We examined the formation of ultra-thin ferroelectric ZrO2 films on the TiN bottom electrode using a sputtering method in conjunction with post oxidation treatment. We found that high-temperature sputtering and post plasma oxidation at room temperature successfully forms ferroelectric ZrO2 films. Furthermore, we demonstrated the formation of ferroelectric ZrO2 in the ultra-thin region down to 6 nm. Finally, we discussed the possibility of further improvement of the ZrO2 ferroelectricity.