Microsensor Applications Research Articles

Adjusting the distance of electrochemical microsensors from secreting cell monolayers on the micrometer scale using impedance.

There are emerging applications of electrochemical microsensors where the distance of the sensor from an insulating plane needs to be adjusted and/or accurately known. The plane may be merely an obstruction or the source of a species whose release rate needs to be measured. An example is in cell secretion studies where a monolayer of cells is stimulated to secrete ions and/or other biochemical species which then diffuse away from the cells while being measured by a microsensor. Sensor response will thus depend on both the rate of release and the distance of the sensor from the cells. To obtain accurate release rates, the precision of the scheme to control electrode distance from the monolayer needs to be on the micrometer scale for species with ionic diffusivities. Optical (stereomicroscope and microruler) and mechanical (precalibrated micrometer screw) methods to precisely position the electrode are difficult to use under realistic circumstances (due to opaqueness of the chamber, and/or the medium, or irreproducible chamber depth). In this work we propose to correlate electrochemical cell impedance with sensor distance. This scheme has been used to adjust the distance of a chloride (tip diameter approximately 250 microns) and a potassium (tip diameter approximately 1000 microns) ion-selective microelectrode in the 0-250- and 0-2500-micron range, respectively, from a planar obstruction as well as from a monolayer of cells with a best precision of +/- 5 microns (n = 6) for the chloride and about +/- 20 microns for the potassium sensor. Larger electrodes have a broader range of distances over which they are sensitive, albeit with a poorer spatial resolution. This was verified by using Ag disk electrodes of 250 and 500 microns in diameters in AgNO3 solution.

Micro-environments and mass transfer phenomena in biofilms studied with microsensors

Direct observations on chemical micro-environment and microbial composition in biofilms are rare. The combination of microsensor and molecular techniques is highly useful for studies on the microbial ecology of biofilms. We shortly describe some applications of microsensors to study mass transfer phenomena and microbial processes in biofilms. It has recent been recognized that biofilms are not always flat layers of cells, but can consist of complex structures allowing liquid flow. Thus the classical view, that transport in biofilms is diffusional, is challenged. In laboratory grown biofilms the effect of convection on mass transfer was demonstrated. The microsensor technique has improved, so that direct in situ measurements in living biofilms are possible. By direct measurements of liquid flow with microsensors we show that in biofilms grown in bioreactors heterogeneity and convectional transport must also be taken into account. For the description of the microbial population we use molecular techniques, such as in situ hybridisation with 16S rRNA-targeted oligonucleotide probes. In a nitrifying-denitrifying biofilm we found a complex nitrifying community consisting of members of the genera Nitrosomonas, Nitrosospira, Nitrobacter and Nitrospira. Their occurrence was correlated with nitrification activity as determined by microsensor measurements.

Some particular aspects of the thin membrane by boron diffusion processes

Micromachining techniques have an increasing importance in the development of microsensors applications. For free standing 3D structures bulk and surface micromachining are used. This paper presents experimental results concerning thin membranes, realized by p + etch stop techniques. We obtained a concentration of 1×10 20 cm −3 through thermal diffusion from boron + solid source by a performed program (thermal cycles). This value is enough for stop-etch layer in anisotropic etching of silicon (100) in KOH solution at 80°C. Spreading resistance has measured the boron profile. The wafers were processed in two ways. Some of them were boron diffused over the entire area of the wafer and other were diffused only in pattern geometries, defined in silicon after thermal oxidation. In this case, we have used double side alignments. We define square geometries for the membranes, with 1.5 and 2.5 mm 2 areas. The performance result could be noticed from the thickness of the membranes of 10–12 μm with good uniformity on wafers.

A fully integrated, untrimmed CMOS instrumentation amplifier with submicrovolt offset

A low-noise CMOS instrumentation amplifier intended for low-frequency thermoelectric microsensor applications is presented that achieves submicrovolt offset and noise. Key to its performance is the chopper modulation technique combined with a bandpass filter and a matching on-chip oscillator. No external components or trimming are required. The achievable offset performance depends on the bandpass filter Q and the oscillator-to-bandpass filter matching accuracy. Constraints are derived for an optimum Q and a given matching accuracy. The improvement of common-mode rejection ratio (CMRR) in chopper amplifiers is discussed. The amplifier features a total gain of 77/spl plusmn/0.3 dB and a bandwidth of approximately 600 Hz. The measured low-frequency input noise is 8.5 nV//spl radic/Hz and the input offset is 600 nV. The measured low-frequency CMRR is better than 150 dB. The circuit has been implemented in a standard 1-/spl mu/m single-poly CMOS process.

