J.A. Voorthuyzen

The influence of tensile forces on the deflection of circular diaphragms in pressure sensors

It is known that the deflection of a diaphragm is determined by two mechanisms, bending moments or bending stress and tensile forces or membrane stress. Usually the influence of tensile forces is not taken into account when calculating the mechanical properties of thin diaphragms. Hence the mathematical description thus obtained will only be valid if the deflection of the diaphragm is small compared with its thickness and if lateral stress is absent. In this paper we will consider uniformly loaded circular diaphragms, which are assumed to be isotropic, and present the results of a study on the influence of bending stress and tensile stress, which together determine the diaphragm deflection. Starting from theoretical considerations, a simulation program is developed of which several results are presented and discussed.

Investigations of the surface conductivity of silicon dioxide and methods to reduce it

In this paper we describe our investigations of the electrical conductivity of the silicon dioxide-air interface. It appears that this conductivity is caused by the adsorption of water vapour on the oxide surface and strongly depends on the relative humidity of the surrounding air. Considering this fact we have decided to investigate the possibility to reduce the surface conductivity by means of a chemical modification of the oxide surface, which reduces the adsorption of water vapour on it. To measure the conductivity we have used a so-called open-gate FET structure. The performance characteristics of this structure and the experimental results obtained with it are presented. We have found that by using silane agents like HMDS and DCDMS the surface conductivity of silicon dioxide can be reduced with at least a factor 1000 and 10000 respectively.

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. >

Investigation of attractive forces between PECVD silicon nitride microstructures and an oxidized silicon substrate

A troublesome phenomenon encountered during the realization of free-standing microstructures, for example, beams, diaphragms and micromotors, is that initially released structures afterwards stick to the substrate. This effect may occur during wafer drying after the etching process has been completed, as well as during normal operation as soon as released structures come into contact with the substrate. In this paper the most important types of attractive forces are discussed with respect to their possible influence on the performance of micromachined structures. It is concluded that the main reason for sticking of PECVD silicon nitride micromachined structures is adsorption of water molecules. The water molecules, adsorbed on both surfaces, attract each other as soon as the surfaces come into contact. It is shown that a chemical surface modification, in order to achieve hydrophobic surfaces, is an effective method for avoiding adsorption of water, and therefore reduces sticking. Sticking of micromachined structures during drying is reduced by rinsing with a non-polar liquid before wafer drying.

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. >

PECVD silicon nitride diaphragms for condenser microphones

The application of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride as a diaphragm material for condenser microphones has been investigated. By means of adjusting the SiH4/NH3 gas-flow composition, silicon-rich silicon nitride films have been obtained with a relatively low tensile stress. Aluminium can be etched selectively with respect to the silicon nitride films. Using aluminium as a sacrificial layer, 300 × 300 μm silicon nitride diaphragms have been made. Admittance measurements on silicon nitride capacitances have shown that the insulating properties are sufficiently good for application as a microphone diaphragm.

Photoelectric effects in Ta2O5SiO2Si structures

Investigating the behaviour of ISFETs with a Ta2O5SiO2 dielectric it has been observed that their long-term stability is strongly influenced by previous exposures to daylight. A Ta2O5 layer, as prepared for ISFET application, will become conductive by exposure to optical radiation. This conduction is due to bulk traps with a depth of 1.6–3.2 eV. It is shown that a low temperature anneal step in the presence of an A1 layer on top of the Ta2O5 layer may eliminate these traps and reduce the associated threshold voltage instability.

The pressfet: an integrated electret-mosfet based pressure sensor

We present a new MOSFET-based pressure sensor, incorporating an air gap that is a function of pressure and a permanently charged dielectric layer (electret) between the gate and bulk of the MOS structure. In this way we obtain a MOSFET with a pre-charged variable gate capacitance. The theory of this sensor and the NMOS-compatible process for its realization are given. We present also the first experimental results on this new sensor. We have determined the characteristics of sensors with outer dimensions of 1 mm × 2 mm × 0.3 mm and measured a maximum sensitivity of about 10 mA/A/100 mm Hg, which is about ten times higher than the sensitivity of piezoresistive pressure sensors with comparable dimensions. Drawbacks of the present sensor design are a relatively high temperature sensitivity and a rather complex fabrication process.

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.