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Electric Techniques

The two most common electric techniques used with the Dimension Icon microscope are Electric Force Microscopy (EFM) and Surface Potential Detection. Both modes make use of Interleave and LiftMode procedures. Ensure you are familiar with before attempting electric measurements.

Electric techniques are similar to Magnetic Force Microscopy (MFM). The two-pass LiftMode measurement allows the imaging of relatively weak but long-range electrostatic interactions while minimizing the influence of topography. In the case of MFM, the system is measuring long-range magnetic fields. LiftMode records measurements in two passes, each consisting of one trace and one retrace, across each scan line. First, LiftMode records topographical data in TappingMode on one trace and retrace. Then, the tip raises to the Lift Scan Height, and performs a second trace and retrace while maintaining a constant separation between the tip and local surface topography.

Cantilever measures surface topography on first (main) scan (trace and retrace).
Cantilever ascends to lift scan height.
Cantilever follows stored surface topography at the lift height above the sample while responding to electric influences on second (interleave) scan (trace and retrace).

Electric Force Microscopy Overview

Electric Force Microscopy (EFM) measures variations in the electric field gradient above a sample. The sample may be conducting, nonconducting, or mixed. Because the surface topography shapes the electric field gradient, large differences in topography make it difficult to distinguish electric field variations due to topography or due to a true variation in the field source. The best samples for EFM are samples with fairly smooth surface topography. The field source could be trapped charges, applied voltage, and so on. Samples with insulating layers (passivation) on top of conducting regions are also good candidates for EFM.

Surface Potential Imaging Overview

Bruker provides several methods for surface potential imaging:

Surface Potential Detection (aka AM-KPFM) measures the effective surface voltage of the sample by adjusting the voltage on the tip so that it feels a minimum electric force from the sample.
PeakForce KPFM-AM Imaging combines the Surface Potential Detection mode with Bruker's proprietary Peak Force Tapping Mode.
FM-KPFM Imaging uses a frequency modulation technique to measure surface potential. FM-KPFM is generally more accurate and has higher spatial resolution than AM-KPFM.
PeakForce KPFM Imaging combines FM-KPFM Imaging with Bruker's proprietary Peak Force Tapping Mode, combining the benefits of both modes.
PeakForce KPFM-HV Imaging combines Bruker's proprietary Peak Force Tapping Mode with an AC technique that enables measurement of surface potentials up to approximately ±200 V.

Electrical Sample Preparation

The sample should be electrically connected directly to the chuck, so that it can be held at ground potential (normal operation) or biased through the chuck. The sample can either be mounted directly on the chuck or onto a standard sample puck using conductive epoxy or silver paint as shown below:

Figure 1: Schematic diagram showing how to electrically connect a sample onto a sample puck.

HINT: If the surface of your sample is conductive and the base of the sample is insulative, you will need to ensure that the conductive epoxy or paint contacts one edge of the sample surface and one edge of the conductive mount (either sample puck or the chuck itself). Ensure that the large "glob" of glue/paint required is NOT located directly underneath the cantilever substrate, as the substrate may come in contact with the glue/paint, completing the circuit and preventing the tip from contacting the sample:

Figure 2: Schematic diagram showing how NOT to electrically connect a sample onto a sample puck.

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