Showing papers in "Journal of Micromechanics and Microengineering in 1994"

Microflow devices and systems

Abstract: Microflow devices including microvalves, micropumps and microflow sensors fabricated by micromachining are reviewed from the point of view of the actuating principle and structures. Integration of microflow control devices and microflow sensors allowed very precise control of small flow. High performance liquid dosing microsystems and sophisticated chemical analysing microsystems were demonstrated by the combination of microflow devices and microsensors. Applications of microflow devices and systems are also introduced.

Electroosmotic pumping and electrophoretic separations for miniaturized chemical analysis systems

Abstract: Electroosmotic pumping is highly efficient in capillaries of less than 100 mu m inner diameter bearing an immobilized surface charge. Electric fields in the kV cm-1 range allow for liquid motion of several mm s-1 in the case of an aqueous electrolyte. This pumping mechanism is used for miniaturized chemical analysis systems. Flow and mixing behaviour in branched channels are characterized. A capillary electrophoresis device allows for repetitive, electroosmotic injections of 100 pL samples, for efficiencies of up to 200000 theoretical plates in less than a minute, and for external laser induced fluorescence detection at any capillary length of choice between 5 and 50 mm.

Post buckling of micromachined beams

Abstract: The static deformation of micromachined beams under prescribed in-plane compressive stress is studied through analytical and experimental means over the prebuckling, transition, and postbuckling load ranges. The finite amplitude of the beam in its postbuckled state is predicted by modeling the non-linear dependence of the out-of-plane deformation on the compressive stress. In addition, the model explicitly considers the net effect of slight imperfections, which can include fabrication defects, geometric irregularities, or non-ideal loading, on the beam's behavior in the near-buckling regime. As an application, clamped-clamped silicon dioxide beams are fabricated through conventional bulk micromachining, and their deflected profiles are measured through three-dimensional optical profilometry. The measurements are compared to the postbuckled amplitudes and shapes that are predicted by the model, and by existing simpler models that do not include the effects of either non-linearity or imperfection. As borne out by the data, when imperfections are considered, the beams exhibit continuous growth of the out-of-plane amplitude during transition from the prebuckled state to a postbuckled one, in contrast to sudden bifurcation at a critical load. By accounting for this behavior, the estimate of residual stress in the thin film from which the beams are fabricated can be improved, and the amplitude of common postbuckled micromachined structures can be predicted during the design phase.

Towards integrated microliquid handling systems

Abstract: In this paper we describe components for integrated microliquid handling systems such as fluid injection analysis, and first results of planar integration of components. The components discussed are channels, passive and active valves, actuators for micropumps, micromixers, microflow sensors, optical detectors, pumps and dosage systems. The dosage system described comprises a flow sensor and a pump micromachined on a single silicon wafer sandwiched between Pyrex wafers. The liquid pump is of the reciprocating type with a thermo-pneumatic actuator. The microliquid flow sensor is based on the thermal anemometer type. Both pump and flow sensor are realized in a 3 inch (100)- Si wafer using a KOH bulk etch from both sides of the wafer. The dosing system allows accurate dosing of liquid in the mu l regime and can easily be integrated with components as mixers and detectors to microliquid handling systems. A new concept for micromixing of liquids is introduced and its feasibility is demonstrated. The mixer allows fast mixing of small amounts of two liquids and it is applicable to microliquid handling systems. The mixer has a channel for the liquid, an inlet port for the reagent, and a mixing area, the bottom of which has 400 micronozzles (15 mu m*15 mu m). Through these nozzles, a reagent is injected into the sample liquid, making many microplumes. These plumes speed up mixing by diffusion over a short distance.

Travelling wave-driven microfabricated electrohydrodynamic pumps for liquids

Abstract: The basic principles of high-frequency travelling wave-driven micropumps are explained. We describe the design and construction of closed pump systems which contain planar and three dimensional components fabricated by semiconductor technology. Theoretical and experimental results and further perspectives are discussed.

