Effect of annealing in processing of strontium titanate thin films by ALD

SrTiO3 thin films were prepared from Sr(thd)2 (thd=2,2,6,6-tetramethyl-3,5-heptanedionate) and titanium isopropoxide (Ti(O-i-Pr)4) on Si(1 0 0) substrates by atomic layer deposition (ALD) followed by rapid thermal annealing. In order to optimize the growth parameters for the ternary oxide growth, binary films were deposited at first from Sr(thd)2 using ozone as an oxygen source. The resulting films were identified as crystalline SrCO3 by XRD. A self-limiting growth mechanism of SrCO3 was observed at 280–410 °C with a constant growth rate of 0.53 Å per cycle. The effect of reactant pulsing times was investigated at 325 °C confirming the surface-controlled deposition mechanism. SrTiO3 thin film depositions were then performed at 290 and 325 °C by combining the strontium process with the titanium dioxide ALD process developed earlier. The resulting films were analyzed by XRD, RBS, XRF, FTIR, XPS and AFM to determine film crystallinity, stoichiometry, possible impurities and surface morphology, respectively. As-deposited mixed oxide films were amorphous containing a carbon impurity, present probably as SrCO3. Nevertheless, crystalline SrTiO3 phase was obtained by an RTA treatment at 650–900 °C and stoichiometric films resulted from using a Sr:Ti pulsing ratio of 1:1. The film deposition rate was also found to be greatly dependent on the pulsing ratio of the Sr and Ti precursors. Apparently the TiO2 surface was inhibiting the formation of the subsequent Sr layer.