Self-limiting surface reactions

Klaus J W, Sneh O, Ott A W and George S M 1999 Atomic layer deposition of Si02 using catalyzed and uncatalyzed self-limiting surface reactions Surf. Rev. Lett. 6 435-48... 

FIGURE 12.1 Schematic representation of ALD using self-limiting surface chemistry and an AB binary reaction sequence (From Ref. 15, J. Phys. Chem., 100 (1996) 13121. 1996 American Chemical Society). See insert for colour representation of the figure.)... 

An important point is that both of the reaction steps are self-limiting. In reaction step 1, once all available AlOH(s) surface sites have reacted with the Al(CH3)3(g) precursor, the reaction comes to a stop because the CH3 groups that now terminate the surface are unreactive toward Al(CH3)3(g). Thus, only a single atomic layer of... 

Tin deposited directly over copper, whether by electrolytic or electroless methods, should be avoided except for low-end/low-cost, short-field-life products. Sn alloys with copper, even at room tenqjerature, form a brittle intermetaUic compound, albeit slowly. It has limited solderable shelf-Ufe at room tenqjerature and the Sn-Cu intermetaUic can inhibit soldering and may result in incomplete solder wetting or solder joints of inferior strength. For tin to be used effectively, an appropriate non-porous barrier metal, such as Ni, should be deposited between the Cu and the Sn. Immersion Sn can be plated only as a very thin layer since its deposition is by a self-limiting replacement reaction. When there is no more exposed Cu on the surface of the PWB during the surface-plating operation, Sn deposition wUl cease.Therefore, it is impossible to plate enough Sn by tins method to extend the solderable sheU-hfe or reduce the brittleness of the solder joint. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

The presence of corrosion products is not always a negative event some small degree of surface corrosion of all steel heat exchanger surfaces is generally beneficial. Under the reducing conditions normally found on the surfaces of pre-boiler FW heaters, FW lines, and boiler surfaces, black magnetite naturally forms by the direct thermal reaction of water with steel. The development of this self-limited magnetite film is most desirable, and optimum formation is achieved at pH levels of 10.5 to 11.5. 

The most comprehensive experiments have been performed in low-pressure CVD hot tube reactors.24 When WF6 is reduced on clean, flat silicon surfaces, the deposition rate is very rapid (>1000 A/min) and self limiting. Generally, a tungsten film of less than 200 A (grown at 300° to 425°C and 500 mTorr) is sufficient to completely block this reaction, as shown in Figure 14. 

Next, an example of CG-KMC from pattern formation on surfaces is presented. Another application to relatively thick membranes was given in Snyder et al. (2004). In the example considered here, atoms adsorb from a fluid reservoir on a flat surface. Subsequently, they may desorb back to the fluid, diffuse on the surface, or be annihilated by a first-order surface reaction, as shown in Fig. 11a. Attractive interactions between atoms trigger a phase transition from a dilute phase (a low coverage) to a dense phase (a high coverage) (Vlachos et al., 1991), analogous to van der Waals loops of fluid vapor coexistence. Surface reactions limit the extent of phase separation the competition between microphase separation and reaction leads to nanoscopic patterns by self-organization under certain conditions (Hildebrand et al., 1998). 

Ensure fast completion of the surface reactions and thereby short cycle times Lead to high film purity No problems of gas phase reactions No competing reaction pathways Would prevent the film growth Would destroy the self-limiting film growth mechanism To avoid corrosion... 

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