M. Reiche and T. Wilhelm
Hydrophillic wafer bonding
Hydrophilic wafer bonding of silicon wafers was first discussed in 1986 [1,2]. Results on the adhesion mechanisms on silicon and other materials refer to the strong influence of OH groups on the surface. A model for hydrophilic wafer bonding was first described by Stengl et al.  using the analogy of surface chemistry of silica and oxidized silicon. Based on results of infrared spectroscopy, a 3-dimensional hydrogen bonded network of water molecules was assumed. The water is primarily bonded via Si–OH groups on the silica surface. During heating above 180 °C the adsorbed water molecules desorb under atmospheric pressure leaving a hydroxylated silica surface, on which most of the SiO groups are linked via hydrogen atoms. Annealing at high temperatures causes the formation of Si-O bonds via the interface (Fig. 2).
Hydrophobic wafer bonding
When the oxide layer from a crystalline silicon substrate is removed with HF, a hydrophobic surface with unique properties is obtained, i.e., having a good resistance to chemical attacks and a low surface recombination velocity, which means a surface with a very low density of surface states. The etching of the oxide is assumed to be a 2-step process. First, most of the oxide layer is rapidly dissolved in HF, forming SiF62– ions in solution. In the second step, anodic dissolution of the last monolayer of oxidized silicon (Sin+ with n = 1, 2, 3) occurs, resulting in a hydrogen-passivated surface.
Detailed analyses of interfaces of bonded hydrophobic wafers were carried out by Bengtsson and Engström in 1989 . A first concept for hydrophobic wafer bonding was presented by Bäcklund et al. [5,6] suggesting van der Waals forces as the origin of the attraction forces. Further investigations assume the formation of hydrogen bonds via Si–F groups on the hydrophobic surface [7,8].
The behaviour of the interface energy on the annealing temperature is quite different for bonded hydrophobic wafers (Fig. 3). The interface energy is nearly constant for annealing temperatures up to 150 °C. At higher temperatures the interface energy increases. There are 2 different regimes. For 150 °C ≤ T ≤ 300 °C the increase of the interface energy is characterized by an activation energy of 0.21 eV, while an activation energy of 0.36 eV was determined for annealing at higher temperatures . Both activation energies correlate to different interface processes. Si-Si bonds are formed via the interface after annealing at high temperatures (Fig. 4).
Fig. 2. TEM cross-sectional image of a hydrophyllical bonded wafer pair
Fig. 3. Dependence of the interface energy on the annealing temperature for hydrophobic and hydrophilic wafer bonding 
Fig. 4. TEM cross-sectional image of a hydrophobically bonded wafer pair
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