Characterization of bubbler performance for low-volatility liquid precursor delivery

The performance of a bubbler to deliver the low-volatility, liquid cobalt precursor mu(2)-eta(2)-(Bu-t-acetylene) dicobalthexacarbonyl (CCTBA) for reduced-pressure chemical vapor deposition and atomic layer deposition processes was characterized. A relatively large process window was investigated by varying carrier gas flow rate, system pressure, and bubbler temperature. For this range of conditions, the CCTBA partial pressure was measured using a custom-designed nondispersive infrared gas analyzer, and the CCTBA flow rates were derived from these partial pressure measurements. The dependence of CCTBA flow rate on these process parameters was modeled to obtain a deeper understanding of the factors influencing bubbler performance. Good agreement between measured and modeled CCTBA flow rates was obtained using a model in which the pressure drop between the bubbler head space and the pressure measurement location was included and in which a constant CCTBA partial pressure in the bubbler head space for a given bubbler temperature was assumed. The dependence of CCTBA head space partial pressure on temperature was parameterized in the form of the August equation, which is commonly used to describe the temperature-dependence of vapor pressure. While this report was focused specifically on CCTBA, the results of this study indicate that this method for estimating the precursor delivery rate from a bubbler should be applicable to other low-volatility, liquid precursors.