Pyrophoric Effluent Treatment for the Prevention of Fires and Explosions
Samir Shiban – Intel (SSA Journal – March 1993 pp. 42 – 46)
Pyrophoric gases such as silane (SiH4), dichlorosilane (CI2SiH2) and phosphine (PH3) are commonly used in the semiconductor industry. In some processes pyrophoric gases are used in combination with flammables such as hydrogen (H2) or oxidizers such as nitrous oxide (N2O) and nitrogen trifluoride (NF3). They are generated as waste products in the gas supply areas during purging cycles of piping control panels and as a result of venting prior to replacement of cylinders. Pyrophoric waste gases are also generated from a variety of processes in a semiconductor facility, such as CVD (chemical vapor deposition) and diffusion. These processes are conducted in the clean room inside vacuum chambers which are kept under negative pressure by vacuum pumps located in the mechanical support areas.
Several factors must be taken into consideration prior to disposing of these gases. These factors should govern the selected treatment of the gases, depending on the specifics of the situation. The most prevalent risks associated with pyrophoric gases are fires and explosions. In addition to being flammable, pyrophorics can ignite spontaneously, under some flow conditions at ordinary ambient temperatures.
Another factor that complicates the treatment of pyrophoric gases is that, depending on the process requirements, they could be mixed with flammable or toxic gases in the gas supply system, in the process tools or in the exhaust stream. Flammability and toxicity factors must also be taken into consideration when selecting a method of treatment for those gaseous mixtures, prior to being discharged as harmless products into the environment.
Several methods of treatment are available to reduce the risk of fire and explosions and meet the requirements from an environmental standpoint; these options include water or chemical scrubbers, active oxidation systems, dilution systems and pyrophoric/air reaction systems. In order to maximize safety and achieve environmental compliance, all the factors involved in each particular condition must be taken into consideration.