On the other hand, the aperture had to be
large enough to ensure a high secondary-elec-tron yield for good signal-to-noise ratio in
the SEM image. An efficient clamping mechanism of the sample enabled a good thermal
contact and an easy-to-handle in situ sample exchange at low temperatures. Additional design measures were required, including
a specialized bath cryostat that permits probe
tip navigation during SEM operation and
superconducting leads to minimize thermal
load in the magnetic coil. The combined effect
of these innovations is the ability to conduct
multiple microscopy modes, including AFM,
STM and SEM on a single sample with ultra-high-vacuum at low temperatures.
A wave of nanoindentation
One of the most useful techniques for testing small, or nanoscale, volumes of materials is depth-sensing indentation. More commonly known
as nanoindentation, this approach involves continuously acquiring direct
measurements of penetration depth (as well as measures of a contact
load) while indenting a sample, usually, with a sharp indenter.
A tip of certain geometry is applied to a sample surface and electronics record the effect, which can include data on physical characteristics
like toughness, hardness and elasticity.
Instrumented indentation increased in popularity after the development of equipment that can record load and displacement from small
volumes with high precision, and now is a $50 million market. Agilent
Technologies’ Nano Indenter G200, a traditional nanoindentation tool,
is emblematic of mechanical nanoindenters in that it is uncomplicated,
flexible and approachable, offering a wide dynamic range of force, deformation and displacement measurements, from nanometers to millimeters.
But its ability to conduct rapid measurements of certain characteristics,
such as elastic modulus, is limited to just 30 measurements an hour.
Previously, the workaround was to perform a scan of the surface using
an AFM-like modulus mapping technique. Howevever, this approach-compromises the determination of contact area and elastic modulus
when used on a rough or plastic surface. It also prevents hardness testing
because the surface must be elastic for modulus mapping to work.
Working with engineers at Nanomechanics Inc., Oak Ridge, Tenn.,
Agilent Technologies developed the 2013 R&D 100 Award-winning
Express Test, which adopts electromagnetic actuation to allow the
indentation tip to quickly perform 100 tests at 100 different surface sites,
greatly speeding up modulus determination while still relying on tried
and tested load displacement techniques.
The instrument differs from a regular G200 in that it’s equipped with
an Agilent Dynamic Contact Module II indentation head and Agilent
Technologies’ NanoVision stage option. The rapid actuation of the
head, tracked by the stage, can perform the complete indentation cycle,
including approach, contact detection, load, unload and movement, in
a single second. The full test creates a map of points that can be used to
quantitatively calculate Young’s modulus and hardness without the lim-
itations of prior scanning methods. The new method does increase tip
wear and the indentation point spacings are limited, but the increase in
throughput is substantial.
For a comprehensive understanding of the
nanoscale behavior of materials at extremely
small volumes, down to small collections of
molecules or atoms, researchers require more
ways to analyze materials. One of the major
innovations pioneered in the 2000s was the
development of an in situ nanoindentation
platform, which allowed study, in real time, of
the interplay between mechanical, thermal and
electrical effects at the nanoscale.
Miniaturization of the indenter apparatus
allowed developers to place the indenter tip
directly opposite the sample and perpendic-
ular to the electron beam. In this way, the tip
could be integrated into the optical path of a
variety of beam instruments, including SEMs.
As these systems have improved, research-
ers have asked for more capabilities, including testing materials at
various temperature levels. High-temperature testing of micro- and
nanoscale materials has been limited by deleterious effects like oxidation
or thermal drift, but demand has led to heating stages as options on
many commercially available nanoindentation systems. In an effort at
greater integration, Nanomechanics Inc. has built a system that allows
materials testing under load up to 500 C in an electron microscope or
other vacuum environment. The InSEM HT is a first in the industry for
vacuum-environment testing, and has led to a 2013 R&D 100 Award for
“For the academic community,” says Warren Oliver, president of
Nanomechanics Inc., “in situ SEM has been a fairly interesting innova-
Prior to development of in situ nanoindentation techniques, he
continues, the difficulty researchers had was that the sample area of the
indenter was too small to measure accurately. Techniques were devel-
oped to allow scientists to use load displacement information to calcu-
late mechanical properties from load displacement curves.
The key mechanical component of InSEM HT is an actuating transducer that applies the load and measures displacement using a three-plate capacitive displacement sensor. The system delivers isothermal
heating of the probe tip and sample, which are independently controllable because of multi-location thermocouple feedback. By designing this
thermal control system around its existing InSEM, Nanomechanics still
offers three testing implementation options: dynamic, in situ and high
temperature. InSEM permits compression testing of specimens as small
50 nm dia and a few hundred nanometers long, with a peak load of 30
mN, and a resolution of 3 nN.
Information about temperature, says Oliver, is important to the processing use of materials, because at the nanoscale small thermal changes
can add up to significant changes in materials behavior.
“The InSEM system and nanoindentation systems in general have driven fundamental changes in how we understand materials at a very small
scale,” says Oliver. “What people have discovered is that materials at the
macroscale and materials at the nanoscale behave in very different ways.”
— Paul Livingstone