Showing papers in "Journal of Micromechanics and Microengineering in 2000"

Ferroelectric thin films for micro-sensors and actuators: a review

Abstract: This paper reviews deposition, integration, and device fabrication of ferroelectric PbZrxTi1-xO3 (PZT) films for applications in microelectromechanical systems. As examples, a piezoelectric ultrasonic micromotor and pyroelectric infrared detector array are presented. A summary of the published data on the piezoelectric properties of PZT thin films is given. The figures of merit for various applications are discussed. Some considerations and results on operation, reliability, and depolarization of PZT thin films are presented.

RF MEMS from a device perspective

Abstract: This paper reviews the recent progress in MEMS for radio frequency (RF) applications from a device perspective. RF MEMS devices reviewed include switches and relays, tunable capacitors, integrated inductors, mechanical resonators and filters, and some representative microwave and millimetre-wave components. Important device parameters are highlighted, as they have significant contributions to the performance of the final products in which the devices are used. The challenges and statuses of these RF MEMS devices are outlined and discussed. The intent of this topical review is to provide perspective to newcomers in the field, and empower potential end-users with an overall device picture, current status, and a vision of their ultimate performance capabilities.

A pneumatically-actuated three-way microvalve fabricated with polydimethylsiloxane using the membrane transfer technique

Abstract: In this paper, a three-way microvalve system composed of three independent one-way valve units is presented. Each valve unit has a membrane, which is actuated by external negative air pressure. Intervals between the valve units are smaller than 780 µm, which opens up the possibility of realizing a high-density microvalve array. The small intervals were realized by providing the system with a layer of microchannels to conduct the air pressure to the valve units. In spite of the extra layer of microchannels, the device has been fabricated through a simple process by adopting polydimethylsiloxane (PDMS) as the material for the microchannel chips as well as the membrane. In particular, a newly developed technique for wafer level transfer of a PDMS membrane has been proven to be effective. Flow characteristics of the microvalve system for water are presented. The microvalve works in an on-off manner with hysteresis. No leakage has been observed in the closed state. In the open state, measured flow resistances (pressure drops) are within the range of 1.65-2.29 kPa (µl min -1)-1, and consistent with an electric circuit model.

Micro-systems in biomedical applications

Abstract: In this paper we analyse the main characteristics of some micro-devices which have been developed recently for biomedical applications. Among the many biomedical micro-systems proposed in the literature or already on the market, we have selected a few which, in our opinion, represent particularly well the technical problems to be solved, the research topics to be addressed and the opportunities offered by micro-system technology (MST) in the biomedical field. For this review we have identified four important areas of application of micro-systems in medicine and biology: (1) diagnostics; (2) drug delivery; (3) neural prosthetics and tissue engineering; and (4) minimally invasive surgery. We conclude that MST has the potential to play a major role in the development of new medical instrumentation and to have a considerable industrial impact in this field.

Microfabrication technology for polycaprolactone, a biodegradable polymer

Abstract: In this paper, we report the development of micromachining techniques for a biodegradable polymer for the first time. By virtue of their ability to naturally degrade in tissues, biodegradable polymers hold immense promise as new materials for implantable biomedical microdevices. This work focuses on the establishment of microfabrication processes for biodegradable microstructures and microdevices. Three unique fabrication processes have been established: (1) a micro-molding process to form 3D microstructures in polycaprolactone (PCL) via a silicon micromachined mold, (2) a method of transferring metal patterns to surfaces of PCL substrates, (3) techniques for sealing both dry and liquid-filled PCL micro-cavities with a metal thin film (e.g. gold). The chemical compatibility of PCL with common micromachining chemicals have been investigated.

