Design and operation of reactive gas delivery systems for IC fabrication plants

This article describes equipment design criteria and equipment operation for ultra-high purity (UHP) reactive gases. Selection of material and proper operation are critical for consistent and reliable gas delivery. Corrosion and subsequent contaminants are detrimental to downstream processes. The HBr case is used to illustrate the influence of moisture content on HBr corrosivity toward common materials of construction. Proper drydown and system operation at lower pressure to avoid vapor condensation alleviate the corrosion problem and maintain reliable gas delivery. We propose a drydown model to predict the drydown time, and perform experiments to confirm the predicted drydown time. The moisture is modeled as a chemisorbed monolayer on equipment and several physisorbed layers of water molecules on top of the chemisorbed layer. Addition of moisture not only aggravates corrosion, but also reduces the saturation pressure and causes vapor condensation. A previous study shows that higher operating pressure results in more severe corrosion. A better practice is to operate the system much below the saturation pressure at room temperature. This allows a reasonable safety margin. The gas system manifold is discussed, and basic heat transfer and fluid dynamics models are used to examine the system. It is shown that with proper design and operation, steady delivery can be maintained over a long distance.