Sol-gel bonding of silicon wafers Part 1: Influence of the processing temperature on final bond morphology and interfacial energy
Sol–gel bonds have been produced between smooth, clean silicon substrates by spin-coating solutions containing partially hydrolysed silicon alkoxides. The two coated substrates were assembled and the resulting sandwich fired at temperatures ranging from 60 to 600 -C. The sol–gel coatings were characterised using attenuated total reflectance Fourier transform infrared spectroscopy, ellipsometry, and atomic force microscopy, while the corresponding bonded specimens were investigated using scanning electron microscopy and cross-sectional transmission electron microscopy. Mechanical properties were characterised using both microindentation and tensile testing. Bonding of silicon wafers has been successfully achieved at temperatures as low as 60 -C. At 300 -C, the interfacial fracture energy was 1.55 J/m2. At 600 -C, sol–gel bonding provided superior interfacial fracture energy over classical hydrophilic bonding (3.4 J/m2 vs. 1.5 J/ m2). The increase in the interfacial fracture energy is related to the increase in film density due to the sintering of the sol–gel interface with increasing temperature. The superior interfacial fracture energy obtained by sol–gel bonding at low temperature is due to the formation of an interfacial layer, which chemically bonds the two sol–gel coatings on each wafer. Application of a tensile stress on the resulting bond leads to fracture of the samples at the silicon/sol–gel interface.