Auger Emission Spectroscopy (AES)

 

Auger Emission Spectroscopy (AES), or Auger Analysis, is a failure analysis technique used in the identification of elements present on the surface of the sample.  Like EDX and WDX analysis, AES involves the bombardment of the sample with an energetic primary beam of electrons.  This process generates, among other things, a certain class of electrons known as Auger electrons.

   

The mechanism by which an Auger electron is released starts with an electron being ejected by the primary electron beam from its shell, say, the K-shell. Another electron from an outer shell (say, the L1-level) of the same atom emits energy in the form of a photon in order to go down to the K-shell position vacated by the ejected electron.  The photon released by the second electron will either get lost or eject yet another electron from a different level,  say, L2.  Auger electrons are electrons ejected in this manner, such as the third electron from L2 in the example.  

  

Thus, the generation of an Auger electron requires at least three electrons, which in the example above are the 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.

    

Figure 1. Example of an Auger Analysis Equipment from JEOL

   

The energy content of the emitted Auger electron is unique to the atom where it came from.  Thus, AES works by quantifying the energy content of each of the Auger electrons collected and matching it with the right element.  

    

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.  As such, AES can only provide compositional information about the surface of the sample.  In order to use AES for compositional analysis of matter deep into the sample, a crater must first be milled onto the sample at the correct depth by ion-sputtering.

 

AES has the ability to provide excellent lateral resolution, allowing reliable analysis of very small areas (less than 1 micron).  It also offers satisfactory sensitivity, detecting elements that are less than 1% of the atomic composition of the sample.  Coupled with ion-sputter milling capability and raster scanning, AES can even be used to generate 3-dimensional maps of elemental distributions of a volume of the sample.  This analysis technique is known as scanning auger microscopy (SAM).     

    

The output of AES is referred to as an Auger spectrum. This spectrum would show peaks at Auger electron energy levels corresponding to the atoms from which the auger electrons were released. 

         

The uses of AES as an analytical tool in the semiconductor industry include but are not limited to the following: 1) identification of surface contaminants; 2) detection of very thin SiO2 and other oxide layers on surfaces; 3) determination of contamination levels in barrier metals; 4) analysis of corrosion failures; and 5) detection of P, B, and AS concentrations in SiO2 layers. 

   

AES has the following limitations:  1) charging up of insulative surfaces when struck by the primary electron beam; 2) damage to certain materials, especially organic ones, when struck by the electron beam; 3) occurrence of matrix effects, i.e., signal alterations when some elements are present in particular matrices. 

  

See Also:  Failure AnalysisAll FA TechniquesEDX/WDX Analysis;

FTIR SpectroscopySIMS/LIMSESCA or XPSChromatography;

FA Lab EquipmentBasic FA Flows Package FailuresDie Failures

                

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