Amorphous thin film materials are more and more commonly adopted in the emerging new applications of MEMS. However, their mechanical behavior is still not yet nearly well understood as compared with their crystalline counterparts. The overall objective of this work is to contribute to the scientific understanding of the causal mechanisms of the mechanical responses of amorphous thin film materials so as to permit the design and construction of efficient MEMS structures and devices. Specifically, mechanical responses of the plasma-enhanced chemical vapor deposited silicon oxide thin films will be probed under different thermal conditions, stress levels, size scales, and in both elastic and plastic regions. Various distinctive characteristics of the mechanical behaviors of the thin films will be characterized systematically, and the underlying physical causal mechanisms will be analyzed in depth.
Educationally, given the broad interdisciplinary nature of this research, graduate students will have an outstanding opportunity to work in an area that bridges basic research and application. In addition, two to three undergraduate researchers will be hired to work on this project, and women and minority students will be included via the University's collaborative relationship with the Society of Women Engineers and Minority Engineers Society.
The importance of successful development of Microelectromechanical Systems (MEMS) and its descendent NEMS is clear to our community, which is also aware of the need for leading researchers in the field to identify promising new materials and structures for MEMS/NEMS and to master and characterize technologies that will qualify these materials and structures for robust and reliable system applications. From the time of the earliest research in MEMS, researchers have concentrated most heavily on silicon and the technologies derived from the fabrication of integrated circuits. Although further research based on IC fabrication methods is certainly still very active, it is also clear that new opportunities for MEMS and its descendent NEMS are likely to be realized by exploiting new materials and processes. In MEMS/NEMS many of the most important failure mechanisms such as fracture, buckling and delamination are mechanical in nature. Successful design and fabrication of MEMS/NEMS requires a fundamental understanding of the mechanics and physics of thin films. Traditionally, most of the previous research efforts have been focusing on crystalline/polycrystalline thin films. While amorphous thin film materials are more and more commonly adopted in the emerging new applications of MEMS/NEMS, their mechanical behavior is still not yet nearly well understood as compared with their crystalline counterparts. In particular, amorphous silicon oxide, silicon nitride, silicon oxynitride, and silicon oxycarbide films are four most commonly used ones, and they often have severe stress-related reliability problems under different processing conditions. Further, their mechanical responses are found to be influenced by the physical dimensions of the structures, such as film thickness. The overall objectives of this project is to contribute to the scientific understanding of the causal mechanisms of the mechanical responses of amorphous thin film materials so as to permit the design and construction of efficient MEMS/NEMS. Mechanical responses of the amorphous thin films are probed at: 1) different size scales (from wafer level down to micro/nanoscale, and different length-scale within each realm; 2) different temperatures (from room temperature up to high temperatures); and 3) with a combination of different experimental techniques (such as substrate curvature measurements, nanoindentation tests, and a novel 'microbridge testing' technique). Various distinctive characteristics of the mechanical behaviors of the thin films are systematically characterized, and the microstructural causal mechanisms for each of them are analyzed in depth. In this project, we have achieved significant results on the following five topics of research, namely, 1) Systematic study of the intrinsic stress evolution and related material properties changes during thermal processing (thermal cycling and annealing); 2) Investigation of the temperature-induced deformation mechanisms of the amorphous thin film materials, with a focus on the influence of both the thermal processing conditions and the film thickness; 3) Development of new and improved experimental techniques for measuring the microscale residual stress and elastic properties of the thin film materials; 4) Comprehensive characterization of the plastic responses of the thin films, including at different size scales as well as under different stress levels and time-scales; and 5) Analysis of the stress-induced plastic deformation mechanisms in the amorphous thin films. Of specific interests are character of plastic flow, and cause of rate-sensitivity and size-effect. The scientific insights from the in-depth study of the causal mechanisms of the mechanical responses of the amorphous thin film materials will contribute to the analysis of many other similar materials at micro- or nano-scales commonly used in MEMS/NEMS. Our vision is to transform the science and engineering community’s capacity to probe amorphous thin film materials at micro and nano scales by developing both new theoretical models and experimental methodologies. The resulting engineering base will address important societal needs ranging from materials design and surface engineering to MEMS/NEMS and nanotechnologies. These characterization techniques and platforms will allow scientists to ask profound and previously intractable questions related to amorphous thin film materials, and to obtain quantitative empirical answers with resolution sufficient to characterize amorphous thin film materials in small scales. This project helps to build national and international communities of students and faculty through broadening participation and collaborations in multidisciplinary research. We have attracted and retained diverse excellent undergraduate and graduate students, particularly minorities and women to work on this project. We have established a productive collaboration with National Cheng Kung University (Taiwan) and Hong Kong University of Science and Technology (Hong Kong). Given the broad interdisciplinary nature of the project, undergraduate and graduate students have had an outstanding opportunity to work in an area that bridges basic research and applications. This project also served as an excellent vehicle for the teaching of a contemporary MEMS/NEMS course that the PI has developed at Boston University for both graduate students on-campus and from the industry, through the University’s Distance Learning Program. In addition, our work has been published in top journals, and presented at a dozen of national and international conferences.
