Low-temperature bonding of ceramics by sol-gel processing

Sol-gel bonding was produced between smooth, clean substrates of silicon and polycrystalline alumina by spin-coating solutions containing partially hydrolyzed silicon alkoxides. The two coated-substrates were assembled and the resulting sandwich was fired at temperatures ranging from 300 to 600 °C. The coatings and bonded substrates were investigated using SEM, TEM and micro-indentation. For silicon wafers, an optimum water-to-alkoxide molar ratio of 10 and hydrolysis water pH of 2 was found. Such conditions led to relatively dense films (>90%), resulting in bonds with a fracture energy of 3.5 J/m 2, which is significantly higher than those obtained using hydrophilic wafer bonding (typically 1.5 J/m 2). Poly-crystalline alumina substrates were similarly bonded at 600 °C; the optimized silica sol-gel chemistry yielded interfaces with fracture energy of 4 J/m 2