Atomic layer deposition (ALD) of Al₂O₃-inserted SrTiO₃ (STO) dielectric thin films were investigated in metal-insulator-metal capacitors for dynamic random access memory. STO thin films exhibit much higher dielectric constant compared with currently used ZrO₂-based films, but the leakage current density is generally higher due to their low band-gap. To decrease the leakage current density, insertion of the ALD-Al₂O₃ could be adopted as for the current ZrO₂-Al₂O₃-ZrO₂ dielectric layer. One ALD cycle of the Al₂O₃ film was performed during the ALD of the bottom region of the STO film. As a result, the leakage current of the STO thin film with a thickness of 8nm decreased from 8.1 X 10^-6 A/cm² to ~2.8 X 10^-8 A/cm² at 0.8V, whereas the equivalent oxide thickness (EOT) of the (top) RuO₂/STO/Ru (bottom) capacitor slightly increased from 0.8nm to 0.9nm. Al₂O₃ can exhibit a sufficient leakage current reduction effect at a small insertion ratio, but the increased Al portion deteriorates the dielectric constant and leakage current due to the inhibited crystallization. The lower crystallization during the ALD requires higher annealing temperature, which caused the nano- or micro-cracks in the film and consequent increase in the leakage current. Therefore, careful optimization of the Al insertion was performed to improve the leakage current performance while maintaining the high dielectric constant.