Radical Chemistry on Silicon Surfaces

Silicon is the most technologically important material utilized today owing to its imique role in the fabrication of semiconductor devices and microprocessor chips. The understanding and control of silicon surfaces is of great importance in the production of silicon-based electronic devices, since the fraction of atoms 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter- 

It is worth mentioning that the photooxidation of porous silicon behaves differently [49]. Indeed, ETIR spectra show that there is a tremendous increase in vsi o, without a correspondingly large loss of vsi H peak intensity. The decrease of the vsi H band is offset by an increase in the vosi—h band, resulting in no net loss of hydride species on the surface during the course of the photooxidation reaction. These data apparently suggest that oxidation does not result in the removal of H atoms, implying that Si—Si bonds are attacked directly. 

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Dissociative excitation of molecules by electrons is a key process in many industrially important plasmas because it is the mechanism that provides the activated radicals that initiate the surface chemistries of interest. For example, many of the gases used in the etching of silicon do not display any reactivity in the absence of plasma. The construction of detailed models of these plasmas relies on a reliable data base of cross sections. Unfortunately, electron-impact dissociation cross sections are extremely difficult to measure and there are only a handful of cases where good data exist. Chlorine gas, which is widely used in the plasma etching of semiconductors, is one such example. Cross sections for ionization and dissociative electron attachment were measured during the 1970s and there has been one experimental study of electron impact dissociation. Cross sections for other dominant electron collision processes have been derived from Boltzmann analysis and early swarm measurements. ... 

The most important factor is that, unlike the silicon to carbon bond, the titanium to carbon bond is very unstable and cannot be used to permanently attach organic groups for surface treatment applications. The organo-titanates are thus generally based on tetra-substimted titanium, where all the substituents are linked by titanimn-oxygen-carbon bonds, and their chemistry is dominated by the hydrolytic sensitivity of these. Alkoxy radicals are rapidly hydrolysed, with the rate in water decreasing as the chain length increases, due to reduced solubility. Acyloxy derivatives are also fairly readily hydrolysed. 

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