Spotlight: Non-destructive testing

Materials World magazine
,
1 Oct 2017

Dr Ian R Holton, Acutance Scientific Ltd & BLG Vantage Ltd, UK, explores strain mapping on the 100-nanometre scale using HR EBSD.

Why map on such a small scale?

Type 3 strain (strain mapped within a microscopic grain, crystal or device) is a growing research and development topic both in advanced university Materials Science and Mechanical Engineering departments and in top engineering companies. This is because it is only an understanding of what is happening at this microscopic spatial scale that can explain behaviour at a coarser scale. It is the recent development of high-angular resolution electron backscatter diffraction (HR EBSD) that has generated this possibility and growth.

An example is the improved lifetime of semiconductor devices. Often this life is cut short by strain, on the 100 nanometre (nm) to 10 micron scale, introduced by processes during manufacturing – a map of Type 3 strain can show where the microscopic scale strain is introduced, allowing redesign of the processes in order to remove it. There are also questions about early crack propagation mechanisms, about how strain is terminated at grain boundaries, the design of new materials (necessarily at this scale) to optimise strain properties and processes to prevent introduction of certain strains, all of which requires being able to map those strains in the first place.

How does it work?

When the electron beam of an SEM meets a crystalline material, it is diffracted by it, being bounced in directions mostly determined by the crystal structure. The spacing between atomic planes in the material and their orientation determines the directions in which the beam is diffracted. From such patterns of diffraction, the crystal structure and orientation can be determined using EBSD. The EBSD technique is now mature after about 30 years. It has, however, a relatively poor angular resolution, which means it cannot measure useful levels of strain.

Whereas EBSD has an angular resolution of around 0.5°, HR EBSD has no problem in resolving 0.006o. This high-angular resolution directly enables elastic strain measurement with a sensitivity of around one part in 10,000.  

It can be imagined that, as strain is introduced to the material, the spacing and orientation of the atomic lattice planes are distorted, hence the diffracted electron beam changes direction subtly. This changes the diffraction pattern minutely, but HR EBSD software is capable of measuring these shifts of up to 1/20th pixel sensitivity. By measuring the diffraction patterns over the whole scan surface, the software can back-calculate the entire deformation tensor – all nine components.

In order to study the interplay between heat processes and tensile processes, HR EBSD measurements are increasingly made under in situ conditions.

Which measurements can be made using this technique?

All nine components of the deformation tensor are measured to an accuracy of about one part in 10,000. Lattice rotations, normal and shear elastic strains can all be measured with a spatial resolution of 50nm, giving a complete picture of the relative strain state. 

Additionally, from these measurements HR EBSD is used to produce maps of geometrically necessary dislocation densities, and high-resolution kernel average misorientation maps. 

A few other techniques such as X-ray diffraction or neutron scattering may deliver higher strain sensitivity, but with much worse spatial resolution, so are unable to measure a Type 3 strain. Transmission electron microscope (TEM) diffraction – a crystallographic experimental technique – can measure to higher spatial, but with worse strain resolution. HR EBSD therefore occupies a niche in the spatial resolution/strain resolution chart.

What are the limitations of HR EBSD strain mapping?

As this is an scanning electron microscope (SEM) based technique, a number of companies use local university groups with SEM to strain-map such samples. Also, it only works on materials that, at this microscopic scale, have a crystalline structure, which the great majority of structural and functional materials do not.

HR EBSD is a software technique applied to the data output from standard EBSD instruments, so any department that has a scanning electron microscope with this capability and HR EBSD software can do it. 

The samples require preparation for EBSD, which is a routine preparation procedure for SEM laboratories. Because HR EBSD measures differences between neighbouring diffraction patterns, the strain is measured with respect to a local reference location on the sample, chosen either by the user or automatically by the software.

 

ECLIPSE is evolving to the next stage

The ECLIPSE microscope body has been modularised to meet industrial microscope applications in diverse fields of industry, from semiconductor devices, packaging and FPDs, to electronic components, materials and precision moulds. A range of options is available for the ECLIPSE LV Series, including stand units and illumination units to meet a variety of observation methods and purposes.

The ECLIPSE LV Series has also gained a new optical system and various new features, such as diverse observation methods and advanced optical performance. 

More efficient observation and image capture are achieved using the latest Nikon USB 3.0 digital cameras, along with image analysis software and intelligent microscope functions. 

Rigaku introduces Simultix 15

With an update to its multi-channel simultaneous wavelength dispersive X-ray fluorescence (WDXRF) spectrometer system, the Rigaku Corporation, Japan, has introduced the Simultix 15 high-throughput WDXRF spectrometer. 

The new system was developed to meet changing needs and customer requirements across a range of industrial applications, offering improved performance, functionality and usability as an elemental analytical tool for process control in industries that require high throughput and precision, such as steel and cement. The analyser has a standard 30-fixed channel configuration, optionally upgradeable to 40 channels, and multiple discrete and optimised elemental channels and 4kW of X-ray tube power. 

