Impurity Drift and Variations in High-Purity Gas Delivery Systems

Junsheng Wu, Farhang Shadman


Moisture in process and inert gases, even in parts-per-billion (ppb) levels, can affect many semiconductor manufacturing processes and fabrication plants (fabs). Additionally, slight variations in moisture level can drastically offset the calibration of various process and result in loss of performance. Therefore, it is critical to minimize moisture level and at the same time provide a stable purity in the gas delivery systems. In this study, a process model is developed to characterize different mechanisms that affect the moisture level at the Point-of-Use (POU) for high purity gases. In particular, the dynamics and the relative contributions of the two major sources of drift in the Ultra-High Purity (UHP) gas-delivery systems are analyzed: a) the moisture build-up and drift in the concentration delivered from cryogenic sources, and b) the moisture adsorption and desorption on the surfaces of the pipes and other components of the gas delivery systems. A method, based on experimental data and comprehensive process modeling, is developed and used to investigate the effects of system physical properties (size, configuration, and surface properties) and operating conditions (initial gas purity, flow rate, pressure, temperature, and the changes in these properties with time) on moisture variations at the manufacturing POU.

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