Table of contents

Crystalline silicon has become more and more important for optical MEMS. The increased need of bandwidth in optical communication networks has led to a number of new optical MEMS devices such as moving-fibre and moving-waveguide switches, optical cross-connects, mirrors, resonators, optical benches and fibre alignment structures which are based on wet anisotropic micromachining techniques and deep reactive ion etching of single-crystal silicon. The excellent mechanical properties as well as the optical properties and orientation-dependent etching behaviour of crystalline silicon attract attention for reliable optical components based on surface micromachining, silicon-on-insulator, integrated optics and bulk micromachining. Anisotropic etching is especially useful for the batch fabrication of large opto-mechanical devices with sub-μm precision.

We have investigated the electrical damage by anodic bonding on CMOS-quality gate oxide and methods to prevent this damage. n-type and p-type MOS capacitors were characterized by quasi-static and high-frequency CV-curves before and after anodic bonding. Capacitors that were bonded to a Pyrex wafer with 10 μm deep cavities enclosing the capacitors exhibited increased leakage current and interface trap density after bonding. Two different methods were successful in protecting the capacitors from such damage. Our first approach was to increase the cavity depth from 10 μm to 50 μm, thus reducing the electric field across the gate oxide during bonding from approximately 2 × 10⁵ V cm⁻¹ to 4 × 10⁴ V cm⁻¹. The second protection method was to coat the inside of a 10 μm deep Pyrex glass cavity with aluminium, forming a Faraday cage that removed the electric field across the cavity during anodic bonding. Both methods resulted in capacitors with decreased interface trap density and unchanged leakage current after bonding. No change in effective oxide charge or mobile ion contamination was observed on any of the capacitors in the study.

The use of SU-8 high aspect ratio, thick, photoresist as a functional material for MEMS applications is described in this paper. SU-8 processing is developed to implement low-stress SU-8 structures as permanent and functional material incorporated with silicon-on-insulator technologies. Silicon micromachined cantilevers were fabricated with SU-8 structures on the cantilevers as added masses. Separation of material function can be achieved in this way. Silicon provides excellent mechanical properties, while SU-8 is used as extra mass to adjust the mechanical behaviour. The resonance behaviour of the cantilever structure with SU-8 is characterized through measurement, simulation and calculation, and the strength of the SU-8 material for this purpose is evaluated. The results show that SU-8 is well suited as a permanent material in mechanically active MEMS devices, and several applications are suggested. 3D MEMS architectures can also be achieved in this manner.

We present a new method for the low-cost manufacture of micro fluidic devices from polymers for single use. Within a one-step or two-step process inside a hot embossing press, micro channels are thermoformed into a thin plastic film and welded on to a thicker plastic film or sheet. Sterile, hermetically sealed micro fluidic structures were fabricated from polystyrene for easy opening immediately before use. It even appears to be possible to produce micro fluidic analysis chips from polymers on a coil from which single devices are cut off for use.

We describe the fabrication of silicon carbide layers for micromechanical applications using low-pressure metal–organic chemical vapour deposition at temperatures below 1000 °C. The layers can be structured by lift-off using silicon dioxide as a sacrificial layer. A large selectivity with respect to silicon can be exploited for bulk micromachining. Thin membranes are fabricated which exhibit high mechanical quality, as necessary for applications in harsh environments.

We present a low-temperature post-processing module, utilizing polyimide as a sacrificial layer and novel materials such as PECVD SiC and metals (sputtered aluminium and titanium) as structural layers. The use of spin-on polyimide allows an all-dry final release step overcoming stiction problems often encountered in wet sacrificial etching processes. The spinning and curing procedure has been tailored to the specific needs of the IC-compatible post-process module. For the patterning of the polyimide, thin films of aluminium, PECVD silicon oxide or silicon carbide are employed as a mask layer. Anisotropic etching of the mask film and of the polyimide layer is accomplished by RIE. After patterning the structural layer, sacrificial etching of the polyimide is done using an isotropic dry etch process in high-density oxygen plasma. An underetch rate of 4 μm min⁻¹ is achieved. Compatibility with different structural materials is tested and test structures are designed and realized in a fully post-processing surface micromachining module.