Three-dimensional micro flow manifolds for miniaturized chemical analysis systems

Abstract: A three-dimensional (3D) stack concept for the assembly of photolithographically fabricated microfluidic components is presented and discussed. The system uses silicon-based micropumps and simple planar structures which mimic standard elements of conventional flow systems. Detection is provided either by solid state electrochemical sensors or small volume optical detection. The general advantages of using micromachined flow manifolds for microchemical analysis are addressed. The particular benefits to be derived from this an approach compared with other assembly methods are also examined.

Gallium arsenide as a mechanical material

Abstract: The aim of this work is to introduce GaAs as a mechanical material to those working primarily with silicon in micromechanics, and to give an update of the micromechanical properties of GaAs. Mechanical properties, some promising response mechanisms for micromechanical sensors, and recent micromechanical applications are reviewed for GaAs, and its best developed alloying system, the AlxGa1-xAs ternary.

Electrically activated normally closed diaphragm valves

Abstract: Thermal actuation of integrated valve structures is attractive due to the relative simplicity of the actuator design, the moderate activation time possible, the relatively low total power required, and the ability to fully integrate the actuator with the flow control component. Of the many thermal actuation schemes, the use of bimetallic elements is preferred, particularly when a low cost, fully integrated structure is desired. Bimetallic structures allow both normally-open and normally-closed valves by varying the boundary conditions at the support for the actuator, which is difficult to achieve with many alternate technologies. One such valve has been designed to provide fully proportional control of flows in the range of 0-150 cc min-1 at input pressures between 1 and 50 PSIG. The valves are batch fabricated using silicon micromachining techniques. By combining these valves with pressure or flow sensing elements, closed loop pressure or flow control is easily accomplished.

Simulation of microfluid systems

Abstract: The dynamic behavior of microfluid devices like miniaturized diaphragm pumps, microvalves and flow channels will be discussed. Special interest is directed to the dynamics of microminiaturized diaphragm pumps. A formalism will be given, which allows the evaluation of the interaction between these pumps and the connected fluid system. The results of the simulation will be compared with transient pressure measurements on pneumatically and electrostatically actuated diaphragm pumps on the millisecond time scale. It will be outlined how the dynamic behavior of more complex fluid systems can be described. Hence the hydraulic simulation of chemical analysis systems, consisting of several micropumps, flow channels, sensing and mixing elements becomes possible in the future.

Microfluidic components in LIGA technique

Abstract: The transport of small amounts of fluids and gases must be controlled with a view to automated drug dosage, chemical analysis with microsensors, and hydraulic as well as pneumatic microactuators. Components for microfluidic systems have been developed at Karlsruhe Nuclear Research Center which are manufactured by the LIGA technique. Micropumps have been fabricated by combining thermoplastic molding with membrane techniques. It is envisaged to integrate these micropumps into a module for handling gases and liquids. The module can then be coupled with different sensor modules to form microsystems, e.g. for the detection of toxic chemicals in the environment or for bedside analysis in hospitals. Microvalve systems and fluidic beam amplifiers have been developed for hydraulic and pneumatic actuators. These devices are capable of controlling a liquid or gas stream and may be used to steer e.g. instruments in a catheter tip for minimally invasive surgery.

An electrostatically actuated gas valve with an S-shaped film element

Abstract: A new microvalve actuator driven by electrostatic force is described and shown to be suitable for controlling the flow of a rarefied gas. The actuator consists of a pair of planar electrodes sandwiching a conductive film that has an S-shaped bend in the middle. The S-bend moves back and forth as voltage is alternately applied between each of the electrodes and the film. A manually assembled macromodel actuator having a film 5 mu m thick and 12 mm wide is operated between electrodes separated by 2.5 mm. The propagation speed is 4.0 m s-1 at an applied voltage of 150 V. The large vertical displacement of the film enables the large valve seat lift necessary to allow a certain amount of gas flow. An experimental model of the valve allowed at airflow rate of 10 sccm at a pressure of 60 Pa.

