Environmentally-Assisted Fatigue Cracking in Silicon MEMS Structures

0071483 Soboyejo该项目的目的是发展一个基本的理解环境辅助裂纹萌生和传播机制，以及表征微机电系统（MEMS）结构制造的单晶和多晶硅。 在初始阶段之后，将通过扫描和透射电子显微镜研究MEMS结构的微观结构，用原子力显微镜技术检查样品的表面形貌。 然后，在对单晶和多晶结构进行微拉伸实验以确定它们在单调或循环载荷下的本构行为之前，将使用取向成像显微镜测量多晶硅结构中的初始微观织构。 该计划的第二阶段将集中在二氧化硅层中的环境辅助裂纹萌生机制的表征，该二氧化硅层在暴露于空气时形成在硅MEMS结构的表面上。 该计划将探讨裂纹成核发生的假设，应力辅助溶解或破裂过程，是由于与水蒸气的表面反应引起的。 为了检验这一假设，自然裂纹萌生实验将在实验室空气中进行，并控制水蒸气分压的环境。 二氧化硅层的表面形貌的变化将被监测作为时间和应力的函数，使用原子力显微镜技术。 裂纹萌生的开始将从微型测试仪/试样配置的共振条件的变化和高放大率扫描电子显微镜检测。 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米缺口或压痕裂纹的试样进行研究。 与裂纹萌生实验一样，将在实验室空气和相对湿度受控的环境中研究裂纹扩展。 裂纹扩展的增量将从微型测试仪/试样组件的共振条件的变化中检测到。 将使用原位和非原位扫描电子显微镜技术的组合来确定裂纹增长和裂纹/微观结构相互作用的增量。 在进行扫描电子显微镜分析以确定断裂模式之前，还将通过取向成像显微镜表征裂纹生长的结晶方向。 然后将开发基于机制的力学模型，用于预测裂纹的萌生和扩展。 这些模型将与Jean普雷沃斯特教授和Tim Baker博士在普林斯顿的一个单独资助的NSF项目中合作开发。 因此，这两个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，并为首席研究员提供资金，继续开展过去五年来由NSF资助的外展和教学项目。 在高中阶段，PI将与当地的高中教师互动，他们将在夏天从事基于MEMS的教学和研究材料的工作，并带回课堂。 这位教师将通过NSF少数民族高中教师奖获得资助。 PI还将在夏季雇用两名少数民族工程专业的学生从事拟议的MEMS项目的各个方面的工作。 学生们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。 该计划旨在激发高质量的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的研究生课程。 在研究生阶段，PI将使用NSF的一些资金为高级结构材料的新课程开发基于网络的教学材料，该课程将与Anthony Evans教授共同授课。 将开发的教学材料，包括，基于网络的每一个类的开销，和案例研究，说明力学和材料的结构材料设计的应用。 图1 -该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