Wouter Olthuis

The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications

Polydimethylsiloxane (PDMS) is a commercially available physically and chemically stable silicone rubber. It has a unique flexibility with a shear elastic modulus due to one of the lowest glass transition temperatures of any polymer . Further properties of PDMS are a low change in the shear elastic modulus versus temperature , virtually no change in G versus frequency and a high compressibility. Because of its clean room processability, its low curing temperature, its high flexibility, the possibility to change its functional groups and the very low drift of its properties with time and temperature, PDMS is very well suited for micromachined mechanical and chemical sensors, such as accelerometers (as the spring material) and ISFETs (as the ion selective membrane). It can also be used as an adhesive in wafer bonding, as a cover material in tactile sensors and as the mechanical decoupling zone in sensor packagings.

A review of silicon microphones

Silicon micromachining has successfully been applied to fabricate piezoelectric, piezoresistive and capactive microphones. The use of silicon has allowed the fabrication of microphones with integrated electronic circuitry and the development of the new FET microphone. The introduction of lithographic techniques has resulted in microphones with very small (1 mm2) diaphragms and with specially shaped backplates. The application of corrugated diaphragms seems a promising future development for silicon microphones. It is concluded from a noise consideration that the FET microphone shows a high noise level, which is mainly due to the small sensor capacitance. From this noise consideration, it can be shown that integration of a capacitive microphone and a preamplifier will result in a further reduction of the noise.

A plastic micropump constructed with conventional techniques and materials

A plastic micropump which can be produced using conventional production techniques and materials is presented. By applying well-known techniques and materials, economic fabrication of micropumps for various applications is feasible even at low production volumes. The micropump is capable of pumping both liquid and gas at a considerable high pump rate and is self-priming, which means that it can start pumping gas in a dry state and automatically fills with liquid. Pump rates, at actuation frequencies between 2 and 500 Hz, were around 2 ml/min for water and up to 50 ml/min for air. A differential pressure of 1.25×104 Pa (125 cm water column) was reached. Basically, the micropump consists of two parts, a flat valve assembly with two passive membrane valves and an actuator placed on top. The valves were made by sandwiching a punched thin polymer film between two plastic valve parts containing the valve seats. The latter parts are made by reactive injection molding of an epoxy resin. Two types of actuators have been applied to drive the pump; an electromagnetic actuator consisting of a magnet placed in a coil and secondly a disk. The first actuator, when combined with a flexible polymer pump membrane, showed a very large pump rate for gas, up to 40 ml/min at the resonant frequency of the actuator system. A disadvantage of the electromagnetic actuator was the relatively large volume occupied by the coil giving the micropump final dimensions of 10×10×8 mm3. Application of the piezoelectric actuator reduced these dimensions down to 12×12×2 mm3 with comparable performance.

Procedure for in-use calibration of triaxial accelerometers in medical applications

In the medical field triaxial accelerometers are used for the monitoring of movements. Unfortunately, the long-term use of accelerometers is limited by drift of the sensitivities and the offsets. Therefore, a calibration procedure is designed which allows in-use calibration of a triaxial accelerometer. This procedure uses the fact that the modulus of the acceleration vector measured with a triaxial accelerometer equals 1g under quasi-static conditions. The calibration procedure requires no explicit knowledge of the actual orientation of the triaxial accelerometer with respect to gravity and requires only quasi-static random movements. The procedure can be performed within several minutes. Considering practical applications, it is found possible to obtain an average error smaller than 3% (of 1 [g]) with the calibration procedure for inputs caused by ‘standing straight and moving slightly’ when the offsets only are estimated. For the input caused by the sequence that occurs when going to bed, it is found possible to obtain errors smaller than 3% for estimating all parameters.

