Claude Pellet

Fast response humidity sensors for a medical microsystem

A fast response humidity sensor was fabricated to equip a medical microsystem for diagnosis of pulmonary diseases. Its main characteristics are reported in this paper. The sensor is based on a capacitor made of a divinyl siloxane benzocyclobutene (BCB) thin film in between parallel plate electrodes. It was fabricated with compatible CMOS technology. It exhibits good linearity, good sensitivity, and a short response time. Equilibrium capacitance variations versus humidity can be considered linear with a linearity error less than 2% of the humidity level. The sensitivity is 0.1 pF by per cent of the humidity level. The device displays a typical adsorption time of 650 ms, a minimum adsorption time of 400 ms, and a desorption time of a few seconds at ambient temperature. Its performances were compared to other types of capacitive humidity sensors fabricated for the same medical use. The static behavior and the dynamic behavior of the device are reported. They are interpreted according to physical processes of gas adsorption and diffusion in and through glassy polymers. The suitability of the conventional Dual-Mode model to explain water sorption in BCB is discussed. This investigation is a starting point in a modeling process to improve the design of the sensor interface circuitry.

Comparison of three humidity sensors for a pulmonary function diagnosis microsystem

Three capacitive humidity sensors developed for a portable clinical application are presented and compared. The first structures consist of interdigitated electrodes covered by a polyimide sensitive layer. The second structures have the same geometry but include a benzocyclobutene sensitive layer and a heating. resistor. The third structure has been developed with a new geometry, with the electrodes being stacked. Humidity measurement results are presented, in particular sensor response time in absorption that must be very small (less than 500 ms). The influence of the heating on the response time is described. In conclusion, the three sensors are compared and the most suitable structure for our application is indicated. Although polyimide has been widely used for realization of capacitive humidity sensors, we demonstrate here that it is not the best sensitive material for our application.

The Microcantilever: A Versatile Tool for Measuring the Rheological Properties of Complex Fluids

Silicon microcantilevers can be used to measure the rheological properties of complex fluids. In this paper two different methods will be presented. In the first method the microcantilever is used to measure the hydrodynamic force exerted by a confined fluid on a sphere that is attached to the microcantilever. In the second method the measurement of the microcantilever’s dynamic spectrum is used to extract the hydrodynamic force exerted by the surrounding fluid on the microcantilever. The originality of the proposed methods lies in the fact that not only may the viscosity of the fluid be measured but also the fluid’s viscoelasticity, i.e., both viscous and elastic properties, which are key parameters in the case of complex fluids. In both methods the use of analytical equations permits the fluid’s complex shear modulus to be extracted and expressed as a function of shear stress and/or frequency.

Dynamic behavior of a chemical sensor for humidity level measurement in human breath

This paper presents a model of the transient response of a short response time capacitive humidity sensor to evaluate its ability to measure the humidity level in exhaled air. This sensor will be used to increase diagnosis accuracy of pulmonary deficiencies. A dedicated humid air blower was developed to measure the transient response of the humidity sensor exposed to abrupt humidity variations. The model is proposed for extraction of humidity data from experiments or for the sensor response prediction in any humidity conditions. This model relies on the theory of gas diffusion in polymer films. It describes a bidimensional diffusion process as the combination of two one-dimensional mechanisms.

Modeling and optimization of a fast response capacitive humidity sensor

This paper presents a model which predicts the temporal response of capacitive humidity sensors made of parallel electrodes and a polymer sensitive coating. This model is used for the simulation of the response of the sensor subjected to specific conditions. The model includes electrostatic and molecular diffusion calculation. As a validation, the model is confronted with finite-element simulation and experimental results. Experiments were carried out for divinyl siloxane benzocyclobutene (DVS-BCB) polymer films with different film thicknesses and upper electrode dimensions. As an application example, the model is used to simulate the response of several humidity sensor structures exposed to the conditions of a medical application related to breath analysis. The model may also be helpful to predict the effects of fabrication process uncertainties. Because the model used to describe water diffusion in DVS-BCB is based on Henrys law and Ficks law, it could be extended to the optimization of capacitive sensors for other vapors

All‐Organic Microelectromechanical Systems Integrating Specific Molecular Recognition – A New Generation of Chemical Sensors

Cantilever-type all-organic microelectromechanical systems based on molecularly imprinted polymers for specific analyte recognition are used as chemical sensors. They are produced by a simple spray-coating-shadow-masking process. Analyte binding to the cantilever generates a measurable change in its resonance frequency. This allows label-free detection by direct mass sensing of low-molecular-weight analytes at nanomolar concentrations.

Collective fabrication of all-organic microcantilever chips based on a hierarchical combination of shadow-masking and wafer-bonding processing methods

This paper describes a new collective microfabrication process of all-organic microcantilever chips. This method is based on the hierarchical combination of shadow-masking and wafer-bonding processes. The shadow-masking combines deposition and patterning in one step thanks to spray-coating through a polymer microstencil that allows patterning of thermosensitive materials such as PMMA. The shadow-masking parameters have been optimized to obtain suspended microcantilevers characterized by a convenient thickness profile. The resulting PMMA structures were then transferred onto SU-8 chips by using an SU-8 wafer-bonding process. The effect of the UV exposure dose of both SU-8 layers in contact on the bonding quality has been investigated and optimized. With the optimized bonding process, we have achieved the large-scale transfer of microstructures with a yield of 100% and a bond strength of 50 MPa. These microcantilevers were also tested at resonance to determine Youngs moduli of patterned polymers. The low values obtained (below 5 GPa) make these organic MEMS structures strong candidates for highly sensitive sensing applications when used in the static mode.

Water Solubility and Diffusivity in BCB Resins used in Microelectronic Packaging and Sensor Applications

This paper presents a simple measurement method to estimate water solubility and diffusivity in dielectric polymers which have low sorption rates and high diffusivities. The polymer dielectric is sandwiched between electrodes to create a specific capacitive structure. It is then exposed to step-like humid air stimuli. The resulting transient capacitance increase is recorded and the sorption rate and the diffusivity of water in the polymer are numerically extracted from experiments. The extraction model is derived from the theory of gas diffusion in polymer coatings. After experimental validation on cyclotenetrade 4024 processed with hard cure, the method is used to compare the impact of different cure methods on the solubility and the diffusivity of this material and of aqueous developable photoBCB. Hence good candidates are determined for microelectronic packaging and humidity sensor applications

Accurate model of the dynamic response of a capacitive humidity sensor

This paper presents a model simulating the transient response of a capacitive humidity sensor made of a polymer sensitive film. The model is based on the theory of molecule diffusion in polymer films. The confrontation of this model with finite element simulation and experimental results demonstrates its accuracy. This model could be implemented in a microprocessor for data extraction.

Capacitive humidity sensors based on oxidized PhotoBCB polymer films: enhanced sensitivity and response time

A fast response capacitive humidity sensor was previously designed with a PhotoBCB film, cured in inert atmosphere with O2 plasma surface modification. Satisfactory sensitivity (0.15 pF/%RH) and response time (less than 2 s) were due to the oxidized film surface which improved water adsorption. In this paper, full oxidation of the film has been considered to design a new fast response humidity sensor with enhanced sensitivity and response time. The sensitivity of the new sensor is 1.3 times higher, and its response time is less than 350 ms at ambient temperature, thus being more than four times faster than the previous sensor. The enhancement of the sensitivity is particularly noticeable under high humidity levels, and this new sensor could be suitable for medical applications. Future improvements consider decreasing the thickness of the film, with support of predictive simulations