Scanning electron microscope installation requirements

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,X, spatial resolution of 50 to 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 EDS , crystalline structure, and crystal orientations using EBSD.

The design and function of the SEM is very similar to the EPMA and considerable overlap in capabilities exists between the two instruments. Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons that produce SEM images , backscattered electrons BSE , diffracted backscattered electrons EBSD that are used to determine crystal structures and orientations of minerals , photons characteristic X-rays that are used for elemental analysis and continuum X-rays , visible light cathodoluminescence--CL , and heat.

Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples i.

X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete ortitals shells of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength that is related to the difference in energy levels of electrons in different shells for a given element. Thus, characteristic X-rays are produced for each element in a mineral that is "excited" by the electron beam.

SEM analysis is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates. 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.

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. While this contribution is most concerned with geological applications, it is important to note that these applications are a very small subset of the scientific and industrial applications that exist for this instrumentation.

Most SEM's are comparatively easy to operate, with user-friendly "intuitive" interfaces. Many applications require minimal sample preparation. Modern SEMs generate data in digital formats, which are highly portable. Samples must be solid and they must fit into the microscope chamber. Maximum size in horizontal dimensions is usually on the order of 10 cm, vertical dimensions are generally much more limited and rarely exceed 40 mm.

For most instruments samples must be stable in a vacuum on the order of 10 -5 - 10 -6 torr. Samples likely to outgas at low pressures rocks saturated with hydrocarbons, "wet" samples such as coal, organic materials or swelling clays, and samples likely to decrepitate at low pressure are unsuitable for examination in conventional SEM's.

However, "low vacuum" and "environmental" SEMs also exist, and many of these types of samples can be successfully examined in these specialized instruments. Most SEMs use a solid state x-ray detector 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.

An electrically conductive coating must be applied to electrically insulating samples for study in conventional SEM's, unless the instrument is capable of operation in a low vacuum mode.

This is done by using a device called a "sputter coater. The sputter coater uses an electric field and argon gas. The sample is placed in a small chamber that is at a vacuum. Argon gas and an electric field cause an electron to be removed from the argon, making the atoms positively charged. The argon ions then become attracted to a negatively charged gold foil. The argon ions knock gold atoms from the surface of the gold foil.

These gold atoms fall and settle onto the surface of the sample producing a thin gold coating. The radiation safety concerns are related to the electrons that are backscattered from the sample, as well as X-rays produced in the process. Most SEMs are extremely well shielded and do not produce exposure rates greater than background. However, scanning electron microscopes are radiation-generating devices and should be at least inventoried.

It is also important that the integrity of the shielding is maintained, that all existing interlocks are functioning, and that workers are aware of radiation safety considerations. Purdue Police Phone: Purdue Fire Phone: Sign up for Emergency Text Messages. Quick Links. Radiological and Environmental Management. How does a SEM work? How is a sample prepared? What are the radiation safety concerns? Diagram courtesy of Iowa State University The SEM is an instrument that produces a largely magnified image by using electrons instead of light to form an image.

Each machine should be key controlled when not in use. Interlocks, if present, must remain operational unless approved by the RSO. Shielding must be sufficient to maintain exposure rates less than 0. The Radiation Safety Office will keep inventory and survey information on file in their offices. The SEM user should keep logbook of any maintenance done on machine. RSO must be notified if any modifications are made to the interlocks or any other safety devices.