Energy Analysis of Scattered Primary Ions

Ion Scattering Spectroscopy. The principle of ISS is the collisional elastic momentum and energy transfer of a primary ion of mass m1 and energy E0 (1 to 5 keV) with an atom of mass m2 in the topmost layer of the sample (Ref 27). For a given scattering angle, the energy E1 of the backscattered primary ion (generally He) is a unique function of the ratio m2/mi. For example, at 90° scattering angle the relation

is valid and shows how the energy scale (Ei) can be calibrated in a mass scale (m2). The single-collision binary scattering described by Eq 6 occurs only at the surface. Therefore ISS is unique in determining the composition of only the topmost surface layer. However, the limitations in sensitivity and mass resolution at higher masses (see Eq 6), along with its difficult quantification, have confined ISS to more fundamental research.

Rutherford backscattering spectroscopy is one of the techniques first applied to thin-film analysis (Ref 28). The reason is that due to their high energy (>300 keV, up to several MeV), the primary ions penetrate between several 100 nm and some 10 pm of a solid until they lose their energy and are stopped. Along their way, a considerable fraction undergoes Rutherford backscattering, provided that the nuclei in the target have a higher mass than the primary ions. (These are usually He+ ions, so hydrogen and helium cannot be detected by RBS.) However, forward ejection of these atoms, that is, elastic recoil detection, can be applied to detect hydrogen (Ref 33). The backscattered primary ions lose a specific amount of energy, which is determined by both the mass m2 of the scattering atomic nuclei in the sample and by the well-known energy loss that occurs mainly through electronic interaction, which is proportional to the totally traveled distance z and the loss rate dE/dz. The measured energy Ej/(m2, z) of the primary ion (m1) after backscattering is given by the following two terms:

E1(m2, z) = fm2, E0) - z • fm2, E0, dE/dz) (Eq 7)

Because of the second term, the width of the peak due to scattering at m2 is a measure of the thickness of the respective material, and its intensity is proportional to the (areal) concentration. It is obvious that the achievable depth resolution depends on the energy resolution of the analyzer. Thin-film analysis with high depth resolution is enabled by modern electrostatic energy analyzers. An example is shown in Fig. 9, where the measured and calculated intensity-energy relations for a 10.4 nm thick niobium layer on sapphire is depicted (Ref 34). The notch of the measured profile is due to a native niobium oxide layer of about 2 nm thickness. The main advantages of RBS are that it is a nondestructive and quantitative method. The main disadvantages are the relatively large analyzed spot (typically 1 mm diameter), the necessity of expensive particle accelerators, and the lack of chemical information. For a summary of the typical features of RBS in comparison with other methods, see Table 1.

Fig. 9 High-resolution Rutherford backscattering spectroscopy of a 10.4 nm niobium layer on sapphire (calculated solid lines) that was oxidized in air (shoulder in the experimental points distribution). 1 MeV4He+. Source: Ref 34
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