Thin-Film Electrostatic Actuators on Flexible Plastic Substrates
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Thin-Film Electrostatic Actuators on Flexible Plastic Substrates J. Gaspar1,2, V. Chu1 and J. P. Conde1,2 1 INESC Microsistemas e Nanotecnologias, Lisbon, Portugal 2 Dept. of Materials Engineering, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT Thin-film silicon micromachined bridge actuators are fabricated at temperatures below 110ºC on flexible polyethylene terephthalate plastic substrates. The micromechanical structures are electrostatically actuated both at the resonance frequency and at below-resonance frequencies, and the resulting deflection is optically monitored. Deflections up to 100 nm are measured below the resonance frequency with subnanometric precision. Resonance frequencies in the MHz range are observed in vacuum with quality factors of the order of 100. The movement is studied as a function of the geometrical dimensions of the actuators, of the actuation voltage and of the measurement pressure. The experimental data are analyzed using an electromechanical model. The performance of hydrogenated amorphous silicon based resonators on PET substrates is compared to that of similar microstructures on glass substrates.
INTRODUCTION Thin-film microelectromechanical systems (MEMS) combine the low-temperature processing, large-area capability, and CMOS compatibility of thin-film silicon technology with the mechanical functionality of MEMS. MEMS devices based on hydrogenated amorphous silicon (a-Si:H), such as electrostatic and thermal actuators on glass substrates [1,2] and bolometers [3], have been developed. Microelectronics processing on large area and flexible substrates is of great interest for applications requiring low-weight integrated circuits, mechanical flexibility, biocompatibility, or inexpensive, disposable electronics. Because of the low temperatures used in most thin-film processes, a wide variety of substrates can be used. Thin-film transistors (TFTs) and image sensors fabricated on plastic substrates have already been demonstrated [4]. Electrostatic microactuators are technologically important in applications such as switches, resonant radio-frequency (RF) filters and sensitive mass detectors [5]. This work presents MEMS electrostatic actuators fabricated on flexible PET substrates using surface micromachining and thin-film silicon technology.
EXPERIMENTAL PART Fabrication of electrostatic microactuators on plastic Microbridges with an underlying gate electrode are fabricated at temperatures below 110ºC on 250-µm-thick flexible polyethylene terephthalate (PET) substrates. The fabrication sequence is shown in figure 1. The substrate [fig. 1(a)] is cleaned with a degreasing solution at 100ºC for 1 hour. A 0.15-µm-thick TiW gate electrode is deposited by DC magnetron sputtering and
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Figure 1. Fabrication sequence of n+-a-Si:H microbridges on flexible plastic substrates.
patterned by lift-off using acetone [fig. 1(b)]. A 0.75-µm-thick sputtered Al sacrificial layer is patterned using wet etching [fig. 1(c)]. This is followed by the deposition of a 0.4-µm-thick phosphor
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