Oxide layers

The behavior in the presence of air is quite different. For example, Tingle [22] found that the friction between copper surfaces decreased from a fi value of 6.8 to one of 0.80 as progressive exposure of the clean surfaces led to increasingly thick oxide layers. As noted by Whitehead [23], several behavior patterns... 

Diamond behaves somewhat differently in that n is low in air, about 0.1. It is dependent, however, on which crystal face is involved, and rises severalfold in vacuum (after heating) [1,2,25]. The behavior of sapphire is similar [24]. Diamond surfaces, incidentally, can have an oxide layer. Naturally occurring ones may be hydrophilic or hydrophobic, depending on whether they are found in formations exposed to air and water. The relation between surface wettability and friction seems not to have been studied. 

Another approach is to use the LB film as a template to limit the size of growing colloids such as the Q-state semiconductors that have applications in nonlinear optical devices. Furlong and co-workers have successfully synthesized CdSe [186] and CdS [187] nanoparticles (<5 nm in radius) in Cd arachidate LB films. Finally, as a low-temperature ceramic process, LB films can be converted to oxide layers by UV and ozone treatment examples are polydimethylsiloxane films to make SiO [188] and Cd arachidate to make CdOjt [189]. 

For some materials, the most notable being silicon, heating alone sufiBces to clean the surface. Commercial Si wafers are produced with a thin layer of silicon dioxide covering the surface. This native oxide is inert to reaction with the atmosphere, and therefore keeps the underlying Si material clean. The native oxide layer is desorbed, i.e. removed into the gas phase, by heating the wafer in UHV to a temperature above approximately 1100 °C. This procedure directly fonus a clean, well ordered Si surface. 

This is demonstrated by the XPS spectra in figure B 1.25.5(a) which show the Si 2p spectra of a silicon crystal with a thin (native) oxide layer, measured under take-off angles of 0° and 60° [12]. When the take-off angle is... 

Monolayers can be transferred onto many different substrates. Most LB depositions have been perfonned onto hydrophilic substrates, where monolayers are transferred when pulling tire substrate out from tire subphase. Transparent hydrophilic substrates such as glass [18,19] or quartz [20] allow spectra to be recorded in transmission mode. Examples of otlier hydrophilic substrates are aluminium [21, 22, 23 and 24], cliromium [9, 25] or tin [26], all in their oxidized state. The substrate most often used today is silicon wafer. Gold does not establish an oxide layer and is tlierefore used chiefly for reflection studies. Also used are silver [27], gallium arsenide [27, 28] or cadmium telluride wafer [28] following special treatment. 

Finally, in 1985, the results of an extensive investigation in which adsorjDtion took place onto an aluminium oxide layer fonned on a film of aluminium deposited in vacuo onto a silicon wafer was published by Allara and Nuzzo 1127, 1281. Various carboxylic acids were dissolved in high-purity hexadecane and allowed to adsorb from this solution onto the prepared aluminium oxide surface. It was found that for chains with more than 12 carbon atoms, chains are nearly in a vertical orientation and are tightly packed. For shorter chains, however, no stable monolayers were found. The kinetic processes involved in layer fonnation can take up to several days. 

The diagram gives regions of existence, i.e. for a particular combination of pH and redox potential it can be predicted whether it is thennodynamically favourable for iron to be inert (stable) (region A), to actively dissolve (region B) or to fonn an oxide layer (region C). 

If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

Lohrengel M M 1993 Thin anodio oxide layers on aluminum and other valve metals high field regime Mater. Sc/. Eng. R... 

Strong oxidising acids, for example hot concentrated sulphuric acid and nitric acid, attack finely divided boron to give boric acid H3CO3. The metallic elements behave much as expected, the metal being oxidised whilst the acid is reduced. Bulk aluminium, however, is rendered passive by both dilute and concentrated nitric acid and no action occurs the passivity is due to the formation of an impervious oxide layer. Finely divided aluminium does dissolve slowly when heated in concentrated nitric acid. 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

The result is the formation of a dense and uniform metal oxide layer in which the deposition rate is controlled by the diffusion rate of ionic species and the concentration of electronic charge carriers. This procedure is used to fabricate the thin layer of soHd electrolyte (yttria-stabilized 2irconia) and the interconnection (Mg-doped lanthanum chromite). 

Sihcon dioxide layers can be formed using any of several techniques, including thermal oxidation of siUcon, wet anodization, CVD, or plasma oxidation. Thermal oxidation is the dominant procedure used in IC fabrication. The oxidation process selected depends on the thickness and properties of the desired oxide layer. Thin oxides are formed in dry oxygen, whereas thick (>0.5 jim) oxide layers are formed in a water vapor atmosphere (13). 

Step 5. The remaining Si N, pad oxide, and resist are stripped away and a thin, precisely controlled Si02 gate oxide layer is thermally grown. 

Fig. 9. Fabrication sequence for an oxide-isolated -weU CMOS process, where is boron and X is arsenic. See text, (a) Formation of blanket pod oxide and Si N layer resist patterning (mask 1) ion implantation of channel stoppers (chanstop) (steps 1—3). (b) Growth of isolation field oxide removal of resist, Si N, and pod oxide growth of thin (<200 nm) Si02 gate oxide layer (steps 4—6). (c) Deposition and patterning of polysihcon gate formation of -source and drain (steps 7,8). (d) Deposition of thick Si02 blanket layer etch to form contact windows down to source, drain, and gate (step 9). (e) Metallisation of contact windows with W blanket deposition of Al patterning of metal (steps 10,11). The deposition of intermetal dielectric or final...

Probably the most important powder property governing the formation of atomic bonds is the surface condition of the particles, especially with respect to the presence of oxide films. If heavy oxide layers are present, they must be penetrated by projections on the particles. This results in only local rather than widespread bonding. A ductile metal such as iron which has a heavy oxide layer may not form as strong or as many bonds as a less ductile metal. 

In the anodizing stage electrolytic reactions produce a uniform aluminum oxide layer across the aluminum surface. This anodic film is transparent and porous. The underlying matte or bright surface can be seen. After anodizing, the aluminum part can be colored or sealed. 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

The BLM layer uses a glue layer of chromium or titanium. These metals stick well to other metals and most dielectrics, but they are not solderable. Copper, nickel, and silver have been used as the solder-wetting layer for BLM in appHcations involving 95% lead—5% tin solders. Gold is commonly used as the oxidation layer on account of its resistance to oxidation and its excellent solderabiUty. 

For many electronic and electrical appHcations, electrically conductive resias are required. Most polymeric resias exhibit high levels of electrical resistivity. Conductivity can be improved, however, by the judicious use of fillers eg, in epoxy, silver (in either flake or powdered form) is used as a filler. Sometimes other fillers such as copper are also used, but result in reduced efficiency. The popularity of silver is due to the absence of the oxide layer formation, which imparts electrical insulating characteristics. Consequently, metallic fibers such as aluminum are rarely considered for this appHcation. 

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