It has been discovered that poor TDDB reliability of microelectronic device capacitors with organic polymer material in the capacitor dielectric is due to water molecules infiltrating the organic polymer material when the microelectronic device is exposed to water vapor in the operating ambient. Water molecule infiltration from water vapor in the ambient is effectively reduced by a moisture barrier comprising a layer of aluminum oxide formed by an atomic layer deposition (ALD) process. A microelectronic device includes a capacitor with organic polymer material in the capacitor dielectric and a moisture barrier with a layer of aluminum oxide formed by an ALD process.