Surface reactions microelectronic fabrication

Due to the historical importance of the initial stages of silicon oxidation to microelectronics fabrication, there has been a great deal of interest in the reaction of the water oxidant on the Si(100)-2 x 1 surface. A number of studies have shown that water adsorbs in a dissociated state consisting of OH(a) and H(a) species adsorbed on the Si surface dimer at room temperature [60-69]. More recent studies have closely investigated the mechanism of water oxidation. A series of density functional theory calculations (DFT) calculations by Konecny and Doren indicated that water first molecularly adsorbs through one of its lone pairs in a weakly bound precursor state, then transfers a proton to form OH(a) and H(a) species on the surface dimer [43]. The pathway to proton transfer is found to be unactivated with respect to the entrance channel, which suggests that OH(a) and H(a) are the dominant surface species at room temperature, in agreement with the previous experimental work [60-69]. 

Thin layers of catalyst can be deposited onto the surface of silicon microchannels by physical vapour deposition. Silicon is the preferred material, because the equipment for physical vapour deposition is available at microelectronics fabrication sites, which can also produce silicon microreactors. Physical vapour deposition such as cathodic sputtering, electron beam evaporation and pulsed laser deposition but also chemical vapour deposition create uniform metal surfaces with thicknesses in the nm range. Such coatings are rarely suitable as catalysts. However, a few exceptions such as hydrogen oxidation [145] and reactions in the very high temperature range do exist. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

Plasmas interact with bounding surfaces in a manner that is largely unknown. A later section of this chapter summarizes electrochemical aspects that involve chemical reactions coupled with charge transfer processes. Such phenomena are utilized extensively in the fabrication of microelectronic devices. 

For thin-film metallization, a thin metallic film is first deposited onto the surface of the substrate. The deposition can be accomplished by thermal evaporation, electronic-beam- or plasma-assisted sputtering, or ion-beam coating techniques, all standard microelectronic processes. A silicon wafer is the most commonly used substrate for thin-film sensor fabrication. Other substrate materials such as glass, quartz, and alumina can also be used. The adhesion of the thin metallic film to the substrate can be enhanced by using a selected metallic film. For example, the formation of gold film on silicon can be enhanced by first depositing a thin layer of chromium onto the substrate. This procedure is also a common practice in microelectronic processing. However, as noted above, this thin chromium layer may unintentionally participate in the electrode reaction. 

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