PHYSICS AND CHEMISTRY OF SILICON-WAFER BONDING INVESTIGATED BY INFRARED-ABSORPTION SPECTROSCOPY

Silicon wafer bonding is achieved by joining two particle-free silicon wafers and annealing to elevated temperatures (similar to 1100 degree s C). We have used multiple internal transmission infrared absorption spectroscopy to probe the interface between the wafers upon initial jo ining and also during subsequent annealing steps. For atomically flat hydrophobic wafers (H passivated), we observe a pronounced shift in th e Si-H stretching frequency due to the physical interaction (van der W aals attraction) that occurs when the surfaces come into intimate cont act. The hydrogen eventually disappears at high temperatures (1000 deg rees C) and Si-Si bonds are formed between the two surfaces. For hydro philic wafers (oxide passivated), we initially observe three to five m onolayers of water at the interface (providing the initial attraction through H bonding), as well as the presence of hydroxyl groups that te rminate the oxide at low temperature. Upon moderate heating (< 400 deg rees C), the water trapped at the interface dissociates and leads to t he formation of additional oxide. Between 400 and 800 degrees C, the h ydroxyl groups disappear, resulting in a corresponding increase in oxi de and the formation of Si-O-Si bridging linkages across the two surfa ces. (C) 1996 American Vacuum Society.