De-embedding transmission line of SiO2 thin-film measurements for accurate characteristics

This paper is aimed to present the exact electrical characterization of the SiO2 thin film coplanar waveguide (CPW) transmission line. SiO2 is one of the promising dielectric material candidates in the semiconductor industry, and it has been paid a great attention by various parties. At millimeter-wave (mm-wave) frequencies, S-parameters can comprehensively and intuitively indicate the performance of the devices and systems. In this study, a series of wafer-probe measurements and electromagnetic simulations (EMs) are performed with the same design layouts but different lengths of the transmission lines up to 6 GHz. However, due to the probe doesn´t directly use for contacting transmission line, extra test fixtures such as probing pads are designed, but these fixtures can produce noise for test results. To achieve the “real” S-parameters of the transmission lines, “L-2L” de-embedding technique based on the cascade approach and an equivalent-circuit approach is used to remove these parasitics of the instrument itself. It is a process of mathematically subtracting networks from the measured results by the algorithm between S-parameters and ABCD-parameters transmission matrix, and the probe pads are modeled a lumped series impedance and a limped shunt admittance in this method. ADS (Advanced Design System 2012.08) simulations are adopted to verify the correctness of the de-embedding method. This simulation model is defined as a CPW transmission line without pads. Through the analysis of simulations and de-embedding result, different results are found of the S-parameter before and after de-embedding. It shows a same trend between electromagnetic simulations and results after de-embedding. In summary, the accurate characteristics such as propagation constant γ and characteristic impedance ZC of the SiO2 thin-film transmission lines are obtained after the de-embedding process is added.