AES Instrumentation: Related Techniques

Three other analytical techniques use some of the same key instrument components as Auger electron spectrometers. Scanning electron microscopy and electron probe microanalysis both employ focused electron beams to excite the sample and X-ray photoelectron spectroscopy employs an electron energy spectometer to measure energies of emitted electrons.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) instruments and Auger spectrometers use similar primary electron columns. In fact,SEM capabilities are usually incorporated into Auger instruments. Separate detectors are required for secondary and backscattered electrons. To produce images, these electron signals are measured as a function of primary beam position while the beam is scanned in a raster pattern over the sample.

The scintillator-photomultiplier electron detector (called an Everhart-Thornley detector, after its inventors) measures the secondary electrons. Higher voltages on the Faraday cage draw in more secondary electrons with more diverse trajectories. Off-axis detector placement favors secondary electrons with trajectories leading toward the detector. This provides the topographical information characteristic of secondary electron images.

The backscattered electrons are usually measured with a solid state detector located on the primary beam pole piece. The detector consists of a diode with a thin gold conductor across the front surface. Backscattered (but not secondary) electrons have sufficient energy to pass through the front surface and produce electron hole pairs which produce a current in the diode.

The secondary and backscattered electron signals are much more intense than the Auger signals. Therefore, Auger electron measurements require more intense primary beams to provide sufficient Auger electron signals. Auger instruments provide higher primary beam intensities and accept larger beam diameters as a trade-off. This maximizes the Auger electron signals at the expense of lateral resolution. Since the secondary and backscattered signals are more intense, a primary column dedicated only to SEM can be optimized for lateral resolution. Nevertheless, modern Auger instruments provide reasonably high resolution (< 10 nm) SEM images. Most stand-alone Auger instruments come equipped with both secondary and backscatter detectors.

For a thorough discussion of scanning electron microscopy (and electron probe X-ray microanalysis) see J. I. Goldstein, D. E. Newbury, P. Echlin, D. C. Joy, C. Fiori, and E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, Plenum Press, New York, 1981, and D. E. Newbury, D. C. Joy, P. Echlin, C. E. Fiori, and J. I. Goldstein, Advanced Scanning Electron Microscopy and X-Ray Microanalysis, Plenum Press, New York, 1986.

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Electron Probe X-Ray Microanalysis

The sample is bombarded with an electron beam in electron probe X-ray microanalysis. The X-rays formed in competition with the Auger process serve as the measurable signal. Two kinds of X-ray detectors are in wide use. Energy dispersive spectrometry (EDS) relies on semiconductor detectors, usually lithium drifted silicon. The EDS detector converts the X-rays into electron-hole pairs by inelastic scattering. The principle of operation is similar to the surface barrier detectors used in RBS. Wavelength dispersive spectrometry (WDS) depends on Bragg diffraction of X-rays incident on a crystal. Any X-ray wavelength can be selected by adjusting the crystal angle and/or changing the crystal to provide different diffraction plane spacing. The two detectors are complementary.

The EDS system detects all X-ray energies simultaneously and accepts a wide solid angle of the X-ray emission. Thus, EDS is faster and better for survey spectra when the sample is totally unknown. The WDS system provides higher energy resolution, useful for separating overlapping peaks. Furthermore, bremsstrahlung contributes less background to the narrower peaks. In addition, WDS detectors accept a wider range of signal intensities. The WDS system is useful for mapping the locations of specific elements over a sample surface and for elemental quantitation down to the 100 ppm level.

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X-Ray Photoelectron Spectroscopy

X-Ray photoelectron spectroscopy (XPS) measures the energy distribution of photoelectrons produced by sample irradiation with X-rays. The photoelectron energies follow the Einstein photoelectric law (kinetic energy = photon energy - binding energy). The Auger process also contributes peaks to XPS spectra. The theory and instrumentation of XPS are described in another section.

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