Editing Auger electron spectroscopy (section)

==Uses==

There are a number of electron microscopes that have been specifically designed for use in Auger spectroscopy; these are termed [[scanning Auger microscope]]s (SAMs) and can produce high resolution, spatially resolved chemical images.<ref name="grant2003"/><ref name="briggs_sheah1983"/><ref name="davis1980"/><ref name="oura2003"/><ref>{{cite book |last1=Attard |first1=Gary |last2=Barnes |first2=Colin |title=Surfaces |date=January 1998 |publisher=Oxford Chemistry Primers |page=47 |isbn=978-0-19-855686-2 }}</ref> SAM images are obtained by stepping a focused electron beam across a sample surface and measuring the intensity of the Auger peak above the background of scattered electrons. The intensity map is correlated to a gray scale on a monitor with whiter areas corresponding to higher element concentration. In addition, [[sputtering]] is sometimes used with Auger spectroscopy to perform depth profiling experiments. Sputtering removes thin outer layers of a surface so that AES can be used to determine the underlying composition.<ref name="briggs_sheah1983"/><ref name="thompson1985"/><ref name="davis1980"/><ref name="feldman_mayer"/> Depth profiles are shown as either Auger peak height vs. sputter time or atomic concentration vs. depth. Precise depth milling through sputtering has made profiling an invaluable technique for chemical analysis of nanostructured materials and thin films. AES is also used extensively as an evaluation tool on and off fab lines in the microelectronics industry, while the versatility and sensitivity of the Auger process makes it a standard analytical tool in research labs.<ref name="chao_yang">{{cite journal |last=Chao |first=Liang-Chiun |author2=Shih-Hsuan Yang |date=June 2007 |title=Growth and Auger electron spectroscopy characterization of donut-shaped ZnO nanostructures |journal=Applied Surface Science |volume=253 |issue=17 |pages=7162–7165 |doi=10.1016/j.apsusc.2007.02.184|bibcode = 2007ApSS..253.7162C }}</ref><ref name="jang2007">{{cite journal |author=Soohwan Jang |date=May 2007 |title=Comparison of E-beam and Sputter-Deposited ITO Films for 1.55 μm Metal–Semiconductor–Metal Photodetector Applications |journal=Journal of the Electrochemical Society |volume=154 |issue=5 |pages=H336–H339 |doi=10.1149/1.2667428 |display-authors=1 |last2=Kang |first2=B. S. |last3=Ren |first3=F. |last4=Emanetoglu |first4=N. W. |last5=Shen |first5=H. |last6=Chang |first6=W. H. |last7=Gila |first7=B. P. |last8=Hlad |first8=M. |last9=Pearton |first9=S. J.|bibcode=2007JElS..154H.336J }}</ref><ref name="xu2006">{{cite journal |author=Mingjie Xu |date=March 2006 |title=Biomimetic silicification of 3D polyamine-rich scaffolds assembled by direct ink writing |journal=[[Soft Matter (journal)|Soft Matter]] |volume=2 |pages=205–209 |doi=10.1039/b517278k |display-authors=1 |last2=Gratson |first2=Gregory M. |last3=Duoss |first3=Eric B. |last4=Shepherd |first4=Robert F. |last5=Lewis |first5=Jennifer A. |issue=3|pmid=32646146 |bibcode = 2006SMat....2..205X }}</ref><ref name="gondran2006">{{cite journal |last=Gondran |first=Carolyn F. H. |author2=Charlene Johnson |author3=Kisik Choi  |date=September 2006 |title=Front and back side Auger electron spectroscopy depth profile analysis to verify an interfacial reaction at the HfN/SiO<sub>2</sub> interface |journal=[[Journal of Vacuum Science and Technology B]] |volume=24 |issue=5 |doi=10.1116/1.2232380 |pages = 2457|bibcode = 2006JVSTB..24.2457G }}</ref> Theoretically, Auger spectra can also be utilized to distinguish between protonation states. When a molecule is protonated or deprotonated, the geometry and electronic structure is changed, and AES spectra reflect this. In general, as a molecule becomes more protonated, the ionization potentials increase and the kinetic energy of the emitted outer shell electrons decreases.<ref name="pmid26295900">{{cite journal | vauthors = Kryzhevoi NV, Cederbaum LS | title = Exploring Protonation and Deprotonation Effects with Auger Electron Spectroscopy | journal = J Phys Chem Lett | volume = 3 | issue = 18 | pages = 2733–7 | date = September 2012 | pmid = 26295900 | doi = 10.1021/jz301130t }}</ref>