A cryogenic SF₆/O₂ plasma process has been used to investigate the etching of deep holes in silicon wafers. The influence of crystallographic and aspect ratio dependence of the etch rate on the holes profile have been explored. It was found that wafer temperature, during the etching process, played a crucial role in controlling the anisotropy and deteriorative faceting due to crystal orientation dependent etching. High anisotropy and switching of the process to crystallographic independent etching was achieved by controlling the temperature. Aspect ratio dependent etching was also identified as a serious limitation for the required homogeneity in etched depth.

Through-wafer electrical connections are becoming increasingly important for three-dimensional integrated circuits, microelectromechanical systems packaging and radio-frequency components. In this paper, we report our current results on the formation of through-wafer metal plugs using the copper electroplating technique. Several approaches for via filling are investigated, such as filling before or after wafer thinning. Among the methods experimented, the one-side Cu plating and bottom-up filling appears to be the most suitable technique for copper filling into high aspect ratio vias. Using this method, we demonstrate the successful filling of vias with an aspect ratio of up to 7. Copper plugs as small as 20 × 20 μm² are obtained uniformly over 4 inch Si wafers.

The electrodeposition process has been studied and optimized in order to obtain homogeneous Co-Ni deposits on Si/SiO₂/Ti/Ni substrates. The electrochemical and magnetic characterization of the deposited layers shows the features of a soft magnetic material, i.e. high saturation magnetization (1.2 T) and low coercivity, which may be varied as a function of the electrodeposition parameters. Good adherence, brilliant aspect, smooth surfaces and a constant deposition rate have been obtained in all the processed samples. The compatibility of the Co-Ni plating process with the main microelectromechanical systems (MEMS) fabrication techniques, surface and bulk, has been evaluated and these results widely demonstrate the versatility of this magnetic layer in MEMS production. Due to the high selectivity and homogeneity of the deposition process, it has been possible to pattern the deposits with a definition down to 3 μm and an aspect ratio of 4.6 has been achieved. A novel method to liberate the patterned structures by sacrificial etching of the seed layer has been developed. Uniform and homogeneous deposits of Co-Ni alloy have also been obtained in devices fabricated by bulk technologies. For these devices an anisotropic wet etching procedure in tetramethyl ammonium hydroxide (TMAH) has been optimized to release the magnetic layer. Low stressed free-standing structures have been obtained by both surface and bulk methods and are presented in this paper.

We report on a novel device-scale packaging by transferring an ultra-thin silicon (UTSi) capping structure, which may encapsulate microdevices. The UTSi was fabricated to form a recessed structure on the carrier glass, and then transferred on to a host substrate, thus forming a micro encapsulation. In this approach, the flip-chip assembly and tether-broken techniques are successfully utilized in demonstrating the transferred microcap on the selective area of the host substrate that is aligned through the transparent glass. Furthermore, the robustness of the UTSi microcap is even superior to those used in the poly-Si surface micromachining technique due to the high stiffness structure. As a result, the micro encapsulation presents a new packaging technique with advantages of area-selective three-dimensional micropackaging, hermetic sealing, robustness, integrated circuit packaging process compatibility and, potentially, wafer level packaging.

Thermal vertical bimorph actuators consist of silicon beams side-coated with aluminium. Upon heating they bend like a bimetal and produce movement in the wafer plane. We have measured the time constant describing the deflection response of the actuator to input pulses. Depending on the dimensions of the actuator, the time constant ranges from 0.5 ms to 3.8 ms. Lateral resonances of the first mode have been measured using thermal excitation and are found to be between 20 and 85 kHz.

