Table of contents

This special issue of the Journal of Micromechanics and Microengineering is devoted to the 14th Micromechanics Europe Workshop (MME'03), which was held at Delft University of Technology, The Netherlands on 2–4 November 2003. Papers have been selected from this workshop for presentation in this special issue.

After a careful review by the MME'03 programme committee, 53 submissions were selected for poster presentation at the workshop in addition to 6 invited presentations. These covered the many aspects of our exciting field: technology, simulation, system design, fabrication and characterization in a wide range of applications. These contributions confirm a trend from technology-driven towards application-driven technological research. This trend has become possible because of the availability of mature fabrication technologies for micromechanical structures and is reflected by the presentations of some of the invited speakers. There were invited lectures about applications in the medical field, automotive and copiers, which provide evidence of the relevance of our work in society. Nevertheless, development of technologies rightfully remains a core activity of this workshop. This applies to both the introduction of new technologies, as was reflected by invited presentations on new trends in RIE and nanotechnology, and the addressing of manufacturing issues using available techniques, which will be demonstrated to be crucial in automotive applications. Out of these 59 papers 21 have been selected for presentation in this special issue. Since the scope of the workshop is somewhat wider than that of the journal, selection was based not only on the quality of the work, but also on suitability for presentation in the journal. Moreover, at the workshop, student presentation of research at an early stage was strongly encouraged, whereas publication of work in this journal requires a more advanced level.

I would like to express my appreciation for the outstanding efforts made by all involved in the workshop: the steering committee for its support, the programme committee for the review and the local organization for all the detailed planning required to make it both an interesting and enjoyable meeting. Last, but not least, I would like to thank the authors for preparing significant and exciting papers that reflect the progress made in the field of micromechanics and the 80 or so attendees for their enthusiastic participation.

This paper reviews the impact of scaling on the system performance of mechanical inertia sensors. Permanent cost pressure will result in continuous efforts to integrate more functions into further miniaturized systems. As a consequence microsystems (MEMS) will also incorporate functional nano devices such as carbon nanotubes and nanosystems might replace microsystems for certain applications in the future. Therefore, a review of the application of carbon nanotubes for sensors with a focus on mechanical sensors is provided. Nanosystems are defined as systems that involve electronic and non-electronic elements and functions on the nano scale. Surface properties of materials and the influence of texture will outbalance bulk properties on the micro and nano scale. Therefore, a discussion of mechanical material properties is added to the review. Finally it is concluded that mechanical sensors will go nano provided that self-assembly of nanostructures becomes a well-controlled fabrication technology.

It has been shown that electrical stimulation of retinal ganglion cells yields visual sensations. Therefore, a retina implant for blind humans suffering from retinitis pigmentosa based on this concept seems to be feasible. In Germany, there are two projects funded by the government working on different approaches namely the subretinal and the epiretinal approaches. This paper describes the epiretinal approach for such a system. The extraocular part of this system records visual images. The images are transformed by a neural net into corresponding signals for stimulation of the retinal ganglion cells. These signals are transmitted to a receiver unit of an intraocular implant, the retina stimulator. Integrated circuitry of this unit decodes the signals and transfers the data to a stimulation circuitry that selects stimulation electrodes placed onto the retina and generates current pulses to the electrodes. By this, action potentials in retinal ganglion cells are evoked, causing a visual sensation. This paper concentrates on the MEMS part of this implant.

This paper outlines the capabilities of RF MEMS tunable microinductors designed using mechanical displacements that change the magnetic coupling coefficient between circuits. The design and fabrication of a first and an optimized second tunable microinductor prototype are presented. We report a 50% inductance variation from 1.5 to 5 GHz measured on the first test wafers. Moreover, these tunable inductors, which have continuous variations, can be integrated with tunable capacitors into reconfigurable RF systems suitable for future wideband RF communications. The design of basic functions, such as a simple phase shifter cell or a tunable impedance, is lastly described.

