Semiconductors are produced using different long-lived fluorinated gases during plasma etching (patterning) and plasma enhanced chemical vapour deposition (PECVD). About one hundred process steps requiring fluorinated gases are used to produce the semiconductor products such as devices or chips from silicon wafers.
Plasma etching is applied to provide pathways for conducting substances which connect the circuit components of the semiconductors. Plasmagenerated fluorine atoms are used. These atoms chemically react with exposed dielectric films. During this process certain portions of the film are selectively removed. Some residual undissociated fluorinated gases remain. They together with removed material are partly emitted as waste gas, and are partly treated in emission abatement systems.
PECVD chambers are periodically cleaned by using fluorinated gases, which in plasma, are converted to fluorine atoms. They move away the residual material from chamber walls, electrodes and hardware. Residues and reaction products, e.g. CF4, are emitted.
For these purposes, depending of the specifity of the products, mainly the following gases are used: Trifluoromethane (HFC-23, CHF3), perfluoro-methane (CF4), perfluoroethane (C2F6), nitrogen trifluoride (NF3), and sulphur hexafluoride (SF6), moreover perfluoropropane (C3F8), and perfluorocyclobu-tane (c-C4F8).
About 500 t of these substances were applied in the U.S. semiconductor industry in 2004. This seems to be a relative low amount. But these gases are highly potent greenhouse gases (see table 7.1), and thus end up as about 4.7 Mio ton CO2-eq. The numbers increased during the last 15 years, due to the growth of the industry and the higher complexity of the semiconductors, which needs more PFCs. But there has been a declining growth rate of PFCs in recent years due to process optimisation and abatement technologies. The decline in growth was about one-third during the last 5 years.
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