【作者】 郭育华；

【导师】 田扬超； 刘刚；

【作者基本信息】 中国科学技术大学 ， 核技术及应用， 2007， 博士

【摘要】 近年来，随着聚合物基MEMS芯片在生物、化学和微光学方面的广泛应用，相应的制作技术成为当前微系统技术的一个研究热点。同时，由于器件性能对大高宽比微结构的需求，运用LIGA技术制作MEMS模具并利用热模压成型技术进行微塑铸复制就成为很有前途的一种加工方法。然而，在热模压成型技术中大部分问题不是由模压成型过程引起，而是发生在脱模过程当中。由于LIGA制作的MEMS模具与热塑性材料之间脱模力的存在，对模压复制品的质量造成了很大的影响，聚合物微结构有可能拉断、变形甚至毁坏。随着微结构高宽比的增加，这些现象越来越明显，当高宽比达到一定数值时，某些微结构的脱模变得不可能进行，几乎很难得到质量好的结构。为了对脱模过程进行系统深入的分析，设计和制作具有高脱模性能的MEMS模具，并进行相应的脱模性能研究和评价，本论文主要开展了以下几个方面的工作：1．对热模压成型中的脱模力分析及高性能MEMS模具的设计在热模压成型技术中，MEMS模具和聚合物微结构的脱模过程是摩擦剪切的过程。本工作重点分析了即脱模力的组成，包括热收缩力和表面粘着力，从理论分析、实验论证以及有限元分析多个角度分析了脱模力的影响，并提出了相应的解决方案。以微摩擦学及表面粘着力理论分析了热模压成型脱模过程主要缺陷的成因，如微结构图形边的拉起、拔断以及大高宽比图形的无法模压等问题；同时利用有限元数值软件分析冷却过程中热应力对脱模初始阶段的影响，如切根现象的成因，并提出使用热应力阻隔器的优化设计。最后通过对T型微结构脱模过程的个案分析，排除了热应力的影响后，认定由模具与聚合物的表面能决定的表面粘着力是影响脱模的重要因素，据此我们制定了高性能MEMS模具的制作方案—Ni-PTFE的一种抗粘着、低摩擦系数的复合材料LIGA模具，并设计出测试结构的模具掩模。2．同步辐射深度X射线光刻工艺的研究中国科学技术大学国家同步辐射实验室二期工程结束以后，LIGA线站的同步辐射X深度光刻研究得以开展。本研究工作首先对同步辐射深度光刻掩模制作、样品制备工艺进行了探索，在此基础上利用NSRL的LIGA站开展了曝光剂量与显影速率关系的研究，获得了稳定的深度光刻工艺条件，为制作出LIGA光刻胶微结构提供了前提；同时，为了获得高质量的光刻胶结构以满足后续电铸工艺的进行，特别研究了微结构图形在不同基底的粘附性能；最后还研究了基于Su8胶的紫外光刻(UV-LIGA)技术，作为LIGA工艺的有益补充。3．精密微电铸技术研究在微电铸模具制作过程中，工程人员发现电流密度受到深宽比掩模微结构的影响有一定的再分布规律，往往出现不希望得到的表面轮廓。而采用overplating的模具制作技术能够有效的避免电流密度再分布带来的对微结构图形的影响，获得的微结构图形部分表面平整性较好。当前对overplating电铸的研究很少，本工作利用Cell-Design软件并结合微电铸实验分析了overplating电铸时的电流密度分布规律，提出Ni在overplating电铸过程中的生长模型，发现了电极的“相邻调制效应”，在理论和实验论证上给出了在各种线宽＼间距比条件下其可能造成的缺陷，并提出了相应的解决办法。这部分工作对利用overplating微电铸技术提供了部分设计思路及制作经验。4．Ni-PTFE的复合微电铸技术研究在微电铸Ni模具的基础上，研发Ni-PTFE的复合微电铸工艺是本论文工作的一个重要内容，它扩展了传统LIGA的模具制作技术，为获得高性能MEMS模具开辟了一条新的途径，为国内外MEMS模具制作技术上的首创。工作对Ni-PTFE的复合微电铸工艺进行了深入的探索和研究，克服了复合电铸中的种种困难，包括复合电铸装置的搭建，采用FC型阳离子表面活性剂使PTFE粒子在镀液中保持良好悬浮及共沉积，以及镀液组成和电铸操作条件的摸索，克服了铸层中的针孔问题，最终制作出高性能的复合材料的LIGA模具。在这些工作的基础上，实验最终获得了掩模设计的Ni及Ni-PTFE复合材料的MEMS模具，并通过形貌学、成分分析以及侧壁粗糙度、摩擦系数的测量，表明制作的模具不但符合设计要求，也完全符合热模压脱模的工艺条件。5．HEX02真空热模压成型技术研究脱模过程中的温度及脱模速率也是影响摩擦的主要因素。目前，专门针对脱模过程中工艺参数的研究报道很少，往往局限在具体应用上的参数选择。本研究工作首先弥补了这一缺憾，利用德国JENOPTIK MikrotechniK GmbH公司生产的Hot embossing system HEX02塑铸仪开展了真空热模压成型技术研究，分析了聚合物微结构在不同脱模温度和脱模速率条件下的力学行为，如低温下脱模情形和快速脱模情形，在理论和实验上给予论证，以此实现了对脱模工艺条件的优化。6．脱模性能评价与对比分析确定优化的脱模工艺条件后，对Ni和Ni-PTFE模具的高宽比微结构、连续模压寿命进行了脱模性能的对比研究，结果表明Ni-PTFE模具不仅能够有效的减小模具与聚合物之间的表面粘着力引起的摩擦力，而且能够有着良好的抗粘着能力。它能够模压高宽比更大的微结构图形，并且连续模压的能力更强，因此是一种很有前途、很具实用价值的MEMS模具制作方法。更多还原

【Abstract】 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.更多还原