Renishaw offers the Structural and Chemical Analyser (SCA), which benefits from the structural, chemical and electronic information obtained from optical spectroscopy, while observing the sample with a scanning electron microscope (SEM) or using energy dispersive spectroscopy (EDS).
It is possible to connect the SCA to any Renishaw spectrometer and it supports multiple or single techniques. Using a single instrument, detailed in situ sample characterization is possible. The SEM-SCA system is an excellent tool, which redefines effectiveness, convenience, and productivity with the unique combination of two established technologies.
There are many different types of optical spectrometer available, which are based on absorption, emission, scattering or reflection of light by materials. Generally, a vibration or a transition induced in the sample leads to the emission or absorption of light.
The light spectrum is represented normally as a plot of light intensity against light frequency with troughs or peaks signifying the absorption or emission of light. A spectrometer is needed to study these phenomena.
Raman spectroscopy is the principal method supported by SCA. A Raman spectrum indicates the fingerprint of a specific sample and also reveals factors such as crystal orientation, chemical composition, degree of crystallinity and strain.
The spatial resolution of Raman spectroscopy is similar to EDS wherein it is possible to focus a laser spot of micron-size into or onto the sample and collect spectra from the sampled volume.
The Raman effect does not depend on the operating environment and works well under vacuum SEM conditions. A crystal mixture on a SEM stub is shown in Figure 1a. This kind of sample is prepared by forensic scientists trying to determine the nature of a suspicious material. It is apparent, given the high depth of field of the SEM image, there are two crystal classes, large cubic types and small trigonal ones.
On most SEMs, energy dispersive x-ray analysis is available and is the regular choice for analyzing unknown samples.
Figure 1. a. low magnification image of crystals on an SEM stub; b.EDS spectra from the cubic-and trigonal-type crystals; c. Raman spectra from both types of crystal.
There are two crystal types shown. The EDS spectra depicted in Figure 1b show one crystal type contains carbon and oxygen and the other contains oxygen, sodium, and chlorine. However, the crystals are not identified conclusively in these elemental analyses.
In-SEM Raman analysis using the SCA produces the spectra presented in Figure 1c. These spectra, upon comparison against Renishaw's Raman spectral databases, reveal that the crystals are sucrose and sodium chlorate - sugar and weed-killer, which are commonly found in “home-made" explosives.
Structural and Chemical Analyzers (SCA) for SEMs from Renishaw
Raman spectroscopy is the principal method supported by SCA
The SCA offers a new in-SEM analytical technique, which complements light microscope-based Raman spectroscopy, and overcomes certain limitations of the traditional in-SEM analytical technique - EDS
Photoluminescence (PL) Spectroscopy
Similar to Raman spectroscopy, PL spectroscopy involves illuminating the sample with laser light. However, the spectra result from an absorbtion-emission process, rather than a scattering process.
PL spectroscopy is sensitive to the sample’s physical properties such as degree of crystallinity, lattice defects and strain, trace impurities and electronic properties. PL spectroscopy offers a spatially resolved and sensitive means of studying some of the physical and electronic structure of materials. An example of PL spectroscopy is shown in Figure 2.
Figure 2. The power of the SEM-SCA combination is illustrated here with the analysis of diamond films prepared by chemical vapour deposition (CVD):a) SEM micrograph showing a linear feature; b) PL spectra collected from two bulk samples ('x' and 'y') at -172 °C; and c) CL spectra collected at -172 across the linear feature in sample x.
Cathodoluminescence (CL) Spectroscopy
The excitation source in CL spectroscopy is an electron beam. While high-energy photons are generated by high-energy primary electrons exciting inner shell electrons (as detected by EDS), interactions between secondary electrons (typically with energy less than 50eV) and the valence band electrons result in CL peaks in the visible and UV regions of the spectrum.
CL offers several added benefits when compared to PL. Changing the accelerating voltage of the electron beam changes the depth from which CL signals are excited within the sample (as much as several micrometres) - so "depth profiles" can be generated for defects, impurities, and carriers (dopants). There is also a decrease in the CL excitation volume as the beam voltage is reduced so that sub-micrometre spatial resolution is possible.
