Mechanical Behavior of Ceramic/SAM Bilayer Coatings
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Mechanical Behavior of Ceramic/SAM Bilayer Coatings Quan Yang, Guangneng Zhang, Kaustubh Chitre and Junghyun Cho Department of Mechanical Engineering State University of New York at Binghamton Binghamton, NY 13902, U.S.A. ABSTRACT Ceramic/self-assembled monolayer (SAM) bilayer films can provide adequate protection and/or act as a multipurpose coating for microelectronics and MEMS applications, due to synergistic effects of the hybrid film structure. The organic SAM acts as a “template” for the growth of the ceramic film while the hard ceramic can provide protection from environmental and mechanical impact. To process the bilayer films, a low-temperature solution deposition technique (biomimetic process) is employed using phosphonate-based SAM and zirconium oxide precursors. A particulate zirconium oxide film is formed by enhanced hydrolysis of zirconium sulfate solutions in the presence of HCl at about 80°C, and its particle size and thickness effects are discussed. In addition, microstructure and micromechanics involved in the synthesis of both zirconia films and SAM are systematically assessed. Especially, mechanical properties such as Young’s modulus and hardness are analyzed using a nanoindenter, as well as with the aid of theoretical models. Further, the substrate effects resulting from a large indentation depth relative to the film thickness are eliminated to obtain the “film-only” properties. This study also highlights the role of compliant SAM layer in forming a strain-tolerant bilayer film. INTRODUCTION A common issue of MEMS is the need for hermetic packaging in order to isolate and protect the device against adverse environmental effects. Hermetic packaging, however, not only increases the size of a MEMS device, but also raises the cost. In addition, proper surface modifications are essential for specific MEMS applications [1]. One sensible way to tackle these problems is to produce robust surface coatings on the surface of silicon. Such features would in turn benefit the MEMS packaging development by relaxing the stringent requirements for the assembly, thereby drastically reducing the packaging cost. Ceramics, due to good mechanical and thermal properties, are good candidate materials for surface coatings. However, the major challenge to producing ceramic films on silicon is to avoid the brittleness such as crack formation and spallation of the films while having a capability to process at low temperatures. In an attempt to grow the strain-tolerant ceramic films, self-assembled monolayer (SAM) with phosphonate surface functionalities is pre-deposited on single-crystal silicon substrates. This self-assembly technique has been used to grow an organic monolayer on the Si/SiO2 substrate [2-5]. The underlying SAM acts as an organic template for the subsequent growth of ceramic film, as well as a buffer layer to relieve the stress in the ceramic film. This process copies the biomineralization of inorganic (ceramic) materials frequently found in biological environments (called ‘biomimetic’ process). In
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