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Auger Tutorial: Instrumentation

In this Auger Tutorial from EAG Laboratories, we present the history of Auger, as well as the scientific principles behind the instrumentation and data provided by this analytical technique.

History

In 1923, Pierre Auger discovered the Auger Process and Auger electrons while irradiating samples with X-rays. The idea of using electron-stimulated Auger signals for surface analysis was first suggested in 1953 by J. J. Lander. However, it wasn’t until 1967 that Larry Harris demonstrated the use of differentiation for enhancing the Auger signals. This development provided the necessary sensitivity for useful measurements. Early differentiating instruments used analog circuits and lock-in amplifiers to provide differentiated spectra directly, but more modern instruments acquire electron intensities directly and use computer Display Algorithms to provide differentiated spectra. Today Auger electron spectroscopy is the most frequent analytical method for surfaces, thin-films, and interface compositions. This utility arises from the combination of surface specificity (0.5 to 10 nm), good lateral surface resolution (as little as 10 nm), periodic table coverage (except hydrogen and helium), and reasonable sensitivity (100 ppm for most elements).

Electron Energy Analyzers

Electron energy analyzers measure the number of ejected electrons as a function of the electron energies. The analyzers must be located in a high vacuum chamber and isolated from stray magnetic fields (including the earthís) that deflect electrons. Past Auger spectrometers used several types of electron energy analyzers, including spherical sector and cylindrical mirror analyzers. However, modern instruments nearly always incorporate cylindrical mirror analyzers because their high transmission efficiency leads to better signal-to-noise ratios. The schematic shows a cross section of a cylindrical mirror analyzer in red. The primary electron beam hits the sample surface at the source point of the analyzer. Auger electrons move outward in all directions and some pass through the grid covered aperture in the inner cylinder. A variable negative potential on the outer cylinder bends the Auger electrons back through a second aperture on the inner cylinder and then through an exit aperture on the analyzer axis. The energy of transmitted electrons is proportional to the voltage (-V) on the outer cylinder.

Primary Electron Sources

Three kinds of primary electron sources are in common use in Auger electron spectrometers.

  1. A tungsten cathode source consists of a wire filament bent in the shape of a hairpin. The filament operates at ~2700 K by resistive heating. The tungsten cathodes are widely used because they are both reliable and inexpensive. Lateral resolution is limited because the tungsten cathode current densities are only about 1.75 A/cm2.
  2. Lanthanum hexaboride (LaB6) cathodes provide higher current densities because LaB6 has a lower work function and greater emissivity than tungsten. At 2000 K, current densities of ~100 A/cm2 are available. Higher current densities provide narrower electron beams useful for analyzing smaller features.
  3. Field Emission electron sources consist of very sharp tungsten points at which electrical fields can be >10E7 V/cm. At these fields, electrons tunnel directly through the barrier and leave the emitter with near zero work functions. Field emission guns provide the brightest beams (1E3 to 1E6 A/cm2). However, the low work functions are only obtained with extremely clean tips. A single atom on the tip can increase the work function and reduce electron emission. Ultrahigh vacuum and continuous heating (~2000 K) keep the tip clean and the electron beam stable. Electron beams as narrow as 10 nm provide Auger analysis of small features.

 

Auger instruments have primary electron beam columns similar to electron microscopes. The columns may include both electrostatic and magnetic lenses for beam focusing as well as quadrupole deflectors for beam steering and octopole lenses for beam shaping.

Detectors

Auger electron spectrometers use electron multipliers similar to those used in SIMS. The Auger electron multipliers usually operate in pulse counting mode which records every electron as it arrives at the detector.

Modern Auger Instruments

One modern Auger electron spectrometer incorporates both a field emission electron source and a parallel electron detector. The parallel detector simultaneously records electrons at eight separate energies. The detector consists of a dual microchannel plate with a hole in the middle to accommodate the primary beam. Eight concentric ring anodes detect electrons arriving with eight separate energies.

Auger Data Display Algorithms

Displaying Auger spectra as differentials enhances the sharp Auger peaks and deemphasizes the relatively intense and structureless background. Strong interfering signals accompany the Auger electrons in all regions of the Auger spectrum. The interference includes contributions from secondary and backscattered electrons, and from Auger electrons that have lost energy as they escape from beneath the sample surface. The first figure shows a typical Auger spectrum (of iron) plotted as total electron signal, N(E), versus electron energy. The Auger peaks are obscure even after expanding the vertical scale.

Plotting the spectrum as the differential of the electron signal, dN(E)/dE, clarifies some of the spectral details.

Both of the first two plots under emphasize the high energy end of the spectrum. Multiplying the total electron signal by the electron energy, E x N(E), accentuates the high energies as shown in the next figure.

Finally, plotting the differential, d[E x N(E)]/dE, of the above function provides for clear display of the features in an Auger electron spectrum. This d[E x N(E)]/dE format is the most common mode for presenting Auger data.

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.

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.

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.