Application of Thin-Film Micromachining for Large-Area Substrates
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M. BOUCINHA', V. CHU', V. SOARES' AND J. P. CONDEE 1 Instituto de Engenharia de Sistemas e Computadores (INESC), R. Alves Redol, 9, 1100 Lisboa, Portugal, [email protected] 2 Department of Materials Engineering, Instituto Superior T~cnico (IST), Av. Rovisco Pais, 1049-001 Lisboa, Portugal ABSTRACT Surface micromachining is used with amorphous silicon, microcrystalline silicon, silicon nitride and aluminum films as structural materials to form bridge and cantilever structures. Low temperature processing (between 110 and 250 'C) allowed fabrication of structures and devices on glass substrates. Two processes involving different materials as the sacrificial layer are presented: silicon nitride and photoresist. The mechanical integrity of the fabricated structures is discussed. As examples of possible device applications of this technology, air-gap thin film transistors and the electrostatic actuation of bridges and cantilevers are presented. INTRODUCTION Microelectromechanical systems (MEMS), which extend the silicon microelectronics technology to the fabrication of 3-dimensional micromachined structures, is a fast growing field and is already competitive against the traditional sensor and actuator field [1-4]. An important advantage of MEMS lies in the possibility of producing a sensor, an actuator and their control electronics together in the same substrate (and, potentially, also the functions of communication) [4,5]. Various mechanisms of sensing and actuation can be used, such as electrostatic, magnetic and electromagnetic, piezoelectric, shape-memory alloys or thermoelectromechanical. MEMS applications to a variety of fields are being developed using these mechanisms: micromotors, biosensors, optical systems, pressure sensors, fluid systems and accelerometers among others [511]. Some are already being marketed [10]. The fabrication processes used in MEMS have been heavily based on those of silicon microelectronics with some extra demands [12]. The first similarity is the extensive use of c-Si as the base substrate. From this starting point, a variety of processing techniques are possible, most relevant of which are bulk micromachining and surface micromachining. In bulk micromachining, the mechanical structures are created within the silicon wafer by selectively removing wafer material. For example, the c-Si wafer is covered with a patterned mask film (silicon carbide or silicon nitride) and then it is etched either isotropically (typically, using wet etch with a HF/Nitric acid mixture or dry etch using XeF 2) or anisotropically (typically, using wet etch with KOH, TMAH or EDP) to produce the electromechanical structure [1,13]. In surface micromachining, the c-Si substrate is first coated with a thin-film isolation layer (usually silicon nitride). Then alternate layers of controlled-stress polysilicon (typically deposited at 580 'C by LPCVD and annealed at 1050 'C) and sacrificial silicon dioxide are deposited and photolithographically patterned. The sacrificial layer is removed by HF. In principle, it is poss
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