Chemical Vapor deposition Technology
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The growth of thin films via chemical vapor deposition (CVD) is a powerful technique for bioassay, microarray, medical and microelectronic device (MEMS) fabrication. Chemical vapor deposition (CVD) is carried out by passing a volatilized precursor (such as a silane, organometallic or metal coordination complex) over a heated substrate. Thermal decomposition of the precursor produces a thin-film deposit, the ligands associated with the precursor are cleanly lost to the gas exhaust. Compared to other thin-film production techniques, CVD offers several significant advantages, most notably the potential for very thin monolayer or atomic layer deposition (ALD) or angstron level coatings, selective deposition and low process temperatures (average 50 to 120C). Many metal CVD depositions are autocatalytic. Growth of such thin films is characterized by an induction period, which is a consequence of the higher barriers that relate to the activation of the precursor on a non-native substrate.Silanes can be applied to substrates under dry conditions by CVD, which favors monolayer deposition. Although under proper conditions almost all silanes can be applied to substrates in the vapor phase, those with vapor pressure >5 torr at 100°C have achieved the greatest number of commerical applications. In closed chamber designs, substrates are supported above or adjacent to a silane reservoir and the reservoir is heated to achieve 5mm vapor pressure. Alternatively, vacuum can be applied until silane evaporation is seen. The silane can also be prepared in toluene solution, which when refluxed allows sufficient silane to vaporize through its inherent partial pressure. Here, the substrate temperature should be maintained between 50°-120°C to promote reaction. Cyclic azasilanes deposit the quickest (usually <5 mins). Amine functional silanes also deposit rapidly (~30 mins) without a catalyst. Other silanes require extended reaction times, usually 4-24 hours. Such silane reactions can be promoted by adding catalytic amounts of amines.

Using differing precursor and reactive gases allows films of varied composition to be made. SnO2, ZnO, and Pt films can be grown using the following precursors and reactants respectively: tetramethyltin and oxygen; diethylzinc and oxygen; and dimethylplatinum(II) cyclooctadiene ({COD}PtMe2) and hydrogen. Precursors are available for commonly employed metals, such as silicon, copper, aluminum, titanium, tantalum and tungsten. These precursors are volatile solids or liquids and can be used to deposit metal thin films in typical CVD or atomic layer deposition (ALD) processes. Typical applications are for metal interconnects or diffusion barriers. In addition, metal-organic precursors that are volatile and reactive in the gas phase are employed for CVD. A wide variety of co-reactants and reaction conditions can be used to deposit mainly metal, metal oxides or metal nitrides.


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