Acoustic Wave Mass Sensors

The application of acoustic wave microsensors for mass sensing will be reviewed with focus on the quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices. The use of QCM and SAW devices in chemical sensing as well as in the determination of solid and liquid properties will be described. In chemical sensing, it is unlikely that a single sensor with a single coating will display a selective and reversible response to a given analyte in a mixture. Alternative strategies such as the use of sensor arrays and the use of sampling devices can be used to improve performance. QCM sensors (QCMs) will oscillate under liquids; their use in under-liquid sensing will be discussed.

Micro-environments and mass transfer phenomena in biofilms studied with microsensors

Direct observations on chemical micro-environment and microbial composition in biofilms are rare. The combination of microsensor and molecular techniques is highly useful for studies on the microbial ecology of biofilms. We shortly describe some applications of microsensors to study mass transfer phenomena and microbial processes in biofilms. It has recent been recognized that biofilms are not always flat layers of cells, but can consist of complex structures allowing liquid flow. Thus the classical view, that transport in biofilms is diffusional, is challenged. In laboratory grown biofilms the effect of convection on mass transfer was demonstrated. The microsensor technique has improved, so that direct in situ measurements in living biofilms are possible. By direct measurements of liquid flow with microsensors we show that in biofilms grown in bioreactors heterogeneity and convectional transport must also be taken into account. For the description of the microbial population we use molecular techniques, such as in situ hybridisation with 16S rRNA-targeted oligonucleotide probes. In a nitrifying-denitrifying biofilm we found a complex nitrifying community consisting of members of the genera Nitrosomonas, Nitrosospira, Nitrobacter and Nitrospira . Their occurrence was correlated with nitrification activity as determined by microsensor measurements.

Development of a GaAs monolithic surface acoustic wave integrated circuit

An oscillator technology using surface acoustic wave (SAW) delay lines integrated with GaAs MESFET electronics has been developed for GaAs-based integrated microsensor applications. The oscillator consists of a two-port SAW delay line in a feedback loop with a four-stage GaAs MESFET amplifier. Oscillators with frequencies of 470, 350, and 200 MHz have been designed and fabricated. This oscillator technology is most suitable for sensor applications but can logically be extended to radio-frequency oscillator and filter applications by methods well known for other piezoelectric substrates.

A study on the thermal behavior in microaccelerometer manufacturing processes

Single crystal silicon (SCS) is used in a variety of microsensor applications in which stresses and other mechanical effects may dominate device performance. One of major problems associated with the manufacturing processes of the microaccelerometer based on the tunneling current concept is temperature gradient and thermal stressess. This paper deals with finite element analyses of residual stresses causing popping up, which are induced in micromachining processes of a microaccelerometer. The authors model temperature-dependent mechanical properties during focused ion beam (FIB) cutting and Pt deposition processes. The maximum thermal strain of 0.0012 occurred at the readout gap of the microaccelerometer during the Pt deposition process. The stress produced during the heating process of FIB cut was also found to be about 33–36 MPa and cooling process the maximum equivalent stress of about 34MPa still at the right corner of readout gap. The calculated maximum displacement occurred at the readout gap was 0.14 μm. This is still smaller than the popping up of about 2 μm observed in the experiment. The reason for this popping up phenomenon in munufacturing processes of microaccelerometer may be the bending of the whole wafer or it may come from the way the underetch occurs. Different SOI material and a new geometry of the accelerometer are under investigation. We want to seek after the real cause of this pop up phenomenon and diminish this by changing munufacturing processes of microaccelerometer.

Structural and functional dynamics of sulfate-reducing populations in bacterial biofilms

We describe the combined application of microsensors and molecular techniques to investigate the development of sulfate reduction and of sulfate-reducing bacterial populations in an aerobic bacterial biofilm. Microsensor measurements for oxygen showed that anaerobic zones developed in the biofilm within 1 week and that oxygen was depleted in the top 200 to 400 &mgr;m during all stages of biofilm development. Sulfate reduction was first detected after 6 weeks of growth, although favorable conditions for growth of sulfate-reducing bacteria (SRB) were present from the first week. In situ hybridization with a 16S rRNA probe for SRB revealed that sulfate reducers were present in high numbers (approximately 10(8) SRB/ml) in all stages of development, both in the oxic and anoxic zones of the biofilm. Denaturing gradient gel electrophoresis (DGGE) showed that the genetic diversity of the microbial community increased during the development of the biofilm. Hybridization analysis of the DGGE profiles with taxon-specific oligonucleotide probes showed that Desulfobulbus and Desulfovibrio were the main sulfate-reducing bacteria in all biofilm samples as well as in the bulk activated sludge. However, different Desulfobulbus and Desulfovibrio species were found in the 6th and 8th weeks of incubation, respectively, coinciding with the development of sulfate reduction. Our data indicate that not all SRB detected by molecular analysis were sulfidogenically active in the biofilm.