The piezoelectric effect of GaAs used for resonators and resonant sensors

Abstract: The piezoelectric effect in GaAs is investigated for potential use in sensor and resonator applications. Flexural resonant vibrations of GaAs tuning-fork structures have been excited at 33 kHz, with measured performance in excellent agreement with predictions. Longitudinal vibrations can equally well be activated. High Q-values at atmospheric pressure (Q=570000 at 4 MHz) have been measured for thickness shear vibrations of (100) GaAs wafers, using lateral electric field excitation. The temperature and stress dependences of the resonance frequency are important features when designing GaAs sensors. The temperature dependences of the frequency range from -59 ppm degrees C-1 to -48 ppm degrees C-1, and the stress dependences are at least 200 ppm MPa-1 and 39 ppm MPa-1 for tensile stress of flexural and shear vibrations, respectively. The semiconducting nature of GaAs affects the piezoelectric behaviour only at high temperatures. The successful experiments open up many possibilities of new resonator and resonant sensor concepts.

A study of wear on MEMS contact morphologies

Abstract: The fabrication of many micro electro mechanical systems (MEMS) is mainly based on silicon and its compounds and, for moving structures such as micromotors, tribological behaviour plays a key role in the performance. In this paper the wear of MEMS-compatible materials has been investigated for a range of contact areas and contact forces typical of MEMS. Special test specimens incorporating a range of micromachined micro structures on their top surfaces have been fabricated in order to simulate those conditions. These single crystal silicon (SCS) micro structures were coated with a range of materials used in MEMS; diamond-like carbon (DLC), silicon nitride (Si3N4), silicon dioxide (SiO2), and doped and undoped polysilicon. A specimen-on-disc arrangement (a development of the macroscopic pin-on-disc) was used for the wear experiments and dead weight loading was applied using micro loads specially calibrated for this purpose. The results show that the wear rates of DLC and SCS sliding on DLC decrease with increasing sliding distance whereas Si3N4 and SiO2 showed approximately linear wear behaviour. The effect of contact morphology and contact pressure was investigated for doped polysilicon sliding on DLC and for doped and undoped polysilicon sliding on Si3N4. The results can be attributed to the differing mechanical and chemical properties of the materials leading to wear mechanisms ranging from asperity fracture to asperity deformation.

Morphology of etch hillock defects created during anisotropic etching of silicon

Abstract: Etch hillocks are defects created during anisotropic etching of (100) silicon. From direct measurements of defect micrographs, we have determined the structure of etch hillocks to be pyramids bounded by (567) planes. The defects planes are different from those which are observed to emerge as anisotropic etch facets, such as (111), (211), (212), (133) and (411). We also describe the process by which defects are annihilated in high-concentration etchant. The relative stability of various defect edges is in agreement with calculations of the degree of backbonding of Si atoms along hillock edges. Our results suggest that the mechanisms responsible for hillock formation are distinct from those causing faceting during undercutting of convex corner masks.

Design rules and test of electrostatic micromotors made by the LIGA process

Abstract: Electrostatic stepping micromotors have been designed with regard to low rotor friction on the axle. The design rules are determined using two-dimensional FEM calculations and taking into consideration the particular rotor position during motion. The motors are fabricated by a fully integrated LIGA process. The minimum driving voltages needed are measured to be about 60 V. Determination of the coefficients of friction in operation completes the rules for future designs.

A GaAs pressure sensor based on resonant tunnelling diodes

Abstract: A GaAs-AlAs resonant tunnelling diode (RTD) is incorporated in a 1 mu m thick membrane and used as a pressure sensor. The fabrication technology of the membrane is based upon the selective etch of GaAs with AlAs as an etch stop layer. An external pressure introduces stress in the layers of the RTD and modifies the position of the conduction band, the value of the effective mass and the Fermi level. These variations will change the peak current and voltage of the RTD. Measurements at room temperature show that the effect of an applied pressure on a symmetric RTD is asymmetric. This asymmetric is explained theoretically by the difference in the sign of the stress between the top and bottom layers of the RTD.

