1. Field of the Invention
This invention pertains in general to a system and method for processing residual gas and, more particularly, to a system and method for processing and disposing of residual toxic gas in a semiconductor manufacturing process.
2. Description of the Related Art
A variety of process gases are used in various process equipment in a semiconductor manufacturing process. For example, a Chemical Vapor Deposition (CVD) process often uses SiH4, B2H6, NH3 and H2 as process gases. Because many process gases are toxic and explosive, and because process gases are seldom completely reacted during a manufacturing process, handling of residual gases, i.e. process gases that remain after completion of the manufacturing process, is an important issue in semiconductor manufacturing. In addition, environmental concerns and government legislating prohibit toxic gases and harmful particles from being vented to the atmosphere or disposed of with waste water.
FIG. 1 is a flow chart of a conventional process for handling residual gas in a semiconductor manufacturing process that uses silane (SiH4) as the processing gas. Referring to FIG. 1, process gas, for example, silane, is selected at step 1. Some of the characteristics of silane gas are described in the following table:
AtomicBoilingMoleculeNameWeightColorSmellpoint (° C.)SiH4Silane32NoneRepulsive−112MeltingStatus inPointDensityRoomSafety(° C.)(g/L)TemperatureCharacteristicsToxic Indication**−1850.68GasE/F/P*TLVIDLH5 ppm—*E: explosive; F: inflammable; P: toxic**TLV: Threshold Limit Value (time weighted average exposure for an 8-hour day in a 40-hour workweek)IDLH: Immediate Danger to Life and Health
Source: R. J. Lewis, Sr., Hazardous Chemicals Desk Reference, 3rd Ed., Van Nostrand Reinhold, 1993.
These characteristics show that silane is toxic and explosive, and therefore great care should be taken in handling and disposing of silane gas. The process gas is then introduced at step 10 to a process chamber through a connecting pipe. After a semiconductor manufacturing process is performed in the process chamber at step 10, a pump propels the remaining silane gas that did not fully react during the process, i.e., the residual gas, from the process chamber to a wet scrubber through another connecting pipe at step 20. As described above, the residual gas introduced to the wet scrubber still contains non-reacted silane gas. Upon entering the wet scrubber, oxidation ensues and powdered silicon dioxide (SiO2) is formed. Water is then added to the wet scrubber so that both soluble and non-soluble SiO2 powders are further processed at a waste water facility drain at step 42. The resultant waste water is expelled to the environment. Any remaining residual gas and powders are vented at step 40 to a waste gas facility exhaust to be further processed and then be expelled into the atmosphere.
This conventional technique, however, cannot ensure that all of the silane gas that passes through the wet scrubber is reacted. Therefore, an explosion is still possible if the silane gas were to come into contact with oxygen in one of the connecting pipes. Furthermore, the non-reacted silane gas expelled to the atmosphere may still exhibit a toxic level higher than the legally prescribed safety level. In addition, the powders produced through oxidation of the silane gas may result in the blockage of inlets and outlets to and from the wet scrubber.
As a proposed improvement to ensure complete reaction of the residual gas, an alternative conventional technique employs catalysts to breakdown the residual gas, or absorbents to absorb toxic materials or particles so that the gas expelled into the atmosphere is harmless. Such a method, however, requires complex chemical reaction processes. In addition, catalysts and absorbents are usually expensive and cannot be repeatedly used, resulting in an additional cost to the manufacturing process. Moreover, the catalysts and absorbents themselves become toxic from the process and become industrial wastes, of which cannot be easily disposed.