A closed-loop controlled electrochemically actuated micro-dosing system

Abstract: In this paper a closed-loop controlled micromachined dosing system is presented, for the accurate manipulation of liquids in microsystems down to the nanoliter range. The applied driving force to dispense liquids originates from the electrochemical generation of gas bubbles by the electrolysis of water. The proposed dosing system comprises a micromachined channel/reservoir structure in silicon, capped with a Pyrex® cover on which a set of platinum electrodes is patterned. By adopting an interdigitated electrode geometry, the electrodes can be used for electrochemical gas generation as well as for the simultaneous determination of the total gas bubble volume, via an impedance measurement of the gas/liquid mixture in the reservoir. As this measured gas bubble volume equals the dosed liquid volume, active control of dosed volumes can be obtained. It will be shown that the cell impedance can be applied to accurately determine the generated gas volume and that by using this parameter in a closed-loop control system, dosed volumes can be controlled in the nanoliter range.

Optimization of a circular piezoelectric bimorph for a micropump driver

Abstract: Piezoelectric bimorph actuation has been successfully used in numerous types of microdevices, most notably micropumps. However, even for the simple case of circular geometry, analytical treatments are severely limited. This study utilized the finite-element method to optimize the deflection of a circular bimorph consisting of a single piezoelectric actuator, bonding material and elastic plate of finite dimensions. Optimum actuator dimensions were determined for given plate dimensions, actuator-to-plate stiffness ratio and bonding layer thickness. Dimensional analysis was used to present the results for fixed- and pinned-edge conditions in a generalized form for use as a design tool. For an optimally-thick actuator, the optimum actuator-to-plate radius ratio ranged from 0.81 to 1.0, and was independent of the Young's modulus ratio. For thin plates, a bonding layer minimally affected the optimum dimensions. The optimized actuator dimensions based on a model of an actual device were within 13% of the fixed-edge condition.

Towards a force-controlled microgripper for assembling biomedical microdevices

Abstract: This paper presents recent results on the development and control of a microgripper based on flexure joints, fabricated by LIGA and instrumented with semiconductor strain-gauge force sensors. The microgripper is the end-effector of a workstation developed to grasp and manipulate tiny objects such as the components of a typical biomedical microdevice. The development of the force control in the microgripper is of fundamental importance in order to achieve the dexterity and sensing capabilities required to perform assembly tasks for biomedical microdevices. As a step towards the definition of the force control strategy, system identification techniques have been used to model the microgripper. Results indicate that a proportional integral (PI) controller could be used to assure, at the same time, closed-loop stability of the system, and a bandwidth suitable for the intended applications. The force control is based on strain-gauge sensors which have been integrated in the microgripper and experimentally characterized. Sensor response in the idling condition and during grasp showed that they can provide useful information for force control of the microgripper.

Micro-discharge and electric breakdown in a micro-gap

Abstract: Very weak light emission due to micro-discharges in the micro-gap of an electrostatic actuator under a high electric field can be imaged using a highly sensitive CCD camera. In this paper, we focus, in particular, on the relation between the micro-discharge and the electric field breakdown. Micro-discharges attributed to local breakdown is observed even if the electric field strength is below the breakdown threshold. The observations of the micro-discharge give important information on the irregularity and inhomogeneity of the electric field. It is found that irregular instability and inhomogeneous distribution of the electric field develop micro-discharges. The micro-discharge evaporates the electrode material, which results in increasing pressure in the gap, and finally grows to the breakdown. This effect seems to be remarkable, especially in narrow gaps. Furthermore, the electric breakdown threshold depends on the electrode material. A silicon-to-silicon gap configuration shows a higher breakdown threshold as well as the prebreakdown threshold in comparison to a silicon-to-metal gap.

The future of MEMS in telecommunications networks

Abstract: The move toward dense wavelength-division-multiplexed networks provides an application-rich environment for MEMS components and systems. In many crucial component applications, there are no clear technological winners, leaving room for new technologies such as MEMS to create inroads. In this paper, wavelength-division-multiplexed networks and their evolution are discussed and examples of several MEMS devices being developed for telecommunication systems are presented.