Based on the use of a resonant cantilever, a mass sensitive gas sensor for the detection of volatile organic compounds (VOC) has been developed. Analyte gases are absorbed by a sensitive layer deposited on the cantilever: the resulting mass change of the system implies the cantilever resonant frequency decreases. In this paper, the process technology, based on the use of SOI wafer, is described. To integrate the measurement, piezoelectric and electromagnetic excitations are investigated and for the detection of microcantilever vibrations, piezoresistive measurement is performed. Then, the polymer choice and the spray coating system are detailed. Using various geometrical microcantilevers, the frequency dependence on mass change is measured and allows us to estimate the mass sensitivity (0.06 Hz ng⁻¹). In gas detection the first experiments exhibit the sensor response, then by calculating the partition coefficient (K = 977), the minimum detectable concentration of ethanol is deduced and permits us to estimate the gas sensor resolution (14 ppm). Finally a comparison between millimeter size and micrometer size cantilevers shows the importance of noise in the design of an integrated sensor.

Polycrystalline-Si microactuators based on electrothermal principles exhibit many interesting features but their practical use is severely limited by permanent damage that may occur due to accidental overheating. Under these conditions, polycrystalline-Si structures will display irreversible structural changes ranging from slight geometrical deformations to complete damage. In this paper, an approach is presented to avoid permanent structural deformation of B-doped polycrystalline-Si based electrothermal actuators by overheating. The method allows us to distinguish reversible and irreversible actuation conditions and is demonstrated under environmental and vacuum conditions. It enables full utilization of the capabilities of B-doped polycrystalline-Si based electrothermal actuators with reproducible performance.

A careful analysis of the dynamics of the pull-in displacement reveals a metastable transient interval for devices with a Q factor lower than 1.2. The duration of this metastable regime could be up to 20 ms for the structure used in this work, depending on the damping. For typical device dimensions this regime dominates pull-in dynamics. This paper explicitly focuses on the metastable regime. The results of numerical simulations are confirmed with measurement results with the purpose of providing a better understanding of the underlying mechanisms. This may contribute to both improved actuator design and enhanced sensitivity of pressure sensors and accelerometers operating on pull-in time interval measurement. The sensitivity of the pull-in time to external accelerations is 6 × 10⁻² s/ms⁻² (∼0.6 ms mg⁻¹) for current devices and can be increased by design.

The RF-power handling capability is an important characteristic for RF-MEMS switching devices. Apart from excessive heat dissipation, the power handling capability is mainly limited by the so-called self-biasing and/or RF-latching. These two phenomena result from the fact that the available RF-power from the source induces a non-zero electrostatic pulling force on the suspended structure. So far, self-biasing of RF-MEMS switches has always been studied assuming a perfect match of the device to the network in the ON-state (i.e. no reflection) and thus a fixed dc-equivalent rms voltage on the capacitor. If the RF-power exceeds a critical value, pull-in or self-biasing occurs. In practice, however, the assumption of the perfect match is not correct as the switch capacitance increases with increasing RF-power. This will cause a change in the reflected signal and thus a decrease in the dc-equivalent voltage source. This paper gives a new insight into the RF-power handling of RF-MEMS shunt switches and, per extension, of RF-MEMS shunt tunable capacitors. We analytically show how the negative feedback on the electrostatic force introduced by the capacitive mismatch changes the pull-in characteristics of the structure and can even stabilize it, totally avoiding the pull-in phenomenon.

A closed-form relationship between the insertion loss, the externally applied mechanical shock and the RF signal voltage of a capacitive RF-MEMS shunt switch is derived. It is shown that, based on this relationship, the minimum required mechanical stiffness of the suspended structure can be calculated. This allows determination of the minimum electrostatic switching voltage in a given process flow. The results are illustrated for specifications regarding shock resistance of electronic equipment as set out in MIL-STD-883. Even under the least severe test conditions, the shocks can affect the insertion loss of RF-MEMS switches, and can provoke self-biasing. This paper gives guidelines to avoid such false operation modes. The method can also be extended to yield the sensitivity of RF-MEMS devices to harmonic vibrations.