Micro torque measurements for a prototype turbine

Abstract: Many applications for microengineered devices can be envisaged for actuators capable of doing work or transferring power. Millimetre order turbines are considered in this study for the development of torque and the possibilities for the delivery of work. A prototype microturbine, with overall thickness of less than a millimetre, was studied for its torque capabilities. The initial prototype was realized using precision mechanics although implementation of the turbine is planned using microengineering techniques. A viscous braking method was developed to measure the shaft torque of the turbine, demonstrating shaft coupling and the possibilities for power transfer. In order to validate the viscous braking method for torque measurement, a mechanical friction brake (dynamometer) was developed to compare the measurements obtained for a miniature electric motor of known characteristics. The results from this series of calibration experiments were then used to evaluate the performance of a microturbine prototype. The dynamometer torque measurements were found to closely agree with the manufacture's stated stall torque for the miniature motor of 1.8*10-4 N m. The viscous brake torque measurements were found to underestimate the motor torque by around 20% with slight variation related to the angular velocity of the shaft. Shaft torque measurements for the prototype microturbine were possible using the viscous brake but not the dynamometer. It was felt that 10-5 N m represented the lower limit for the dynamometer torque measurement while the viscous brake could address torques down to 10-8 N m. The fluid brake produced measurements of torque in the range of 10-7 N m for the microturbine. At this level only an order of magnitude accuracy is claimed because of some uncertainties with the fluid model used for the viscous brake torque calculation. The shaft torque range for the viscous brake was from 10-4 N m down to 10-8 N m; this might be extended by optimizing the fluid model.

Defined crystal orientation of nickel by controlled microelectroplating

Abstract: Controlled microelectroplating allows the fabrication of microstructures of nickel with defined crystal orientation within the LIGA process. The change in the crystal orientation due to different wetting agents and current densities in the electroplating process has been examined by X-ray diffraction. A careful adjustment of these parameters leads to the electrodeposition of nickel with perfect (110) and (100) orientations or the electrodeposition of nickel with a complete isotropic orientation. The defined crystal orientation permits the control of the magnetic properties of the materials for microactuators based on electromagnetic principles. A new method of analysis has been developed, which results in a quick and easy determination of the crystal orientation by the use of X-ray spectra obtained with a conventional powder diffractometer without the necessity for a special apparatus for texture analysis.

Friction measurements on LIGA-processed microstructures

Abstract: A cantilever microactuator made of a ceramic fibre was used to load LIGA processed microstructures by pushing them over different substrates. Static and kinetic friction coefficients in air were determined for the interfaces between the LIGA-processed microstructures and the different substrates. Microrotors disassembled from electrostatic micromotors made of Cu and Ni have been used for the experiments. The substrate materials were amorphous Al2O3 and sputtered Ag on Cr. The experiments were carried out in a clean environment under class 100 conditions. Different cleaning methods and surface treatments have been applied and the influence on the coefficient of friction studied. It was found that the cleanliness of the surface and adsorbed gas layers have a substantial effect on the static friction results. This dependence was higher for Cu than for Ni. The kinetic and static friction appeared to be lowest for sliding on the Ag substrate and sliding of Ni on sputtered Ag exhibited the lowest coefficients of friction.

Micromotor dynamics in lubricating fluids

Abstract: This paper presents results from step-transient measurements on salient-pole, side-drive, polysilicon micromotors operated in different viscosity silicone lubricating oils under different temperatures and excitation voltages. The step transient has been found to be overdamped, with the rise time decreasing with decreasing viscosity of the oil. The rise time, which is in the range of 1 to 20 msec, is around one to two orders of magnitude larger in oil than that in gaseous environments because of the larger viscous drag contribution. In oil, voltages as low as 12 V across 2.5 mu m rotor/stator gaps are sufficient to operate the micromotors. Micromotor speeds up to 12500 rpm are achieved with 110 V excitations across 1.5 mu m rotor/stator gaps. Based on the experimental findings, a model for micromotor dynamics in lubricating oils is examined in which the Coulomb frictional torque is neglected, since micromotor operation in oil is dominated by viscous drag.

