Showing papers in "Sensors and Actuators A-physical in 2002"

Recent achievements in miniaturised humidity sensors—a review of transduction techniques

Abstract: A review of miniaturised humidity sensors is presented. Recent achievements in capacitive-, hygrometric-, gravimetric-, optical- and integrated sensors are discussed. Attention is paid to a general perspective of miniaturised humidity sensors, emphasising on integration issues and technological challenges. A table summarising most of the reviewed devices is included.

Active microfluidic mixer and gas bubble filter driven by thermal bubble micropump

Abstract: A microfluidic mixer with a gas bubble filter activated by a thermal bubble actuated nozzle-diffuser micropump is successfully demonstrated. The oscillatory flow generated by the micropump can induce wavy interface to increase the contact area of mixing fluids to accelerate the mixing process. The microfluidic mixing channels are 200 μm wide, 50 μm deep and the speed of the mixing liquids are measured at 6.5 μl/min. The optimal mixing result is found when the actuating frequency of thermal bubble reaches 200 Hz. Normalized gray-scale values that correspond to the completeness of the mixing effect are observed to be proportional to the one-third power of the input pulse frequency. Furthermore, a gas bubble filter is integrated and successfully demonstrated in the microfluidic mixing system. A model based on the principle of threshold pressure with respect to the geometry of microchannels is established.

Effect of cubic nonlinearity on auto-parametrically amplified resonant MEMS mass sensor

Abstract: Parametric resonance has been well established in many areas of science, including the stability of ships, the forced motion of a swing and Faraday surface wave patterns on water. We have previously investigated a linear parametrically driven torsional oscillator and along with other groups have mentioned applications including mass sensing, parametric amplification, and others. Here, we thoroughly investigate the design of a highly sensitive mass sensor. The device we use to carry out this study is an in-plane parametrically resonant oscillator. We show that in this configuration, the nonlinearities (electrostatic and mechanical) have a large impact on the dynamic response of the structure. This result is not unique to this oscillator—many MEMS oscillators display nonlinearities of equal importance (including the very common parallel plate actuator). We report the effects of nonlinearity on the behavior of parametric resonance of a micro-machined oscillator. A nonlinear Mathieu equation is used to model this problem. Analytical results show that nonlinearity significantly changes the stability characteristics of parametric resonance. Experimental frequency response around the first parametric resonance is well validated by theoretical analysis. Unlike parametric resonance in the linear case, the jumps (very critical for mass sensor application) from large response to zero happen at additional frequencies other than at the boundary of instability area. The instability area of the first parametric resonance is experimentally mapped. Some important parameters, such as damping co-efficient, cubic stiffness and linear electrostatic stiffness are extracted from the nonlinear response of parametric resonance and agree very well with normal methods.

A laser-micromachined multi-modal resonating power transducer for wireless sensing systems

Abstract: This paper presents the development of a vibration-induced power generator with total volume of ∼1 cm 3 which uses laser-micromachined springs to convert mechanical energy into useful electrical power by Faraday’s law of induction. The goal of this project is to create a minimally sized electric power generator capable of producing enough voltage to drive low-power ICs and/or microsensors for applications where ambient mechanical vibrations are present. Thus far, we have fabricated generators with total volume of 1 cm 3 that are capable of producing up to 4.4 V peak-to-peak, which have a maximum RMS power of ∼830 μW with loading resistance of 1000 Ω. The mechanical vibration required to generate this electrical energy has frequencies ranging from 60 to 110 Hz with ∼200 μm amplitude. The generator was shown to generate sufficient power at different resonating modes. We have demonstrated that this generator can drive an infrared (IR) transmitter to send 140 ms pulse trains every minute, and also a 914.8 MHz FM wireless temperature sensing system.

