Silicon oxidation thin-oxide case

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

Equation (6.25) not only allows us to calculate the Hamaker constant, it also allows us to easily predict whether we can expect attraction or repulsion. An attractive van der Waals force corresponds to a positive sign of the Hamaker constant, repulsion corresponds to a negative Hamaker constant. Van der Waals forces between similar materials are always attractive. This can easily be deduced from the last equation for 1 = e2 and n = n2 the Hamaker constant is positive, which corresponds to an attractive force. If two different media interact across vacuum ( 3 = n3 = 1), or practically a gas, the van der Waals force is also attractive. Van der Waals forces between different materials across a condensed phase can be repulsive. Repulsive van der Waals forces occur, when medium 3 is more strongly attracted to medium 1 than medium 2. Repulsive forces were, for instance, measured for the interaction of silicon nitride with silicon oxide in diiodomethane [121]. Repulsive van der Waals forces can also occur across thin films on solid surfaces. In the case of thin liquid films on solid surfaces there is often a repulsive van der Waals force between the solid-liquid and the liquid-gas interface [122],... 

In another study, the so-called sohd-liquid-solid (SLS) technique was used to prepare a silicon oxide nanofiber surface [64]. The amorphous sihcon oxide nanofibers shown in Fig. 10b were made by heating a silicon substrate coated with a thin gold layer at 1100°C for 3 h in a lutrogen atmosphere in contrast to the previous example, no additional source of silicon materials was used in this case. As before, the nanofiber growth starts at the interface of the Au/Si alloy droplet and the Si substrate and is maintained by the diffusion of the Si atoms from the substrate to the interface [65]. The surface was then UV/ozone-treated to generate surface hydroxyl groups that were subsequently reacted with perfluorodecyltrichlorosilane. The chemically modified nanofiber surface exhibited a WCA of 152°. 

Microelectronics device fabrication entails lithography in order to transfer a layout pattern onto the surface of a substrate, such as a silicon wafer. In this technique, the surface of a Si wafer is oxidized, typically at 800-1200 °C in steam or dry oxygen, to form a thin layer of silicon oxide as shown in the upper left portion of Figure 1.23. Then, a thin film of a polymer known as a photoresist is deposited on the oxide layer. This material is sensitive to ultraviolet radiation but resistant to chemical attack by etchants and, in some cases, to electrons and x-rays. A photomask, which comprises a glass... 

Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

For the case of Si02 etching, HF, (HF)2 and HF2- are assumed to be the active species [Vel, Jul]. If HC1 is added to the solution the concentration of the HF2-ion becomes negligible, which leaves HF and its polymers to be the active species [Ve3]. Because for high current densities the electrochemical dissolution of silicon occurs via a thin anodic oxide layer it can be concluded that, at least for this regime, the same species are active. This is supported by the observation that F- is... 

Usually the nanotube arrays have been made from a titanium thick film or foil, in which case the resulting nanotubes rest upon an underlying Ti substrate as separated by a barrier layer. The nanotube arrays have also been fabricated from a titanium thin film sputtered onto a variety of substrates, such as silicon and fluorine doped tin oxide (FTO) coated conductive glass. This extends the possibility for preparing technical catalysts by deposing a thin Ti layer over a substrate (a foam, for example) and then inducing the formation of the nanostructured titania film by anodic oxidation. ... 

In fabrication of the catalysts by laser electrodispersion, thermally oxidized silicon wafers with a thickness of Si02 oxide layer of 1 pm were used as a substrate. A substrate with so thick an oxide layer can be regarded as an insulator. In some cases, wafers of crystalline (1 0 0) Si were used, which had on their surface only a thin (l-2nm) layer of a natural oxide. This layer is tunnel-transparent for electrons, and, therefore, charge exchange between supported nanoparticles and silicon is possible. 

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