New features include an RX85 synthetic multi-layer crystal (producing approximately 30% greater intensity than existing multi-layers for Be-Ka and B-Ka), an available XRD channel for quantitative analysis by XRD and software featuring a quantitative analysis flow-bar.

Raman-SEM imaging available for Sigma 300

Witec Wissenschaftliche Instrumente und Technologie GmbH and Carl Zeiss GmbH, Germany, have jointly developed a fully integrated OEM product featuring a standard, unmodified vacuum chamber and SEM column along with a complete confocal Raman microscope and spectrometer. 

The companies state that the microscope expands the range of choices available to researchers in imaging and advanced structural analysis. The objective and sample stage can both remain under vacuum for all measurements, and the sample is transferred between the Raman and SEM measuring positions using the stage of Sigma 300. 

Bruker reveals the D8 DISCOVER Plus

Bruker, Japan, has announced its new X-ray diffraction system, the D8 DISCOVER Plus. The system combines the TXS-HE high-efficiency Turbo X-ray Source with the ATLAS goniometer. 

The TXS-HE is optimised to deliver an high-intensity X-ray beam, reducing data collection time compared to conventional X-ray sources and, Bruker claims, deliver better data quality, giving new insights into structural properties of investigated materials. The ATLAS provides angular positioning accuracy, increasing the reliability of measured data for all applications. 

The D8 DISCOVER Plus has applications in analytical tasks in materials research and analysis.

Covering all bases 

Trisha Rice, Vice President and General Manager of Materials Science Solutions at Thermo Fisher Scientific, introduces the Quattro ESEM.

What is the Quattro ESEM?

The Thermo Scientific Quattro field-emission environmental scanning electron microscope (ESEM) provides high-throughput imaging and analysis for a wide variety of academic, industrial and government labs that want a versatile, easy-to-use scanning electron microscope for multiple users of different experience levels and disciplines. It has a range of analytical capabilities and accommodates different types of samples. The system’s software is simple and allows even the most novice user to get started quickly. 

Complementing the static imaging capability, Quattro’s environmental mode allows materials science researchers to study dynamic processes in situ. Nanomaterials can be studied in real-world conditions during processes such as wetting, annealing, melting and chemical reactions. This in situ capability provides scientists with the opportunity to study materials in conditions not otherwise accessible.

What are the benefits?

The Quattro ESEM is built on a new platform optimised for productivity. The chamber accommodates large samples, an accurate stage and ample room for analytical accessories. The user interface ensures the shortest time to results with advanced automation, and new detection options allow access to more data from more samples and in more conditions.

Quattro supports a unique range of fully integrated cooling and heating stages – also capable of heating samples in a clean environment, a reactive environment, with short ramp rates or with simultaneous energy dispersive spectroscopy (EDS) or EBSD analysis. This combination makes Quattro ESEM applicable for construction, automotive, packaging, coatings and energy sectors. 

What additional data can the product provide?

Quattro’s chamber fits the optional new RGB Cathodoluminescence detector. This detector provides real-time, real-colour images from the sample, highlighting sample properties not visible with conventional electron or X-ray imaging techniques. The detector is compatible with simultaneous SEM detection and EDS detection. It is integrated in the user interface and requires no optical alignments.

There are more features on the detection side. Data from Quattro’s segmented directional backscatter detector can now be directly used for 3D surface reconstruction with TopoMaps, a new software option available with Quattro.

Do the automation options improve speed?

While our four-quad user interface was already easy to use, Quattro improves it further with an interactive help function called user guidance, which not only instructs, but also directly interacts with the microscope. With the undo functionality, novice users are encouraged to experiment, while expert users can shorten time-to-results.

For users requiring unattended SEM operation, or for those who desire more accurate control over dynamic experiments, the Quattro offers AutoScript, a Python-based automation tool. AutoScript allows users to program imaging and stage movements, supported by extensive image processing capabilities that are available with Python. This opens up many possibilities to further boost productivity.

How are the heating stage capabilities improved?

Studies of surfaces at high temperature are often limited by oxidation or contamination, as heating stages usually operate in low vacuum. Oxidation on the surface prevents the study of, for example, recrystallisation, as an oxide blocks the view of the surface. With the new high vacuum heating stage, heating experiments become cleaner as the stage does not produce significant outgassing, enabling clean heating experiments at temperatures up to 1,100°C. The stage is compatible with in situ EBSD at temperatures of up to 900°C.

Other heating experiments are limited by sample drift or temperature ramp rate. For these experiments, we offer the microHeater, which heats 50µm samples up to 1200°C in under 0.1 seconds. On Quattro, it offers an opportunity to study the dynamic behaviour of nanoparticles at high temperatures