Stimulus-sensitive hydrogels and their applications in chemical (micro)analysis

In this tutorial review the use of stimulus-sensitive hydrogels as sensors and actuators for (micro)analytical applications is discussed. The first part of the article is aimed at making the reader familiar with stimulus-sensitive hydrogels, their chemical composition and their chemo-physical behavior. The prior art in the field, that comprises a number of sensors ranging from metal ion-sensitive sensors to antigen-sensitive sensors and a few actuators, is also treated in this part. The second part of the article focusses on the use of stimulus-sensitive hydrogels for microsensors and microactuators as well as their application in micro total analysis systems. The benefits of stimulus-sensitive hydrogels, their miniaturisation and the use of 365 nm UV-photolithography as a fast economical manufacturing technique are discussed.

Theoretical and experimental determination of cell constants of planar-interdigitated electrolyte conductivity sensors

In this paper, an analytical expression is presented for the cell constant of planar-interdigitated electrodes used as electrolyte conductivity sensors. The result of this expression is compared with results of measurement carried out with several differently shaped planar probes provided with a thin Ta2O5 insulating film, showing good agreement. More than 10 different devices have been fabricated with predicted cell constants ranging from 0.14 to 4.44 cm?1. The measured cell constants are typically 10?20% smaller, possibly due to fringing effects.

A closed-loop controlled electrochemically actuated micro-dosing system

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

The design, fabrication, and testing of corrugated silicon nitride diaphragms

Silicon nitride corrugated diaphragms of 2 mm/spl times/2 mm/spl times/1 /spl mu/m have been fabricated with 8 circular corrugations, having depths of 4, 10, or 14 /spl mu/m. The diaphragms with 4-/spl mu/m-deep corrugations show a measured mechanical sensitivity (increase in the deflection over the increase in the applied pressure) which is 25 times larger than the mechanical sensitivity of flat diaphragms of equal size and thickness. Since this gain in sensitivity is due to reduction of the initial stress, the sensitivity can only increase in the case of diaphragms with initial stress. A simple analytical model has been proposed that takes the influence of initial tensile stress into account. The model predicts that the presence of corrugations increases the sensitivity of the diaphragms, because the initial diaphragm stress is reduced. The model also predicts that for corrugations with a larger depth the sensitivity decreases, because the bending stiffness of the corrugations then becomes dominant. These predictions have been confirmed by experiments. The application of corrugated diaphragms offers the possibility to control the sensitivity of thin diaphragms by geometrical parameters, thus eliminating the effect of variations in the initial stress, due to variations in the diaphragm deposition process and/or the influence of temperature changes and packaging stress. >

Fabrication of silicon condenser microphones using single wafer technology

A condenser microphone design that can be fabricated using the sacrificial layer technique is proposed and tested. The microphone backplate is a 1- mu m plasma-enhanced chemical-vapor-deposited (PECVD) silicon nitride film with a high density of acoustic holes (120-525 holes/mm/sup 2/), covered with a thin Ti/Au electrode. Microphones with a flat frequency response between 100 Hz and 14 kHz and a sensitivity of typically 1-2 mV/Pa have been fabricated in a reproducible way. These sensitivities can be achieved using a relatively low bias voltage of 6-16 V. The measured sensitivities and bandwidths are comparable to those of other silicon microphones with highly perforated backplates. The major advantage of the new microphone design is that it can be fabricated on a single wafer so that no bonding techniques are required. >

pH Sensor Properties of Electrochemically Grown Iridium Oxide

The open-circuit potential of an electrochemically grown iridium oxide film is measured and shows a pH sensitivity between −60 and −80 mV/pH. This sensitivity is found to depend on the state of oxidation of the iridium oxide film; for a higher state of oxidation (or more of the oxide in the high valence state), the sensitivity is also higher. This high sensitivity can be explained on the basis of the extra proton release as a result of the acidic character of the porous hydrous oxyhydroxide, in combination with the redox behaviour. The response time to a pH step is measured and is found to depend mainly on the thickness of the oxide; it varies from 40 ms to 0.35 s due to the porous nature of the film. Drift measurements show that an iridium oxide film in reduced state is slowly oxidized by dissolved oxygen, whereas a pre-oxidized film in a pH = 4.01 buffer solution in contact with air shows a long-term drift of <0.3 mV/h.