Notes to practice about the SEM & EDX lab (compi l ed by Dr. Stefan Boyadjiev )

Definitions and general theory of the methods

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. In most applications, data are collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20x to approximately 30,000x, spatial resolution of 50 to 100 nm). The SEM is also capable of performing analyses of selected point locations on the sample; this approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions, using Energy-dispersive X-ray spectroscopy (EDX or EDS ), also crystalline structure and crystal orientations, using diffracted backscattered electron detectors (EBSD). The design and function of the SEM is very similar to the Electron probe microanalyser (EPMA) and considerable overlap in capabilities exists between the two instruments, so they can be combined in one. Electron probe microanalysis (EPMA) is a term for an analytical technique that is used to establish the composition of small areas on specimens. The types of signals produced by a SEM include secondary electron (SE) , back-scattered electron s (BSE), characteristic X-rays, light (cathodoluminescence, CL), specimen current and transmitted electrons. Secondary electron detectors are standard equipment in all SEMs, back-scattered electrons detectors are also usually present. But it is rare that a single machine would have detectors for all possible signals. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. In the most common or standard detection mode, secondary electron imaging or SEI, the SEM can produce very high-resolution images of a sample surface, revealing details up to less than 1 nm in size. Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding also the surface structure of a sample. Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. BSE are often used in analytical SEM along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images (BSEI or BSI) can provide information about the distribution of different elements in the sample. For this reason, for example, BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, which would otherwise be difficult or impossible to detect in secondary electron images in biological specimens. It is also very useful for tracing distribution of metal ions in polymeric matrix. In SEI the signal is from around the top 5 nm in metals, and the top 50 nm in insulators. In BSI the signal comes from the top ~100 nm of the surface or even more. Characteristic X-rays are emitted when the electron beam removes an inner shell electron from the sample, causing a higher-energy electron to fill the shell and release energy. These characteristic X-rays are used to identify the composition and measure the abundance of elements in the sample.EMPA is one of several particle-beam techniques. A beam of accelerated electrons is focused on the surface of a specimen using a series of electromagnetic lenses, and these energetic electrons produce the characteristic X-rays within a small volume (typically 1 to 9 cubic microns) of the specimen. The characteristic X-rays are detected at particular wavelengths, and their intensities are measured to determine concentrations. All elements (except H, He, and Li) can be detected because each element has a specific set of X-rays that it emits. This analytical technique has a high spatial resolution and sensitivity, and individual analyses are reasonably short, requiring only a minute or two in most cases.

Additionally, the electron microprobe can function like a SEM and perform mapping of the surface producing magnified secondary- and backscattered-electron images of the sample employing at the same time data for the composition (making “elemental composition images”). In SEM and EPMA is required high vacuum, which is usually created by 2 vacuum pumps (for pre-vacuum and high vacuum), most commonly rotary pump + turbomolecular pump. SEMs always have at least one detector (usually a secondary electron detector), and most have additional detectors. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates. The Everhart-Thornley Detector (E-T detector or ET detector) is a secondary electron and back-scattered electron detector used in scanning electron microscopes (SEMs). It is named after its designers, Thomas E. Everhart and Richard F. M. Thornley who in 1960 published [1] their design to increase the efficiency of existing secondary electron detectors by adding a light pipe to carry the photon signal from the scintillator inside the evacuated specimen chamber of the SEM to the photomultiplier outside the chamber. The E-T secondary electron detector can be used in the SEM's back-scattered electron mode by either turning off the Faraday cage or by applying a negative voltage to the Faraday cage. However, better back-scattered electron images come from dedicated BSE detectors rather than from using the E-T detector as a BSE detector. The semiconductor backscattered electron detector is energized by incident high energy electrons (~90% E0), wherein electron-hole pairs are generated and swept to opposite poles by an applied bias voltage. This charge is collected and input into an amplifier. It is positioned directly above the specimen. Most SEMs use a solid state x-ray detector for EDS, and while these detectors are very fast and easy to utilize, they have relatively poor energy resolution and sensitivity to elements present in low abundances when compared to wavelength dispersive x-ray detectors (WDS) on most electron probe microanalyzers (EPMA).

Construction of SEM

Essential components of all SEMs include the following:- Electron Source ("Gun")- Electron Lenses- Sample Stage- Detectors for all signals of interest- Display / Data output devicesInfrastructure Requirements:- Power Supply- Vacuum System- Cooling system- Vibration-free floor- Room free of ambient magnetic and electric fields

Applications

The SEM is routinely used to generate high-resolution images of shapes of objects (SEI) and to show spatial variations in chemical compositions: 1) acquiring elemental maps or spot chemical analyses using EDS, 2) discrimination of phases based on mean atomic number (commonly related to relative density) using BSE, and 3) compositional maps based on differences in trace element "activitors" (typically transition metal and Rare Earth elements) using CL. The SEM is also widely used to identify phases based on qualitative chemical analysis and/or crystalline structure. Precise measurement of very small features and objects down to 50 nm in size is also accomplished using the SEM. Backescattered electron images (BSE) can be used for rapid discrimination of phases in multiphase samples. SEMs equipped with diffracted backscattered electron detectors (EBSD) can be used to examine microfabric and crystallographic orientation in many materials. There is arguably no other instrument with the breadth of applications in the study of solid materials that compares with the SEM. The SEM is critical in all fields that require characterization of solid materials. Most SEM's are comparatively easy to operate, with user-friendly "intuitive" interfaces. Many applications require minimal sample preparation. For many applications, data acquisition is rapid (less than 5 minutes/image for SEI, BSEI, spot EDX analyses.) Modern SEMs generate data in digital formats, which are highly portable.

Sample Preparation

Sample preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of "low vacuum" operation.

References

[1] Everhart, T E and R F M Thornley (1960). "Wide-band detector for micro-microampere low-energy electron currents". Journal of Scientific Instruments 37 (7): 246–248. doi:10.1088/0950-7671/37/7/307.[2] http://www.geosci.ipfw.edu/sem/semedx.html