Reducing airborne molecular contamination by efficient purging of FOUPs for 300-mm wafers - The influence of materials properties

The control of airborne molecular contamination (AMC) plays an increasing r ole in semiconductor manufacturing processes. A method to reduce AMC is pur ging of wafer boxes with inert gas. In this study, data on the practicabili ty and optimization of purging a front opening unified pod (FOUP), a wafer box for 300-mm wafers, are presented. Different parameters for the purge pr ocess are evaluated experimentally. Key values for the assessment of effici ency are the time-dependent content of oxygen and humidity in the FOUP. The increase in the key values after the purge was measured and the constructi on of the FOUP was modified in order to obtain sufficient tightness, Spatia lly resolved measurements reveal the homogeneity of the purge. Experimental data are compared to data obtained by a simulation using a computational f luid dynamics program. Values for oxygen are in agreement with the calculat ed curves. In contrast to this, an additional, long-lasting contribution th at was not taken into account in the simulations makes depletion of humidit y slower than expected. This contribution is explained with the desorption and permeation of humidity through the plastic walls of the FOUP. The prese nce of both effects, desorption and permeation, is proved and quantified. M aterials properties turn out to heavily affect purge effectiveness and the postpurge ingress of certain contaminants in a wafer box.