Scanning Auger Microscopy (SAM)
Scanning Auger microscopy (SAM) is also known as Auger electron spectroscopy (AES). In this technique, the sample surface is bombarded with a high energy (3 – 10 kV) primary electron beam, which results in the emission of secondary, backscattered and Auger electrons from the area of bombardment and these can be readily detected and analyzed. The secondary and backscattered electrons are used for imaging purposes similar to scanning electron microscopy (SEM). The Auger electrons are emitted at discrete energies that are characteristic of the elements present on the sample surface. The characteristic energies of the Auger electrons are such that only the electrons from the outer 0.5 to 5 nm can escape and be detected. This makes SAM an extremely surface sensitive technique and the analysis volume is typically 106 to 108 times smaller than that excited in SEM/EDX analysis. All elements in the periodic table, except hydrogen and helium, can be detected and their concentration can be determined. Applications of SAM include materials characterization, thin film analysis, corrosion product chemistry, failure analysis, particle identification for semiconductor and thin film head manufacturing, and mineral chemistry.
The scanning Auger microprobe available at Surface Science Western is a Physical Electronics Model PHI 660 instrument. The LaB6 filament in the electron gun is capable of producing a stable electron beam diameter of 500 nm or less in diameter. The cylindrical mirror energy analyzer is coaxial with the electron beam. This geometry is considered to be crucial for accurate representation of Auger intensity distributions on irregular surfaces. Ion bombardment for sputtering is performed using Ar+ ions. Other features include: sample rotation; freeze-fracture and parking stages; high efficiency V/F preamp for improved S/N ratio.
Several modes of analysis (examples below) can be performed in the SAM. 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/absence of contaminants and other elements of interest 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. With argon ion bombardment, the surface layers can be removed in a controlled manner, and analysis can be carried out on new layers exposed after each sputtering cycle. This is known as depth profiling, and it provides the relative concentrations of elements of interest as a function of depth. Finally, Auger elemental maps display the presence and the surface distribution of elements of interest within the area examined.
Physical Electronics Model 660
- Survey scan analysis of elements from Li to U at outer 0.5 – 5 nm of the surface
- Depth profiling (Ar+ ion gun 1 to 3 kV)
- Imaging (secondary and backscattered electron images)
- Mapping the presence and spatial distribution of elements at the very outer surface
- Small point analysis (0.5 μm or lower spot size) and multiple point analysis
- Sample size typically 25 x 25 x 5 mm
- Auger analysis can be conducted on metals and alloys, semiconductors, minerals, some polymers and glasses. Not suitable for highly insulating samples
Selected Applications in Industry:
- Stain on gold-plated lid
- High contact resistance in relay contacts (Ag-Sn-In alloy)
- Hazy region found on a chrome-plated part
- Stain found on Inconel 690 sample
- Dark-coloured contaminants on aluminum wire bond pads