Research On The Theory And Equipment Of Micro Hot Embossing

Fabrication of micro structures with hot embossing is an important technology due to its numerousadvantages such as low cost, high efficiency and parallel operation. Supported by the National Hi-TechResearch and Development Program (863) of China—"Research on the manufacturing device of the micromold used in microfluidic chips" (No. 2002AA421150) and the Research Foundation of DoctorialProject—"Study of new methods and process of manufacturing of 3D polymer microfluidic chips"(RFDP20030335091), actively absorbed the original thoughts, new theories and technologies of correlativesubjects, the model in embossing process was established, the rheological character of polymer with respectto the imprint quality and the imprint efficiency was expounded, the strategy of mold topology optimizationof structures was proposed and hot embossing process was optimized with theoretical analysis, numericalsimulation and experiments.In chapter 1, the background and significance of the research are introduced, the development trend andthe current research situations of hot embossing mechanism, hot embossing process and the device of hotembossing were expatiated, and then the study contents of this dissertation were proposed.In chapter 2, the flow mechanism of the polymer in hot embossing was described from the mechanicalbehavior in macroscopy to the evolvement of structure in microscopy. Three phenomena in macroscopy,dependency of time, dependency of temperature and time temperature equivalence principle were imported.Based on the polymer network theories and the molecule theories, the rheological behavior of polymer and itsmolecule movement were explained, which established the theoretical base of the theorical analysis,numerical simulation and experiment study in the following chapters.In chapter 3, the mechanical model of microfluidic chip in hot embossing based on the rhologicalanalysis was established with theoretical analysis and numerical simulation. For better describing theviscoelastic behavior in hot embossing process, the rule of the polymer material's constant n changing withtime was acquired. The result deducted from the Navier-Stokes (NS) equation accords with that of rhologicalanalysis, which proves the right of this model on the other hand. Based on the hot embossing model ofmicrofluidic chips, normal hot embossing process was modeled with the coupling of two flow fields. Thetime required for completely filling and the time development of micro pattern height were got.In chapter 4, the change of mold structures with respect to flow behavior in the embossing step wasstudied systematically. The factors influencing the polymer flow were analyzed with the DEFORM code. Thedifferent flow profile and flow character with the different mold duty ratio, aspect ratio, width to thicknessratio and the mold attitude were expounded. The difference of flow character between isothermal hotembossing process and non-isothermal hot embossing process was investigated and the reason was analyzed.The strategy of topology optimization of mold structures was proposed to improve the filling efficiency in hot embossing process. With the arrangement of some flow barriers at the mold boundary, the flow speed at theboundary can be decreased and the filling efficiency can be accelerated. Numerical simulation results showthat this strategy has better effect on the accelerating filling speed.In chapter 5, the reason that caused the defects in hot embossing was studied systemically. The finiteelement method (FEM) was used to analyze the cooling step and the demolding step. The numericalsimulation results show that profile precision is largely influenced by the topology of mold structures duringthe embossing step. Inadequate holding time will result in low pattern fidelity. Inapposite demoldingtemperature could induce large thermal stress at the bottom of the micro pattern, while keeping imprintpressure during the cooling step will aggravate this phenomenon. The demolding direction with respect to theproduct quality was studied in the first time, and the large lateral stress will be induced by the little error ofthe demolding direction, then the fracture and other defects would happen. To keep the right demoldingdirection, a new pneumatic demolding device was designed. The optimization process of hot embossing wasproposed with the decreasing the imprint pressure in the cooling step until rearching the demoldingtemperature. This optimized process can not only reduce the stress concentration, but also avoid the recoveryand the distortion of micro structures induce by releasing the imprint pressure too early.In chapter 6, a hot embossing system based on the Peltier effect was designed to improve some problemsthat existing in the current hot embossing device with the help of theoretical analysis and numericalsimulation. Thermoelectric cooler is used for heating/cooling and the heat of the cooler surface is taken awayby the flowing water when cooling. A fuzzy-PID control was designed for the precision control oftemperature. The temperature control precision can reach 0.2℃and temperature increasing/decreasing speedis larger than 1℃/s. The imprint pressure is applied with hydraulic pressure system, which can decrease thenon-uniform distribution of pressure in the embossing step and can improve the imprint quality.In chapter 7, the control precision of the temperature, the pressure and the vacuum was tested to verifythe performance of self-designed hot embossing device. The experiment results show the device accords wellwith the design specification. Two types of micro structures, microfluidic chips and the light guide plate werechosen as the experimential object and the process parameters with respect to the imprint quality wereanalyzed. By analyzing the error of channel sizes, the main reason of the error was assumed as the recoveryof polymer. Orthogonal method was used in the analysis of relationship between the process parameters andthe imprint quality. Then the optimized process parameters were got.In chapter 8, the study contents and conclusions of the dissertation have been summarized and thefurther research works have been forecast.