Despite the advantages of high spatial resolution and precise chemical sensitivity attributed to AES, there are several factors that can limit the applicability of this technique, especially when evaluating solid specimens. One of the most common limitations encountered with Auger spectroscopy are charging effects in non-conducting samples.<ref name="carlson"/><ref name="briggs_sheah1983"/> Charging results when the number of secondary electrons leaving the sample is different from the number of incident electrons, giving rise to a net positive or negative electric charge at the surface. Both positive and negative surface charges severely alter the yield of electrons emitted from the sample and hence distort the measured Auger peaks. To complicate matters, neutralization methods employed in other surface analysis techniques, such as [[secondary ion mass spectrometry]] (SIMS), are not applicable to AES, as these methods usually involve surface bombardment with either electrons or [[ions]] (i.e. [[flood gun]]). Several processes have been developed to combat the issue of charging, though none of them is ideal and still make quantification of AES data difficult.<ref name="briggs_sheah1983"/><ref name="feldman_mayer"/> One such technique involves depositing conductive pads near the analysis area to minimize regional charging. However, this type of approach limits SAM applications as well as the amount of sample material available for probing. A related technique involves thinning or "dimpling" a non-conductive layer with [[argon|Ar<sup>+</sup>]] ions and then mounting the sample to a conductive backing prior to AES.<ref name="yu_jin">{{cite journal |last=Yu |first=Ling |author2=Deling Jin |date=April 2001 |title=AES and SAM microanalysis of structure ceramics by thinning and coating the backside |journal=[[Surface and Interface Analysis]] |volume=31 |issue=4 |pages=338–342 |doi=10.1002/sia.982|s2cid=98258140 |doi-access=free }}</ref><ref name="cazaux1992">{{cite journal |last=Cazaux |first=Jacques |date=December 1992 |title=Mechanisms of charging in electron spectroscopy |journal=Journal of Electronic Spectroscopy and Related Phenomena |volume=105 |issue=2–3 |pages=155–185 |doi=10.1016/S0368-2048(99)00068-7}}</ref> This method has been debated, with claims that the thinning process leaves elemental artifacts on a surface and/or creates damaged layers that distort bonding and promote chemical mixing in the sample. As a result, the compositional AES data is considered suspect. The most common setup to minimize charging effects includes use of a glancing angle (~10°) electron beam and a carefully tuned bombarding energy (between 1.5 keV and 3 keV). Control of both the angle and energy can subtly alter the number of emitted electrons vis-à-vis the incident electrons and thereby reduce or altogether eliminate sample charging.<ref name="carlson"/><ref name="davis1980"/><ref name="feldman_mayer"/>

In addition to charging effects, AES data can be obscured by the presence of characteristic energy losses in a sample and higher order atomic ionization events. Electrons ejected from a solid will generally undergo multiple scattering events and lose energy in the form of collective electron density oscillations called [[plasmon]]s.<ref name="carlson"/><ref name="oura2003"/> If plasmon losses have energies near that of an Auger peak, the less intense Auger process may become dwarfed by the plasmon peak. As Auger spectra are normally weak and spread over many eV of energy, they are difficult to extract from the background and in the presence of plasmon losses; deconvolution of the two peaks becomes extremely difficult. For such spectra, additional analysis through chemical sensitive surface techniques like [[x-ray photoelectron spectroscopy]] (XPS) is often required to disentangle the peaks.<ref name="carlson"/> Sometimes an Auger spectrum can also exhibit "satellite" peaks at well-defined off-set energies from the parent peak. Origin of the satellites is usually attributed to multiple ionization events in an atom or ionization cascades in which a series of electrons is emitted as relaxation occurs for core holes of multiple levels.<ref name="carlson"/><ref name="briggs_sheah1983"/> The presence of satellites can distort the true Auger peak and/or small peak shift information due to chemical bonding at the surface. Several studies have been undertaken to further quantify satellite peaks.<ref name="went2006">{{cite journal |last=Went |first=M. R. |author2=M. Vos |author3=A. S. Kheifets |date=November 2006 |title=Satellite structure in Auger and (''e'',2''e'') spectra of germanium |journal=Radiation Physics and Chemistry |volume=75 |issue=11 |pages=1698–1703 |doi=10.1016/j.radphyschem.2006.09.003|bibcode = 2006RaPC...75.1698W }}</ref>

Despite these sometimes substantial drawbacks, Auger electron spectroscopy is a widely used surface analysis technique that has been successfully applied to many diverse fields ranging from gas phase chemistry to nanostructure characterization. A new class of high-resolving electrostatic energy analyzers, face-field analyzers (FFA)<ref>Ilyin, A. M.; N. R. Guseinov; M. A. Tulegenova (2022). "Conical Face-Field electrostatic energy analyzers for investigating nanomaterials". ''J. Electr. Spectr. Relat. Phenom.'' 257.</ref><ref>Ilyin, A. M. (2003). "New class of electrostatic energy analyzers with a cylindrical face-field". Nuclear Instruments and Methods in Physics Research Section A. 500 (1–3): 62–67. Bibcode:2003NIMPA.500...62I. doi:10.1016/S0168-9002(03)00334-6.</ref> can be used for remote electron spectroscopy of distant surfaces or surfaces with large roughness or even with deep dimples. These instruments are designed as if to be specifically used in combined [[scanning electron microscope]]s (SEMs). "FFA" in principle have no perceptible end-fields, which usually distort focusing in most of analysers known, for example, well known CMA.