Passivation silica films

SiC heating elements, depending upon the grade, are routinely used at temperatures up to about 1500 °C in air, and even up to about 1650 °C for short periods. The unglazed variety rely on a thin native passivating silica film for their protection against oxidation. Service life therefore depends strongly on the atmosphere in which they operate, which affects the stability of the film, and on temperature which, of course, affects reaction rates. 

Nitric acid is also withstood by high-silicon iron. The concentrated acid is believed to reinforce the silica film by the formation of a passive iron oxide... 

The AO reaction efficiency of the Kapton H reference sample was used to calculate the Kapton equivalent fluence and erosion yields of each sample exposure. For various exposures, the step heights (or etch depths) of POSS-PI films were plotted as a function of the step height of the Kapton H film [18, 26]. The derivative functions indicated that the 3.5 and 7.0 wt % SigOn POSS polyimide films reached erosions rates of 3.7 and 0.98%, respectively, of the erosion rate for Kapton H after 395,000 beam pulses (8.47 x 1020 atoms cm-2) [9, 10]. 8.75 wt% Si80 MC-POSS-PI samples had an erosion rate that was 0.3 percent of the erosion rate for Kapton H , and 1/3 of 7.0 wt % POSS-PI at a fluence of 8.5xl020 atoms cm 2. These results support the formation of a passivating silica layer that is a result of the nano-dispersed POSS moieties reacting with AO. 

As silica is not attacked by any acid other than hydrofluoric it might be expected to act as an effective barrier to attack by any other acid solutions, but in fact, while the high-silicon iron is resistant to attack by most acids, it is corroded relatively severely by hydrochloric, hydrobromic and sulphurous acids. The aggressive character of the two halogen acids may be ascribed to the readiness with which their relatively small anions can penetrate a passive film. 

Thin films (qv) of vitreous silica have been used extensively in semiconductor technology. These serve as insulating layers between conductor stripes and a semiconductor surface in integrated circuits, and as a surface passivation material in planar diodes, transistors, and injection lasers. They are also used for diffusion masking, as etchant surfaces, and for encapsulation and protection of completed electronic devices. Thin films serve an important function in multilayer conductor insulation technology where a variety of conducting paths are deposited in overlay patterns and insulating layers are required for separation. 

The 64k, 80 pm x 80pm sized tilting mirrors are built on the top of a CMOS-based control ASIC. In order to reduce the topography of the underlying metallization/passivation structures, a 2.5pm-thick PECVD oxide film is first deposited on the ASIC. An ILD oxide CMP step based on Klebosol 30N50 colloidal silica slurry is used for planarization. In order to connect the ASIC with the deflection electrodes above (see Fig. 14.10), vias have to be etched into the planarized dielectric film. Then, a copper metal stack including a TaN barrier has to be deposited and a two-step Cu damascene CMP process has to be performed. As this process is equivalent to Cu damascene in microelectronics fabrication, standard Cu CMP slurries can be used. 

Thermal oxidation is an old and common method of forming a primary passivating film of Si02 on silicon. The metal is heated in dry oxygen, in wet oxygen, or in steam. A silica layer grows inwardly from the surface by a thermal oxidation mechanism. The silicon wafer is heated to 600-1200°C to achieve 1 pm thick films in about an hour. 

Yu et al. (2003) synthesized hybrid thin films containing nanosized inorganic domains from poly(acrylic) and monodispersed colloidal silica with bound 3-(trimethoxysilyl)propyl methacrylate (MSMA) for potential applications as passive films in optical devices. The latter was polymerized with acrylic monomer to form a precursor solution, which was spin-coated and cured to form hybrid films. The silica content in the hybrid thin films was varied from 0 to 50 wt% (Figure 1.302). The coverage area of silica particles by the MSMA decreased with increasing silica content and resulted in the aggregation of silica particles in the hybrid films, and the silica domains were 20-35 nm in size. 

The reaction of Eq. 16 is the chemistry that occurs during the sol-gel process, used to prepare colloids, films, or monoliths of porous silica from solution precursors (Brinker and Scherer 1990). This reaction explains why elemental silicon does not corrode appreciably at pH values <7 the oxide is insoluble at low pH and so provides a protective, passivating layer. The same is not tme in highly basic solutions here the solubility of silicon oxide drives sdicon dissolution by Eqs. 9 and 10. 

Shen, Y.-L., Suresh, S., He, M. Y., Bagchi, A., Kienzle, O., Riihle, M. and Evans, A. G. (1998), Stress evolution in passive aided thin films of Cu on silica substrates. Journal of Materials Research 13, 1928-1937. 

Popali M., Kappel J., PUz M., Schulz J., Feyder G. A new inorganic-organic polymers for the passivation of thin film capacitors. J. Sol-Gel Sci.. Technol. 1994 2 157-160 Pope E.J.A. Gel encapsulated microorganismus Saccharomyces crevisiae-silicA gel biocomosites. J. Sol-Gel Sci. Technol. 1995a 4 225-229... 

Increased Si content at the surface of steel leads to a beneficial effect improving the anti-oxidation behaviour. The surface protection is achieved by several mechanisms that take place at the same time, including the formation of silicon oxide films, increased Cr diffusion from the bulk to the surface, formation of phases such as y-Fe, Cr203, Si02, Si FCy, passivation of the surface by oxidation and surface diffusion. As mentioned in references [1-4], silicon seems to retard breakaway in the presence of water vapour in the environment and may facilitate Cr rediffusion from the bulk which would help repassivation observed after breakaway. As a consequence the level of the Cr reservoir possibly may be kept lower than for Si-free steels. At least, a continuous silica layer is not the reason for the improved oxidation behaviour. The positive effect of silicon seems to stabilise at values above 0.5% Si. A possible reason for the influence of silicon seems to be that silicon enhances the diffusion of Cr in the metal matrix. 

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