Secondary Ion yields

Like the chemical composition of the primary beam, the chemical nature of the sample affects the ion yield of elements contained within it. For example, the presence of a large amount of an electronegative element like oxygen in a sample enhances the positive secondary ion yields of impurities contained in it compared to a similar sample containing less oxygen. 

Several ion sources are particularly suited for SSIMS. The first produces positive noble gas ions (usually argon) either by electron impact (El) or in a plasma created by a discharge (see Fig. 3.18 in Sect. 3.2.2.). The ions are then extracted from the source region, accelerated to the chosen energy, and focused in an electrostatic ion-optical column. More recently it has been shown that the use of primary polyatomic ions, e. g. SF5, created in FI sources, can enhance the molecular secondary ion yield by several magnitudes [3.4, 3.5]. 

Cesium ions are also sometimes used to enhance the secondary-ion yield of negative elemental ions and that of some polymer fragments [3.6]. They are produced by surface ionization with an extraction technique similar to that of FI sources. 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

The advantages of SIMS are its high sensitivity (detection limit of ppms for certain elements), its ability to detect hydrogen and the emission of molecular fragments that often bear tractable relationships with the parent structure on the surface. Disadvantages are that secondary ion formation is a poorly understood phenomenon and that quantification is often difficult. A major drawback is the matrix effect secondary ion yields of one element can vary tremendously with chemical environment. This matrix effect and the elemental sensitivity variation of five orders of magmtude across the periodic table make quantitative interpretation of SIMS spectra oftechmcal catalysts extremely difficult. 

Coverage and Secondary Ion Yield Relationship for Ni(100)/CO. We showed above the enormous variation in yields that occurred on going from adsorption to oxide nucleation. In the case of Ni(100)/CO, one can perform more subtle bonding changes by changing the CO coverage. Below 0 O.4 ML, no ordered LEED structure is formed, and vibrational spectroscopy (HRELS) indicates... 

Secondary ion yields vary by more than six orders of magnitude over the periodic table. 

Secondary Ion Yields. The most successful calculations of secondary in yields are based on the local thermal equilibrium (LTE) model of Andersen and Hinthorne (1973), which assumes that a plasma in thermodynamic equilibrium is generated locally in the solid by ion bombardment. Assuming equilibrium, the law of mass action can be applied to find the ratio of ions, neutrals and electrons, and the Saha-Eggert equation is derived ... 

The exact mass determination of emitted secondary ions yields informations about the molecular structure of the thick film. In the low mass area the fragmentation of the quasimolecular cation m/e = 163, i.e. (EtO)3Si+ derived from the alkoxysilane monomer gives rise to the positive ions m/e = 135, 119, 107, 91, 79, 63. The resulting fragmentation pattern can be described as follows ... 

The secondary ion yield depends both on the energy and the nature of the primary ions. It has been demonstrated that the use of clusters instead of monoatomic ions improves the secondary yield. Classical primary ions, like Ga+ and Cs+, are increasingly replaced by Au3+, Bi3+ or C60+ which has permitted great progress in organic ToF-SIMS studies [Kollmer 2004, Touboul et al. 2004, Winograd 2005]. 

K. Keune and J. J. Boon, Enhancement of the static SIMS secondary ion yields of lipid moieties by ultrathin gold coating of aged oil paint surfaces, Surface and Interface Analysis, 36, 1620 1628 (2004b). 

The ionization probabilities It vary over some five decades across the elements in the periodic table. In addition, they vary also with the chemical environment of the element. This effect, usually referred to as the matrix effect, makes quantitation of SIMS spectra extremely difficult. As illustrated in Table 4.1, positive secondary ion yields from metal oxides are typically two orders of magnitude higher than those of the corresponding metals. A similar increase in yields from metals is observed after adsorption of gases such as oxygen or carbon monoxide. 

Table 4.1 Ionization potential, /, work function, (p, sputter yield, Y, and secondary ion yield of positive ions, R Y, of selected elements and their oxides.

Expression (4-2) accounts qualitatively for the observed variations of secondary ion yields with ionization potential. It also describes correctly that the yields of positive secondary ions from metals increase when molecules such as CO or oxygen, which increase the work function, cover the surface. Although the model elegantly predicts a number of trends correctly, it is not detailed enough to be a basis for quantitative analysis of technical samples. 

One of the most significant uses of LMI sources in connection with SIMS of organic compounds may be as probes in performing measurements of secondary particle yields. Such measurements are important for understanding the processes of secondary ion emission from solid and liquid organic samples. Total particle yields reflect directly the dynamical aspects of emission processes variations in primary beam energy, incident flux density, and primary particle mass, for example, are all manifested in changes in total particle yields. The ratio of secondary ion yield to total particle yield and the ratio of secondary ion yields from two different species can be sensitive, quantitative monitors of the chemistry and kinetics, respectively, of ionization processes. 

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