An all-dry silicon-etch based micromachining process for neural probes was demonstrated in the manufacture of a probe with a 32-site recording electrode array. The fork-like probe shafts were formed by double-sided deep reactive ion etching (DRIE) of a silicon-on-insulator (SOI) substrate, with the buried SiO₂ layer acting as an etch stop. The shafts typically had the dimensions 5 mm × 25 μm × 20 μm and ended in chisel-shaped tips with lateral taper angles of 4°. An array of Ir electrodes, each 100 μm², and Au conductor traces were formed on top of the shafts by e-beam evaporation. An accompanying interconnect solution based on flexible printed circuitry was designed, enabling precise and flexible positioning of the probes in neural tissue. SEM studies showed sharply defined probes and probe tips. The electrical yield and function were verified in bench-top measurements in saline. The magnitude of the electrode impedance was in the 1 MΩ range at 1 kHz, which is consistent with neurophysiological recordings.

In this paper a piezoelectrically driven silicon membrane pump with passive dynamic valves is described. It is designed to pump gases and liquids and to be tolerant to gas bubbles. Reducing the dead volume within the pump, and thus increasing the compression ratio, one achieves the gas pumping. The main advantages and novel features of the pump described in the paper are the self-aligning of the membrane unit to the valve unit and the possibility of using screen-printed PZT as actuator, which enables mass production and thus low-cost micropumps. A liquid pump rate of 1500 μl min⁻¹ and a gas pump rate of 690 μl min⁻¹ were achieved.

Micro-pellistors and conductivity-type gas sensors are among the most promising candidates for the monolith integration of gas-sensing elements in olfactory gas detection. Both types have to be operated at elevated temperatures of 200–600 °C; therefore, the formation of thermally isolated integral micro-hotplates is the key element in the development of the array. In this paper, the alternative processes are discussed with emphasis on thermal isolation, selection of the appropriate structural materials, formation of stable contacts to the filaments and deposition of gas-sensitive layers. The results of thermal and mechanical model calculations were confirmed by experimental measurements.

The handling and assembling of microcomponents is usually considered as a bottleneck in the fabrication process of hybrid microsystems. This is especially true for very small components that require high positioning tolerances. In this paper we present gripping solutions for various applications concerning the handling and assembling of microcomponents as well as the major microgripper manufacturing technologies.

The suitability of titanium for use in microelectromechanical applications is investigated. A range of titanium microdevices, including free-standing fixed–fixed beams and cantilevers, has been successfully fabricated using a fully CMOS compatible, dry-release surface micromachining process. Finite-element simulations have been used to extract a semi-analytical model which describes the pull-in behaviour of fixed–fixed beams, while taking into account the effects of the non-ideal beam anchors. This method has been used to obtain an estimate of the Young's modulus and residual stress in the metal. Capacitance monitoring has shown that the beams remain flat after sacrificial layer release, and interferometry imaging has been used to investigate the stability of beam anchors during device actuation. Furthermore, titanium beams have remained stable under repeated actuation in initial cycle testing and may be suitable for use as a major component of microswitches. One such possible design is outlined.

This paper reports the fabrication of coils for micro-magnetic devices on silicon using thick photoresists commonly used in the manufacture of microelectromechanical systems. A comparison of three photoresists, EPON SU-8, AZ 4562 and AZ 9260, is presented for the fabrication of high aspect ratio conductors. With a thickness of 81 μm, aspect ratios of 6:1 are obtained using the AZ 9260 photoresist. RF inductors and micro-transformers for power conversion applications are fabricated using this technology. The quality factor of the RF inductors shows maximum values of 23 at 0.4 GHz. Very good measurement is also obtained with the micro-transformers: the resistance of the electroplated copper windings is 0.3 Ω up to 2 MHz.

