EDAX -Energy Dispersive X-ray analysis

EDAX analysis

1. Auger Spectroscopy ( Energy Dispersive X-ray analysis) Two views of the Auger process. (a) illustrates sequentially the steps involved in Auger deexcitation . An incident electron creates a core hole in the 1s level. An electron from the 2s level fills in the 1s hole and the transition energy is imparted to a 2p electron which is emitted. The final atomic state thus has two holes, one in the 2s orbital and the other in the 2p orbital. (b) illustrates the same process using spectroscopic

2. Energy Dispersive X-ray (EDX) analysis

3. Inside the detector, each absorbed X-ray quantum is converted into a cloud which consist of pairs of charge- carriers (electrons) and holes. A strict proportionality of the number of the charge carriers to the photon energy is given. Due to a high voltage, which is between both ends of the sensitive area of the detector, the charge carriers are moved and collected at the ends of the sensitive zone. With a charge-sensitive preamplifier (PA) the charge quantity is finally converted into a voltage-pulse . With a detector-capacity of approx. 2 , a X-ray quantum of 1 keV energy produces a voltage-pulse of approximately 20 microvolt (!). (multi-channel analyser) analog-digital conversion Impulse Density meter peak-stretcher

4. The pulse is amplified and shaped in a spectroscopic amplifier (SPA). Thus the signal-to-noise ratio is increased. With an analog-digital converter (ADC) each pulse is measured individually. Then the current channel contents of an pulse-height spectrum will be incremented at concerned channel (with knowledge of the ADC-data word). Before the ADC is able to measure the value, the maximum of the intensified pulse must be detected or stretched for measurement (peak-detector or peak-stretcher PS ). If the ADC is fast enough, the point of the maximum signal is determined only for analog-digital conversion (the PS is not needed).

5. EDX Analysis EDX Analysis stands for Energy Dispersive X-ray analysis. It is sometimes referred to also as EDS or EDAX analysis. It is a technique used for identifying the elemental composition of the specimen, or an area of interest thereof.  The EDX analysis system works as an integrated feature of a scanning electron microscope (SEM) , and can not operate on its own without the latter.      During EDX Analysis, the specimen is bombarded with an electron beam inside the scanning electron microscope. The bombarding electrons collide with the specimen atoms' own electrons, knocking some of them off in the process. A position vacated by an ejected inner shell electron is eventually occupied by a higher-energy electron from an outer shell. To be able to do so, however, the transferring outer electron must give up some of its energy by emitting an X-ray. The amount of energy released by the transferring electron depends on which shell it is transferring from, as well as which shell it is transferring to. Furthermore, the atom of every element releases X-rays with unique amounts of energy during the transferring process. Thus, by measuring the amounts of energy present in the X-rays being released by a specimen during electron beam bombardment, the identity of the atom from which the X-ray was emitted can be established.

6. Figure 1.    Elements in an EDX spectrum are identified based on the energy content of the X-rays emitted by their electrons as these electrons transfer from a higher-energy shell to a lower-energy one The output of an EDX analysis is an EDX spectrum (see Figure 2 ). The EDX spectrum is just a plot of how frequently an X-ray is received for each energy level . An EDX spectrum normally displays peaks corresponding to the energy levels for which the most X-rays had been received. Each of these peaks is unique to an atom, and therefore corresponds to a single element. The higher a peak in a spectrum, the more concentrated the element is in the specimen. Figure 2.    Example of an EDX Spectrum

7. Principles of the technique EDS makes use of the X-ray spectrum emitted by a solid sample bombarded with a focused beam of electrons to obtain a localized chemical analysis. All elements from atomic number 4 (Be) to 92 (U) can be detected in principle, though not all instruments are equipped for 'light ' elements (Z < 10 ). Qualitative analysis involves the identification of the lines in the spectrum and is fairly straightforward owing to the simplicity of X-ray spectra. Quantitative analysis (determination of the concentrations of the elements present) entails measuring line intensities for each element in the sample and for the same elements in calibration standards of known composition.

8. By scanning the beam in a television-like raster and displaying the intensity of a selected X-ray line, element distribution images or 'maps' can be produced. Also , images produced by electrons collected from the sample reveal surface topography or mean atomic number differences according to the mode selected. The scanning electron microscope (SEM), which is closely related to the electron probe, is designed primarily for producing electron images, but can also be used for element mapping, and even point analysis , if an X-ray spectrometer is added. There is thus a considerable overlap in the functions of these instruments.

9. X-ray intensities are measured by counting photons and the precision obtainable is limited by statistical error. For major elements it is usually not difficult to obtain a precision (defined as 2σ) of better than ± 1% (relative), the overall analytical accuracy is commonly nearer ± 2%, owing to other factors such as uncertainties in the compositions of the standards and errors in the various corrections which need to be applied to the raw data. As well as producing characteristic X-ray lines, the bombarding electrons also give rise to a continuous X-ray spectrum, which limits the detachability of small peaks , owing to the presence of 'background'.

10. Spatial resolution - EDAX Spatial resolution is governed by the penetration and spreading of the electron beam in the specimen. Since the electrons penetrate an approximately constant mass, spatial resolution is a function of density. In the case of silicates (density about 3 g cm-3), the nominal resolution is about 2 μm under typical conditions, but for quantitative analysis a minimum grain size of several micrometers is desirable. Better spatial resolution is obtainable with ultra-thin (~100 nm) specimens, in which the beam does not have the opportunity to spread out so much. Such specimens can be analyzed in a transmission electron microscope (TEM) with an X-ray spectrometer attached, also known as an analytical electron microscope, or AEM.

11. In principle, specimens of any size and shape (within reasonable limits) can be analyzed. Holders are commonly provided for 25mm (1") diameter round specimens and for rectangular glass slides. Standards are either mounted individually in small mounts or in batches in normalsized mounts . Many samples are electrically non-conducting and a conducting surface coat must be applied to provide a path for the incident electrons to flow to ground. The usual coating material is vacuum-evaporated carbon (~10nm thick), which has a minimal influence on X-ray intensities on account of its low atomic number, and (unlike gold, which is commonly used for SEM specimens ) does not add unwanted peaks to the X-ray spectrum.

12. An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well. For example, a peak corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-Alpha peak. The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-Beta peak and so on. See Figure 1. When performing EDX analysis, the following must be observed: 1) The probe current must be adjusted such that data collection is just between 10%-30% dead. 2) Spot Mode operation must be used for contaminants suspected to be concentrated in very small regions. Failure Mechanisms/Attributes Tested for by EDX Analysis:   Inorganic Contamination, Elemental Composition

13. Images produced by electrons collected from the sample reveal surface topography or mean atomic number differences according to the mode selected. Advantages Qualitative and Quantitative analysis