The Heat is On
Nanoprobes and nanoindenters have dramatically improved physical measurements at the nanoscale. Now, researchers want faster testing under a wider array of conditions.
The effort to better understand nanoscale properties has produced large-scale government and industrial research organizations, such as the National Nanotechnology Initiative (NNI) and the Nanoelectronics Research Initiative (NRI). These efforts, each funded in the billion-dollar range, depend on the ability of researchers from around the world to effectively use the analytical tools at their disposal to learn as much as possible about the properties of materials at small scales.
The difficulty, however, is that answers often generate new questions. As imaging and mechanical testing technologies have improved, so has the desire by researchers to understand how materials act at the smallest, most fundamental levels. By doing this, a set of knowledge could be generated that would allow engineers to understand or even construct materials from the most basic of building blocks: atoms.
This race to discover more about nanoscale materials has generated a number of highly sophisticated technologies, many of which have, over the years, earned R&D 100 Awards. 2013 is no different. Three R&D 100 Awards address specific nanoscale measurement challenges: electrical behavior, thermal changes and throughput. And each of them also represents an effort to improve the return of reproducible results.
Probe techniques charge forward
Scanning probes are among the most useful tools at the disposal of a researcher working with nanoscale materials. The development of highly precise and sensitive piezoelectric actuators has allowed a wide variety of techniques, or modes, to be developed that can acquire physical, electrical, magnetic and sometimes even chemical information. Dozens of microscopy techniques rely on the use of scanning probes, including atomic force microscopy (AFM) and scanning tunneling microscopy (STM).
Laboratory researchers are always on the hunt for more information, which has prompted instrument developers to combine as many of these methods as possible. Modern scanning probe-based instrumentation can involve the use of several scanning probes, a plethora of optics and various detector technologies. The 2013 R&D 100 Award-winning LT Nanoprobe from Oxford Instruments Omicron NanoScience, Taunusstein, Germany, offers a good example of how far these instrument platforms have progressed to fulfill the needs of today’s nanotechnologists.
Designed to bring a new level of precision to nanoscale electrical transport measurements, the LT Nanoprobe offers the ability to analyze samples under vacuum at very low temperatures of less than 5 K. This is a key capability, because it provides fundamental electrical information about sample materials without the need for integrating nanoscale structures with larger-scale electrical circuits for test purposes. This saves considerably and accelerates research into molecular electronics and other areas.
To achieve a flexible platform for comprehensive electrical testing at such small scales, Oxford Instruments’ LT Nanoprobe combines four individual-controlled scanning probes with a scanning electron microscope (SEM). According to Andreas Frank, marketing manager at Oxford Instruments, the availability of several probes means they can be navigated to any region of interest within the measurement range.
With this setup, he says, “nanostructures of any geometrical structure can be analyzed. Furthermore, it is possible to analyze only some regions of interests of the nanostructure. Another advantage of their independence is that we can use each probe module like a conventional low-temperature STM/AFM.”
However, uniting the four nanoprobes with the SEM while still permitting low-temperature analyses created a design challenge for Oxford Instruments’ developers, says Frank. They first had to design both the scanning probe and the thermal shield compartment in as little space as possible to obtain a small working distance for the SEM. This distance, which falls between the front end of the SEM column and sample surface, is essential for achieving optimal resolution, but means little separation from a room-temperature environment from 5 K. This created two points of potential thermal weakness: the aperture in the shield compartment that allows the SEM electron beam to enter the shield compartment and the sample-clamping mechanism.
The solution, says Frank, was “to make the access port for the SEM as small as possible in order to reduce the thermal radiation onto the sample so that the sample temperature is below 5 K during illumination of the sample with the electron beam.”
On the other hand, the aperture had to be large enough to ensure a high secondary-electron 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 ultrahigh-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. However, 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 limitations 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 perpendicular 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, researchers 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 the system.
“For the academic community,” says Warren Oliver, president of Nanomechanics Inc., “in situ SEM has been a fairly interesting innovation.”
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 developed to allow scientists to use load displacement information to calculate 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.”