Moisture Prevention in a Pre-Purged Front-Opening Unified Pod (FOUP) During Door Opening in a Mini-Environment

Contamination in a wafer transportation box [e.g., a front-opening unified pod (FOUP)] can influence device yield and performance. Additional precautions might be required to prevent outside contamination during FOUP door opening. This paper experimentally examines moisture evolution in a pre-purged FOUP with an open door in a mini-environment. Air curtains of different widths (WA) were employed to prevent outside moisture intrusion, and their performance was compared with that of a conventional purge method. Clean dry air was used as the supplied gas in experiments. A moisture prevention index (MPI) was further introduced as a tool for interpreting results. When the conventional purge method was used, moisture in the mini-environment was rapidly transferred into the FOUP, and relative humidity (RH) reached approximately the same levels as the mini-environment, indicating poor moisture prevention performance. RH values were much lower when the air curtain was used. The best moisture prevention performance was observed for a system using an air curtain with W
_A
= 50 mm. The MPIs for the conventional purge method were approximately 8.0%-8.4%, whereas those for the air curtain application ranged from 40%-100%. Most importantly, the MPIs for WA = 50 and 70 mm reached or exceeded 90%.