Enhancing the high-frequency signal performance through surface morphological modification of Cu interconnects

Abstract

Recently, the mobile communication community has expanded its operating frequency bands to the millimeter wave (mmWave) range to increase the transmission bandwidth, to meet the requirements of higher data transfer rates, lower latency, and greater data transmission capacity for wireless communications. In mmWave transmission, the skin effect might cause the majority of signals to be delivered near the conductor periphery, inducing noticeable conductor loss (signal degradation) due to signal scattering/reflections and surface inductance as a result of a rough conductor (Cu) surface. This study was conducted to modify the surface morphology of Cu interconnects through the Cu clad laminates (CCL) process utilizing different Cu foils, including high-temperature-elongation (HTE) foil, two types of reverse-treated foils (RTF), and hyper-very-low-profile (HVLP) foil, to promote the signal transmission performance of differential striplines in mmWave frequency bands. The signal loss (1–43.5 GHz) on differential striplines with different Cu interconnect roughness was characterized using the Groisse and Huray models through the use of a high-frequency structure simulator (HFSS). Furthermore, experimental measurements utilizing a vector network analyzer (VNA) were conducted to evaluate the signal loss resulting from various Cu foils, with the aim of validation the numerical simulation results. The HFSS simulation and VNA measurement results revealed that the Huray model enables to the characterization of high-frequency transmission behavior more accurately in rough Cu foils than the Groisse model does. Moreover, the HVLP-type Cu foils exhibited better high-frequency characteristics than the other Cu foils examined because of lower signal scattering/reflections and surface inductance as a consequence of their low-profile surface morphology. A detailed comparison between the VNA measurements and simulation models (Groisse and Huray) was made and the dependence of transmission loss on the Cu interconnect roughness was quantitatively analyzed in this study. These new data will not only advance our own fundamental knowledge of the high-frequency materials but also be highly beneficial for the development of mmWave transmission technologies.