An OXO-type inter-poly insulator (where X is a high-K metal oxide and O is an insulative oxide) is defined by forming an amorphous metal oxide layer on a silicon-based insulator (e.g., a silicon oxide layer) and then nitridating at least upper and lower sub-layers of the amorphous metal oxide with a low temperature plasma treatment that maintains temperature below the recrystallization temperature of the amorphous material. Such a plasma treatment has been found to improve breakdown voltage characteristics of the insulator. In one embodiment, the metal oxide includes aluminum oxide and it is fluorinated with low temperature plasma prior to nitridation.