13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Wear Life Test and Mechanisms of Silicon MEMS Devices under Different Gas Environments Sihan Shen and Yonggang Meng* State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China * Corresponding author: mengyg@tsinghua.edu.cn Abstract: Microelectromechanical systems (MEMS) are usually fragile to wear problems. In this research, a bulk-fabricated side-wall Si-MEMS tribotester with the feature of on-chip buckle loading mechanism was designed and used to study the wear life and the mechanisms of early stage wear of resonant MEMS devices in different gas environments. Two distinguishable wear mechanisms are recognized: (1) in dry N2 or O2/N2 mixture environments, wear exhibited an adhesive feature that the instantaneous wear rate is inversely proportional to the wear depth; (2) if corrosive vapors, such as fatty alcohols, are introduced into the environment, chemical reaction can limit the wear rate as a constant. In more complex situation, such as trifluoroethanol vapor, the rubbing process shows a transition from a short adhesive wear phase to a corrosive wear process. The thermal- and tribo-solvolysis could not give a full explanation of the life-time tests for the tested fatty alcohols. Therefore, there must be derivative reactions of grafting groups at silicon surfaces, which cause the instability of such groups. A mechanochemical mechanism is established to understand the stability of grafting groups in friction situation. Compared with fatty alcohols, fluoroalcohol is much more stable and can be used as vapor lubricant for Si-MEMS devices. Key words: MEMS, wear, solvolysis, mechanochemistry 1. Introduction Microelectromechanical systems (MEMS) are a class of IC-compatible sophisticated mechanisms to realize highly controllable precision motions at micro/nano scales. Compared with the failure modes of fracture and fatigue, tribological problems, including adhesion, friction and wear, are the main factors which give rise to device failure at such scales owing to the high surface-to-volume ratios of these devices [1]. Until now the adhesion problem can be solved by lowing down the surface energy using self-assembly monolayers (SAM), such as fluoro-substituted tris(dimethylamino)silane [2]. However, SAM could not provide persistent protection of contacting surfaces from wear to achieve enough life-time [2-4]. To overcome the wear problem, lubricant needs to be able to diffuse onto the moving part continuously. Two instances of vapor phase have been successfully used to lubricate tribo-MEMS devices, one is perfluorodecanoic acid vapor using for protecting the aluminum protrusion in Digital Micromirror DeviceTM [5], and the other is 1-pentanol vapor which is effective to prolong the life-time of silicon micro-tribotester fabricated by Sandia National Laboratory [6]. The possible reasons for the good lubricity of these two lubricants are not exactly the same. For the perfluorodecanoic acid, the competitive adsorption may reduce the corrosion of water on aluminum material, while the 1-pentanol can react with silicon in a soft manner to avoid adhesive wear, which is the most common wear mechanism of silicon materials [7]. In order to expand applications of MEMS, looking for a suitable lubricant and understanding the wear mechanism at micro scales becomes particularly important. For tribo-MEMS, devices are easy to fail at the early stage of wear, only allowing for blunt rough asperities and a small number of wear debris. To study the early stage of wear, a
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