| In recent years, the increasing demand for polymer based devices resulted in the strong need of mature polymer fabrication technology. Many polymer based micro-fabrication techniques have been explored for applications in bio-, chemical-and optical-MEMS (Micro Electro Mechanical System). In many of these applications, a trend towards high aspect ratio microstructures can be seen recently. Thus, fabrication of MEMS mold inserts using LIGA technology and producing polymer replicas by hot embossing become a popular way. Most difficulties in polymer micro molding are not caused by the filling of the mold, but by demolding. Microstructures may be torn apart, deformed and even destroyed in the demolding process. If the microstructure is designed improperly or if unsuitable process parameters are chosen, some delicate parts will not survive after the first demolding process especially when the aspect ratios of microstructures increase. For this purpose, the demoldmg process has been analyzed and a novel high-performance MEMS mold insert has been designed and fabricated using extended LIGA technology in this paper. At last the demolding properties of mold inserts are studied comparably. The main work and innovations are described as the following:1. Analysis of the demolding forces and design of high-performance MEMS moldinsertIn hot embossing, the friction between the mold insert and polymer is involved in the demolding process. In our model, the demolding forces mainly consist of thermal shrinkage stress and adhesive forces. The demolding process is studied theoretically or by FEM simulations and also with experimental supports. This work proposes several methods that can optimize the demolding process and introduces some good suggestions for mold tool design. Firstly, regarding the adhesion and friction forces, the defects in the demolding process such as pull-up edges, pull-off edges and deformed etc, are analyzed using by adhesive friction theory and micro tribology about micro contact and adhesive phenomena. And the adhesion and friction forces in demolding cycle are evaluated and simulated by FEM using ABAQUS/Standard. Then, Finite Elements Method (FEM) is applied to analyze thermal stress caused by the shrinkage differences between the mold and polymer using ABAQUS/Standard, and thermal stress barrier is proposed as an auxiliary structure to protect against the converging stress at the bottom corner of microstructures. At last, in the case of T-structure demolding behavior, analysis identified interface adhesion between the mold and the polymer as the main source of the demolding forces. And Ni-PTFE is recommended as the mold material for achieving lower surface energy and lower friction force and a mask layout of test structures is also designed. 2. Study of deep X-ray lithography in NSRLThe study of deep X-ray lithography (DXRL) has been developed since the end of Phase II project in National Synchrotron Radiation Laboratory (NSRL). This work firstly carries out the researches on the fabrication of DXRL masks and making samples. Based on these work, the study of measurement of the development rate of irradiated PMMA in deep X-ray lithography under different doses is performed at the LIGA beamline station in NSRL. The aim of this work is to fix appropriate dose range for exposure and to find out a stable experiment condition for the development, which results in a predictable development process and better microstructure quality. Besides, the adhesion properties of microstructures on different metal seed layer and substrates are studied considering the requirement in subsequent electroforming process. At last, the ultraviolet (UV) lithography of Su8 photo resist is also studied which is supplement for DXRL.3. Study of micro-electroformingIn nickel micro-electroforming, the current is usually affected by the deep resist molds and results in irregular top surfaces on the metal. Fabrication of nickel molds using the overplating process can avoid this and achieving better flatness. In overplating process how the metal grows up across the top of resist molds has been relatively neglected. Typical defects like holes formation at the top of cavities of electroplated metal mold usually occur due to improper process control especially when the space/linewidth ratio of microstructure increases. To help understand these problems, overplating process has been investigated using Cell-design. A model is developed to compute current density distribution based on LIGA mold feature using electroplating simulation tools. And the growing model of nickel in the overplating is proposed and the modulation effect is pointed out. Corresponding solutions for the defects are also given. This work will help the design and fabrication of metal microstructures by overplating.4. Study of Ni-PTFE composite micro-electroformingThe R&D of Ni-PTFE composite micro-electroforming is the main work in this paper, which extends the LIGA process and is a novel fabrication method for the MEMS mold insert. A model for the Ni-PTFE codeposition system has been proposed and various difficulties are conquered including the building of experimental set-ups, keeping the suspension of PTFE particles using FC cationic surfactant and pitting problems in coatings etc. At last the Ni-PTFE compound material mold inserts is fabricated by special treated galvanic bath and operation conditions. And the results show that the mold inserts are well qualified by the measurement of morphology, content analysis, sidewall roughness by AFM and measurement of frictional coefficients etc.5. Study of vacuum hot embossing using HEX02The process parameters including demolding temperature, demolding velocity and the material properties may influence the demolding very much. The study of process parameters is rarely reported and in practice, the demolding temperature and demolding velocity are usually chosen according to the specific applications. In this work, the demolding process parameters are studied under different demolding temperatures and demolding velocities using HEX02 hot embossing system for JENOPTIK MikrotechniK GmbH. The mechanical behavior of PMMA is analyzed theoretically and with experimental supports especially under low demolding temperature and fast demolding velocity. At last, the demolding process is optimized with fixed demolding temperature and velocity.6. Study of the demolding behaviors of the mold insertsBased on the optimized demolding process parameters, the comparison of embossed microstructures with various aspect ratios and the comparison of the embossing lifetimes of mold inserts have been carried out between Ni and Ni-PTFE mold inserts. The results show that the Ni-PTFE LIGA mold insert can reduces the friction force and adhesion between the mold insert and polymer replica effectively compared to nickel at the same dimension. And the lower friction and adhesive forces let the PMMA replicas easily separate from the mold insert. It will improve the quality of polymer replica and also the embossing lifetime of mold insert, which is a promising way for the fabrication of MEMS mold inserts.
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