Edge Bonding Voids Management Using Humid Helium Bonding Atmosphere
Authors: Fournel Frank , Larrey Vincent , Morales Christophe , Michaud Laurent Gaëtan 
Published: February 2024

As far as hydrophilic direct wafer bonding is concerned, there are, beside classical bonding voids induced by particle contamination or surface topography, edge bonding voids due to the bonding wave dynamic itself. Indeed, as shown by Castex et al. [1], during the adiabatic expansion of the bonding wave at the wafer edge, humidity inside the bonding wave gas can liquefy and small water droplets nucleate. This is shown in Figure 1a, with the bonding of a 300mm silicon wafer on another one having a 145 nm thermal oxide on top and after a post bonding thermal annealing at 300°C. These wafer edge droplets result in bonding defects without the presence of particle contamination or surface topography. Even if bonding is performed under a dry gas like Nitrogen or Argon, these defects remain at the hydrophilic direct bonding interface. Indeed, there is always a small amount of water on a hydrophilic surface. During the first bonding wave propagation stage, gas compression increases the temperature and enables the gas to take water from the surfaces, as explained V. Larrey et al. [2]. The humid gas will result in edge bonding voids at the end of the propagation. There are only two methods to get rid of this phenomenon. The first one is to perform bonding under vacuum (<0.1 mbar). The second is to use a gas with a negative Joule-Thomson coefficient.
Such gases will be heated during the adiabatic expansion when the bonding wave will go out of the bonding interface. Three gases have such a negative coefficient: Helium, Hydrogen and Neon. Helium has the lowest joule-Thomson coefficient at ambient condition and is the saver and cheapest one. Bonding under Helium atmosphere will then prevent water nucleation, as shown in Figure 1b. Usually, pure Helium is used which is then a dry gas. Some water will then be removed from the surface. As shown by Radisson [3], the adhesion will then be less important as well as the adherence as shown by Fournel et al. [4]. Adding water vapor to increase the relative humidity (RH) of helium could then enable us to recover standard bonding conditions regarding the amount of trapped water.
In our EVG850LT bonding tool, a humidifier has thus been installed on the Helium gas line in order to purge the bonding chamber with a varying RH gas mixture (from 0 to 80%). As shown in Figure 1c, even with 68% of RH, no bonding voids are detected by scanning acoustic microscopy. It will be shown also that the bonding wave speed increases with the RH value, highlighting an adhesion energy recovery. The evolution of the adherence with the RH bonding conditions and after the post bonding annealing will be also presented.

REFERENCES

[1] A. Castex, M. Broekaart, F. Rieutord, K. Landry, and C. Lagahe-Blanchard, ECS Solid State Lett. 2, p. 47 (2013).
[2] V. Larrey, G. Eleouet, C. Bridoux, C. Morales, F. Fournel, F. Rieutord, D. Landru, Proceeding of The International Conference on Wafer Bonding 2019-WaferBond'19, p.91 (2019).
[3] D. Radisson, Direct Bonding of Patterned Surfaces, PhD thesis, Université Grenoble Alpes, (2014).
[4] F. Fournel, C. Martin-Cocher, D. Radisson, V. Larrey, E. Beche, C. Morales, P.A. Delean, F. Rieutord, and H. Moriceau, ECS J. Solid State Sci. Technol. 4, P124 (2015).

Figure 1