Scanning Electron Microscope Technique

Scanning Electron Microscopy (SEM) and

X-ray Energy Dispersive Spectroscopy

1. Description of Technique

The scanning electron microscope (SEM) is used to analyze the surface of specimens over a wide range of magnifications. A focused beam of electrons is either scanned across the surface of a specimen to form an image or stopped on a fixed location to perform one of a variety of spectrographic or analytical functions. The interaction of the beam with the specimen results in the generation of secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies.

Secondary electrons and backscattered electrons are collected by their respective detectors and their signals are amplified. These electrons are used in imaging. Secondary electrons mainly provide surface topographic imaging and backscattered electrons can form both images and supply atomic number information about the sample. Information may also be obtained as a result of the beam interjecting charge carriers into the specimen, which may be used to assess semiconductor material properties. Alternatively, the specimen’s surface potential may be measured by the “voltage contrast” method. Thus the position of a crystal defect or a p-n junction may be correlated with surface topography by comparing images obtained by different methods. Crystal perfection may be studied by electron channeling contrast, Kikuchi patterns and electron backscatter diffraction patterns.

The elemental make-up of the portion of the specimen illuminated by the beam may be determined via x-ray energy dispersive spectrometer (EDS) or wavelength dispersive spectrometer (WDS) attachments on the SEM. Utilizing a fixed beam generates an X-ray spectrum. Rastering the beam on the surface provides an elemental map of the surface.

2. Sample preparation for analysis

Little specimen preparation is needed other than assuring that the specimen’s surface is clean, free of artifacts, and conductive. Non-conducting specimens are usually carbon coated.

3. Information Obtained

Information comes from the surface and the portion of the specimen close to the surface. How close is a function of the instrument’s accelerating voltage, the composition of the specimen, and the signal in question.

  • The secondary electron signal comes from the top 20 - 50 Angstroms. Utilizing very low instrument accelerating voltages for very shallow penetration, resulting in a true surface image.
  • The BSE signal come from depths in the micron range and will provide elemental information arising in the entire volume of specimen involved.
  • The x-ray signal used to form spectra or element maps arises in a volume within the specimen on the order of microns in depth. Both qualitative and quantitative elemental analysis is possible. All elements from B on up in the periodic table may be detected. Our Lithium drifted Silicon detector (from EDAX - USA) can be operated in a windowless mode, ECON III, allowing 100% transmission of x-rays for all elements. Due to a very low noise circuitry of the data acquisition system, and the windowless detector, the light elements B, C, N, O, F can easily be detected. EDS detectability limit is about 0.1%.

5. Statistics/Miscellaneous

  • Data collection time: SEM images take about 1 min/photograph (development time), observation and adjustment of features in image (magnification needed, contrast enhancement, etc.) may take from one hour to many hours.
  • Quantitative analysis and x-ray maps will take from one to many hours.

Scanning Electron Microscope Equipement

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