Chemical profiling with depth

These two techniques may be applied in a gentle manner using a near static ion beam to produce little change in the surface and also in a mode in which chemical profiling with depth is possible. ISS can detect all of the elements heavier than helium in the periodic table. The sensitivity variation across the periodic table is probably less than one order of magnitude. 

The application of XPS to microelectronic materials typically focuses on two areas. First, as a surface investagatlve technique, XPS can be used to establish the chemical Interactions between two materials or between a material and the ambient atmosphere. Second, in conjunction with depth profiling, the changes in chemical composition with depth can be followed. 

Many chemicals escape quite rapidly from the aqueous phase, with half-lives on the order of minutes to hours, whereas others may remain for such long periods that other chemical and physical mechanisms govern their ultimate fates. The factors that affect the rate of volatilization of a chemical from aqueous solution (or its uptake from the gas phase by water) are complex, including the concentration of the compound and its profile with depth, Henry s law constant and diffusion coefficient for the compound, mass transport coefficients for the chemical both in air and water, wind speed, turbulence of the water body, the presence of modifying substrates such as adsorbents in the solution, and the temperature of the water. Many of these data can be estimated by laboratory measurements (Thomas, 1990), but extrapolation to a natural situation is often less than fully successful. Equations for computing rate constants for volatilization have been developed by Liss and Slater (1974) and Mackay and Leinonen (1975), whereas the effects of natural and forced aeration on the volatilization of chemicals from ponds, lakes, and streams have been discussed by Thibodeaux (1979). 

Langmuir-Blodgett films offer a natural means to achieve controlled chemical variations with depth of the film. Molecules of different types may be deposited sequentially, thus making a profile of known chemical composition on a length scale of the thickness of a molecular layer. To make a chemical profile on a smaller scale of depth, one can make substitutions within the molecule of a single layer. We utilize both of these approaches in the present work. 

In summary, the forte of SNMS is the measurement of accurate compositional depth profiles with high depth resolution through chemically complex thin-film structures. Current examples of systems amenable to SNMS are complex III-IV laser diode structures, semiconductor device metallizations, and magnetic read-write devices, as well as storage media. 

Sect. 2.1.6.6). The extent of interaction and interdiffusion can be established by AES with depth profiling, although chemical information is normally absent, but the development of the interface is usually studied by continuous recording of Auger spectra during film deposition. 

SIMS has superb surface sensitivity since most of the secondary ions originate within a few nanometers of the surface and since high detection efficiency enables as little as 10 " of a monolayer to be detected for most elements. Because of its very high surface sensitivity, SIMS can be used to obtain depth profiles with exceptionally high depth resolution (<5 nm). Since the beam of primary ions can be focused to a small spot, SIMS can be used to characterize the surface of a sample with lateral resolution that is on the order of micrometers. Elements with low atomic numbers, such as H and He, can be detected, isotope analysis can be conducted, and images showing the distribution of chemical species across... 

Imaging MS is and will become increasingly critical for many aspects of materials science. One example is in the semiconductor industry, where the ability to provide spatial and chemical information on the length scales of current integrated circuit fabrication (50 nm or better) with depth profiling to provide layer-by-layer maps of the fabricated layers is critical for the continued advancement of the computer industry. Maps of any heterogeneous surface are important in other areas of materials science. For example, using various laser desorption techniques, information about the molecules found in specific inclusions in meteorites or defects in reactive surfaces can be obtained. 

If groundwater is being used, it is important to know where wells and tubewells are located, because there may be localized areas where chemicals in the groundwater are of concern. Also, groundwater may be stratified, with water at different depths having different chemical profiles. In such cases, it is important to understand the potential variation in chemicals caused by the stratification. 

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