K V GOPINATH M Pharm PhD,CPhTTirumala Tirupati Devasthanams

TIRUPATIe-mail:gopinath.karnam@gmail.com

SCANNING ELECTRON MICROSCOPE- ENERGY – DISPERSIVE X – RAY MICROANALYSIS (SEM-E-DAX)

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Introduction

SEM - is a powerful and mature technique in the examination of materials, widely in metallurgy, geology, biology and medicine.

It can obtain high magnification images, with a good depth of field. It can also analyze individual crystals or other features.

A high-resolution SEM image can show detail down to 25 Angstroms, or better.

When used in conjunction with the closely-related technique of energy-dispersive X-ray microanalysis (EDX, EDS, EDAX), the composition of individual crystals or features can be determined.

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The scanning electron microscope

SEM operates at a high vacuum. The principle is that a beam of electrons is generated by a suitable source, typically a tungsten filament or a field emission gun.

The electron beam is accelerated through a high voltage (e.g.: 20 kV) and pass through a system of apertures and electromagnetic lenses to produce a thin beam of electrons., then the beam scans the surface of the specimen by means of scan coils .

Electrons are emitted from the specimen by the action of the scanning beam and collected by a suitably-positioned detector.

The microscope operator is watching the image on a screen. Imagine a spot on the screen scanning across the screen from left to right. At the end of the screen, it drops down a line and scans across again, the process being repeated down to the bottom of the screen.

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The scanning electron microscope

The electron detector controls the brightness of the spot on the screen - as the detector "sees" more electrons from a particular feature, the screen brightness is increased. When there are fewer electrons, the spot on the monitor generally on digital screen gets darker.

The magnification of the image is the ratio of the size of the screen to the size of the area scanned on the specimen. If the screen is 300 mm across and the scanned area on the specimen is 3 mm across, the magnification is x100. To go to a higher magnification, the operator scans a smaller area; if the scanned area is 0.3 mm across, the magnification is x 1000, and so on.

There are different types of electron image. The two most common are the secondary electron image (sei) and the backscattered electron image (bei).

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The scanning electron microscope

The sei is used mainly to image fracture surfaces and gives a high resolution image.

The bei is used typically to image a polished section; the brightness of the bei is dependent on the atomic number of the specimen (or, for compounds, the average atomic number). For example, lead will appear brighter than iron and calcium oxide will appear brighter than calcium carbonate. The bei is, in essence, an atomic number map of the specimen surface.

All SEM images are in black-and-white, although they may subsequently have false colors applied to them for aesthetic reasons or to aid interpretation.

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Principle

An SEM is essentially a high magnification microscope, which uses a focused scanned electron beam to produce images of the sample, both top-down and, with the necessary sample preparation, cross-sections.

The primary electron beam interacts with the sample in a number of key ways- Primary electrons generate low energy secondary electrons, which tend to emphasize the topographic nature of the specimen.- Primary electrons can be backscattered which produces images with a high degree of atomic number (Z) contrast.- Ionized atoms can relax by electron shell-to-shell transitions, which lead to either X-ray emission or Auger electron ejection. - The X-rays emitted are characteristic of the elements in the top few μm of the sample and are measured by the EDX detector. 

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Components of SEM-E-Dax

Tungsten gun Sample

- Conductive samples (hiVac)- Nonconductive, mixed or contaminating samples (Low Vac)- Wet samples (use H2O gas medium) ESEM

Detector- Everhart Thornley Detector (ETD)- Backscattered Electrons (BSED)- Large Field Detector (LFD)- Gaseous Secondary Electron Detector (GSED)

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SEM – E - Dax

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Components of SEM-E-Dax - Tungsten gun

SEM have a tungsten gun which is capable of imaging the samples under different vacuum regimes such as High-vacuum (10-2 to 10-4 Pa), Low-vacuum (10 - 130 Pa) and ESEM-vacuum (10 - 2600 Pa).

The scanning electron microscope (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons. These electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties. The types of signals made by an SEM can include secondary electrons (SE) back scattered electrons (BSE), characteristic X-rays and light. These signals are captured by various detectors.

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Components of SEM-E-Dax - DETECTORS

Everhart Thornley Detector (ETD) :It is a scintillator photo-multiplier type detector monitoring electrons generated by the primary beam interaction with the specimen surface. It is permanently mounted in the chamber above and to one side of the sample. It works in two modes:- Secondary Electrons (SE) & -Backscattered Electrons (BSE).

