Low temperature bonding of ceramics by sol-gel processing

Sol-gel bonds were produced between smooth, clean silicon or polycrystallin e alumina substrates by spin-coating solutions containing partially hydroly sed silicon alkoxides onto both substrates. The two coated substrates were assembled and the resulting sandwich was fired at temperatures ranging from 300 to 600 degreesC. The influence of the sol-gel chemistry on the film mi crostructure and interfacial fracture energy was investigated using a wide range of techniques, including ellipsometry, FTIR, TG-DTA, rheology, TEM an d micro-indentation. For silicon wafers, an optimum water-alkoxide molar ra tio of 10 and hydrolysis water pH of 2 were found. Such conditions led to r elatively dense films (> 90%), resulting in bonds with significantly higher fracture energy (3.5 J/m(2)) than those obtained using classical water bon ding (typically 1.5 J/m(2)). Aging of the coating solution was found to dec rease the bond strength. Poly-crystalline alumina substrates were similarly bonded at 600 degreesC; the optimised silica sol-gel chemistry yielded int erfaces with fracture energy of 4 J/m(2).