Microsystem Devices Research Articles

Silicon-carbide-based MEMS for gas detection applications

Gas sensors are devices that can detect and/or discriminate gases in their surroundings. Some of these devices are based on vibrating structure covered with a coating sensitive to the species to detect. But such a layer can cause device failures issues like ageing, low reliability and high response time. Nonetheless, gas sensors are of importance for industrial environments in many applications. In addition, in some cases, the sensors must operate in harsh environments, that can lead to a severe degradation of the devices.In this paper, we propose to review different MEMS devices, without any sensitive layer, for gas detection applications. The objective is to measure a physical property of the gas in order to determine its concentration. With the microsystem devices, limits of detection as low as 0.2 % has been obtained, illustrating the capabilities of the structures elaborated. And in our case, due to the absence of sensitive film that must be adapted according to the species to detect, it leads to generic sensors, compatible with many different gases. Moreover, by combining the measures of 2 physical parameters, the discrimination of the gases, with their respective concentrations, is accessible.

CMOS Compatible Al‐Doped ZnO Sol–Gel Thin‐Film Properties

Zinc oxide (ZnO) is a low‐cost class of n‐type inorganic semiconductors with a large exciton binding energy (≈60 meV), wide direct bandgap (3.37 eV), and the most important material for various fields of industrial and deep‐tech applications. Herein, a complementary metal–oxide–semiconductor (CMOS) temperature compatible (400 °C)‐ integrated circuit (IC) process based on the sol–gel method is described. The properties of aluminum (Al)‐doped ZnO (AZO) thin films were investigated. The Al content, Al/(Al+Zn) ratio, varies from 0 to 10% and exhibits compressive stresses from −4 to −1.8 GPa. At low dopant concentrations, the Al content acts as an electrical dopant, while at higher dopant concentrations, it acts as an impurity. The electrical resistivity, which was only 3 × 10−3 Ω cm, is inversely related to the orientation of the thin film, which was preferably along the (0 0 2) direction. The optical bandgap energy of AZO thin films was determined to be in the range of 3.34–3.87 eV. Herein, a novel method to change the Al content of doped AZO thin films to improve their properties is described, which is suitable for next‐generation flexible, microsystem, and optoelectronic devices.

Controlling the Fractality and Size of Silver Clusters in the One-Step Synthesis of Ag–ZnO Heterostructures

On the basis of the sol–gel method, techniques for the combined and simultaneous synthesis of fractal silver nano- and microclusters and a matrix of the ZnO metal–oxide semiconductor material have been developed. The possibility of flexibly controlling the size and fractal dimension of metal structures by changing the concentration of silver nitrate in the initial sol is demonstrated. The proposed approaches can be used in the manufacture of photocatalysts, thermistors, biosensors, and other nano- and microsystem devices and tools.

Ultrathin electrochemically driven conducting polymer actuators: fabrication and electrochemomechanical characterization

Electronic conducting polymer based-actuators have attracted lots of interest as alternative materials to traditional piezoelectric and electrostatic actuators. Their specific characteristics such as their low operating voltages and large strains should allow them to adapt better to soft microstructures. Recently, poly (3,4-ethylenedioxythiophene) (PEDOT) – based ionic actuators have overcome some initial stumbling blocks to widespread applications in the microfabricated devices field. These trilayer bending microactuators were fabricated (i) by sequential stacking, using a layer by layer polymerization (LbL) of conducting polymer electrodes and a solid polymer electrolyte and (ii) by micro-patterning, using standard microsystems processes. While microfabrication processing of a trilayer actuator, involving no manual handling has been demonstrated, their bending performances remain limited for practical applications. Moreover, the complete characterization of their electrical, electrochemical, and mechanical properties has never been investigated. This paper describes the optimization of PEDOT electroactive electrodes synthesized with a vapor phase polymerization process. Influence of synthesis parameters on thickness, electronic conductivity and volumetric charge density were studied to determine the guidelines for synthesizing highly efficient electrodes. Afterwards, these parameters are used to guide the LbL synthesis process of ultrathin trilayer actuators. Electrochemical and mechanical properties of the resulting microactuators have been thoroughly characterized. Bending deformation and output force generation have been measured and reached 0.5% and 11 μN respectively. This constitutes the first characterization of ionic PEDOT-based microactuators operating in air of such a thin thickness (11 μm dry and 18.3 μm swelled in 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImTFSI)). These actuators and their actuation properties are promising for future soft microsystem devices where the use of polymer actuators should be essential.

