INFRARED-SPECTROSCOPY AS A PROBE OF FUNDAMENTAL PROCESSES IN MICROELECTRONICS - SILICON-WAFER CLEANING AND BONDING

In this paper, we review our recent infrared studies of the fundamenta l physical and chemical processes occurring al the interface of bonded silicon wafers, as a function of surface preparation and annealing te mperature. We present a brief overview of the practical aspects of sil icon-wafer bonding and the techniques used to evaluate the interface i ntegrity, which highlight the need for fundamental studies of the micr oscopic interface phenomena. Importantly, we show that the interface b etween two silicon wafers approximates an ideal spectroscopic environm ent, in that there is a 28-fold enhancement in the sensitivity to the normal component of the interface absorption over any other surface op tical geometry. Furthermore, the interface region is almost infinitely stable at room temperature, but can exhibit partial pressures ranging from near vacuum to several atmospheres upon annealing. We present re sults for two distinct types of wafer bonding: hydrophobic (hydrogen-t erminated and hydrophilic (oxide-terminated), since the origin of the initial attraction between the opposing surfaces is quite different in the two cases. Specifically, we show that ideally hydrogen-terminated Si(111) surfaces come within a few Angstrom, under the influence of a Van der Waals attraction, as evidenced by a pronounced perturbation o f the isolated Si-H stretch mode. In contrast, the initial attraction between hydrophilic surfaces is via hydrogen bonding, which is mediate d by the presence of 2-4 monolayers of water that are trapped at the i nterface upon room-temperature joining. We demonstrate that vibrationa l spectroscopy provides unprecedented mechanistic insight into the the rmal evolution of the molecular interface, which necessarily has a pro found influence on the bonding process. Throughout the paper, emphasis is given to the need for a wide variety of additional (fundamental) s tudies of the surface phenomena occurring in these novel, technologica lly important systems.