Sputtering of ions

The sputtering of neutral species, which are not detected by the mass spectrometer, is probably the major process occurring at the surface. Similarly, the simple sputtering of ions that are naturally present in the sample, undoubtedly, must constitute an important source of ions in FAB mass spectra. Consequently, sample preparation methods such as spiking with various additives which lead to an increase in the concentration of ionic material in the sample, can lead to an enhanced sensitivity (81AC25). Despite considerable speculation on the subject, the mechanism of ionization in the sputtering of nonionic compounds is uncertain. A recent review paper states that Good FAB mass spectra may be obtained with ease from compounds... 

Barish E L, Vitkavage D J and Mayer T M 1985 Sputtering of chlorinated silicon surfaces studied by secondary ion mass spectrometry and ion scattering spectroscopy J. AppL Phys. 57 1336-42... 

There are, however, continuing difficulties for catalytic appHcations of ion implantation. One is possible corrosion of the substrate of the implanted or sputtered active layer this is the main factor in the long-term stabiHty of the catalyst. Ion implanted metals may be buried below the surface layer of the substrate and hence show no activity. Preparation of catalysts with high surface areas present problems for ion beam techniques. Although it is apparent that ion implantation is not suitable for the production of catalysts in a porous form, the results indicate its strong potential for the production and study of catalytic surfaces that caimot be fabricated by more conventional methods. 

Experimental values for tire sputtering efficiency tend to show lower values of a for elements, such as aluminium and mngsten which form stable oxides, compared with the metals such as gold and platinum which do not under normal experimental conditions. This is probably due to the presence of a surface oxide, since industrial sources of argon, which are used as a source of ions for example, usually contain at least 1 ppm of oxygen, which is more than enough to oxidize aluminium and tungsten. 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

The essentials of SNMS are illustrated in Figure 1. The surface of the solid sample is sputtered by energetic ion bombardment. Generally, at eneigfes above a few hundred eV, several particles are ejected from the surface for each incident particle. A very small fraction of the particles are sputtered as ions, the so-called secondary ions... 

Crater Bottom Roughening. Depth resolution is also limited by roughening of the crater bottom under the action of ion bombardment. On polycrystalline samples this can be because of different sputter yields of different crystal orientations, because the sputter yields of single crystals can vary by a factor of two depending on their orientation. Because of this type of roughening, depth resolution deteriorates with increasing sputter depth. 

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. 

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