Characterization of vapor draw vessel performance for low-volatility solid precursor delivery

Low volatility precursors are widely utilized in chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. Compared to gases and high volatility liquid precursors, delivery of low volatility liquid and solid precursors can be problematic, with solid precursors being particularly so. To investigate some of these delivery issues, the performance of a vapor draw vessel was characterized for the delivery of pentakis(dimethylamido) tantalum (PDMAT), a low-volatility solid precursor at preferable delivery temperatures, for reduced-pressure cyclical CVD and ALD processes. Vessel characterization involved determining (1) a source efficiency as a function of process conditions and (2) the degree of PDMAT decomposition as a function of temperature and vessel idle time. The PDMAT partial pressure, flow rate, and mass per injection used to determine the source efficiency were determined from measurements obtained using a custom-designed non-dispersive infrared gas analyzer. For a series of injections after an idle/purge sufficiently long to saturate the vessel head space, the source efficiency decreased from a maximum slightly less than unity for the first injection until a consistent value was reached that was approximately one half to one third of the maximum value. A comparable trend was observed for mass delivered per injection. For the conditions used in this investigation, the source efficiency decreased when the injection time was increased to longer than 1 s, when pressure was decreased, and when the carrier gas flow rate was increased. Although the corresponding mass per injection increased with these changes, the increase in mass was less than that predicted had the carrier gas been saturated. The source efficiency did not depend strongly on temperature and only moderately on vessel idle durations (4–16 s). The degree of PDMAT decomposition was evaluated by measuring the partial pressure of dimethylamine (the primary PDMAT decomposition product under the conditions of this investigation) using the same gas analyzer. For a given idle time, the amount of dimethylamine delivered more than doubled as vessel temperature was increased from 68 to 78 °C.