Additive electroplating technology as a post-CMOS process for the production of MEMS acceleration-threshold switches for transportation applications

Abstract: This paper presents an acceleration-threshold sensor fabricated with an electroplating technology which can be integrated on top of a pre-processed CMOS signal processing circuit. The device can be manufactured using a standard low-cost CMOS production line and then adding the mechanical sensor elements via a specialized back-end process. This makes the system especially interesting for automotive applications, such as airbag safety systems or transportation shock monitoring systems, where smaller size, improved functionality, high reliability and low costs are important.

Design and fabrication of travelling wave dielectrophoresis structures

Abstract: The design and fabrication of a generic travelling wave dielectrophoretic separation system is presented. A range of technical issues have been addressed and resolved. These include the optimization of the electrical insulation and electrical contact between different electrode layers and the choice of materials. A new method for fabricating microfluidic liquid channels, the fluidic channel cover and the seal within the channel, as well as an effective sample introduction system has been developed. A complete particle handling and fractionation system has been fabricated and the functioning of the device demonstrated with latex particles.

Mask materials for powder blasting

Abstract: Powder blasting, or abrasive jet machining (AJM), is a technique in which a particle jet is directed towards a target for mechanical material removal. It is a fast, cheap and accurate directional etch technique for brittle materials such as glass, silicon and ceramics. The particle jet (which expands to about 1 cm in diameter) can be optimized for etching, while the mask defines the small and complex structures. The quality of the mask influences the performance of powder blasting. In this study we tested and compared several mask types and added a new one: electroplated copper. The latter combines a highly resistant mask material for powder blasting with the high-resolution capabilities of lithography, which makes it possible to obtain an accurate pattern transfer and small feature sizes (<50 µm).

Silicon-processed plastic micropyramids for brightness enhancement applications

Abstract: A combination of silicon anisotropic etch and mechanical hot embossing techniques is used to demonstrate the feasibility of manufacturing plastic micropyramids. These micropyramids have the desirable optical characteristics to enhance the brightness of LCD (liquid crystal displays) in battery-powered laptop computers, personal TVs and camcorders. Silicon mold inserts, 4-inch in diameter, are first micromachined and used directly in the micro hot embossing process by applying plastic sheets made of polymethyl methacrylate (PMMA) or polyvinyl chloride (PVC). Micropyramids which have base widths of 30 µm, heights of 21 µm and apex angles of 70 ◦ are hot embossed on the surface of plastic sheets. The fabricated plastic films are tested optically and the results show that up to 20% brightness enhancement within the front viewing angles of ±35 ◦ have been achieved. An optical model has been established to simulate the brightness enhancement effects and it is concluded that the optimal pyramid angle for the brightness enhancement application is 90 ◦ .

Electrodeposition for the synthesis of microsystems

Abstract: Electroplating is an emerging technique for the production of microsystems. This is due to advantages such as high rate of deposition, high resolution, high shape fidelity, simple scalability, and good compatibility with existing processes in microelectronics. Materials ranging from high-conductivity metals over soldering connections to ferromagnets can be deposited. In this paper the basics of electroplating are reviewed and examples of recent applications of electroplating in the processing of microsystems are presented.

A robust micro conveyer realized by arrayed polyimide joint actuators

Abstract: A new micro-robotic conveyance system based on arrays of movable robust silicon legs has been developed and investigated. Motion is achieved by thermal expansion in polyimide joint actuators using electrical heating. Successful experiments on moving and rotating flat objects in the millimeter range have been performed with high load capacity. The conveyer consists of a 15×5 mm2 chip having 12 silicon legs, each with a length of 500 µm. The maximum load conveyed on the structure was 3500 mg. Both transverse and rotational movements have been demonstrated experimentally. Conveyance velocities up to 12 mm s-1 have been measured. Accelerated lifetime measurements demonstrate the long-term stability of the actuators. The functionality of the polyimide joint actuators is unaffected after more than 2×108 load cycles.