Polycrystalline silicon (poly-Si) based microstructures, with boron doping (B-doping), are studied, with reference to their built-in stress, for different germanium implantation (Ge-implantation) doses. The microstructures are studied free standing and under static and dynamic deformations, by a combination of macroscopic (pull-in voltage, resonance frequency) and microscopic (micro-Raman) experimental techniques, in comparison with numerical calculation methods. The counterbalancing effect of Ge-implantation versus the B-doping, with respect to the built-in stress, is examined in poly-Si. Measurements, with three different experimental methods, and calculations, on bridges designed and fabricated for micromachining applications, show, consistently, the same maximization trend for the built-in stress, with a maximum at a Ge-dose of 10¹⁵ ions/cm², in agreement with a similar non-monotonic Ge-dependence of the growth rate and the crystalline quality of poly-Si, from the literature.

In this paper, a closed-form expression for the pull-in voltage of fixed–fixed beams and fixed–free beams is derived starting from the known expression of a simple lumped spring-mass system. The effects of partial electrode configuration, of axial stress, non-linear stiffening, charge re-distribution and fringing fields are all included in the final expression. Further, the results obtained are summarized and validated with other existing empirical and analytical models as well as with finite element simulation results. The model agrees well with finite element simulation results obtained with COVENTORWARE software.

GaAs/AlGaAs and InGaP membrane bridges micromachined on GaAs substrates have been developed for use as supporting micromechanical structures for coplanar waveguides. The internal mechanical stresses potentially induced in these micromechanical devices are evaluated analytically, and also by both the acoustic pressure bulging method and free cantilever deformation measurement. The microwave transmission properties of the micromechanical coplanar waveguides are investigated. The amplitude attenuation at a frequency of 20 GHz is found to be 1.3 dB mm⁻¹ for GaAs/AlGaAs bridge-based devices of a length of 900 μm and a slot of 3 μm. We discuss the potential applications of the fabricated micromechanical devices.

We present a new hybrid technology for the realization of three-dimensional millimetre-size inductors for low-power (0.1–1 W) applications. Our devices consist of electroplated planar Cu coils, realized within a high-resolution (5 μm) polyimide flex-foil process, and mm-size ferrite magnetic cores, obtained by three-dimensional micropatterning of ferrite wafers using powder blasting microerosion. Our devices range in volume between 10 and 1.5 mm³ and we characterize their inductive and resistive properties as a function of frequency.

We report quantitative results on the load-deflection response of compressively prestressed square membranes under differential pressure. The membranes consist of 1.9 μm thick silicon nitride films with a compressive prestrain ε₀ = −(1.6 ± 0.1) × 10⁻³. For these square membranes we observed a new symmetry transition of the deflection profile from a state without reflection symmetries at small loads to a state with reflection symmetries at sufficiently large loads. The load-deflection response was modeled by finite element simulations covering a wide range of prestrains ε₀ and pressures using various geometries. From the symmetry transition process, Young's modulus E = (150 ± 7) GPa of the membrane material was extracted.

Vibration analysis of axle box bearings is addressed for monitoring the condition of wagon wheel sets, which is crucial for the safety of high-speed railway traffic. We investigated the application of piezoresistive microelectromechanical silicon sensors designed for low-frequency resonant operation. Sensor fabrication using a low-cost planar technology and characterization are described. We show the feasibility of our method by vibration measurements with wheel sets of high-speed trains under the harsh conditions of railway operation leading, e.g., to intense background interference. Incipient fatigue damage could be detected on the cups of axle box bearings which were operated at constant and variable rotational speed.

