D. Hauden

AlN/silicon Lamb-wave microsensors for pressure and gravimetric measurements

Abstract We have studied a pressure and a gravimetric sensor using an AlN film over a silicon membrane. The membrane is micromachine using both anisotropic and isotropic chemical etching. The AlN thin film is deposited by the sputtering method. Elastic waves are generated and detected through interdigital transducers (IDTs) calculated at the frequency of 90 MHz for the S 0 Lamb-wave mode. The behaviour of such a sensor is presented. We have analysed and experimentally tested the pressure and have calculated the gravimetric sensitivity. By using the opposite temperature behaviours of silicon and AlN, a satisfactory temperature compensation is achieved. Results obtained from such experiments show sensitivities that agree with theoretical sensitivities provided by a theoretical model which includes the effect of the mass loading of the deposited thin-layer material.

Acoustic wave thermal sensitivity: Temperature sensors and temperature compensation in microsensors

Abstract In the field of temperature measurements, microsensors are based on different physical transduction principles, such as semiconductor parameters (conductivity, p–n junction voltage), metallic junction voltage (thermocouple) or elastic wave velocity variations (bulk or surface acoustic waves). This paper discusses acoustic-based temperature microsensors. Thermal effects are modelled and applications to bulk and surface acoustic wave microsensors are presented. Moreover, the use of temperature sensing for other measurements (thermal flow and humidity sensors) is described. Improvements of sensitivities and response time by varying the operating temperature are also pointed out for gas detectors. Possibilities of temperature compensation in silicon acoustic wave microsensors are discussed.

Microstereophotolithography and shape memory alloy for the fabrication of miniaturized actuators

Abstract A new microassembly technique has been developed to combine polymer micro-objects fabricated by stereophotolithography with shape memory alloy (SMA) wires. This technique is based on a local laser-aided potymerization allowing SMA wires to be associated with polymer structures. Since stereophotolithography allows the fabrication of real three-dimensional objects, different mechanical structures have been designed and manufactured to develop miniaturized mechanical devices (in the centimetre range) with multiple degrees of freedom actuated by SMA wires (diameter 120 µm). The actuation principle has been successfully tested. Experimental measurement procedure and results are presented and discussed. Further developments and improvements of the actuators are proposed.

A fast and reproducible method for gas sensor screening to flavour compounds

Abstract The use of gas sensor arrays has taken a growing place in the field of gas sensor research. One of the main drawbacks of this method is the large amount of time necessary to collect the amount of calibration data, for a convenient task [1]. The purpose of this paper is to present a brand new method for fast and reproducible characterization and calibration of gas sensors. Surface acoustic wave (SAW) gas sensors are very sensitive [2,3], nevertheless their chemical adsorbant layer must still be optimized with respect to the gas species to be detected. For food control purposes, it has been shown that SnO2 sensors may be useful [4–6]. This paper therefore deals with integrated semiconducting sensors and their ability to detect flavours. Experimental results will be presented and discussed and finally some aspects of sensor array processing, using artificial neural networks, will be explored.

Three-dimensional silicon electrostatic linear microactuator

In this paper, the fabrication of a three-dimensional (3D) silicon linear microactuator is described. This actuator is realized by anisotropic chemical etching of a (110)-oriented silicon wafer. The silicon monolithic structure of the microactuator comprises a fixed part bonded onto a glass substrate and a movable part suspended from two elastic beams. The displacement of the movable part is consequently frictionless, driven by electrostatic forces. We present the steps of the etching process and initial results concerning mechanical and electrostatic tests. Finally, some projected applications are discussed.

