About the importance of purge time in molecular layer deposition of alucone films

Abstract

The deposition rate and properties of MLD films are for a large part determined by what happens during the reactant exposure step. In some cases, however, the purge step is of equal importance, for example in MLD of alucone using trimethylaluminum (TMA) and ethylene glycol (EG). We show that infiltration of TMA into the alucone film followed by its continuous outgassing during the subsequent EG exposure step can lead to undesired CVD effects. To avoid the CVD effects, very long TMA purge times are required which in turn significantly impact the obtainable deposition rates. We also developed a kinetic model that correlates process parameters like reactant partial pressures, exposure times, purge time and deposition temperature to the CVD component in the film growth. We observed that the overall GPC decreases exponentially with TMA purge time attributed to the decreasing CVD component and after a long enough purge time reaches a steady-state value of growth only due to the MLD component. It was also observed that the CVD contributions reduced with decreasing partial pressure of TMA and increasing deposition temperature. With an intention to improve the outgassing efficiency of TMA, the influence of purge gas flow on the CVD growth component is also briefly discussed. Moreover, to mitigate the problem of infiltration, we show that a bulkier substitute of TMA like dimethylaluminum isopropoxide (DMAI) shows no infiltration and can improve the alucone deposition rate by at least an order of magnitude.