A medium-k Metal-Insulator-Metal (MIM) capacitor based on a 11 nm thick ZrO2/Al2O3/ZrO2 dielectric layer stack was performed using Atomic Layer Deposition (ALD) in silicon trenches with a high aspect ratio of 40 to enhance capacity density. The two electrodes were made of TiN using NH3 and TiCl4 as precursors while the insulating layer was made of Al2O3 and ZrO2 deposited using O3 and respectively Al(CH3)3 and ZrC5H5(N(CH3)2)3. Spectral ellipsometry and X-ray reflectometry were performed to characterize the ALD windows of TiN, Al2O3 and ZrO2 processes. They were found to be respectively between 210 and 310°C, 210 and 310°C and 390 and 450°C. Scanning electron microscope (SEM) images of trench cross-sections show conformal TiN, Al2O3 and ZrO2 layer deposition with non-uniformities respectively lower than 5%, 1% and 3%. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) demonstrated a fully crystallized ZrO2 layers in a cubic structure and amorphous Al2O3 layers. The effects of reducing the thickness of the Al2O3 layers in the ZrO2 stack were also studied to evaluate the evolution of electrical characteristics of the MIM stack. As the thickness of the Al2O3 layer decreased from 1.5 nm to 0.2 nm, a capacitor value increasing from 17.2 to 23.6 nF/mm² was measured, while, in the meantime, the breakdown field decreased from 4.7 to 3.8 MV/cm. Moreover, a study of the conduction mechanisms showed that a layer of 1.5 nm thick Al2O3 reduced the Fowler-Nordheim current and consequently decreased the total leakage current from 101 nA/cm² to 7 nA/cm² at 2.1 V (compared to stacks comprising 0.2 nm thick Al2O3 layer). Time-dependent dielectric-breakdown measurements were also carried out to evaluate the reliability resulting from thinning of the Al2O3 layer in the MIM capacitor.