Backscattered Electrons (BSED) : These are two-segment low-voltage diodes. The BSED is designated for a HiVac large field of view.

Gaseous Secondary Electron Detector (GSED)It is used for general wet imaging and for high pressure imaging with auxiliary gases.

Large Field Detector (LFD)The signal from the LFD contains more BSE information. This detector is ideal for general imaging at LoVac mode.

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The main features and benefits of the SEM

Image magnification range X 15 – X 200,000 and resolution 2 nm Accelerating voltage 1 - 30 keV Secondary and backscatter electron imaging Stereo imaging and stereo height measurement EDX analysis of known or unknown materials Automatic particle counting and characterisation Qualitative and quantitative analysis for all elements from carbon

upwards Quantitative analysis of bulk materials and features ≥ 2 μm Qualitative analysis of features ≥ 0.2 μm

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The main features and benefits of the SEM

Detection limits typically 0.1 - 100 Wt% for most elements Multi-element X-ray mapping and line scans Multi-layer, multi-element thin film analysis - Thickness and

composition Particle / Phase analysis - Detection, analysis, morphology and size Image Analysis Automatic particle counting and characterisation

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When performing SEM inspection, the following must be observed:

     The EHT must be high enough to provide a good image but low

enough to prevent specimen charging.

 To maximize contrast due to material differences, use as low an EHT as possible.

If possible, sputter-coat the specimen to prevent specimen charging. Sputter-coating is considered destructive. Never sputter-coat units that still need to undergo electrical testing, curve tracing, EDX analysis, inspection, etc.

The probe current must be set to its default value, unless a higher probe current is needed to focus the point of interest properly.05/01/23

Sample Images/data

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Energy Dispersive X-ray analysis (EDX or EDAX) Analysis

A SEM may be equipped with an Edax analysis system to enable it to perform compositional analysis on specimens. 

EDX analysis is useful in identifying materials and contaminants, as well as estimating their relative concentrations on the surface of the specimen.

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.

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Theory: Energy Dispersive X-ray analysis

During EDX Analysis, the specimen is bombarded with an electron beam inside the SEM. 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.

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Theory: Energy Dispersive X-ray analysis

The output of an EDX analysis is an EDX spectrum . 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 are 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.

EDX spectrum05/01/23

Theory: Energy Dispersive X-ray analysis

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.    Elements in an EDX spectrum are identified

Fig :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

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X- Ray Detector

The EDS detector is a solid state device designed to detect x-rays and convert their energy into electrical charge. This charge becomes the signal which when processed then identifies the x-ray energy, and hence its elemental source.

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Important EDS Parameters

Count Rate Accelerating Voltage Take of Angle Dead time & Time

constants

Count Rate: For a good quality spectrum (i.e. good resolution and fewest artifacts) use the 50 or 100 μs time constant (pulse processing time) with a dead time of 20 to 40%, and 500 to 2500 cps. These are good numbers if the sample consists largely of high energy peaks (> 1 keV), but if the spectrum is dominated by low energy peaks (< 1 keV) then a count rate of 500 - 1000 cps is better and the 100 μs time constant should be used.

Accelerating Voltage: The overvoltage is a ratio of accelerating voltage used to the critical excitation energy of a given line for an element. Typically, the overvoltage should be at least 2 for the highest energy line and no more than 10 to 20 times the lowest energy line of interest. We use the number 10 for quantitative applications and the 20 for qualitative applications. 05/01/23

Important EDS Parameters

Count Rate Accelerating

Voltage Take of Angle Dead time &

Time constants

Take of Angle: Typical take-off angles will range from 25 to 40 degrees. This angle is a combination of the detector angle, its position, sample working distance and sample tilt. The sensitivity for very low energy x rays and/or signals characterized by high absorption can be enhanced by increasing the take-off angle.

Dead Time & Time Constants: In an EDS system the real time (or clock time) is divided into live time and dead time. The live time is the time when the detector is alive and able to receive an x-ray event (i.e. the time when it is doing nothing) and the dead time is when the detector or preamplifier is unable to accept a pulse because it is busy processing or rejecting an event(s)

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When performing EDX analysis, the following must be observed:

The probe current must be adjusted such that data collection is just between 10%-30% dead.

Spot Mode operation must be used for contaminants suspected to be concentrated in very small regions.

The EHT level used during the analysis must be higher than the energy peaks corresponding to the elements of interest.

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