Showing papers in "Journal of Micromechanics and Microengineering in 1996"

Comb-drive actuators for large displacements

Abstract: The design, fabrication and experimental results of lateral-comb-drive actuators for large displacements at low driving voltages is presented. A comparison of several suspension designs is given, and the lateral large deflection behaviour of clamped - clamped beams and a folded flexure design is modelled. An expression for the axial spring constant of folded flexure designs including bending effects from lateral displacements, which reduce the axial stiffness, is also derived. The maximum deflection that can be obtained by comb-drive actuators is bounded by electromechanical side instability. Expressions for the side-instability voltage and the resulting displacement at side instability are given. The electromechanical behaviour around the resonance frequency is described by an equivalent electric circuit. Devices are fabricated by polysilicon surface micromachining techniques using a one-mask fabrication process. Static and dynamic properties are determined experimentally and are compared with theory. Static properties are determined by displacement-to-voltage, capacitance-to-voltage and pull-in voltage measurements. Using a one-port approach, dynamic properties are extracted from measured admittance plots. Typical actuator characteristics are deflections of about at driving voltages around 20 V, a resonance frequency around 1.6 kHz and a quality factor of approximately 3.

Stiction in surface micromachining

Abstract: Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces.

A survey on the reactive ion etching of silicon in microtechnology

Abstract: This article is a brief review of dry etching as applied to pattern transfer, primarily in silicon technology. It focuses on concepts and topics for etching materials of interest in micromechanics. The basis of plasma-assisted etching, the main dry etching technique, is explained and plasma system configurations are described such as reactive ion etching (RIE). An important feature of RIE is its ability to achieve etch directionality. The mechanism behind this directionality and various plasma chemistries to fulfil this task will be explained. Multi-step plasma chemistries are found to be useful to etch, release and passivate micromechanical structures in one run successfully. Plasma etching is extremely sensitive to many variables, making etch results inconsistent and irreproducible. Therefore, important plasma parameters, mask materials and their influences will be treated. Moreover, RIE has its own specific problems, and solutions will be formulated. The result of an RIE process depends in a non-linear way on a great number of parameters. Therefore, a careful data acquisition is necessary. Also, plasma monitoring is needed for the determination of the etch end point for a given process. This review is ended with some promising current trends in plasma etching.

Equivalent circuit representation of electromechanical transducers: I. Lumped-parameter systems

Abstract: Lumped-parameter electromechanical transducers are examined theoretically with special regard to their dynamic electromechanical behaviour and equivalent circuits used to represent them. The circuits are developed starting from basic electromechanical transduction principles and the electrical and mechanical equations of equilibrium. Within the limits of the assumptions on boundary conditions, the theory presented is exact with no restrictions other than linearity. Elementary electrostatic, electromagnetic, and electrodynamic transducers are used to illustrate the basic theory. Exemplary devices include electro-acoustic receivers (e.g., a microphone) and actuators (e.g., a loudspeaker), electromechanical filters, vibration sensors, devices employing feedback, and force and displacement sensors. This paper forms part I of a set of two papers. Part II extends the theory and deals with distributed-parameter systems.

Determining mean and gradient residual stresses in thin films using micromachined cantilevers

Abstract: A technique that provides a first approximation to the mean, , and gradient, , components of residual stress in a thin-film material is discussed. In this method, measurements are made on a single micromachined cantilever, as opposed to an array of structures as used in the related critical-length buckling approach, to find tensile, compressive and gradient stresses. The measured deflection profile of a cantilever is reduced to rotation and curvature components, which are shown to derive independently from and , respectively. Essential to this method is the observation that a micromachined structure with a `nominally-clamped' boundary undergoes subtle rotation at its junction with the portion of the thin film that remains bonded to the substrate but is contiguous with the structure. This boundary rotation effect occurs through in-plane expansion or contraction of the bonded film following relief of residual stress. Thus, the deformation of the micromachined cantilevers considered here and of more general bulk- or surface-micromachined devices, can be strongly influenced by the state of stress in the still-bonded film.

