All Nanograined Copper Is Not Created Equal

Enabling robust low-temperature Cu-to-Cu bonding is essential for future semiconductor growth because it promises to play a pivotal role in heterogeneous integration. Although multiple approaches have been proposed and investigated extensively over the years, a robust, low-temperature bonding process is still missing from high volume manufacturing (HVM) offerings. Recent work has suggested that it is desirable and also possible to obtain nanograined copper deposits [1-5]. Compared with large-grained copper, the nanograined copper was shown to bond at significantly lower temperatures such as 180°C and 150°C [2], [3], [4] with good bonding strength. Since a stable nanograined microstructure is prerequisite for Cu-to-Cu hybrid bonding, we carried out a systematic study with the goal to understand the key factors that influence nano grain growth and its stability. In this paper, we demonstrate how plating additives, plating parameters, substrates, and annealing conditions influence grain growth as well as final microstructure stability. We will attempt to delineate the relationship between bonding temperature and nanograin size. In addition, we shall show that there is a close correlation between nanograin size and self-anneal behavior, which is absent when nanograin is smaller than a threshold. This is significant as it may be one of the most important factors that enables the HVM implementation of Cu-to-Cu low-temperature bonding.