(Invited) Application of Impedance Spectroscopy to Analysis of Plasma-Surface Interaction

The impedance spectroscopy method is a way to investigate the electrical properties of the target system by applying an AC voltage and measuring the amplitude and phase of the current. From measured impedance spectra and a physics- and/or chemistry-based electrical circuit model, we obtain physical and/or chemical information about the target systems. This methodology has been widely used in electrochemical research fields, such as batteries. It can monitor the degradation of elements in the target system independently by continuously scanning the AC voltage frequency. In this study, we applied impedance spectroscopy to the investigation of electrical properties and degradation features of dielectric materials during plasma exposure. Considering the plasma generation system, which has some critical differences from conventional DUT of impedance spectroscopy, we developed an in-situ impedance spectroscopy system to measure the impedance of materials under plasma exposure. We applied the in-situ impedance spectroscopy system to measure the degradation features of SiO2/Si structures (SiO2 film on Si substrate) exposed to a low-temperature Ar plasma. By analyzing the measured data based on an equivalent circuit model considering the plasma and SiO2/Si structures, we obtained the resistance (R) and capacitance (C) values for the SiO2 film and SiO2/Si interface independently. One example is the sputtering of SiO2 film by high-energy ion irradiation. In addition to the decrease in thickness, we found dielectric-property degradation by ion irradiation based on the variations in the R and C values of the SiO2 film. Another example is the temporal variation of film and interface properties during continuous plasma exposure. The thickness-dependent degradation in the SiO2 film and SiO2/Si interface was analyzed and compared with the results of previous ex-situ measurement studies. Further studies on the in-situ impedance spectroscopy are ongoing. We recently found a way to diagnose not only material properties but also plasma properties by measuring currents at harmonic frequencies of AC voltage. This harmonic spectroscopy method was demonstrated to capture surface oxidation of metal probe and plasma parameters (electron temperature and ion density) simultaneously. These studies have revealed the potential of impedance spectroscopy to contribute to fundamental and application research on plasma-material interaction.