Challenges in surface analysis

Abstract

Many techniques have been developed since the 1960s to study the surfaces of materials. Some of them provide information on the chemical composition of surfaces and three have achieved widespread application. These three are X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS). XPS is also referred to as electron spectroscopy for chemical analysis (ESCA), and photoemission spectroscopy (PES). Histories and the backgrounds of XPS (Briggs and Grant, 2003; Kelly, 2004), AES (Burhop, 1952; Briggs and Grant, 2003) and SIMS (Benninghoven et al., 1987; Benninghoven, 2001) have been written, and these techniques continue to be developed today, providing increased sensitivity, spatial resolution, and automation. XPS and AES measure the kinetic energies of electrons leaving the surface from incident X-rays (XPS) or incident electrons (AES). Auger electrons can also be produced by other means such as X-rays, positrons (Ohdaira and Suzuki, 2013), and even ions (Grant, 2003). SIMS measures the mass spectrum (actually the mass-to-charge ratio) of positively or negatively charged ions ejected from the surface of a material following impact by energetic ions. A variation of SIMS, sputtered neutral mass spectrometry (SNMS), measures the mass spectrum of the neutral species emitted. In SNMS, ionization of the neutral species is made after they leave the surface. Other surface analysis techniques include ion scattering spectroscopy (ISS) and Rutherford backscattering spectrometry (RBS), and these have been compared with the other techniques mentioned above (Powell et al., 1991). The most commonly used technique for surface analysis is XPS as it provides the simplest spectrum and is the easiest to quantify. XPS instruments also have a much lower cost than AES, SIMS, etc., so when groups are financially constrained, they tend towards XPS. In most cases, XPS also provides excellent information on the chemical state of surface atoms. AES can sometimes provide superior chemical information, such as the chemical state of carbon onmetal surfaces (Haas et al., 1972; Hooker and Grant, 1977).While XPS and AES do not directly detect hydrogen and helium in materials, the effect of hydrogen on other elements in the surface can sometimes be observed with XPS (Smentkowski et al., 1995) and AES (Bevolo, 1985). On the other hand, SIMS can detect all elements as well as distinguish isotopes; spectra can be quite complex as large molecular fragments from the material are also formed. Isotope detection can be very useful when an oxygen beam is used for analysis, where oxygen from the surface of the material can be distinguished from the oxygen in the beam. The number of papers published each year in AES and SIMS has been fairly constant for the past 20 years, whereas those published in XPS continue to increase. This is illustrated in Figure 1, which is plotted on a logarithmic scale to better show their growth since the early years. The publications using the terms ESCA, PES, HAXPES, NAP-XPS, ARXPS (angle resolved XPS), and ARPES (angle resolved PES) have been included along with those using XPS to compare with the other techniques. Figure 2 illustrates the use of the different terms for XPS and shows that ESCA was the preferred term in the 1960s and 1970s, but was overtaken by the term XPS in the 1980s and remains dominant. The term PES is often used OPEN ACCESS