Atomistic State of Hydrogen in Electrochemically Deposited Cu Films

Electro and electroless deposition of Cu films are widely used in the print circuit boards production. These electrochemical Cu deposition process are accompanied with the simultaneous hydrogen evolution, and the hydrogen co-deposition sometimes causes the voids and blisters1). However, the detail mechanism of such hydrogen-induced phenomena have not been clarified. We previously reported that the primary cause of the room-temperature grain growth observed in electrodeposited Cu films was the formation of superabundant vacancies induced by the hydrogen co-deposition2). In this study, the atomistic state of hydrogen co-deposited in the Cu films obtained by electro and electroless deposition and the influence of hydrogen on the microstructure of Cu films were investigated. Cu films were electrodeposited from six different baths (sulfate bath, additive-containing bath, chloride bath, pyrophosphate bath, EDA bath, and EDTA bath) and electrolessly deposited from an EDTA bath. The amount and atomistic state of hydrogen in the Cu films were analyzed by thermal desorption spectroscopy (TDS). The microstructural analysis of the Cu films was performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). In the electrodeposited Cu films with high hydrogen concentrations (x = H/Cu = 5.4 ~ 98.2×10-4), the room-temperature grain growth proceeded with the desorption of hydrogen during 7 ~ 21 days after deposition2). A pronounced desorption peak at 600 ~ 700 K due to the break-up of vacancy-hydrogen clusters3) was observed in the thermal desorption spectra of hydrogen from these Cu films. In the thermal desorption spectra of hydrogen from electrolessly deposited Cu films, a pronounced peak at around 420 K and a small peak at 600 ~ 700 K were observed and the hydrogen concentration was x = 13.6×10-3. From the XRD measurements, the lattice expansion of about 0.14% was observed in the as-deposited Cu films, which was reduced with the desorption of hydrogen at room temperature after 7 days. These results suggest that the desorption peak at 420 K may be assigned to the desorption of hydrogen from the regular interstitial sites. In the TEM images of these Cu films, many nano-voids distributed in the crystal grains were observed. We consider that the hydrogen molecules trapped in the nano-voids should be desorbed at high temperature above 900 K. The high-temperature TDS measurements will be performed. References 1) T. Sharma, A. E. Landry, A. Leger, D. A. Brown, T. Bernhard, S. Zarwell, F. Brüning, and R. Brüning, Thin Solid Films, 666, 76 (2018). 2) N. Fukumuro, H. Yoshida, T. Yamazaki, Y. Fukai, and S. Yae, J. Japan. Inst. Met. Mater., 80, 736 (2016). 3) Y. Fukai, M. Mizutani, S. Yokota, M. Kanazawa, Y. Miura, and T. Watanabe, J. Alloy. Comp., 356-357, 270 (2003).