Rough Surface Adhesion Mechanisms for Wafer Bonding

Abstract

Wafer bonding can be viewed as an example of rough surface adhesion. Although silicon surfaces are among the flattest surfaces available, we will show that formalisms developed to describe rough surface adhesion [1] can be rescaled to nanometer range and applied to silicon wafer bonding, with results that fit well with experimental observations.The models are based on the compression of roughness asperities that balances attractive forces (Fig.1). Different type of models can be considered for the type contact of the asperities (Hertz, JKR, DMT [2]) and for the type of compression (elastic or plastic).The models allow first a prediction of the equilibrium distance between the bonded wafers. This was tested against X-ray reflection experiments, that provide directly this information through the fringe spacing of interface reflectivity (Fig.2).They also predict the real contact area between the wafers. Although this data is more difficult to measure in experiments directly, it is related to the closure of the interface, also measured in X-ray reflection experiments.The real contact area is then correlated to the mean adhesion energy between the wafers. Hence, the results on the interface closure can be matched to the bonding energy values (Fig.3), obtained using other standard techniques such as blade insertion [3] or bonding velocity measurements [4].This allows to build-up a master curve which can be used to measure adhesion energy. References[1] J.A. Greenwood, J.B.P. Williamson, Proc. Roy. Soc. A295, 300 (1966).[2] C. . Fig.1: balance between Van der Waal attraction (solid line) and asperity compression repulsion (dashed line) as calculated using Gaussian roughness models. The equilibrium distance and bond strength can be predicted.Fig.2 X-ray reflection on a bonding interface. The Fringe spacing gives the gap width while the contrast (intensity) gives the closure of the interface 2.5 2.0 1.5 1.0 0.5 0.0 Adhesion energy (J/m 2 ) 1.0 0.8 0.6 0.4 0.2 0.0

Elastohydrodynamics of contact in adherent sheets

“…At the scales involved in such instances as the adhesive bonding of silicone wafers, substrate roughness can lead to contact at asperities (Rieutord et al. 2006; Moriceau et al. 2011) or can even induce capillary condensation due to the Kelvin effect (de Boer & de Boer 2007; Fournel et al.…”

“…Role of substrate roughness Our model of adhesion assumes a perfectly smooth substrate and sheet, and neglects the effects of surface roughness. At the scales involved in such instances as the adhesive bonding of silicone wafers, substrate roughness can lead to contact at asperities (Rieutord et al 2006;Moriceau et al 2011) or can even induce capillary condensation due to the Kelvin effect (de Boer & de Boer 2007;Fournel et al 2015). These effects are challenging to model in our lubrication model, which relies on a long-wavelength analysis.…”

Overview of recent direct wafer bonding advances and applications

“…proposed to understand bonding mechanisms at nanometer scales [17,35]. These models have been shown to be well suited for most bonded structures.…”

Influence of water diffusion in deposited silicon oxides on direct bonding of hydrophilic surfaces

“…As direct bonding operates through surface asperities deformation, specific mechanical properties of such contact asperities can lead to a stronger bonding by increasing bonding area (Fournel 2015, Rieutord 2006Ventosa 2008;Ventosa 2009). As the mechanical properties of silicon oxide material are greatly influenced by internal water concentration, aging and water diffusion on asperity have a real impact in term of direct bonding energy.…”

“…Stresses evolutions and water penetration are characterized and a correlation is made between the kinetics of both stress variations and water diffusion through the oxide thin films. Impacts of room temperature (RT) storage prior to bonding and thermal treatments applied for strengthening the bonding is shown.As direct bonding operates through surface asperities deformation, specific mechanical properties of such contact asperities can lead to a stronger bonding by increasing bonding area (Fournel 2015, Rieutord 2006Ventosa 2008;Ventosa 2009). As the mechanical properties of silicon oxide material are greatly influenced by internal water concentration, aging and water diffusion on asperity have a real impact in term of direct bonding energy.…”