Auger electron spectroscopy and its use for the characterization of titanium and hydroxyapatite surfaces

This review paper provides the basic background and underlying theory behind Auger electron spectroscopy (AES). Among the many surface analytical tools, AES has been shown to be very effective for surface composition analysis. These analyses are critically needed to better understand the interactions between the host and implant. The use of AES for titanium (Ti) and hydroxyapatite (HA) biomaterials characterization is demonstrated in this paper. The relative peak heights of TiL(2,3)M(2,3)V can be used as 'fingerprints' for TiO2 surfaces which have undergone different degrees of reduction. Similarly, for HA coatings, a shift in the phosphorus Auger peaks to a higher kinetic energy indicates the presence of a phosphate group, with strong P-O bonds. Depth compositional profiling and thin-film analysis can be performed using AES. In our studies, oxide thicknesses on Ti surfaces range from 36.8 +/- 7.4 A to 436 +/- 49 A depending on the surface treatment. Depth profiling can also be used to determine the subsurface composition of biomaterials. For HA coatings, a phosphorus concentration at the oxide/metal interface has been observed to be higher than at the outermost oxide surface. The HA coatings have also been observed to coexist within the titanium oxide, suggesting the occurrence of chemical bonding between the coatings and the metallic substrates. However, like other analytical tools, AES has its limitations. The electron beam damage can severely limit useful analysis of organic and biological materials and occasionally ceramic materials. Carbide buildup during long beam exposure times has been shown to affect the relative peak-to-peak intensities of the oxygen and metal Auger signals. The determination of film thickness requires a standard of known thickness and depth profiling of overlapping peaks can be very problematic. Even with these limitation, AES can be a powerful analytical tool for the characterization of biomaterial surfaces.