Development of fast atom beam source with bidirectional magnetic field for enhanced surface activated bonding

Surface activated bonding (SAB) is a promising room-temperature bonding technique for fabricating composite wafers used in surface acoustic wave (SAW) filter devices and three-dimensional semiconductor integration, particularly hybrid bonding. In SAB, fast atom beam (FAB) irradiation is employed to remove native oxide layers and activate wafer surfaces under ultra-high vacuum. In FAB sources, Ar gas is internally ionized to form Ar+, which are subsequently neutralized via charge exchange and surface recombination processes before being irradiated as an Ar-FAB to sputter the wafer surface. However, conventional FAB sources suffer from internal erosion caused by Ar+ sputtering, which generates particles, induces bonding defects, and necessitates frequent electrode replacement. Previous studies have shown that applying a bidirectional magnetic field can extend electrode lifetime and improve irradiation performance by steering Ar+ toward the irradiation port through controlled electron trajectories, with the improved efficiency enabling comparable surface activation at lower power. However, these findings were obtained only in large-scale prototypes; scaling down to practical-sized sources introduces spatial and magnetic constraints. To address this, we systematically investigate design factors—anode configuration, magnetic-field strength, and irradiation-port size—through oxide-film removal tests and two-dimensional particle-in-cell Monte Carlo collision simulations. Among the tested conditions, the best-performing configuration achieved irradiation performance exceeding previously reported values. We also found that an excessively strong magnetic field over-confines Ar+ between the anodes, reducing the effective Ar-FAB emission area and thereby degrading irradiation performance—a trend not captured by prior simulation-based discussions. The limitations of the two-dimensional model, including its inability to represent fringe-field effects and non-uniform gas distributions, are discussed, and directions for three-dimensional model development are outlined. These results provide design guidelines for bidirectional-magnetic-field FAB sources and support the further development of SAB-based integration technologies.