Modeling the thermal behavior of a surface-micromachined linear-displacement thermomechanical microactuator

Abstract: Thermomechanical microactuators possess a number of desirable attributes including ease of fabrication and large force and displacement capabilities relative to other types of microactuators. These advantages provide motivation for improving thermomechanical microactuator designs that are more energy efficient and thus better suited for low-power applications. To this end, this paper describes the development and experimental validation of a finite-difference thermal model of a thermomechanical in-plane microactuator (TIM). Comparisons between the model and experimental results demonstrate the importance of including the temperature dependence of several parameters in the model. Strategies for reducing the power and energy requirements of the TIM were investigated using model simulations as a guide. Based on design insights gained from the model, the energy efficiency of the TIM has been improved significantly by operating in a vacuum environment and providing short-duration, high-current pulse inputs. These improvements have been validated experimentally.

Miniaturized Thermoelectric Generators Based on Poly-Si and Poly-SiGe Surface Micromachining

Abstract: We report on miniaturized thermoelectric generators which are being developed to convert waste heat into a few µW of electrical power sufficient to supply microelectronic circuitry. A BiCMOS realization using standard materials is favored to make these generators amenable to low cost applications. In order to optimize our device, the design and the material properties have been studied. The use of micromachining techniques allowed us to improve the thermal efficiency of the generator significantly. Low thermal conductivity of the thermoelectric materials proved to be the most important factor to increase the output power. The materials we have investigated are poly-Si and poly-SiGe. Experimental results of the fabricated devices show good agreement with the predictions of thermal simulations.

Development of AFM-based techniques to measure mechanical properties of nanoscale structures

Abstract: Mechanical property evaluation of nanometer-sized structures is necessary to help design reliable MEMS/NEMS devices. Most material properties are known to exhibit dependence on specimen size and such properties of nanoscale structures are not well characterized. Silicon and SiO 2 nanometer-scale beams (nanobeams) with a 6 μm length and widths ranging from 200 to 600 nm were fabricated using lithography-based techniques. A quasi-static bending test technique for these nanobeams was developed using an atomic force microscope (AFM). This technique was used to evaluate elastic modulus, bending strength and estimate fracture toughness of the beams and beam materials. The beams failed in a linear elastic and brittle manner. Results indicate that elastic modulus and fracture toughness values are comparable to bulk values, whereas bending strength appears to be much higher for these nanobeams than for larger scale specimens, thus revealing a size effect. We also report results from monotonic cyclic loading tests of the nanobeams that reveal nanoscale fatigue performance of the beam materials. SEM observations of the fracture surfaces suggest cleavage as the fracture mechanism for both Si and SiO 2 beams.

A 12 MHz micromechanical bulk acoustic mode oscillator

Abstract: We demonstrate a bulk acoustic mode silicon micromechanical resonator with the first eigen frequency at 12 MHz and the quality factor 180 000. Electrostatic coupling to the mechanical motion is shown to be feasible using a high bias voltage across a narrow gap. By using a low-noise preamplifier to detect the resonance, a high spectral purity oscillator is demonstrated (phase noise less than −115 dBc/Hz at 1 kHz offset from the carrier). By analyzing the constructed prototype oscillator, we discuss in detail the central performance limitations of using silicon micromechanics in oscillator applications.

Polysilicon: a versatile material for microsystems

Abstract: The initial attraction of polysilicon was the ability to deposit semiconductor layers on a wide range of substrates. This leads to the development of polysilicon gate MOS, polysilicon emitters and a range of passive devices. In the field of sensors and actuators, the strain sensors, based on piezoresistive effect, was one of the first successful applications. However, it was probably the development of surface micromachining, which received the most attention. The ability to deposit polysilicon on oxide meant that free-standing structures could be fabricated and this has lead to a wide range of successful devices. Polysilicon has a structure comprising small single-crystal grains with many orientations separated by thin grain boundaries. The size of these grains is highly dependent upon processing and therefore a wide range of electrical and mechanical properties can be achieved. This yields greater flexibility but this sensitivity to process parameters can lead to problems in obtaining a stable process. This paper will look back at the development of polysilicon, its structure, fabrication and both mechanical and electrical properties.

