Environmentally-Assisted Fatigue Cracking in Silicon MEMS Structures

该项目旨在对由单晶硅和多晶硅制成的具有良好特性的微电子机械系统(MEMS)结构中的环境辅助裂纹引发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结构的微结构的初始阶段之后，将使用原子力显微镜技术来检查样品的表面形貌。然后使用取向成像显微镜测量多晶硅结构中的初始微织构，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加载下的本构行为。该计划的第二阶段将侧重于表征暴露在空气中时在硅MEMS结构表面形成的二氧化硅层中的环境辅助裂纹引发机制。该计划将探索裂纹形核是通过应力辅助溶解或破裂过程发生的假设，这些过程是由于表面与水蒸气反应而引起的。为了验证这一假设，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。使用原子力显微镜技术将监测二氧化硅层表面形貌的变化作为时间和应力的函数。裂纹萌生的开始将从显微测试仪/样品配置的共振条件的变化和高倍率扫描电子显微镜中检测到。然后将测量的起裂条件与应力辅助溶解和裂纹形核模型的预测进行比较。在第二阶段，还将使用包含纳米或微缺口或压痕裂纹的试件来研究环境辅助裂纹扩展的机制。正如在裂纹萌生实验中一样，将在实验室空气和相对湿度可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/试件组件的共振条件的变化中检测到。裂纹扩展的增量和裂纹/微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相结合来确定。在进行扫描电子显微镜分析以确定断裂模式之前，还将通过定向成像显微镜来表征裂纹扩展的晶体方向。然后将开发基于机理的力学模型来预测裂纹的萌生和扩展。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NSF项目中合作开发。因此，这两个NSF项目将作为一个微型中心运作，涉及与普林斯顿大学的索志刚教授和Anthony Evans教授的密切互动。该项目还将为首席研究人员提供资金，以继续开展过去五年NSF财政支持下发起的外展和教学项目。在高中层面，PI将与当地一名高中教师互动，这名教师将在夏季从事基于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高中教师少数族裔奖获得资助。PI还将在夏季雇用两名少数族裔工程学学生，从事拟议的MEMS项目的各个方面的工作。学生的暑期工资将由普林斯顿大学研究生研究主任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族裔本科生对研究生学习和未来学术生涯的兴趣。因此，PI将努力指导学生，并鼓励他们继续攻读力学和材料方面的研究生课程。在研究生层面，PI将使用NSF的一些资金为一门新的高级结构材料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节课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-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