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.