With CL spectroscopy, it is also possible to collect spatially resolved data.
Information Available from the SEM-SCA System
The characterizations of a sample in situ using the SCA when attached to a SEM with EDS are as follows:
- Morphology from the SEM secondary electron image
- Composition information from the SEM (mean atomic number from backscattered electron imaging)
- Electronic structure from CL and PL spectroscopies
- Physical structure and properties (crystallographic and mechanical data) from Raman, CL and PL
- Elemental composition from EDS
- Chemical composition and identification from Raman spectroscopy
Advantages of the SCA
The benefits of the SCA are listed below:
- The SCA offers a new in-SEM analytical technique, which complements light microscope-based Raman spectroscopy, and overcomes certain limitations of the traditional in-SEM analytical technique - EDS.
- With EDS, micrometre-scale elemental analysis mapping of a regular operation is possible but the method is not sensitive to lighter elements such as B, C, N, O, F and a sample’s chemical characterisation is mostly guesswork.
- The field depth and the SEM’s spatial resolution are much higher than optical microscopy and samples can be imaged with SEMs using several contrast mechanisms arising from the sample’s composition and morphology. Areas and features of interest can be determined rapidly with the SEMs and these can be characterized with the EDS and SCA.
- SEM stages are developed to move samples in five axes, (x, y, z, tilt, and rotate), and several SEMs are developed to accommodate heavy and large samples. This sample handling capability enables highly topographic and complicated objects to be rapidly surveyed.
- SEM is an established technique and there are a large number of accessories to support experiments in the SEM chamber such as cooling, heating, mechanical-testing, electrical testing.
- When used in conjunction with these accessories, PL, Raman and CL spectroscopy offer dynamic physical, structural, chemical and electronic information in combination with SEM observation, such as changes in strain in fibres or oxide layers, phase transformations in polymers, and polymerisation reactions.
The applications of the SCA are:
- Nanotechnology - Analyzing self-assembling nanostructures, characterisation of carbon nanotubes
- Materials science – Examining composite materials, corrosion and oxidation studies
- Electronic materials – Lattice, defect and impurity studies, band-gap and free carrier studies
- Polymer science – Characterizing mixtures and blends, phase transformation and polymerisation studies
- Mineralogy - Characterisation of ores and tailings
- Gemmology - Identification of thermally treated (HPHT) diamonds
- Life sciences – Studying teeth, bone and bio-crystals, biomedical materials
- Device failure - Thin film inspection and metrology analysis, lattice, defect, and impurity studies
- Pharmaceutical - Detection and characterisation of polymorphs, distributing components within tablets
- Paints/coatings - Distributing components within paint films, non-destructive studies of "Old Masters"
- Contaminants -Analysis of particles/contaminants on wafers and devices, examination of filtered materials
- Forensic sciences -Identification of explosives and drugs, characterisation of inks, fibres, and paint chips
- Environmental -Analyzing airborne contaminants, Characterisation of asbestos fibres
Features and Benefits
The features and benefits of the SCA are:
- The sample is co-located, so there is no need to transfer the sample between instruments
- Using the SCA collection optics inserted, secondary electron imaging and EDS analysis are possible
- There is no compromise in the normal functioning of multiple-user SEMs
- It is possible to completely retract the SCA in-beam collection optics and there is no need for a dedicated SEM
- The analysis position is known always as the positioning of the collection optics is with sub- micrometre accuracy and the laser spot is seen in the white light image
- Analytical and observation techniques that require line-of-sight to the sample are unhindered. The SCA collection optics are retracted quickly by the standby mode, enabling BE imaging and EDS analysis
- Servicing costs and down time are reduced
- The SCA has a number of safety features, which include vacuum interlock, developed to protect itself, the SEM, and samples against accidental damage
- The grease-free operation of the SCA ensures low-maintenance and cleanliness
- In order to satisfy present and future analytical needs, systems can be customized
- The SCA is completely configurable and can support multiple laser wavelengths and/or techniques. Several spectrometers from turn-key solutions to full research grade capability are compatible
- The SCA features a high level of automation and minimal training is required for SEM operators to use the system