Abstract
Defect free direct bonding of rigid and large area glass samples, such as prisms, becomes increasingly important for the manufacturing of modern optical and optomechanical components. Typically, in order to apply a static load during the annealing step, specialized heat-resistant pressure mountings are required. This makes manufacturing effortful and cost-intensive. In this paper, we present plasma activated bonding experiments conducted on fused silica plates where residual stress has been introduced prior to the contacting step and where annealing is performed with and without a static load. We find that in case of a sufficiently smooth surface, bonding strength is insensitive towards residual stress or static load, or more precisely, towards the interface stress. Furthermore, the residual Fresnel reflection losses of the realized bonding interface were optically measured and they amount to only 10^{-6}. We propose that a consideration of the change in Gibbs free energy, dG, allows qualitatively predicting the resulting bonding strength and its spatial distribution, where dG is determined by surface energy and interface stress. At the end of this article, conceivable applications are discussed.
Highlights
Direct bonding is a bonding technique which allows permanent and high strength joining of a variety of similar and dissimilar materials without introducing any intermediate layer [1,2,3,4,5]
Some sample pairs are subject to a homogeneous static load, P, of several MPa, a value that is typical for wafer direct bonding, and some are subject to no static load
Our motivation was the investigation of direct bonding of large area and stiff fused silica samples, because of its important role for state-of-the-art large-aperture-optics applications
Summary
Direct bonding is a bonding technique which allows permanent and high strength joining of a variety of similar and dissimilar materials (including optical glasses, silicon, germanium, metals and laser crystals) without introducing any intermediate layer [1,2,3,4,5]. Due to more temperature sensitive parts in the fields of microelectronics, three-dimensional device integration, micro-electro-mechanical systems [23, 24], micro-fluidics [25] and optical, optomechanical and photonics applications [19, 26, 27], increased interest has emerged on direct bonding techniques at lower annealing temperatures in order to avoid thermal stress but still obtain permanent and high bonding strength This is typically made possible by employing a plasma activation step prior to the contacting step, allowing annealing temperatures about or below 250 ◦ C [28,29,30]. Considering the static load, from a theoretical point of view there is no reason to do so: Once a full-area contact between two planarized surfaces is established, the initial “weak” adhesion should maintain this contact throughout the annealing step. For our findings to be relevant for the application, we examined these properties via performing PAB experiments on large area and stiff fused silica samples with focus on the effects of static load and residual stress
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