Micromachined transformers are complex devices. Conductors, insulators and magnetic materials are manufactured on the same substrate, thus leading to transformer structures often with many manufacturing levels. We describe here a new low cost solution to make quasi-planar transformer structures. The primary and secondary coils are manufactured separately and assembled by flip-chip technology. This novel structure allows a single conductor level process. The influence of different parameters of the design on expected performances is discussed. A set of transformers was made with 1 µm thick Al conductors to assess the process validity. First measurements were carried out. A current mode operation was tested and showed an output voltage proportional to the operation frequency up to 10 MHz. Tests were carried out under no-load conditions and with resistive load (1 kΩ and 470 Ω values). A resonance mode appeared for frequency above 15 MHz but was not measured.

The demand for compact power sources with high energy density is increasing. A direct methanol fuel cell (DMFC) is a renewable energy source which works at near room temperature, and allows for easier liquid fuel storage, which makes it a potential candidate. We report the design, fabrication and characterization of a self-driven DMFC made by micromachining techniques and macro-assembly. Several designs were created on the basis of state-of-the-art DMFCs. A simplified mathematical model was used mainly to design the flow channels and verify the polarization curves, which reveal the output power of a cell. Silicon was used as a substrate for the fabrication of electrodes, and the membrane electrode assembly was provided by Ion Power, Inc. A 0.25 cm² cell showed a performance of 0.29 mW cm⁻² and an open circuit voltage of 0.7 V. Ten microliters of 6 M methanol solution is sufficient to operate the cell for more than 1 h.

An investigation into the pumping flow rates and the time-resolved membrane actuation of a microperistaltic pump integrated within a micro total analysis system (μTAS) is presented. The results include (i) the design of the driver circuit to operate the peristaltic micropump, (ii) Michelson interferometer measurements of the pump displacement and (iii) pump flow rate measurements. The peristaltic micropump, configured with three PZT actuated glass membranes and silicon channels, is integrated within the μTAS device with microfluidic reaction chambers. The micropump pumps a 1 µl droplet back and forth between the reaction chambers.

A bulk micromachining technology for fabrication of micro electro mechanical systems (MEMS) in a standard silicon wafer is presented. A fabrication process, suitable for full integration with on-chip electronics, employs advanced plasma processing to etch, passivate and release micromechanical structures in a single plasma system, and vertical trench isolation to obtain electrical isolation between the released components. Distinct electrical domains can be defined even on movable parts. The sophisticated electrical isolation between high-aspect-ratio single-crystal silicon (SCS) components allows simplification of the fabrication and improvement of the performance of existing devices and design of entirely new MEMS. The presented technology is an attractive platform for both fabrication and rapid prototyping of MEMS. This is due to a short processing time, a large freedom of design, high process flexibility and low-cost of the starting SCS substrate relative to SOI substrates. Several example microstructures demonstrating the capabilities of this technology have been successfully fabricated.

A batch fabrication technology of high-precision metal-on-insulator microspires has been developed. In the fabrication process, only a self-convergent bulk micromachining by hydrofluoric acid isotropic etching is employed to sharpen fused silica. In the isotropic wet-etch process, convex corners, once they appear on the etched spires, are kept even in over-etching. This convex-corner preservation effect allows us to define the nanometer-scale profiles at the apex of the spires with micrometer-scale mask patterns. By sputtering platinum on the fabricated silica spires, we have succeeded in building exceptionally tall (0.15 mm) and nanopointed (50 nm curvature radius) metal-on-insulator spires having our designed apex structure. Clear scanning tunneling microscopy (STM) images of highly oriented pyrolytic graphite surface have been obtained by applying the microspire to an STM probe, thus verifying their applicability to an STM tip.

