Stability of Charge Distributions in Electret Films on the nm-Scale

Electret materials find use in various applications, such as microphones or filter media. In recent years, electrets have been used also increasingly on the micrometer scale, for example, in MEMS or for nano-xerography. However, for these applications, it becomes more important to prepare defined charge structures with sub-micrometer features. On the macroscopic level, the technique of isothermal potential decay at elevated temperatures has been developed to study aging effects and charge retention capabilities in electret materials. Here, we extend this technique to the nm-level by means of AFM-based methods, such as contact charging by AFM and the Kelvin probe force microscopy. Defined charge distributions in polyetherimide (PEI) ULTEM 1000 thin-film electrets have been studied for the first time with a high lateral resolution on the nanometer scale. We found a linear correlation between externally applied contact charging potential on the AFM-tip and the resulting relative surface potential on the PEI film. Charge decay at elevated temperatures is independent from the length scale. The same time dependence as for macroscopic, homogenously charged films could be established. We observe a potential decay only at an elevated temperature of 120 °C and no significant lateral charge transport. Thus, we propose a thermally enhanced charge carrier release from surface traps and a subsequent charge migration to the back electrode as the dominant mechanism. This finding is in-line with the observation that potential decay can be reduced also on the nm-level by pre-annealing the film slightly below the glass transition temperature. In contrast to many polymeric or inorganic electrets, no lateral charge migration is observed. Therefore, the charge patterns are preserved for PEI ULTEM 1000 thin-film electrets, which makes it a good candidate as electret for applications in MEMS or similar applications.