Mechanism of Thermal Silicon Oxide Direct Wafer Bonding

Abstract

Thermal silicon oxide-to-oxide bonding was investigated at the nanometer level using X-ray reflectivity, transmission electron microscopy, and infrared absorption spectroscopy. The measurements reveal the stages of the closure mechanism, which are different from standard silicon bonding. Upon annealing, interface water pockets are formed, the contents of which are further dissolved into the oxide, demonstrating that the buried thermal oxide-silicon interface acts as a barrier against water reaction with silicon.Direct wafer bonding consists in joining two wafers at room temperature without any adhesive or additional materials, followed by annealing. Despite a wide technological interest, the detailed mechanism of the sealing of such model solids is poorly known due to the lack of experimental tools to investigate a nanometer-wide interface buried under millimeter-thick materials. Yet, the bonding technology is increasingly used in microelectronics, e.g., for siliconon-insulator mass production, microelectromechanical systems manufacturing, or in three-dimensional level integration. In many cases, it represents an alternative to deposition for building heterostructures with additional capabilities. For most applications, high quality bonding is required, driving many studies on bonding interfaces. 1-4 Water removal has been widely described in the literature through H 2 outgassing due to a low temperature oxidation occurring through the native oxide. We show here that although macroscopically oxide/oxide, silicon/oxide, and silicon/silicon bondings show similar behavior, the interface evolutions at the nanometer scale are entirely different. We used interfacial X-ray reflectivity ͑XRR͒, 5 high resolution transmission electron microscopy ͑HR-TEM͒, and Fourier transform infrared spectroscopy in multiple internal reflection ͑FTIR-MIR͒ 6 to demonstrate our point. The results of similar techniques applied to silicon/silicon or silicon/oxide bonding can be found for comparison in Ref. 7.The wafers used were 100 mm in diameter ͑001͒-oriented Czochralski-grown Si wafers. HF-etch was first performed on the wafers to remove the native oxide from the surface. Then the oxide films were thermally grown on the Si-H terminated wafers at 800°C in dry O 2 flow. The typical SiO 2 thickness used in this study was 10 nm. Wafers were then cleaned in sulfoperoxide mixture ͑H 2 SO 4 ,H 2 O 2 ͒ and rinsed in deionized water. An RCA cleaning treatment was then performed, followed by a deionized water rinse and spin-drying. Finally, hydrophilic wafers were bonded at room temperature in a clean room atmosphere and annealed at various temperatures in the range from room temperature to 400°C for 2 h. Figure 1 presents the evolutions of ͑a͒ bonding interface density and ͑b͒ width as a function of the annealing temperature, obtained from XRR analyses. Above 100°C, together with the bonding energy, our data show that the interface density sharply increases corresponding to interface closure. This mechanism of interface sealing in wafer bonding h...

A Review of Hydrophilic Silicon Wafer Bonding

“…These results have been repeated several times subsequently 34,35 utilizing the same technique. The 1994 publication by Q. Y. Tong et al marks the next significant stage in the wafer bonding literature.…”

Direct Bonding Mechanism of ALD-Al2O3Thin Films

“…Nevertheless, face to scale reduction of device, specific attention should be paid to thermal SiO 2 thickness sublayer. Indeed, following to Ventosa and Vincent studies, 25,26 40 nm of oxide is not sufficient to withstand the produced hydrogen due to interfacial trapped water after 600…”

“…In the bonding structure, the alumina compressive stress can hardly induce debonding because of stiffener effect of both silicon substrate. If we care about direct bonding mechanism, 15,25,26 it is known that hydrogen can be generated during silicon oxydation by trapped water at bonding interface. One possible interpretation would be the following : as soon as the water reaches the silicon, it starts to oxidize it, interfaces adherence between alumine and silicon susbtrate is then degraded.…”

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.…”