This study performs surface reduction treatments, including hydrogen plasma treatment and rapid thermal anneal (RTA) in hydrogen ambient, to reduce the nitrogen content in the surface layer of the TaN x ultrathin film deposited by plasma-enhanced atomic layer deposition (PE-ALD). A four-point bend delamination test and a pull-off tensile test are used to study the interfacial strength of the PE-ALD thin film with copper. According to X-ray photoelectron spectroscopy and Auger electron spectroscopy, a new chemical phase with a very small nitrogen content, possibly Β -TaN x , is formed on the PE-ALD TaN x thin film after the RTA treatment. The increase in the Ta/N atomic ratio in the RTA-treated TaN x thin film significantly improves the adhesion of the TaN x film with the sputter-deposited Cu layer. However, the hydrogen-plasma-treated TaN x thin film shows a slight decrease in nitrogen content but still demonstrates better adhesion with the Cu layer compared with the as-deposited one. While the Cu overlayer on the RTA-treated PE-ALD TaN x thin film can sustain the thermal anneal at 600°C, the one on the as-deposited TaN x thin film exhibits voiding even at a temperature as low as 400°C.