AUGER ELECTRON SPECTROSCOPY

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Slide 1:

By SD.SHAFROSE UNDER THE GUIDENCE OF THE Dr. Prafulla Kumar Sahu M. pharm, Ph.d

INTRODUCTION:

INTRODUCTION Auger Electron Spectroscopy (AES ),is one of the most commonly employed surface analytical technique for determining the composition of the surface layers of a sample, and a widely used technique in the identification of elements present on the surface of the sample. AES involves the bombardment of the sample with a high energetic (2 - 10 keV) primary electron beam. This bombardment results in the emission of backscattered, secondary, and Auger electrons that can be detected and analyzed. This process generates, among other things, a certain class of electrons known as Auger electrons . And process known as auger emission or auger process.

HISTORY:

HISTORY AES is a surface analytical technique First discovered in 1923 by Lise Meitner and later independently discovered once again in 1925 by Pierre Auger. and deriving its name from the effect observed by Pierre Auger, a French Physicist. Lise Meitner Pierre Victor Auger

PRINCIPLE:

PRINCIPLE Finally the atom is left with two vacancies. The main principle involves three steps

MODES OF OPERATION:

MODES OF OPERATION The Auger analysis can include Survey Scans, High-Resolution spectra, Depth Profiles, Imaging, Mapping, and Point Analysis. Survey scans of the entire range of Auger electron energies, carried out by detecting and counting the number of Auger electrons, could reveal the presence of contaminants on the sample surface. By taking into account the sensitivity factors of the elements detected, quantification is possible. This is useful in identifying the unknown elements and estimating their concentration on the surface.

CONDITIONS FOR AUGER SPECTRA:

CONDITIONS FOR AUGER SPECTRA The generation of an Auger electron requires at least three electrons, that are K, L1, and L2 electrons. In this example, the emitted Auger electron is referred to as a KLL Auger electron.  Hydrogen and Helium atoms have less than three electrons, and are therefore undetectable by AES. The energy of Auger electrons is usually between 20 and 2000 eV.  The depths from which Auger electrons are able to escape from the sample without losing too much energy are low, usually less than 50 angstroms.  Thus, Auger electrons collected by the AES come from the surface or just beneath the surface.

The Auger Process & Auger Spectroscopy :

The Auger Process & Auger Spectroscopy the Auger process is illustrated using the K, L 1 & L 2,3 levels. These could be the inner core levels of an atom in either a molecular or solid-state environment. Auger spectroscopy can be considered as involving three basic steps : (1)   Atomic ionization (by removal of a core electron) (2)   Electron emission (the Auger process) (3)   Analysis of the emitted Auger electrons

I. Ionization :

I. Ionization The Auger process is initiated by creation of a core hole, this is typically carried out by exposing the sample to a beam of high energy electrons (typically having a primary energy in the range 2 - 10 keV). Such electrons have sufficient energy to ionise all levels of the lighter elements, and higher core levels of the heavier elements. Ionization

II. Relaxation & Auger Emission :

II. Relaxation & Auger Emission The ionized atom that remains after the removal of the core hole electron is in a highly excited state and will rapidly relax back to a lower energy state by one of two routes : X-ray fluorescence Auger emission

Slide 10:

In this example, one electron falls from a higher level to fill an initial core hole in the K-shell and the energy liberated in this process is simultaneously transferred to a second electron ; a fraction of this energy is required to overcome the binding energy of this second electron, the remainder is retained by this emitted Auger electron as kinetic energy . In the Auger process illustrated, the final state is a doubly-ionized atom with core holes in the L 1 and L 2,3 shells.

Slide 11:

We can make a rough estimate of the KE of the Auger electron from the binding energies of the various levels involved. In this particular example, KE = ( E K - E L1 ) - E L23 Note : the KE of the Auger electron is independent of the mechanism of initial core hole formation.

Auger Transitions:

Auger Transitions Auger electrons designated by x-ray notation as KLL , KL 1 L 2,3 , L 2,3 M 2,3 M 4,5 or KVV … etc. First letter - initial core hole location Second letter - initial location of relaxing electron Third letter - location of second hole (initial location of Auger electron) KL 1 L 2,3 = K(1s) L 1 (2s) L 2,3 (2p) Location of origin of relaxing Auger electron core hole electron

INSTRUMENTATION:

INSTRUMENTATION It contains the following components: A source of radiation,(electron gun) An electron energy analyzer, An electron detector, A read – out system, and A high vacuum system. In addition, the entire system must be shielded from the earth’s magnetic field. Figure 1. Example of an Auger Analysis Equipment from JEOL

Electron gun::

Electron gun: The nature of the electron gun used for AES analysis depends on the following factors: • The speed of analysis (requires a high beam current) • The desired spatial resolution. • Beam-induced changes to the sample surface. The range of beam currents normally used in AES is between 10 –9 and 5 * 10 –6 A. The lower current gives high spatial resolution whereas the higher current may be used to give speed and high sensitivity where spatial resolution is of little concern. In certain samples the high current used may induce surface damage to the specimen and should be avoided.

