Study of low-temperature direct bonding of (111) and (100) silicon wafers under various ambient and surface conditions

摘要

Wafer bonding of commercially available (100) and (111) silicon wafers was performed in the range of temperatures from 80℃ to 400℃ in nitrogen, oxygen and low-vacuum atmosphere. Surface preparation with modified RCA cleaning method and hot nitric acid provided extremely clean and hydrophilic surfaces that were later brought into intimate contact. Various combinations of surface terminations such as thin chemical native silicon dioxide and thick thermal silicon dioxide on (111)- and (100)-oriented silicon wafers were prepared and investigated. Bonding quality evaluated by the tensile strength measurements showed the highest values obtained for strengthening in nitrogen atmosphere, reaching 12 MPa. Correlation between prebonding treatment, initial surface roughness and microroughness was made revealing the influence on the bonding energy. Interface imperfections of bonded samples were investigated by infrared transmission imaging revealing bubbles at the bonded interface only in case when bonding was performed in oxygen ambient. Thickness of the chemical native oxide after surface preparation step necessary for good bonding was found to be at least 0.8 nm. Wafers in the (111) orientation exhibited higher bonding abilities compared to (100) in case of bonding wafers with native oxides. This is believed to be due to higher density of available bonding sites as a consequence of enhanced chemical oxide growth rate and its homogeneity on (111) surface. Moreover, it is believed that due to higher positive charge of oxides grown on (111)-oriented silicon compared to (100), desorption of interfacial water is accelerated thus increasing the bonding energy at lower temperature. In conclusion, the best bonding results were obtained by bonding wafers with thick thermal oxide to (111) wafers with native oxide and annealed in nitrogen ambient.