PROCESSING AND CHARACTERIZATION OF PIEZOELECTRIC MATERIALS AND INTEGRATION INTO MICROELECTROMECHANICAL SYSTEMS

▪ Abstract Piezoelectric materials have been integrated with silicon microelectromechanical systems (MEMS) in both microsensor and microactuator applications. Thin-film materials selection and processing routes are reviewed. Some recent and emerging applications of piezoelectric MEMS are presented including acoustic emission microsensors, vibration monitors, molecular recognition biosensors, precision positioners, micropumps, and linear stepper motors.

Magnetoelastic characterization of thin films dedicated to magnetomechanical microsensor applications

This paper discusses the results of measurements on the saturation magnetostriction constant λ s, magnetoelastic coupling coefficient b γ,2 and magnetostriction strain coefficient d for Ni-evaporated and Fe 70B 20Si 6C 4-sputtered thin films on silicon and mica substrates. These results are presented in correlation with other magnetic properties (i.e., saturation magnetization, Curie temperature and anisotropy field) which characterize magnetoelastic sensing applications. We have demonstrated that the relevant properties of the magnetoelastic media used for magnetomechanical microsensors can be considerably improved by using amorphous Fe-metalloid thin films sputtered onto a silicon substrate in comparison with conventional magnetostrictive thin films evaporated onto silicon and mica substrates.

Microsensors as a tool to determine chemical microgradients and bacterial activity in wastewater biofilms and flocs.

Microsensors used in microbial ecology are reviewed with emphasis on new sensor developments (NO3-, NO2-, NH4+, CO2, H2, H2S and CH4 microsensors as well as fiberoptical microsensors for O2, temperature and pH). Examples of microsensor applications in biofilms and activated sludge flocs are presented, in which sulfate reduction and denitrification were studied.

Fabrication of low-stress dielectric thin-film for microsensor applications

A method of fabricating low-stress dielectric thin film as the supporting material of micromachined devices is reported. The film is processed by post thermal oxidation of silicon-rich nitride (SN) deposited on a silicon substrate by LPCVD. Due to the compensation on the nitride by its top oxide, an ultra-low residual stress less than 10 MPa can be obtained with a proper oxidation scheme. Characteristics of the oxidized nitride were analyzed by Auger electron spectroscopy (AES) and ellipsometry. Large floating membranes of 4/spl times/4 cm/sup 2/ and 400-nm thick can be made by this method with TMAH etching.

A comparative study of signal processing techniques for clustering microsensor data (a first step towards an artificial nose)

Microsensor technology has progressed to the point where it is now feasible to place several hundred sensors on a computer chip. Such a sensor array can potentially be used in many applications including detecting hazardous chemical emissions, food processsing, and fire detection. This paper addresses an important aspect involved in microsensor applications, namely how the sensor signals are processed. The problem treated involves classifying whether a sensed signal is generated by one of four chemicals. Two broad approaches to processing the sensor signals are discussed, one based on classical signal processing approaches, and one based on a model of how the olfactory system in animals functions. The classical approaches used include: Gram Schmidt orthogonalization, fast Fourier transforms, and Haar wavelets. For the experimental signals treated, the classical approaches give superior results compared to those produced by the olfactory model.

IC-compatible silicon wafer-to-wafer bonding

In this paper a fully IC-compatible silicon wafer-to-wafer fusion-bonding process is described. Before the bonding, the silicon surfaces are treated by chemicals which do not attack the electronic circuits or aluminium patterns on the silicon. The prebonding of the two silicon wafers is performed at room temperature. after which bonding takes place through annealing at temperatures between 120 and 400°C without affecting the performance of the electronic circuitry. The bonding is sufficiently strong for microsensor applications.

