Sacrificial oxide

Figure 19. Polypyridylruthenium(II) complexes that upon irradiation producephenoxyl radicals in the presence of a sacrificial oxidant. Compound A (184), Compound B (186), and Compound C (186, 187).

The galvanized nail (bottom) is protected from rusting by the sacrificial oxidation of zinc. 

Electron Transfer Between Co-adsorbed Species. So far, the most notable examples for photo-induced electron transfer on clay surfaces occur between co-adsorbed species as illustrated by the sacrificial oxidation of water. In water the system is the following (31) ... 

A material with nitrogen-coordinated Ru was obtained from a silica-linked 2-(phenylazo)pyridine ligand. Results for cyclobutanol oxidation with 02 and the sacrificial oxidant isobutyraldehyde indicate that one- and two-electron oxidations occur simultaneously. The stability of the catalyst is not always guaranteed, probably because acids may be formed in oxidations of alcohols (284). Leaching problems are also encountered with a polymer-bound Ru Schiff base complex, used in oxidation of benzyl alcohol (285). 

For metal complexes, which promote only the initial SET process 31 >31A, but do not couple efficiently subsequently, stabilization of the final radical often occurs by hydrogen transfer from the reaction medium. In such reactions the catalyst can be regenerated similarly. Using this principle, many transition metal complexes become efficient catalysts for radical reactions. Similar oxidative processes using a catalytic SET oxidant, such as Fe(III) or Mn(III) in combination with a cheap sacrificial oxidant, are also attractive, but less developed. Electrochemical catalyst regeneration may serve well for this purpose. 

Ohmori, T., H. Takahashi, H. Mametsuka and E. Suzuki (2000). Photocatalytic oxygen evolution on 0 -I,e2O films using Fe3+ ion as a sacrificial oxidizing agent. Physical Chemistry Chemical... 

The vibrating ring/disk structure as well as the drive mechanism consists of 1 l- rm-thick poly-Si, which has been structured by deep RIE and released from the sacrificial oxide layer underneath by HE vapor phase etching. For the deposition of the thick poly-Si, a modified epitaxy deposition process (EPI poly) has been used [24]. However, as can be seen in Fig. 14.6, the deposition process leads to a rough poly-Si surface with Ra 100nm. For the removal of underlying topography, the surface has to be planarized by CMP in order to... 

FIGURE 14.7 The roughness of the EPI-poly layer can be planarized by using CMP. After removal of the sacrificial oxide layer by vapor phase etching, the sensor structures are released. 

Centers re-examined the published information about the performance of antimony trioxide, and rejected both the sacrificial oxidation theory and the oxide interaction concept. He concluded that antimony trioxide and certain other oxides and sulphides improve performance by providing a soft component which enables molybdenum disulphide more readily to acquire the optimal basal plane orientation for low friction and wear. However, he used compacts, and it is difficult to understand how his conclusion could apply to burnished films or well run-in films which already approach perfect crystallite orientation. His rejection of the sacrificial oxidation theory is based on a report by Gardos and McConnell , which does not include all of... 

Jamison found that low concentrations of intercalated copper or silver in niobium disulphides and diselenides promoted good lubricating performance. Higher concentrations increased the resistance to inter-lamellar shear, and therefore the friction, but improved high temperature performance due to the reduced intracrystalline shear and some sacrificial oxidation of the intercalated metals. 

By these same routes, olefins, vinyl ethers, and a,y5-unsaturated enones could be photoelectrochemically hydrogenated on Ti02 in the presence of ethanol as sacrificial oxidant [166-168]. With 2-methyl-2-pentene, hydrogenation occurred in 63% yield, without side products (Eq. 28). 

Neohexene is by far the more expensive of the two starting materials, and so the process shown in Scheme 4.45 is unsatisfactory, since it consumes twice as much of this reagent as is desirable. One way of overcoming this problem is to add a sacrificial oxidant that is less expensive than neohexene. For example, /-butyl chloride, under the influence of aluminium chloride, loses its chlorine to produce the /-butyl cation, which can abstract the hydride ion from p-cymene and thus save one molar equivalent of neohexene. 

Sacrificial oxide. The sacrificial oxide layer is deposited by low-pressure chemical vapor deposition (LPCVD) or plasma-enhanced chemical vapor deposition (PECVD). The layer is made of either undoped glass or phosphorous-silicon glass (PSG)-doped resulting in a final thickness of 1.5-2.5 pm. Low defect density, good etch rate control, uniformity and stress control have been accomplished for updoped oxide with a Novellus Concept One tool. 

Anchor. The next step is to open contact holes through the sacrificial oxide to the ground-plane polysilicon layer and/or the n+ runners. These contacts become the electrical and mechanical interconnections or anchors for the mechanical polysihcon structure (Fig. 5.2.6). A wet-dry etch combination provides a... 

Fig. 5.2.6 Wafer cross section after deposition of sacrificial oxide and contact via s...

Microstructure stabilization. Before complete removal of the sacrificial oxide, small cavities are etched around and under the mechanical polysilicon and down to the ground-plane polysilicon below the sacrificial oxide layer. These cavities are then backfilled with photoresist, resulting in pillars that support the polysilicon. In a subsequent masking operation, strips of photoresist are placed across the micromachined elements. This results in a web of photoresist material that holds the polysilicon elements in place after complete removal of the sacrificial oxide layer. Subsequent etching of the sacrificial oxide with a buffered... 

A large body of research exists on the deposition and processing of structural polysilicon [8-10]. The key parameter for films made of structural polysilicon is residual stress and therefore the flatness of the films. The deposition of polysilicon from silane (SiH4) gas in a horizontal furnace has been extensively used. The addition of phosphorus as a dopant for the polysilicon has been used to control resistivity and stress. Including phosphorus dopant in the sacrificial oxide layer under the structural polysilicon layer and/or as ions implanted after deposition of the polysilicon is the most common methods used. 

The sacrificial oxide layer is initially formed on top of the silicon substrate by thermal or plasma processing (Fig. 5.3.1a). If insulation of the functional layer from the silicon substrate is required, the sacrificial layer may be preceded by deposition of an insulating film, for example, silicon nitride. The thickness of the sacrificial oxide layer determines the height of the released cantilever above the silicon substrate. It is typically a few micrometers thick, depending on the specific requirements and application. 

One approach suitable for high-volume production is to stabilize the movable beam structure with pedestals of photoresist [19]. Rectangular openings are created in the sacrificial oxide layer and are isotropically underetched to create circular holes. These holes are filled with photoresist that is patterned to allow the etching liquid to enter the entire area under the movable beam structure. Subsequently the pedestals are removed in an oxygen plasma. Temperature control during this step is essential to avoid capillary forces due to low viscosity in the resist. 

Typically, the vapor phase of hydrofluoric acid is used instead of HF liquid. HF vapor is known to strip native oxides during cleaning of silicon wafers [21] and was suggested for sacrificial oxide etching [22]. However, even using the vapor phase can be critical in terms of stiction, because a byproduct of the chemical reaction is water ... 

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