Spatial and Temporal Distributions of a Gaseous Pollutant During Simulated Preventive Maintenance and Pipe Leaking Events in a Working Cleanroom

Sulfur hexafluoride (SF₆) of > 99.9% in purity was artificially released to simulate the emission sources in the etching/thin-film area of a working cleanroom in a semiconductor fab at the rate of 492 g/h. The temporal and spatial dispersion patterns of the gas pollutant were studied during the simulated preventive maintenance (PM) and pipe leaking exhaust events experimentally and numerically. Three mobile Fourier transform infrared spectrometers (FTIRs, detection limit: 10 ppb) were used simultaneously to measure the real time SF₆ concentrations in different locations of the etching/thin-film area of the cleanroom. An additional open-path FTIR with very low detection limit of 0.4 ppb was installed before the recirculation duct in the lithography area to monitor if the pollutant drifted from the etching/thin-film area to the lithography area. The results show that the 3-D numerical model predicts the unsteady gas concentration accurately in both the peak concentration and the time required to reach the peak concentration. Due to high dilution of the pollutant in the cleanroom, the current gas sensors may not be sensitive enough and a better monitoring system and strategy is needed to protect workers from injury and ensure good product yield. The well-mixed model predicts the peak pollutant concentrations within a reasonable range which is 0.34-1.33 times the experimental values except when the monitored distance is very close to the release point. Although it is not able to predict the time required to reach the peak concentration and the time for the concentration to drop below a small level, the simple well-mixed model can be used to obtain an estimation of the peak concentration quickly when the emission rate and the ventilation condition of the cleanroom are different than the current study.