Voltage Contrast
The interaction of a charged particle beam with structures of different electric conductivity in a microelectronic
circuit locally changes the electric potential at its surface. In a scanning electron microscope (SEM), this effect
leads to a distinct voltage contrast superimposed on the image. It is very useful for failure analysis and device
debugging in microelectronic research and development.
Introduction
Bulk insulating specimens, or insulated layers on a conductive
substrate with a thickness larger than the electron range,
charge up when irradiated by the electron beam in the SEM.
If the combined drain of backscattered and secondary
electrons exceeds the supply of primary beam electrons,
a positive net charge results that generates a positive electric
surface potential. The trajectories of primary, secondary
and backscattered electrons are influenced by this potential.
Part of the secondary electrons will be prevented from
reaching the detector. Thus, insulated structures will appear
darker in the secondary electron image than grounded
structures that do not charge (Fig. 1). This phenomenon
is called passive voltage contrast. Electrical opens, for instance
in contact chains, can be easily found with this method.
In integrated circuits, electric surface potentials can also
be actively controlled by applying voltages to selected
interconnects. The connected structures will then be visible
with different contrast in the secondary electron image.
Similar to passive voltage contrast, positive regions appear
dark, while negative regions appear bright (Fig. 2). This
active voltage contrast is used to detect dielectric leakage
or metallisation shorts. Moreover, active elements in the
device under test can be dynamically driven, enabling
waveform and timing measurements.
Active voltage contrast on microelectronic devices requires
precise probing using micromanipulators. Interconnects can
be contacted using a single microelectrode tip to apply a voltage
against the grounded substrate, and closed electrical circuits
can be constructed by contacting with two or more tips
simultaneously. A tip positioning accuracy in the nano-metre
range, low drift, low backlash, and insusceptibility to vibrations
of the micro-probing setup are indispensable for successful
use of this method on modern nanoscale interconnects.
Topographic Contrast
Difference in emission and difference in detection
of electrons depending on surface topography (also
specimen tilt)
Topographic Contrast
• In general, the most frequent application of the SEM is the visualization of the
topography (shape, size, surface texture) of three-dimensional objects. Topographic
contrast has a complex origin with strong number and trajectory components of both
BSE and SE.
• Topographic contrast arises because the number and trajectories of BSE and the
number of SE depend on the angle of incidence between the beam and the specimen
surface.
Several effects can contribute to the formation of topographic contrast:
• The backscatter coefficient increases as a monotonic function of the specimen tilt.
The more highly inclined the local surface is to the incident beam, the higher the
backscatter coefficient. This effect produces a number component contribution to
topographic contrast in BSE.
• The angular distribution of backscattered electrons is strongly dependent on the local
surface tilt. This directionality of backscattering from tilted surfaces contributes a
trajectory component to the backscattered electron signal.
• The SE coefficient varies with the specimen tilt angle in a monotonic fashion. Tilted
surfaces thus yield more SEs than a surface normal to the beam. This effect
introduces a number component to topographic contrast in the SE signal.
• The topographic contrast that is actually observed depends on the exact mix of
backscattered and secondary electrons detected.
Topographic Contrast