Sensitivity comparison between gas sensors using SAW and shear horizontal plate-mode oscillators

Abstract Gas sensors using SAW oscillators are now well known. However, the corresponding devices based on shear horizontal (SH) plate-mode oscillators have not received so much attention. Although both of them work on a mass-loading effect due to adsorption properties of a chemical coating, SH plate-mode delay-line oscillators are considered to be more sensitive. Few results are available to support this assertion. Using computer simulation, the responses of the two sensor types are calculated and compared when their active chemical layer is the seat of gas-induced physical modifications. The simulation requires a knowledge of several physical constants for both materials (quartz and coating). The theoretical responses are expressed in terms of acoustic-wave phase velocity and calculated for different quartz crystal cuts by a perturbation method involving thickness, density and stiffness of the substrate and its coating. Used under the same conditions, the SH plate-mode device is found to be more sensitive than the SAW-mode one within a particular thickness domain.

Elastic wave sensors—quartz technological channel and silicon technology

Abstract Elastic wave sensors use the vibrating mechanical properties of materials, generally monocrystalline and piezoelectric, such as quartz crystal or lithium niobate. Within such materials, elastic waves are easy to generate and to detect after propagation by means of plate electrodes (bulk waves) or interdigital transducers (IDT) (surface waves or bulk waves). The sensitive signal of such sensors can be measured in terms of intensity (attenuation of the signal) or frequency, with oscillating electronic loop, or in-phase versus an arbitrary or out of phase reference. Among quartz crystal sensors, a particularly interesting new family has been studied: sensors using transverse horizontal waves (TH sensors). In recent years, elastic wave sensors on silicon have been studied. The piezoelectric layer deposited on the silicon substrate must be compatible with the usual silicon process used in microelectronics, and also with the micromachining made generally chemically in 3-D.

Feasibility of surface acoustic wave (SAW) sensor array processing with formal neural networks

Abstract The poor selectivity of sensors, used to detect different species mixed in a real gas mixture, requires efficient signal processing devices to improve it. We have tried to apply neural techniques to this problem in order to obtain some ‘electronic’ selectivity. This work is the first step of a feasibility study of an electronic device using SAW gas sensors. Using linear approximation of experimental and static results of SAW sensors given in previous data, we also attempted to apply neural networks for multisensor array signal processing. In order to extend the operating range, the saturation effects yield by chemical layers was considered. To optimize the neural network, several activation functions were tested. A design close to radial basis functions networks was successfully applied. A new network connectivity led to increased interpolation capability. For real time processing, it is important to escape from slow kinetic adsorption of chemical layers. In this paper, the theoretical possibility of response time compensation has been shown by using a deconvolution process.

A study of CuPC/water interactions using surface acoustic wave devices

Abstract The aim of this paper is to report on the results obtained by a surface acoustic wave (SAW) technique used in measuring humidity sorption. Dual SAW delay-line oscillators are used. One of the two paths is coated with an organic compound in order to achieve a gas sensor (copper phthalocyanine, CuPC, for NO 2 sensing). It appears, however, that if a polar molecule is present in the vector gas (air or nitrogen) and sorbs on the SAW path, it can interact with these waves and modify the mechanical and electrical fields. This results in Rayleigh wave velocity changes and as a consequence produces frequency shifts. Quartz substrates (ST-cut, X propagating) are used for this study. Responses are obtained for many relative humidity values ranging from 0 to 90%. Each SAW delay-line velocity change, as well as the difference between both of them, is monitored. We consider that mass loading is the most important effect (Δ f = k Δ m ): water molecules induced only physical modifications on both the organic layer and quartz substrate. This assumption is reinforced by the cell time responses. A diffusion law is obvious from the SAW oscillator frequency-transient analysis and measurements of the corresponding diffusion coefficients.

Transmission Thermoacoustic Imaging Without Contact

Thermoacoustic imaging techniques are considered as a means of detecting surface and subsurface thermal inhomogeneities in solids. Localized heating of the sample is usually produced by means of a modulated laser or an electron beam. Thermal waves1,8 or acoustic waves4,9 may be detected in several ways. Thermal waves essentially depend on optical and thermal properties of the material and elastic waves bring out extra information depending on its elastic constants. Scanning of the sample produces a thermal and/or acoustical image of inhomogeneities in the tested material.