Dr. Mizuno received his PhD in engineering from Tohoku University, where he conducted research on silicon capacitive sensors that can simultaneously detect acceleration and angular acceleration. Since then, he has been conducting research on organic EL using liquid semiconductors, SAW devices using low-temperature bonding technology, 3D electronics packaging technology, low-damage surface treatment machines, and recently, implant materials and regenerative medicine of epithelial mucosa in collaboration with medical engineering. During his three years at NCKU, he conducted collaborative research projects with various companies and institutions, including AGC, ASE, ASML, Bosch, Canon, Creative Coating, ITRI, Lintec, NIPRO Corporation, Nissan Chemical, NSTC Project, Okuno Chemical Industry, Omron, Toray, and Toray Engineering.

Surface cleanliness is critical for Cu interconnection because mechanically prepared Cu rapidly acquires adventitious hydrocarbons, oxygen-containing species, adsorbed moisture, and native oxide, modifying the chemistry and wettability of the outermost interface. Vacuum-ultraviolet (VUV) irradiation offers a dry, non-contact route for organic removal without wet-chemical residues. A Xe₂ excimer source emitting at 172 nm corresponds to a photon energy of approximately 7.21 eV, exceeding representative dissociation energies of common C-C, C-H, C-O, and O-H single bonds in organic contaminants [1,2]. This work investigates time-dependent VUV activation of Cu coupons using X-ray photoelectron spectroscopy (XPS) and water contact-angle measurements.

As illustrated in Figure 1, VUV absorption can initiate electronic excitation, bond scission, radical/fragment formation, secondary fragmentation, and desorption of low-molecular-weight products. Oxygen-assisted reactions may additionally contribute when O₂ or H₂O is present; therefore, loss of carbonyl- or carboxyl-related XPS species is interpreted as the overall outcome of fragmentation rather than direct cleavage of every C=O bond. Polished Cu samples were irradiated at 172 nm for 0, 5, 10, 15, and 20 min at approximately 1 mm working distance and 17.03 mW cm⁻² irradiance. Surface chemistry was analysed by Al Kalpha XPS using C 1s, O 1s, Cu 2p, and Cu LMM spectra.

The untreated polished Cu surface contained approximately 19.3 at.% Cu, 24.7 at.% O, and 56.1 at.% C, giving a C/Cu ratio of 2.91. After 20 min VUV irradiation, carbon decreased to 5.9 at.% while Cu increased to 64.9 at.%, reducing C/Cu to 0.091; this was the lowest detected carbon concentration within the investigated range. C 1s initially showed a dominant C-C/C-H component near 284.8 eV with oxygenated-carbon components near 286.3, 287.8, and 288.5 eV. Their progressive attenuation is consistent with VUV-induced bond scission, fragmentation, and removal of the adventitious carbonaceous overlayer. The increased Cu signal reflects reduced photoelectron attenuation from the underlying Cu/Cu-oxide surface rather than generation of additional metallic Cu.

O 1s spectra (Figure 3) further distinguish organic cleaning from oxide removal: higher-binding-energy contributions associated with adsorbed or organic oxygen decreased, whereas a Cu-O-related contribution remained. Thus, the present treatment is primarily an organic-contaminant removal and surface-activation process rather than complete native-oxide removal; oxide reduction under 172 nm irradiation depends strongly on the surrounding atmosphere [3].

Because Cu⁰ and Cu⁺ cannot be reliably distinguished using Cu 2p alone, Cu LMM was used for additional chemical-state sensitivity [4]. The contribution near 916.8 eV after prolonged irradiation is persistence of a Cu(I)/Cu₂O-rich near-surface state. Meanwhile, water contact angle decreased from 76.3 deg to 30.6 deg after 20 min (Figure 4), consistent with removal of a hydrophobic hydrocarbon-rich overlayer and increased surface wettability.

172 nm VUV irradiation effectively removed adventitious organic contamination from mechanically prepared Cu. After 20 min, carbon decreased from 56.1 to 5.9 at.% and C/Cu from 2.91 to 0.091. C 1s evolution supports photon-induced excitation, bond scission, fragmentation, and desorption, while O 1s and Cu LMM indicate persistence of a Cu(I)-rich oxide. The concurrent reduction in contact angle confirms increased surface wettability, establishing 172 nm VUV as a dry photochemical surface-activation method for Cu interconnect surfaces.

1. N. Shirahata, K. Oda, S. Asakura, A. Fuwa, Y. Yokogawa, T. Kameyama and A. Hozumi, J. Vac. Sci. Technol. A 22, 1615-1619 (2004), DOI: 10.1116/1.1692318
2. B. Ruscic, J. Phys. Chem. A 119, 7810-7837 (2015), DOI: 10.1021/acs.jpca.5b01346
3. J.-X. Li and A.-H. Liu, J. Electron. Mater. 40, 2105-2110 (2011), DOI: 10.1007/s11664-011-1714-x
4. M. C. Biesinger, Surf. Interface Anal. 49, 1325-1334 (2017), DOI: 10.1002/sia.6239