What Is Abatement Technology?
Abatement Technology refers to equipment designed to remove hazardous gases from the exhaust stream of process tools. This equipment is referred to as “scrubbers” and is common in several industries. The most efficient abatement technology depends heavily on the process gases and application requiring the abatement. This article is intended to serve as a general guide to current abatement technologies and is focused primarily on the semiconductor industry.
When abatement was initially implemented into the semiconductor industry, the standard design was a large scale, end-of-line scrubber which would handle all the toxic gases used by a fab. These tended to be wet scrubbers or charcoal/carbon filled drums. The idea was to route all hazardous gases to a single point of abatement and support the entire facility. Unfortunately, this method posed several downsides such as corrosive gases degrading long distance piping, limited ability to expand, and the inefficiency of general abatement targeting all hazardous gases.
These concerns led to the development of small point-of-use scrubbers which offered several advantages. Smaller systems could be located closer to the tool to minimize facility piping that actively had hazardous gases and could limit the strain on a single facility wide abatement system. In addition to this, you can design the scrubber for a specific application to improve efficiency. With the transition to small scale abatement, several technologies developed: dry bed, wet, thermal, and plasma.
Dry bed abatement makes use of physical and chemical adsorption to trap hazardous gases and release inert gas. By minimizing the scope of the abatement from facility wide to a particular process, the hazardous gases could be targeted directly. Instead of using a general carbon drum, the media was modified to improve abatement efficiency and scrub only the necessary gases. This means you can get the highest abatement efficiency of any technology and can target a wide variety of gases rather than having multiple technologies for a single process. The strength of dry bed abatement at having highly efficient, targeted media also means that if there is not media effective at targeting a specific gas, it will not be abated as efficiently. This is common for H2 and PFCs with low carbon counts, like CF4 and C2F6, which are not abated by dry bed scrubbers.
Wet scrubbing was also modified to handle point of use applications. These scrubbers make use of recirculating water and chemical injection to treat water soluble gases, typically corrosive acid gases and Ammonia. The priority of a wet scrubber is to maximize gas and liquid mixing through high surface area contact and promote transferring gas into the liquid phase. Chemicals like NaOH or KOH can be implemented to help control pH and improve efficiency for gases with lower solubility like Cl2. Generally, wastewater must be kept around a pH of 6-8 to discharge to municipal water treatment. While wet scrubbers are effective at handling water reactive gases and particulates, you will want to avoid organic compounds that could contaminate the aqueous effluent from the scrubber. The wastewater from point-of-use wet scrubbers usually proceeds into a facility wide neutralization or water treatment plant before being released to general waste.
Thermal units are most common for applications with high loads of flammable or pyrophoric gases. Thermal oxidizers commonly use fuels, such as H2 or CH4, to provide the necessary energy to burn hazardous gases. Alternatively, there are thermal scrubbers that use electrically powered furnaces to help avoid the generation of NOx. These systems are susceptible to fluctuations in process gas flow and ineffective against non-flammable gases. The process of burning also produces particulate which can clog the scrubber and exhaust piping, requiring additional cleaning and maintenance.
Finally, you have plasma scrubbers which are the newest form of abatement technology. The method of using plasma to break apart hazardous gases has long been in the industry in the form of the process tool itself. PFC gases, like CF4, are often used as a cleaning or etch gas and the plasma chamber will break apart the PFC to access the fluorine. The plasma scrubber uses the same concept, and reimplements it to break apart any remaining gases that make it past the process chamber. This is most common for PFC gases that will then produce either F2 or HF in the presence of Hydrogen or moisture. Water or oxygen can also be injected into the plasma chamber to encourage the formation of these byproducts. The fluorinated byproducts are then treated with a packed wet scrubber or separate dry canister after the plasma torch.