Showing papers by "Lionel Buchaillot published in 2006"

Variation of absorption coefficient and determination of critical dose of SU-8 at 365 nm

Abstract: The absorption coefficient of thick-films of the negative photoresist SU-8 is observed to be time dependent during photolithographic exposure by I-line ultraviolet light (λ=365nm); varying linearly from 38±1cm−1 to 49±1cm−1 for a surface exposure dose of 415mJ∕cm2. We develop a general model which enables the exposure dose to be calculated at a given photoresist depth for a given exposure time. We determine the critical exposure dose for the subsequent polymerization of SU-8 having an arbitrary thickness to be 49.4±3.9mJcm−2.

The stability and pull-in voltage of electrostatic parallel-plate actuators in liquid solutions

Abstract: This paper deals with parallel-plate electrostatic actuators in liquids. We study the stability conditions of such actuators and show that the pull-in effect can be shifted beyond one-third of the gap, and can even be suppressed. We demonstrate that the insulating layers of the actuator plates, which are originally designed to avoid any current leakages or short-circuits, play a major role in this phenomenon. Experiments are performed on fabricated devices; silicon nitride layers are used to completely encapsulate the actuator plates. The voltages required to close the actuator gap are measured in various liquids and compared to the values obtained by analytical calculations. This study gives guidelines for the design of parallel-plate actuators featuring in liquids either a binary-state operation when the pull-in effect occurs, or a continuous displacement within the full gap.

Ultra-Low Voltage MEMS Resonator Based on RSG-MOSFET

Abstract: 16MHz and 91MHz micromechanical resonators based on the Resonant Suspended-gate MOSFET (RSG-MOSFET) architecture are demonstrated. A fabrication process using a polysilicon sacrificial layer and an aluminum-silicon alloy (AlSi 1%) suspended-gate was developed. Static and dynamic electrical characteristics of the Clamped-Clamped beam (CC-beam) resonator have been investigated in order to explain the device behavior. The lowest reported actuation voltage for a MEMS resonator, less than 1V, makes the resonator compatible with standard CMOS voltages. The actuation voltage dependence on the MOSFET characteristics and the threshold voltage is explained. A resonant frequency tuning of 750kHz was achieved by a 240mV DC voltage variation, and a quality factor of Q= 641 was calculated from measurements in vacuum.

Suppression of the pull-in instability for parallel-plate electrostatic actuators operated in dielectric liquids

Abstract: We study electrostatic parallel-plate actuators operated in dielectric liquids. The stable operation zone of the actuators in common liquids is extended far beyond one-third of the initial gap, which is the typical limit of stability for such actuators operated in air. The pull-in effect can even be totally suppressed. In spite of higher dielectric constants in liquids than in air, which is expected to magnify the electrostatic forces, the required voltages to actuate are not significantly reduced and may even be larger. These results are discussed with regard to the dielectric constant of the liquids and to the electrical insulating layers of the parallel plates which appear to play a major role. Finally, the analytical approach is compared to experiments performed on fabricated devices.

Coupled-resonator micromechanical filters with voltage tuneable bandpass characteristic in thick-film polysilicon technology

Abstract: This paper reports on the design and characterization of 2.4 and 9.2 MHz fourth and sixth order passband microelectromechanical coupled-resonator filters implemented in a 15 μm thick-film epitaxial polysilicon technology. The work uses a novel approach in the building of coupled-resonator electromechanical filter structures using on-line controlled electrostatic coupling. Mechanical springs commonly used to couple micromechanical resonators were replaced by electrostatical links created by pairs of biased electrostatical transducers. To control the coupling strength, an original biasing scheme of electrostatical transducers is proposed. The idea is to maintain a fixed charge on the floating-potential middle electrode of the electrostatical coupler. This electrode does not need any external electrical connection, thus does not suffer from parasitic capacitances. The coupling factor is directly controlled by coupling transducer bias voltages, allowing the control of the filter pole frequencies. A voltage controlled bandpass filter at 9.2 MHz showed a bandwidth with a tuning range of 3.18–47.4 kHz.

Alteration of superconductivity and radial breathing modes in suspended ropes of carbon nanotubes by organic polymer coatings

Abstract: We have altered the superconductivity of a suspended rope of single walled carbon nanotubes, by coating it with organic polymers. Upon coating, the normal state resistance of the rope changes by less than 20%. But superconductivity, which on the bare rope shows up as a substantial resistance decrease below $300\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, is gradually suppressed. We correlate this to the suppression of radial breathing modes, measured with raman spectroscopy on suspended single and double-walled carbon nanotubes. This points to the breathing phonon modes as being responsible for superconductivity in ropes of single walled carbon nanotubes.

Electrostatic actuators operating in liquid environment: suppression of pull-in instability and dynamic response

Abstract: This paper presents results about fabrication and operation of electrostatic actuators in liquids with various permittivities. In the static mode, we provide experimental and theoretical demonstration that the pull-in effect can be shifted beyond one third of the initial gap and even be eliminated when electrostatic actuators are operated in liquids. This should benefit to applications in microfluidics requiring either binary state actuation (e.g. pumps, valves) or continuous displacements over the whole gap (e.g. microtweezers). In dynamic mode, actuators like micro-cantilevers present a great interest for Atomic Force Microscopy (AFM) in liquids. As this application requires a good understanding of the cantilever resonance frequency and Q-factor, an analytical modeling in liquid environment has been established. The theoretically derived curves are validated by experimental results using a nitride encapsulated cantilever with integrated electrostatic actuation. Electrode potential screening and undesirable electrochemistry in dielectric liquids are counteracted using AC-voltages. Both experimental and theoretical results should prove useful in micro-cantilever design for AFM in liquids.

Remote Actuation of Independent Electrostatic Distributed Micromechanical Systems (DMMS) for a Wireless Microrobot

Abstract: The realization of microrobots is one of the important exploration fields of research in microsystems. Many applications are expected, particularly in the field of microassembly and test of circuits in confined environments (M. Takeda, 2001). Towards the realization of a wireless microrobot, we report in this paper for the first time the successful asynchronous remote operation of two distributed micro mechanical systems (DMMS). The DMMS are made of a new version of the large stepwise motion electrostatic actuator realized in doped polysilicon and reported at MEMS 2002 (P. Basset et al., 2002). Each DMMS is composed of 1,700 actuators, presents a capacitance of 2 nF and a pull-in voltage of 20 V. Remote control is obtained by inductive coupling at 13.56 MHz thanks to an IC specially dedicated to that purpose. This circuit makes it possible to power but also to remote control a double electrostatic ciliary motion system (CMS) (M. Ataka et al., 1993), allowing a two degrees of freedom displacement. Voltages up to 100 V can be used. The receiver antenna, realized in thick electroplated gold on an epoxy substrate to prevent eddy current, is also presented. The emitter system needs 9 W to fully actuate the DMMS at a distance of 1 cm. The maximum actuation frequency is 20 Hz