Reduction of Post-Combustion Abatement System Particulate Material
Adam Stover
(centrotherm Clean Solutions)
Particulate matter less than 2.5 microns in diameter (PM 2.5) is a health hazard that is gaining increasing attention in the semiconductor industry. Long-term exposure to fine particulate is correlated to increased cardio-pulmonary and lung cancer mortality and decreased life expectancy. Furthermore, PM 2.5 is more dangerous than coarse particulate (PM 10). Various regulatory bodies are limiting PM 2.5 release because of the health risk. For example, German TA Luft regulations limit PM 2.5 to 20 mg/m3, while other HVM manufacturers have taken the lead to reduce PM 2.5 emission to 2 mg/m3. In general, PM 2.5 is generated as a combustion byproduct. A common gas abatement approach in the semiconductor industry is a combustion/water wash combination to first react the gas phase species and then capture the acid and particle byproducts. However, particles less than 10 microns in diameter are difficult to capture in a wet scrubber, as the capture rate increases with increasing particle size. For example, a polycrystalline Si deposition process with a SiH4 flow of 500 sccm would generate approximately 1.3 grams of SiO2 per minute in a combustion abatement unit, for a post-combustion particle concentration of 1.3 g/m3. The particle removal efficiency (PRE) would need to be >99.85% to reach 2 mg/m3, but most standard wet scrubbers struggle to achieve more than 90% PRE. Therefore additional powder capture technologies must be incorporated to increase PRE to acceptable levels. One such approach is a wet electrostatic precipitator system (WESP) that charge the incoming particles between two capacitor plates to capture them, however these systems bring a number of technical challenges. Most importantly, the quoted PRE is only >99%, leaving a potential gap in performance. Furthermore, these systems have high facilities usage for power and water and the capacitor plates can break down over time, requiring replacement. Taken together, these factors cause these systems to have a high TCOO, with high downtime probability and potential performance shortcomings. An alternative method is the use of fine particle filters. Filter systems have superior PRE compared to WESP systems (>99.95%), fewer facilities needs and high uptime. Sophisticated systems have automated filter cleaning with N2 or CDA sprays to minimize downtime as the filters themselves have a 3-5 year lifetime. Filter systems can be designed using a modular approach and can be deployed point of use or point of area, minimizing either subfab piping requirements or footprint. This presentation will first discuss the important of managing particulate emissions from semiconductor fabs, the mechanisms of capture and finally compare field performance data for both WESP and particulate filters.