Environmentally-Assisted Fatigue Cracking in Silicon MEMS Structures

该项目旨在对单晶和多晶硅制成的具有良好表征的微机电系统（MEMS）结构的环境辅助裂纹萌生和扩展机制有一个基本的了解。在通过扫描和透射电子显微镜研究MEMS结构的微观结构的初始阶段之后，将使用原子力显微镜技术检查样品的表面形貌。在对单晶和多晶结构进行微拉伸实验以确定其在单调或循环载荷下的本构行为之前，多晶硅结构中的初始微织构将使用取向想象显微镜进行测量。该项目的第二阶段将重点研究暴露在空气中的硅MEMS结构表面形成的二氧化硅层中环境辅助裂纹起裂机制的特征。该计划将探索裂纹成核是由应力辅助溶解或破裂过程引起的假设，这些过程是由表面与水蒸气反应引起的。为了验证这一假设，将在实验室空气和控制水蒸气分压的环境中进行自然裂纹起裂实验。利用原子力显微镜技术监测二氧化硅层表面形貌随时间和应力的变化。裂纹起裂的开始将从微观测试器/试样配置的共振条件和高倍扫描电子显微镜的变化中检测出来。然后将测量的起始条件与应力辅助溶解和裂纹成核模型的预测进行比较。环境辅助裂纹扩展的机制也将在第二阶段使用含有纳米或微缺口或压痕裂纹的样品进行研究。与裂纹萌生实验一样，裂纹扩展将在实验室空气和控制相对湿度的环境中进行研究。裂纹扩展的增量将从微测试仪/试样装配的共振条件的变化中检测出来。裂纹扩展的增量和裂纹/微观结构的相互作用将使用原位和非原位扫描电子显微镜技术的组合来确定。在进行扫描电镜分析以确定断裂模式之前，还将通过取向成像显微镜来表征裂纹扩展的晶体学方向。基于力学的力学模型将被用于预测裂纹的起裂和扩展。这些模型将与Jean Prevost教授和Tim Baker博士在普林斯顿的一个单独资助的国家科学基金会项目中合作开发。因此，这两个NSF项目将作为一个小型中心运作，与普林斯顿大学的索志刚教授和安东尼·埃文斯教授密切互动。该项目还将为首席研究员提供资金，以继续在过去五年中由美国国家科学基金会资助的外展和教学项目。在高中阶段，PI将与当地一名高中教师进行互动，该教师将在暑假期间研究基于mems的教学和研究材料，并将其带回课堂。该教师将获得美国国家科学基金会少数族裔高中教师奖的资助。该项目还将在夏季聘请两名少数族裔工程专业的学生来研究拟议的MEMS项目的各个方面。学生的暑期工资将由普林斯顿大学研究生院院长办公室组织的一个正在进行的项目提供。该计划旨在激发高素质少数民族本科生对研究生学习和未来学术生涯的兴趣。因此，PI将尝试指导学生，并鼓励他们攻读力学和材料方面的研究生课程。在研究生阶段，PI将使用NSF的部分资金开发基于网络的先进结构材料新课程的教学材料，该课程将与Anthony Evans教授共同教授。将开发的教学材料包括，每节课的网络管理费用，以及说明力学和材料在结构材料设计中的应用的案例研究。图1 -计划的三个阶段示意图

0071483SoboyejoThe project aims to develop a fundamental understanding of environmentally-assisted crack initiation and propagation mechanisms in well characterized Micro-Electro-Mechanical Systems (MEMS) structures fabricated from single crystal and polycrystalline silicon. Following an initial phase in which the microstructure of the MEMS structures will be studied via scanning and transmission electron microscopy, the surface topography of the specimens will be examined with atomic force microscopy techniques. The initial micro-textures in the polycrystalline silicon structures will then be measured using orientation imagining microscopy, before conducting micro-tensile experiments on single and polycrystalline structures to determine their constitutive behavior under monotonic or cyclic loading. The second phase of the program will focus on the characterization of environmentally-assisted crack initiation mechanisms in the silica layer that is formed on the surfaces of the silicon MEMS structures upon exposure to air. The program will explore the hypothesis that crack nucleation occurs by stress-assisted dissolution or rupture processes that are induced as a result of surface reactions with water vapor. To test this hypothesis, natural crack initiation experiments will be performed in laboratory air, and environments with controlled partial pressures of water vapor. The changes in surface topography of the silica layer will be monitored as a function of time and stress using atomic force microscopy techniques. The onset of crack initiation will be detected from changes in the resonance conditions of the micro-tester/specimen configuration, and high magnification scanning electron microscopy. The measured initiation conditions will then be compared with predictions from stress-assisted dissolution and crack nucleation models.The mechanisms of environmentally-assisted crack growth will also be studied in Phase II using specimens containing nano- or micro-notches, or indentation cracks. As in the crack initiation experiments, crack growth will be investigated in laboratory air, and environments with controlled relative humidities. The increments of crack growth will be detected from changes in the resonance conditions of the micro-tester/specimen assembly. The increments of crack growth and the crack/microstructure interactions will be determined using a combination of in-situ and ex-situ scanning electron microscopy techniques. The crystallographic directions of crack growth will also be characterized via orientation imaging microscopy before performing scanning electron microscopy analyses to identify the fracture modes. Mechanism-based mechanics models will then be developed for the prediction of crack initiation and propagation. The models will be developed in collaboration with Prof. Jean Prevost and Dr. Tim Baker in a separately funded NSF program at Princeton. The two NSF programs will, therefore, operate as a mini-center that will involve close interactions with Prof. Zhigang Suo and Prof. Anthony Evans of Princeton University.The program will also provide the Principal Investigator with the funds to continue outreach and teaching programs initiated with NSF financial support over the past five years. At the senior high school level, the PI will interact with a local high school teacher who will spend the summer working on MEMS-based teaching and research materials to take back to the classroom. The teacher will be funded through an NSF Minority Award for high school teachers. The PI will also employ two minority engineering students to work on aspects of the proposed MEMS project during the summer. The students' summer salaries will be provided by an ongoing program that is organized by the Office of the Dean of Graduate Studies at Princeton University. The program is designed to stimulate the interest of high quality minority undergraduate students in graduate studies and future academic careers. The PI will thus try to mentor the students, and encourage them to pursue graduate studies in mechanics and materials. At the graduate level, the PI will use some of the NSF funds to develop web-based instructional materials for a new course on Advanced Structural Materials that will be co-taught with Prof. Anthony Evans. The instructional materials that will be developed, include, web-based overheads for every class, and case studies that illustrate the application of mechanics and materials to the design of structural materials. Figure 1 - Schematic of the Three Stages of the Program