Shadowing vapor deposition technique

A vacuum technique closely related to CVD is PVD (physical vapor deposition), in which a solid (metal or compound) is evaporated in a vacuum by heating or by a plasma (called sputtering) and condensed on a substrate to form a coating. Often there is no chemical reaction during deposition—hence the name. PVD is a line-of-sight process, and unlike CVD it suffers from shadowing on profiled surfaces if the substrates are stationary with respect to the source. 

An important topic of research is the introduction of the catalyst in the microreactor. In brief solid catalysts can be incorporated on the interior of micromachined reaction channels, prior to or after closure of the channel, by a variety of strategies anodic oxidation, plasma-chemical oxidation, flame combustion synthesis, sol-gel techniques, impregnation, wash coating, (electro-)plating, aerosols, brushing, chemical vapor deposition, physical vapor deposition and nanoparticle deposition or self-assembly. Some of these methods can be applied in combination with photolithography or shadow masking. 

Physical vapor deposition with shadow masks is known for its simplicity for creating defined areas of thin-film catalytic material in microreactors. This technique was used to deposit silver in the reaction manifolds of microreactors for small-scale synthesis of valuable fine chemicals (Figure 1.4a). The manifolds consisted of a network of 16 parallel channels (19 mm x 600(im x 60-220 tm), in which the oxidative dehydrogenation of 3-methyl-2-buten-l-ol to aldehyde was carried out successfully for temperatures up to 464 °C [31]. The conversion increased smoothly with temperature and low oxygen and high alcohol concentrations were beneficial for selectivity, in addition to less deep channels (higher catalyst surface area to reaction channel volume). For temperatures >400°C, the selectivity deteriorated due to CO and CO2 formation. 

Stencil lithography (shadow masking) is an old technique in microelectronics. A hole array mask placed above the substrate allows localized flux of material from above. Traditionally, the deposition was evaporation of metals. In the microfluidic version of the technique, the stencil is a flexible polymer film (e.g., parylene) attached to the surface of the substrate (which can be, e.g., glass or polymer). The sample can then be exposed to liquids or vapors that will modify the open areas defined by the stencil. 

Electron microscopy contributed to the study of virus structure with two techniques (a) heavy-metal shadowing, in which heavy metal vapors are deposited upon a grid with the virus particles this reveals structural details where the metal atoms have been deposited, (b) Negative staining, in which the virus suspension is treated with phospho-tungstate, an electron-dense material, which fills the empty spaces and crevices of the particle. 

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