Surface Activation and Planarization with Gas Cluster Ion Beam for Wafer Bonding

Abstract

Surface activated bonding (SAB) has been widely used for various semiconductor wafers. In the SAB process, atomic Ar ions bombard wafer surface to form activated surface. In order to form activated surface on materials which are susceptible to irradiation damage, we have been investigating the gas cluster ion beam (GCIB) as an alternative to atomic ion beams. Gas clusters are aggregates of several thousands of gaseous atoms. Therefore, energy per atom of a gas cluster ion can be easily reduced to several eV. Although energy/atom of a gas cluster ion is low, thousands of low-energy atoms bombard a target surface at the same location simultaneously. As a result, the energy density on the target surface became extremely high and non-linear collisions of atoms are induced. They induces unique irradiation effects of GCIB such as high-yield sputtering, surface analysis of organic materials, thin film deposition at low temperature, and surface smoothing, which are beneficial for surface activated bonding. In this paper, preliminary irradiation effects of surface activation and planarization with glancing incidence GCIBs will be reported. From in-situ XPS analysis of O1s peak intensity on Si(100) with native oxide, Ar-GCIB irradiation at 70o removed native oxide efficiently. On the contrary, mixing of native oxide by normal incidence GCIB might occur, which caused residual oxide layer formation. In addition, the surface roughness at oblique incidence showed lowest value (Ra < 0.5 nm) without ripple formations. Cross-sectional TEM observation with oblique incidence GCIB irradiation showed that the sub-surface damage was almost negligible. These results indicated the possibility of use of GCIB as low-energy ions for wafer bonding.