High-Resolution Kelvin Force Microscopy

Sergei Magonov, PhD, John Alexander, & Mattias Fenner PhD
European Microscopy Conference 2008

Kelvin Force Microscopy (KFM) is a very powerful tool for mapping of surface charges, surface potentials, and doping profiles. [1] This technique is implemented in amplitude modulation and frequency modulation Atomic Force Microscopy (AFM) modes. [2] In many applications surface electric properties are measured with two-pass technique in which a “spill over” of topographic response to the probe motion is reduced by lifting the probe over a sample surface during detection of electric signals. Such approach has also severe limitations in sensitivity and lateral resolution due to a remote probe position during the lift scan. A separation of topographic and electrostatic responses is also possible by operating topography and electric-response servo loops at different frequencies that enable single-pass KFM with simultaneous studies of sample topography and surface potential. The latter is measured by voltage applied to the probe that nullifies its electrostatic interaction with a sample.

We consider several practical implementations for the electric-response servo loop. In search for one that provides best spatial resolution [3] and highest sensitivity. Different inputs and frequencies were applied in this search. We have also employed AFM probes with different cantilever geometries (Figure 1, left and right) and tip dimensions. The results of this study will be presented and illustrated by KFM images of different materials: semiconductor structures with different level of doping, polymer composites, graphite, Au (111) and fluoroalkanes. Doped electric passes (Figure 2, right), negatively charged self-assemblies of fluoroalkane layer on graphite (Figure 3, right) and contamination patches, which are grown on the graphite surface in air (Figure 4, right) are visualized in surface potential images. It was possible to achieve lateral resolution better than 5 nm and sensitivity of few tens of milliVolts.

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