Co-sputtered Cu(Ti) films on SiO2 substrates exhibit the phase separation of Cu and Ti after annealing at 400 °C. In this study, the diffusion behavior and phase separation were investigated using X-ray photoelectron spectroscopy (XPS), grazing angle incident X-ray diffraction (GIXRD), and transmission electron microscopy (TEM). The GIXRD pattern revealed that the Cu peak replaced the Cu2O peak after annealing. The formation Gibbs free energy of TiO2 (−812 kJ) was lower than that of Cu2O (−109 kJ). The calculated diffusivity of Ti in Cu was one order of magnitude smaller than that of Cu in Cu. Thus, Ti was the dominant diffusing species that accumulated near SiO2, whereas Cu atoms were segregated near the surface. A few Cu islands were initially formed, and these Cu islands grew and formed continuous Cu layers after long-term annealing at 400 °C. The Cu(Ti) films were bonded face-to-face through thermo-compression. The nearly oxide-free Cu yields strong Cu bonding and a void-free interface.