How Point-of-Use Waste Gas Treatment Reduces Fab Emissions


What is Point-of-Use Waste Gas Treatment?

Many industrial and research production procedures use process gases and generate waste gases. These waste gases are toxic and/or highly flammable and very often pose a significant risk to production facilities and the environment. The semiconductor industry, for instance, uses perfluorocarbons, whose global warming potential (GWP) is extremely high. Combining and transporting different gases into a fab’s central waste gas system might produce highly flammable and highly explosive gaseous mixtures, which in the past has occasionally caused the total loss of entire production facilities. Particles contained within gases may also cause exhaust blockages. To eliminate these risks, process waste gases need to be treated at the “Point of Use” (POU) where harmful exhausts are abated immediately. 

Subfub and Fab - Point of Use Waste Gas Treatment Principle
Subfub and Fab - Point of Use Waste Gas Treatment Principle

Typically POU gas treatment tools are situated in the subfab, and placed after the vacuum pumps. There are very few atmospheric processes in use, with vacuum based processes being in the majority. Best practice installation results in the gas abatement tool being placed as close to the pumps as possible. In turn, the pumps are also placed as close to the process tool as possible. 

The basic concept of waste gas treatment systems are the same although several different technologies are available. Most tools use two stages in treating the waste gas flows, however depending on the process gases, one stage maybe more than adequate, be it a dry burner or a wet scrubber. In two stage systems the waste gases are broken down thermally to their constituent elements, oxidized to form a stable by product before being washed in a scrubber to remove any acidic components. The first stage of decomposition can be performed by several differing technologies such as "Burn", "thermal" and "plasma". The last two use electricity as an energy source while burn technologies use natural gas, LPG, or H2 as a fuel source.

Burn/Wet Principle
Burn/Wet Principle

The function of the scrubber is to transfer the remaining hazardous gases into the scrubbing liquid where they can be more easily treated. Often, lye is used to neutralize the scrubbing liquid prior within the system, this generally results in a lower water consumption and consumable usage. A strongly acidic scrubbing liquid has a large impact on the lifetime of internal components and therefore COO of the tool.

 

What are the common semiconductor fab emissions?

Semiconductor manufacturers use a variety of high GWP gases to create intricate circuitry patterns upon silicon wafers and to rapidly clean chemical vapor deposition (CVD) tool chambers. Semiconductor manufacturing processes use high GWP fluorinated compounds including perfluorocarbons (e.g., CF4, C2F6 and C3F8), hydrofluorocarbons (CHF3, CH3F and CH2F2), nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6). Semiconductor manufacturing processes also use fluorinated heat transfer fluids and nitrous oxide (N2O).

It is now known that under normal operating conditions, anywhere between 10 to 80 percent of the GWP gases pass through the manufacturing tool chambers unreacted and are released into the air if not treated by Point-of-Use abatement systems.

 

Benefits of DAS Point-of-Use Waste Gas Treatment

There are three main reasons and benefits why gas treatment is installed in a factory:

Health and Safety

Many of the process gases used are flammable or explosive such as H2 or SiH4 while others are poisons such as COF2, HBr, F2 and so must be treated before coming into contact with people or wildlife. 

Environmental Protection

The second reason is that many gases used are dangerous for the environment such as CF4 which has a lifetime of approx. 50,000 years in the atmosphere and has a Global Warming Potential of 6,500 times that of CO2

Optimization of the Process Tool Uptime

The third reason for gas abatement is that a lot of waste gases contain a lot of particles or dust in the gas flow. For example this dust is often created during Silane (SiH4) based deposition processes, the reacted Silane produces a Silicon Dioxide as a very fine dust as a waste product. This dust has the possibility to disrupt the facility exhaust leading to unintended downtime in the manufacturing plant. What makes the dust difficult to treat is that the particle size is often submicron and easily passes through wet scrubbers or physical filters without being trapped in the scrubbing liquid. 

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