Mechanisms of sodium hydroxide (NaOH)-catalyzed sealing of SiO2/SiO2 direct bonding interfaces

The introduction of NaOH catalysts at an SiO2/SiO2 hydrophilic bonding interface is investigated. Combining bonding energy measurements, interface hard x-ray reflectometry, transmission electron microscopy, and time of flight secondary ion mass spectroscopy, the change of interface morphology, and composition as a function of surface preparation is evidenced. The hydroxide catalyzed interface shows the formation of interface nanoclusters, associated with the low temperature interface sealing. A reference interface (bonded without a catalyst) is sharper. These contrasting behaviors highlight the issue of water management at the interface and the balance between interface sealing and water out-diffusion. The additional impact of oxide thickness in interfacial water management is further investigated. Finally, the catalytic interface sealing is compared with other surface preparations that achieve high bonding energies at low temperatures.