Pascale Godts

Simultaneous fabrication of superhydrophobic and superhydrophilic polyimide surfaces with low hysteresis.

Polyimide is of great interest in the field of MEMS and microtechnology. It is often used for its chemical, thermal, mechanical, and optical properties. In this paper, an original study is performed on controlled variation of polyimide film wettability. A two-step microtexturing method is developed to transform hydrophilic polyimide surfaces into a superhydrophobic surface with low magnitude of hysteresis (Δθ ≈ 0° and contact angle θ ≈ 158°). This method is based on the conception of a new kind of fakir surface with triangular cross-section micropillars, the use of a two-scale roughening, and a C(4)F(8) coating. We demonstrate that the absence of hysteresis is related to a combination of two scales of structuring and the pillar shape. The technology that has been developed results in the simultaneous fabrication of adjacent superhydrophobic and superhydrophilic small areas, which allows an effect of self-positioning of water droplets when deposited on such a checkerboard-like surface.

Thin foil planar radiometers: application for designing contactless sensors

This paper is devoted to describing a new sensor allowing one to measure the net radiant flux exchanged by the wall surface it is mounted on. The device is constructed by mounting a thermopile-type radiometer on a larger thin metallic foil support. When the emissivity of the paint covering the support is the same as that of the wall surface on which the sensor is applied, a direct reading (positive or negative emf) of the radiant flux (absorbed or emitted) by the wall surface is given, whatever the convective losses. The calibration is carried out in a simple and useful apparatus designed to produce a prescribed total radiant exchange between two metallic plates at different temperatures and is estimated to be accurate to within two per cent. Simplicity and ruggedness make the radiometer appropriate for direct measurement of heat exchanged between surfaces heated up to 500 K. Notable applications include use as a traditional total hemispheric radiometer and a contactless temperature difference sensor.

Direct measurement of the latent heat of evaporation by flowmetric method

This paper describes an original and very simple device dedicated to measure the latent heat of evaporation of various liquids at ambient temperature. This apparatus is composed of two very thin heat flow sensors glued to a heat sink. One of these sensors is covered with oiled textile, and the other one with an identical dry textile intended to receive the liquid to be characterized. This differential system allows compensating the disturbances due to environment. A thermodynamic model is established to determine the heat flow induced by evaporation. By integrating this flow, one calculates the quantity of latent heat absorbed by evaporation. Numerous experiments with different liquids such as alcohol, acetone, water, and trichloroethylene have been carried out. The results are compared with theoretical values, and the maximum error is less than 6%.

Design of Textile Heat Flowmeter Combining Evaporation Phenomena

This article describes the development of a new thermal flowmeter based on the principle of thermopiles measuring heat flow through a wall. The principal advantage of this sensor is the use of a textile auxiliary wall, which gives it flexibility and enables it to be applied to rounded, deformable surfaces. Another property, related to this type of wall, is that it is permeable to moisture. In fact, this sensor takes into account the phenomena of evaporation which, until now, was not carried out. This allows a better measurement of the energy during heat exchange.

Thermoelectric infrared microsensor using suspended membranes made by silicon micromachining

The main activity of our laboratory is based on the design and the fabrication of thermal microsensors whose infrared radiometers are a significant part. These microsensors work by converting infrared radiation into spatial periodical temperature gradients patterned on substrate. An array of plated bimetallic microthermocouples is designed to produce the output voltage from the maximum generated temperature differences. Until now, planar sensors featuring a metering area from 3 to 100 mm2 have been realized on glass or kapton (submitted for SPIE 2001) substrates. To take advantages of silicon technology, it is necessary to adapt the structure of the sensors to such a material. Indeed, the sensitivity of this kind of sensor is approximately proportional to the thermal resistance between hot and cold junctions. Thus, taking into account the low thermal resistivity of silicon, membranes must be recessed what makes it possible to strongly increase the thermal resistance of the zone located under the junctions of the thermocouples. However, to preserve the advantage of immunity against convection, these membranes must present quite particular properties of symmetry and periodicity compared to the thermocouples. In this case, the microthermopile is constituted of alternatively doped N and P polysilicon layers. Using a simple model, operation principle is analyzed and the microradiometer sensitivity can be computed. The microsensors are manufactured within the framework of a national project (CNRS Interlab) and the successive steps of the technological process are described. Lastly, the first experimental results are presented.

A new method to improve the efficiency of the heat flow path of a micro thermoelectric generator

This work proposes a new method to improve the efficiency of the heat flow path of a micro thermoelectric generator (μTEG). A silicon heat concentrator is placed on a planar μTEG after alignment to guide the heat flow from the heat source to the hot junctions. The results show that the heat concentrator can efficiently isolate the heat source from the cold junctions and guide the heat flux to pass through the planar thermocouples in order to generate a temperature gradient. When a 4W/cm2 power is injected to a fabricated device, it builds an output voltage of 29V/cm2 which can supply an output power of 41μW/cm2 to a matched load resistance.

