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Comparison of saturator designs for low volatility liquid precursor delivery



James E. Maslar, William A. Kimes, Vladimir B. Khromchenko, Brent Sperling, Ravindra Kanjolia


Low volatility precursors are widely utilized in chemical vapor deposition and atomic layer deposition processes. Such precursors are often delivered from one of two common saturator designs: a bubbler or a flow over vessel. Previous reports concerning precursor delivery from such vessels have focused primarily on continuous delivery of moderate to high volatility liquids and solids. Few reports have focused on cyclical delivery of low volatility precursors at reduced pressures. This lack of knowledge concerning such processes can be a hindrance to efficient selection of deposition conditions and vessel design. The objective of this investigation was to compare the performance of these two saturator designs for pulsed injection at reduced pressures using the low volatility liquid precursor μ2-η2-(tBu-acetylene) dicobalthexacarbonyl (CCTBA). The basis of this comparison was the measurement of CCTBA mass carryover per injection as a function of injection number, injection time, carrier gas flow rate, system pressure, and vessel idle time. The mass carryover was determined from absorbance measurements performed using a non-dispersive infrared gas analyzer. The measured mass carryover for both vessels was compared to the theoretical mass carryover determined using a simple analytical model based upon the "bubbler equation". In the case of the bubbler, this model described the vessel performance well with knowledge of the precursor vapor pressure and vessel head space pressure. In the case of the flow over vessel, this model described the overall vessel performance poorly unless an additional vessel efficiency factor was included, a factor that is difficult to predict a priori. Furthermore, the efficiency factor was not necessarily constant for a series of injections: the efficiency factor tended to decrease from the first injection until a stable value was achieved, a value that depended on the process conditions. This limitation of the model was attributed to the specific flow dynamics associated with the flow over vessel design. Computational fluid dynamics simulations were able to reproduce the mass carryover of the flow over vessel, after estimating the CCTBA-argon binary diffusion coefficient. These simulations also showed that a larger binary diffusion coefficient and a higher vapor pressure both led to an increase in mass carryover but vessel efficiency could not equal that of the bubbler. While these results were obtained with CCTBA, the general relationships between mass carryover and the various process parameters in these saturators are expected to be similar for other low-volatility precursors.
Journal of Crystal Growth


ALD, atomic layer deposition, bubbler, CVD, chemical vapor deposition, flow over vessel, non-dispersive infrared gas analyzer, NDIR, precursor delivery


Maslar, J. , Kimes, W. , Khromchenko, V. , Sperling, B. and Kanjolia, R. (2023), Comparison of saturator designs for low volatility liquid precursor delivery, Journal of Crystal Growth, [online],, (Accessed February 29, 2024)
Created April 1, 2023, Updated March 16, 2023