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Direct wafer bonding is becoming more attractive to achieve heterostructures,(such as Silicon-on-Insulator and Germanium-on-Insulator)as it allows different materials to be stacked together without concern of the structure of different materials.[1] In the bonding process,a lower bonding temperature is needed to minimize the thermal stress in the final substrates.[2] However,at low temperature bonding annealing,the bonding strength also reduces.To solve this problem,improving the wafer surface hydrophilicity has been proved to be effective.[3] In this study,the N2,O2 and Ar plasma treatments were performed to show the impact on the Ge surface hydrophilicity,as a measure of contact angle.We found that the hydrophilicity was better after O2 and Ar plasma exposure than N2 plasma.A thin GeOx film was formed after O2 and Ar plasma exposure,while a GeOxNy film was formed after N2 plasma.Considering the hydrophilicity of GeOx film is greater than GeOxNy film,the properties of transition layers formed by plasma exposure were responsible for the Ge surface hydrophilicity.Furthermore,an additional diluted NH-4OH rinsing(NH4OH:H2O=1:10 for 30s)just after plasma treatments was performed for the improvement of surface hydrophilicity.Finally,a flat and high hydrophilicity(contact angle reduced from 24.7° to smaller than 3°)Ge surface was achieved,that was significative for the Ge wafer hydrophilic bonding.