Electron Energy Analyzer::

Electron Energy Analyzer: The function of an electron energy analyzer is to disperse the secondary emitted electrons from the sample according to their energies. An analyzer may be either magnetic or electrostatic. Because electrons are influenced by stray magnetic fields (including the earth’s magnetic field), it is essential to cancel these fields within the enclosed volume of the analyzer. Electrostatic analyzers are used in all commercial spectrometers because of the relative ease of stray magnetic field cancellation

Electron Detector::

Electron Detector: Detectors used in AES. I.Single-Channel Detector (SCD): The detector used in conventional instrumentation is a channel electron multiplier. It is an electrostatic device that uses a continuous dynode surface. It requires only two electrical connections to establish the conditions for electron multiplication. The output of this detector consists of a series of pulses that are fed into a pulse amplifier/discriminator and then into a computer. The advantage of such a detector is that it can be exposed to air for a long time without damage. It counts electrons with a high efficiency, even at essentially zero kinetic energy, and the background is 0.1 count/sec or lower.

II. Multi Channel Detector (MCD):

II. Multi Channel Detector (MCD) A multiple detection system can be added at the output of the analyzer. The system may be in the form of a few multiple, parallel, equivalent detector chains or position-sensitive detectors spread across the whole of the analyzer output slit plane. Such an arrangement can be devised in a number of ways using phosphor screens and TV cameras, phosphor screens and charge-coupled devices, resistive anode networks, or discrete anodes.

Read – out system:

Read – out system The Auger electrons appear as peaks on a smooth background of secondary electrons. If the specimen surface is clean, the main peaks would be readily visible and identified. However, smaller peaks and those caused by trace elements present on the surface may be difficult to discern from the background. Because the background is usually sloping, even increasing the gain of the electron detection system and applying a zero offset is often not a great advantage. Therefore, the Auger spectra are usually recorded in a differential form. In the differential mode it is easy to increase the system gain to reveal detailed structure not directly visible in the undifferentiated spectrum

Vacuum system::

Vacuum system: Electron spectrometers must operate under a vacuum of 10 -6 torr or lower, and 10 -10 torr is ideal. At pressures higher than 10 -6 torr, the electrons would be scattered from the sample to the detector. Pressures of these orders can be achieved by a variety of techniques. However, the most common system is a getter-ion pump complemented by a sublimation pump, with a cryogenic shroud and sorption forepump. The vacuum system is of stainless steel construction, with crushed metal gaskets.

Slide 20:

Auger electron spectrophotometer

Slide 21:

The basic technique of auger spectroscopy, which is primarily a surface-sensitive technique use for elemental analysis of surfaces, has also been adapted for use in: Auger depth profiling: providing quantitative compositional information as a function of depth below the surface. Depth profiles are obtained by employing a controlled sputtering process which enables elemental concentration to be plotted as a function of depth. Sputtering is a process in which an ion gun is used to remove a few angstroms of the top most surface of a sample. Sputtering and analysis or alternated until the desired depth is reached. Scanning auger microscopy(SAM): providing spatially-resolved compositional information on heterogeneous samples. Therefore, the auger multiprobe is capable of producing elemental composition spectra, surface images, selective elemental line scans and map , and depth profiles.

Slide 22:

Scanning electron micrograph (SEM) image of surface defect on stainless steel

AES spectrum from a Cu grid: on the top is the measured spectrum, below is its derivative :

AES spectrum from a Cu grid: on the top is the measured spectrum, below is its derivative Auger spectra from a ‘Cu’ grid

General Uses :

General Uses • Identification of elements on surfaces of materials • Quantitative determination of elements on surfaces • Depth profiling by inert gas sputtering • To determine the chemical reactivity at a surface • Auger electron elemental map of the system qualitative analysis through fingerprinting spectral analysis • Identification of different chemical states of elements • Determination of atomic concentration of elements • Adsorption and chemisorption of gases on metal surfaces

Applications :

Applications Identification of surface contaminants . Detection of very thin sio2 and other oxide layers on surfaces. Determination of contamination levels in barrier metals. Analysis of corrosion failures; and detection of P, B, and AS concentrations in sio2 layers. Analysis of mayerthorpe meteorite. Imaging of Ag nanoclusters on silicon. Palladium nanowires on silicon. Oxidation of grain boundaries in steels.

AES limitations::

AES limitations: Charging up of insulative surfaces when struck by the primary electron beam; Damage to certain materials, especially organic ones, when struck by the electron beam; Occurrence of matrix effects, i.e., signal alterations when some elements are present in particular matrices.

REFERENCES:

REFERENCES ^ IUPAC, Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Auger effect". ^ IUPAC, Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Auger electron". ^ Duparc, Olivier Hardouin (2009). "Pierre Auger – Lise Meitner: Comparative contributions to the Auger effect". International Journal of Materials Research (formerly Zeitschrift fuer Metallkunde) 100 (09): 1162. doi:10.3139/146.110163.. ^ "The Auger Effect and Other Radiationless Transitions". Burhop, E.H.S., Cambridge Monographs on Physics, 1952. ^ "The Theory of Auger Transitions". Chattarji, D., Academic Press, London, 1976. Hand book of “Analytical instruments”. Second edition by R.S.Khanpur . “Instrumental methos of analysis”.seventh edition by willard, merritt, dean, settle. Hand book of” Instrumental techniques for analytical chemistry” by Frank settle.

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THANK YOU ALL