A modular microfluid system with an integrated micromixer

Abstract: Microsystems are gaining more and more interest in several technological areas. The complete integration of different basic elements like pumps, valves and reaction chambers is possible by the use of microtechnologies. In the field of micro analysis systems a mixing procedure of different solutions is often required. A static micromixer built up in silicon has been developed. The design and fabrication procedures of the micromixer allow the integration of other microcomponents like pumps, valves or sensors. The micromixer consists of two structures silicon wafers. The structured surfaces are bonded together by using a low temperature silicon direct bonding process. The structure of the mixer consists of several identical mixing elements. The size of one element is about . Each element consists of two or more microchannels for different liquids to be mixed. The arrangements of the microchannels in the elements lead to a mixing of the liquids. We found a homogeneous mixing after about 5 mixing elements for mixable liquids. Not mixable liquids will be emulsified after about 16 - 20 mixing elements.

Microvalves with bistable buckled polymer diaphragms

Abstract: A microvalve with a bistable buckled diaphragm has been developed which can be employed for microfluidic systems. The bistable diaphragm is used to open or close the inlet of the microvalve. There is no power consumption either in the closed or in the open state, and the valve is closed tight against an inlet gas pressure of up to 470 hPa. The case of the valve is made by molding in PMMA and joined to a polyimide diaphragm thick by adhesive bonding. The temperature treatment and mechanical loading during bonding, and the different thermal expansions of PMMA and polyimide, result in a compressive stress and in buckling of the diaphragm. The microvalve has an outer diameter of 5 mm and is 1 mm thick.

LPCVD against PECVD for micromechanical applications

Abstract: After discussion of the basic aspects of CVD and its reaction kinetics LPCVD and PECVD will evolve as techniques commonly used at high temperature and lower temperature , respectively. Films deposited by these two techniques differ in several aspects, i.e., thickness, uniformity, purity, density, electrical properties, adhesion, step coverage, etc. Reactor designs are discussed in brief for optimization of the process parameters to yield optimized film properties. Then each of the major film materials such as polysilicon, SiN, , , SiC and some exotics such as diamond films are discussed with respect to their application in microstructures and their film properties in dependence on the deposition technique and follow-on processing, e.g., internal stresses due to imperfection in structure and composition or clamping, film density, pinhole density, and etchability. The discussion then moves to the application of LPCVD and PECVD in microstructures. A few typical examples will be presented for functional layers: films for membranes, cantilevers, etc in mono- and heterostructures, or ion sensitive films including passivation films as used in many sensors (e.g., microphones) and actuators (e.g., micromotors), especially such as fabricated by surface micromachining. Some room is also given to SiC, a new micromechanical material. A summary and weighting of the two CVD techniques is given.

Porous silicon as a sacrificial material

Abstract: Porous silicon is emerging in micromachining technology as an excellent material for use as a sacrificial layer. This is largely due to the ease of fabrication and freedom of design it allows. The rate of pore formation is heavily dependent upon the doping type and concentration of the silicon, allowing patterned porous silicon formation through selective doping of the substrate. Etch-rates above have been reported for highly doped material. Silicon that has been made porous can be quickly and easily removed in a dilute hydroxide solution, as low as 1%. Porous silicon technology offers the unique ability to fabricate free-standing structures in single-crystal silicon with separation distances from the substrate ranging from a few microns to over one hundred microns. A review of the development of porous silicon for micromachining applications is given.

Modeling micropumps with electrical equivalent networks

Abstract: The aim of this paper is to present a rapid modeling method for micropumps, based on electrical analogies and equivalent networks. The different parts of a pump are modelled by means of lumped elements which are calculated using appropriate formulae. These elements are linked together to constitute a network which describes the entire electromechanical system. The networks built were implemented in an electrical simulation tool for rapid analysis. In the case of pneumatic actuation, the method was validated by a qualitative and quantitative comparison with results obtained by another method. The particular case of electrostatic actuation was also investigated, leading to the observation of specific features of this actuation mechanism.

Elastic properties and microstructure of LPCVD polysilicon films

Abstract: This paper investigates the influence of film thickness, high-temperature annealing and doping by ion implantation on Young's modulus and the residual stress of LPCVD polysilicon. Films with thicknesses between 100 nm and 800 nm were deposited at with a pressure of 100 mTorr. For annealing investigations, films were annealed in a nitrogen atmosphere for 2 hours at temperatures between 600 and . The implantation doses of boron and phosphorus varied between and . This corresponds to a doping concentration of and . Young's modulus and the residual stress were determined by load-deflection measurements with suspended membranes and by the use of ultrasonic surface waves. The microstructure of the film and grain size was studied by TEM analyses and texture variations were investigated by x-ray deflection. Although Young's modulus was found to be very stable, it showed a small dependence on film thickness and annealing temperature. It varied between 151 GPa and 166 GPa. The residual stress could be strongly influenced by film thickness (-420 MPa to -295 MPa), annealing temperature (-350 MPa to -20 MPa) and ion implantation (-560 MPa to +30 MPa). The as-deposited film always showed compressive stress, a pronounced texture and a grain size of around 55 nm. Strong correlations between the variations of the elastic properties and the variations in the film thickness, annealing temperature, grain size, mass density and refractive index were found. These correlations and the observed microstructure are used to develop a model for the origin of the compressive stress and for the mechanism of stress variation. A theoretical value for Young's modulus of textured polySi was calculated and corresponds well with the measured values.

