Contamination Control for Gas Delivery from a Liquid Source in Semiconductor Manufacturing
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Gas delivery from a liquid source, common in semiconductor manufacturing, raises contamination control concerns not only due to impurity levels in the source. In addition, the lower vapor pressure of impurity species compared to that of the host (source) species causes impurity concentrations in delivered gas to increase as the source is used up. A physics-based dynamic simulator to describe the time-dependent variation of impurity level in such a gas delivery system has been developed and applied to important case of CHCIF2 impurities in host CHF3 liquid, as routinely used for dry etching processes. For a cylinder of CHF3 liquid with 100 ppm of CHCIF2 at 21.1o C (70o F), the concentration of CHCIF2 in the delivered gas is initially ~ 21 ppm, and rises slowly to ~ 100 ppm with ~ 25% of the initial material remaining. With further usage, the CHCIF2 level increases quickly to ~ 350 ppm when ~ 15% of the initial source material is left; at this point, the source has reached the liquid-dry point, i.e., all the remaining source material is gaseous, and the impurity concentration in delivered gas remains constant at 350 ppm until all material is gone. The time- dependence of CHCIF2 impurity concentration is also dependent on the operating temperature of the liquid source: for higher temperatures, the fast rise in impurity concentration and the liquid-dry point occur earlier, while the final impurity level after this point is lower. The dynamic simulator represents a useful tool for avoiding contamination problems with liquid delivery systems and for optimizing materials usage (for cost and environmental benefits) by structuring source usage procedures consistent with contamination-sensitivity of the process. The results also suggest benefits in materials usage if specific source temperatures (different from room temperature) were imposed. The physical basis of the dynamic simulator allows more general application to other systems.