Ozone-based atomic layer deposition of alumina from TMA: Growth, morphology, and reaction mechanism

We examine the effect of growth temperature in the 150-300 degrees C range on the structural and morphological properties of Al2O3 films deposited using atomic layer deposition, contrasting the effect of H2O and O-3 as oxygen sources. Trimethylaluminum (TMA) is the metal source. A mechanism for the O-3 reaction is investigated using ab initio calculations and provides an explanation for the observed temperature dependence. The simulations show that hydroxyl groups are produced at the surface by the oxidation of adsorbed methyl groups by O-3. This is confirmed by the measured rates; both H2O and O-3 processes show molar growth rates per cycle that decrease with increasing reactor temperature, consistent with a decrease in hydroxyl coverage. At no temperature does the O-3 process deposit more Al2O3 per cycle than the H2O process. Morphological characterization shows that O-3 as the oxygen source yields lower-quality films than H2O; the films are less dense and rougher, especially at low growth temperatures. The existence of voids correlates with the low film electronic density. This may indicate the low mobility of surface hydroxyl at low temperatures, an effect that is washed out by repeated exchange with the vapor phase in the H2O case.