J. Courbat

Making environmental sensors on plastic foil

With the emergence of the printed electronics industry, the development of sensing technologies on non conventional substrates such as plastic foils is on-going. In this article, we review the work performed and the trends in the development of environmental sensors on plastic and flexible foils. Our main focus is on the integration of temperature, humidity, and gas sensors on plastic substrates targeting low-power operation for wireless applications. Some perspectives in this dynamic field are also provided showing the potential for the realization of several types of transducers on substrates of different natures and their combination with other components to realize smart systems.

Inkjet printing on paper for the realization of humidity and temperature sensors

We report on inkjet printing of silver nanoparticles on paper for the design of resistive temperature and capacitive humidity sensors. Temperature and humidity have shown to have an influence on the electrical and mechanical properties of the Ag printed structures on paper. The passivation of the paper substrate and the Ag structures with parylene was investigated to reduce these effects and improve the stability of the structures. The lines were printed with an average thickness of 0.8 µm and a resistivity of 30 µΩ·cm. The Ag thermometer coated with parylene showed a good linearity with a TCR of 0.0011°C−1, while the capacitive humidity sensor exhibited an exponential response. This study foresees the possibility of using paper as substrate with inkjet printing of silver for the design of low-cost environmental sensors.

Design and fabrication of micro-hotplates made on a polyimide foil: electrothermal simulation and characterization to achieve power consumption in the low mW range

The design of ultra-low power micro-hotplates on a polyimide (PI) substrate supported by thermal simulations and characterization is presented. By establishing a method for the thermal simulation of very small scale heating elements, the goal of this study was to decrease the power consumption of PI micro-hotplates to a few milliwatts to make them suitable for very low power applications. To this end, the mean heat transfer coefficients in air of the devices were extracted by finite element analysis combined with very precise thermographic measurements. A simulation model was implemented for these hotplates to investigate both the influence of their downscaling and the bulk micromachining of the polyimide substrate to lower their power consumptions. Simulations were in very good agreement with the experimental results. The main parameters influencing significantly the power consumption at such dimensions were identified and guidelines were defined allowing the design of very small (15 × 15 µm) and ultra-low power heating elements (6 mW at 300 °C). These very low power heating structures enable the realization of flexible sensors, such as gas, flow or wind sensors, for applications in autonomous wireless sensors networks or RFID applications and make them compatible with large-scale production on foil such as roll-to-roll or printing processes.

Failure Analysis of Micro-Heating Elements Suspended on Thin Membranes

We report on the degradation of platinum micro-heating elements operating at high temperatures. Devices with platinum heaters suspended on micro-machined dielectric membranes were self-heated at high temperature until failure. Optical and SEM observations combined with mechanical deformation measurements and Thermal Laser Stimulation techniques were used to analyze the failure mechanisms of the micro-heating elements. Platinum atoms migration and breaking of the membrane were two failure modes observed. At high temperature, the migration of the platinum atoms was linked to the mechanical stress in the dielectric membrane. The Thermal Laser Stimulation technique revealed the formation of vertical as well as lateral thermocouples at mechanically deformed areas. One explanation proposed is that those thermocouples are the result of Si diffusion from the Si3N4 membrane into the platinum heater as well as electro-stress migration of platinum atoms.

Ultra-low power metal-oxide gas sensor on plastic foil

We report on the design and fabrication of ultra-low power metal-oxide (MOX) gas sensors on plastic foils envisioning their fabrication at large scale and low cost. A complete sensor solution is presented including its packaging at the foil level and the driving/readout circuitry. The latter allowed the sensor to operate in pulsed temperature mode to reduce the power consumption in the sub-mW range. Gas measurements under CO, CH4 and NO2 have proven the proper operation of the sensor. These devices are being developed targeting wireless applications.

