L. Buchaillot

Gallium Nitride as an Electromechanical Material

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnsons figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

Amplified piezoelectric transduction of nanoscale motion in gallium nitride electromechanical resonators

A fully integrated electromechanical resonator is described that is based on high mobility piezoelectric semiconductors for actuation and detection of nanoscale motion. We employ the two-dimensional electron gas present at an AlGaN/GaN interface and the piezoelectric properties of this heterostructure to demonstrate a resonant high-electron-mobility transistor enabling the detection of strain variation. In this device, we take advantage of the polarization field divergence originated by mechanical flexural modes for generating piezoelectric doping. This enables a modulation of carrier density which results in a large current flow and thus constitutes a motion detector with intrinsic amplification.

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

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.

Size and shape effects on creep and diffusion at the nanoscale

In this paper, we have investigated the size and shape effects on creep and diffusion phenomena at the nanoscale. From a classical thermodynamic model, the higher diffusion of nanostructures is explained. As creep is particularly due to diffusion processes, it is therefore important to consider it at the nanoscale. Therefore, to be able to control creep in the nanoworld, temperature and stress thresholds, taking into account the size and shape of the nanostructure, are defined.

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

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.

Design, realization and testing of micro-mechanical resonators in thick-film silicon technology with postprocess electrode-to-resonator gap reduction

This paper presents the design, fabrication and testing of high-Q high-frequency lateral-mode clamped–clamped beam micro-resonators driven by parallel-plate electrostatic transducers fabricated in a thick epipoly technology. An innovative approach is employed to reduce an intrinsically high transducer gap value (>3.0 μm) required by the need of 15 μm thick structural layer etching down to 0.2–0.4 μm after the fabrication. This is achieved by employing an electrostatic motor that approaches the actuating and sensing electrodes close to the resonator. The electrode motor is driven with 30 V dc voltage, without any dc current consumption. Two resonators having a resonance frequency of 10 MHz have been fabricated with gap values of 0.2 and 0.4 μm respectively. A comparative analysis of performances of the two resonators is given in the paper.

Electromechanical Transconductance Properties of a GaN MEMS Resonator With Fully Integrated HEMT Transducers

We investigate the response of a GaN microelectromechanical resonator where the strain detection is performed by a resonant high-electron mobility transistor (R-HEMT). The R-HEMT gate located above the 2-DEG (two-dimensional electron gas) appears to enable a strong control of the electromechanical response with a gate voltage dependence close to a transconductance pattern. A quantitative approach based on the mobility of the carriers induced in the device by the piezoelectric response of the GaN buffer is proposed. These results show for the first time the electromechanical transconductance dependence versus external biasing and confirm that active piezoelectric transduction is governed by the AlGaN/GaN 2-DEG transport properties.

Design and operation of a silicon ring resonator for force sensing applications above 1 MHz

We present an integrated force probe based on a silicon bulk-mode MEMS resonator. This device uses a silicon ring with symmetrical tips vibrating in the elliptic vibration mode. The tips enable us to make mechanical interactions with surfaces or external objects. Both excitation and detection of the resonator are integrated thanks to electrostatic actuation and capacitive detection. Apart from optical and electrical characterizations of the fabricated device, we report for the first time on the interaction between the resonator tip and a hydrodynamic force applied thanks to a water droplet. This demonstrates a first step toward high frequency atomic force probes for liquid medium applications.

Design, Fabrication, and Operation of Two-Dimensional Conveyance System With Ciliary Actuator Arrays

In this paper, we present the design, fabrication, and operation of a two-dimensional (2-D) microconveyance system. Conceptually, this system was designed as a mechanical part of an autonomous decentralized system composed of integrated micro actuator-sensor-controller cells. The presented system is a 2-D ciliary motion system (2-D CMS) composed of arrayed cantilever actuators. The actuators are made of two types of polyimide with different thermal expansion coefficients. They are thermally driven by flowing current in laminated heaters between the polyimide layers. We investigated the 2-D conveyance characteristics of the 2-D CMS consisting of 20times20 cells. Each cell has a pitch of 1420 mum consisting of four actuators of 500 mum in length. The conveyance was performed in a voltage range of 19-32 V, which corresponds to approximately 48-136 mW/cell, in a frequency range of 1-100 Hz, which corresponds to approximately 2-18 mum of minimum step size. We operated it in a feedback control scheme. In feedback operation mode, the 2-D CMS was controlled with a charge-coupled device (CCD) camera regulated by a PC and a programmable logic device (PLD). We succeeded to convey an object to a predetermined target point on the surface of the CMS.

SOI-based nanoelectrospray emitter tips for mass spectrometry : a coupled MEMS and microfluidic design

We present here micromachined nanoelectrospray emitter tips based on a microfluidic capillary slot fabricated using silicon-on-insulator (SOI) technology. We couple microelectromechanical systems (MEMS) and microfluidic design rules to ensure the rigidity of the structures by taking into account the effect of capillary forces generated by the introduction of liquids into MEMS. The SOI-based microtechnology fabrication process uses four simple steps: photolithography, dry etching, wet etching and dicing. This optimized fabrication process enables cost-effective batch production of chip-based micromachined nanoelectrospray emitter tips. The characteristics of these nanoelectrospray emitter tips were investigated using mass spectrometry. We have detected the protein lysozyme at a concentration as low as 100 fmol µL−1; our results indicate that micromachined electrospray emitter tips are useful in biomolecular analysis.