Controlling Crystallization Structures in Thin Si Film for Improving Characteristics of MEMS Resonators
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Controlling Crystallization Structures in Thin Si Film for Improving Characteristics of MEMS Resonators Shinya Kumagai1,3, Hiromu Murase1, Takashi Tomikawa1, Syohei Ogawa1, Ichiro Yamashita2,3, Yukiharu Uraoka2,3, and Minoru Sasaki1,3 1
Toyota Technological Institute, 2-12-1 Hisakata, Tenpaku, Nagoya, 468-8511, Japan Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0101, Japan 3 CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan 2
ABSTRACT An approach to control the tensile stress and Q factor of thin Si film beams in MEMS resonators was investigated. Metal-induced lateral crystallization (MILC) using Ni nanoparticles that were synthesized within a cage-shaped protein, apoferritin, was applied to a thin morphous Si film for making a MEMS resonator with thin film beams. The MILC produced a thin polycrystalline Si (poly-Si) film with large crystallized domain (50-60 μm) with nearly the same crystalline orientation, whereas the poly-Si film obtained by conventional annealing (without MILC) consisted of small grains (less than 1 μm) with random orientation. The MEMS resonator with a beam made of poly-Si film by MILC was fabricated. The large domain size and the improved crystallinity increased the tensile stress, and resulted in 20% increase in Q factor in the resonant characteristics. INTRODUCTION Thin Si film is widely used in MEMS/NEMS devices. Especially, it is used in the application of surface-micro/nano-machined devices because the thin Si film has nearly the same property with crystalline Si [1,2]. However, characteristics of the thin Si film significantly changes depending on the phase, amorphous or polycrystalline. Even in the polycrystalline phase, grain structures affect the mechanical properties of the thin Si film. Since single crystalline Si has the best mechanical strength and actuation properties, a polycrystalline Si (poly-Si) film with larger grains and high cyrstallinity is required. The smaller grains or more grain boundaries in Si film leads to the lower strength and more energy dissipation which degrades the actuation characteristics of the MEMS/NEMS devices. Therefore, in the effort to obtain the same good properties with single-crystalline Si film, first amorphous Si film has been prepared and then annealed to be polycrystalline. The phase change has another merit that the volume of the Si film shrinks to generate tensile stress, which is called crystallization-induced stress [3,5], and the tensile stress improves the MEMS/NEMS device characteristics [5]. So far, annealing at 650-800 ˚C was adopted to transforms the amorphous Si film into polycrystalline [3,4]. One of the technical challenges to obtain the Si films with high crystallinity is to control the generation of crystallization nuclei. In conventional annealing, the nuclei were randomly generated in the Si film. Growing grains collided with neighboring ones to stop expanding their grain size. As a consequence, there were many grains, and many grain boundaries existed
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