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

The consequences of the application of a floating gate on d.c.-MISFET characteristics

In the literature the influence of the conducting layer, sometimes called a floating gate, upon d.c.-MISFET characteristics is ignored or only treated in a phenomenological way. Our intentions in this paper are to present a study of the consequences of a conducting layer in an exact way by using the MISFET theory described earlier.\ud \ud It is found that the d.c.-characteristics are influenced by parasitic capacitances from the conducting layer to source and drain and the charge-voltage relations along the channel of the MIS transistor.\ud \ud The theoretical considerations are verified by simulations with Spice 2 and some experimental results and are in agreement with the characteristics already given in the literature referred to

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.\ud \ud 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.\ud \ud 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.\ud \ud Starting from theoretical considerations, a simulation program is developed of which several results are presented and discussed

Micromachining of electret materials, advantages and possibilities

A process is described for the micromachining of organic electret materials like Teflon FEP (fluorinated ethylene propylene). The authors have used photolithography and plasma etching, by which electrets can be etched selectively. The advantages of micromachining electrets in such a way are numerous. Miniature sensors like microphones and pressure sensors, containing local areas using Teflon, have been realized in the authors' laboratories. The process can also be used for electret research. An example is the overall reduction of electret film thickness, by which the authors have determined the average penetration depth of accelerated electrons in Teflon FE

An electret-based pressure sensitive MOS transistor

The operation of the MOSFET is based on the fact that the lateral conductivity of silicon at the silicon-dioxide-silicon interface strongly depends on the transverse electric field in the oxide. Adding a small air-filled spacer between the metal gate and the oxide, and applying a voltage across the insulator on top of the silicon, the lateral conductivity can become pressure sensitive. The generation of the electric field in the insulator can also be provided by means of an electret. The theory, realization, and performance of an integrated electret-MOSFET-based pressure sensor are presente

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

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

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.\ud \ud This structure is basically an MOS transistor with a movable gate and can thus be considered as a pressure-sensitive field effect transistor.\ud \ud It is shown that the addition of a metal layer on top of the semiconductor-oxide increases the sensitivity due to charge density multiplication.\ud \ud 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.\ud \ud 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

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 (SiO2), 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 PTF

The tonometric sensor, a new device for the measurement of intraocular pressure

In this paper we present a new sensor for the measurement of intraocular pressure. We have applied the indentation principle, in which the eye is indented by exerting a force on it, while the size of the indented area is monitored. To measure the force we have used a commercial force sensor. The sensor to measure the indentation of the eye has been developed in our laboratories. We have performed measurements using an experimental set-up; these show that an accuracy of 90% will be possible if the sensor is designed in the proper way