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 MEMS. The bond characteristics were measured under different bonding conditions of HF concentration, pressure, chemicals and so on. The bond strength depends on the applied pressure during bonding. HF concentration can be reduced to 0.1%. The bonding is also observed using KOH solution in stead of HF, TEM, SIMS, RI and EPMA were applied to evaluate the bonded interface. From the TEM results, an interlayer is formed between SiO₂-SiO₂. The thickness of the interlayer depends strongly on the applied pressure during bonding. The SIMS results showed that hydrogen and fluorine partially exist in the interlayer. Considering the result of the RI analysis, surplus HF solution is squeezed out from the interface as the bonding progress. From these results, both surfaces of the SiO₂ are solved by HF and an interlayer, which is a binding layer, is formed. Formation of the interlayer plays a very important role for the characteristics of HF-bonding.