Integrated chemical analysis microsystems in space life sciences research

Abstract: A review of life sciences research in space, and the experimental infrastructure available or proposed for future development is presented. The monitoring of a wide range of chemical parameters is required in contrasting experimental configurations, ranging from autonomous experimental modules in sounding rockets, to bioregenerative life support systems on manned planetary bases. Microsystem technologies (MST) are proposed as a solution to the development of integrated chemical analysis systems which will meet both the diverse requirements for chemical monitoring systems, and the constraints of size and power imposed on instrument design by deployment on space missions. It is proposed that these technologies will transform the data yield from experiments, and expand the scope for more creative experimental protocol. The use of MST is illustrated with design concepts for cell microincubators, and a chemical analysis facility. The review concludes that the application to space life science research provides a challenging and unique opportunity for the development of demonstrator prototype microsystems.

Optical pressure sensor using two Mach-Zehnder interferometers for the TE and TM polarization

Abstract: We have fabricated an optical pressure sensor consisting of two neighbouring Mach-Zehnder (MZ) interferometers which have the same structure and with their measurement arms running in parallel over a thin membrane. A standard plasma-enhanced chemical vapour deposition process (PECVD) was used to deposit the oxinitride waveguide and the upper cladding layer, respectively. The rib-waveguides are designed to be single-mode both for the TE and TM polarizations. Through proper design of the membrane and by measuring the optical intensity of the TE mode in one interferometer and of the TM mode in the neighbouring one, it is possible to calibrate the sensor for absolute value measurement.

Radiation chemistry of poly(lactides) as new polymer resists for the LIGA process

Abstract: Recently poly(lactides) have been discovered to be a promising new polymer family for applications in the LIGA (the German acronym for Lithography, Galvanoplating and Plastic moulding, German: Abforming) process, as they show a considerably enhanced sensitivity and reduced tensile stress corrosion with respect to the standard poly(methyl-methacrylate) (PMMA) resist. The irradiation chemistry was studied by Fourier-transform infrared and mass spectrometry. An irradiation chamber comprising a quadrupole mass spectrometer and a multiple sample holder was built, allowing us to analyse in situ radiation-chemical reaction products escaping from the polymer samples. This apparatus was installed at the SR beamline BN1 of ELSA, the Electron Stretcher and Accelerator of the Physics Institute of the University of Bonn. The FTIR spectra were measured from samples before and after irradiation at the same beamline. Through these two complementary methods, considerable insight could be gained on the radiation chemical processes of this polymer family. The observed gaseous fragments resulting from main chain scissions of the lactides indicate a dominant elimination of CO2. At the same time, the FTIR spectra showed a reduction in absorption strength of the C-O-main chain bonds. A statistical copolymer of 90% L-lactide and 10% glycollide with an alcoholic solution of NaOH in water as a developer shows structure precision comparable to PMMA together with a photospeed enhancement of a factor 2 over the conventional PMMA/GG system.

Finite-element analysis of rotor stability in an axial-drive micromotor

Abstract: The effect of axial electrostatic forces on the rotor deflection of an axial-drive double-stator micromotor is studied with respect to the limits of stability against elastic rotor collapse using the finite-element method (FEM). Forces obtained from an electrostatic FE model have been applied to a structural FE model using the indirect coupled-field analysis approach. This technique may be applied to other MEMS structures with conducting components. A technical description is given of the design and the fabrication process of the axial-drive double-stator micromotor investigated. Two different axial rotor supports (bearing and bushing) are investigated. The results are discussed as a function of the applied voltage and rotor thickness considering polysilicon and aluminium as possible rotor materials. The effects on the stability of the rotor of supporting the rotor with an additional bushing, and of increasing the rotor thickness, are investigated quantitatively.