High-Q factor RF planar microcoils for micro-scale NMR spectroscopy

Abstract: We present the design, fabrication and test of high-Q factor radiofrequency planar microcoils for nuclear magnetic resonance (NMR) spectroscopy in small volume samples. The coils are fabricated on glass wafers using high-aspect ratio SU-8 photoepoxy and copper electroplating. On-wafer electrical characterization shows quality factors up to 40 at 800 MHz. A 500 μm diameter microcoil with a measured quality factor of 24 at 300 MHz is mounted on a printed circuit board and electrically contacted using aluminum wire bonding. This probe is inserted in a 7 T-superconducting magnet, and a 1H-NMR spectrum of 160 nl ethylbenzene contained in a capillary placed over the microcoil is acquired in a single scan. This work is an important step towards the integration of NMR detection into micro-total analysis systems (μTAS).

Application of MEMS force sensors for in situ mechanical characterization of nano-scale thin films in SEM and TEM

Abstract: We present a novel tensile testing technique utilizing MEMS force sensors for in situ mechanical characterization of sub-micron scale freestanding thin films in SEM and TEM. Microfabrication techniques are used to cofabricate the thin film specimens with force sensors to produce the following unique features: (1) small setup size to fit in SEM and TEM for in situ experiments, (2) ability to measure tensile pre-stress in specimen, (3) alignment between specimen and applied loading axes with lithographic precision, (4) no extra gripping mechanism required, and (5) ability to measure creep strain in the material. The technique allows single or multilayers of materials that can be deposited/grown on silicon substrate to be tested. We demonstrate the technique by testing a 100 nm thick, 8.8 μm wide and 275 μm long freestanding aluminum specimen (average grain size about 50 nm) in situ inside an environmental SEM chamber, and present another setup for similar experiment in TEM. Experimental results strongly suggest that at this size scale: (1) elastic modulus does not change, (2) size effects on yield strength are pronounced (63 times the bulk pure aluminum yield stress), and (3) permanent strain hardening effects are absent.

Magneto-hydrodynamic (mhd) pump fabricated with ceramic tapes

Abstract: The use of Magneto Hydro Dynamics (MHD) to circulate fluids in conduits fabricated with low temperature cofired ceramic tapes is described. Conduits shaped like toroidal and rectangular loops were fabricated. Electrodes printed on the ceramic substrate along the conduits' walls facilitated transmission of electric currents through the test fluids. When the devices were subjected to a magnetic field, the resulting Lorentz forces propelled the liquids. The paper details the fabrication process and describes experiments with mercury slugs, saline solution, and deionized water. The measured fluid velocities were compared with theoretical predictions.

Design of a pulsed eddy current sensor for detection of defects in aircraft lap-joints

Abstract: This paper presents a new type of pulsed eddy current (PEC) sensor that has been designed for defect detection in aircraft lap-joint structures. The sensor employs a new excitation circuit that requires no additional signal amplification and the paper also reports compensation techniques that improve the sensing resolution and stability. A new hybrid feature of the peak value in time domain and the maximum frequency magnitude in frequency domain has been investigated. A test rig has been built and some results from aircraft samples are presented.

Wireless, passive, resonant-circuit, inductively coupled, inductive strain sensor

Abstract: This paper presents a non-contact, wireless, passive, inductively coupled strain sensor. The sensor itself is a parallel-connected LC tank circuit. Small geometric changes in a stressed solenoidal inductor were predicted to affect its inductance and as a consequence the resonant frequency of the LC circuit. Using a gate dip meter as a sensitive detector, this was experimentally confirmed. There was found to be a consistent relationship between relative strain with shifted resonant frequency independent of whether the sensor was embedded or not.

Submicron stereolithography for the production of freely movable mechanisms by using single-photon polymerization

Abstract: Our group has developed an advanced micro stereolithography system by using single-photon polymerization with an He–Cd laser of 442 nm. This system takes advantage of pinpoint solidification generated at a tightly focused spot inside a liquid photopolymer. This three-dimensional (3D) pinpoint exposure technique allows true 3D microstructures to be fabricated without relying on layer-by-layer processing unlike conventional micro and macro stereolithography. The resolution has also been improved to 0.43 μm, which is close to the diffraction limit of light, without any nonlinear optical process such as two-photon absorption. Moreover, the direct drawing technique inside the liquid photopolymer makes it possible to fabricate freely movable micromechanisms without support parts or sacrificial layers. Using this process, we successfully fabricated several movable microstructures such as micro rotators and micro gears.

