Invited: Water Stress Corrosion in Bonded Structures
Authors: Fournel Frank , Martin-Cocher Chloé , Radisson Damien , Larrey Vincent , Beche Elodie , Morales Christophe , Delean Pierre-Antoine , Rieutord François , Moriceau H. 
Published: February 2020

Direct bonding is now a well-known technique to join two flat surfaces without any additional material. This technique is used in many application and especially in SOI (Silicon on insulator) elaboration or in some backside imager manufacturing process which is now almost a mass production process. Direct bonding mechanism study is then very important in order to clearly understand this bonding behaviour. Especially in the case of silicon or silicon dioxide hydrophilic surface, the role of water is essential. Water could lead to lots of different reactions at the bonding interface and especially the water stress corrosion could play an important role in the direct bonding mechanism.
After having reviewed the different silicon dioxide water stress corrosion in the literature [1,2], its influence in the direct bonding area will be pointed out. The direct bonding energy measurement, G, using the double cantilever (DCB) beam technique is for instance highly affected by the water stress corrosion [3, 4, 5, 6]. In addition to the direct bonding energy, this DCB technique is also very interesting to allow improvement of the direct bonding mechanism. Indeed, as the water stress corrosion only appears with siloxane bonds, it could be an interesting characterization of the classical silicon direct bonding reaction:
Si-OH+Si-OH => Si-O-Si+H2O
We will then show that siloxane bonds could appear already at room temperature and not only during the classical post bonding annealing process using for instance plasma activated bonding by changing the relative humidity of the characterization atmosphere. This water stress corrosion is also put into evidence during adhesion bonding energy and subsequent G measurement in which the hysteresis cycle which appears between these two measurements shows that specific siloxane bonds appear after the bonding wave propagation [7, 8]. Moreover, they put in evidence the effect of the atmosphere humidity in such hysteresis cycle.
In addition to this, the water stress corrosion could be used to put into evidence the remaining water at the bonded interface [9]. Indeed in specific dioxide/dioxide bonding, Ventosa et al. [10] show that the water could remain at the interface until post bonding annealing treatment of 400°C during 2 hours and we will show that this can be seen by the water stress corrosion which takes place even in anhydrous atmosphere for this bonding type. Thus, the stress corrosion study in anhydrous atmosphere can deeply participate to the elaboration of a direct bonding mechanism.
Moreover, one could think that the water stress corrosion plays an active role in the direct bonding evolution. Indeed, in other silicon dioxide processes, as CMP for instance, the water stress corrosion helps to hydrolyze the surface and to remove the polished material. The water stress corrosion might help the surface asperities to deform in order to increase the bonding area and thus the bonding energy. Furthermore, as the water stress corrosion rate depends on the temperature [11], it could also participate to the bonding energy evolution versus annealing temperature. The direct bonding mechanism could then be seen as a mechanical model of asperity deformation greatly helped by the trapped water at the bonding interface.
Besides the bonding energy evolution, we will also discuss how the water stress corrosion study of bonded structure could be very interesting for the characterization of bonded interface chemical resistance. Indeed a good relation is seen between the chemical interface resistance and the water stress corrosion sensitivity [12].
The water stress corrosion study in bonded structure and especially in silicon or silicon dioxide bonding is then very fruitful and should deserve to be extended to other type of bonding interface and materials.

REFERENCES

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