Detection of Sputtered Particles

Secondary ion mass spectroscopy (SIMS) is probably the first technique used for depth profiling (Ref 29). A primary ion beam of about 0.5 to 15 keV energy causes sputter erosion of the surface by emission of neutrals and ions. The ejected secondary ions are separated in a mass spectrometer (electric quadrupole or magnetic sector or, in time-offlight SIMS, by flight time measurements of pulse-accelerated secondary ions). The precision of identification of atomic and molecular species depends on the mass resolution, (m/Am), which typically is 500 to 2000 for quadrupole, up to 10,000 for magnetic sector, and up to 15,000 for time-of-flight spectrometers. The basic equation for quantitative SIMS can be written as (Ref 29):

where the measured count rate at mass i of the positive or negative secondary ions, II± , is determined by the primary ion intensity I0±, the total instrumental transmission Tu the sputter yield Y, (atoms/ion), the ionization probability (< 1) of the sputtered particles (i), and their mole fractionX, in the sample.

The main advantages of SIMS are its:

• Ability to detect all elements, including hydrogen

• Ability to detect elements in the parts-per-billion range

• Ability to perform microanalysis and imaging in the submicrometer range

• High dynamic range of up to 8 orders of magnitude in concentration

Besides its intrinsic destructiveness, the main disadvantage of SIMS is its difficult quantification. The strong matrix dependence of the ionization probability (up to 5 orders of magnitude) is the main reason for the notorious difficulty of quantifying SIMS in a multicomponent matrix with varying composition. Because the sputter yield Y, is much less matrix dependent, ionization of practically all sputtered neutrals (i.e., approaching unity) would drastically reduce the matrix effect. Indeed, this is done in the so-called SNMS method, where the sputtered neutrals are ionized by a low voltage of plasma or an electron beam ("positonization") (Ref 30). The simplest way to achieve enhanced ionization is a direct-current glow discharge on the sample surface, which at the same time generates sputtered particles and ionizes them (GDMS method) (Ref 31).

A similar method that also uses a direct-current glow discharge is GDOES (Ref 32), where the excitation of optical emission is used as a means to analyze the sputtered species. As in GDMS, the main advantages are the high sputter rate due to the high current density and therefore the speed of thin-film depth profile analysis and high sensitivity. However, lack of spatial resolution and a generally lower depth resolution than with SIMS and SNMS are disadvantageous.

Table 1 compares the most important surface and thin-film analysis methods: AES, XPS, SIMS, and ISS, together with the generally non-UHV methods for thin-film analysis, RBS and GDOES.

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