Distribution-patterned radiometers: a new paradigm for irradiance measurement

This paper introduces a new approach to measuring infrared radiant flux density. Thermal sensors featuring a significant metering area from 3 mm2 to 25 cm2 can be achieved by way of distributing the sensing surface upon an appropriate plated-planar thermopile. Despite a low figure of merit regarding bimetallic structures, low noise, rugged, thin and even flexible devices are made in the laboratory. Such sensors have neither to be covered with a protective widow nor to be placed in an insulating gas, thanks to their inherent immunity against convection afforded by the differential behavior of their structure. Hence wide spectrum infrared measurements, and experiments undergoing a wide range of pressure, are allowed with distribution-patterned radiometers. Current techniques of manufacture are reviewed together with the philosophical arguments concerning the distributed layout of monolithic thermopiles. Since such devices can be directly deposited upon various dielectric materials, many an application in military and space research can be expected. As regards industrialization, those multipurpose sensors meet the necessary requirements of self-calibrating ability, good reproducibility, fast response (#20 ms), ruggedness, and low cost. It is expected that the versatility of the device will result in a wide number of industrial applications.

New developments on IR distribution-patterned microradiometer family

In this paper the last results obtained in the field of design and realization of planar infrared microsensors are presented. These sensors can be considered as a pattern of cells electrically serialized. For each cell, the incident radiation is converted into heat transfer by way of alternatively absorbent and reflecting areas. The center of each area is crossed with a thin microthermocouple whose hot and cold junctions are submitted to the superficial thermal field. By using micromachining, cell dimensions can be shrinked and 5 X 5 cm2 microsensors have been manufactured with more than 3000 cells. KaptonTM is used as substrate and a liquid resin polyimide intended to constitute the infrared absorbing layer. To determine the intrinsic absorption spectrum of this resin, processing a membrane of some cm2 was needed. In this case, the spectral transmittance of this membrane was measured with an infrared spectrometer (Perkin - Elmer) and absorptivity can be mathematically deduced. The sensitivity represented by the ratio between the voltage delivered by the sensor and the absorptive heat flux is calculated by the way of a monodimensional analytical model dependent on a parameter representing the penetration depth of heat in the monolithic substrate. This parameter is computed from 2D finite elements modeling and takes into account the geometrical characteristics of each basic cell constituting the sensor. Finally, by multiplying the absorption spectrum with the sensitivity, the curves of sensors spectral sensitivities in the range 5 - 20 μm can be deduced.

A new self calibrating radiation planar microsensor. Application to contactless temperature measurement

Compared to conventional radiometers, this new microsensor, realized with thin film technologies presents a different location of the hot and cold junctions as well as symmetrical structure of the latter. Hot junctions are achieved by way of an infrared absorbing layer whereas the cold ones are coated with a thin golden reflecting film. The structure is designed so as to allow the microsensor to be manufactured onto a plane substrate with no need of etching cavities as for classical thermopiles. The latter often consists of a small number of thermocouples with enhanced thermal resistances while this new sensor trades this feature against numerous thermocouples fitted with very low internal thermal resistances. This microradiometer is therefore largely immune against convection mechanisms and remains operative in most circumstances without any encapsulation attachment. These overriding characteristics allow a direct control of the microsensor temperature by an independent heating for calibration or measurement purposes. Thus the device includes a planar thermocouple directed to measuring the average temperature of the sensor. The specific structure of this microradiometer enables a calibration-free and contactless temperature measurement which are original features of prime importance. The main purpose of this study is the realization or a contactless microthermometer allowing the effect of emissivity variations to be screened out. This device will prove to be instrumental in numerous industrial applications.

Optimisation théorique et expérimentale des caractéristiques géométriques et électriques du transistor à effet de champ à grille submicronique

RésuméLes auteurs présentent ici les résultats d’une étude théorique et expérimental de l’influence des caractéristiques géométriques et électriques d’un transistor à effet de champ sur les performances dynamiques. En particulier ils montrent que pour obtenir les performances maximales, il faut travailler à canal ouvert, réduire la distance entre le début de la zone creusée du côté source et le début de la grille, désymétriser la grille dans le canal creusé pour diminuer la conductance et la capacité de drain, enfin réduire les résistances d’accès en particulier la résistance de grille par l’utilisation d’une géométrie en Té. En utilisant ces principes, nous avons réalisé desmesfet de 0,2 μm de longueur de grille avec des transconductances hyperfréquences supérieures à 1,4 S/mm et des fréquences de coupure proches de 110 GHz.AbstractIt is shown that the performance of submicronic gateMesfet’s strongly depends on the geometrical and electrical characteristic of the source-gate and gate drain access region. Significant performance increase is obtained by: i) working in enhancement mode at high positive gate bias voltage; ii) reducing as much as possible the distance between the source-end of the recess and the gate; iii) reducing drain conductance and capacitance making a correct positioning; iv) using a T-shaped gate in order to reduce gate access resistance. We report on the fabrication and characterization of GaAsmesfet’s with 0.2 μm gate lengths yielding microwave transconductances up to 1.4 S/mm and cut-off frequencies up to 110 GHz.