Sacrificial etching of III - V compounds for micromechanical devices

Abstract: Sacrificial etching of III - V compounds is reviewed with emphasis on its use in surface micromachining of micromechanical structures. A table is presented with 23 different combinations of stopping layers, sacrificial layers, and etchants for sacrificial etching of GaAs- and InP-based epitaxial heterostructures. Examples of surface micromachined microstructures of III - V compounds are shown, and some possible applications are discussed.

Precise mask alignment to the crystallographic orientation of silicon wafers using wet anisotropic etching

Abstract: A design tool for fast and precise determination of the crystallographic orientation in (001) and (011) silicon wafers using anisotropic wet etching is introduced The design takes advantage of the symmetric under-etching behaviour around, but not at (!), the -directions The pattern needs to be etched only for a short time, and after a very quick optical inspection it can be used for aligning subsequent masks, using the same masking layer, more or less automatically Two effects were investigated in a number of common anisotropic etchants: KOH, KOH with isopropyl alcohol (KOH/IPA), ethylenediamine based solutions (EDP), and tetramethyl ammonium hydroxide (TMAH) The precision of the method was found in most cases to be better than

Sacrificial etching of III-V compounds for micromechanical devices : Sacrificial etching papers

Abstract: Sacrificial etching of III-V compounds is reviewed with emphasis on its use in surface micromachining of micromechanical structures. A table is presented with 23 different combinations of stopping layers, sacrificial layers, and etchants for sacrificial etching of GaAs- and InP-based epitaxial heterostructures. Examples of surface micromachined microstructures of III-V compounds are shown, and some possible applications are discussed.

Modelization and characterization of asymmetrical thermal micro-actuators

Abstract: Structures of very high aspect ratio which are mechanically stiff in the substrate direction and flexible in the direction parallel to the substrate are studied. Such structures can be exploited to produce thermal flexure actuators which are capable of large motion produced by thermal expansion. The magnitude of the deflection depends strongly on geometry and material properties. Structures fabricated by laser micro-machining are characterized and compared to numerical simulations.

AFM probes with directly fabricated tips

Abstract: Different fabrication technologies for atomic force microscopy (AFM) probes have been investigated, emphasizing the fabrication of versatile tip shapes, which can be integrated with micromachined cantilevers. By successive reactive ion etching processing steps, novel high-aspect-ratio tip shapes have been produced and integrated with AFM cantilevers. The performance of the fabricated tips has been investigated by scanning test structures in an atomic force microscope.

A valve-less planar pump isotropically etched in silicon

Abstract: The first valve-less diffuser fluid pump in silicon is presented. It consists of a planar double-chamber arrangement fabricated in a silicon wafer anodically bonded to a glass wafer. The pump uses fluid-directing diffuser - nozzle elements which have a depth of and a neck width of . The pump chamber diameter is 6 mm. Pump cavities and diffuser - nozzle elements are etched with an isotropic HNA silicon etch. Pumps with three different diffuser lengths are compared reaching a maximum pump capacity of and a maximum pump pressure of 1.7 m at a resonance frequency of 1318 Hz for methanol.

Micromechanical engineering: a basis for the low-cost manufacturing of mechanical microdevices using microequipment

Abstract: Microelectronics-based micromechanics is rather limited for the construction of 3D micromechanisms with moving parts. We propose to use microequipment to transfer the technologies of mechanical engineering to the microdomain. We show that equipment precision increases linearly with decreasing size. To make microequipment, we suggest a series of equipment generations with gradually decreasing dimensions. Miniaturization of equipment will reduce power consumption and floor area occupied. Coupled with automation, it will drastically reduce the cost of microequipment. This in its turn will reduce the cost of micromechanical devices manufactured by microequipment. Microequipment-based manufacturing will also increase throughput by the concurrent operation of large numbers of low-cost microequipment pieces. The low cost and high productivity of microequipment-based manufacturing will widen the range of feasible micromechanical applications, both single-unit and mass. We propose designs for microvalve fluid filters, capillary heat exchangers, electromagnetic and hydraulic step motors that could be easily implemented by micromechanical engineering technologies. Hybrid technologies combining massively parallel microequipment based manufacturing and batch manufacturing may also be promising.

