Energy Dispersive Spectroscopy
In conjunction with our ESEM CAPPA can also carry out Energy Dispersive X-Ray Spectroscopy [EDS] which compliments the high resolution imaging and also aids product and process analysis for a wide variety of samples. To stimulate the emission of characteristic X-rays from a specimen, a high energy beam of charged particles [electrons in our case] is focused into the sample being studied. The number and energy of the X-rays emitted from a specimen can be measured by an energy dispersive spectrometer. As the energy of the X-rays is characteristic of the atomic structure of the element from which they were emitted, this allows the elemental composition of the specimen to be measured. This can be used in a wide variety of applications from fundamental study of materials to more applied uses such as analysis of coating processes or determining sources of contaminant.
Attached to the centre’s Hitachi S-3700N ESEM is an Oxford Instruments X-Max 80mm2 SDD Energy Dispersive Spectrometer. The detector is a Silicon Drift Detector [SDD] with an extremely large active area [80mm2] which greatly increases the collection angle. This results in a detector which performs measurements either more quickly or at higher resolution than traditional systems. In addition the increased sensitivity allows the system to operate at much lower beam currents which has many benefits including reducing the risk of sample damage. The capabilities of this system also complement our Chemical Imaging suite in terms of sample analysis.
Below in Fig. 1 is an example of the capabilities of EDS. In this example the sample is a plastic pouch material used in product packaging and is expected to have a bi-layer structure. The Electron Microscope image reveals a confusing picture, however despite this, EDS can be used to look beyond the physical and utilise the chemical composition to show the hidden structure. The top layer is believed to a Polyester and have a much higher Oxygen content than the underlying layer. Using the Oxygen peak in the Energy Dispersive Spectrum we can clearly see the bi-layer structure with the top layer exhibiting a much stronger response. The technique behind such an image is explained in the second example further below.
Fig. 1: Top is an ESEM Image of a plastic pouch for product packaging. Bottom is an EDS image of the Oxygen Peak. The bi-layer structure is impossible to discern in the electron image whereas the EDS image reveals the strong oxygen signal from the top Polyester layer.
EDS Characterisation & Mapping
The sample in this case has multiple elements with the investigation illustrated in the images below. It is a compound semiconductor with metal structures, namely Ni (Nickel) dots on a GaN (Gallium Nitride) Substrate, and has uses in photonic applications. Firstly in the top image several dots are imaged using the ESEM and a region of interest around a single dot is selected. The EDS system can interface with the ESEM to control the location from which it collects the spectrum and uses this to build up an elemental image by point mapping. The result is an elemental map of the sample, similar to the Chemical Images achieved using FT-IR and Raman. The results below show the single element maps for Ga, N and Ni and a combined map which shows the distribution of all elements. In this example the dot is ~120μm in diameter but Oxford have demonstrated greater than 50nm resolution performance with this detector system.
This is the ESEM image of the Ni dots on the GaN substrate. Highlighted is the region of interest for the single dot that was mapped by EDS.
A typical EDS spectrum for the sample, with the Ga, N, Ni and O peaks labelled. The Gallium peak is by far the strongest given that this element dominates the substrate.
Monochrome intensity map of the Ga spectral peak. The substrate, rather than the dot area, shows a high signal as expected.
Monochrome intensity map of the N spectral peak. Not as evident as the Ga peak but the substrate shows a higher signal than the dot region.
Monochrome intensity map of the Ni spectral peak. The dot area shows the high signal as expected.
In this map information from the Ga, N and Ni maps are combined in a false colour image. Ga is in blue, N as red and Ni as green.