A.J. Sprenkels

Semiconductor-based electret sensors for sound and pressure

The theory and experimental results for integrated electret-based silicon sensors for the detection of sound and pressure are presented. A silicon electret microphone for use in hearing-aids is described. It has an experimentally determined sensitivity of 19 mV/Pa in the frequency range of 50 Hz to 10 kHz. The realization of a pressure sensor for application in catheter-tip blood-pressure measurements is discussed. This type of electret sensor appeared to have a pressure sensitivity of 1.6 mu V/Pa in the required frequency range of 0 to 100 Hz. >

Development of an electret microphone in silicon

We describe a subminiature electret microphone, which has been realized in silicon using wafer processing techniques. The microphone consists of a rigid backplate fabricated in silicon and a 2.5 μm thick metallized Mylar foil (PETP) acting as the diaphragm. Between the diaphragm and the backplate a 20 μm thick air cavity and a 1.1 μm thick charged SiO2 layer are present. The SiO2 layer is used as the electret and generates an electric field in the air gap. The electret has been changed to 300 V using a corona-charging set-up. The time constant of the charge decay amounts to more than 100 years at ambient laboratory conditions. The microphone cartridge, which measures 3 × 3 × 0.3 mm, shows an open-circuit sensitivity of about 2.5 mV/μbar at 1 kHz.

Research and development of miniaturized electrets

A description is given of the realization of small electrets, using techniques generally applied in the fabrication of integrated circuits and microsensors. Attention is paid to the different electret decay mechanisms and their relative contributions to the overall stability of miniaturized electrets. A process is described by which polymer electrets such as Teflon-FEP and PTFE (polytetrafluoroethylene) can be deposited and shaped in a predefined pattern on a silicon wafer. Results on the application of new materials, especially silicon dioxide (SiO/sub 2/), for use in electret applications, are presented. It appears that after an appropriate treatment of the oxide surface, its charge-stability is at least equal to that of polymer electrets such as Teflon-FEP and PTFE. >

Planar interdigitated electrolyte-conductivity sensors on an insulating substrate covered with Ta2O5

Interdigitated electrolyte-conductivity sensors with an added top layer of insulating Ta2O5 have been realized. The electrode-substrate structure under the Ta2O5 film has been planarized in order to obtain a totally flat top surface. In addition, the electrodes have been applied on a totally insulating substrate, thus reducing the parasitic sensor capacitance by a factor of ten.

A theoretical analysis of the electret air-gap field-effect structure for sensor applications

In order to develop a capacitive solid state sensor that makes use of an electret, a theoretical analysis is given of an electret air-gap field-effect structure. This structure is basically an MOS transistor with a movable gate and can thus be considered as a pressure-sensitive field effect transistor. It is shown that the addition of a metal layer on top of the semiconductor-oxide increases the sensitivity due to charge density multiplication. All calculations are based upon the displacement sensitivity S, which is independent of the mechanical properties of the diaphragm and thus independent of a specific application. Based upon the calculated sensitivities of the several configuration, a well-considered decision can be made as to which configuration is best suited for a specific application. In this paper this has been done for a solid state microphone and a pressure sensor as examples.

Optimization of Capactive Microphone and Pressure Sensor Performance by Capacitor-electrode Shaping

In many designs of capacitive microphones or pressure sensors the electrode size is chosen to be equal to the diaphragm size. In this paper it will be discussed whether an electrode size or shape that differs from that of the diaphragm is attractive for obtaining a maximum value for the sensor sensitivity and the signal-to-noise ratio. A theoretical analysis will be given for circular diaphragms and electrodes, from which it can be shown that for maximum sensitivity the electrode should be located at the centre of the diaphragm, with a radius depending on the value of the amplifier input capacitance.

Preliminary results of a silicon condenser microphone with internal feedback

In order to overcome the problem of a too large bias voltage and to increase the bandwidth of silicon condenser microphones with small air gaps, an electrostatic feedback system is presented that will reduce the movement of the diaphragm of the microphone. It is shown that a 10-fold reduction of the movement of the diaphragm is possible for microphones with an air gap of 35 mu m. However, to get a reduction of about ten times over a wide range of frequencies, an actuator with a larger sensitivity is needed. This can be achieved by the use of a smaller air gap, which may also increase the sensor sensitivity.<<ETX>>

A Closed-Loop Calibration System for a Microdialysis-Based Lab-On-A-Chip

In this contribution a closed loop calibration facility for a microdialysis based lab-on-a-chip is presented. Each facility consists of a channel structure etched in silicon, closed by a Pyrex® cover comprising two small platinum electrodes. In the silicon structure the calibration liquid is stored in the meander shaped channel that starts in an electrolyte filled reservoir. By applying an electric current via both electrodes through the electrolyte in the reservoir, gas bubbles are generated by electrolysis, forcing the calibration liquid into the main carrier channel leading to the sensor area of the lab-on-a-chip. Above the electrolyte reservoir two additional interdigitated electrodes are present that are used to measure the gas bubble volume by measuring the impedance of the gas/liquid mixture in the reservoir. Since the volume of the gas bubbles is equal to the dispensed calibration liquid, its total volume can be controlled by a closed loop impedance measuring system. In this way precise plugs of 60nl calibration liquid have been dispensed into the main carrier channel.

An Integrated Microdialysis-Based System

In this paper the integrated (bio)chemical analysis system for microdialysis, consisting of several functional blocks: microdialysis probe, electrochemical sensor array and calibration facilities, as well as results on calibration of the Potentiometric sensor by means of the integrated calibration facilities are presented.

On the integration of a microdialysis-based /spl mu/TAS with calibration facility on a silicon-glass sandwich

The integration is discussed of all parts of a microdialysis-based micro Total Analysis System or /spl mu/TAS. In particular a microdialysis probe, a potentiometric and amperometric ion- and enzyme sensor and a calibration dosing pump have been developed separately using different precision machining techniques. By modifying and adapting these parts they can be realized in one generic technology consisting of a stack of a silicon and a glass wafer. The silicon wafer contains the double lumen microdialysis probe connections, a dosing pump chamber with meander formed cavities containing the calibration solutions and small cavities for both the potentiometric and amperometric sensor. The glass wafer contains all the electrical contacts and wires for the sensors, the pump and Interconnections. Both wafers are anodically bonded to each other, yielding a hermetically sealed liquid handling system.