Analysis of vacancies in wafer-bonding interface via positron annihilation lifetime spectroscopy

Plasma-activated bonding is employed in stacked device manufacturing processes. It has been verified that the bonding strength increases as the thermal expansion of the oxide films used for bonding increases. The proposed mechanism indicates that the thermal expansion during the heat treatment fills the small gaps that exist at the bonding interface because of the surface roughness. This results in an increase in bonding sites. However, these small gaps have not been observed directly. Therefore, this study aimed to detect the small gaps corresponding to the bonding interface by measuring the bonding samples using positron annihilation lifetime spectroscopy (which has a high sensitivity for detecting small vacancy-type defects) to elucidate the mechanism of bonding strength development. Previous studies have revealed that surface moisture affects the bonding strength. Moreover, investigations were conducted after bonding under both atmospheric and vacuum conditions. These studies verified that the detection of bonding interface gaps using positron annihilation lifetime spectroscopy is feasible when the surface moisture is low. However, interface gaps filled by surface moisture are not detected as vacancies. Furthermore, evaluating the distribution of vacancies within the film using positron annihilation lifetime spectroscopy clarified the mechanism of vacancy formation owing to surface and internal moisture and the subsequent increase in bonding strength.