Thermal drift of piezoresistive properties of LPCVD polysilicon thin films between room temperature and 200 °C

Investigations of the thermal drifts of the resistivity and gauge factors of polysilicon are needed to extend its use as a transducing material for high temperature piezoresistive microsensor applications. Longitudinal gauge factors of LPCVD B-doped and rapid thermal-annealed polysilicon films, patterned onto oxidized silicon using a clamped beam technique, have been measured in the 20–200 °C temperature range as a function of the doping concentration between 8 × 10 18 and 10 20 cm −3. The gauge factors have been simulated as a function of the B-concentration and the temperature, starting from experimental data for the polysilicon resistivity and literature data for the hole resistivity and the piezoresistive effect in monocrystalline silicon. Only a model involving a significant piezoresistive effect at grain boundaries can adequately fit the temperature dependent gauge factors of polysilicon.

Microsensor techniques for cardiovascular systems

Summary. Progress in semiconductor technology has been very fast in the last few years. One important fabrication process is the CMOS (Complementary Metal Oxide Semiconductor) process. This technology has been developed for the fabrication of electronic circuits, but is now also used for the fabrication of microsensors and microsystems by adding some compatible process steps. This contribution reports upon some examples for the application of CMOS-compatible physical microsensors with on-chip integrated read-out electronics in a cardiovascular ambient. Especially monolithically integrated pressure sensors and flow sensors for blood pressure and blood flow measurement are presented, representing the state-of-the-art techniques of CMOS-compatible surface micromachining and bulk micromachining. Due to the flexible design possibilities, the surface micromachined pressure sensor with a chip size of 0.7 mm × 7 mm or even smaller can be placed in a heart catheter with a diameter of 1 mm for blood pressure measurement in arterial vessels. Additionally a temperature sensor is integrated on the chip. In the same catheter application a flow sensor chip based on a hot-film anemometry principle measures blood flow dynamically with a fast response and low power consumption combined with small dimensions. A system with integrated circuits used for data acquisition with wireless data transfer of cardiovascular sensor signals from inside the body is also discussed.

Piezoresistance characterization of commercial CMOS gate polysilicon and its application in biomass microsensors

This paper presents experimental results on the piezoresistance characterization of gate polysilicon available from two commercial CMOS processes. It is shown that the gate polysilicon is very strain-sensitive, and a gauge factor of about 25 can be readily achieved. This value can allow standard gate polysilicon to be used as a strain-sensing element for integrated microsensor applications. As an example, a sub-nanogram mass sensor was fabricated using commercially available CMOS technology and is presented. The device incorporates gate polysilicon of the CMOS process as the sensing material, and is subjected to low levels of strain in order to measure small masses (< 10−9 g). A potential application for this sensor is to monitor the growth of biological cell cultures in a liquid environment.

A novel low-field electron-emission polycrystalline diamond microtip array for sensor applications

Electron field emission from an array of patterned pyramids of polycrystalline diamond has been investigated for microsensor applications. Selective deposition and molding of a polycrystalline diamond film in a silicon cavity mold and subsequent creation of a freestanding polycrystalline diamond diaphragm with diamond pyramidal microtips has been achieved. The processing techniques are compatible with IC and micromachining technologies. High current emission from the patterned diamond microtip arrays is obtained at low electric fields. Field emission for these diamond microtips exhibits significant enhancement in total emission current compared to silicon emitters. Moreover, field emission from patterned polycrystalline diamond pyramidal-tip arrays is unique in that the applied field is found to be lower than the required for emission from S 1, Ge, GaAs, and metal surfaces. The findings suggest the potential use of diamond-based field-emitter arrays in microsensors such as pressure sensors, accelerometers, and tactile sensors.

Surface acoustic wave microsensors.

Surface acoustic waves (SAWs) excited on the surface of piezoelectric crystals form the basis of a family of microsensors that is sensitive, portable, cheap, and small. These microsensors can be used in a wide variety of applications to sense low levels of gases, physical quantities such as mass, temperature, pressure, flow and humidity, biological processes, corrosion, ions and particulate matter, and a variety of mechanical and electrical quantities. In contrast to other microsensor technologies, SAW microsensors can sensitively detect both electrical and mechanical phenomena. This is due to the fact that the SAW has both mechanical and electrical fields. In those applications requiring sensitivity, such as gas sensing and biosensing, the selectivity is provided by a receptor film applied to the piezoelectric substrate. In this paper different types of SAWs are described and prototype sensing geometries are presented. Particular attention is devoted to one of the most common SAW microsensor applications, namely, gas sensing. Finally, results obtained using SAW microsensors in a variety of applications are presented and discussed.