The 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