Thermal and gas-sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications

Abstract: This work describes the design and characterization of a micromachined thermal conductivity sensor for detection of hydrogen in automotive fuel cell systems. Thermal and gas-sensing properties are investigated via simulations and experiments. The manufactured sensors consist of a thin dielectric membrane as carrier structure for a platinum heater and temperature sensor. Membrane and heater size were varied to examine their influence on sensitivity, power consumption and thermal response. Based on a sensor element with a membrane size 1.5 mm ×1.5 mm a hydrogen sensor prototype with a detection limit of 0.2% hydrogen in air was achieved.

SMA microgripper system

Abstract: A microgripper system is presented, which consists of a monolithic shape memory alloy (SMA) device of 2 × 5.8 × 0.23 mm3 size and an integrated optical position sensor. By use of an integrated gear actuator, the motion of gripping jaws is transmitted into a linear motion of an integrated optical slit, which is detected by change of optical transmission. The maximum stroke and force of the gripping jaws are 300 µm and 35 mN, respectively. In the range between 10 and 90 % of the maximum stroke, positioning is achieved within 140 ms with an accuracy of about 2 µm.

A novel six-component force sensor of good measurement isotropy and sensitivities

Abstract: In this work, a novel six-component force sensor with its force-sensing member in the form of four identical T-shaped bars is presented. The force-sensing member is subjected to finite element analysis in conjunction with a design optimization for high measurement sensitivities. Although significant measurement couplings exist in this six-component force sensor, however, they distribute only in a few sparse places in the calibration matrix, making the calculations for the force components relatively easy and quick. The condition number under the full rated loading conditions for this sensor is 1.543, which represents a rather good measurement isotropy, as compared to approximately 2–4 for a Maltese crossbar sensor under similar conditions. In addition, only 20 strain gauges are required in the design, which is less than that used in a Maltese crossbar type sensor, in which at least 24 strain gauges are used.

Arrayed ultrasonic microsensors with high directivity for in-air use using PZT thin film on silicon diaphragms ☆

Abstract: An ultrasonic microarray sensor using piezoelectric diaphragms has been fabricated and applied to recognition of a three-dimensional object. The sensor consists of piezoelectric PZT (Pb(Zr,Ti)O 3 ) thin film on 18 diaphragms of micromachined SOI wafer. The array is designed to obtain a sharp directivity considering wavelength of ultrasound in air and the chip size for micromachine applications. Three-dimensional images have been successfully obtained by scanning the sensor.

Fabrication and investigation of in-plane compliant SU8 structures for MEMS and their application to micro valves and micro grippers

Abstract: This paper demonstrates the versatile application potential of SU8 photoepoxy for micromechanical components Its outstanding aspect ratio and attainable film thicknesses enable the design of structures such as beams and hinges that are compliant in the wafer plane The applicability of this technology is demonstrated on shape memory alloy and pneumatically actuated micro grippers [1] , [2] as well as a micro fluidic channel with integrated check valve Besides reporting on the improved technology for 300–400 μm SU8 structures, we will present elastic material properties measured on micro compliant beams and flexural hinges employing a tactile force sensor [3] The material properties measured on micro specimen are compared to large scale standard tensile test results

Comparison of porous silicon, porous polysilicon and porous silicon carbide as materials for humidity sensing applications

Abstract: This paper investigates the suitability of porous polysilicon and porous SiC as materials for sensing humidity. The investigation is a continuation of earlier work on porous single-crystalline silicon, where it was shown that this material was appropriate for humidity sensing, and could be easily integrated with standard Si processing. It was also shown that membrane structures enable the integration of a heating device to ‘reset’ the system. The best microstructure for humidity sensing was obtained for low-doped p-type silicon. The advantage of using polysilicon is that it is possible to tune its response (by doping) so that it has only a very small temperature coefficient of resistance. The idea is that a humidity sensor with a very small temperature dependence could be realised. The advantage of using SiC is that it offers the possibility of a humidity sensor that could withstand very harsh chemical environments.