The formation of very flat silicon sidewalls by dry etching

Abstract: A dry etching system producing very flat wall profiles in bulk silicon is suggested. Wall angles down to 1 degrees were obtained at accurate aligned structures with well defined dimensions. The surface roughness is nearly the same as that prior to the etch. Pattern transfer was carried out in a conventional RIE system. A resist mask was not used but instead a special prepared bonded mask wafer with holes and diffusion channels for the reactive species from the plasma. After the etch this mask wafer is reusable by separating it from the substrate wafer.

Current fluctuations of a monocrystalline (100) silicon field emitter

Abstract: Current fluctuations of a gated, n-type, (100) silicon field emitter were recorded from initial emission, through the forming process, and then cycled between low ( approximately 0.4 mu A) and high ( approximately 10 mu A) currents. After the first excursion from high to low current, the signature of the fluctuations changes from being very noisy to near stable, indicating electron-induced desorption of the surface impurities during the high-current operation. The near-stable condition becomes noisy again over a time period of 100 min at a pressure of 3*10-9 Torr. This indicates re-adsorption of impurities such as oxygen, hydrogen, carbon, and carbon monoxide from the background gas and/or the formation of a new native oxide layer.

Fluorinated polyimide fabrication by magnetically controlled reactive ion etching (MC RIE)

Abstract: This paper describes the etching rates and morphologies of etched surfaces of fluorinated polyimides having fluorine concentrations, when a magnetic field parallel to the cathode surface was varied during magnetically controlled reactive ion etching (MC RIE). The fluorine concentration of fluorinated polyimides is a key parameter used to control the etching rate in MC RIE and the etching rate can also be increased by applying a high magnetic field. The etching rate was maximum when the fluorine concentration was 15-23 wt%, and the peaks became sharper as the magnetic field increased. The surface morphology also became smoother with increasing magnetic field, and SEM observation revealed that the surface morphology of 23 wt% fluorine polyimide was smoothest when a high etching rate and a high magnetic field were used.

Optical micro displacement sensor using a composite cavity laser diode integrated with a microlens

Abstract: We demonstrated an extremely small optical displacement sensor using a laser diode integrated with a microlens and based on the composite cavity principle, which is that the light output power from the laser diode changes with reflectance and displacement of the third mirror. The light output power from the device was modulated by the reflectance of the third mirror with the displacement in the lateral direction. The modulation depth was 0.1 when the third mirror reflectances were 70% and 4%, and the resolution in the lateral direction was 3 mu m. The light output power was also modulated by the third mirror displacement in the axial direction with a period of 0.4 mu m, and a modulation depth of 0.047 was obtained.

A dual-gate field emitter and its integration into a flat panel display

Abstract: Single tip and arrays of n-type, (100) monocrystalline silicon field emitters were fabricated with a dual-gate structure. The field at the tip can be expressed as E= beta (Vgl+Vgr), in which Vgl and Vgr are the left and right gate voltages. By applying a gate voltage below the threshold of emission to one of the gates, the device can then be activated by the second gate. Integration of this device into a matrix-addressable array can be accomplished via a single cross-over structure without the need of emitter isolation.

Attachment of optoelectronic components to silicon submounts by Nd:YAG laser soldering of indium

Abstract: A fluxless soldering technique has been developed for the attachment of optoelectronic components to silicon submounts by laser soldering of gold-coated indium solder pads. By keeping the solder time per pad at 60 msec, and by adhering to a solder schedule that minimizes the formation of indium oxide, it is possible to solder without flux or the need of an inert atmosphere. The method is demonstrated on a high speed data link consisting of a laser, an external modulator, and a laser back facet detector attached to a silicon submount.