| Polymer microfluidic chips not only have the advantages of high-speed, high-throughout, small sample consumption and easy to realize automatic operation as other microfluidic devices, they also have their inimitable advantages such as wide choices of materials, ease of fabrication, low cost, disposability and good bio-compatibility and so on. Therefore, polymer microfluidic chips are considered as the most potential for the commercialization and industrialization of portable analytical instruments. Many techniques for the fabrication of polymer microfluidic chips have been studied in the last decay. And small batches of chips used for research in the lab can be realized by these techniques. However, there are still some technical issues in the mass production. Bonding is one of the bottlenecks among these issues. In order to meet the bonding requirements of microfluidic chips and other polymer MEMS devices, ultrasonic bonding methods and the bonding mechanism were systematically studied in this thesis.Based on theoretical analysis and numerical calculation, the heating mechanisms of polymer under ultrasonic vibration in glassy state and viscoelastic state were studied. The temperature field during ultrasonic bonding process was measured. Based on the calculation and experiment results, a new frictional composited with viscoelastic heating mechanism was presented. It reveals that facial friction rather than viscoelastic heat initially start the bonding process in glassy state. Viscoelastic heat becomes dominant when temperature reaches Tg (glass transition temperature) of the material. Viscoelastic heat provides most required heat during the process. Temperature fields at different ultrasonic amplitudes were also discussed based on numerical calculations and temperature measuring experiments. The results indicates that there exists a critical amplitude for the given material and dimensions of energy director. The peak temperature become constant and the polymer does not melt when ultrasonic amplitude is lower than the critical amplitude no matter how long the bonding time is. These findings give a more clear understanding of heating mechanisms in ultrasonic bonding. And it will contribute to the improvement of bonding quality and method innovation of ultrasonic bonding.In order to improve the bonding quality of melt based ultrasonic bonding method, different bonding auxiliary microstructures were designed. The fabrication of these complex 3-D microstructures was also studied. Based on bonding experiments, the mating style design including molten bath and flowing block was successfully used to hermetically bond the microchannels with characteristic dimension of 30μm. This method shows many advantages such as very high bonding strength (near the body strength of the material), short bonding time (less than 0.5 s) and stable bonding quality. However, the melt of polymer make it difficult to improve the bonding accuracy further using this method.Based on the research of heating mechanism, the non-melt ultrasonic bonding method was firstly proposed. And two novel non-melt ultrasonic bonding methods were presented in this thesis. They were local solubility activated non-melt ultrasonic bonding method and thermal assisted non-melt ultrasonic bonding method.The bonding mechanism of the local solubility activated non-melt ultrasonic bonding method was studied according to the heating process under low amplitude ultrasonic and molecular dynamics simulation. PMMA microfluidic chips were successfully bonded with isopropanol (IPA) as the assisted solvent. Comparison to traditional ultrasonic bonding method, lower amplitude was used in this method, which prevented melting of polymer and deformation of microchannel. Additionally, most part of the PMMA substrates kept at room temperature except for the interface between the energy director and cover substrate during the bonding process. So IPA did not damage the microstructures as in common solvent bonding method. The experimental results indicated many advantages of this method such as easy to fabricate the substrates, good controllability of the process, low deformation of the microstructures and high bonding strength. It is a novel bonding method for polymer MEMS devices. However, high selectivity for the polymer and solvent greatly limited the usage of this method.A thermal assisted non-melt ultrasonic bonding method without bonding auxiliary structures was proposed according the heating properties during ultrasonic bonding and the bonding mechanism of traditional thermal bonding method. Temperature of the interface rose to around Tg as a result of friction heating from ultrasonic vibration, while the bulk temperature of the substrates was still well below Tg. Combined with ultrasonically oscillating pressure, bonding formed at the interface with little deformation of microstructures. Bonding experiments indicated some advantages of this method such as needless of bonding auxiliary structures, low deformation of the microstructures, good controllability of the process, suitable for almost all thermoplastics and automatically focus of ultrasonic energy at the interface. Considering the special advantages of this method, planar nanochannel array with depth of 200 nm was successfully bonded using this method. And multilayer microfluidic devices with as many as 12 layers were also bonded together at one time using this method.
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