Characterization of Strain-Tolerant Ceramic/SAM Bilayer Coatings
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Characterization of Strain-Tolerant Ceramic/SAM Bilayer Coatings Q. Yang*, T. O. Salami†, K. Chitre*, S. R. Oliver†, and J. Cho* Dept. of Mechanical Engineering* Dept. of Chemistry† State University of New York, Binghamton ABSTRACT Ceramic coatings can provide an ideal protection for MEMS (MicroElectroMechanical Systems) structures while imposing a great challenge in processing a prime, reliant coating due to their inherent brittleness and defects formed during processing. In an attempt to compensate for the weakness of the ceramic coating, we have developed a low-temperature solution precursor process to create strain-tolerant, protective bilayer coating consisting of an integrated ceramic-organic hybrid material. The top ceramic coating offers an inert, protective layer whereas the underlying nanometer scale self-assembled organic coating provides compliance for the overlying hard coating. Together, these bilayers minimize mechanical and thermal stresses. In addition, organic self-assembled monolayers(SAM) act as a ‘template’ by forming a proper surface functionality for the subsequent growth of hard ceramic coatings. Molecular level understanding of the microstructure and micromechanics involved in the synthesis and processing of the coating is systematically studied by a variety of characterization techniques such as XRD, AFM, SEM/EDS and nanoindentation. This work is also complemented by numerical simulation to provide a clearer understanding of the stress development in the ceramic coating and its interfacial properties. INTRODUCTION MEMS have received a great deal of attention because of their promising applications spanning the navigation, automotive, military, and seismic markets [1-3]. However, the cost and reliability issues with MEMS have yet to be optimized. A common issue in MEMS is the need of high-vacuum hermetically sealed packaging to isolate and protect the device against adverse environmental effects. Hermetic packaging, however, not only increases the size of a MEMS device, but it also raises the cost. It is estimated that hermetic packaging is responsible for 7095% of the overall cost of a MEMS [4,5]. Cost reduction in the fabrication of high-performance and automotive electronic and MEMS devices is indeed a main driving force for their application. One sensible way to tackle the current limitations imposed on MEMS is to deposit a protective coating layer on the surface of silicon. Protected silicon structures in MEMS may no longer need a hermetic sealing to protect them from environmental effects while providing a protection from a variety of environments. Good candidate materials for surface coatings are ceramics due to their high degree of hardness and modulus. Ceramics have indeed been employed for protective coatings in numerous applications. Particular examples include oxide ceramics such as Al2O3 and ZrO2 as well as non-oxide ceramics such as TiN, TiC, TiAlN, and cubic boron nitride (c-BN) [6-10]. Conventional ceramic processing, however, involves high temperature sintering and c
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