Navier-Stokes simulations of gas flow in micro devices

Abstract: A numerical study of flow in micro channels and micro pipes is described. The simulations are performed by solving Navier-Stokes equations with a slip velocity boundary condition, using the LU-TVD implicit algorithm. A second-order slipping model incorporating pressure gradient is proposed and investigated. The numerical results obtained using the new slipping model are presented and found to compare well with available experimental data and numerical results from other references.Our computations also show that compressibility and the rarefied effects of gas flows are present in both micro channel and micro pipe flows. It is also found that the effect of rarefaction tends to mitigate the negative curvature of pressure distribution that can be attributed to compressibility.

Focused ion beam induced deposition: fabrication of three-dimensional microstructures and Young's modulus of the deposited material

Abstract: In this work, some of the possibilities of focused ion beams for applications in microsystem technology are explored. Unlike most previous studies, the emphasis is on `additive' techniques, i.e. localized maskless deposition of metals and insulators. More precisely, we will show the possibility of fabricating small three-dimensional structures, using focused ion beam deposition of silicon oxide. Deposition examples will show that the technique is most promising for small post-processing steps or prototyping, because of its high degree of flexibility. Furthermore, an investigation into the mechanical properties of the deposited material is presented. More specifically, the Young's modulus of the deposited silicon oxide is determined.

On the design of piezoresistive silicon cantilevers with stress concentration regions for scanning probe microscopy applications

Abstract: In this paper, the design of silicon based cantilevers for scanning probe microscopy has been described in detail. ANSYS software has been used as a tool to design and model the mechanical properties of the silicon based cantilevers. The incorporation of stress concentration regions (SCRs) with a thickness smaller than the cantilever thickness, to localize stresses, has been explored in detail to enhance the piezoresistive displacement, force, and torque sensitivity. In addition, SCRs of widths less than the cantilever width have also been explored. Two basic designs were studied, i.e. a rectangular cantilever and a U-shaped cantilever. The placement of the SCR was found to be critical, and optimal placement and thickness of the SCR can result in a 2× and 5× improvement in piezoresistive displacement and force sensitivity, respectively, for the rectangular cantilever. For the U-shaped cantilever, the torsional piezoresistive sensitivity was found to increase by 5×, depending on the SCR thickness. Process flows and associated fabrication challenges for the proposed cantilever structures are also presented.

Electrodeposited copper inductors for intraocular pressure telemetry

Abstract: A microsystem for wireless long-term measurement of the intraocular pressure is presented. The sensing element is a novel distributed parallel-resonant inductive-capacative circuit, with a pressure-dependent resonance frequency. This circuit is based upon a twofold on-chip deposited inductor. The high Q inductor is deposited by electrodeposition of copper on a micromachined chip incorporating a pressure-sensitive diaphragm. Test structures were fabricated and characterized. Q factors of 30 at 45 MHz and inductance values of 0.4 µH are obtained for 3×3 mm2 structures.