A piezoelectric triaxial accelerometer

Abstract: A new piezoelectric silicon accelerometer design is presented. The design allows sensing in three directions. The accelerometer is designed to have a detection level of and a resonance frequency of 10 kHz.

Three-axis capacitive accelerometer with uniform axial sensitivities

Abstract: A three-axis capacitive accelerometer which has uniform sensitivities to three axes was developed using a micromachining technique. This sensor has a seismic mass whose center of gravity is raised above suspending beams so that longitudinal and lateral accelerations can be detected by parallel shift and tilt of a seismic mass, respectively. Uniform axial sensitivities without cross axis sensitivity could be realized by a three-dimensional sensor structure.

Surface micromachining by sacrificial aluminium etching

Abstract: Sacrificial aluminium etching enables micromechanical structures integrated with circuitry to be fabricated using standard IC processes followed by simple post-processing. In this paper, the etching characteristics of CMOS aluminium in four etch solutions are reported. The solutions are (A) a commercially available aluminium etchant, (B) Krumm etch, (C) diluted hydrochloric acid, and (D) diluted hydrochloric acid with hydrogen peroxide. The etching of narrow channels is studied as a function of time and temperature. Initially, the etching process is reaction-rate controlled and then crosses over to a diffusion-controlled regime with reduced etch rate. Underetching distances larger than are readily achieved with etchants `A', `B', and `D'. The commercially available aluminium etchant has a low initial underetch rate of at but offers best control. The initial etch rate of hydrochloric acid with hydrogen peroxide is at . However, irregular etch fronts are obtained. Reliable protection of aluminium pads against etchants `A', `B', and `D' is guaranteed by Shipley's photoresist S1828 spun at 3000 rpm and hardbaked at .

Polydimethylsiloxane as an elastic material applied in a capacitive accelerometer

Abstract: Polydimethylsiloxane is a silicone rubber. It has a unique flexibility, resulting in one of the lowest glass-transition temperatures of any polymer. Furthermore, it shows a low elasticity change versus temperature, a high thermal stability, chemical inertness, dielectric stability, shear stability and high compressibility. Because of its high flexibility and the very low drift of its properties with time and temperature, polydimethylsiloxane could be well suited for mechanical sensors, such as accelerometers. A novel capacitive accelerometer with polydimethylsiloxane layers as springs has been realized. The obtained measurement results are promising and show a good correspondence with the theoretical values.

Fabrication of InP-based freestanding microstructures by selective surface micromachining

Abstract: InP-based microstructuring methods are presented with a view to develop micro opto electro mechanical systems (MOEMS). Fabrication parameters and dimensions of the freestanding structures are determined for specific technological constraints (etching selectivities, anisotropy, sticking phenomena). thick InGaAs deformable cantilevers, bridges and membranes have been fabricated by elimination of around -thick InAlAs sacrificial layers. Showing high aspect ratio, smooth surfaces and high accuracy in thicknesses, these microstructures are perfectly suitable for optical applications.

A fluid handling system for a chemical microanalyzer

Abstract: In this paper we present the design and fabrication of the two fundamental components of a fluid handling system intended to be the core of a chemical microanalyzer for measuring heavy metals in ground water. The first fluidic component is a micropump with built-in passive valves. The pump body is fabricated by thermoplastic moulding, whereas the passive valves are obtained by reaction injection moulding. The second fluidic component is an active valve, also fabricated by thermoplastic moulding. Both devices are driven by miniaturized electromagnetic actuators. Experiments demonstrate that the head pressure produced by the pump is 5.5 x 10 3 Pa and the maximum flow rate is 13 ml s -1 . The active valve is designed to be normally closed till an external pressure of 6.5 x 10 3 Pa.