Study of bending reliability and electrical properties of platinum lines on flexible polyimide substrates

We have experimentally studied the variation in electrical resistance of flexible platinum lines patterned on polyimide foil when they are subjected to circular bending constraints. The lines were patterned by means of standard photolithography and sputtering deposition. Two different photolithography masks were used for comparative evaluation: an un-expensive transparency mask and a standard chromium mask. Measurements of the temperature coefficient of resistance (TCR) and time stability of the resistance have been acquired for lines bent down to 1.25 mm radius of curvature on a customized bending setup, showing good reliability results. The robustness of the lines has been also assessed by registering their change in resistance while bending at different radii of curvature. The lines showed reliability issues for radii of curvature below 1.25 mm, presenting a resistance variation of 19% for transparency mask-fabricated lines and 9% for chromium mask-fabricated lines. The worse reliability performances of transparency mask lines, compared to the chromium mask ones, was found to be due to their imperfect edges, which promoted the formation and propagation of cracks during bending. The results of the experiments in this work permitted to compare the performances of flexible conductive lines with different geometry and fabricated with two different masks, establishing quantitative and qualitative bending limits for their appropriate operation in flexible electronics systems.

Residual stress and buckling patterns of yttria-stabilised zirconia thin films for micro-solid oxide fuel cell membranes

Introduction Micro-solid oxide fuel cells (micro-SOFCs) have drawn much attention in the last 5 years as promising energy conversion systems for powering small portable electronic devices [1-4]. Micro-SOFCs consist of a thin freestanding cathode-electrolyte-anode assembly (typically <1 μm) prepared by microfabrication techniques used in silicon technologies (Fig. 1). One crucial factor is the thermomechanical stability of these thin-film membranes during microfabrication and device operation. In this study, the thermomechanical properties of freestanding yttria-stabilised-zirconia (YSZ) thin-film membranes deposited by pulsed laser deposition (PLD) for application in micro-SOFCs are investigated and the experimental observations regarding the buckling patterns are simulated.

Inkjet printed colorimetric ammonia sensor on plastic foil for low-cost and low-power devices

A polymeric-based colorimetric gas sensor with its associated electronics targeting ammonia detection at low-cost and low-power is presented. The gas sensitive layer was inkjet printed on a plastic foil used as an optical waveguide. The planar configuration of the sensor makes it compatible with large scale fabrication techniques, such as roll-to-roll processes. Its power consumption and noise level were measured. The sensor exhibited a good sensitivity to ammonia with a theoretical limit of detection of 104 ppb in a constant operation mode. The power consumption was reduced to the sub-mW range when operating in pulsed mode. It foresees applications in the field of wireless systems, for environmental and safety monitoring.

Microfluidic Channels in Porous Silicon Filled with a Carbon Absorbent for GAS Preconcentration

This paper presents the development of a gas pre-concentrator based on a micro-reactor in porous silicon filled with carbon nanopowders in solution. The particularity of this microsystem is its applicability in the fields of atmospheric pollution monitoring and explosives detection. The performances of the device are closely related to the design of the micro-reactor and the choice of the adsorbent material. The benefits of using porous silicon to ease the fixing of the carbon absorbent in the reactor and to enhance gas adsorption are also investigated. Preliminary results on a device based on a carbon adsorbent powder filled in a porous silicon micro-reactor for benzene pre-concentration are reported.

Ink-jet printed colorimetric gas sensors on plastic foil

An all polymeric colorimetric gas sensor with its associated electronics for ammonia (NH3) detection targeting low-cost and low-power applications is presented. The gas sensitive layer was inkjet printed on a plastic foil. The use of the foil directly as optical waveguide simplified the fabrication, made the device more cost effective and compatible with large scale fabrication techniques, such as roll to roll processes. Concentrations of 500 ppb of NH3 in nitrogen with 50% of RH were measured with a power consumption of about 868 μW in an optical pulsed mode of operation. Such sensors foresee applications in the field of wireless systems, for environmental and safety monitoring. The fabrication of the planar sensor was based on low temperature processing. The waveguide was made of PEN or PET foil and covered with an ammonia sensitive layer deposited by inkjet printing, which offered a proper and localized deposition of the film. The influence of the substrate temperature and its surface pretreatment were investigated to achieve the optimum deposition parameters for the printed fluid. To improve the light coupling from the light source (LED) to the detectors (photodiodes), polymeric micro-mirrors were patterned in an epoxy resin. With the printing of the colorimetric film and additive patterning of polymeric micro-mirrors on plastic foil, a major step was achieved towards the implementation of full plastic selective gas sensors. The combination with printed OLED and PPD would further lead to an integrated all polymeric optical transducer on plastic foil fully compatible with printed electronics processes.