Thick-layer resists for surface micromachining

Abstract: Interest in thick-photoresist applications is steadily growing. In addition to bump fabrication and wire interconnect technology (WIT), the process of patterning thick-layer photoresists by UV lithography is specially qualified for applications in microelectromechanical systems (MEMS). Specialized equipment and new photoresists have been developed or are under development to cope with the new challenges in the field of preparing extremely thick photoresist layers, the process of patterning these thick resists, and to deal with the difficulties of the following galvanoplating step. As one of the most critical steps in thick-photoresist processing, the baking procedure was investigated. Positive tone photoresists (AZ 4562, ma-P 100) were processed by means of three different baking methods: air-forced oven, ramped hotplate, and IR radiation. It could be shown that IR baking is advantageous compared to the other methods with respect to process duration and energy consumption. As for edge steepness, resolution, edge loss, and surface roughness, all methods deliver nearly the same results. A minimum width of 2-3 µm for the resist bars was found to be necessary to withstand the fabrication process of lines and spaces in about 15 µm thick resists. For thicker layers, high aspect ratios of about 10 as well as steep edges of more than 88° could be fabricated. The development of SU-8, a chemically amplified negative tone photoresist for the 300-450 nm region opened totally new dimensions for the UV depth lithography. Even under development, SU-8 delivers results otherwise only achievable by x-ray lithography. The deposition of photoresist on highly-structured surfaces demands advanced methods. Electrodeposition of resist is one solution. PEPR 2400 was used for patterning by UV light in order to generate resist patterns around a free standing silicon bar. The achieved resist patterns were moulded by using electroplating. For microsystem applications some metals and alloys were deposited. Three-dimensional micro components were fabricated as demonstrators for the new technique. Electrodeposition allows the use of materials with interesting properties which could not be provided by standard processes in microelectronics.

A laterally driven symmetric micro-resonator for gyroscopic applications

Abstract: Conventional microgyroscopes of the vibrating type require resonant frequency tuning of the driving and sensing modes to achieve high sensitivity. These tuning conditions depend on each microgyroscope fabricated, even though the microgyroscopes are identically designed. A new micromachined resonator, which is applicable to microgyroscopes with self-tuning characteristics, is presented. Since the laterally driven two-degrees-of-freedom resonator was designed as a symmetric structure with identical stiffness in two orthogonal axes, the resonator is applicable to vibrating microgyroscopes, which do not need mode tuning. A dynamic model of the resonator was derived considering gyroscopic applications. The dynamic model was evaluated by experimental comparison with fabricated resonators. The resonators were fabricated using a simple process of a single polysilicon layer deposited on an insulator layer. The feasibility of the resonator as a vibrating microgyroscope with self-tuning capability is discussed. The fabricated resonators of a particular design have process-induced non-uniformities that cause different resonant frequencies. For several resonators, the standard deviations of the driving and sensing resonant frequencies were as high as 1232 and 1214 Hz, whereas the experimental average detuning frequency was 91.75 Hz. The minimum detuned frequency was 68 Hz with 0.034 mV s-1 sensitivity. The sensitivity of the microgyroscope was low due to process-induced non-uniformity; however, the angular rate bandwidth was wide. This resonator could be successfully applicable to a vibrating microgyroscope with high sensitivity, if improvements in uniformity of the fabrication process are achieved. Further developments in improved integrated circuits are expected to lower the noise level even more.

Characteristic modes of electrostatic comb-drive X-Y microactuators

Abstract: The design of dual axis microengineered comb-drive electrostatic actuators is investigated. Experimental measurements of diode-isolated bulk micromachined x -y stages with a folded, single-flexure suspension show the presence of unwanted two-dimensional vibrational mode patterns. A combination of coupled-mode and finite-element analysis is used to explain the existence of the spurious modes, and good agreement is obtained with the experimental response provided that the intrinsic stress is taken into account. It is shown that improved performance can be obtained simply by doubling the number of suspension flexures.

Industrial MEMS on SOI

Abstract: The industrial use of silicon on insulator (SOI) substrates is quickly spreading. For microelectronics applications, this material brings new functionality, such as radiation-hard, high-voltage or low-voltage and low-consumption integrated circuits. Industrial wafers are now commercially available and new technologies, such as Smart Cut® from SOITEC company, will provide low prices in high-volume production. SOI also provides very important features for microelectromechanical systems (MEMS) and new industrial products, such as high-temperature and low-noise piezoresistive pressure sensors or miniature high-performance capacitive pressure and acceleration sensors, are now proposed. These new generic technologies will provide various industrial products in the near future.

