Low Temperature Thin-Film Microelectromechanical Devices on Plastic Substrates
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Low Temperature Thin-Film Microelectromechanical Devices on Plastic Substrates M. Boucinha1, P. Brogueira2,V. Chu1, P. Alpuim1 and J. P. Conde3 1 Instituto de Engenharia de Sistemas e Computadores (INESC), R. Alves Redol, 9, 1000-029 Lisboa, Portugal 2 Department of Physics, Instituto Superior Técnico (IST), Av. Rovisco Pais, 1049-001 Lisboa, Portugal 3 Department of Materials Engineering, Instituto Superior Técnico (IST), Av. Rovisco Pais, 1049-001 Lisboa, Portugal ABSTRACT Air-gap micromachined structures such as bridges and cantilevers were fabricated on 50 and 125 µm-thick polyethylene terephthalate (PET) plastic substrates. The maximum processing temperature using PET is limited to 110 ºC. Two surface micromachining processes on PET which used two different sacrificial layers - photoresist and Al - were developed. Several materials were used as structural layers in the microstructures including Al, TiW, amorphous silicon (a-Si:H) and a bilayer of a-Si:H and Al. The maximum length of free-standing bridges and cantilevers is discussed as a function of the fabrication process. The bridge structures were actuated electrostatically, in a DC switch setup configuration, and the critical voltage as a function of the length was measured. Mechanical actuation and optical detection were used, in an AC mode, for the measurement of the resonance frequency of bridge structures. INTRODUCTION Microelectromechanical systems (MEMS) represent a class of 3-dimensional devices which can be fabricated using standard silicon technology and that bring with them the extra capacity of mechanical movement [1]. The possibility of shrinking the dimensions of sensors and actuators to the micrometer size, and also the possibility of the total integration with its control electronics on the same substrate have been powerful motivation for the explosive growth of this technology [2-7]. The base substrate of choice for MEMS has been crystalline silicon. In bulk micromachining, the mechanical structures are created by selectively removing wafer material. In surface micromachining, alternate sacrificial and structural materials are deposited on the c-Si layer. The most common structural material has been low-stress poly-Si which requires an anneal above 900 ºC. Recently, thin-film MEMS have been developed using surface micromachining on glass substrates [8,9]. Thin-film MEMS have some advantage over standard MEMS in that they can potentially be fabricated on large area substrates, they increase the options available in the choice of layers and processing techniques, and they are inherently more economical (in particular for the fabrication of small series of devices). Plastic substrates, which are flexible, lightweight, unbreakable and inexpensive are a natural substrate to expand the applications of thin-film MEMS. In this paper, two different surface micromachining processes for the fabrication of airgap bridge and cantilever structures on a inexpensive plastic substrate, PET, at low temperatures, A21.2.1
are presented. The structures are cha
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