A micromechanical switch with electrostatically driven liquid-metal droplet

Abstract: This paper reports the design, fabrication and test of a micromechanical memory cell (essentially a switch) using a microscale liquid-metal droplet as the moving and contact part. The droplet is driven by electrostatic force between a grounded liquid metal and imbedded driving electrodes, all placed inside of an anisotropically etched silicon cavity. The electrodes inside the silicon cavity and 111 side walls are patterned by a new shadow masking technique using thin wafers.

A Hybrid Cold Gas Microthruster System for Spacecraft

Abstract: MEMS components forming a complete hybrid cold gas microthruster system for spacecraft, designed to deliver maximum thrusts in the range of 0.1 to 10 mN, have been demonstrated. The system includes three different micromachined subsystems: a nozzle unit comprising four nozzles generating supersonic flows, four independent piezoelectric proportional valves with leak rates below 10−5 sccm He, and two particle filters. The performances of all these MEMS subsystems have been evaluated.

A low-temperature CVD process for silicon carbide MEMS

Abstract: A low-temperature, single precursor CVD process for the realization of SiC-based MEMS and SiC-coated MEMS is described using 1,3-disilabutane. With this deposition method, the fabrication of an all-SiC cantilever beam array is demonstrated using standard microfabrication processes. Also, SiC coating of released Si micromechanical structures is realized using this process. The SiC-coated microstructures are shown to have superior chemical stability when compared to their Si analogs, as well as exhibit highly favorable mechanical properties.

Novel ionic polymer–metal composites equipped with physically loaded particulate electrodes as biomimetic sensors, actuators and artificial muscles

Abstract: Described is a novel fabrication process of manufacturing ionic polymer–metal composites (IPMCs) equipped with physically loaded electrodes as biomimetic sensors, actuators and artificial muscles. The underlying principle of processing this novel IPMCs is to first physically load a conductive primary powder layer into the polymer (ionomeric) network forming a dispersed particulate layer. This primary layer functions as a major conductive medium in the composite. Subsequently, this primary layer of dispersed particles of a conductive material is further secured within the polymer network with smaller secondary particles via chemical plating, which uses reducing agents to load another phase of conductive particles within the first layer. In turn, both primary and secondary particles can be secured within the polymer network and reduce the potential intrinsic contact resistance between large primary particles. Furthermore, electroplating can be applied to integrate the entire primary and secondary conductive phases and serve as another effective electrode. In this paper, we describe the details of this newly developed technique to efficiently produce an IPMC loaded with spherical silver particles ( D 10 D 50 D 90 A sur 2 /g) and subsequently secured by palladium ( D p ∼50 nm, via a chemical reducing process). It has been established that such an IPMC is quite comparable in force and displacement performance with the traditional platinum loaded and gold electroplated IPMCs but can be manufactured at about 1/10th of the cost. Yet it produces a low surface resistivity (less than 1 Ω per square), which is highly desirable in creating more uniform deformation.

Microplastic embossing process: experimental and theoretical characterizations ☆

Abstract: Microlens hot embossing process conditions, including processing pressure and temperature with respect to the penetration height and radius of curvature have been characterized in this work. Polycarbonate is used as the plastic material and silicon mold inserts with circular openings of 100 to 200µm are used to fabricate plastic microlenses. The height and radius of curvature of microlenses with respect to temperature changes can be modeled by an Arrhenius function and an activation energy derived from experimental data. It is found that the activation energy is a constant independent of processing temperature or size of opening holes on the mold insert. Experimentally, the processing pressure has little effect on the radius of curvature of microlens under a fixed processing temperature at the steady state. The relationship between the height of the microlens and the applied pressure can be modeled linearly to derive the interfacial stress empirically.

