Bonding Performance and Interface Contamination in Atomic Diffusion Bonding of Wafers Using Ti Thin Films

Room-temperature bonding performance of wafers using thin Ti films was examined as a function of waiting time t w in vacuum between film deposition and bonding. The surface free energy at bonded interface γ decreased as t w increased. However, results indicated that a γ value greater than 6.1 J m −2 is obtainable even at t w of 1 h or more at vacuum chamber base pressures lower than 1 × 10 –6 Pa. Bonding apparatuses of three types with different vacuum base pressures were used for this study, but γ values obtained using the equipment showed an almost quantitatively identical reduction as the calculated total amount of residual gas adsorption on Ti film surfaces, G a , increased. This finding indicates that the interface contamination evaluated as G a played a dominant role in determining the γ reduction. Structural analysis revealed that the formation of reactive Ti–O layers on film surfaces prevented the crystal lattice rearrangement between Ti film surfaces, thereby leading to γ reduction. A few Langmuir of H 2 O adsorption prevented the crystal lattice rearrangement to a considerable degree. Moreover, surface roughness reduction of Ti films was effective at maintaining high bonding performance for a long waiting time.