In situ measurement of residual stress in micromachined thin films using a specimen with composite-layered cantilevers

Abstract: A new concept of measuring residual stress in micromachined thin films using a specimen with composite-layered cantilevers is proposed. The reliable fabrication process of a composite-layered cantilever and its corresponding modeling are introduced and tested. A few errors that may exist in the previous measurements can be removed by using the proposed method. The residual stress of an additional thin film is obtained by comparing the deflections of the states before and after the deposition of the additional film. Since the structure and fabrication processes described in this paper are widely used in surface micromachining, in situ measurement can be undertaken easily and simply by using the proposed method and a specimen for measurement.

A silicon shadow mask for deposition on isolated areas

Abstract: A shadow mask with high pattern flexibility is realized by deep reactive ion etching (RIE) on Si wafer. The novel features of the mask are the presence of a mechanical alignment structure and of patterns with isolated islands inside them. The advantage of this shadow mask is the possibility of deposition of any kind of pattern shape by evaporation or sputtering on a sample that is precisely positioned. Moreover, by this technique, deposition is realized without damaging electronic devices or micromachined structures on the sample. Precise positioning of the sample with respect to the shadow mask is allowed by the mechanical alignment structures. Some doughnut-shape-like patterns are obtained by deposition through the patterns with isolated islands inside them. In this article we will describe the realization and the application of such a shadow mask.

Three-dimensional microfabrication for a multi-degree-of-freedom capacitive force sensor using fibre-chip coupling

Abstract: The design and fabrication of a novel multi-degree-of-freedom force sensor is described. The three-dimensional structure of the sensor is a result of combining several microfabrication techniques: wet bulk micromachining, fusion bonding, chemical mechanical polishing, deep RIE, LPCVD, PECVD and thermally evaporated thin films. The sensor is designed to operate in the 0-500 µN force range and the 0-10 µNm torque range. The flexibility of the process to create overhanging structures with arbitrary lengths and heights is illustrated by the integration of micro-tweezers directly onto the force sensor. Among other advantages of the developed process is a dicing-free self-release of wafer structures. This allows very fragile structures, such as micromirrors and other optical components, to be individually packaged.

Heat-transfer analysis of microfabricated thermocapillary pumping and reaction devices

Abstract: A heat-transfer analysis was performed on thermocapillary pumping, a surface-tension-based drop pumping mechanism. The analysis, which incorporates both fluid flow and energy transport through multiple device layers, reveals effective material, design, and operational choices that result in improved pumping performance. Important design factors include thermal conductivity, channel/substrate thickness, and the velocity of the liquid drops. Results for pumping a drop of water on a fused silica substrate with a glass channel show that uniform interface temperatures can be achieved with bottom heating as long as drop velocities remain below ~0.1 cm s-1 . The analysis was also extended to include thermal reaction channels. The reactor analysis verified that relatively uniform reaction temperatures were attainable with bottom heating as long as channel heights were below ~100 µm.

Bulk micromachined relay with lateral contact

Abstract: A bulk micromachined relay has been developed with silicon-glass wafer bonding and deep reactive ion etch technologies. The microrelay has a lateral contact structure, and it is laterally driven by electrostatic actuators. The processing is very simple, and only two masks are used. To fulfill different demands, we designed various structures of the microrelay including sizes, contact structures and actuating structures. The mechanical structures are made by single-crystal silicon, which has perfect mechanical performance. Using sputtered gold as contact materials, the measured contact is below 1 Ω.

Design and fabrication of a micromachined silicon accelerometer with thick-film printed PZT sensors

Abstract: An accelerometer fabricated by a combination of screen printing and silicon micromachining is reported. This is the first device of its kind fabricated in this manner. Details of the finite element model (FEM) of the sensor and its method of fabrication are described. Initial results indicate a sensitivity of 16 pC g-1, which compares very favourably with a sensitivity of 0.15 pC g-1 reported for thin-film devices. Experimental results show a good level of agreement with the FEM results. Analysis of the results highlight the need to further improve the mechanical properties and consistency of the thick-film PZT layer.