Proper airflow contains VOCs

SINGAPORE – While numerous studies have focused on design methods to ensure cleanliness inside a minienvironment, the area of VOC pollution caused by the manufacturing process has not been well addressed.

With funding from the Taiwan Government and the private sector, two researchers at the National Taipei University of Technology (NTUT) conducted a one-year experimental study into control of particle contamination and VOC pollution.

Assistant professor Shih-Cheng Hu and associate professor Yew-Khoy Chuah, both with the Department of Air-Conditioning and Refrigeration at NTUT presented their findings at Cleanrooms/Datastor Asia '99 in Singapore July 27-30.

The manufacturing process undertaken in the research was the application of color filter coatings for liquid crystal displays (LCDs). The color filter resin must be uniformly coated on a glass substrate and the process requires an ultra-clean environment.

During the study occasional high levels of particle contamination affected the coating process. Particle concentration counts in the hundreds were detected in the vicinity of the color filter spin coater. In addition, the color filter resin and the coating process released odorous vapors and VOCs and ? in a real life environment ? these gases would cause health hazards to the cleanroom workers.

The flow field was studied using a Kaijo FA600 3-D ultrasonic anemometer, while the particle concentration in the minienvironment was measured by a particle counter, PMC uLPC-110. The light source for the particle counter was a 2mW Helium Neon laser. VOC concentration was measured using a Cosmos XP-329 trace gas detector.

Typically an exhaust fan is installed in the equipment to suck out pollution. However, sometimes one fan is not enough. In the original equipment obtained by the researchers only one exhaust fan was installed. The researchers added an enclosure on top with a side door to allow access. A second exhaust fan was installed in order to contain the VOC within the minienvironment.

The minienvironment used the exhaust fan to maintain negative pressure. While the exhaust was effective in containing the VOC pollution, it did somewhat enhance the horizontal flow transportation which was detrimental to the control of particle contamination.

Experimental conditions * There were four experimental conditions in the study, involving closed door and exhaust fan on/off, and open door with exhaust fan on/off. The highest particle contamination counts were recorded when the exhaust fan was on. When the door was open and exhaust fan on, particle count per cubic foot was 141, with most particles under 1.0 micron in size. The lowest particle count was recorded when the door was closed and exhaust fan off: only 15 particles per cubic foot.

VOC concentration was measured under various conditions. An acetone source was used to simulate the VOC emissions, with the source measurement reading 1900. The worst case was when the door was open and the exhaust off, with VOC measurements of 780 near the door and up to 1000 near the source. Best case was with the door open and exhaust on, with zero VOC measured near the door and 254 near the source.

In order to further improve the performance of the minienvironment, several design parameters were studied by experimental measurements and by computer models.

During their research Hu and Chuah also sought to ascertain the ideal height for a partition to contain particles. A total of 12 cases were studied using different opening sizes for the door (from 0.18m to 0.93m) and different partition heights (from no partition to 1.2m). There were three zones in the cleanroom; control zone, local clean zone and equipment zone.

When the opening was higher than 0.48m (about half the height of the equipment zone) there was risk that submicron particles would be diffused into the equipment zone. Therefore, it was found that the partition was effective in shielding the equipment zone from contamination by particles generated in the control zone.

With regard to partition height, no significant difference was observed in the case of a 1.2m and 1.5m high partition in terms of the velocity vectors and particle diffusion. However, the researchers reported that the case of a partition height of 1.5m was the most favorable for the working conditions of operators. “The traditional design of a minienvironment is not enough to keep away the particles. We need isolation by introducing the partition,” says Hu.

More research needed * One of the conference participants, Peter Maguire, Taiwan-based sales manager for Lighthouse Worldwide Solutions, applauded the NTUT study and says more research needed to be done on airflow dynamics for minienvironments.

“If you don't have proper airflow you'll create areas like recirculation zones where particles will get trapped and recirculate…and they can dislodge from this recirculation zone and onto your product,” he says. Turbulence created by opening and closing doors and robotic machines moving products around is an often overlooked source of contamination. “We've seen particles shedding from specific tools form doors opening and closing. If you have laminar airflow those particles will be taken out through the cleanroom, but if you don't those particles can stay trapped in the minienvironment,” notes Maguire.

The research by Chuah and Hu was conducted in collaboration with Dr C.H. Tsai of the Electronics Research and Services Organization, part of Taiwan's Industrial Technology Research Institute.

Craig Addison


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