Katir Ziouche

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.

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.

Dispersion of Heat Flux Sensors Manufactured in Silicon Technology.

In this paper, we focus on the dispersion performances related to the manufacturing process of heat flux sensors realized in CMOS (Complementary metal oxide semi-conductor) compatible 3-in technology. In particular, we have studied the performance dispersion of our sensors and linked these to the physical characteristics of dispersion of the materials used. This information is mandatory to ensure low-cost manufacturing and especially to reduce production rejects during the fabrication process. The results obtained show that the measured sensitivity of the sensors is in the range 3.15 to 6.56 μV/(W/m2), associated with measured resistances ranging from 485 to 675 kΩ. The dispersions correspond to a Gaussian-type distribution with more than 90% determined around average sensitivity Se¯ = 4.5 µV/(W/m2) and electrical resistance R¯ = 573.5 kΩ within the interval between the average and, more or less, twice the relative standard deviation.

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.