Smoothing of ultrasonically drilled holes in borosilicate glass by wet chemical etching

Abstract: Besides silicon various types of glass play an important role in microsystem technology. One requirement of the glasses is that they have to be microstructurable. In many applications for microsensors and microstructures the glass must be suitable for anodic bonding. Usually borosilicate glass (e.g. Corning Pyrex code 7740 glass) is used, which has a thermal expansion coefficient that is almost equal to the thermal expansion coefficient of silicon and which has the necessary electrical conductivity at the temperature at which the bonding process occurs. For many applications, e.g., microfluidic systems, it is necessary to have fluids flown through ultrasonically drilled holes in the Pyrex glass to the silicon chip. The main objective of the investigation was to obtain smooth walls of ultrasonically drilled holes because every contamination influences the performance of the microsystem. In this paper the ultrasonic drilling method is compared with two other procedures for machining holes (sand blasting and laser machining). Other ways of structuring glass have been presented previously. A process was developed to smooth the walls of the drilled holes with different concentrations of hydrofluoric acid.

Packaging of physical sensors for aggressive media applications

Abstract: Micromechanical sensing elements have a high potential for a variety of applications detecting various properties. To date the commercial success has been essentially limited to pressure and acceleration detection. To extend the range of applications we believe the critical hurdle is to develop packaging schemes that ensure reliability and reduce packaging costs. For sensing in aggressive media, sensor packaging can be carried out at various levels. Packaging attempts are critically discussed and compared. The discussion is exemplified by using pressure sensors since this is the only sensor type at a sufficiently mature level of development. In particular a packaging scheme using protective thin films, mounting the sensing element in direct contact with the media, is discussed. Limitations on the traditional materials in chip processing are presented, concluding that the chemical stability is inadequate.

Manufacturing of three-dimensional micro-parts by UV laser induced polymerization

Abstract: This research deals with the development of a simple and practical method for manufacturing high-aspect-ratio (height/width) micro-parts and three-dimensional micro-parts. The desired product shape is obtained by writing along the contour of the product on the surface of a liquid photopolymer using a focused UV laser beam. The written pattern then becomes solid so as to produce a solidified polymer mold or structure. This paper examines the effect of physical parameters such as wavelength of laser beam, focal length of lens, absorption coefficient and writing speed on the shapes of the solidified polymer. The examination is done theoretically, and the optimum conditions to produce high-aspect-ratio micro-parts and three-dimensional micro-parts are also verified. Using these conditions, various microstructures are produced.

Microfabricated components for heterogeneously catalysed reactions

Abstract: Microsystems offer considerable industrial potential for unit operations, e.g. heat transfer, mass transport, and mixing of gases and liquids. The development of techniques for performing chemical reactions in microsystems has high priority. Only a few examples are known of using microreactors for performing homogeneous chemical reactions and heterogeneously catalysed liquid-phase enzyme reactions. However, heterogeneously catalysed gas-phase reactions in microreactors have not been reported at all. This is due to the lack of surfaces having enough catalytically active sites. The present article describes a method for treating the surfaces of channels in a microreactor in order to achieve adequate numbers of catalytically active and selective sites - which is the main prerequisite for the performance of heterogeneously catalysed reactions. The experimental procedure and the measurements of characteristic parameters are described. An envisaged scheme for a chemical microsystem similar to a chemical microplant is presented, and its scaling up via replication is addressed.

Thermal conductivity measurements on thin films based on micromechanical devices

Abstract: A new, highly accurate technique to measure the thermal conductivity k(T) of thin films (thickness ranging from up to several hundred micrometres) parallel to the surface over a wide temperature range is presented. The silicon substrate on which the films were deposited is completely removed in a defined area leaving membranes or free standing beams. A thin-film heater generates a temperature profile which is measured by several thermoresistors. The shape and dimensions of the structures have been optimized using computer simulations (FEA). The measurement is carried out in a vacuum chamber. First measurements on etched silicon membranes show a good agreement with literature values for bulk silicon. Measurements on polycrystalline diamond films show a maximum value of the thermal conductivity of between 100 and , which can be explained by the theory for thermal conductivity as an effect of the small grain size of the diamond film.

Development of surface micromachining techniques compatible with on-chip electronics

Abstract: Silicon has long been known to have excellent mechanical properties although the full potential of the material was not realized until micromachining techniques enabled the fabrication of true mechanical structures. The first types of structure were fabricated using bulk micromachining techniques, where the silicon wafer is etched to leave free-standing mechanical structures. More recently surface micromachining techniques have received considerable attention. In this case the mechanical structures are fabricated in thin films deposited on the silicon wafer surface. This development has yielded structures considerably smaller than those fabricated in bulk micromachining. In this paper the development of surface micromachining techniques is reviewed. This includes the modifications required to achieve compatibility with standard electronic circuitry.