Pulsed DC magnetron sputtered titanium nitride thin films for localized heating applications in MEMS devices

Titanium nitride (TiN) thin films are deposited on Si/SiO2 substratesby using Pulsed DC magnetron sputtering and are characterized for their structural, mechanical and electrical properties for their application as localized heating elements in microsystem devices. The influence of substrate temperature on the properties of TiN films has been investigated. The correlation between the structural orientation with mechanical and electrical properties has been established. The films deposited at a substrate temperature of 300 °C have shown better structural, mechanical and electrical properties. This film has been chosen for the fabrication of microheater and its characterization. A maximum temperature of 250 °C is achieved by applying a power of 2.8 W to the microheater.

Immobilized Multifunctional Polymersomes on Solid Surfaces: Infrared Light-Induced Selective Photochemical Reactions, pH Responsive Behavior, and Probing Mechanical Properties under Liquid Phase.

Fixing polymersomes onto surfaces is in high demand not only for the characterization with advanced microscopy techniques but also for designing specific compartments in microsystem devices in the scope of nanobiotechnology. For this purpose, this study reports the immobilization of multifunctional, responsive, and photo-cross-linked polymersomes on solid substrates by utilizing strong adamantane-β-cyclodextrin host-guest interactions. To reduce nonspecific binding and retain better spherical shape, the level of attractive forces acting on the immobilized polymersomes was tuned through poly(ethylene glycol) passivation as well as decreased β-cyclodextrin content on the corresponding substrates. One significant feature of this system is the pH responsivity of the polymersomes which has been demonstrated by swelling of the immobilized vesicles at acidic condition through in situ AFM measurements. Also, light responsivity has been provided by introducing nitroveratryloxycarbonyl (NVOC) protected amine molecules as photocleavable groups to the polymersome surface before immobilization. The subsequent low-energy femtosecond pulsed laser irradiation resulted in the cleavage of NVOC groups on immobilized polymersomes which in turn led to free amino groups as an additional functionality. The freed amines were further conjugated with a fluorescent dye having an activated ester that illustrates the concept of bio/chemo recognition for a potential binding of biological compounds. In addition to the responsive nature, the mechanical stability of the analyzed polymersomes was supported by computing Young's modulus and bending modulus of the membrane through force curves obtained by atomic force microscopy measurements. Overall, polymersomes with a robust and pH-swellable membrane combined with effective light responsive behavior are promising tools to design smart and stable compartments on surfaces for the development of microsystem devices such as chemo/biosensors.

Photochemical Synthesis and Versatile Functionalization Method of a Robust Porous Poly(ethylene glycol methacrylate-co-allyl methacrylate) Monolith Dedicated to Radiochemical Separation in a Centrifugal Microfluidic Platform.

The use of a centrifugal microfluidic platform is an alternative to classical chromatographic procedures for radiochemistry. An ion-exchange support with respect to the in situ light-addressable process of elaboration is specifically designed to be incorporated as a radiochemical sample preparation module in centrifugal microsystem devices. This paper presents a systematic study of the synthesis of the polymeric porous monolith poly(ethylene glycol methacrylate-co-allyl methacrylate) used as a solid-phase support and the versatile and robust photografting process of the monolith based on thiol-ene click chemistry. The polymerization reaction is investigated, varying the formulation of the polymerisable mixture. The robustness of the stationary phase was tested in concentrated nitric acid. Thanks to their unique “easy-to-use” features, centrifugal microfluidic platforms are potential successful candidates for the downscaling of chromatographic separation of radioactive samples (automation, multiplexing, easy integration in glove-boxes environment, and low cost of maintenance).

Multifunctional and Dual-Responsive Polymersomes as Robust Nanocontainers: Design, Formation by Sequential Post-Conjugations, and pH-Controlled Drug Release

Robust, multiresponsive, and multifunctional nanovesicles are in high demand not only as carrier systems but also for applications in microsystem devices and nanotechnology. Hence, multifunctional, pH-responsive, and photo-cross-linked polymersomes decorated with adamantane and azide groups are prepared by mixed self-assembly of suitably end-modified block copolymers and are used for the subsequent postconjugation of the polymersome surface by using covalent and noncovalent approaches. For the covalent approach, nitroveratryloxycarbonyl-protected amine (NVOC) molecules as light-responsive moieties are introduced into the polymersomes through an azide–alkyne click reaction. After photocleavage of NVOC units, functional dye molecules react with the now freely accessible amine groups. The noncovalent approach is performed subsequently to introduce further moieties, making use of the strong adamantane-β-cyclodextrin host–guest interactions. It is quantitatively proven that all reactive groups have sufficient accessibility as well selective and orthogonal reactivity throughout these stepwise processes to allow the successful establishment of aimed pH- and light-responsive multifunctional polymersomes. Moreover, this sequential methodology is also applied to obtain doxorubicin-loaded multifunctional polymersomes for an efficient pH-controlled drug release. Overall, tunable membrane permeability combined with the potential for introducing multiple targeting groups by light-exposure or host–guest interactions make these smart polymersomes promising nanocontainers for many applications.

