Chemical Vapor Deposition Safety

Safety issues are particularly important for CVD since many of the source compounds are toxic and disposal of waste products, e.g., HCl, is often problematic. Additional problems can occur if the reactants are pyrophoric (ignite in contact with air). Table 28.4 lists some of the source gases used in CVD and their potential hazards. Silane is widely used in the semiconductor industry and was the cause of a major explosion and fire at a manufacturing plant in Moses Lake, WA. The incident caused a number 

Experimentally, evaporation is a very simple method for forming thin films. The source is either a liquid or a solid that is heated to produce a flux of atoms or molecules. In general, it will be necessary to melt the material if the vapor pressure is 10 torr at its melting temperature. Most metals fall into this category and so liquid sources 

Gas Corrosive Flammable Pyrophoric Toxic Bodily hazard 

Evaporation of metals is generally straightforward because they evaporate either as atoms or as clusters of atoms. However, most compounds dissociate when heated and therefore the vapor composition will be different from that of the source. Consequently, the stoichiometry of the deposited film will also be different from that of the source. An example of an oxide that dissociates on heating is ZrOi 

Films formed directly from evaporation of Zr02 tend to be metal rich. To maintain the desired stochiometry it is necessary to perform the evaporation in an oxygen-rich environment. This is called reactive evaporation (RE). A similar approach needs to be used with Si02, Ge02, Ti02, and Sn02. 

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This chapter introduced and briefly described a number of industrial surface treatments. These treatments are made to material surfaces for various reasons. One is to enhance the appearance of a finished product to attract potential buyers. Another is to improve the safety of products. Other purposes are to promote corrosion control and wear resistance, and to improve mechanical and electrical properties. The treatments discussed include electroplating, electroless plating, spray coating, galvanization, painting, anodizing, physical vapor deposition (PVD), and chemical vapor deposition (CVD). 

Rhoades, B. J., Sands, D. G., Mattera, V. D., Safety and Enviromnenlal Control Systems Used in Chemical Vapor Deposition (CVD) Reactors at AT T-Microelectronics—P adaag,Appl Ind. Hyg., 4(5) 105-109 (1989)... 

The majority of lasers used in semiconductor processing equipment are low powered (Class 2 or 3A) helium-neon (He-Ne) lasers used for alignment purposes. T q)ical fab equipment using these lasers includes some photolithographic steppers, wafer surface scanners, chemical vapor deposition (CVD) equipment, and some aligners. Safety precautions needed with these lasers are minimal assuming collecting optics are not used in the system. 

Figure 12.5 A schematic diagram of some of the major elements of a typical low-pressure chemical vapor deposition system. Note that the entire process is typically contained within a safety enclosure because many of the reactants as process gases are either flammable or toxic. A toxic gas detector scans the interior of the enclosure, especially around the gas piping, for leaks. The scrubber reacts the source materials that are left over after passing through the reactor tube to render them harmless. Typically the valves for all sources are operated electronically so that the user does not need to open the safety enclosure except to load substrates on the susceptor. The system shown here includes a single substrate on a tilted susceptor. Batch processes are similar except in the arrangement of substrates in the reactor tube. Other reactor tube designs are also used. More specific reaction examples are given in Section 12.8.

Whichever heating method is employed, CVD has to provide a volatile precursor containing the elements that compose the deposited film, transport the precursor towards the substrate surface, enhance or reduce reactions in the gas phase, and provide the surface reaction needed to form the film. The setup consists of a reaction chamber gas/vapor delivery lines the energy source vacuum systems (LPCVD) an exhaust system and gas flow, pressure and temperature monitoring systems. Hazardous vapors are also frequently used and may be produced by chemical reactions. Thus, safety equipment may be necessary. The advantages of CVD films are good... 

The chemical species present in the electrolyte are actually more complex than that described. In solution, elemental bromine exists in equilibrium with bromide ions to form polybromide ions, Br, where = 3, 5, 7. Aqueous zinc bromide is ionized, and zinc ions exist as various complex ions and ion pairs. The electrolyte also contains complexing agents which associate with polybromide ions to form a low-solubility second liquid phase. The complex reduces the amount of bromine contained in the aqueous phase 10 to 100-fold, which, in addition to the separator, also reduces the amount of bromine available in the eeU for the self-discharge reaction. The complex also provides a way to store bromine at a site remote from the zinc deposits and is discussed further in the next section. Salts with organic cations such as iV-methyl-iV-ethylmorpholinium bromide (MEMBr) are commonly used as the complexing agents. One researcher has proposed a mixture of four quaternary ammonium salts for use in zinc/bromine batteries. The proposed electrolyte has favorable properties with regard to aqueous bromine concentration, resistivity, and bromine diffusion and does not form solid complexes at low temperatures (5°C and above). Complexes with quaternary ammonium ions are reversible and also have an added safety benefit due to a much reduced bromine vapor pressure (see Sec. 35.6). 

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