Characterization of sputtered ZnO thin film as sensor and actuator for diamond AFM probe

Abstract: In order to develop a diamond atomic force microscopy (AFM) probe with a piezoelectric sensor and actuator, we fabricated piezoelectric zinc oxide (ZnO) thin film and measured its piezoelectric constant. First, we developed a simple measurement method for the piezoelectric constant of the thin film, d 31 . This was based on the free vibration theory of cantilever beams. The values of d 31 were determined by measuring an electric charge induced in the piezoelectric thin film on the vibrating cantilever beam and its displacement. Using this method, we evaluated d 31 for ZnO thin film sputtered at various substrate temperatures. The ZnO thin film deposited at temperatures of less than 350 °C was highly c -axis oriented and showed a high piezoelectric constant d 31 of −3.5 pC/N. Using this value, we calculated properties of the diamond cantilever AFM probes of various dimensions and of 5 μm in thickness with a ZnO sensor and actuator of 1 μm in thickness. The resolution of displacement and actuation force for a probe of 150 μm in length and 50 μm in width were estimated to be about 1.5 nm at a resolution of charge measurement of 1×10 −15 C and 7 μN at an applied voltage of 10 V, respectively.

A novel topology design scheme for the multi-physics problems of electro-thermally actuated compliant micromechanisms

Abstract: In addition to the advantages of embedded actuation, large forces and displacements, and ease of microfabrication, one more attractive feature of electro-thermally actuated compliant mechanisms is their suitability for systematic synthesis directly from behavioral specifications. We and other groups have presented topology optimization methods for similar problems. In this paper, we describe a new design parameterization scheme for the topology optimization problem involving three energy domains (electrical, thermal, and elastostatic) and multiple materials. We also show how topology-dependent modeling such as convection through side surfaces can be dealt with in a design technique wherein the topology is varied.

Carbon coated liquid crystal elastomer film for artificial muscle applications

Abstract: New liquid crystal elastomer materials with properties that mimic the action of a muscle have been developed recently. Uniaxial contraction of a free standing film of the material can be achieved by heating the film through the nematic to isotropic phase transition. In this paper, carbon coating of such a material is shown to be an effective approach for enhancing the response of a free standing elastomer film to an external stimulus. The heat is generated in the carbon coating by absorption of infrared laser radiation and then conducted through the bulk. It is shown that this leads to a significant reduction in the actuation response time without substantially affecting mechanical properties. This demonstration opens up new opportunities for application of liquid crystal elastomers as actuators.

Piezoelectric shear mode drop-on-demand inkjet actuator

Abstract: We report on comprehensive characterization of piezoelectric shear mode inkjet actuators micromachined into bulk Pb(Zr0.53Ti0.47)O3 (PZT) ceramics. The paper starts with an overview of different drop-on-demand inkjet systems, whereas the main attention is then turned on particular Xaar-type piezoelectric shear mode inkjet printheads. They are an example of complex microelectromechanical system (MEMS) and comprise a ferroelectric array of 128 active ink channels (75 μm wide and 360 μm deep). Detailed information about fabrication process and principles of operation are given. Since each actuating wall of 128 channels is a piezoelectric capacitor metallized from both sides to be animated by electric pulse, electrical properties of channel walls (CWs) are easy to test and serve as a fingerprint of actuator performance in the virgin state as well as after high voltage/elevated temperature heavy duties. We present several techniques to control manufacturing process and fatigue effects. So, continuous wave and pulsed spectroscopy and hysteresis P–E loop tracing showed that compared to a virgin PZT ceramics state, dielectric permittivity (e′) was reduced three times, the loss factor (tan δ) increased from initial 4.8 to 6.6%, remnant polarization decreased by 43%, coercive field increased by 38%, whereas Curie temperature increased from 508 to 560 K after 90,000 cycles of ferroelectric hysteresis P–E loop tracing at 50 Hz at electric field of 88.5 kV/cm. Heat treatment also results in PZT ceramics degradation: appreciable reduction of the coupling coefficient (k15) and the degradation of inkjet performance were revealed by optical stroboscope technique: 8.7 and 14% reduction of drop velocity and volume in electrically fatigued actuator, 2.5% reduction of drop velocity and unchanged drop volume in temperature-treated actuators.