Optimization Study for Work Function Based CO2 Sensing Using CuO-nanoparticles in Respect to Humidity and Temperature

The detection of CO2, based on metal oxide nanoparticles has a lot of potential due to their higher surface area and easy implementation in microsystem devices. In this work, we investigated semiconducting copper oxide nanoparticles (CuO-NPs) with respect to different conditions affecting the stability and sensitivity of the CO2 response. The adsorption induced changes have been recorded through work function (WF) based alterations using Kelvin probe measurements. It is shown that an optimized CO2-sensing behavior for CuO-NPs by using combined effect of humidity and temperature is achievable and can play a pivotal role towards the development of low cost CO2 gas sensors.

Mechanically activated oxidizer-fuel energetic composites

Preparation of mechanically activated energetic composites (MAECs) based on solid fuels (Al, Mg, and Si) and oxidizers (S, MoO3, (-C2F4-) n , KClO4, NH4 ClO4, etc.) is considered. Compared to conventional mechanical mixtures, the burning rate of MAECs is significantly increased, and in some cases high-velocity detonation can be obtained. The propagation of the reaction in MAECs is accompanied by high energy release comparable to the heat of explosion of powerful aluminized explosive materials. The composites are highly sensitive to heat treatment and are capable of rapid transition from combustion to detonation. The results obtained in this work show that MAEC based formulations are promising energetic materials for a wide range of applications, from igniting and initiating compositions to components to small charges in microsystem devices.

Analysis of the Material Removal Rate of Nanosecond Laser Ablation of Aluminium Using a Parallel Hatching Mode

Laser ablation is a new, very flexible process for micro-fabrication of micro-moulds and other micro-system devices. This process is suitable for machining difficult-to-machine materials, like ceramics, dielectrics, carbide and hardened steel with excellent productivity and surface. The paper presents the investigation on the material removal rate of aluminium 5754 using a nanosecond Nd:YAG laser with a wavelength of 1064 nm.

Promising energetic materials composed of nanosilicon and solid oxidizers

A method for production of mechanically activated energetic compositions consisting of nanosil� icon and solid inorganic oxidizers is developed. For the compositions prepared, both highspeed burning and detonation are observed. The propagation of the reaction is accompanied by a high energy release, compara� ble to the heat of explosion of aluminized high explosives. The compositions are highly sensitive to thermal stimuli and capable of rapid deflagrationtodetonation transition. The results obtained in the work suggest that nanosiliconbased formulations as promising ener getic materials for a wide range of applications, from initiating compositions in blasting caps to compositions for small charges in microsystem devices.

Selective growth and photoelectrochemical properties of Bi2S3 thin films on functionalized self-assembled monolayers

A convenient method for preparing highly crystalline Bi2S3 thin films by combining chemical bath deposition with self-assembled monolayers (SAMs) is described. The surface morphology, structure and composition of the resultant Bi2S3 thin films were characterized by XPS, XRD and FESEM, respectively. The optical and photoelectrochemical properties of as-prepared Bi2S3 thin films with different reaction times were investigated by UV-vis absorption and photocurrent action spectroscopy. It was demonstrated that these properties were dependent on the film thickness which increased with the deposition time. According to the different deposition mechanisms on the NH2 and CH3 terminal groups, patterned Bi2S3 microarrays with different feature sizes (50, 130, 250 μm) were successfully deposited on –NH2-terminated SAMs regions of –NH2/–CH3 patterned SAMs surfaces. Clearly, with a decrease in feature size, the absorption intensity and photocurrent density of the patterned Bi2S3 thin film increased, which was due to the increased Bi2S3 surface area as well as the increased optical path length within the patterned Bi2S3 thin film, resulting from multiple reflection of incident light. The photocurrent density of the patterned thin film for the 50 μm-size feature exhibited an almost 9 times larger photocurrent density than a similar thin film without patterning. This work indicates that patterned Bi2S3 thin films are attractive systems for surface tailoring and also provide a novel approach to effectively control the photoelectrochemical properties of nanostructured Bi2S3 thin films with promising applications in microsystem devices for solar energy conversion.

