Electric Force Microscopy, Surface Potential Imaging,
and Surface Electric Modification
with the Atomic Force Microscope (AFM)
Introduction
Electric Force Microscopy (EFM) and
Surface Potential (SP) imaging are two
AFM techniques, which characterize
materials for electrical properties. A
conductive AFM tip interacts with the
sample through long-range Coulomb
forces. These interactions change the
oscillation amplitude and phase of the
AFM cantilever, which are detected
to create EFM or SP images (see
Resonance Shift, page 8). In an EFM
image (Figure 1) the phase, frequency,
or amplitude of the cantilever
oscillation is plotted at each in-plane
(X,Y) coordinate. This phase,
frequency, or amplitude is related
to the gradient of the electric field
between the tip and the sample. In a
SP image (Figure 2), variations in the
surface potential on the sample are
plotted. A voltage carrying AFM tip
also enables electrical modification of
materials on or beneath the surface,
such as depicted in Figures 1 and 10.
Applications include electrical failure
analysis, detecting trapped charges,
quantifying contact potential difference
(CPD) between metals and/or
semiconductors, mapping relative
strength and direction of electric
polarization, testing electrical
continuity, and performing electrical
read/write.
Electric Force Microscopy (EFM)
EFM is used to map the vertical (Z)
and near-vertical gradient of the
electric field between the tip and the
sample versus the in-plane coordinates
X and Y. This is done using LiftModeTM
(see page 8). The field due to trapped
charges—on or beneath the sample
surface—is often sufficiently large to
generate contrast in an EFM image.
Otherwise, a field can be induced by
applying a voltage between the tip
and the sample. The voltage may be
applied directly from the microscope’s
electronics under AFM software
control, or from an external power
supply with appropriate current-limiting
elements in place. EFM is performed
in one of three modes: amplitude
detection, phase detection, or
Figure 1. E