Studies on SiO₂-SiO₂ bonding with hydrofluoric acid. Room temperature and low stress bonding technique for MEMS

Studies on SiO₂-SiO₂ bonding with hydrofluoric acid (HF) are described. This method has a remarkable feature that bonding can be obtained at room temperature. Advantages of this method are low thermal damage, low residual stress and simplicity of the bonding process, which are expected for the packaging and assembly of micro-electro-mechanical systems (MEMS). The bond characteristics were measured under different bonding conditions of HF concentration, applied pressure, another chemicals for bonding and so on. The bond strength depends on the applied pressure during bonding. To achieve reliable bonding, HF concentration of higher than 0.5 wt.% and a large applied pressure of 1.3 MPa are required. The bonding is also observed using KOH solution in stead of HF. Transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), radioactive isotope (RI) analysis and electron probe micro analysis (EPMA) were applied to evaluate the bonded interface. The results of these analysis indicated that an interlayer of a silicon oxide complex including hydrogen and fluorine atoms is formed between bonded SiO₂ to SiO₂. The thickness of the interlayer depends strongly on the applied pressure during bonding. Large bond strength is obtained when the interlayer is thin. The bonding mechanism is expected when the SiO₂ at both surfaces is dissolved in HF solution, and that the interlayer, which is a binding layer, is formed between substrates by resolidification of dissolved silicon dioxide. Formation of the interlayer plays very important roles for the characteristics of HF-bonding.