ENGINEERING CURRICULUM DEVELOPMENT IN MICROSYSTEMS

Microsystems are a rapidly developing technology that integrates Micro-electro-mechanical systems (commonly known as MEMS) and microfluidics devices with microelectronics and optoelectronics circuits. Due to increasing demand and opportunities from these fields, universities are introducing or have introduced MEMS/Microfluidics courses to their students at undergraduates and graduate levels. By its nature, the field is multidisciplinary and requires various backgrounds when integrating all components of Microsystems as described above. The current challenges in this area are design, fabrication and testing the microsystem devices. This brings various constraints on both the instructors and the students from the point of teaching and learning aspects. Designing a typical microsystem requires the use of modeling and simulation tools from multi energy domains (circuit simulators, multiphysics analysis, 3D modeling, fluidic flows etc). Fabrication methods have recently been evolved from traditional microelectronics fabrication techniques to more complicated micromachining technologies (bulk vs. surface micromachining, LIGA vs. Deep Reactive Ion etching etc). Testing of Microsystems goes beyond traditional electrical testing to the motion analysis of the moving parts on the microsystems. In recent years, the application of microsystems have also been moving away from traditional telecommunication and entering into new areas like health care, energy, environment, automobile and biotechnology. This makes the teaching of microsystem technologies more challenging to fulfill the needs of students entering in Microsystems from existing disciplines (electrical, mechanical, physics, etc.) with limited background on all parts of microsystems. In addition to the multidisciplinary nature of Microsystems, limited resources (small number of design platforms versus large number of students), absence of supportive funding vs. high cost of prototyping, limited time frame vs. long fabrication periods, restrictive opportunities for students to have hands-on design experience make it difficult to offer such courses at graduate and undergraduate levels. In this paper, we discuss developing a new Microsystems course curriculum with emphasis on MEMS in university environment, particularly at Queen’s University in Canada. This curriculum is designed in a team project-based manner but in modular form. The lecture will be delivered by two main instructors and some guest instructors for different topics. The team members for each project have complementary background or coming from different discipline. The pertinent resources at Queen’s University, including such in CMC Microsystems will be available to the students which solves the shortage of resources for teaching.

Bio-inspired polymeric patterns with enhanced wear durability for microsystem applications

At micro/nano-scale, friction force dominates at the interface between bodies moving in relative motion and severely affects their smooth operation. This effect limits the performance of microsystem devices such as micro-electro-mechanical systems (MEMS). In addition, friction force also leads to material removal or wear and thereby reduces the durability i.e. the useful operating life of the devices. In this work, we fabricated bio-inspired polymeric patterns for tribological applications. Inspired by the surface features on lotus leaves namely, the protuberances and wax, SU-8 polymeric films spin-coated on silicon wafers were topographically and chemically modified. For topographical modification, micro-scale patterns were fabricated using nanoimprint lithography and for chemical modification, the micro-patterns were coated with perfluoropolyether nanolubricant. Tribological investigation of the bio-inspired patterns revealed that the friction coefficients reduced significantly and the wear durability increased by several orders. In order to enhance the wear durability much further, the micro-patterns were exposed to argon/oxygen plasma and were subsequently coated with the perfluoropolyether nanolubricant. Bio-inspired patterns with enhanced wear durability, such as the ones investigated in the current work, have potential tribological applications in MEMS/Bio-MEMS actuator-based devices.

Synthesis and Characterization of Ammonium Functionalized Porous Poly(glycidyl methacrylate-co-ethylene dimethacrylate) Monoliths for Microscale Analysis and Its Application to DNA Purification

A systematic study of parameters affecting the nucleophilic addition of secondary and tertiary amines on reactive epoxy groups was conducted on porous polymer monoliths. Reaction of small amines like diethylamine (DEA) or triethylamine (TEA) on poly(glycidyl methacrylate-co-ethylene dimethacrylate) monoliths (poly(GMA-co-EDMA)) allows to prepare anion exchange media. This study aimed to determine optimal and suitable conditions to prepare anion-exchange porous monolith inside 100 microm internal diameter capillary. The reaction kinetic of amine nucleophilic addition on porous poly(GMA-co-EDMA) monoliths was followed by FTIR-ATR spectroscopy. The reactivity of such epoxy-functionalized porous polymers was first determined through a study in pure amine solutions. Thereafter, conditions of reactions (i.e., temperature and time of reaction, solvent composition, concentration of amine) with respect to its further implementation at nanoscale, were optimized through a factorial analysis. The optimization allowed extending conversion yields of epoxy groups up to more than 90% in dilute amine solution within less than 4 hours of reaction for TEA addition. This ion-exchange support with respect to the in-situ light-addressable process of elaboration is specifically designed to be incorporated as biomolecular sample preparation module in microsystem devices. The high loading capacity obtained for the preconcentration of DNA demonstrate the attractivity of this functionalized polymeric porous monolith as solid-phase support to improve the quantity and the efficiency of DNA extraction applied into microscale format like capillaries or lab-on-chip.

