Low temperature bonding of ceramics by sol-gel processing

Sol-gel bonds were produced between smooth, clean silicon or polycrystalline alumina substrates by spin-coating solutions containing partially hydrolysed silicon alkoxides onto both substrates. The two coated substrates were assembled and the resulting sandwich was fired at temperatures ranging from 300 to 600 degrees Celsius. The influence of the sol-gel chemistry on the film microstructure and interfacial fracture energy was investigated using a wide range of techniques, including ellipsometry, FTIR, TG-DTA, rheology, TEM and micro-indentation. For silicon wafers, an optimum water-alkoxide molar ratio of 10 and hydrolysis water pH of 2 were found. Such conditions led to relatively dense films (>90 percent), resulting in bonds with significantly higher fracture energy (3.5 J/m^2) than those obtained using classical water bonding (typically 1.5 J/m^2). Aging of the coating solution was found to decrease the bond strength. Poly-crystalline alumina substrates were similarly bonded at 600 degrees Celsius; the optimised silica sol-gel chemistry yielded interfaces with fracture energy of 4 J/m^2