Ions and electrons

Figure A3.10.8 Depiction of etching on a Si(lOO) surface, (a) A surface exposed to Br2 as well as electrons, ions and photons. Following etching, the surface either becomes highly anisotropic with deep etch pits (b), or more regular (c), depending on the relative desorption energies for different surface sites [28].

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

A plasma of electrons, ions, and neutrals produced in gas flowing through concentric tubes is maintained and heated to 5000 to 8000 K by inductive coupling to a high (radio) frequency... 

An approximate equilibrium is set up in the plasma, with the electrons, ions, and atoms having velocity distributions similar to those of a gas that has been heated to temperatures of 7,000 to 10,000°C. Since the plasma is ignited toward the end of the concentric tubes from which argon gas is issuing, the plasma appears as a pale-blue-to-lilac flame coming out of the end of the tube, which is why the system is referred to as a torch, as in a welding torch. 

K. H. Brown, Proc. Soc. Photo-Opt. Instr. Eng. 2438, 33 (1995). A good source of information on the new exposure technologies can be found in the Proceedings of the International Conference on Electron, Ion and Photon Beam Technology and Nanofabrication, pubhshed annuaUy in Issue 6 of the Journal of Vacuum Science and Technology B. 

The physical techniques used in IC analysis all employ some type of primary analytical beam to irradiate a substrate and interact with the substrate s physical or chemical properties, producing a secondary effect that is measured and interpreted. The three most commonly used analytical beams are electron, ion, and photon x-ray beams. Each combination of primary irradiation and secondary effect defines a specific analytical technique. The IC substrate properties that are most frequendy analyzed include size, elemental and compositional identification, topology, morphology, lateral and depth resolution of surface features or implantation profiles, and film thickness and conformance. A summary of commonly used analytical techniques for VLSI technology can be found in Table 3. 

Consider electrons of mass m and velocity v, and atoms of mass M and velocity V we have mjM 1. The distribution function for the electrons will be denoted by /(v,<) (we assume no space dependence) that for the atoms, F( V), assumed Maxwellian as usual, in the collision integral, unprimed quantities refer to values before collision, while primed quantities are the values after collision. In general, we would have three Boltzmann equations (one each for the electrons, ions, and neutrals), each containing three collision terms (one for self-collisions, and one each for collisions with the other two species). We are interested only in the equation for the electron distribution function by the assumption of slight ionization, we neglect the electron-electron... 

For each particle j in the discharge volume (electrons, ions, and radicals) the density balance can be written as... 

Raman microscopy has been used for analysis of very small samples or small heterogeneities in larger samples. Recent developments and applications of this technique have been reviewed by Turrell and Corset (1996), including a discussion of the coupling of Raman microscopy with electron, ion and x-ray microscopies, and these authors give a description of a number of prototype instruments with this facility. 

Figure 5.10 Electron-ion and electron-molecule reactions. (Adapted with permission from Fraser, 2002)...

Characterization techniques become surface sensitive if the particles or radiation to be detected come from the outer layers of the sample. Low energy electrons, ions and neutrals can only travel over distances between one and ten interatomic spacings in the solid state, implying that such particles coming off a catalyst reveal surface-specific information. The inherent disadvantage of the small mean free path is that measurements need to be carried out in vacuum, which conflicts with the wish to investigate catalysts under reaction conditions. 

R. E. Honig and J. R. Woolston. Laser-Induced Emission of Electrons, Ions, and Neutral Atoms from Solid Surfaces. Appl. Phys. Lett., 2(1963) 138-139. 

In this chapter, we introduce some of the most common spectroscopies and methods available for the characterization of heterogeneous catalysts [3-13], These techniques can be broadly grouped according to the nature of the probes employed for excitation, including photons, electrons, ions, and neutrons, or, alternatively, according to the type of information they provide. Here we have chosen to group the main catalyst characterization techniques by using a combination of both criteria into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characterization of real catalysts, and toward the end make brief reference to studies with model systems. Only the basics of each technique and a few examples of applications to catalyst characterization are provided, but more specialized references are included for those interested in a more in-depth discussion. 

The r + d algorithm produces integers for odd-electron ions and molecules, but non-integers for even-electron ions that have to be rounded to the next lower integer, thereby allowing to distinguish even- from odd-electron species. 

Plasmas can be classified as either thermal or non-thermal. " Thermal plasma is a highly energetic state of matter, characterized by thermal equilibrium between the three components of the plasma electrons, ions, and neutrals. However, it requires high-energy input to achieve high temperatures. Researchers at MIT used a non-catalytic thermal plasma technology to produce H2 from liquid hydrocarbons. Non-catalytic processes are beyond the scope of this work, and will not be discussed. 

During the plasma surface reaction, the plasma and the solid are in physical contact, but electrically isolated. Surfaces in contact with the plasma are bombarded by free radicals, electrons, ions, and photons, as generated by the reactions listed above. The energy transferred to the solid is dissipated within the solid by a variety of chemical and physical processes, as illustrated in Figure 7.95. These processes can change surface wettability (cf. Sections 1.4.6 and 2.2.2.3), alter molecular weight of polymer surfaces or create reactive sites on polymers. These effects are summarized in Table 7.21. 

The Madelung energy, eqn (6.61), can also be expressed as a pairwise sum over coulomb interactions between the point ions, plus a q - 0 contribution arising from the electron-ion and electron-electron interactions. Grouping this together with the band-structure contribution we have... 

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