Electrostatic actuators for misalignment compensation in multi-layered microsystem devices

Electrostatic actuation is a promising approach to compensate for misalignment of bonded, multi-layered microsystem devices. The present work discusses the performance of electrostatic actuators used for in-plane misalignment compensation in an atom chip comprising an optical cavity. Experimental investigation revealed that the central frame suspending the mirrors can be moved between 3-5 m in the in-plane direction for the applied DC voltage of 90 volts. Future work involves characterizing the mirror displacement for optical tuning function.

Combustion and explosion of nanostructured silicon in microsystem devices

Combustion and explosion in layers of nanostructured porous silicon has been studied in relation to the layer thickness and duration of sample storage after electrochemical anodization. The amount of hydrogen adsorbed on the surface of porous silicon after anodic treatment is evaluated. The amount of hydrogen accu-mulated in porous silicon is 4 wt %. Prototype microsystem devices employing the energy of the processes studied have been developed and fabricated. The combustion of porous silicon can be used as an energy source for silicon microactuators and the microexplosion can be employed both in self-destructing silicon chips and for dividing silicon wafers into separate crystals.

Surface Tailoring for Controlled Photoelectrochemical Properties: Effect of Patterned TiO2 Microarrays

In efforts to make use of unprecedented physical and chemical characteristics of titania (TiO2) nanomaterials and investigate their innovative developments in microsystem applications, it is essential to affix them on surfaces or arrange them in an organized network. In this work, a simple, fast, and cheap patterning technique for the fabrication of patterned TiO2 microarrays with different features is presented. As an alternative to typical pattern transfer techniques for microfabrication, this work employed a standard microcontact printing (μCP) process for the fabrication of patterned titania microarrays onto F-doping SnO2 (FTO) conductive glass substrates. During the μCP process, the titania precursor was used as the “ink” and transferred from a pattern-featured poly(dimethylsiloxane) “stamp” onto the pretreated FTO substrate. Following the subsequent thermal oxidation, patterned TiO2 microarrays with different features (100, 200, and 400 μm) were successfully achieved. The surface properties and the photoelectrochemical properties of as-prepared patterned TiO2 microarrays were investigated by scanning electron microscopy, X-ray diffraction, UV−vis absorption spectroscopy, electrochemical impedance spectroscopy, Mott−Schottky spectroscopy, photocurrent action spectroscopy, and photocatalytic degradation. It was demonstrated that these properties were dependent on the feature size of the TiO2 patterns. For the patterned TiO2 thin film photoelectrodes with 100, 200, and 400 μm patterns, the generated peak photocurrent was ca. 5, 2, and 1 nA, and the photodegradation rate constant of methylene blue was found to be 1.747%, 1.415%, and 0.96% min-1, respectively. Clearly, with the decrease of the feature size, the photocurrent action and photocatalytic ability of the patterned TiO2 thin film increased, which was due to the increased TiO2 surface area as well as the increased optical path length within the patterned TiO2 thin film, resulting from multiple reflection of incident light. This work indicates that patterned TiO2 thin films are attractive systems for surface tailoring and also provide a novel method to effectively control the photoelectrochemical properties of nanostructured TiO2 thin films with promising applications in microsystem devices for solar energy conversion, photocatalysis, sensing, and so on.

Synchrotron‐radiation computed laminography for high‐resolution three‐dimensional imaging of flat devices

AbstractSynchrotron‐radiation computed laminography (SRCL) is developed as a method for high‐resolution three‐dimensional (3D) imaging of regions of interest (ROIs) in all kinds of laterally extended devices. One of the application targets is the 3D X‐ray inspection of microsystems. In comparison to computed tomography (CT), the method is based on the inclination of the tomographic axis with respect to the incident X‐ray beam by a defined angle. With the microsystem aligned roughly perpendicular to the rotation axis, the integral X‐ray transmission on the two‐dimensional (2D) detector does not change exceedingly during the scan. In consequence, the integrity of laterally extended devices can be preserved, what distinguishes SRCL from CT where ROIs have to be destructively extracted (e.g. by cutting out a sample) before being imaged. The potential of the method for three‐dimensional imaging of microsystem devices will be demonstrated by examples of flip‐chip bonded and wire‐bonded devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)