (conductivity and thickness) simultaneously. They investigated three different metal films (aluminum, copper, and silver) and discovered that their measured values of conductivities for the metals were significantly different than the already-known bulk material conductivity values. Their measurements of material thicknesses were consistent with values obtained from other measurement methods.
The studies were performed with the measurement power of the terahertz time-domain spectroscopy (THz-TDS) system at Tianjin University with the aid of grants from China’s National Natural Science Foundation and the Natural Science Foundation of Tianjin Province. The analysis system operates mainly in the spectrum from 0.4 to 2.5 THz. Samples were prepared by depositing metal films on 22-μm-thick Mylar substrates by means of thermal evaporation.
The researchers concluded that the differences in the measured values of conductivities and the known bulk materials could stem from a number of factors, such as defects in film metal (e.g., grain boundaries), leading to a reduction in the measured conductivities for those thin metal films compared to bulk metal values. The measurement system and test approach provide convenient means for capturing the two simultaneous conductive metal parameters. The material conductivity and thickness data gathered by the THz-TDS system, while not without some variations, does provide invaluable additional insights to computer-aided-engineering (CAE) design programs, including electromagnetic (EM) simulation software used for circuit designs.
See “Characterization of Thin Metal Films Using Terahertz Spectroscopy,” IEEE Transactions on Terahertz Science and Technology, Vol. 8, No. 2, March 2018, p. 161.