Environmentally-Assisted Fatigue Cracking in Silicon MEMS Structures

0071483Soboyejo 该项目旨在对由单晶和多晶硅制成的微机电系统 (MEMS) 结构中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本的表面形貌。 然后，使用取向成像显微镜测量多晶硅结构中的初始微观纹理，然后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。 该计划的第二阶段将重点研究硅 MEMS 结构暴露于空气后形成的二氧化硅层中环境辅助裂纹萌生机制的表征。 该计划将探讨这样的假设：裂纹成核是通过应力辅助溶解或破裂过程发生的，这些过程是由于与水蒸气的表面反应而引起的。 为了检验这一假设，将在实验室空气和水蒸气分压受控的环境中进行自然裂纹萌生实验。 将使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。 裂纹萌生的开始将通过微型测试仪/样本配置和高放大倍率扫描电子显微镜的共振条件的变化来检测。 然后将测量的引发条件与应力辅助溶解和裂纹成核模型的预测进行比较。第二阶段还将使用含有纳米或微米凹口或压痕裂纹的样本来研究环境辅助裂纹扩展的机制。 与裂纹萌生实验一样，裂纹扩展将在实验室空气和相对湿度受控的环境中进行研究。 裂纹扩展的增量将通过微型测试仪/样本组件的共振条件的变化来检测。 裂纹扩展的增量和裂纹/微观结构相互作用将使用原位和异位扫描电子显微镜技术的组合来确定。 在进行扫描电子显微镜分析以识别断裂模式之前，还将通过定向成像显微镜来表征裂纹生长的晶体方向。 然后将开发基于机制的力学模型来预测裂纹的萌生和扩展。 这些模型将在普林斯顿大学单独资助的 NSF 项目中与 Jean Prevost 教授和 Tim Baker 博士合作开发。 因此，这两个 NSF 项目将作为一个小型中心运作，将与普林斯顿大学的 Zhigang Suo 教授和 Anthony Evans 教授进行密切互动。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 NSF 财政支持发起的外展和教学项目。 在高中阶段，PI 将与当地高中教师互动，该教师将利用整个暑假研究基于 MEMS 的教学和研究材料，然后带回课堂。 该教师将通过美国国家科学基金会 (NSF) 高中教师少数群体奖获得资助。 PI 还将在夏季雇用两名少数族裔工程专业学生来研究拟议的 MEMS 项目的各个方面。 学生的夏季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。 该项目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职业的兴趣。 因此，PI 将尽力指导学生，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 将使用部分 NSF 资金为高级结构材料新课程开发基于网络的教学材料，该课程将与 Anthony Evans 教授共同教授。 将开发的教学材料包括每个班级的基于网络的管理费用，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 - 该计划的三个阶段的示意图

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