This paper describes the development towards a miniaturized analytical system that can perform the major key functions of a flow cytometer. The development aims at diagnostic applications for cell counting and sorting with the ultimate goal of a low-cost portable instrument for point of care diagnosis. The present systems configuration consists of a disposable microfluidic device, that enables injection, single file cell flow through a miniaturized laser induced fluorescence detection system as well as sorting of identified samples. The microfluidic devices were fabricated by means of rapid prototyping technologies based on thick film photo-polymers. This paper reports various approaches on cell sorting and demonstrates sorting of single cells by means of an off-chip valve switching technique. The miniaturized fluorescence detection system employs active and passive micro-optical components, including semiconductor laser and ultra bright LED sources, highly sensitive avalanche photodiodes as well as micro-prism, holographic diffraction gratings and fibre optics for transmission and collection of light. Furthermore we demonstrate the feasibility of integrating solid-state components as part of an on-chip detection system.

A system level model of a force balance accelerometer in which the proof mass is levitated with electrostatic forces is presented. The position of the proof mass is detected capacitively and controlled electrostatically. The mathematical modelling of the system is described. In particular, equations for the capacitances and electrostatic forces are derived and verified by comparison with a finite element model. They are then implemented in a Simulink system level model of the accelerometer.

It is a great pleasure to introduce this issue of Journal of Micromechanics and Microengineering devoted to a selection of papers presented at the 12th Micromechanics Europe Workshop, MME'01. This workshop was held in University College Cork, Ireland, on 16--18th September 2001. The workshop was organized by the National Microelectronics Research Centre, located in the university. This series of workshops on Micromachining, Micromechanics and Microengineering started in 1989 and has continued with the purpose of promoting research and collaboration within the microtechnology and microsystems field and supporting further industrialization. Special attention is given to PhD students, who are encouraged to present their research work. This year the workshop was enhanced by the addition of a one day short course on "Microsystems in Biomedical Engineering" given by Dr Malcolm Wilkinson on September 16th. This course was given in collaboration with FSRM.

The MME'01 workshop took place in the immediate aftermath of the terrible events of September 11th. These events overshadowed the workshop and resulted in a number of delegates not being able to attend. The organizers are especially grateful to the over 100 delegates, from 20 countries, who did attend having overcome difficult flight schedules and security fears to ensure the success of the workshop. In particular we are grateful to Prof. Jed Harrison who managed a transatlantic crossing to attend, Dr Tim Brosnihan, who ensured his invited paper was well presented by Dr Eamon Hynes, and Dr Bob Mehalso whose presentation material was used as a guide for a packaging presentation by Dr Malcolm Wilkinson. We also thank our other invited speakers Prof. Andreas Kaiser and Dr Martin Hoffmann for their contributions. The high quality content and presentation of the invited papers set an excellent standard for all of the poster presentations.

After reviewing 89 high quality abstracts submitted to the workshop, the programme organizers selected 73 papers for poster presentation after short oral presentations. The format once again proved very popular as the level of interaction in the poster sessions was excellent. The papers covered materials, processing, modelling and applications of micromachining and micromachined devices. The diversity of potential applications was reflected by the wide range of topics included in the presentations.

The selection of papers for inclusion in this issue posed considerable difficulty due to the consistently high standard of papers at the workshop. The selected content is the result of a collaboration between the programme committee and the IOP staff and editors and has undergone selective reviewing by the journal's referees. The selected papers cover the important areas of materials and process technology for microsystems, device applications, characterization and modelling. They illustrate the large variety of materials, techniques and processes that can be used in microtechnology. The process and application papers cover the wide range of potential markets for microsystems including RF communications, optics, biomedical sensing and analysis and transportation.

I would like to thank, on behalf of the MME steering committee, my colleagues of the MME'01 programme committee, Alan G R Evans (Southampton University), Ylva Backlund (University of Uppsala) and Alexander Muller (IMT, Bucharest) for their help and advice in establishing the scientific programme of MME'01. I thank all the members of the steering committee for their assistance in informing researchers in all European countries of the MME workshop. I also acknowledge the effort of my friends and colleagues in NMRC who helped with the workshop organization. Finally, I am grateful to all participants of MME'01 for their technical and social contribution to the workshop, where excellent research was discussed and new ideas formulated, and many friendships and collaborations have been fostered.