Effect of Pre-bonding Thermal Treatment on the Bonding Interface Evolution in Direct Si-Si Hydrophilic Wafer Bonding

Direct silicon or silicon dioxide wafer bonding appears as a more and more attractive way to build up stacking structures in micro-technologies. For this, high bond strengths and void-free bonding interfaces are required. Although Si-Si hydrophilic bonding provides suitable bonding energies after low temperature annealing (T<400°C) [1], it reveals also an important defectivity. Therefore, different pre-bonding processes have been tested in order to reduce the thermally generated voids without degrading the bonding energy. For instance, vacuum and hot vacuum bondings have been investigated, but the bond strength was too much affected [2]. Also, plasma activation treatments have been extensively studied prior to direct bonding because they enable one to get both high bonding energies and reduce the interface defects [3]. However, a number of bonding mechanisms remains partially unexplained. In this work, we have studied another type of pre-bonding treatment in order to improve the bonding qualities and complete the understanding of bonding mechanisms. In the hydrophilic Si-Si bonding, the wafers are covered by a native silicon oxide film formed during the cleaning treatment. The native oxide films have hydrophilic properties due to the presence of silanol groups (Si-OH). Under standard bonding conditions (cleaning + bonding in room atmosphere), few monolayers of water are trapped at the bonding interface. We know that this water is the main source of thermally generated voids during the post-bonding annealing treatment [4]. Therefore, one method to reduce the defectivity created by excess water could be to better dry the wafer surfaces before bonding. In this paper, we have studied a basic way to dry silicon wafer surfaces, which is the thermal treatment.