Performance of Thin Film Silicon MEMS on Flexible Plastic Substrates
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0989-A10-02
Performance of Thin Film Silicon MEMS on Flexible Plastic Substrates Samadhan Patil1, Virginia Chu1, and Joao Pedro Conde1,2 1 INESC MN, Rua Alves Redol, 9, Lisbon, 1000-029, Portugal 2 Dept. of Chemical and Biological Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, 1, Lisbon, 1049-001, Portugal ABSTRACT Microresonators based on thin film hydrogenated amorphous silicon microbridges were fabricated by surface micromachining on flexible polyethylene terephthalate (PET) substrates with a maximum processing temperature of 110∫C. An aluminum sacrificial layer is used which is patterned by either wet etching or lift-off. Resonance in the MHz range was observed using electrostatic actuation. Processing of the microbridges on PET with sacrificial layer patterned by lift-off has higher yield than by etching. Bending measurements show that the thin film silicon microbridges on PET can withstand a higher compressive strain (-2.5%) than tensile strain (1.25%). INTRODUCTION Thin film technology used to deposit amorphous silicon by Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) allows semiconductor device processing at temperatures ~100-250˚C [1]. The combination of thin film deposition technology with microelectromechanical systems (MEMS) processing opens the possibility of realizing thin film MEMS devices on large area, low cost, and transparent substrates. Standard MEMS are fabricated on silicon wafers using processing temperatures at or above 600˚C [2,3]. The low temperature processing of thin film MEMS fabrication technology has the advantage of being CMOS compatible. Hydrogenated amorphous silicon (a-Si:H) based MEMS and bolometers have been previously demonstrated on glass substrates [4-6]. Microelectronics on large area, flexible substrates is being pursued because of their low cost, low weight, robustness, mechanical flexibility and their simple processing. Devices on flexible substrate are valuable in medical electronics; they are biocompatible and can be used in disposable devices. The field of flexible electronics has already produced devices such as thin film transistors (TFTs) [7,8], image sensors [9], organic TFTs [10] and LEDs [11] fabricated on flexible plastic substrates. In this study, thin-film silicon MEMS microbridges are fabricated on a 250 µm-thick flexible transparent polyethylene terephthalate (PET), and their performance in the as-fabricated state, as well as after substrate bending, is evaluated. EXPERIMENTAL PROCEDURES The device fabrication process sequence is shown in figure 1. The fabrication sequence depends on the way the sacrificial layer is patterned. First, n+-a-Si:H (1000 ≈) is deposited on the PET substrate using RF-PECVD at 100˚C. This n+-a-Si:H acts as an anchor layer for the TiW (500 ≈) gate electrode, which is deposited on top of it by DC magnetron sputtering. This bilayer is patterned to form the gate electrode. Next, the Al sacrificial layer (7500 ≈) is patterned either by etching or by lift-off as shown in branches (a) and (b) in fig. 1, r
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