Simulation and testing of pollutant dispersion during preventive maintenance in a cleanroom Hui-Ya Shih a , Shih-Hsuan Huang b , Shou-Nan Li a , Sheng-Chieh Chen b , Chuen-Jinn Tsai b, * a Energy and Environment Research Laboratories, Industrial Technology Research Institute (ITRI), Hsin Chu, Taiwan b Institute of Environmental Engineering, National Chiao Tung University, No. 75 Poai Street, Hsin Chu 300, Taiwan article info Article history: Received 16 December 2008 Received in revised form 2 March 2009 Accepted 18 March 2009 Keywords: Airborne molecular contamination Gas sensors Cleanroom Preventive maintenance abstract Sulfur hexafluoride (SF 6 ) of >99.9% 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. Three mobile Fourier transform infrared spectrometers (FTIRs, detection limit: 10 ppb) were used simultaneously to measure the real time SF 6 concentrations at different locations of the cleanroom. A three-dimensional numerical model was also used to predict the unsteady gas concentration distribution and the results were compared with the experimental data. Due to high dilution of the pollutant in the cleanroom, it is found that the current gas sensors may not be sensitive enough and a better monitoring system and strategy is needed to protect workers from injury and to ensure good product yield. After comparison with the validated numerical results, the well-mixed model is found to predict 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. The well-mixed model is shown to be capable of predicting a reasonable attainable maximum concentration once a pollutant leaks in the cleanroom. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Airborne contaminants pose a serious threat to the state-of- the-art manufacturing processes as the feature size continues to shrink in the semiconductor industry. When a gas pollutant leaks from the pipes, fittings or process chambers, it mixes with the recirculation air and disperses in the cleanroom to become an AMC (airborne molecular contamination), which will cause process tool damage, product corrosion, wafer defects and potential worker injury [1]. Many micro-contamination studies in the cleanroom have been conducted in the past. When the concentration of hydrogen chlo- ride was higher than 28 ppb, the corrosion defects were observed on the test wafer [2]. The concentration of ammonia of 20 ppb could cause the critical dimension shift of 25–35% depending on the type of photo-resists [3]. When chemically amplified resist was left in an uncontrolled atmosphere of about 10 ppb NH 3 , patterns on the wafers were either not developed or T-top phenomenon occurred [4]. Hazy optical lens was found in a TFT-LCD fab due to the continuous emission of high concentration of NH 3 into the cleanroom during the preventive maintenance (PM) process of the photo-resist stripper [5]. Ruthenium (Ru) airborne contaminant, which diffused to the cleanroom during cleaning of Ru CVD furnace tubes, was determined as a harmful gas to the MOSFETs process [6]. High concentrations of corrosive and toxic gases were found to emit from the metal etch chambers and downstream pipelines during PM process [7,8]. Without appropriate control, hydrogen chloride (HCl) as high as 343 ppm was detected inside the enclosed chamber, which might cause corrosion on the wafers and the process tools after the chamber was opened. Therefore, to ensure high yield manufacturing in the semiconductor industry, the pollutant concentration must be controlled below a certain limit. For example, the yield enhancement committee of the Interna- tional Technology Roadmap for Semiconductors (ITRS) recom- mended that the concentration of the total inorganic acids and bases be less than 0.5 and 2.5 ppb, respectively, for reticle exposure environment, or less than 5.0 and 50.0 ppb, respectively, for lithography cleanroom environment for the years 2007–2015 [9]. To meet this stringent requirement, chemical filters are often used to reduce the AMC concentration in the cleanroom [2–4]. Mini- environment and SMIF (standard mechanical interface) enclosure are also useful tools to achieve the requirement of cleanliness [10]. The air-pressure differentials between mini-environment space and its surrounding space were found to be very low and yet were effective in maintaining low particle-concentration levels [11]. * Corresponding author. Tel.: þ886 3 573 1880; fax: þ886 3 572 7835. E-mail address: cjtsai@mail.nctu.edu.tw (C.-J. Tsai). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.03.018 Building and Environment 44 (2009) 2319–2326