High Temperature Sealing of Silicon Oxide/Oxide Bonding Interfaces
Authors: Rieutord Francois , Tardif Samuel , Fournel Frank , Larrey Vincent , Landru Didier , Kononchuk Oleg 
Published: December 2020

High temperature (>1000°C) annealing is necessary to completely close a bonding interface. Using interface synchrotron high energy X-ray reflectivity, we have investigated the sealing behavior of silicon thermal oxide/ silicon thermal oxide interfaces, with different surface roughnesses. Interface X-ray reflection is able to measure accurately the width and the depth of the electron density gap across a bonding interface with a sub-nanometer accuracy, whose product is a good marker of the interface closure.
The uncomplete closure of the interface is the result of a balance between attractive and repulsive force between the two bonded solids. These two forces depend on the statistics of asperity and on the mechanical behavior of individual asperity [1]. For attraction, at high temperatures, a key factor is the gain of contact surface area which reduces the surface energy due to uncontacted zones. Repulsion is due to compression of contacting asperities. At high temperature, both elastic and plastic behavior can be expected, with some yield of the asperities compressed above the elastic limit.
In the elastic solid regime, we have shown that equilibrium distance of rough silicon oxide to rough siliconoxide bonding can be predicted using standard adhesive contact models such as Johnson-Kendall Roberts (JKR) or Deryagin-Müller-Toporov (DMT) with Gaussian roughness statistics[2]. The equilibrium distance is dependent and sensitive to the Fuller Tabor adhesion parameter, θ=E σ3/2 R1/2 / 2γR where E is the Young modulus, σ the mean roughness, R the radius of the asperity, while 2γ is the adhesion energy at contact points. When θ>>1 , the equilibrium distance is large and the contact area is small (unclosed interface) . When θ<<1 the adhesion energy is strong and the system will have a strong tendency to seal.
In the plastic regime, both repulsive and attractive curves share the same dependence with distance so that the system is expected to be unstable, between a weakly evolving unsealed interface and a fully sealed interface when the asperity pressure stress is above the elastic limit.
The kinetics of the sealing can also be modeled in this case, monitoring the interfacial gap as a function of time (Figure). It is found that the sealing characteristic time scales directly with the silicon oxide viscosity (Figure insert). This dependence can be understood using the plastic asperity contact model, assuming the fluid part flows according to the Stokes equation around asperities.

References

[1] Rieutord, F., L. Capello, R. Beneyton, C. Morales, A.-M. Charvet, and H. Moriceau. “Rough Surface Adhesion Mechanisms for Wafer Bonding.” In Semiconductor Wafer Bonding 9: Science, Technology, And Applications ECS Transactions, edited by H. Baumgart, S. Bengtsson, H. Moriceau, K.D. Hobart, T. Suga, and C. Colinge, 3:205–15. ECS Transactions, 2006.
[2] Johnson, K. L., Contact Mechanics, Cambridge University Press (1985).

Figure caption

Time dependence of the interface closure for a SiO2//SiO2 bonding interface. Roughness of surfaces was 5.4Å rms as measured using AFM. The color scale gives the temperatures (1000-1200°C). Insert: dependence of the closure time with viscosity.

Figure 1