We have fabricated curved optical micromirrors on silicon. We expect to be able to form open optical cavities between these mirrors and plane mirrors coated on the ends of optical fibres. The curved mirror templates have been prepared with less than 10 nm surface roughness by means of isotropic chemical etching. Two different methods are used for the fabrication of the mirrors: the first method uses a silicon oxide mask, while the second uses dry etching to form an indentation in the silicon surface that is opened out by wet isotropic etching. After coating the silicon substrate with gold, good quality mirrors are obtained with R ∼ 98% in the near infra-red. We find that a reflectivity of approximately 98% can be achieved by this method. Cavities formed from these mirrors will be useful for manipulating single atoms.

This paper presents the improvement of the signal-to-noise ratio of a silicon microphone by utilizing a high-impedance load resistor. A model is described which considers the acoustical and electrical parts of the microphone. Based on an equivalent circuit diagram, the model is able to simulate the sensitivity and the noise thus the signal-to-noise ratio. Describing the noise spectrum differentiated by parts, the thermal noise of the load resistor proves to be a main noise source as long as the resistance is relatively low.

In this paper, a piezoelectric micro-transformer processed in a SOI (silicon-on-insulator) wafer is presented. The micro-transformer consists of a unimorph circular membrane (PZT/silicon). The manufacturing uses an original method consisting of a deposition by sputtering and a etching by lift-off of thin material layers. This transformer is intended to supply micro-systems requiring a very low amount of energy. An analytical model has enabled us to determine the electrical equivalent circuit of the transformer, to express the resonance frequencies and to plot the deflection and strain shapes of the membrane. A mechanical characterization has been carried out by interferometry and an electric characterization has been conducted using a first prototype.

It is demonstrated that stroboscopic optical microscopy combined with image processing is able to detect and map in-plane bi-directional vibrations of microstructures at video rate with a detection limit down to 0.1 nm in the frequency bandwidth 0–4 MHz. Furthermore, this technique was combined with phase shifting stroboscopic interferometry and light pulse delay shifting to provide simultaneously amplitude and phase maps of out-of-plane and in-plane vibrations of micromechanical devices. Application of these techniques is illustrated on Al microdevices with pattern sizes in the micron range.

A modelling study for a tuneable micro-capacitor actuated by piezoelectric film actuator is presented in this paper. Compared to similar sized electrostatically actuated capacitors, piezoelectrically actuated capacitors have the potential to provide a larger tuning range and provide a more linear relationship between actuation voltage and capacitor gap change. FE and analytical models have been established, including both piezoelectric actuation and the electrostatic force due to bias voltage across the capacitor. The predictions of the two models are in good agreement. The models are used to investigate the displacement of the movable plate and the tuning range of a capacitor.

Zener's model for thermo-elastic loss, when applied to uniform beams undergoing flexural vibrations, gives theoretical predictions of mechanical Q-factor that often agree well with experimental measurements. The use of silicon ring resonators in MEMS devices is now becoming increasingly common. This paper considers the application of Zener's theory to thin, circular rings and presents a simple expression for the Q-factor associated with in-plane flexural modes of vibration. The theoretical predictions are shown to be in good agreement with experimental measurements for a practically relevant range of ring sizes. The relationships between ring dimensions, ambient temperature and Q-factor are explored.

This paper studies the principle of a novel voltage step-up converter based on a micromachined variable parallel-plate capacitor in combination with an electrostatic actuator. Electrical equivalent circuit and system-level SIMULINK models have been developed. Based on these models, an analysis of design parameters and expected device performance has been performed to serve as a starting point for a prototype implementation. Possible areas of application are self-powered, stand-alone sensing systems, aerospace applications and any kind of electrostatic or piezoelectric MEMS devices in general.

In this paper, the modelling of sacrificial aluminium etching in complex network geometries is presented. The structures for the etching experiments are arrays of membranes for a capacitive tactile sensor for medical applications. The modelling is based on the two-dimensional diffusion equation. A one-dimensional model for correlating etch rate and diffusion coefficient in silicon dioxide etching is extended to cover two-dimensional geometries. The etch front progression can be derived from the modelled concentration of the etchant within the etch network. This allows for the extraction of numerical values of diffusion coefficients D from the data. The values of D for the three sections of the geometry in both networks studied reflect the differences in the boundary conditions of the etching process.