High-Resolution Molecular Secondary Ion Mass Spectrometry for Absolute Quantification of Materials in Low-Dimensional Structures: Foundation, Perception and Challenges

Abstract

Material surfaces have unique structures and compositions that are strikingly different from the material itself. Atoms on the surface of a crystal are in an anisotropic condition markedly different from the situation experienced by the atoms in a bulk. The symmetry experienced by the individual surface atoms breaks down because of the fact that atoms at the surfaces are under-coordinated. The surface atoms have more energy compared to bulk atoms because the breaking of bonds occurs at the expense of energy. A surface as a natural interface between a material and its surroundings acts as a major counterpart in the interactions between them. Therefore, detailed characterizations of a surface are indispensable. Since the surface occupies a microscopic portion of a material, highly dedicated and specialized techniques are required to examine the surface region. Since the nanostructures hold the major fractions of surface atoms, physical and chemical properties of surfaces have important parts in describing the properties of nanomaterials. As the surface analysis provides a means to correlate the performance with surface composition and structure, it is used not only for the development of new surfaces with high functionality but also for helping materials exhibit their function properly. Amongst all existing analytical methods, “Secondary Ion Mass Spectrometry (SIMS)” provides an exclusive place in quantitative analyses of material surfaces and interfaces. Continuous development in the fundamental and technological aspects of SIMS has made this technique to be extremely sensitive and powerful. Secondary ion emission results predominantly from the sputtering of materials under ion bombardment and is a complex inelastic process of ion-surface interactions. Amongst existing models for explaining ionization probabilities for sputtered species, “electron tunnelling mechanism” has so far proven to have maximum acceptability for all materials. Intensity of secondary ions is directly coupled with “ionization efficiency”, which strongly depends on instantaneous local surface chemistry. This is the so-called “Matrix Effect”, that makes the SIMS technique challenging for quantitative analysis of materials despite the fact that the technique has the highest detection sensitivity (<ppb) and exceptional depth resolution (<1 nm). Thus, the quantification badly requires the compensation of the matrix effect. If an alkali atom (Li, Rb, K, Na, Cs, …etc. (referred to as ‘A’) comes to the proximity of the probing element (M) residing on the sample surface, this alkali ion can attach to a sputtered neutral atom (M⁰) and form a quasi-molecular (MA)⁺ ion. Such a molecular ion formation is strongly correlated to the atomic polarizability of the element M. Since M⁰ emission is completely independent of MA⁺ ion formation, the ‘matrix effect’ drastically decreases; a process similar to the ion formation in “secondary neutral mass spectrometry (SNMS)”. Although these (MA)⁺ molecular ions are the potential for materials analysis without “calibration standards”, they generally suffer from low yields. In that case, the (MA₂)⁺ molecular ions often meet the requirement as their yields have been found higher by orders of magnitude compared to (MA)⁺ ions, thus offering a better detection sensitivity. Molecular ions are often monitored in standard SIMS experiments, especially to detect those ion species, which have poor dynamic ranges or are affected by mass interferences. For example, whilst making SIMS analysis of GaAs samples using Cs⁺ primary ions, (CAs)⁺ molecular ions are routinely monitored instead of C⁻ ions in order to detect carbon as an impurity element, as carbon evolving from residual-gas species produces a high background in the mass spectra. Caesium is highly preferred for MCs⁺ or MCs₂⁺ molecular ions in SIMS because of the strongest reactivity and electropositive nature of caesium. The present chapter deals with a comprehensive description of the ion emission phenomena in sputtering, the ‘matrix effect’ in SIMS and its compensation, and the potential applications of MCs_n⁺-based molecular SIMS method for quantitative chemical analysis of materials. A special emphasis has been given to the quantification of low-dimensional materials, such as thin films, multilayers, superlattices, quantum structures, etc., using this advanced “MCs_n⁺ -SIMS” (n = 1, 2, ….) approach in all complexities.